Note: Descriptions are shown in the official language in which they were submitted.
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CONTINUOUS METAL FIBER BRUSHES
This invention relates to fiber brushes, and in particular, the improvements
in the
design and manufacture of fiber brushes of the type disclosed in commonly
owned U.S.
Patents 4,358,699 and 4,415,635.
Although graphite and metal-graphite brushes have for nearly 100 years
dominated
the field of electrical brushes, for many applications there now exists a
superior form of
sliding electrical conduction; high performance fiber brushes wherein
typically the fibers
axe made of metal for which reason they are called metal fiber brushes. Prime
candidates
for this new technology include sliding electrical systems which require high
current
densities, high sliding speeds, low electrical noise, high efficiency (low
brush losses),
compact size, or long brush lifetimes.
In particular, low voltage electric motors and generators can be made smaller,
more
powerful and longer lasting owing to the increased current capacity, higher
efficiency and
longer wear life. This has a direct bearing on electric vehicular and ship
drive systems as
well as low voltage electrical power generators. Other applications which
require high
currents, such as high-force linear actuators, electromagnetic brakes, and
armatures, are
similarly well suited.
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Many signal-critical electronic devices such as rotating antennae, slip rings
and
shaft pickups for electronic sensors and other transducers could greatly
benefit from the
low noise and low voltage drop characteristics of metal fiber brushes. In
addition, the new
generation metal fiber brushes can be manufactured with dimensions as small as
fractions
of a millimeter with user-selected stiffness (as measured in applied brush
force in Newtons
per millimeter of resulting brush compression, for example), making them
usable as close-
proximity, multiple-pole sliding pickups. They are also superior for delicate
rotating
instruments, since the required brush forces are much lower than for typical
graphite based
brushes. The broad-band electrical "noise" emission spectra of electrical
equipment such as
drills, saws and other power tools can be greatly reduced by the use of metal
fiber brushes,
thereby reducing or eliminating the electrical interference through these
brushes in use near
sensitive electronic equipment.
As an interface, metal fiber structures and material can provide a low loss
connection at greatly reduced forces, thereby providing high-efficiency, low
force electrical
contact. This is particularly important for high-current, low voltage
switching, such as
encountered in variable voltage battery storage systems which are charged at
high voltages.
Based on simple laws of physics, the capability of fiber brushes to
efficiently transfer
electrical current across interfaces which are in relative motion or at rest,
is paralleled by
their capability to similarly transfer heat. Therefore the brushes can also be
used as heat
transducers for cooling or heating purposes. The outstanding features of metal
fiber
brushes and some suggested applications are listed as follows.
High Current Capacity
Because metal fiber brushes can operate at very low losses, and consequently
at low
heat evolution rates, they can conduct higher current with lower losses than
graphite based
brushes. Continuous current densities of over 314 A/cm2 (2000 A/in2) have been
demonstrated and this does not by any means represent an upper limit.
Accordingly,
equipment which operates at high currents and low voltages can be made more
efficient and
in many cases can run at higher power levels. Examples of this type of
equipment include
homopolar motors and generators, which have applications in electric
automotive, rail and
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ship drives, low voltage generators, such as those used with fuel cells and
with such
applications as the hydrolyzation of water for combustible fuel production.
Similarly,
linear high current devices, such as linear actuators, and linear pulse
generators.
Low Electrical Noise
. As already mentioned above, metal fiber brushes can operate at much lower
electr~al noise levels than traditional graphite-based brushes. This can have
dramatic
benefits for signal-critical equipment on two fronts. First, instrumentation
which requires
rotating or linear sliding contacts, such as rotating antennae, can achieve
much higher
signal resolution than with graphite-based brushes. Second, machinery will
give off much
less electrical noise and therefore cause much less induced interference when
located in
close~roximity to sensitive transducers, detectors, and other electronic
equipment if metal
fiber inrushes are used.
Long;Wear Life
Metal fiber brushes can achieve not only low dimensionless wear rates,
measured in
wear length of brush shortening per length of sliding path, but they can also
be constructed
with very long, and in some cases nearly unlimited, permissible wear lengths.
This
translates to extremely long brush life and greatly lengthened service
intervals. For
exan~le, metal fiber brushes have demonstrated a dimensionless wear rate of
2xI0-", and
at this rate a brush will wear by 5 cm of wear length over 2.5x109 meters of
sliding path,
or over 1.5 million miles. Obviously, continuously operated equipment would
greatly
benefit from this feature of metal fiber brushes.
Hig Tiding Speeds
Many applications such as high speed motors and generators require electrical
brushes which can operate at high sliding speeds. Metal fiber brushes have
been
successfully operated at speeds in excess of 70 mls and their theoretical
limit certainly lies
considerably higher than that.
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Com acp t Size
Electronic systems which need close proximity to a moving power or signal
coupling, or space-critical sliding contacts could be further miniaturized by
the use of this
new generation of metal fiber brushes because these brushes can be made in
sizes down to
fractions of millimeters in thickness or diameter. This has a particular
application relating
to signal power, and control-line pickups from rotating shafts such as are
found in
satellites, aircraft, periscopes, or many kinds of rotor testing systems.
Low Heat Dissi~tion
Because they operate at low loads and have very low resistance, metal fiber
brushes
dissipate much less heat than typical brushes in high-current or high-sliding-
speed
applications. This could be of great benefit in insulated or temperature
sensitive equipment
such as refrigeration systems or devices that incorporate compact rotating
electronics.
Cl~erating
Unlike graphite-based brushes, metal fiber brushes do not generate fine carbon
dust,
which can cause problems not only with appearance and clean-up but also with
long-term
fouling and shorting. Metal fiber brush wear debris is heavy enough to be
easily trapped
or filtered making it therefore much easier to keep the system clean.
In addition, an advantage of metal fiber brushes is the smaller production of
presumably more benign wear debris as compared to that of graphite-based
brushes. At
anticipated similar dimensionless wear rates of conventional and metal fiber
brushes,
reduction of wear debris volume from the latter is due to smaller running
areas on account
of increased current densities in combination with the fact that typically 80
% to 90 % of the
brush is voidage, ( 1 - f) with f the "packing fraction" of the volume
occupied by fibers,
which does not produce wear debris. The extreme limits of packing fraction
range between
1 % and 90 % .
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neccrintion of the Invention
a General Considerations
The previous metal fiber brushes suffered from the following problems;
- difficulty of manufacture
- limitations on the achievable relationship between macroscopic brush
stiffness and microscopic fiber compliance
- problems associated with the necessity of using a removable constituent
during manufacturing
- limitations on the types of metals usable as conductors in the brushes on
account of the need for differential etchability or dissolution of the matrix
material.
The ideal, therefore, are fibers assembled into the form of rods (brush-
stock),
typically but not necessarily straight and of constant cross section, which
locally leave the
fibers within them individually flexible such that the properties at the
interface to the
conducting surface do not change if run end-on even for long periods of time
so as to cause
considerable wear.
~i General Characteristics o~3rush Stock
The most important feature of fiber brushes is that at any one moment a large
number of fibers, electrically connected to a current supply or sink, touch
the interface (the
rotor or substrate) which is electrically connected to the opposite pole. This
requires that
the fiber ends are at least somewhat independently mobile so as to be free to
"track° the
substrate contours. The efficient production of fiber brushes is therefore
possible through
the construction of "brush-stock" incorporating a multitude of electrically
conducting fibers
(preferably of 0.2mm diameter or less) in a mechanically stable arrangement,
which fibers
extend along the brush stock for individual lengths not shorter than the
brushes to be cut
from the brush stock, and are substantially evenly spaced with a packing
fraction f ranging
as high as 70 % or as low as 2 % for special applications, but more typically
varying
between 10% and 20%. In the previous Patents 4,358,699 and 4,415,635,
otherwise
comparable brush stock included a matrix material in which the fibers were
embedded and
which had to be etched away or dissolved in order to expose the fibers. The
present
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invention substitutes empty space, i.e. "voidage", for such matrix material
and the
improvements which are necessary in order to accomplish this.
In principle, making such brush stock including voidage instead of a matrix
material, requires the production of tows, felts, weavings, ropes, spooled
layers or braids
of fibers, in any combination, and to shape these into brush stock of a
predetermined shape
which without imposed forces includes a predetermined voidage and is
mechanically strong
enough to withstand the lengthwise brush pressures (typically up to a few
newtons per
square centimeter) without being crushed, and the bending forces on the
brushes made
from the brush stock which result from the friction between brush and rotor or
other
substrate. It also requires means by which to cut the brushes from the brush
stock and
producing working surfaces at which the fiber ends are individually flexible.
Note,
however, that high flexibility in regard to bending can be an advantage in
case long pieces
of brush stock are guided through suitable "guides" or apertures, if desired
arranged so as
to be pushed forward against the contacting surface through their own internal
stress, much
like a constant-force spring.
Such brush stock is characterized by the common feature that its cross
section, or
the cross section of its outer shell, is shaped to suit the intended
application conditions of
the brushes cut from it.
c. Fiber Materials
The basic requirement for the fibers is that they be electrically conductive.
This
means that they also are good heat conductors and that the brushes may be used
for heat
transfer across interfaces in the same manner as for current conduction.
However, not all
fibers within a given brush stock have to conduct current but some may have
the purpose
of increasing the mechanical stability of the brush ("support fibers"), and
also for various
other reasons fibers of different materials, cross sectional shapes and
diameters may be
used in the same brush.
In applications involving high current densities, the fibers are preferably
made of
the traditional metal conductors, specifically copper, silver, gold and their
various alloys
including brasses, bronzes and monels as commonly used in technology. On
account of low
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cost and low intrinsic electrical resistivity, aluminum could in principle be
useful,
especially for physically large brushes, but it is prone to a high film
resistivity and cannot
be commercially obtained in fiber diameters thin enough for most purposes.
Under demanding conditions when cost is of little concern, besides gold, ~a
variety
of noble metal and metal alloys comprising silver, gold, rhodium, palladium
and/or
platinum in various proportions, a number of these which are available
commercially, will
be very useful. For protection from oxidation and corrosion of the base
metals, platings of
these noble metals are valuable. For use in conjunction with liquid metals,
especially the
sodium-potassium eutectic which is fluid at room temperature, niobium fibers
are superior
and would be difficult to replace. For commutating applications, prospects are
good for
cadmium or cadmium alloy fibers, and for use in rail transportation iron and
its alloys, i.e.
steels, importantly among them stainless steels are useful. Further, for some
purposes,
e.g. tarnish resistance, reduction of friction, provision of a protective
layer for the
substrate or rotor surface, wear rate reduction or facilitation of alloy shape
fixing or
eutectic bonding (see below) fibers are advantageously provided with suitable
platings, e.g.
of copper, silver, nickel, gold or other suitable metals or non-metals. Also,
carbon/graphite may be used as fiber material and graphite or diamond plating
can be
invaluable for some applications. Finally, especially at high temperatures
semiconductors
could also be used, among them germanium and silicon.
d Fiber Shades Internal Brush Friction
The cross sections of fibers will ordinarily be circular but they may be
arbitrarily
shaped, e.g. be elliptical, triangular, quadratic, polygonal, strip-like with
or without
curvature, and tube-like with one or multiple bores and have arbitrary
external cross
sections, as may be suitable for different purposes. In particular, strip-like
fibers oriented
with their long axis parallel to the sliding direction may facilitate
reversals of sliding
direction during operation, and bores may contain lubricants or be used for
cooling
purposes or delivery of cover gas. Also required are means to establish and
maintain a
desired fairly uniform distribution of the fibers at a predetermined packing
fraction.
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e. "Interior" StrenEthening Through Eutectic Bonding and/or Allo~pe Fixing
Often, especially at low packing fractions as may be desirable in order to
conserve
costs in case of noble metal fibers, one may want to make the brush stock
stiff largely
without regard for internal friction. In fact, the brush stock can be greatly
strengthened by
setting the touching points, or joints, in place through local soldering or
welding.
According to the present invention this is accomplished particularly
effectively through
"eutectic bonding". Stiffening of the brush stock without increasing internal
friction is
accomplished through "alloy shape fixing", wherein the momentary shape of the
fibers is
set into place through annealing at or above the recrystallization
temperature.
f. Surface Treatments
The inventors realized that a rod-like, tube-like or strip-like fiber assembly
as
discussed would perhaps not necessarily need, but would mostly benefit from,
some
"surface treatment" to counteract the tendency for unraveling of the fibers
about the
circumference and at the rotor surface. "Surface treatments" include any and
all treatments
which will join the peripheral fibers more firmly together than interior
fibers or to provide
some kind of strengthening "skin" . The effect of such surface treatments is
to protect the
macroscopic brush shape against splaying apart under the applied lengthwise
force,
preventing fibers at the surface to fluff out or unravel, and to increase the
resistance of the
brush stock against imposed forces, e.g. bending on account of friction
against the
tangentially moving rotor surface.
Surface treatments can take the form of an external casing of a material or
geometrical construction different from that of the rest of the brush stock,
into which the
fibers are inserted or which is formed about the fibers. A surface layer can
be applied
through some treatment of the outermost layers of fibers, e.g. through
spraying onto the
brush stock a material which hardens. A sheath can be applied through wrapping
the brush
stock with a suitable foil or with metal leaf, with or without subsequent heat
treatment to
induce eutectic bonding and/or alloy shape fixing (see below) on the surface
layers.
Alternatively, surface treatments may be applied through rolling in a powder
or slurry,
through dipping in a liquid, or through electro-deposition or electroless
deposition.
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Specifically, eutectic bonding can be used for surface stiffening via any
application of Sn or
In in conjunction with silver, copper, silver alloy and copper alloy fibers.
It can be
accomplished, for example, by wrapping the fiber bundles (in previous
experiments of Cu
or Ag or brass) with an outer sheath of copper or brass foil lined with an Sn
or In foil.
The sheath is then essentially soldered to the fibers on heating to the
melting temperature
of the Sn or In.
g~pa~ial or Complete Filling of Voidage
For the further improvement of fiber brushes the inventors had envisaged to
mix
graphite with the fibers to provide a lubricating and protective film for use
in the open
atmosphere. However, problems have been encountered with the intended
admixture of
graphite powder in the process of brush stock manufacture since it interferes
with the
eutectic bonding of silver and copper. However, graphite can be injected into
the brushes
as a slurry after completion.
h. Brush Loading
A further consideration in the use and operation of metal fiber brushes is the
mechanical loading applied to the brushes during use. Metal fiber brushes can
conduct
very high current densities but require much lighter mechanical loading than
conventional,
"monolithic" brushes. Moreover, the brush force has to remain constant within
reasonably
close, predetermined limits, independent of the length of brush wear. This
causes a
problem because 1), the constant-force springs widely used for conventional
brushes have a
much too high electrical resistance for the purpose, especially if they are
designed for low
loads, and 2), conventional current leads capable of conducting the required
high currents
to and from the brushes, are stiff and interfere with the intended light
mechanical loading.
Furthermore, for practical mass applications, fiber brushes will eventually
have to be
soldldistributed in a packaged form which protects them from damage during
storage,
shipment and handling, and which is designed for fool-proof installation by
private persons
or unskilled workers, much like light bulbs or printer cartridges.
U.S. Patent 4,415,635 envisaged metal fiber brushes composed of hair-like
metal
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fibers protruding from a matrix material and conducting current to an
electrically
conducting surface (typically in relative motion to the brushes) against which
the fiber ends
were lightly, mechanically pressed. U.S. Patent 4,358,699, greatly elaborated
on different
possible configurations of the concept of using hair-fine wires in electrical
brushes,
including the fibers contacting the conductor along their long surfaces, being
felted or
woven together, and strengthened in various manners, including by the
incorporation of
"support fibers°, being fibers which are substantially more rigid and
of a length a little
shorter than the average fibers so as to protect these from accidental damage.
The
drawback of other than end-on contact between fibers and opposing conducting
surface is
too short a wear-life. Namely, wear by one fiber diameter shortens a fiber
little if it occurs
end-on but cuts off a whole length of fiber if it occurs on a lengthwise
surface.
Accordingly, one object of this invention is to solve the problems associated
with
the prior art metal fiber brushes.
The present invention concerns a brush stock for an electrical fiber
brush, comprising:
plural conductive elements including at least one of plural conductive fibers
and
plural conductive strands of fibers; and
said conductive elements having contacting engagements with each
other at irregularly longitudinally spaced contact points with the contacting
engagements maintaining elastic stresses between said conductive elements
and maintaining irregularly longitudinally extended voids between said
conductive elements-
The present invention also concerns A brush stock for an electrical fiber
brush, comprising:
plural conductive elements including at least one of plural conductive fibers
and
plural conductive strands of fibers; and
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said conductive elements having contacting engagements
interconnected by longitudinally extending fixed in shape segments of
saidconductive elements so as to maintain irregularly longitudinally extended
voids between said conductive elements.
A further object of this invention is to provide a new and improved electrical
fiber
brush stock from which electrical brushes can be cut having low electrical
contact
resistance, and associated therewith low interfacial heat generation and a low
sliding wear
rate.
A further object of this invention is to provide novel fiber brushes in which,
at the
interface to the conducting surface, the fibers are individually flexible.
Yet another object of this invention is to provide a new and improved method
of
manufacturing metal fiber brushes.
Yet another object of this invention is to provide a fiber brush that has a
long wear
life and does not change its characteristics through wear.
Another object of this invention is to provide a fiber brush which is compact
in size.
Yet another object of the invention is to provide an electrical brush which
emits
little electrical noise.
Yet another object of the invention is to provide an electrical metal fiber
brush
which can. be used with high current densities.
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which can be used with high current densities.
Still a further object of this invention is to provide a new and improved
brush
holder and loading device which maintains constant brush force while the brush
wears.
These and other objects are achieved according to the present invention by
providing a new and improved metal fiber brush including a brush stock having
plural
conductive elements and a cross section shaped in accordance with the intended
use of the
fiber brush. Some of the fibers may have plural bends along the length
thereof. In
addition, there is provided a new and improved method of making a conductive
fiber brush
including providing fibers, and bundling the fibers into a brush stock in
which the fibers
are in contacting engagement with each other maintaining voids between the
fibers. This
can be accomplished by means of a suitable die or form, within which the fiber
arrangement concerned is constrained, or compressed, or into which it is
permitted to
expand, so as produce the desired cross-sectional form of the brush stock. The
brush stock
shaping may in commercial production be replaced or complemented by extrusion,
continuous rolling or other reshaping methods, all while producing the final
desired
voidage.
According to yet another aspect of the present invention, there is provided a
hydrostatically controlled brush holder mounting a conductive brush, and a
conductive
hydrostatic fluid coupled under pressure to the brush holder to control the
force application
to the brush as well as lead the current to it.
Still another aspect of the present invention, there is provided a brush
holder which
uses the elasticity of the brush stock to guide the brush stock forward
against the contacting
surface.
Brief Descn~tion of the Drawings
A more complete appreciation of the invention and many of the attendant
advantages thereof will be readily obtained as the same becomes better
understood by
reference to the following detailed description when considered in connection
with the
accompanying drawings, wherein:
Figure la is a schematic side view illustrating a use of the fiber brush
according to
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the present invention. Figure lb illustrates a strip-like fiber inclined to
the substrate
surface in the plane normal to the sliding direction;
Figure 2 is a schematic illustration of a kinked fiber mass of a brush of the
present
invention showing the multiple touching points caused by the kinking, waving,
spiraling,
etc. These touching points cause elastic stresses which tend to keep the
fibers from
bunching together during sliding, and also serve as possible bonding sites;
Figures 3a and 3b are side and end views, respectively, of one possible
embodiment
of an electrical fiber brush made using kinked fibers. Figure 3c is a
perspective view of
the casing surrounding the fibers in Figure 3b, and Figure 3d is a perspective
view of
triangular casing. Typically, but not necessarily, a casing consists of a
bonded kinked
metal fibers. Figure 3e shows a sheath in the process of being applied through
wrapping a
foil strip of Width Ds about the cylindrical brush stock at an inclination of
angle y against
the brush stock axis. Instead of a foil, the sheath can consist of a wrapping
of fibers or,
conversely, a wide foil, or any combination of these. Figure 3f shows the
cross section of
a rectangular brush stock including a surface layer which might have been made
through
dipping the brush stock into a suitable medium or spraying it. Alternatively,
the surface
layer could have been formed by arranging the fibers near the brush stock
surface to be
more densely spaced than for the average of the brush stock or to be more
strongly kinked,
or the joints to be bonded by any means including irradiation,
electrophoresis, or
electroplating. The non-sliding end of the brush can be soldered to a mounting
plate or
stub, plated solid, or crimped to create the finished assembly.
Figures 4a-4k show examples of the different types of fiber bending as regular
spiraling, irregular spiraling, regular waving, irregular waving, curling,
regular saw-tooth,
irregular saw-tooth, rectangular bending, regular V-crimping, irregular V-
crimping, and
waving with intervals, respectively. Figures 41 and 4m are illustrations of
waved fiber
strands. one containing three, the other four fibers. Figure 4n shows a
twisted fiber strand
which may be composed of two or more different metals each of which the brush
stock
may be composed partly or wholly. Figure 4o shows two different twisted
strands which
are twisted together;
Figure Sa is a three-dimensional view of a piece of brush stock whose cross
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sectional shape is of a truncated triangle. Figure Sb is a semi-schematic
cross sectional view
of the possible arrangement of parallel spiral-shaped fiber strands of which
the brush body in
Figure Sa could be composed. Figure 5c shows nested concentric spiraled fibers
of which the
brush stock of Figure Sa could be composed in the arrangement of Figure Sb.
Figure Sd shows
a brush stock in the form of a wavy strip;
Figure 6 illustrates "support fibers" as first introduced in U.S. patent
4,358,699;
Figure 7a is a schematic cross-sectional view illustrating a novel mechanical
loading
applied to the metal fiber brush of the present invention. Figure 7b and
Figure 7c show
two different embodiments of loading devices using flexible brush stock of the
present
invention. Figure 7d shows a guide used to guide a free end of the brush stock
in Figures
7b and 7c;
Figure 8a illustrates accordion-pleated layer of fibers or fiber felt. Figure
8b shows
a possible casing or sheath or other surface layer for the accordion-pleated
brush stock in
Figure 8a as compacted into the form of Figure 8c. Figures 8d and 8e show
alternative
arrangements of pleats;
Figure 9 illustrates production of fiber or strand layers by winding, for
future
rolling up or pleating;
Figure 10 illustrates a method of stitching used to stiffen the brush stock;
Figures l la and l lb illustrate other forms of making a brush stock of one or
more
layers of fibers, strands, or felt; and
Figure 12a illustrates production of fibers or strand layers by winding,
similar to
Figure 9, for making nested concentric spiraled fibers. Figure 12b illustrates
the direction
of the fibers or strands in Figure 12a. Figure 12c illustrates rolling up a
layer of fibers or
strands into a cigarette-shaped brush stock to yield nested spirals all of the
same
handedness, e.g., left-handed. Figure 12e illustrates the same method as in
Figure 12c but
illustrates using two layers of opposite inclination (with arbitrary
inclination angles labeled
a and Vii) so as to yield nested concentric spirals of alternating handedness,
and Figure 12d
illustrates an example of a cigarette-shaped brush stock resulting from
rolling up a layer of
fibers or strands as in Figure 12c.
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Best Mode for CarrxinQ out the Invention
neral
The present invention provides metal fiber brushes which at the sliding
interface
operate in the same manner as previously patented metal fiber brushes but
which, unlike
those, are not painter's style but are cut from indefinite lengths of "brush
stock" in the
shape of rods or strips of arbitrary cross section, and which after shaping
and/or surface or
other treatment as described hereinafter have a running surface ready for use
(in contrast to
the prior art which required matrix material to be removed from among the
fibers}. The
brush stock is composed of substantially parallel fine metal fibers (of
diameter < 0.2
mm, most typically about 50 ~cm within a range of 25 ~cm to 100 ~,m) whose
lengths are at
least several millimeters and more typically extend through a substantial part
of the brush
stock if not its whole length. The fibers are constructed so as to preserve,
through
potentially unlimited wear lengths, the characteristic metal fiber brush
running surface,
being composed of a multitude of individually flexible fiber ends. It is this
structure of the
running surface which, provided the film resistivity (i.e. the resistance of
unit area of film,
a critical quantity) is low, conveys the desirable metal fiber brush
properties of (i) low
electrical contact resistance, (ii) low electrical noise, (iii) ability to run
at high speeds, (iv)
ability to be used at high current densities, and (v) ability, indeed need, to
run at light
mechanical pressure and thus low mechanical loss; in all of these respects
greatly
outperforming conventional graphite-based brushes. While in most cases the
fibers will be
made of metal, in some cases they may be of carbon (graphite) or of
semiconductors such
as germanium and silicon, especially if operation at high temperatures is
desired. For
example, Figure 1 shows a schematic side view of a brush (1) in a typical
working mode.
The brush (1) has an indefinite length, an interface at the rotor or other
substrate (4), and
an surface layer or casing (10).
At the sliding interface, during use, the brushes contact the side to which
electrical
contact is being made (the "rotor" or °substrate") via a multiplicity
of individually
moveable fiber ends. Although Figure la schematically shows the fiber brush
with a
normal orientation to the rotor surface, typically a brush is oriented at an
arbitrary angle to
the rotor surface (e. g. , 15 °-20 ° in trailing orientation
and/or up to, say, 45 ° in the plane
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rotor surface. As another example, Figure lb shows a strip-like fiber brush
(8) in a
working mode, wherein it is inclined to the substrate surface (4) in the plane
normal to the
sliding direction. .
The requisite low wear rate of the brushes depends on running them with
elastic
contact spots under access of moisture (as is normally present in the free
atmosphere and
otherwise must be provided). If the simple fiber brush theory holds true, to
this end the
brush pressure must be below p~"S~3x10'~ fH, where p~"$ is the critical force
at the
transition between elastic and plastic contact spots, f is the packing
fraction, i.e., fraction
of metal in the brush volume, and H is the Meyer hardness of the fiber
material (see eq.
lOb of "Electrical Fiber Brushes - Theory and Observations", D. Kuhlmann-
Wilsdorf,
ICEC-IEEE Holm 95, 41st Holm Conference on Electrical Contacts, IEEE,
Montreal,
Canada, Oct. 2-4, 1995, pp. 295-314; reprinted as "Electrical Fiber Brushes -
Theory and
Observations", D. Kuhlrnann Wilsdorf, IEEE Trans. CPMT Part A, _1Q (199G) pp.
360-375 .
Preferably the brush pressure is p =~3 ptrans with 1/4 <~i<'h which, under
otherwise proper running conditions, will lead to dimensionless wear rates
in the 10-11 range (see Figure 2 in the cited paper). The brush pressure is
adjusted so that the typical contact spots) between any single fiber and the
rotor is/are only
elastically, but not plastically deformed. That condition of elastic contact
spots depends on
a low load per individual fiber and is attained at ~i < 1. Correspondingly,
very fine fibers
are desirable, and as discussed above are typically less than 0.2 mm thick. If
the condition
of elastic contact spots is met, both the electrical contact resistance and
the sliding wear
rate are low, as is essential for superior electrical brushes. (The described
nature of the
sliding interface is the same as for the previously patented brushes except
that the role of
adsorbed moisture was not yet known). In addition, for high current densities
and high
sliding speeds the optimum packing fraction range, at time of writing is
between 12-15
for brushes made in the laboratory but, in agreement with the appended paper,
it is
anticipated that it will be near 20% in commercial production.
Preferred Fiber Materials
All conductive materials which can be formed into fibers are potential
candidate
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All conductive materials which can be formed into fibers are potential
candidate
materials for fiber brushes. Preferred choices include the traditional
technological metal
conductors, including copper, silver, gold and their alloys, including among
the copper
allays, brasses, bronzes and monels, all of the named metal fiber choices with
and without
platings, among these in particular gold, silver and nickel. Also preferred
materials are
niobium, rhodium, platinum, and in general noble metal alloys such as are
commercially
available for operating electrical contacts in the open atmosphere, among them
Paliney
alloys. Further, carbon (graphite) and semiconductors including germanium and
silicon
are preferred materials. The choice depends on purpose, serviceability and
cost; e.g. gold,
platinum and rhodium are excellent fiber materials for almost all purposes but
are very
expensive and rhodium (and the harder noble metal alloys) tend to cut the
rotor or other
substrate surface. Among the noble metals, palladium is a preferred
replacement for gold
because it is lighter and much less expensive per troy ounce, with the further
advantage
that it plates well on other metals. As a major drawback, according to best
previous
laboratory experience, palladium tends to catalyze the formation of contact
polymers
which, if present, raise the film resistivity to an unacceptably high level.
Niobium is
almost irreplaceable for use in conjunction with liquid NaK. Nickel and nickel
alloys are
very corrosion resistant and have excellent mechanical elasticity. Further,
nickel as an
under-plate serves to prevent the diffusion of thin gold platings, in
particular, but also a
number of other platings, into the underlying copper. Semiconductors such as
germanium
and silicon are potentially valuable at high temperatures (in that case
probably for high-
cost applications with hard rotor surfaces such as rhodium or platinum group
alloys) but no
experience with these does as yet exist, albeit iridium has been successfully
tried on a very
small scale. In addition, research is occurring on conductive plastic
materials that may be
used. The lower cost of plastic materials and their resistance against
environmental attack
are expected to be major advantages of using conductive plastic materials in
fiber brush
stock.
Control of Brush Stock Strength Through Touching_Points
As in the previous brushes, the individual movability of the fiber ends, on
which
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such that the fibers occupy only a fraction (the "packing fraction") of the
macroscopic
brush volume. Previously, this was attained through letting the fibers
protrude from a
matrix material, typically by a length which was on the order of 100 times the
fiber
diameter. However, use of parallel fibers protruding from a rigid matrix
material a la a
painter's brush has the disadvantage that already relatively minor wear
lengths (compared
to the macroscopic length of the brush) substantially change its running
characteristics and
thereby cause relatively short brush life-times.
According to the present invention, empty space, i.e. "voidage", is
substituted for
matrix. material and the proper packing fraction, "f', may be controlled by
providing bends
in the individual fibers along the length of the fibers, e.g., by crimping,
kinking, waving,
spiraling or curling the fibers in a regular or irregular pattern, so as to
impart "loft". This
facilitates the desired fairly uniform distribution of the fibers and the
desired constant
packing fractions which are maintained in spite of compressive forces in use.
The effect is
due to the establishment of touching points (or "joints") as shown, for
example, in Figure 2
where fibers touch mechanically, e.g. neighboring substantially parallel
fibers, or mutually
inclined fibers at crossing points. For otherwise same fiber morphology and
arrangement,
the average spacing of the touching points along each fiber is controlled by
the manner of
distorting the fibers; for example as is shown in Figures 4a-4k , the fibers
can be modified
through bending, kinking, curling, spiraling, waving, etc. , alone or in any
combination,
with the bending or kinking imparting arbitrary shapes with arbitrary
amplitude and
wavelength .
The conductive elements have contacting engagements with each other
at irregularly longitudinally spaced contact points with the contacting
engagements maintaining elastic stresses between the conductive elements and
maintaining irregularly longitudinally extending voids between the conducting
elements.
A further tool in the construction of brush stock is the use of fiber strands
in lieu of
or in combination with individual fibers. Fiber strands are any bundled or
twisted
groupings of two or more fibers which are used together, e.g. taken off one
spool. A
major advantage of the use of strands is the increased speed of brush stock
construction,
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resulting in cost savings. Another advantage of strands is that they can be
employed as a
further means to control the density and nature of the touching points in the
brush stock.
The fibers in any Tone strand are not necessarily all of the same size,
morphology or
material. Figure 41 shows a bundled fiber strand composed of three individual
similarly
waved fibers and Figure 4m shows a strand containing four fibers. A fiber
strand made
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waved fibers and Figure 4m shows a strand containing four fibers. A fiber
strand trade
through twisting of either individual fibers or of fiber strands is shown in
Figure 4n.
The effect of deviations from linearity of the fibers is to impart "loft" in
much the
same way as is the case for hair or textile fibers. This is due to an increase
of "touching
points" or "joints" among the fibers. The number of touching spots increases
with the
number of bends per unit length of fiber or strand as well as their amplitude,
i.e. the
magnitude of the deviations from linearity. The number of touching points or
joints
decreases with the number of fibers per strand. Geometrically a pre-determined
distribution of fiber joints may be obtained through twisting of two or more
fibers together
into twisted strands as is shown in Figure 40, which may be further processed
like single
fibers, e.g. be bundled, spooled, or layered, or if desired two or more
bundled or twisted
strands may be twisted together once again and the process repeated at will to
effect
roping. In this way a further control of the density and distribution of
touching points, e.g.
among fibers of different materials, diameters or shapes, is achieved. Or else
pre-
determined touching spots can be achieved through bundling, or arranging into
layers,
fibers which have been curled, waved or kinked in any way.
If desired, a roughly uniform distribution of touching points is achieved
through
regular self contained elastic stresses. One example here is weaving and
braiding of
straight fibers. The same effect with a lower density of touching points is
obtained in brush
stock in the form of a set of nested, graded concentric spirals, for example
as is shown in
Figure Sc, made of intrinsically straight fibers, with either the same or
alternating sense of
rotation from the center outward, or any arbitrary sequence of sense of
rotation. Brush
stock which is composed of spirals with only one sense of rotation will, on
brush force
application, tend to twist about the lengthwise axis. This effect is avoided
when employing
alternating handedness of spiraling as achieved through the method of Figure
12e.
Similarly, brush stock may be composed of cells of single or nested Spirals as
iS Shown m
Figure Sb, or in a related geometry the fibers may be loosely roped for
obtaining a low
density of contact spots. Crimping, kinking, waving, etc. of the fibers in any
of these
geometries increases the density of touching points correspondingly.
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Control of Internal Brush Stock Friction
While the effect of the touching Spots is to keep fibers apart through normal
forces
at them, thereby aiding in the even distribution of the fibers and
mechanically stiffening
the brush stack, at the same time through local friction the touching points
impede
lengthwise relative motion between the fibers and thereby interfere with the
desired
individual fiber-end mobility needed for tracking the substrate contour. Those
undesirable
internal friction forces which interfere with fiber-end mobility rise with the
number of
touching spots as well as the average force with which the fibers are pressed
together. Both
of these rise with packing fraction. Therefore in practice the upper limit of
f is controlled
by the degree to which proper brush operation depends on individual fiber end
mobility,
e.g. higher f's may be used at low speeds rather than at high speeds, for
smooth rather
than for rough substrates, for high brush pressures rather than for low brush
pressures.
It may be noted that the advantage of any of the geometries involving spiraled
or
roped fibers introduced above is that they exhibit reduced internal friction
on account of
relatively few touching points, in combination with high reversible
compressibility in
lengthwise direction. The latter is advantageous because it facilitates
"tracking" of the
fiber ends on the substrate. The brush stock stiffness against bending depends
on specific
construction and is evidently low for roping and much higher for the spiral
cell structure.
Lack of stiffness against bending is not necessarily a disadvantage but
requires that
brushes be guided through apertures which fix their position relative to the
contacting
surface at a distance which decreases with increasing brush stock flexibility
in bending.
Given a certain morphology of the fibers, e.g. kinked or waved in a particular
manner to impart "loft", the packing fraction may still be varied
independently, and with
increasing f as well as "loft", the macroscopic stiffness of the brush
increases.
Simultaneously, the ability of the average fiber tip to remain in contact with
the rotor
surface diminishes on account of the increasing number of, and increasing
forces at, the
three-dimensional connections among the fibers, i.e. the touching points,
either through
rigid or frictional bonding, as "joints" which are distributed along the
fibers so as to leave
some average free fiber length between them which shrinks with increasing
packing
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fraction.
In line with these considerations, it is often useful to reduce the
coefficient of
friction at the average touching points so as to reduce the friction among the
fibers and
thereby improve individual fiber end flexibility as well as the length-wise
elastic
compressibility of the brush stock. This can be done through rinsing with a
lubricant. A
diluted colloidal graphite solution has been found to be very suitable in this
regard. Even
minute amounts of such lubrication, amounting on average to small fractions of
1 ~.m layer
thickness on the fibers, have been found to be very effective to reduce
internal brush
friction, and also to be capable of reducing the friction between the brush
and substrate.
aping Brush Stock and Hardening Effect of Partial Filling of Voidage
Brush stiffness is increased by filling the void space ("voidage", i.e., the
fraction
(1-f) of the brush volume not occupied by fiber material) between the fibers
wholly or
partially with a suitable filler material. While this increases internal
friction and for this
reason is mostly undesirable, the filler material may be chosen to serve as a
lubricant,
abrasive, polishing agent or other surface conditioner of the rotor surface,
to be further
discussed below.
In any case, unless roped or spiraled, the brush stock is ordinarily shaped
via some
mold or die. As a result, brushes according to this invention can have ail of
the same
desirable characteristics as the previous brushes but can be worn to
indefinite lengths
without change of properties.
As already indicated, the mechanical firmness of frictional bonding increases
with
packing fraction as well as with the degree of curling/kinking and is thus
controllable;
e.g., for high packing fractions of very thin fibers (for high-performance
brushes with very
low contact resistance), less curling or kinking will be used than for low
packing fractions
(e.g., as for general purpose, low-cost brushes). Examples of the different
shapes of a
brush stock are shown in Figures Sa and Sd. Figure Sa shows a brush stock with
a
triangular shape and Figure Sd shows a brush stock in the farm of a wavy
strip.
Methods for Internal Stren eni s of Brush Stock
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a - Eutectic Bonding
Brushes according to the previous invention, made from brush stock comprising
fibers embedded in a matrix material, had the additional disadvantage that the
fibers tended
to splay apart, exactly as the bristles in a painter's brush, if pressed down
too firmly.
Similarly, when pressed against the rotor or other moving surface, also
brushes obtained
from continuous fiber brush stock will splay apart and in addition tend to
bend. In order
to prevent excessive bending andlor in order to contain the fibers at the
interface more or
less within the macroscopic geometrical brush stock profile, the brush stock
is typically
stiffened at least at its perimeter. In the present invention mechanical
strength, most
importantly against lateral extension or splaying of the brushes during
installation or use,
independent of or beyond that which may be achieved through control of
touching points
on account of friction among the fibers where they touch, or be due to a
filler material, can
be increased either through "interior bonding" (or "interior stiffening") or
through "surface
treatment".
"Inferior stiffening", throughout the volume of the brush stock independent of
void
filling, may be effected through bonding of varying degrees of firmness at the
touching
points, or joints. Entirely rigid bonding may be obtained through what amounts
to
soldering or welding at the joints via "eutectic bonding". In this method a
eutectic
comprising the fiber, plating andlor stiffening material is allowed to form at
about and
above the melting temperature of the eutectic. If the molten eutectic wicks
into re-entrant
corners at fiber touching points, they are effectively soldered when the
eutectic solidifies
on cooling. The copper-silver eutectic, melting at about 800°C, is
particularly suitable for
this method. Eutectic bonding requires physical touching among the
constituents of the
eutectic, e.g. takes place among silver-plated copper fibers, among copper-
plated silver
fibers, or among mixed silver and copper fibers, or mixed fibers of any
suitable alloys of
these metals. A disadvantage here is that on account of the high melting
temperature of the
silver-copper eutectic, the requisite high annealing temperature tends to
destroy the
"spring" of the fibers which is needed for the elastic bending of the fiber
tips in tracking
the surface profile of the opposing surface. Albeit this may be counteracted
by the
simultaneous alloy formation which is the basis of alloy shape fixing,
especially if the
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annealing is followed by a quench (see Alloy Shape Fixing below).
The low-melting (about 200°C) eutectics of copper with tin or indium do
not suffer
from this disadvantage. However, they must be induced in relatively high
concentrations
locally, say through a tin or indium foil embedded between fibers. This is for
the reason
that low-melting eutectics tend to have a low surface tension (since
thermodyamically the
surface free energy is roughly proportional to the melting temperature).
Therefore, if
layered on the higher-melting copper or silver, indium and tin remain spread
rather than
wicking into re-entrant corners and thereby exposing the copper or silver
surfaces of
higher energy. As a result low-melting eutectics tend to only set joints which
are wetted in
the course of forming the eutectic, meaning when a significant excess of
molten eutectic
exists before cooling. Further, the experiments made by the inventors so far
suggest that
both Sn and In can leave a damaging, relatively high-resistance deposit on the
brush track.
This in turn tends to cause over-heating whereupon the Sn (or In) melts and
fuses the fiber
ends together so as to make the brush surface stiff and cause bouncing,
effectively
destroying the brush. It therefore seems, but has not yet been fully explored,
that there
exists a limiting concentration, depending on use of the brushes, above which
tin and
indium eutectics should not be used.
By the use of twisted strands comprising different metals, e.g. silver and
copper in
various proportions alone or together with bundled strands or single fibers of
either or both
of the pure metals, the distribution and concentration of rigid bonds can be
controlled
within the interior of the brush stock.
Instead of directly bonding fibers, one may also use metal powder mixed with
the
fibers, e.g., silver powder with copper fibers or vice versa, in which case
the eutectic
soldering takes place between the powder particles (which typically will
dissolve or, at a
high enough temperature, will melt in the process) and fibers which they
touch. In lieu of
powders one may similarly intersperse metal foil or metal leaf with the
fibers. All of these
methods may be used together in any combination, if desired involving
different metals for
the platings, powders and foils.
b - Allo.~pe Fixing
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In case of very small concentrations of one of the two components used in the
process which otherwise leads to eutectic bonding, e. g. silver leaf on copper
fibers, the
treatment causes the "setting" of the fiber geometry and an apparent
stiffening of the fibers
inspite of the high annealing temperature used, even though optical
microscopic
examination reveals no wicking of eutectic into re-entrant corners and the
joints are in fact
not bonded at all. The inventors have concluded that ( 1 ) this mechanical
stiffening of the
fibers and (2) setting them into place is due to two distinct effects which
happen to occur
simultaneously but can in principle be used independently. Firstly, the
mechanical
stiffening occurs through the diffusion of the low-concentration constituent
(in this case the
silver) into the fibers (in this case the copper fibers), thereby forming the
corresponding
harder alloy. Meanwhile, simultaneously recrystailization took place to set
the now much
stiffer alloyed fibers into the imposed "brush stock" configuration. Simple
arithmetic
suggests that in the present example only the first, say, n < 5 layers of
fibers could have
been so alloyed, which at, say, f = 0.2 packing fraction, and d = SOmm fiber
diameters
with a net film thickness of t = 2mm could have given rise to a silver
concentration in the
copper of c~ = tl(fnd) = 4vol % , i.e. enough alloying to confer considerably
increased
strength to the fibers. Actually, it is questionable whether the alloying was
uniformly
spread through the fibers, although with the speeding up of diffusion via
concurrent
recrystallization this could have been so.
The above leads to an improved method of forming fiber brush stock, via
annealing
plated fibers or fibers mixed with metal leaf or metal powders at their
recrystallization or
alloying temperature, whichever is higher, long enough to let some or all of
the plating
leaf or powder dissolve in the fibers. This simultaneous alloying and
recrystallization is
expected,to increase the fiber strengthlelasticity while it sets into
permanent place the
shape that is concurrently imposed on the fibers via compressing in the brush
stock form,
or as rolled or twisted e.g. as in Figures 12d and e. Beyond this, the
invention includes
the possibility of simultaneously or subsequently using other metallurgical
techniques, e.g.
of establishing concentration gradients in the fibers, or quenching and age-
hardening, to
improve the mechanical or other properties of the fibers. Also, setting into
place may be
done through heating to the recrystallization temperature independent of any
diffusion
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treatment and, conversely, diffusion treatments are possible below the
recrystallizatian
temperature and therefore without setting the momentary shape into place. It
is
conjectured that ordinary eutectic bonding, e.g. with copper fibers plated
with a normal
thickness of silver, did not lead to observed alloy strengthening because the
liquid eutectic
layer was so thick that it quickly contracted into re-entrant corners before
significant
diffusion of the low-concentration constituent (e.g. silver) into the rest of
the fibers could
take place. Correspondingly, the optimal conditions for alloy shape fixing
still require
exploration.
Suitable plated wires for allay shape stiffening are expected to include: (i)
copper-
plated silver, (ii) silver-plated copper, (iii) nickel-plated copper, (iv)
gold-plated copper
with an under-plate of nickel, to name those which are commercially available
(i.e. (ii)) or
can be readily made even in our own laboratory . For maximum hardening at
minimum
loss of electrical conductivity, a zirconium plate on copper or a chromium
plate on copper
would be desirable. As implied by the preceding explanations, it is
anticipated that the
plating thickness and annealing times can be adjusted to either yield an
optimal alloy at full
dissolution of the plating material in the fiber (e.g. for (i) copper into the
silver so as to
reduce oxide formation) or to leave a remnant plating as probably advantageous
in the
other three cases. The particular advantage of (iv) gold-plated copper with an
under-plate
of nickel, is to harden the copper by means of the nickel, and retain a gold-
plate to lay
down on the wear track a thin protective gold layer. Many other combinations
are
doubtlessly possible. Nor is the method restricted to two components, but
three or more
may be utilized, e.g. copper and silver may be diffused into gold alloy
fibers,
simultaneously or consecutively. Also non-metals can be employed, e.g. carbon
can be
diffused into iron or steel fibers.
c - Layering. Rolling-up or Pleating_Fiber Lovers or Fiber Felt
A disadvantage of interior eutectic bonding is that it raises interior
friction. Other
methods in lieu of or in addition to alloying through diffusion described in
the preceding
section may therefore be used to mechanically strengthen the bulk of the brush
stock with
lesser impact on internal friction. One method consists of placing a layer of
fibers or
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strands, not necessarily all parallel, on a flat surface and rolling it up, as
is shown in
Figure I la, or folding or pleating it to the desired shape of the brush
stock. A fiber felt,
consisting of a thin layer of mutually misoriented fibers bonded at a suitable
concentration
of touching points, can take the place of the layer of fibers. Similarly, one
may layer
fibers, felts andlor foils on top of each other and roll them up. Likewise, as
is shown in
Figure 8a, one may pleat the fibers, felts, andlor foils (13) into desired
morphologies, e.g.
by accordion pleating (14) parallel to the long axis of the brush stock,
wherein the
individual fibers may be inclined at moderate predetermined angles, e.g. t
30° to that
axis. Figures 8c, 8d and 8e show alternative arrangements of pleats to achieve
different
brush stock shapes.
Any of these methods strengthen the brush against bending even while internal
friction may be kept low, depending on construction. For example, in lieu of
or in
addition to, internal eutectic bonding or alloy shape fixing, one may spread
straight or
kinked, waved, etc., fibers and/or fiber strands out over a thin eutectically
bonded skin, or
over any suitable foil of, say, 0.1 mm thickness, and roll up the assembly
(Figure l la) or
fold it (Figure l lb) appropriately into the desired brush stock shape. One
may then either
rely on the extra strengthening effect through the skin or foil, or one may
with appropriate
choice of fibers continue with a eutectic bonding or alloy shape fixing heat
treatment. In
addition, in Figure 8b, a possible casing ( 15) or other surface treatment for
accordion-
pleated brush stock {1) with according pleats (14) may be made of foil or a
layer of bias-
oriented fibers or strands, perhaps eutectically bonded as with any
combination of Ag, Cu,
Cu-plated, an Ag-plated fibers. Alternatively, one may interleave for example
copper
fibers destined for the brush stock interior with silver leaf of only l~cm
thickness or less
and use the alloy shape fixing treatment. The requisite heating is such that
the soldering
and welding might be performed by rf induction heating, furnace heating or any
other
suitable means.
Winding fibers or strands into layers for future rolling up or pleating is
illustrated
in Figure 9. A spool of fibers or strands (12) is wound around a winding frame
(10) of
arbitrary shape. The frame (10) can have a rotation axis (lla) in an arbitrary
orientation
and be rotated to an alternative rotation axis (1lb) for production of bias
windings. A
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stiffener, e.g., a thin layer of eutectically bonded fibers may be inserted
between the fibers
on opposite sides of the frame (10).
If desired, fibers or strands may be made into nested concentric spirals as is
shown
in Figures 12a-12e. To create nested concentric spiraled brush stock of one
single
handedness, e.g. left-handed, for example, one may begin with a layer of
copper fibers or
strands which is wound on a frame (10) as shown in Figure 12a. The angle of
the fibers
(a) could be anywhere from 1 to 80 degrees or so, limited only by what can be
mechanically produced, but is most suitable in the range between 5 and 40
degrees. Next,
one may place a silver Ieaf (e.g. 0.5 ~cm thick) on the fibers or strands and
roll the fibers
or strands, (Figure 12c), into a cigarette-shaped brush stock (Figure 12d),
albeit, in
commercial production the cigarette shaped brush stock could be indefinitely
long. As
shown in Figure 12d, all of the fibers or strands will spiral around the
"cigarette axis in the
same sense", thus creating nested spiral concentric spiraled fibers all of
same handedness,
i.e. left-handed in Figure 12d. This configuration of fibers combines a
minimum number
of contact points (joints), i.e., low internal friction and therefore good
independent
flexibility of fiber ends, with excellent elastic compressibility in a
direction of a brush
stock axis. In order to reduce or avoid the already discussed tendency of the
brush stock
to twist on brush force application, two or more layers with opposite fiber
inclinations may
be rolled up together, characterized by the bias angles a and p as shown in
Figure 12e, to
obtain concentric layers of spirals with alternating handedness. Note also
that such nested
spirals (cigarette-shaped) can be combined in parallel arrangements to form
larger diameter
brush stock of arbitrary cross section as is shown in Figure Sb. After rolling
the fibers or
strands into the cigarette-shaped configuration, a surface treatment may be
needed to keep
the brush stock from unrolling and to keep individual brushes which are cut
from such a
brush stock from unrolling. However, by heating to the eutectic temperature of
copper
and silver, for example, or mildly below, the silver will dissolve in the
copper fibers
thereby hardening them, and then the fibers will recrystallize during
annealing, thereby
fixing the shape of concentric spirals. Or else, with any fiber material
whatsoever, the
shape may be fixed simply through holding at the recrystallization temperature
until
recrystaliization is substantially or entirely complete. As a result,
depending on the
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particular treatment chosen, a brush stock which is elastic, composed of hard
fibers, and
does not need a surface treatment can be achieved. Other materials may be used
besides
copper and silver leaf, as was used in the example of Figures 12a-e.
y Set_ective Grading of Bonded Joints
Decreased distances between joints in the brush stock periphery will
strengthen it
relative to that in the interior, and as a result will increase stiffness
against bending.
Bonded joints can be given predetermined values by the use of twisted strands
fram tight
twisting of multiple strands of the kind in Figure 4n together, up to using
only uncrimped
fibers in the center with only as much twisting, roping or spiraling as may be
needed to
prevent the interior fibers from bunching together. Joint spacings along the
length of any
one fiber or twisted strands can thereby be graded from one or a few fiber
diameters to one
inch or more.
g~ Use of "Su~ort Fibers"
Mixing of "support fibers", meaning fibers of substantially greater stiffness
than the
majority of the fibers into the brush stock, uniformly or with any desired
gradation or
distribution, will correspondingly mechanically strengthen the brush stock.
For example,
Figure 6 shows support fibers (9) and ordinary fibers (8) in an unloaded
state. Support
fibers tray be of the same material as the regular brush fibers but thicker,
or they may be
of any suitable material including non-metals such as graphite, or may even be
non-
conducting; they may be straight, crimped, spiraled, waved, etc., all as may
be deemed to
be most suitable for imparting macroscopic strength to the brush stock with
optionally the
smallest possible interference with individual fiber mobility or largest
macroscopic brush
stock elasticity in the direction of the brush stock axis. When a brush force
is applied, the
support fibers should touch the rotor or substrate surface only lightly.
Other strengthening through geometrical arrangement of the fibers can take the
form of grading the packing fraction from a high level (perhaps as much as 70
% ) about the
periphery to a much lower value in the interior, such as, for example, a
packing fraction
15 % greater on the surface than in the interior. Alternatively, one may
produce a
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systematic variation of two different fiber types (i.e. a slow increase in
amount of one
relative to the other of different material, waviness andlor thickness) from
the periphery to
the center of the brush stock, e.g., so as to increase the density of bonding
points
progressing from the brush stock axis outward. .
Surface Treatments
Surface treatments are used for any of the following purposes: To prevent the
unraveling of fiber arrangement at the working surface and about the brush
stock surfaces;
to fix the geometrical shape of the brush stock; to mechanically strengthen
the brush stock
against bending; to insulate the brush stock and the brushes cut therefrom;
from the
surroundings, including from electrical contact, physical or chemical
contamination, or
magnetic fields.
In addition to the already mentioned surface strengthening methods through
gradation of fiber geometry and/or strengthening of joints, the following are
methods to
stiffen the brush stock by means of surface treatments which may be applied to
part or all
of the brush stock surfaces:
a) the use of a sheath or casing surrounding the bulk of the fibers, as is
shown in
Figure 3b, Figure 3c, Figure 3d and Figure 8b.
b) wrapping the outer surface
c) Spraying, dipping, electroplating, electrophoresis, plasma spraying and
irradiation
d) stitching, as is shown in Figure 10.
a) Cad
Strengthening through surface treatment may be achieved, through filling an
independent casing with bundled, twisted, spiraled, kinked, braided, woven,
roped or
felted, or a combination of any of these, fibers or strands according to the
pertinent points
above. A casing of any predetermined shape and size may be made of fibers
which are
eutectically bonded or be made through alloy shape fixing or recrystallization
fixing. For
example, Figure 3d depicts a triangular shape casing and Figure 8b a
rectangular shape
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casing.
b) Wrapping
Successful forms of mechanical strengthening via surface treatments include
wrapping the fibers, with foils, strips, felt or fibers in any combination and
fastening the
wrapping in any number of ways. Fastening can be done, for example, by an
additional
wrapping of a thin foil of tin or indium and briefly heating, including up to
the melting
point of the lowest-melting component.
The dimensions and kind of wrapping material may be freely chosen, constrained
only by the requirements that the rotor surface not suffer unacceptable damage
through the
wrapping or be covered by a residue which interferes with the brush operation
in an
unacceptable manner, e.g. through increasing the film resistivity or the
coefficient of
friction. Conversely, the wrapping may be used to aid in a brush operation,
e.g. through
containing some lubricant or mild abrasive. In the cases of strips and fibers,
the individual
turns may be inclined relative to the brush stock longitudinal axis at any
chosen angle,
from 90° to as shallow an angle as may still permit the wrapping to
stay in place, which
depends on the degree of fiber crimping or spiraling at the surface but will
rarely be less
than 20°. Favorably, such wrapping may be done in two or more thin
layers of fibers or
matted fibers, alternatively biased in orientation, e.g., ~ 45°
inclined against the brush
stock longitudinal axis, or it may be done with thin metal foil or metal leaf.
In either case,
alloy shape fixing, soldering or eutectic bonding may be used to obtain
additional
strengthening, or in the case of wrapping with a metal leaf followed by
annealing the only
significant strengthening that is obtained.
The inventors have successfully used indium or tin foil in combination with
copper,
silver and brass fibers, besides silver leaf and the already indicated choices
of copper or
silver foil. They do not doubt that besides brass other copper alloys
including bronzes and
monels will be suitable.
~) Sprayj~g ~~gLelectro In ating electr~horesis and irradiation
Other surface treatments, some of which have been used with varying degrees of
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success, include spraying the brush stock, e.g. with a slurry of metal powder
or flakes or
graphite or any suitable semi-conductor, or mild abrasive or other surface
conditioner.
These slurries may be thickened, or caused to set in place either on natural
aging or
subsequent mild heat treatment, by an admixture of agar-agar, waterglass, or
cornstarch,
or such liquids which have the effect of gluing fibers in place. Any of the
latter may be
used with or without the addition of graphite or other powders or flakes. The
application
of these surface treatments may be similarly achieved by dipping the brush
stock into any
of the above liquids. Should it be desired to treat only part of the brush
stock surface, the
remainder can be temporarily masked. Alternatively, more viscous constituents
than may
be applied through spraying or dipping may be applied through rolling the
brush stock in
them, e.g. as would apply to various powders, or slurries of the same kinds as
already
enumerated above. Enriching the brush stock surface by a powder or dough, e.g.
by
rolling or patting, could perhaps be assisted by application of a pressure
difference
between the inside and outside of the intended brush to speed up the process
or in order
not to damage the fiber arrangement.
Very importantly, too, surface treatment may be applied by thermal spraying
including plasma spraying, flame deposition or other. Also used may be
electroplating or
electrophoresis, by which joints can be set into place and voidage be reduced
at the surface
at about room temperature and therefore without annealing the fibers. For
example, electro
copper plating of copper fiber brush stock would selectively strengthen the
surface with
little other effect. One of the goals of surface treatments, namely protection
from
contaminants, and as part thereof from chemical attack, could be effected
through gold
plating. Electrophoresis can have especially good applicability on account of
the wide
range of substances which can thereby be deposited on brush stock surfaces.
Joints can also be welded together, and new joints be created, through local
melting
at the surface. One method for this is use of a high-frequency furnace,
another important
one is irradiation through lasers.
a is '
Stitching in the manner used for textiles or making shoes, for example, may be
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used for internal bonding or as one form of °surface treatment".
Stitching may be
employed in lieu of, or complementing other forms of, internal bonding or
surface
treatment and be applied before or after other surface treatments or eutectic
bonding or
alloy shape fixing, if any. For example, Figure 10 shows a method of stitching
used to
stiffen the brush stock or individual brush (1). The threads (17) in such
stitching are
typically single metal fibers or strands of metal fibers and by the proper
choice of thread
material relative to the fiber material may be set through eutectic bonding or
alloy shape
fixing. Stitching can be in any orientation, be distributed over the whole
brush or
concentrated where needed, e.g. near the running surface. The thread can be
single fibers
or strands, whether twisted or not.
Ordinarily, all of the above treatments are used, or are contemplated to be
used, on
brush stock or brushes not covered by a casing, but optionally they can also
be used on a
casing before or after insertion of the fibers.
It may be noted that surface treatments by any of the above means, on part or
all of
an outer layer and/or a component in the outer layer, may be used temporarily,
to be
removed before completing the brush construction or just before brush use.
Such removal
may be done mechanically, through dissolution, etching or other means. It is
further noted
that the "surface treatment" may be used on any parts) which are assembled
into the final
brush. For example, in a set of brushes constructed by the inventors, parallel
layers of
fiber material were interspersed with thin foils.
Rotor Surface Conditionine Through Void Fillers
In one embodiment of the present invention, all or part of the void space is
filled
with a suitable material, mostly injected in the form of a slurry of any of
the kinds akeady
enumerated in relation to dipping, spraying and roiling for surface
treatments, which then
solidifies in place. The result is a considerable strengthening of the brush
stock which may
be desired in case of rather low packing fractions. Graphite fillings of this
kind have been
successfully used to protect the rotor surface against oxidation (especially
so far of copper
fibers sliding on a silver surface and of silver fibers sliding on copper
surface) when
operating in the open atmosphere. Other useful fillers are possible. Besides
graphite,
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candidate materials include MoS2 and related sulfides (i.e. molybdenites)
which, like
graphite, provide lubrication and are electrically conductive but should best
be used in dry
conditions since MoS2 is attacked by moisture.
Optionally polishing agents or mild abrasives for cleaning the rotor or other
surface
on which the brush slides may be added to those partial void fillers, or they
may be used
alone in the same manner, albeit in only small concentrations in order not to
damage the
surface and not to leave an insulating deposit. Choices of such admixtures, in
any
combination, include aluminum oxide, silicon carbide, colloidal silica and
diamond
powder, either alone or mixed with the already discussed fillers.
A drawback of void fillers is that they strongly reduce the fiber-end mobility
on
which good fiber brush operation depends, with this increase of interior
friction rising
steeply with increasing fraction of voidage filled. Interior lubrication, by
contrast, can be
achieved through rinsing with a lubricant. This could be a thin oil in case
the
accompanying reduction of contact resistance can be tolerated, or can be a
dilute solution
of colloidal graphite which is effective without noticeable increase of brush
resistance.
Other suitable lubricants may well exist and are being actively looked for.
Mechanical Means of Bonding or Stren ening_Fiber Joi_n_ts
In addition to the various means already mentioned, bonding at touching points
may
be achieved through compacting, say in a rolling mill or "forks head" and
subsequent
annealing. Since compacting is incompatible with voidage, it requires use of a
temporary
matrix material which is eventually removed. The introduction of a temporary
matrix
material is a time consuming complication and is applicable to only a
restricted range of
matrix/fiber materials combinations.
Under clean conditions rigid fiber joints may be made through diffusion
bonding
without compacting.
The Role of Humidity
The presence of absorbed water layers on the contact surface is highly
desirable to
prevent sticking and prolong wear. With brush materials which do not oxidize
in the open
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atmosphere, normal atmospheric humidity is sufficient at low and medium
current
densities. Otherwise, moisture has to be provided. The provision of adequate
moisture
for metal fiber brushes, as needed, is therefore another aspect of the present
invention.
The ambient humidity needed rises with the percentage of the rotor or
substrate
surface which is covered by brushes and also with the local heating, i.e. the
current
density. Normally, on continuous slip rings or rotors gaps have to be left
between the
brushes to permit moisture access. In extreme cases, moisture andlor cooling
may have to
be fed through the brushes themselves, either through the brush voidage or,
given suitable
fibers, through channels in some or all of the fibers. "Support fibers" will
be particularly
suitable for this purpose.
Miniature Brushes
For most applications, fiber brushes will be mid-sized, e.g. with
characteristic
dimensions between 0.5 cm to 3 cm. Miniature brushes made of brush stock in
the form
of flat shaped strip are a further aspect of the present invention. Any of the
already
discussed considerations apply except for the small dimensions, easily down to
Il4 mm.
~rge-Sized Annlications of the Fiber Brush Technology
On the other end of the scale, large-sized metal fiber brush stock can be used
for
robust, long wearing, highly efficient cabling and sliding electrical
connections which can
be customized for particular applications and easily constructed with simple
equipment.
Specifically, flexible cables suitable for carrying currents up to hundreds of
amperes (e.g.
as may be needed for the rapid charging of future electrical cars or for
current contacts for
electric trains) could be made of brush stock, insulated from the outside,
optimally
composed of SO~.m or thinner metal fibers, with packing fractions in the order
of f = 10%
or less, and a minimum of touching points and lubrication for reduced internal
friction.
Alternatively or in combination with bundled fibers, thin layers of fiber
felt,
composed of long fibers oriented preferentially parallel to the direction of
intended current
flow, can be used. Similarly, an articulated bus (i.e. a movable jointed
current conductor)
for providing high currents to different locations could use this technology.
The encased
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covered with a metal fiber velvet or metal fiber felt to provide for low
contact resistance
across the relatively moving parts of any one joint, even while keeping the
friction forces
low to make the joints easily rotatable. With proper construction, the fiber
felt or velvet
could be made easily replaceable when necessary. In general, fiber felts
consist of a thin
s layer of mutually misoriented fiber material, bonded at a suitable
concentration of touching
points, optionally without a preferential fiber direction to make the felt
equally electrically
conductive in any orientation within the felt. A fiber velvet has much the
same
construction, and should be made in much the same manner, as textile velvet,
except that
provision may be made for bonding some or many of the fiber joints for
improved
to electrical conductivity.
Electrical brushes for both rotating and linear actuating applications could
be
constructed out of bundled fibers, fiber felts and/or fiber velvet, thereby
providing high
current capabilities, low loss and low noise. Fiber felts or velvets can be
retrofitted into
existing machinery when desired. High power, low voltage, high-current motors
are
~s particularly good candidates for this technology, as are signal-critical
devices such as
rotating antennae slip rings, microphones, video cameras, and other electronic
and
electrical devices.
Also, electrical contactors could greatly benefit from a layer of this felt on
one of
the contacting surfaces, especially when connected in the non-energized
condition. An
2o example of this would be battery contactors which could charge a battery
bank from a low
voltage, high current operating configuration by connection to a high voltage
configuration
for charging.
~pected Uses of Fiber Br hes
25 Fiber brushes are based on the theory disclosed in U.S. Patents 4,358,699
and
4,415,635 and further developed in the paper "Electrical Fiber Brushes -
Theory and
Observations", by D. Kuhlmann-Wilsdorf, ICEC-IEEE Holm 95 (41st. Holm
Conference
on Electrical Contacts, IEEE, Montreal, Canada, Oct. 2-4, 1995), pp.295-314,
reprinted
as "Electrical Fiber Brushes - Theory and Observations", D. Kuhlmann-Wilsdorf,
IEEE
3o Trans. CPMT Part A, 12 (1996) pp. 360-375.
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as "Electrical Fiber Brushes - Theory and Observations", D. Kuhlmann-Wilsdorf,
IEEE
Traps. CPMT Part A, l~ (1996) pp. 360-375, which is incorporated by reference
herein.
This is the general theory controlling current as well as heat transfer across
interfaces, at
rest or in relative motion, and the disclosed construction optimizes the
conditions at the
interface on a microscopical scale. The applicability of fiber brushes is
therefore
unrestricted in regard to size above the dimensions of single contact spots,
as to sliding
speed subject to the limitations only of aerodynamic and hydrodynamic lift, in
regard to
temperature restricted only by the requirement that the fibers remain solid,
and in regard to
current and heat density only to that at which the interface locally melts.
The fiber brushes
are therefore applicable to all conceivable situations of current or heat
conduction across
interfaces, including rotating and reciprocating motions, as well as
indefinite sliding on
one (e.g. rails) or two-dimensionally extended substrates. The fiber brushes
therefore,
also, will in the future make possible technological or scientific
developments which are
still unanticipated or at the moment are stymied for lack of adequate means of
current
and/or heat conduction.
Specifically in terms of applications which are known at present, fiber
brushes have
for example utility in electrical power equipment, in electronic equipment
especially in
light of the superior signal characteristics as well as the capabilities
presented for multiple
close proximity sliding contacts, in electric automotive applications, in
power generation
and distribution systems, and in electrical linear actuators.
Methods to Control Fiber inking
An important aspect of the present continuous metal fiber brush construction
is the
use of kinked fibers. Figures 3a and 3b are examples of fiber brush made using
kinked
fibers. The desired elastic resistance of the fiber bundles against close-
packing is thereby
created via multitudes of mutual friction points of local joints (whether or
not soldered
together through eutectic bonding) among neighboring fibers. The density of
kinks per
unit length of fiber is used to control the "loft" of the bundles. For SOum
diameter fibers,
kinks have been used from a continuous spacing, i.e. making the fibers to be
"waved" with
different amplitudes and wave lengths, to sharp kinks spanning a few
millimeters length
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each spaced nearly 2.5 cm apart, and the amplitude can be varied from
fractions of a
millimeter to a few millimeters. For practical reasons in one embodiment of
this technique,
the inventors have used V-kinks and have controlled the depth of the kinks via
spooling the
fibers under pre-selected tension. Hereby low tension provides deeper kinks
while higher
tension provides more shallow ones. However, it is also the case that a wide
range of
other kink shapes as well as continuous kinking, e.g. in a saw-tooth pattern,
an undulating
pattern, a waving or "lazy" spiraling of the fibers can be similarly used, and
that depth of
initial kink profile can be used instead of spooling tension. For mass-
production, kinking,
curling, spiraling etc., applied to strands, before or after twisting, if any,
whether in
continuous tows or finite lengths, instead of kinking spooled individual
fibers, is also
possible, and indeed will in a majority of cases be more cost effective.
Fiber Brush Stock Shad
Fiber brushes of the present invention, other than obtained by spiraling,
twisting or
roping, have been made in the laboratory by compressing the fibers in a form
to yield the
intended brush stock shape and packing fraction, with or without annealing,
whereby the
chosen surface treatment can be either applied, or if already applied be "set"
, at the same
time. The forms used in the laboratory include, for example, at least once
piece providing
a cavity of the intended shape of the brush stock and a matching lid by which
compression
can be applied to impart the desired packing fraction. The brush stock forms
were made
of stainless steel or graphite, but any other suitable material or combination
of materials
can be used including a variety of metals and ceramics, governed by the
requirements (i)
that they do not dissolve, or are dissolved in, the materials of the brush
stock and (ii) that
the form. maintain its shape independent of the annealing treatments used.
Annealing
treatments can be performed in the open atmosphere if the brush stock form
material is
resistant to oxidation and is firmly closed in use to inhibit oxidation of the
fibers. They
will require a protective atmosphere, e.g. of hydrogen, if the brush stock
form and/or fiber
stock materials are liable to oxidize at the heat treatment temperatures or if
for some
reason the form is not firmly closed, e.g. through leaks about the gaps
between form
components or the form is deliberately left open at one or both of its ends.
In addition to
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the possible use of forms as indicated, extrusion, continuous rolling,
continuous winding
on mandrels, or reshaping is envisioned for large scale production of fiber
brushes.
Cutting_of Brushes from Brush Stock and Shaping Working Surfaces
A further important step in brush construction according to the present
invention is
cutting individual brushes from the "brush stock" and shaping their intended
running
surfaces. In some cases, especially for small dimensions and curved profiles,
laser cutting
may prove to be cost effective. Planar cuts through brush stock of a diameter
which is
comparable to or smaller than the average spacing between touching spots or
joints can be
made with a razor blade. For brush stock with a relatively large diameter,
cutting poses a
problem much like trying to cut a sponge without reducing the size of the
pores in it. The
problem is overcome by infiltrating the brush stock with a hardenable liquid
(if need be at
an elevated temperature), hardening it (e.g. cooling it to freezing or curing
it in case of a
resin, as the case may be), cutting the brush stock andlor shaping the running
surface with
the hardened liquid in it, re-melting or dissolving and removing the liquid
(if need be by
means of a centrifuge), and finally cleaning residues from the brush if
necessary. Good
results have been achieved using water, and cooling the water down to well
below 0°C,
either simply in the freezer compartment of a refrigerator or any lower
temperature, e.g.
of dry ice or liquid nitrogen, so as to reduce superficial melting at the cut
surface during
cutting or shaping. Other fluids that might be used include any aqueous
liquids with
surfactants aimed to increase wetting of the surface, low-viscosity oils, hard
setting
dissoluble gels, frozen carbon dioxide, i.e. dry ice, or commercial
metallographic
embedment resins.
The actual cutting of the brush stock filled with some temporarily hard
substance
can be done by any conventional means but optimally should be done with a
sharp tool and
speedily so as to avoid undue heating. After cutting and clearing the
temporarily hard
substance from the voids, the fibers at the cut face will typically be caked
together. If so,
they must be freed through gentle abrasion, preferentially with some kind of
abrasive
paper mounted on a substrate of the same shape as the intended rotor or
substrate surface.
Alloy shape fixing and solder-bonding of fiber joints via eutectics has been
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employed in surface treatments while the fibers were encased in a fiber brush
form for
imparting the desired brush stock shape and packing fraction. For example,
intended bush
stock of silver fibers or silver-clad copper fibers was wrapped with a few
turns of a 0.5
mm thick copper foil; copper fibers were wrapped with one or a few turns of
silver leaf of
about 0.5 ~.m thickness or the form was lined with the metal leaf prior to
inserting the
fibers. The thickness of the wrapping is chosen depending on the size of the
brush stock
and the depth of hardened layer desired. The forms were then heated to the
required
annealing temperature, typically in a protective atmosphere, meaning a cover
gas which
does not contain oxygen or any chemically aggressive gas.
It is further noted that metal fiber brushes can, and commonly should, conduct
much higher current densities than conventional brushes, and they require much
lighter
mechanical pressure than conventional brushes. In fact, these are important
advantages of
metal fiber brushes, on account of which it is expected that in due course
they will displace
conventional "monolithic" , graphite-based electrical brushes. However, for
proper
operation the brush force has to remain constant within reasonably close,
predetermined
limits, independent of length of brush wear. This creates a problem because,
1) the
constant-force springs widely used for conventional brushes are generally too
stiff and
inaccurate for applying constant light loads, and 2) conventional current
leads capable of
conducting the required high currents to and from the brushes, are stiff and
interfere with
the intended Light mechanical loading.
Furthermore, for practical mass applications, fiber brushes will eventually
have to
be sold/distributed in a packaged form which protects them from damage during
storage,
shipping and handling, and which is designed for fool-proof installation by
unskilled
workers, much like light bulbs or printer camidges.
In a preferred embodiment, the present invention further includes a novel
electrical
brush holder and loading device useful for all types of brushes and
particularly designed to
maintain constant brush force while the brush wears. In "inexpensive"
applications one
makes do with spiral spring loading wherein the brush force slowly drops with
wear. For
more demanding applications one uses "constant force springs" . These are
generally
reliable but far from ideal. In preferred embodiments, the mechanical loading
of the
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brushes is done hydrostatically by means of a liquid metal which at the same
time is used
to conduct the current to and from the brushes. In the particular design of
Figure 7a each
brush (10) is firmly, metallically fastened (e.g. via a screw connection) to a
metal piston
(8) in a cylinder (1) which is at least as long as the brush. On the side of
the piston away
from the brush, the cylinder is filled with the pressurized liquid metal (6).
Such a
combination of a piston whose end is designed for the attachment, e.g., by an
electrically
conducting brush attachment(11) which can be released, of a brush and the
cylinder in
which it glides constitutes a "brush holder" . It may be advantageous to use a
piston liner
(9) andlor a cylinder liner ('n for insulation or low friction. Alternatively,
the piston and
cylinder may be replaced by bellows, not necessarily made of metal except for
the
provision of a conductive plate between liquid metal and the brush.
If the over-pressure in the liquid metal is DP, the force exerted on the brush
will be
PB = A DP, minus the typically negligible friction between piston and
cylinder. Here A is
the cross-sectional area of the cylinder or bellows of whatever shape, albeit
presumably in
most cases of circular cross-section. When the liquid metal over-pressure is
kept at a
constant value, the same brush force will be maintained while the piston
advances in the
cylinder as the brush wears, independent of wear length, or will drop only
slowly in case
bellows are used.
The open end of the cylinder may be shaped to conform, with a predetermined
clearance (12), to the running surface on which the brush slides, e.g. slip
ring, commutator
or rail (IS). Similarly, a guide may be used in conjunction with bellows.
Depending on
conditions, e.g. in connection with fast-moving vehicles, it may be
advantageous to make
that clearance small so as to shield the brush from wind forces. Similarly, in
motors or
generators, it may be possible to shield the brushes from magnetic forces via
a
ferromagnetic cylinder or coverage (16).
Preferably, the holder cylinder or bellows are provided with a stop to limit
the
advance of the piston or bellows and thereby set a minimum brush length so
that the
contact surface (e.g., a rotor) is protected from scratching or gouging by the
piston or the
end of the bellows in the event that the brush inadvertently wears out before
being
replaced.
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In a machine or other device which requires more than one, and perhaps
hundreds
of brushes, any selected group of brush holders may be connected to the same
liquid metal
reservoir. In fact, since the brush force is proportional to the cylinder or
bellows cross
sectional area, and this should ordinarily be close to, though larger than,
that of the
brushes, sets of brushes of the same general construction, and thus same
elastic/plastic
transition pressure, but with arbitrary shapes and sizes could be connected to
the same
reservoir.
Suitable bellows or hydrostatic cylinders and pistons are either directly
available
commercially or can almost certainly be procured from manufacturers since
bellows and
hydrostatic pressure cylinders in a great variety of shapes and sizes are
manufactured in
large numbers and by several firms both domestically and elsewhere. For
storage, sale
and handling, the fiber brushes may be packaged in light metal or plastic
tubes. These
should be suitably matched to the corresponding cylinder or bellows ends.
Various
mechanical mechanisms can be employed to fasten the brushes to the pistons,
e.g. by
sliding into a dovetail while the piston end slightly protrudes from the
piston, or by a
screw and thread arrangement. And similar connections can be made to the ends
of
bellows. Depending on construction, one or two simple valves (5) to control
access of the
fluid to a cylinder or bellows during brush installation may be helpful. For
brush
installation it may be similarly necessary to permit the cylinder or bellows
to slide or
swivel away from the running surface. This can be readily accomplished by the
use of
flexible plastic tubing (4) for the liquid metal, for example. In any event,
the current is to
be conducted through the liquid metal. An optional flexible hose ( I3) for the
supply of
moisture, lubricant, protective atmosphere, coolant, etc., or for exhaust
purposes may be
useful. The flexible hose (13) can be attached to the cylinder by an inlet
(18). An optional
valve (14) to control the access of lubricant, coolant, etc., may also be
helpful. Further, a
release or joint (3) may be used for easier brush installation. Likewise, a
release or joint
(17) for release of the hose (13) may be used for easier brush installation.
In order to keep
the cylinder in a fixed position relative to the slip ring, commutator, rail,
etc. (15), a
releasable or jointed attachment (2) can be used.
The most likely choices for the liquid metal are mercury (Hg) and sodium-
CA 02251379 1998-10-OS
WO 97/37847 PCT/US97/05149
-41-
potassium alloy (NaK). Each have their advantages and disadvantages. In view
of
environmental considerations, NaK is preferred, especially since much
experience with this
liquid alloy is already available. Metals melting modestly above room
temperature may
also be used, such as gallium, provided that there are means to heat them
before or
immediately at the onset of use.
In addition, as depicted in Figure 7b, there is a brush holder which makes use
of an
elastically bent brush stock ( 1 ) fed through a guide (~ towards a substrate
(4) so as to let
it's own elastic compression serve as a brush load. Figure 7c depicts still
yet another
embodiment of the present invention in that a brush holder has a flexible
brush stock (1), a
shell (5) used to contain the brush stock, a rotatable conductive connection
(2), and
connection to power (3). In addition, a fastener (6) is used to secure the
shell containing
the brush stock. The brush stock is guided through an opening (7) in the shell
(5) towards
the substrate (4). Figure 7d illustrates an example of a guide ('n that can be
used in the
brush holder of Figure 7c. Alternatively, the rotatabie brush connection (2)
can be omitted
and instead the inlet end of the brush stock be directly connected to the
power (3),
preferably after one or more complete turns of the brush stock (1) within the
shell (5) and
including a suitable elastic twist be imparted to the brush stock so as to
force the working
end of the brush stock through the guide (7) against the substrate surface
(4).
Particularly advantageous in the present invention is that minor
contaminations in
the liquid metals which would make them unsuitable if used in direct contact
with the
rotor or slip ring surfaces, should be easily tolerable. Moreover, the total
amount of liquid
metal used can be kept relatively small, and the liquid metal flow rates will
be low to
imperceptible even in large systems in which many brushes might be operated
simultaneously .
Obviously, numerous modifications and variations of the present invention are
possible in light of the above teachings. It is therefore to be understood
that within the
scope of the appended claims, the invention may be practiced otherwise than as
specifically
described herein.
