Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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NITINOL BALL BEARING ELEMENT AND PROCESS FOR MAKING
This invention pertains to ball bearings and roller bearings made with Nitinol
bearing elements, and to processes for manufacturing bearing races and rolling
elements, such as balls and rollers, from materials such as Nitinol, Stellite
and carbides
which are not easily formed into balls, and to bearings made from such balls.
Background of the Invention
Conventional "anti-friction" rolling element bearings have been available for
many
years made of numerous materials for numerous different applications. Their
primary
benefit, low starting friction, has earned them a permanent place in the
phalanx of
standard mechanical elements used in most products having moving parts, but
they are
notorious for a host of long-standing intractable problems. Steel, aluminum,
brass,
bronze, Monel, silicon nitride, plastics and ceramics are known materials used
for their
desirable properties in bearings, but all suffer from deficiencies that make
their use in
bearings less than ideal.
One of the most serious problems of conventional rolling element bearings is
corrosion. Even though such bearings are normally used with a lubricant, which
provides some degree of protection from corrosive agents, the lubricant can
itself
become contaminated with those corrosive agents. Even "sealed" bearings are
designed to vent at low atmospheric pressure to avoid blowing out the seal,
and then
corrosive agents can be drawn into the bearing case when the atmospheric
pressure
returns to normal. Also, corrosive particles can become embedded in the
surface of
bearing elements and races which are softer than the particles, and the
embedded
corrosive particles cause rapid corrosive pitting of the bearing elements and
races,
resulting in early failure of the bearing.
Corrosion resistant rolling bearing elements and races have been developed,
but
all suffer from deficiencies. Most seriously, the corrosion resistant bearings
are soft and
have low strength, so even if the corrosion resistance they offer does
actually allow the
bearing to operate in a corrosive environment without corrosion, they have a
limited life
because of accelerated wear and fatigue sensitivity. Moreover, few of the so-
called
corrosion resistant bearings are really corrosion-proof. Corrosion pitting in
a rolling
bearing element or race results in early destruction of the bearing. Replacing
corroded
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bearings reduces the availability of the equipment in which the bearings are
used and
greatly increases operating costs for replacement, maintenance and down time.
Bearings in equipment used in the food processing industry require particular
scrutiny because of the danger of contamination of the food with metal
particles,
lubricant and the likelihood of failure because of corrosion resulting from
use of chloride
cleaners on the equipment. As a result, bearings used in food processing
machinery
are often made of soft "corrosion resistant" materials, but wear particles
from such
bearings are a constant source of concern and require continual costly
maintenance
and down-time to replace the short lived bearings.
Modern equipment often requires non-magnetic bearings. This requirement has
been met in the past by non-magnetic materials such as brass and some
stainless
steels. However, these materials lack the strength of conventional steel
bearings, so
they must be designed over-sized to provide the needed load-carrying capacity.
Moreover, most non-magnetic bearing materials are susceptible to corrosion and
must
be replaced frequently to avoid catastrophic failure from corrosion, resulting
in costly
and disruptive down-time for the equipment in which they are installed. Non-
magnetic
stainless steel is a fair solution to the problem, but the magnetic properties
of stainless
steel are not always predictable and can vary from batch to batch.
Ceramic rolling elements were thought to solve the magnetic and corrosion
problems, and in fact are less susceptible to corrosion than the "corrosion
resistant'
metallic bearings. However, they have introduced their own unique set of
deficiencies
that make their use limited to a small segment of the market. They are very
costly, so
they are used only where there is no other alternative. They are fracture
sensitive and
fracture of a ceramic ball produces instantaneous failure of the bearing, so
the
equipment in which they are used must be designed to isolate the bearing from
impacts.
They are made of material that is very abrasive and can produce accelerated
wear of
the races and each other if allowed to rotate in contact with each other. They
produce
wear products and damage particles that cannot be detected by conventional
equipment, so they are unsuitable for food machinery. They can be used in
ceramic
races, but ceramic races are also expensive and fracture sensitive. When
ceramic balls
are used in metallic races to reduce the fracture sensitivity of the bearing,
the corrosion
and magnetic problems solved by the ceramic rolling elements do nothing to
solve the
same problems with the races.
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Nitinol is a nickel-titanium intermetallic compound invented at the Naval
Ordinance Laboratory in the early 1960's. It is a material with useful
properties, but
manufacturers who have worked with it have had little success in making
Nitinol parts
and semi-finished forms, and have never attempted to make bearings of Nitinol.
Because Nitinol is so extremely difficult to form and machine, workers in the
metal
products arts usually abandoned the effort to make products out of anything
except
drawn wire because the time and costs involved did not warrant the paltry
results they
were able to obtain.
Nitinol, particularly Type 60 Nitinol (60% Nickel and 40% Titanium by weight),
has many properties that are unrecognized as of potential value in bearings.
It can be
polished to an extremely smooth finish, less than 1 microinch rms. It is
naturally hard
and can be heat treated to a hardness on the order of 62Rc or higher. It can
be
processed to have a very hard integral ceramic surface that can itself be
polished to an
even smoother surface than the parent metal. It is non-magnetic, immune to
corrosion
from most common corrosive agents, and has high yield strength and toughness,
even
at elevated temperatures. It is 26% lower density than steel for high
revolution rate
applications and for weight sensitive applications such as aircraft,
satellites and
spacecraft. However, there has hitherto been no attempt to make bearings out
to
Nitinol because it is so difficult to work, because it was known to be
brittle, and because
there has been no known method to make rolling elements and races out of
Nitinol.
Summary of the Invention
Accordingly, this invention provides a bearing having rolling elements and/or
races made of Nitinol. The invention also provides efficient processes for
making Nitinol
bearing races, and for making balls out of Nitinol and other materials that
are difficult or
impossible to form. The Nitinol bearing elements, and the bearings made from
these
elements which this invention provides are capable of performance unmatched by
the
prior art. The bearings are non-magnetic, virtually corrosion-proof, impact
tolerant and
high strength. In addition, they appear to provide better damping capacity,
run quieter,
and to be less susceptible to spalling than steel bearings.
The invention includes making blanks for ball bearings and rods for roller
bearings of Nitinol. Ball blanks can be made by casting balls or cutting cubes
out of
sheets of material of which the balls are to be made, preferably Type 60
Nitinol,
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although cubes can be cut from Stellite, nitrogen steel and even silicon
carbide and
silicon nitride. The blanks are made to dimensions slightly larger than the
desired
dimensions of the bearing elements so they can later be precision ground to
the exact
desired diameter. The cast blanks are treated in a hot isostatic press to
consolidate the
material. The Nitinol cube blanks and ball blanks may be heat treated to
reduce their
hardness from about 62 RC as cast to about 35-40 RC but for some materials and
abrasive combinations, the cube blanks are left hard for more rapid corner and
edge
rounding in the subsequent tumbling operation. The cube blanks are tumbled in
an
abrasive tumbling machine to round the corners and edges enough to allow them
to roll
in a ball grinder. The rounded cube blanks and cast ball blanks are ground on
conventional grinding equipment to the desired dimensions. Rods for roller
elements of
roller bearings may be cast in larger diameters and then heated and drawn or
extruded
to near the desired outside diameter before grinding, or cast in near net size
and then
ground to the precise diameter desired. The rods may be ground in a centerless
grinder
and cut to the desired length of roller elements after grinding. If tapered
roller elements
are desired for thrust bearings and the like, they may be cut initially to
length and
centerless ground individually to the desired taper. The balls may be cast in
an
investment casting process as a "string-of-beads" or grid which can be cut or
broken
apart after casting to produce the individual balls, ready for grinding and
lapping in the
conventional ball grinding and lapping equipment. The cast balls are
preferably cast
each with its own individual gate and riser to avoid the creation of voids
during cooling
in the mold. The use of feeder channels of sufficient cross-section in cast
ball tree grids
is desirable to produce good quality ball blanks. After grinding, the ground
blanks are
heat treated to produce the desired toughness and hardness, e.g. 58 RC. No
final
grinding is necessary after the second heat treat because, unlike steel roller
elements,
the size and shape of the ground elements does not change during heat
treatment, so
the final grinding step necessary with conventional bearing elements may be
entirely
eliminated.
Description of the Drawings
The invention and its many attendant benefits and advantages will become
better
understood upon reading the following detailed description of the preferred
embodiments in conjunction with the following drawings, wherein:
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Fig. 1 is an elevation of a "string of beads" wax form for casting a string of
Nitinol
linked balls in accordance with this invention, and also shows a string of
Nitinol balls
made in the casting operation;
Figs. 1 A and 1 B are sectional elevation and perspective views of a mold for
producing balls, each with its own riser and gate;
Fig. 1 C is a sectional plan of a mold for making a wax ball tree form used in
a lost
wax casting process for casting balls in accordance with this invention;
Fig. 1 D is a stack of ceramic mold stabs made using the wax ball trees made
in
the mole shown in Fig. 1C for casting balls;
Fig. 2 is a schematic elevation of a hot isostatic press in which cast ball
blanks
are treated to consolidate the material;
Fig. 3 is an elevation of a single cast Nitinol ball made by the casting
process of
this invention;
Fig. 4 is a schematic process diagram of the heating steps to produce a medium
hardness, highly elastic condition of Nitinol balls in preparation for
grinding in
accordance with this invention;
Fig. 5 is a schematic elevation of a Nitinol ball, heat treated in Fig. 4,
being
ground;
Figs. 6 and 7 are schematic elevations of a heat treating and quenching
process
to produce a high hardness condition in the ground ball;
Fig. 7A is a schematic elevation of the oven and quench bath to produce
desirable hardness in the Nitinol balls represented in Figs. 6 and 7, showing
one
scheme for providing rapid transit from the oven to the quench bath;
Fig. 8 is a schematic representation of a polishing and/or lapping apparatus
for
producing the final dimensions and a fine finish on the balls after treatment
in the
apparatus of Fig. 6 and 7;
Figs. 9 and 10 are schematic representations of an oven and a quench bath for
producing the desired final hardness and ceramic surface material on the
lapped and
polished balls from the apparatus in Fig. 8;
Fig. 11 is an elevation of the a ball produced by the processes and apparatus
represented in Figs. 1-10;
Fig. 12 is a perspective view of an ingot of material to be made into balls;
Fig. 13 is a perspective view of a plate of material to be made into balls;
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Fig. 14 a perspective view of an industrial laser cutting cubes from the plate
shown in Fig. 13;
Fig. 15 a schematic representation of an abrasive tumbling machine in which
the
cubes cut from the sheet as shown in Fig. 14 are tumbled to produce "rounded
cubes"
shown in Fig. 16;
Fig 16 is a perspective view of a "rounded cube" produced in the tumbler of
Fig.
15;
Fig. 17 is a schematic block representing a conventional ball grinder;
Fig. 18 is a spherical ball ground in the ball grinder of Fig. 21;
Figs. 19-22 are plan views showing a laser cutting pattern for cutting cubical
ball
blanks from the sheet shown in Figs. 13 and 14;
Figs. 23-26 are plan views showing a laser cutting pattern for cutting
cylindrical
ball blanks from the sheet shown in Figs. 13 and 14
Fig. 27 is a perspective view of a Nitinol rod for a roller bearing element in
accordance with this invention;
Fig. 28 is a schematic diagram of a rotary swaging operation on the rod from
Fig.
27;
Fig. 29 is a schematic diagram of a centerless grinding operation on the
Nitinol
rod from Fig. 27 or Fig. 28;
Fig. 30 is a schematic diagram of a rod polishing operation on the ground rod
from Fig. 29;
Fig. 31 is a schematic diagram of a roto-lase cutting operation in which the
rods
from Fig. 30 are laser cut into individual roller elements;
Figs. 32 and 33 are schematic diagrams of the heating and quenching process
for hardening and producing the hard ceramic surface material on the roller
elements;
Fig. 34 is a perspective view of the roller element produced by the process
shown in Figs. 27-33 ready for use or for final polishing
Fig. 35 is a perspective view of a cast Nitinol tube used to make Nitinol
races in
accordance with this invention;
Fig. 36 is a schematic representation of a bearing race being cut by band saw
from the tube shown in Fig. 35;
Fig. 37 is a perspective view of the bearing race cut from the tube as shown
in
Fig. 36;
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Fig. 38 is a schematic representation of a heat treating oven for heating the
race
blanks from Fig. 37 to a medium hardness ultraelastic condition in preparation
for the
machining in Fig. 39;
Fig. 39 is a schematic representation of a machining step for the race blanks
from the heat treating oven in Fig. 38;
Fig. 40 is a schematic representation of a grinding operation on the machined
race blank from Fig. 39;
Figs. 41 and 42 are schematic representations of heat treating and quenching
steps to produce the desired hardness and ceramic finish on the races ground
in Fig.
40; and
Fig. 43 is a schematic representation of the polishing step to produce the
final
surface finish on the races from Figs. 41 and 42.
Detailed Description of the Preferred Embodiments
Turning now to the drawings, wherein like reference characters designate
identical or corresponding parts, and more particularly to Figs. 1-7 thereof,
a process for
making Nitinol ball bearing elements, shown schematically in flow diagram
form, uses
an investment casting process in which a wax form 30 in the form of a "string
of beads"
is made or purchased as a separate item. The wax form 30 is suspended in a
container, not shown, and the container is filled with a fine granular ceramic
powder
surrounding the wax form 30. The ceramic powder has a binder that binds the
power
into a mold when heated, after which the wax is melted and poured out of the
mold,
leaving only a ceramic mold with a "string-of-beads" hollow core.
The ceramic mold may be preheated in an evacuated heater container to prevent
premature quenching and freezing of the molten Nitinol during casting. The
molten
Type 60 Nitinol, an intermetallic composition made of about 60% nickel and 40%
titanium by weight, is poured from a vacuum oven into the hollow core of the
mold.
Type 60 Nitinol has a low viscosity when heated to above its melt temperature
of about
1310°C and flows easily into the mold, filling it completely.
The ceramic mold filled with Type 60 Nitinol is allowed to cool and solidify
into a
string-of balls form 35, also shown in Fig. 1 since it is identical to the wax
string-of
beads mold 30. Removal of the Nitinol string-of beads form 35 from the ceramic
mold is
accomplished by disintegrating the mold, typically by breaking the mold into
fragments
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using ultrasonic or conventional impactors. The mold is intentional made
frangible, so it
fragments readily. The Nitinol string-of-balls 35 is removed from the mold
fragments
and is cleaned of mold fragments. Alternatively, the mold can be made of a
material
that disintegrates when placed in a solvent, which facilitates removal of the
Nitinol form
from the mold.
The cast balls 40 sometimes develop a void in the center, believed to be due
to
the shrinkage of the casting in the molds. The void weakens the ball and
lowers its load
carrying capacity. A technique for avoiding the creation of voids and porosity
in the
balls during casting is to provide each ball cavity 32 with its own individual
gate 33 and
riser 34 depending from a main channel supplied with molten Nitinol in a
"branch of
apples" form as shown in Figs. 1A and 1 B. The gate 33 and riser 34 are made
of
sufFcient diameter to prevent premature hardening of the molten Nitinol in the
gate
during cooling so that molten material may be drawn into the ball 40 as it
cools. Also,
when the balls are removed from the molds, the gates are left attached to the
ball
blanks during processing in a hot isostatic press 43, as described below.
Balls made
and processed in this way have been found to be free of voids and porosity and
much
less susceptible to cracking or breaking under heavy loads.
A high volume arrangement of the casting scheme shown in Figs. 1 A and 1 B is
shown in Figs. 1 C and 1 D. A ball tree mold 31 is shown in Fig. 1 C for
casting wax ball
tree forms for 5/8" balls, although this same arrangement would also work as
well for
balls of other diameters. The wax form for the ball tree has a central trunk
36 and
branches 38 extending from the trunk 36, each with multiple balls 39 depending
from
the branches 38. The ball tree mold 31 for casting the wax ball trees is a
split metal
mold, usually aluminum. After removal from the ball tree mold 31, the wax ball
tree
forms are dipped in the ceramic slurry to make individual ceramic mold slabs
41
encasing the wax ball tree forms, as shown in Fig. 1 D. After setting, the
ceramic mold
slabs 41 are flash heated to melt the wax which is poured out of the ceramic
slab mold.
A multiple-slab stack of slab molds 41 is assembled in a heated, evacuated
chamber
with their inlets aligned, and molten Nitinol is poured into the inlets from
which it flows
freely down the trunks 36, into the branches 38, and fills the individual ball
cavities 39.
To further reduce the chances of voids in the center of the balls and ensure
that
the ball has its full load carrying capacity, the balls are subjected to
heating under high
pressure in a hot isostatic press 43, shown schematically in Fig. 2, a
procedure know as
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NIPPING. Two hours at 1650°F t25°F at 15,000 psi consolidates
the material in the
balls, removing any center voids without distorting the balls. Preferably, the
gates and
risers are not removed from the ball blanks until after NIPPING to improve the
elimination of voids and porosity in the hot isostatic press. Preferably, the
entire ball
tree, or the branches 38 after being cut or broken from the trunks 36, can be
treated in
the hot isostatic press before removing the ball blanks 39, 40 from the string
or branch.
After NIPPING, the ball blanks 39, 40 are broken or cut from the ball forms,
in its
form as a string-of beads as shown in Fig. 1 and 2, or the "branch of apples"
form
shown in Figs. 1A and 1 B, or the tree or branches shown in Fig. 1 D, to
separate the
connected balls into individual balls 39, 40 as indicated in Fig. 3. The
connecting links
37 between the ball blanks 40 are broken or cut, or the ball blanks 39 are cut
free of the
branches 38 to free the individual ball blanks. Type 60 Nitinol in the as-cast
condition is
quite brittle and the links 37 and gates and risers 33/34 break easily. The
length of the
links 37 is preferably made short to facilitate cutting and waste as little
material as
possible. They can be made exactly as long as the thickness of a diamond or
other
abrasive saw that can be used to cut the balls out of the "string-of-beads" or
branches
38, or even shorter if they are to be broken apart. The links 37 and branches
may be as
small as about 1/8" diameter to accommodate the flow of molten Nitinol through
the
mold. Preferably, the links 37 and branches 38 are about 3/16" X 3/8" in cross
section
which provides ample flow area for the molten Nitinol to reach and fill all
the spherical
cavities 39, 40. A significant quantity of material is used in the trunks 36
and branches
38 of that cross-section, but it can be recovered and re-melted, so it is not
wasted.
Type 60 Nitinol of that cross-section in the as-cast condition can be broken,
sometimes
even in the process of removal from the mold, although I prefer that the gates
and risers
remain on the ball blanks for NIPPING. Sawing, especially gang sawing after
NIPPING,
has the advantage of producing a smooth cut surface. If the links are broken
instead of
cut, the broken ends of the links 37 can be ground flush with the balls using
a belt
grinder with 180 grit silicon carbide abrasive particles. It is desirable to
have the balls
40 be as smooth and spherical as possible for the subsequent ball grinding
operation,
although some amount of projections of stubs from links or gates and risers is
acceptable.
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Heat Treating for Ultraelasticity
Balls for ball bearings must be ground to produce a smooth spherical surface.
The Nitinol ball bearing blanks 40 may be ground in the as-cast condition in a
conventional ball grinding machine 42 shown schematically in Fig. 5, but Type
60 Nitinol
in the as-cast condition is very hard, on the order of 62 RC, so grinding
could be slow
and could produce rapid wear of the grinding equipment 42. Moreover, as-cast
Type 60
Nitinol is brittle and can be cracked or broken by impacts. Therefore, I
prefer to perform
an initial heat treating step on the balls blanks 40 before grinding to reduce
the
hardness of the Type 60 Nitinol to about 42-49 RC and to give the material an
elasticity
greatly exceeding as-cast Nitinol and even most other metals. This first heat
treat
process includes heating the ball blanks in a vacuum oven 46, shown
schematically in
Fig. 4, to about 700°C and holding at that temperature for about an
hour to ensure
thorough heating of the balls 40, then letting the balls cool slowly in the
oven, preferably
overnight to ensure against sudden undesirable cooling by premature opening of
the
oven. Preferably the oven is a metal-lined vacuum oven or inert atmosphere
oven to
ensure that the bearing elements remain clean during heat treating to avoid
the
necessity of cleaning them after heat treating. This treatment produces an
elasticity up
to about 6% which I have termed "ultrelasticity" to distinguish over the
"superelasticity"
of Type 55 Nitinol after cold working, but the ultraelasticily is not
temperature sensitive
like the superelasticity of Type 55 Nitinol.
After cooling, the heat treated ball blanks 39, 40 are subjected to a
conventional
ball grinding operation, illustrated schematically in Fig. 5, that is similar
to the grinding
performed on conventional balls used in conventional ball bearings. Because of
the
initial heat treating, the time to remove material from the cast ball blanks
40 is not much
longer than for conventional steel balls. Early prototypes of cast Nitinol had
surface pits
that required removal by removing a surface layer of material, down to a level
in which
the pits were no longer present. Those surface pits are minimized by using a
finer
ceramic powder around the "string-of-beads" form 30 and the molds 41 when
making
the molds. It is a benefit to make the surface of the cast balls 40 is smooth
and free of
pits a possible. Also, the technique noted above of providing each individual
spherical
mold cavity 32, 39 with individual gates and risers of sufficient diameter
avoids
premature solidification of the molten Nitinol in the gates and risers while
the molten
Nitinol in the spherical cavities cools.
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After grinding, the ground balls may be lapped and then polished in a
conventional ball polishing machine 44, shown schematically in Fig. 6,
preferably using
a fine diamond slurry as the polishing medium, although conventional polishing
compounds will also work, if somewhat slower. However, before the final
lapping and
polishing steps, I prefer to heat treat the balls in a second heat treatment
to high
hardness in the oven 46. In this second heat treatment, the balls are placed
in a
vacuum or inert atmosphere oven 46 and are heat treated to about 900°C
and held at
the elevated temperature for about 2-3 hours or until they are heated entirely
through.
They are then removed from the oven 46 and immediately quenched quickly by
immersion in water in a quench bath 48 and agitated in the water, giving them
a of
about 58-62RC, but without the brittleness exhibited by the ball blanks 39, 40
in the as-
cast condition. Small balls can begin to air cool during transit from the oven
46 to the
quench bath 48 and fail to reach the desired hardness produced by immediate
water
quenching from 900°C. An apparatus shown in Fig. 7A shortens the
transit time from
the oven to the water quench bath 48 by heating the balls in a steel tube 50
fitted with a
hinged end cover fashioned in the form of a chute 52. After the balls have
been heated
to the desired temperature, the oven door is opened and the quench bath 48 is
positioned directly in front of the oven door. The end cover/chute 52 is
released to allow
the balls to roll directly from the steel tube 50 in the oven 46 into the
quench bath 48
without allowing time for the balls to air cool enough to prevent attainment
of the desired
hardness.
The lapping a polishing step noted above and represented by the apparatus 44
in
Fig. 6 is pertormed on the palls to give then the desired exact dimensions and
surface
finish.
The lapped and polished balls are thoroughly cleaned with detergent and water,
rinsed and dried. The balls 39, 40 may be used in the hardened condition
following the
second heat treating step, but I prefer to perform one additional operation to
give them
an integral gold or black ceramic surface which is very hard and can be
polished to
extreme smoothness. The ceramic surtace forming step includes washing the
balls 40
after polishing in detergent to remove all residues of polishing medium, and
thorough
rinsing to remove the detergent. The balls are carefully dried to avoid
contamination
and are placed in the oven 46, as illustrated in Fig. 7, for heating to about
950°C, or are
heated by torch with a high temperature flame such as MAP gas to that
temperature.
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The gold ceramic surface is attained at a lower temperature, on the order of
about
600°C. The hot balls 39, 40 are then quenched as indicated in Fig. 8 by
immersion in
flowing water or oil. They may also be cooled by a forced draft of cold air,
below 32°F,
over the balls 40 to produce the black ceramic finish. The hard integral
ceramic surtace
produced on the balls by this process is believed to be a complex mixture of
oxides and
nitrogen compounds of nickel and titanium. Thin surface layers are difficult
to measure
for hardness, but it is believed that the ceramic surface material is on the
order of about
80 on the Rockwell "C" scale. It can be polished to an extremely fine surface
finish if
desired. It is electrically insulating, so it should not be used where
electrical current
through the bearing is needed. However, it improves the resistance to galvanic
corrosion, which is caused by small electrical currents between dissimilar
metals in the
presence of an electrolyte. The ceramic surface can also be applied to Nitinol
ball
races, as described below, to enhance the resistance to galvanic corrosion to
the
bearing and to the structure in which the bearing is mounted.
The balls may now be processed with one final polishing step to produce a
surface finish of less than one microinch rms, on the order of 0.2-0.5
microinch. If
desired, the balls may be again subjected to the heating and forced cooling to
thicken
the ceramic surface.
Nitinol bearings, with or without the ceramic surface finish, are not subject
to
deterioration from contamination in petroleum based lubricants and can be
lubricated
with salt water, which makes them ideal and unique for marine applications.
The
bearings can also be run dry as they retain high hardness even at elevated
temperatures. Tests have shown that Type 60 Nitinol can have a hardness of
about 55
RC at an elevated temperature of about 600°C, a property unique among
metallic
materials.
The balls 39, 40 are useful as ball bearings, and also may be used in many
other
applications where hard and corrosion resistant balls are needed. For example,
check
valves having a spring-loaded ball are often used in flow circuits carrying
corrosive
liquids. The ball is subject to corrosion in that environment and can become
inoperative
as a sealing element in the check valve. A Nitinol ball in accordance with
this invention
solves the corrosion problem because Nitinol is virtually immune to corrosion
by most
corrosive agents.
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The Cube Process for Making Balls
The casting process is best for large balls, on the order 0.5 inch and greater
diameter. However, the casting process is less suitable for making smaller
balls of the
desired quality, so I have developed another preferred process for making
balls of
material like Nitinol which is difficult or impossible to form by conventional
heading
processes. This additional process is shown in Figs. 12-27, wherein a billet
or ingot 60
of the material, shown in Fig. 12, is rolled, cast, or otherwise formed into a
plate or
sheet 62, as shown in Fig. 13. As shown in Fig. 14, the sheet 62 is cut into
cubical ball
blanks 64 and the cubes 64 are reduced to rounded cubes 66, illustrated in
Fig. 16, by
abrasive tumbling in a conventional abrasive tumbling apparatus 68 shown in
Fig. 15.
The rounded cubes 66 are reduced to spherical balls 70, shown in Fig. 18, by
grinding
in the conventional ball grinder 44, illustrated schematically in Fig. 17.
There are many materials which cannot be formed into ball blanks suitable for
ball grinding by known conventional forming processes. Stellite, high-nitrogen
steel
(known by the trade name "Cronidur"), and possibly silicon nitride are
examples of such
materials along with Nitinol. Although there is no known economical process
for forming
these metals into balls, they can be made in sheets or plates 62 by casting,
rolling or
other known techniques. I have discovered that balls can be made by cutting
sheets of
these materials into cubes 64 and tumbling the cubes in a tumbler 68 with
large balls or
grinding media, abrasive grit and a liquid carrier to remove the sharp edges
and corners
of the cubes 64, producing what I call a "rounded cube" 66 illustrated in Fig.
16. The
rounded cube 66 can be ground to a sphere 70 in a conventional ball grinder 44
if
sufficient material is removed from the corners and edges of the cube 64 to
enable the
"rounded" cube 66 to roll in the ball grinder 44. For example, cubes made of
Type 60
Nitinol having six square sides, each 0.170" long, have been ground to make
balls with
a diameter of 0.156" or 5/32. A cube of those dimensions has a maximum corner-
to-
corner dimension of about 0.242", as measured. Reducing the corner-to-corner
dimension in the abrasive tumbler by about 7% to about 0.225" provides
sufficient
rounding of the cube's corners and edges that it can be ground to a sphere 70
in a
conventional ball grinder 44.
The cubes 64 can be cut from the sheet or plate 62 of Type 60 Nitinol by
laser,
following a pattern shown in Figs. 19-22. As the cubes are cut out of the
sheet, they fall
through the support grid on which the sheet lies and fall into a pan below.
The cubes 64
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tend to bounce when they hit the bottom of the pan and the compressed gas from
the
laser head blows the small cubes out of the pan, so the bottom of the pan can
be lined
with a material such as felt impregnated with high temperature grease or a
mesh
material to reduce the tendency of the cubes to bounce and facilitates their
capture and
easy removal from the pan.
The laser cutting speed for cutting small ball blanks could be fastest if the
sheet
62 were cut into a series of parallel bars by parallel cuts on one axis, and
then into
cubes by holding the bars together in a flat array in a jig and feeding the
bar array into
the laser for cutting the bars off at the end into cubes. Naturally, other
laser cutting
schemes and patterns will occur to those skilled in the art in light of this
disclosure.
Although the industrial laser is an excellent tool for cutting Nitinol, there
are other
existing processes that could also be used. Abrasive water jet cuts Nitinol,
although
slower than a laser. EDM also cuts Nitinol, but it is even slower. I believe
plasma
cutting should be fast, although it may produce a wider kerf and more waste.
The
choice of the cutting process is a tradeoff of speed, cost and waste.
Another ball blank form from which balls can be ground is cylinders. A
scalloped
laser-cutting pattern, shown in Figs. 23-26, uses matching semi-circular cuts
instead of
squares to produce cylindrical ball blanks 75 instead of cubes 50. The
diameter of the
cylinders is equal to the thickness of the plate 48 so the three orthogonal
dimensions
through the center of the cylindrical ball blank 75 are equal, as is the case
with the
cubical ball blanks 64. The cylindrical ball blanks 75 have smaller corner and
edge
protrusions and would not require as much time in the tumbler to round off
their edges
to make them ready for the ball grinder. Indeed, the cylinders may not require
any
tumbling time at all. However, the laser time to cut cylinders is considerably
longer than
to cut cubes, and the yield of ball blanks from a sheet or plate of a given
size would be
less. The choice between the two cutting patterns would depend on the overall
efficiency of the ball production process. The opposed semi-circular matching
laser cuts
that produce the cylindrical ball blanks 75 can be left connected at the cusp
77 to
produce a string of cylinders 75 connected by a small rib 78 at adjacent
edges. The
laser travel mechanism is accurate enough to leave a rib that is only a few
thousandths
of an inch thick allowing the cylinders 75 in the string of cylinders to be
easily broken
apart after cutting.
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At least one manufacturer of Nitinol forms has indicated that it has plans to
develop the processes to make drawn rod of Type 60 Nitinol. If they are
successful,
another process for making ball blanks would be to cut lengths of such rod
equal to the
rod diameter. This would produce cylindrical ball blanks like the cylindrical
ball blanks
60. These cylindrical ball blanks would be processed by tumbling in an
abrasive
tumbler if required and ground in a ball grinder to produce spherical balls in
the same
manner as the process used to make spherical balls of the cylindrical ball
blanks 60.
The cost of the rod may be prohibitive and the process selected by the ball
manufacturer would depend on the cost as well as the other usual factors which
influence such decisions.
The Type 60 Nitinol rod would be virtually impossible to shear with tools that
would last long enough to pay their cost, so other processes should be
available to cut
the Type 60 rod into short cylinders. I envision a "rotolase" laser cutter
that rotates a
cylindrical workpiece such as a pipe, and in this case a rod, under the laser
beam to cut
through the cylinder. Another approach would be to use ganged diamond cutter
wires
operating in staged fashion to cut simultaneously but cut off the end closest
to the end
of the rod first. There is little waste with a diamond wire cutter because the
wire is so
narrow and, even though it cuts much slower than a laser, the ganging of
multiple wires
acting on a single length of rod at once would produce a large number of cuts
per unit
time. The rod would be supported in a grooved and slotted support bar to hold
it
against deflection under load by the pressure of the diamond cutting wires
against the
rod. The rod could also be cut off in the desired lengths by plasma cutting,
although
there would be more waste using that process.
Drilling Holes in Hardened Balls
There are uses of balls that require a diametrical hole through the ball.
Typically,
such balls are now made as conventional ball bearing balls that are then
drilled to make
the hole. Because the drilling must be done on conventional steel balls after
the ball is
heat treated and reground, the balls are at full hardness when they are
drilled, which
makes drilling very slow and difficult. This invention provides an effective
way to drill
the hole in the Nitinol ball after it is heat treated to final hardness. The
ground balls can
be heated to a high temperature, on the order of 900°C-950°C,
and placed in split
spherical die having an axial diametrical hole perpendicular to the parting
plane of the
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die halves. The hot 60 Nitinol ball can be punched or, preferably, drilled
while trapped
and squeezed in the spherical die cavity using the through hole as a guide for
the drill or
punch.
Another preferred method of making hard, precision-ground balls with an
diametrical through hole is to make the ball as noted above and position the
finished
and polished ball in a holding jig that has a support for an EDM stinger
positioned axially
aligned above the center of the ball. The stinger is energized and lowered
into contact
with the ball and cuts a diametrical hole exactly through the center of the
ball. If the
oxide finish is desired on the "drilled" ball, it should be applied after the
hole has been
made because the oxide is electrically non-conductive and would adversely
affect the
operation of the EDM stinger.
Roller Bearing Elements
Turning now to Figs 27-34, a process for making Nitinol roller elements for
roller
bearings, shown in flow diagram form, starts with an investment casting
process similar
to the ball casting process described above, but using a wax rod-form made by
injecting
molten wax into in a mold or purchasing it as a separate item since it is a
standard size.
The wax rod form is suspended in a container and the container is filled with
a fine
granular ceramic powder, surrounding the wax rod form. The ceramic powder is
fixed in
the shape encapsulating the rod, with a connecting channel, and the wax is
heated and
poured out of the mold, leaving only a ceramic mold with a cylindrical hollow
core. The
ceramic mold is preheated in the container to prevent premature quenching and
freezing of the molten Nitinol which is melted and poured into the cylindrical
core of the
ceramic mold. The cylindrical core of the mold is filled with molten Nitinol
and the
ceramic mold is allowed to cool.
After cooling and solidifying, the Nitinol rod 80 cast in the ceramic mold is
ready
for removal from the ceramic mold. Removal is accomplished by breaking the
mold into
fragments using ultrasonic or conventional impactors, or the mold is otherwise
disintegrated by solvents or the like. Center voids have not been seen to
develop in the
cast rods 80, but they may be treated in the hot isostatic press if voids are
found to
occu r.
The Nitinol rod 80 may be made with other processes. They may be made by
conventional casting processes by pouring in conventional split molds directly
from the
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draw-down oven where the Type 60 Nitinol is initially made. These rods have
the usual
edge seam marking, but that is removed during the subsequent processing. Hot
rotary
swaging and drawing of larger cast ingots or rotary swaged ingots are other
feasible
processes for making the rods. However it is made, the rod 80 is machined to
the
desired diameter at a temperature of between 600°C and 950°C,
preferably at the upper
end of that range for ease of machining and to achieve a better surface
finish. Good
grades of cutters are required for such machining, preferably grade 5-7 micro-
grain
silicon carbides.
As an alternative to machining, the rods 80 may be mounted on the axis of a
rotary forge 83, shown schematically in Fig. 28, which has a series of forging
hammers
85 disposed in a circle around the axis of the machine. A suitable rotary
forge machine
is produced by American GFM Corp. in Chesapeake, Virginia. The Nitinol rod 80
is
heated by torches, shown schematically at 87, induction heaters, radiant
heaters or the
like to a high temperature of about 800°C - 950°C and hammers 85
are driven against
the Nitinol rod 80 while the rod 80 is axially advanced and rotated through
the ring of
hammers 85. The faces of the hammers are oriented to swage the Nitinol axially
and
not circumferentially. The rod 80 may be passed repeatedly through the rotary
forge
apparatus 83 until the length of the rod 80 has increased and the diameter has
been
reduced to nearly the desired diameter of the roller elements to produce a
semi-finished
rod 90 from which the roller elements can be made.
It may be advantageous for producing a smooth cylindrical rod 90 to perform a
hot drawing operation. The rotary forged rod 90 is heated to about
800°C and is drawn
through one or a series of circular dies, each slightly smaller than the
previous one.
Preferably, the circular dies are made of Type 60 Nitinol to avoid quenching
the rod 90
as it is drawn through the die. The Nitinol bar 60 is reheated to the desired
temperature
of about 800°C -950°C between each drawing step. Only a very
slight reduction in
diameter for each pass is accomplished to preserve the tooling and produce the
desired
smooth cylindrical form.
After the final diameter reduction operation, the Nitinol rod 90 allowed to
cool to
room temperature, and the rod 90 is removed to a centerless grinder 95, shown
in Fig.
29, for centerless grinding to almost the exact desired diameter of the roller
bearing
elements. It is left slightly oversized to allow a small amount of material
from the
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individual roller elements at their ends to be removed for producing crowned
rollers, if
desired, and in the subsequent polishing operation.
The ground, perfectly cylindrical rod 90 is removed to a rod polishing machine
100, shown in Fig. 30, which may be similar to a centerless grinder using
buffing wheels
instead of grinding wheels. The rod 90 is polished to a smooth surface finish
on the
order of 1 microinch. It is then removed to a cutting operation, shown in Fig.
31,
preferably an automated roto-lase cutting machine having a rod support that
rotates the
polished rod 90 under the laser to cut it cleanly into properly sized roller
bearing
element blanks 105 without significant waste of material. The cut roller
bearing element
blanks 105 may be edge trimmed to chamfer and polish the ends of the blanks
105 to
produce finished roller elements 110. The finished roller elements 110 are
heat treated
by heating in a oven to about 800°C - 950°C for 2-3 hours, and
are quenched in a water
bath to produce a deep hardening to about 60-62RC to at least 1/2", and
produce an
integral ceramic finish that is on the order of about 80RC hard. The roller
bearing
elements may be repolished to produce a surface finish of less that 1
microinch RMS.
Bearing Races
Inner and outer bearing race components are manufactured from investment cast
tubular blanks 115, shown in Fig. 35, made of Type 60 Nitinol. The blanks 115
are
made with an outside diameter that is oversize by .010"-.050" to allow
machining and
grinding the blanks to the precise dimensions required for a particular
bearing
application. The tubes 115 are heat treated to the ultraelastic condition
noted above to
produce a hardness of about 40-49RC and a highly elastic crystalline structure
that
resists breakage during handling. The process described herein produces the
bearing
race assemblies.
Race blanks 120, shown in Fig. 37, are cut from the cast tubes 115 by a rotary
laser cutter mentioned above, or by sawing with a carbide band saw 122, as
shown in
Fig. 36. The band saw speed is slower than normal to prevent damage to the saw
and
the tube may be heated to about 900°C to speed the cutting rate. If not
done
previously, the race blanks 120 may be heat treated in the oven 46, shown in
Fig. 38, to
a temperature of about 700° and air-cooled or furnace-cooled to medium
hardness (47
to 50 Rc). In this condition the blanks 120 can be machined and ground to size
with
minimal difficulty. However, the blanks can be heat treated to have high
hardness prior
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to performing the machining and grinding processes. The heat treatment process
required to obtain high hardness is described below.
Nitinol 60 is most satisfactorily machined with carbide tools 124, shown
schematically in Fig. 39, using moderate speeds, light feeds and highly
chlorinated
cutting oils. The material can also be machined without the use of cutting
oils.
However, care must be taken to deflect the particles removed, as they are hot
and
sharp. Carbide tools can wear rapidly and the edges of the cutters will roll
quickly. All
carbide cutting tools should be sharpened or replaced often. A good indication
of tool
wear is when high pitch noise is generated during turning. The machining may
also be
done at high temperature, on the order of 900°C, which allows much
faster removal of
material and produces a very smooth cut surface.
Grinding, employing silicon carbide wheels 126 shown in Fig. 40, is a highly
satisfactory means of machining 60 Nitinol. The ball groove around the
interior of the
race blank may be ground to extremely fine tolerances in Nitinol because it is
a very
stable material. Very fine finishes are possible through very careful control
of the
grinding conditions. Lapping of the final finish with diamond film also
produces superior
finishes. A finish of less than 1.0 microinch RMS is obtainable on the
material.
Nitinol 60 can be heat treated to have high hardness (58 to 62 Rc). The
bearing
races should be machined close to the required dimensions in the medium
hardness
ultraelastic condition then heat treated and quenched for high hardness as
indicated in
Figs. 41 and 42 prior to the final grinding operations. Another option is to
heat treat the
components prior to any machining operations. To obtain high hardness, oven
the
components are heated in the oven 46 and heated to 900°C to
950°C and allowed to
soak at that temperature for at least 15 minutes. They are then quenched in
water in
the quench bath 48. The time between removal from the oven and quenching is
critical,
especially for small races, so the quench bath should be located as close to
the oven as
possible. The quenching from 900°C-950°C will produce a black
ceramic surface finish
on the races.
Type 60 Nitinol can be polished to a high luster finish of less than 0.5
microinch
RMS. The use of diamond paste on a polishing or buffing wheel 128 is
recommended,
although standard polishing compounds such as jeweler's rouge will work.
Diamond
paste called Glanz Wach has been found to provide excellent finishes on the
material.
The paste is manufactured by Menzerna-werk at P.O. Box 60 76468 Otigheim,
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Germany, Tel: +49 (7222) 91 57 0 Fax: +49 (7222) 91 57 10. Bars of the paste
can
also be purchased from Ralph Maltby's The GoItYVorks catalog at 1-800-848-
8358.
Polishing of the material as machined and ground is acceptable as is polishing
of any
applied oxide finish.
The oxide finishes applied to Nitinol 60 provide some advantages over the non-
oxide coated material. The oxide (a ceramic) is an electrical isolator,
increases the
surface hardness to an estimated 70RC-80RC (from the 62 RC hardness of the
base
material). The oxide somewhat improves the corrosion resistance and provides a
lower
coefficient of friction. The oxide also produces an attractive visual
appearance,
especially the gold finish. Final polishing of the oxide using the above
described
compound improves the appearance and produces a smoother surface finish. Gold
and
black finishes are readily attainable, but other colors are also possible,
such as blue,
green or purple. However, a separate the heat treatment necessary to attain
these
colors requires very careful control and is probably not worth the effort in
an industrial
product.
The process for applying the black oxide is the simpler of the two processes.
The black oxide is applied after the components are machined and ground to
size. The
components are machined, ground to size and polished, if the final component
is to be
polished. The components are heated to the 900 to 950°C temperatures as
described
for the high hardness heat treatment process described. The components are
then
water quenched. The black oxide will be formed at the same time as the high
hardness
is obtained. The components are then re-polished using the diamond paste
compound.
The gold oxide is applied on the components upon completion of the final
manufacturing process. In other words, the components are heat treated for
high
hardness, machined and ground to the final dimensions and then polished. The
gold
oxide requires a polished surface. The gold oxide is formed by a low heat
treatment
process. This procedure does not change the hardness of the finished
components, as
long as they are not overheated.
The gold color can be of different shades, from light to dark gold, or almost
brass
in color. The different shades are obtained by controlling the amount of heat
and the
duration of exposure. The gold oxide application process is monitored
visually, but an
automated process to allow precision application of the shades of gold is
feasible.
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As noted above, the components must be cleaned thoroughly with a strong
detergent and rinsed well in fresh water to remove all residue of cutting oil,
polishing
compound, fingerprints, etc. from the components prior to application of the
oxides.
These contaminants would produce discoloration and blotches on the parts if
they were
heated to a high temperature without being cleaned. After cleaning, the
components
should be handled with clean cotton gloves to avoid contaminating the clean
surface
with fingerprints.
The completed components are placed onto a material having low thermal
conductivity such as firebrick, ceramic plate or a 60 Nitinol plate. Using a
MAPP torch,
the components are heated slowly until the gold oxide starts to form. Heat
must be
applied evenly to obtain a uniform color tone. When the desired color is
obtained, the
heat is removed and the component is allowed to air cool. It is not water
quenched. If a
component starts to turn a purple color, the temperature is too high and the
component
will have to be reprocessed for high hardness, then polished and the gold
oxide process
repeated.
The processes disclosed herein make possible for the first time the
manufacture
of ball and roller bearings with Nitinol bearing elements. The high strength
and
chemical inertness or non-reactivity of Nitinol make it a very desirable
material for
bearing elements, especially for applications in chemical and refining plants,
ships and
other applications in and around salt water, geothermal power plants, and many
food
processing, aerospace and defense applications where contaminants and
corrosive
agents and cleaning solutions are present, to name just a few of the myriad
possibilities.
Obviously, numerous modifications and variations of the preferred embodiment
described above are possible and will become apparent to those skilled in the
art in light
of this specification. For example, the balls made of Type 60 Nitinol in
accordance with
this invention have many other applications in addition to ball bearings, such
as ball
valve elements in corrosive or hot applications , e.g. check valves for
corrosive liquids,
or valve elements and valve lifters for internal combustion engines. Moreover,
many
functions and advantages are described for the preferred embodiment, but in
many
uses of the invention, not all of these functions and advantages would be
needed.
Therefore, I contemplate the use of the invention using fewer than the
complete set of
noted features, benefits, functions and advantages. Moreover, several species
and
embodiments of the invention are disclosed herein, but not all are
specifically claimed,
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although all are covered by generic claims. Nevertheless, it is my intention
that each
and every one of these species and embodiments, and the equivalents thereof,
be
encompassed and protected within the scope of the following claims, and no
dedication
to the public is intended by virtue of the lack of claims specific to any
individual species.
Accordingly, it is expressly intended that all these embodiments, species,
modifications
and variations, and the equivalents thereof, are to be considered within the
spirit and
scope of the invention as defined in the following claims, wherein I claim:
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