Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CONDUIT DSVICE FOR A MOTION-TRANSMITTING SYSTEM
Background
This invention relates to a conduit device and
more particularly to a conduit device for a cable or
other motion transmitting element.
Conduit devices for a cable or other motion
transmitting element are known in the art. These
devices comprise a tubular casing and are typically used
to enclose a motion transmitting element such as a wire
core element of a motion-transmitting system.
Illustratively, in a throttle control motion-
transmitting system for a motorboat, the motiontransmitting element (i.e., core elemen~) is attached at
one end to a lever and at the other end to a throttle of
the motor. In addition, the conduit device that
surrounds the core element is attached at one end to a
fulcrum of the lever and at the other end to a mounting
device which mounts the throttle to the motor.
Together, the conduit device and core element with lever
at one end control the throttle of the motor. In
particular, when the lever is pushed or pulled, the core
element moves axially in the conduit device to exert a
corresponding force on the throttle of the motor.
In order for a conduit device to perform
adequately in such a motion-transmitting system, the
conduit device must be designed to meet several
performance criteria. ~amely, (l) the conduit device
must permit the core element to transmit substantial
forces which are typically on the order of tens or
hundreds of pounds. (2) The conduit device must fit
loosely around the substantially inextensible and
incompressible core element in order to permit the core
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member to slide freely within it. In addition, (3) it
must permit the core element to slide freely within it
even when reasonable bends are made in the conduit
device incident to its normal use. Furthermore, ~4) the
conduit device must be flexible, yet must not become
distorted or deformed in cross section when it is bent
along a reasonable radius of curvature, or lengthen or
shorten when it is subjected ~o the forces incident to
its normal use. Moreover, (5) the conduit device must
not crack, stretch, deteriorate, or otherwise fail when
the core element is moved back and forth under a maximum
load within the conduit device. Finally, (6) the
conduit device must form a substantially impervious
covering about the core element to preclude the
penetration of moisture, dust or any other substance
which by corrosion, abrasion, or other effect impairs
the freedom of movement of the core element or otherwise
leads to failure of the conduit device and the core
member.
To date, most conduit devices have not
adequately met these performance criteria. As an
illustration, one prior art conduit device comprises
steel wires that are wrapped about the core element in
the form of a helical coil. Although this conduit
device is mechanically satisfactory in that it meets the
first two performance criteria it is relatively
inflexible to bending around a reasonable radius of
curvature. Additionally, moisture, dust, and other
foreign substances easily penetrate the outer wall of
the device and impair the freedom of movement of the
core element inside the device. Although an outer
sheath of plastic or the like notably improves the
ability of the device to resist such penetration, such
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outer sheath does not, however, improve the
inflexibility of the conduit device to bending around a
reasonable radius of curvature.
Another drawback of this prior art conduit
device is that special lubricants must be used between
the core element and the conduit device in order to
minimize the force that is required to initiate movement
of the core element inside the conduit device, i.e., to
overcome the static friction between the core element
and the conduit device. In addition, once movement of
the core element is initiated, these lubricants are
necessary to permit the core element to continue to move
freely within the steel walls of the conduit device,
i.e. to minimize the dynamic friction between the core
element and the conduit device.
In an attempt to improve the characteristics
of these conduit devices in order to meet the above
performance criteria, other methods have been devised
for encasing the core element of a motion-transmitting
system. In one of these devices, the conduit comprises
laminations of plastic and wire. U.S. Patent No.
3,063,303, discloses one such conduit device comprising
an inner liner, a wire sheath, a nylon winding, and an
outer sheath. The inner liner comprises a
polytetrafluoroethylene material which is commonly sold
under the trademark Teflon and lies contiguous to and
disposed about the outer surface of the core element.
Since the surfaces of the liner have a very low
coefficient of friction, the surface of the liner that
lies contiguous to the outer surface of the core element
eases the axial movement of the core element inside the
conduit device.
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The wire sheath lies contiguous to and is
disposed about the outer surface of the inner liner and
comprises a multiplicity of metallic wires that are
wrapped about the inner liner to form a helical coil
having a high pitch. The wire sheath has a high tensile
strength and resistance to elongation and thus minimizes
backlash in the operation of the conduit device.
The nylon winding of the patent comprises a
multiplicity of nylon fibers that are wrapped about the
wire sheath in the form of a helical coil having a low
pitch. This winding resists any radial deformation or
radial elongation of the conduit device during normal
use, particularly under loads which may be imparted to
bends in thé conduit device by the motion transmitting
core element.
Finally, the outer sheath of the patent
comprises plastic material or other material having a
memory characteristic which causes the material to
return to its original shape after being deformed. This
outer sheath maintains the inner liner, the wire sheath,
and the outer winding in a fixed spatial relationship
with respect to each other without substantially
impairing the bending freedom of the conduit device.
Notwithstanding this construction, which meets
the above performance criteria, the conduit device
disclosed in that patent as well as other similar prior
art devices are plagued with other problems. For
example, material, fabrication, and assembly costs of
these conduit devices are relatively expensive. In
addition, the wire sheath in this conduit device has
multiple turns of wire wrapping in order to provide the
device with a high tensile strength and resistance to
elongation. As a result, the turns of wire wrapping in
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the wire sheath make the conduit device very heavy.
Furthermore, although this conduit device does not
significantly impair the movement of the core element
and does not introduce significant backlash in the
operation of a throttle control system, these
characteristics depend upon the device being in a
neutral bending position i.e., the axis along which
there is no significant change in length when the cable
is bent. When the conduit device is twisted, the
movement of the core member is significantly impaired
and backlash in the operation of the device becomes
noticeable.
Some of these problems stem from using a nylon
winding. Nylon lacks certain characteristics which are
essential to the use of the conduit device and which
must therefore be compensated by other means such as an
expensive wire sheath. In particular, a nylon winding
becomes distorted or deformed in cross section when it
is bent along a reasonable radius of curvature. It also
exhibits minimum tensile strength and resistance to
elongation. Furthermore, over prolonged use, creep
fatigue of the nylon winding greatly reduces the service
life of the nylon winding and hence the service life of
the conduit device.
Another problem with using a nylon winding is
its minimal resistance to crushing or deformation forces
when it is applied in firm gripping relation to the
outer surface of a wire sheath. If too firmly applied,
the wire sheath may cut the nylon winding. If too
loosely applied, the nylon winding as well as the wire
sheath may move longitudinally with respect to the inner
liner. In this latter case, any movament of the wire
sheath creates friction between the wire sheath and the
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inner liner which deteriorates the outer surfaces of the
inner liner and thereby greatly reduces the service life
of the conduit device.
A nylon winding is also wanting in other
physical properties-thermal properties, for example-
that would permit the conduit device to be used under a
variety of conditions. With respect to its poor thermal
properties, when the core element is operated either in
a high temperature environment or in a moderate
temperature environment but under rapid fluctuations of
low to high loads especially around bends over an
extended period of time, the temperature of the nylon
winding often approaches the melting point of nylon.
The change in physical state of the nylon which is
brought about by melting and then hardening of the nylon
winding decreases the tensile strength and resistance to
elongation of the nylon winding. On the other hand, in
a low temperature environment the nylon winding becomes
brittle which makes it susceptible to fatigue or
facture- in short, "cracking"- during normal use.
S[1MMARY
In the present invention, I have devised a
conduit device for a cable or other motion transmitting
element. In accordance with my invention the device
comprises an inner liner, a wire sheath, an outer
winding and an outer sheath.
The inner liner comprises a
polytetrafluoroethylene material which is commonly sold
under the trademark Teflon and lies contiguous to and
disposed about the outer surface of the core element.
The low coefficient of Teflon material eases the axial
movement of the core element inside the conduit device.
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The wire sheath of the invention lies
contiguous to and is disposed about the outer surface of
the inner liner and comprises a multiplicity of metallic
wires that are wrapped about the inner liner in a
helical coil having a high pitch. The wire sheath has a
high tensile strength and resistance to elongation and
thus minimizes backlash in the operation of the conduit
device.
The outer winding of the invention comprises
multiple-strands of aramid fibers which are commercially
available from the DuPont Company under the trademark
Kevlar. The multiple-strands are wrapped around the
wire sheath in the form of a helical coil having a low
pitch. The outer winding is wrapped tightly around the
wire sheath to hold the sheath securely in firm gripping
engagement with the inner liner.
Finally, the outer sheath of the invention
comprises nylon material or other material having a
memory characteristic that returns it to its original
shape after being deformed. This outer sheath is
extruded onto the outer winding and wire sheath and
maintains the inner liner, the wire sheath, and the
outer winding in a fixed spatial relationship with
respect to each other without substantially impairing
the bending freedom of the conduit device.
It is the use of Kevlar material as an outer
winding in the conduit device that makes the present
invention uniquely suitable for use in a motion-
transmitting system. Kevlar is adequately hard and
strong so that an outer winding comprising multiple-
strands of Kevlar does not become distorted or deformed
in cross section when bent along a reasonable radius of
curvature. Revlar also has a high tensile strength and
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resistance to elongation so that an outer winding
comprising Kevlar exhibits a good overall resistance to
compressive, elongation, and tensile forces on the
conduit device that are incident to normal use. In
addition, in comparison to other prior art devices, the
outer winding of the present invention is better able to
resist creep fatigue and therefore has a much longer
service life. This increases the service life of the
conduit device.
Advantageously, Kevlar has adequate mechanical
strength so that the outer liner resists crushing or
deformation forces when it is applied in firm gripping
engagement to the outer casings of sheath wires. Thus,
the outer winding holds the wire sheath in firm gripping
relation to the inner liner in order to minimize
longitudinal movèment between the wire sheath and the
inner liner. As a result, friction between the inner
liner and the contiguous wire sheath which can
deteriorate the surface of the inner liner is minimized
so that the service life of the conduit device is
increased. Kevlar also displays a low coefficient of
friction in combination with steel and other metallic
elements so that if the outer winding moves
longitudinally with respect to the wire sheath it does
not deteriorate as rapidly as windings made of
conventional materials.
~ecause the outer liner of the present
invention advantageously provides the previously
discussed physical properties, it eliminates the need to
use small gage wire in the wire sheath to provide these
same properties. For this reason, the outer winding
makes the conduit device of this invention less
expensive than conventional conduit devices. Moreover,
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the low cost of Kevlar material in the outer winding of
this invention further reduces the cost of the conduit
device.
An outer winding comprising Kevlar can
tolerate temperatures up to 350F which is higher than a
conduit device normally experiences when the core
element is operated either in a high temperature
environment or in a moderate temperature environment but
under rapid fluctuations of low to high loads especially
around bends over an extended period of time. In
addition, an outer sheath of plastic can be extruded
onto the Xevlar winding and the outer surface of the
wire sheath without altering the unique physical
properties that the Kevlar material provides the conduit
device. The Kevlar outer winding also retains its
physical properties at temperatures as low as -50F.
Thus, the Kevlar winding does not become brittle and
susceptible to fatigue or fracture when the conduit
device is used in adverse cold environments.
Brief Description of the Drawing
As shown in fragmentary perspective view in
the drawing, motion-transmitting system 20 comprises
flexible wire core element 22 and conduit device 23 of
the present invention. Wire core element 22 comprises a
stranded cable of steel or other metallic wires.
The core element is contained by conduit
device 23 comprising inner liner 24, wire sheath 26,
outer winding 28, and outer sheath 30. Inner liner 24
comprises a tube of Teflon material. The inner liner
has a moderate wall thickness and has an inside diameter
which is slightly larger than the outside diameter of
core element 22 so that the core element slides freely
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inside inner liner 24. In addition, since the surfaces
of the liner have a very low coefficient of friction,
the surface of the liner that lies contiguous to the
outer surface of the core element eases the axial
movement of the core element inside the liner.
Wire sheath 26 comprises a multiplicity of
steel or other metallic wires that are wrapped about
inner liner 24 in the form of a helical coil having a
high pitch. The wire sheath accommodates tension loads
in the conduit device. In particular, it provides the
conduit device with a high tensile strength and
resistance to elongation, which minimizes backlash
effects in the operation of the conduit device.
Typically, the wires are wrapped tightly about inner
liner 24 in order to prevent the wires from moving
relative to the inner liner along the axis of the
motion-transmitting system or spreading with respect to
each other. Such a tight wire wrapping increases the
service life of the conduit device. Advantageously,
inner liner 24 permits such a tight wire wrapping about
the inner liner. However, although the wires are
wrapped in a firm gripping relation about the inner
liner, the wire sheath does permit small, localized
movements between adjacent wires of the sheath which
normally occur when the cable assembly is bent. As a
result, the wire sheath also provides the cable device
with some bending flexibility.
Outer winding 28 comprises multiple-strands of
aramid fiber which in the preferred embodiment are
commercially available from the DuPont Company under the
trademark Kevlar. These Kevlar strands are wrapped
about wire sheath 26 in the form of a helical coil
having a low pitch. The outer winding is wrapped
tightly about the wire sheath to hold the sheath
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securely in firm gripping engagement with inner liner
24. with respect to the wire sheath and the inner
liner, such out winding under~oes very little axial
change in length when the conduit device is bent. As a
result, it securely grips the wire sheath and inner
liner over the entire length of the motion-transmitting
system without impairing the bending freedom of the
conduit device.
Kevlar material is almost unique in its
suitability for use as an outer winding in a motion-
transmitting system. Numerous other plastic
compositions in tubing form have been found wanting in
one or more of the properties previously discussed in~
the Background which are especially desirable for heavy
duty conduit devices. Some of these, such as nylon or a
soft plastic, are too soft for general use and are
quickly cut through by the longitudinal movement of the
core element when the core element transmits heavy loads
around bends in the cable assembly. Others, such as a
hard resin or a hard plastic, are undesirably stiff and
brittle for use with a cable assembly having the degree
of flexibility generally required in a motion-
transmitting system. Still others, such as wires or
metals, display a high coefficient of friction when they
are contiguous to and move ever so slightly along a wire
sheath.
Kevlar, however, possesses an excellent
combination of properties for use as an outer winding in
a conduit device. Kevlar is adequately hard and strong
so that an outer winding comprising multiple-strands of
Kevlar does not become distorted or deformed in cross
section when it is bent along a reasonable radius of
curvature. As a result, the conduit device resists
large forces that are applied to the outer winding by
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the core member as it transmits a heavy load around
bends in the motion-transmitting system. Kevlar also
has a high tensile strength and resistance to elongation
so that an outer winding comprising Kevlar exhibits a
good overall resistance to compressive, elongation, and
tensile forces on the conduit device that are incident
to normal use. In addition, in comparison to other
prior art devices, the outer winding of the present
invention is better able to resist creep fatigue and
therefore has a much longer service life. This increase
the service life of the conduit device.
Advantageously, Kevlar has adequate mechanical
strength so that the outer winding resists crushing or
deformation forces when it is applied in firm gripping
engagement to the outer casing of sheath wire. Thus,
the outer liner holds the wire sheath in firm gripping
relation to the inner liner in order to minimize
longitudinal movement between the wire sheath and the
inner liner. As a result, friction between the inner
liner and the wire sheath which can deteriorate the
surface of the inner liner is minimized so that the
service life of the conduit device is increased. Kevlar
also displays a low coefficient of friction in
combination with the wire sheath so that if it does move
longitudinally with respect to the wire sheath, it does
not deteriorate as rapidly as windings made of
conventional materials.
Because the outer liner of the present
invention advantageously provides the previously
discussed physical properties, it eliminates the need to
use small gage wire in the wire sheath to provide these
same properties. For this reason, the outer winding
makes the conduit device of this invention less
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expensive than conventional conduit devices. Moreover,
the low cost of Kevlar material in the outer winding
further reduces the cost of the conduit device.
An outer winding comprising Kevlar can
tolerate temperatures up to 350F which is higher than a
conduit device normally experiences either in a high
temperature environment or in a moderate temperature
environment but when the core element is operated under
rapid fluctuations of low to high loads especially
around bends over an extended period of time. In
addition, an outer sheath of plastic can be melted onto
the Kevlar winding and the outer surface of the wire
sheath without altering the unique physical properties
that the Kevlar material provides the conduit device.
The Kevlar outer winding also retains its physical
properties at temperatures as low as -50F. Thus, the
Kevlar winding does not become brittle and susceptible
to fatigue or fracture when the conduit device is used
in adverse cold environment.
outer sheath 30 comprises a tube of plastic
material that is engaged over outer winding 28 and
desirably extends over the entire portion of the wire
sheath. This tube of plastic is extruded onto the
Kevlar winding and wire sheath in order to tailor the
fit of the outer sheath to the outer surface of the wire
sheath and the outer winding. The outer sheath
maintains outer winding 28, wire sheath 26, and inner
liner 24 in fixed spatial relationship with each other
without impairing the bending freedom of the conduit
device. Outer sheath 30 also assists the outer winding
of Kevlar to accommodate compression loads in a manner
which minimizes losses in operating efficiency as a
result of backlash. Although plastic is used in the
preferred embodiment, any material having a memory
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characteristic which causes the material to return to
its original shape after being deformed is suitable
material for the outer sheath. Advantageously, this
material permits the cable assembly to be stored in a
coiled condition and subsequently returned to its
extended, straight condition when installed in a
motorboat or other control system.
While the invention has been described in
conjunction with specific embodiments, it is evident
that numerous alternatives, modifications and variations
will be apparent to those skilled in the art in light of
the foregoing description.
