Fiber brush slip ring assembly

A slip ring and brush assembly comprises a gold plated slip ring surface and a bundle of conductive fibers in the 2 to 3 mil size range. During use, gold transfers from the ring to the fibers, and the resulting gold-on-gold contact interface of ring and brush is extremely noise free and long wearing.

BACKGROUND OF THE INVENTION 
Slip ring assemblies are well known in the prior art. Such assemblies 
generally comprise a rotating conductive ring which is contacted by a 
non-rotating brush mounted in a suitable brush holder. The "brush" is 
often a monolithic element comprising a composite of carbon and other 
materials. The carbon provides lubrication between ring and brush and the 
other materials, such as silver or copper, provide flow paths for 
electrical power or signals. Although the surface of the brush which is in 
contact with the rotating ring is configured to match the curvature of the 
ring, irregularities in the ring surface and uneven wear properties of the 
brush limit contact between the brush and the ring to only a few discrete 
points. 
The "brush" may also be a metallic member which can have a rectangular or a 
cylindrical crossection. In the slip ring industry, this type of 
monofilament member is called a "wire brush". Typical contact geometry for 
a wire brush and ring is shown in U.S. Pat. No. 3,329,923. As is the case 
with the monolithic composite brush, the contact between the ring and such 
a wire brush is limited to only a few discrete points. 
These discrete points of contact between the brush and the ring causes the 
brush biasing force to be concentrated on these few point surfaces. This 
concentration of force results in localized high pressures on these few 
points and this leads to wear of both the brush and ring surface. The 
resultant wear debris contributes electrical resistance to the flow path 
of electricity through the assembly. 
Slip ring assemblies employed in instrumentation systems to transmit signal 
level voltages are expected to operate for long periods of time (years) 
with contact resistance variations in the low milliohm levels. It has been 
known for some time that to achieve this performance, single element wire 
brush assemblies comprising noble metals and noble metal alloys must be 
used in the electrical contact zone rather than base metals. Base metals 
will oxidize if not maintained in an inert environment and the resultant 
semi-conducting oxide layer contributes electrical resistance to the flow 
path of electricity through the assembly. While high contact forces can be 
used to disrupt the oxide layer to achieve better electrical contact, such 
contact forces result in very high wear rates. 
Experience has also shown that a suitable lubricant must be used to reduce 
friction and wear between noble-metal-wire-brushes and noble-metal-rings. 
When these slip ring assemblies are used in vacuum environments, a low 
vapor pressure lubricant is required to prevent cold welding of the 
contacts to the ring. 
Present day research is being directed to slip ring assemblies comprising 
non-noble fiber brushes (e.g. copper, nickel, brass, etc., fibers) which 
ride on a non-noble slip ring surface. In order to prevent the deleterious 
effects of oxide layers on the non-noble slip ring components, the 
assemblies require an environment comprising an inert gas. Such 
environments are producible, but not without elaborate equipment. As an 
example, it has been determined that a humidified inert gas produces a 
greater conductivity between the slip ring components. This is often 
impractical where space is a consideration or where the attendant cost is 
prohibitive. Drawn fibers of solid gold running on a gold slip ring 
surface have also been proposed, but for most applications this approach 
is too costly. 
SUMMARY OF THE INVENTION 
This invention relates to a slip ring and brush assembly comprising a 
multifilament fiber brush in contact with a gold slip ring surface. The 
force which biases the multifilament brush to the slip ring surface is 
distributed over a large number of brush fibers which are in actual 
physical contact with the slip ring surface. This results in a very low 
force being exerted on the ring by each fiber. The low localized pressure 
provides a brush of exceptionally long wearing characteristics and the 
multiplicity of contact points between the multifilament brush and the 
slip ring result in a lower overall electrical contact resistance for the 
assembly. The balance of the filaments comprising the brush which are not 
in contact with the ring provide a damping mechanism to those filaments 
which contact the ring. This mechanism enhances the contact between the 
filaments and the ring by prevention of hydrodynamic and/or pneumatic 
lift, as well as lift or bounce resulting from shock. These additional 
filaments also provide parallel paths for the flow of electricity to the 
vicinity of sliding contact. 
It is advantageous in many instances to initially gold plate both the 
surface of the ring and the fibers of the multifilament brush. The gold on 
the ring should be plated to at least 200 micro-inches thickness and 
should be chosen to have a hardness which is less than the hardness of the 
gold on the filament brushes. During an initial "run-in" stage, the softer 
gold on the ring will transfer from the ring and will cold weld onto the 
harder gold plating on the brush at those points of the brush in actual 
contact with the ring. It will be appreciated that using this technique, 
gold is transferred onto the thin plating of the fibers, rather than being 
worn away. Once this transfer has taken place, the resulting gold-on-gold 
interface of ring and brush is highly conductive, and the tangential force 
between the fibers and the ring surface may be maintained very low. 
This invention is not limited to assemblies in which gold plated fibers 
ride on a gold plated ring, but includes applications in which non-noble 
fibers ride on a gold plated ring whereby a transfer of gold occurs from 
the rotating ring surface to those portions of the non-noble fibers in 
contact with the ring. After the initial oxide layer on the non-noble 
fibers is abraded away by the rotating ring, gold will be transferred to 
the electrical contact zone of the brush where it is most critically 
needed. Such arrangements allow the use of non-noble fibers which may have 
desirable properties of low cost, electrical resistivity, tensile 
strength, corrosion resistance, and the like. This approach has been 
tested whereby nickel fibers have been successfully run on a gold plated 
surface for more than one billion inches of ring travel with current 
densities in excess of 5000 Amps/sq. in. Fibers may also be fabricated 
from copper, copper alloys, nickel, nickel alloys, other metals, and metal 
alloys which can be formed into wire.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows generally a prior art monolithic composite brush 4 in contact 
with a slip ring surface 5. Although the face of the brush 4 is contoured 
to match the shape of the ring, contact exists at only a few discrete 
points 6. These points 6 receive the total force biasing the brush to the 
ring and are areas of abrasion and wear. 
FIG. 1A shows a prior art wire brush comprising a single metallic spring 
element 7. Like the composite brush 4, the spring element contacts the 
slip ring surface 8 at only a few discrete points 9. 
A single element brush exhibits significant electrical losses due to 
constriction resistance. Constriction resistance is proportional to 
n.sup.-1/2, where n is the number of spots which carry current between the 
brush and the ring. It is estimated that in a single element brush, n 
varies between 1 and 20. 
As shown in FIG. 2, a slip ring and brush assembly according to the 
invention may comprise a multifilament brush 10 which is in contact with a 
rotating slip ring 12. The multifilament brush 10 comprises a plurality of 
filaments 11 in the 1 to 3 mil size which are held in a unitary 
relationship by means of a collar 13. The collar 13 may comprise the end 
portion of the wire insulation 14, or may be a separate element 
specifically designed to hold the fibers 11 in a selectively shaped 
bundle. As shown, the fibers 11 extend from the collar 13 a sufficient 
distance to enable them to be in tangential contact with the ring 12, and 
are held in position by a holder 15. 
The surface of the ring 12 may be flat or may be formed with one or more 
channels 16, as shown in FIG. 3. Each channel 16 comprises a plating 17 of 
gold on the base metal of the ring 12. The channels 16 group the filaments 
11 to prevent spreading of the filaments 11 across the surface of the slip 
ring, and the sides of the channels present additional surface area which 
the brush filaments 11 contact. 
Turning now to FIG. 4, it will be seen that the channel may take the form 
of a rectangular trough 19, comprising a gold plating 21 formed on the 
base metal ring 22. An insulating spacer 18 is provided between adjacent 
troughs 19 to create separate circuits on a common ring structure. 
As shown in FIG. 5, the slip ring may comprise a V-shaped channel 26 formed 
in the slip ring surface 27. In each of the embodiments shown by FIGS. 
3-5, the channels are sized so as to be substantially filled by the fibers 
of the brush with which they will be used. In each of the embodiments 
shown by FIGS. 3-5, bidirectional operation of the ring is possible when 
the free length of the fiber is maintained below a critical value. In most 
fiber brush systems under development today, bidirectional operation is 
not possible. 
The fiber brushes of FIGS. 2-5 offer a number of advantages over a single 
element brush. The separate fibers create a large number of current 
carrying spots, thus drastically lowering electrical resistance and 
increasing current density. In a monolithic brush, maximum current density 
is 600 amp per square inch, while with fiber brushes, current densities of 
20,000 amp per square inch can be realized. 
The individual brush fibers are able to adapt to the unevenness of the ring 
surface because of their elasticity and flexibility. The fibers in actual 
contact with the ring are biased by other fibers which comprise the brush. 
These properties also greatly reduce brush bounce caused when the brush 
hits a high spot on the ring surface at high ring speed. 
The fact that brush bounce is greatly reduced and the fact that the need 
for lubrication is minimized because of the very low forces between 
contact members permit the fiber brush contact system to be operated in 
conjunction with very high ring speeds. Tests to date have shown that the 
adventitious lubricants in the environment, i.e., hydrocarbons and other 
airbourn gaseous contaminants, will provide adequate lubrication such that 
fiber brush contact assemblies can be operated for periods of time in 
excess of 50 hours at speeds of 30,000 RPM. 
Slip ring assemblies used in instrumentation systems to monitor a parameter 
such as temperature on the rotating portion of turbine engine may be 
required to operate at speeds of 10,000 to 60,000 RPM. In these systems, 
auxiliary equipment is required to cool a Freon TF.RTM. and oil mixture 
which is circulated throughout the slip ring assembly in order to remove 
the heat generated by friction between the contacts and the ring. In prior 
art slip ring assemblies designed for a high speed, the force between the 
single element wire-brush and the rotating ring is typically 20 grams. 
This force is more than two orders of magnitude greater than the force 
required to hold the fibers of a fiber brush against a ring such that 
electrical noise in the low milliohm levels can be achieved with the 
rotating ring. Thus, such fiber brush contact assemblies designed for high 
speed applications permit instrumentation systems to be employed on 
engines while in flights, whereas prior art systems are limited to ground 
operation because of the bulk of the auxiliary cooling apparatus required. 
The plurality of fibers allow maximum overall brush contact with minimum 
pressure per fiber. A brush life of 1.4 billion inches of ring travel can 
be expected with fiber brushes while monolithic brushes generally cannot 
exceed 10 million inches of ring travel. Since fiber brushes can be biased 
to the slip ring surface with a force which is two orders of magnitude 
less than the force which biases a conventional brush in a similar 
application, the necessity for lubrication othewise necessary to reduce 
friction between the two surfaces is obviated. Film resistance caused by 
the lubricant is eliminated, and since the number of discrete current 
carrying spots for a fiber brush can vary from 50 to 10,000, constriction 
resistance is minimal. 
The low force required to sucessfully use the fiber brush system eliminates 
technological problems in vacuum applications. Typically, the force used 
to bias a single element wire-brush to a slip ring in a vacuum environment 
is sufficient to cold weld the brush to the ring if a lubricant is not 
used. To find a contact lubricant which meets all of the necessary 
requirements of viscosity, vapor pressure, chemical stability, and 
chemical compatibility with the system over a wide temperature range is a 
formidable task. Using fiber brushes of the present invention, gold plated 
fibers, nickel fibers and fibers of a copper silver alloy have been 
successfully run without lubricant on gold plated rings in excess of 1,500 
hours in a minimum vacuum of 2.times.10.sup.-7 torr (500 of these hours at 
6.times.10.sup.-8 torr) without evidence of cold welding. 
As shown in FIG. 6, the fiber brush itself may comprise a gold plating 23 
over a bundle of base filaments 24. The bundle is maintained in a unitary 
relationship by a collar 25 and the base filaments 24 may be formed of a 
plurality of materials but preferably are a conductive metal such as 
beryllium copper, copper, nickel, or phosphor bronze. In actual practice, 
filaments in the 2 to 3 mil size have been used but other sizes may be 
substituted where desired. 
As shown in FIG. 7, a high current carrying capacity brush may comprise a 
plurality of filaments 31 configured by a holder 32 to contact a ring 
surface 33 so that the ends of the filaments are in contact with the ring. 
Such an arrangement provides for a greater number of filaments 31 
contacting the ring 33 than would otherwise occur if the filaments were 
tangential to the ring. In actual practice, the number of fibers in a 
fiber brush may vary between 50 and 10,000. In the configuration shown in 
FIG. 7, a very high percentage of those fibers comprising the brush will 
actually contact the ring. Using such configurations, 20,000 amps per 
square inch of brush surface area can be transferred to a rotating ring 
without deleterious effects to either the ring or the brush.