U.S. patent application number 13/102104 was filed with the patent office on 2011-10-27 for in-situ lubrication of sliding electrical contacts.
This patent application is currently assigned to University of Florida Research Foundation, Inc.. Invention is credited to David L. Burris, Wallace G. Sawyer, John C. Ziegert.
Application Number | 20110263468 13/102104 |
Document ID | / |
Family ID | 36616655 |
Filed Date | 2011-10-27 |
United States Patent
Application |
20110263468 |
Kind Code |
A1 |
Ziegert; John C. ; et
al. |
October 27, 2011 |
In-Situ Lubrication of Sliding Electrical Contacts
Abstract
Embodiments of the present disclosure include electrically
conductive solid lubricant transfer films, methods of making the
same, and the like.
Inventors: |
Ziegert; John C.; (Seneca,
SC) ; Sawyer; Wallace G.; (Gainsville, FL) ;
Burris; David L.; (Gainsville, FL) |
Assignee: |
University of Florida Research
Foundation, Inc.
Gainsville
FL
|
Family ID: |
36616655 |
Appl. No.: |
13/102104 |
Filed: |
May 6, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11908088 |
May 2, 2008 |
7960317 |
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PCT/US2006/008152 |
Mar 8, 2006 |
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13102104 |
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60659719 |
Mar 8, 2005 |
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Current U.S.
Class: |
508/154 |
Current CPC
Class: |
C10M 2201/041 20130101;
C10N 2010/02 20130101; H01R 39/24 20130101; C10M 103/02 20130101;
C10N 2040/17 20200501; H01R 39/56 20130101; H01R 43/14 20130101;
C10M 103/00 20130101; C10N 2010/06 20130101; C10M 2201/06 20130101;
C10N 2030/60 20200501; C10N 2040/14 20130101; H01R 39/58
20130101 |
Class at
Publication: |
508/154 |
International
Class: |
C10M 169/04 20060101
C10M169/04 |
Claims
1-10. (canceled)
11. An electrically conductive solid lubricant transfer film,
comprising: a solid lubricant, and at least one soft metal
intermixed with said solid lubricant, wherein an bulk resistivity
of said lubricant film is no more than 4 times a bulk resistivity
of Cu at 25 C.
12. The lubricant film of claim 11, wherein said soft metal
comprises silver and indium.
13-20. (canceled)
Description
FIELD OF THE INVENTION
[0001] The invention relates to electrically conductive solid
lubricant comprising transfer films including a solid lubricant,
and at least one soft metal, and methods for applying the same
during operation of systems having sliding electrical contacts.
BACKGROUND OF THE INVENTION
[0002] One of the primary goals of lubricants is obtaining low
friction. Solid lubrication offers many benefits over conventional
oil-based hydrodynamic and boundary lubrication. Solid lubrication
systems are generally more compact and less costly than oil
lubricated systems since pumps, lines, filters and reservoirs are
usually required in oil lubricated systems. Greases can contaminate
the product of the system being lubricated, making it undesirable
for food processing, and grease and oil outgas in a vacuum preclude
their use in space applications.
[0003] In some lubrication applications, sliding electrical
contacts connect two electrically conductive members which transmit
high current density from one conductive member to the other
conductive member across the sliding contact. In such applications,
the lubricant material typically must be highly electrically
conductive. These applications include a wide variety of military
hardware, including slip-rings in tilt wing aircraft, antennae,
radar pointing systems, and electrical motors. Conventional solid
lubricants currently available generally provide insufficient wear
protection for some important applications. For example, even with
the use of available solid lubricants to reduce wear rates and
friction, current efforts to develop a Superconducting Homopolar
Motor (SCHPM) for ship propulsion have been hampered by excessive
wear in the brush system which conducts high electrical currents
from the rotor to the stator.
[0004] Since the brushes of SCHPMs are known to wear during use,
designers typically must base design decisions on an assumed wear
rate for the brushes based on past experience and projected
technology development, which generally results in a minimum
required wear length for the brushes. Wear rate is a strong
function of contact force. In order to be able to achieve the
required wear rates, contact forces need to be kept very low, on
the order of about 3 to 4 N. Unfortunately, for electrical contacts
between bulk solids, low contact forces can result in high contact
resistance as well as excessive heating and losses at the
interface. The difficulty is sometimes addressed by the use of
multifilament wire brushes, in an attempt to achieve satisfactory
contact resistance at these low forces. However, when multifilament
brushes are used in the high magnetic fields and high current
densities of the SCHPM, electromagnetic forces on the individual
filaments of the brush are typically high enough to distort the
filaments, often quite significantly, thus changing the true
contact force and altering the brush wear rate in some sections of
the motor.
SUMMARY OF THE INVENTION
[0005] One embodiment of the invention is a method for in-situ
solid lubrication of sliding electrical contacts. The method can
include providing a device comprising a movable electrically
conductive first member and an electrically conductive second
member. The first and second members, according to the method, are
in electrical contact at a slideable electrical contact. The method
can further include automatically applying to the slideable
electrical contact during operation of the device a film of
electrically conductive solid lubricant, the applied film being
defined herein as an electrically conductive solid lubricant
transfer film.
[0006] Another embodiment of the invention is a system having
in-situ solid lubrication of sliding electrical contacts. The
system can include a device comprising a movable electrically
conductive first member and an electrically conductive second
member. The first and second member of the device can be in
electrical contact at a slideable electrical contact. The system
further can include a source of electrically conductive solid
lubricant transfer film. The electrically conductive solid
lubricant transfer film can be automatically applied to the
slideable electrical contact during operation of the device.
[0007] Yet another embodiment of the invention is an electrically
conductive solid lubricant transfer film. The film can comprise a
solid lubricant and at least one soft metal intermixed with the
solid lubricant. The bulk resistivity of the lubricant film is
preferably no more than 4 times the bulk resistivity of copper (Cu)
at 25 C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] A fuller understanding of the present invention and the
features and benefits thereof will be accomplished upon review of
the following detailed description together with the accompanying
drawings, in which:
[0009] FIG. 1 is a perspective view of an in situ conductive solid
lubrication system in which an electrically conductive transfer
film is deposited on the rotor during system operation for reducing
wear between the copper brushes and the rotor, according to an
embodiment of the invention.
[0010] FIG. 2 is a schematic representation of an exemplary film
retention-promoting geometry for an exemplary rotor-stator
configuration having first and second members that each include
triangular teeth which fit together, according to another
embodiment of the invention.
[0011] FIG. 3A is a perspective view of a high-speed rotating
pin-on-disk tribometer for testing the effects of providing in situ
solid lubrication according to the invention.
[0012] FIG. 3B is a graphical presentation of selected measurements
obtained from testing the effects of using in situ solid
lubrication on the device of FIG. 3A.
DETAILED DESCRIPTION
[0013] A method for in-situ solid lubrication of sliding electrical
contacts, according to one embodiment of the invention, includes
the steps of providing a device comprising a movable electrically
conductive first member and an electrically conductive second
member. The first and second member are in electrical contact at a
slideable electrical contact. A sacrificial electrically conductive
solid lubricant transfer film is automatically applied to the
slideable contact during operation of the device. In one
embodiment, the solid lubricant is applied to a surface of the
first member during operation of the device. In this embodiment,
the electrically conductive solid lubricant transfer film is
carried by movement of the first member to the electrical contact
to reduce the wear rate of the first and/or second member.
[0014] The presence of the electrically conductive solid lubricant
transfer film at the slideable contact effectively eliminates wear
by eliminating the need for intimate contact between the surfaces
of the first and second member. Because the solid lubricant is very
soft and highly electrically conductive, large current densities
can be passed through the electrically conductive solid lubricant
transfer film without direct metal to metal contact.
[0015] In a typical but non-limiting example, the second member is
a brush, such as a copper brush. Using the invention, the brush
wear rate is reduced to a low level through the use of a
periodically replenished, electrically conductive, solid lubricant.
In one of many possible applications of the invention, the
invention is applied to a Superconducting Homopolar Motor (SCHPM),
as used in a propulsion system for providing propulsion to an
ocean-going naval ship or other water-borne vessel. As applied to
an SCHPM, the invention may provide a dramatic simplification in
the brush/holder system design, a significant increase in the
reliability of such a system. Such an application of the invention
may remove what is considered by many to be the most significant
remaining technological barrier to the adoption of SCHPMs in naval
propulsion systems. Application of the invention to SCHPMs for ship
propulsion systems also would likely improve national and homeland
security by improving combat readiness of U.S. Navy and other
military defense ships by substantially reducing the downtime of
such ships.
[0016] FIG. 1 is a perspective representation of an situ solid
lubrication system 100, according to one embodiment of the
invention. Copper brushes 105, 106,107 having a plurality of
filaments 108 illustratively provide electrical contact to a copper
rotor 110. As illustrated in FIG. 1, in a particular application,
the rotor 110 can rotate at a moving rate of about 10 m/sec in the
in situ approach, according to this embodiment of the invention, a
sacrificial protective solid lubricant film 112 is deposited on the
surface of the rotor 110 during system operation. A solid block 115
of electrically conductive solid lubricant transfer film is pressed
against the rotor 110 using a normal force to supply transfer film
which is brought to the sliding contact by the movement of rotor
110. The presence of the transfer film 112 under the copper brushes
105, 106, 107 effectively eliminates wear, or substantially reduces
wear by allowing copper brushes 105, 106, 107 and copper rotor 110
to be physically separated during system operation, the
electrically conductive solid lubricant transfer film being between
the brushes and rotor. Although continuous deposition is shown in
FIG. 1, in a preferred embodiment, intermittent application of the
transfer film is used, such as a periodic application of transfer
film for one minute or less for every one hour of protection during
operation.
[0017] Although a frictional source is shown in FIG. 1, a variety
of other deposition sources can be used to deposit the solid
lubricant film 112. The various deposition sources can include, for
example, pulse laser sources as well as a variety of known
powder-delivery sources.
[0018] Although not explicitly shown in FIG. 1, rather than apply
solid lubricant transfer films at predetermined periods, systems
according to the invention can additionally include a sensor and
related feedback-and-control system for determining when to
initiate application of the transfer film as well as when to
terminate application of the transfer film once initiated. Sensors
can include force sensors (e.g. friction), temperature sensors,
acoustic sensors, and/or surface chemistry based sensors. Based on
sensor measurements, such as when the friction coefficient at the
contact exceeds a predetermined value, deposition can be initiated
to provide an electrically conductive solid lubricant transfer
film, or coating, to the slideable electrical contact.
[0019] As noted above, the system shown in FIG. 1 applies a normal
load to solid lubricant block 115 to initiate the transfer film
deposition which travels laterally to reach the slideable contact.
Applied to such a system, once the friction coefficient falls to
below a predetermined friction coefficient, the deposition can be
terminated, such as by removing the normal force applied to end the
transfer film deposition.
[0020] Friction between the block of the electrically conductive
solid lubricant transfer film 115 and the rotor 110 shown in FIG. 1
caused by application of the normal load shown generates the
typically submicron thick transfer film which comprises a plurality
of particles. Because the solid lubricant can be engineered to be
both very soft and electrically conductive, large current densities
can be passed through the thin, generally sub-micron thick transfer
film without direct copper-to-copper contact. A typical thickness
of the transfer film 112 is about 0.2 to 0.8 .mu.m, such as 0.5
.mu.m.
[0021] Regarding the solid lubricant, generally preferred choices
are graphite, molybdenum disulfide (MoS.sub.2), and tungsten
disulfide (WS.sub.2). In a dry form, a powder of these materials
are effective lubricant additives due to their lamellar structure.
The lamellas tend to orient parallel to the surface in the
direction of motion.
[0022] Even between highly loaded stationary surfaces the lamellar
structure is able to prevent contact. In the direction of motion
the lamellas easily shear over each other resulting in low
friction. Large particles generally best perform on relatively
rough surfaces at low speed, while finer particles generally
perform best on relative smooth surfaces at higher speeds. Other
solid lubricants that may be used include, but are not limited to,
boron nitride, polytetrafluorethylene (PTFE), talc, calcium
fluoride, and cerium fluoride.
[0023] The electrically conductive solid lubricant transfer film
112, moreover, can include at least one soft metal. The soft metals
can include gallium, indium, thallium, lead, tin, gold silver,
copper and the Group VII noble metals, and mixtures thereof.
[0024] In one exemplary embodiment, the lubricant film comprises
graphite, silver, and indium. In this embodiment, the lubricant can
comprise 30 to 70 wt % graphite, 15 to 35 wt % silver, and the
remainder indium, such as 50 wt % graphite/25 wt % silver, and the
remainder indium. In one test, using a system arrangement analogous
to that shown in FIG. 1, the graphite/silver/indium film was found
to lay down a uniform 0.5 micrometer thick transfer film on the
copper rotor surface. Using four point probe measurements of the
transfer film at room temperature, the bulk resistivity of the film
was found to be about twice the bulk resistivity of pure
copper.
[0025] Although not necessary for practicing the invention, the
inventors, though not seeking to be bound, propose the following
mechanism for solid lubricants according to one embodiment of the
invention. The film generation appears to be well behaved and the
thickness of the film generated is essentially linearly
proportional to the applied contact pressure (normal load). The
removal mechanisms of the transfer film from the sliding interface
is more complex. Regarding application to the brush/rotor system
illustrated in FIG. 1, the lubricant film has at least three
pathways of consumption and flow: (1) consumption into the pores of
the copper brushes 105-107, (2) side-flow out of the sliding
contact, and (3) flow through and under the copper brushes 105-107.
Performance is improved by minimizing the consumption and side-flow
mechanisms, which maximizes the flow under the copper brushes.
[0026] In one embodiment of the invention, the movable electrically
conductive first member and an electrically conductive second
member are formed in a film retention-promoting geometry that tends
to maximize the flow of the solid lubricant into the slideable
electrical contact as well as the retention at the slideable
contact. For example, FIG. 2 shows an exemplary film
retention-promoting geometry, where the first member 210, such as a
rotor, and second member 220 each include triangular teeth that fit
together. The second member 220 is shown as a solid member
including teeth, rather than being a conventional, multi-filament
brush. One of ordinary skill in the art will appreciate that a
variety of other geometries based on the invention can be used to
help promote retention of solid lubricant at the contact as
compared to a planar rotor surface.
Example
[0027] The present invention is further illustrated by the
following example. The example, however, is only for the purpose of
illustrating aspects of the invention and should not be construed
as limiting the scope or content of the invention in any way.
[0028] Referring to FIGS. 3A and 3B, a demonstration of the in situ
solid lubrication of self-mated copper contacts was performed on a
high-speed rotating pin-on-disk tribometer. The experimental setup
is illustrated in FIG. 3A. Corresponding results are presented
graphically in FIG. 3B. The copper disk 302 shown was sanded to an
initial surface roughness of Ra.about.0.15 .mu.m and had a diameter
of approximately 50 mm. The copper pin 304 had a diameter of
approximately 6 mm and was loaded against the copper disk 302 by a
dead weight load of 5 N. The disk 302 was rotated at a constant
angular speed resulting in a sliding speed at the contact of 1.5
m/s. The solid lubricant pin 306 was unloaded and could be manually
brought into contact as needed.
[0029] As illustrated by the graph in FIG. 3B, at the beginning of
the test, with no solid lubricant present, the friction coefficient
rapidly rose to .mu.=0.8, which is typical for self-mated copper
contacts. The surfaces wore rapidly, with rates exceeding
K=1.times.10.sup.-3 mm.sup.3/(Nm). Additionally, the initially
smooth surface topography was lost resulting in very high surface
roughness. The solid lubricant pin 306 was then pressed against the
wear track and held for approximately 25 m of sliding distance.
Over this period the surfaces were healed, and the friction
coefficient rapidly dropped to below .mu..=0.1. After unloading the
solid lubricant pin 306 the friction coefficient rose to .mu.=0.15
and remained steady for over 10 km of sliding at which time there
was a sudden failure of the tribo-film and the friction coefficient
rapidly rose to a value consistent with self-mated copper contacts.
At this point, the solid lubricant film was again applied and the
process of healing the damaged surfaces and offering prolonged
protection of the copper contacts was demonstrated.
[0030] While the preferred embodiments of the invention have been
illustrated and described, it will be clear that the invention is
not so limited. Numerous modifications, changes, variations,
substitutions and equivalents will occur to those skilled in the
art without departing from the spirit and scope of the present
invention as described in the claims.
* * * * *