Stock material or miscellaneous articles – All metal or with adjacent metals – Having magnetic properties – or preformed fiber orientation...
Reexamination Certificate
1999-02-05
2001-06-12
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Having magnetic properties, or preformed fiber orientation...
C310S251000, C310S252000, C029S826000
Reexamination Certificate
active
06245440
ABSTRACT:
DESCRIPTION
1. Technical Field
This invention relates to fiber brushes, and in particular, the improvements in the design and manufacture of fiber brushes of the type disclosed in commonly owned U.S. Pat. Nos. 4,358,699 and 4,415,635, the disclosures of which are incorporated by reference herein.
2. Background Art
Although graphite and metal-graphite brushes have for nearly 100 years dominated the field of electrical brushes, for many applications there now exists a superior form of sliding electrical conduction; high performance fiber brushes wherein typically the fibers are made of metal for which reason they are called metal fiber brushes. Prime candidates for this new technology include sliding electrical systems which require high current densities, high sliding speeds, low electrical noise, high efficiency (low brush losses), compact size, or long brush lifetimes.
In particular, low voltage electric motors and generators can be made smaller, more powerful and longer lasting owing to the increased current capacity, higher efficiency and longer wear life. This has a direct bearing on electric vehicular and ship drive systems as well as low voltage electrical power generators. Other applications which require high currents, such as high-force linear actuators, electromagnetic brakes, and armatures, are similarly well suited.
Many signal-critical electronic devices such as rotating antennae, slip rings and shaft pickups for electronic sensors and other transducers could greatly benefit from the low noise and low voltage drop characteristics of metal fiber brushes. In addition, the new generation metal fiber brushes can be manufactured with dimensions as small as fractions of a millimeter with user-selected stiffness (as measured in applied brush force in Newtons per millimeter of resulting brush compression, for example), making them usable as closeproximity, multiple-pole sliding pickups. They are also superior for delicate rotating instruments, since the required brush forces are much lower than for typical graphite based brushes. The broad-band electrical “noise” emission spectra of electrical equipment such as drills, saws and other power tools can be greatly reduced by the use of metal fiber brushes, thereby reducing or eliminating the electrical interference through these brushes in use near sensitive electronic equipment.
As an interface, metal fiber structures and material can provide a low loss connection at greatly reduced forces, thereby providing high-efficiency, low force electrical contact. This is particularly important for high-current, low voltage switching, such as encountered in variable voltage battery storage systems which are charged at high voltages. Based on simple laws of physics, the capability of fiber brushes to efficiently transfer electrical current across interfaces which are in relative motion or at rest, is paralleled by their capability to similarly transfer heat. Therefore the brushes can also be used as heat transducers for cooling or heating purposes. The outstanding features of metal fiber brushes and some suggested applications are listed as follows.
High Current Capacity
Because metal fiber brushes can operate at very low losses, and consequently at low heat evolution rates, they can conduct higher current with lower losses than graphite based brushes. Continuous current densities of over 310 A/cm
2
(2000 A/in
2
) have been demonstrated and this does not by any means represent an upper limit. Accordingly, equipment which operates at high currents and low voltages can be made more efficient and in many cases can run at higher power levels. Examples of this type of equipment include homopolar motors and generators, which have applications in electric automotive, rail and ship drives, low voltage generators, such as those used with fuel cells and with such applications as the hydrolyzation of water for combustible fuel production. Similarly, linear high current devices, such as linear actuators, and linear pulse generators.
Low Electrical Noise
As already mentioned above, metal fiber brushes can operate at much lower electrical noise levels than traditional graphite-based brushes. This can have dramatic benefits for signal-critical equipment on two fronts. First, instrumentation which requires rotating or linear sliding contacts, such as rotating antennae, can achieve much higher signal resolution than with graphite-based brushes. Second, machinery will give off much less electrical noise and therefore cause much less induced interference when located in close proximity to sensitive transducers, detectors, and other electronic equipment if metal fiber brushes are used.
Long Wear Life
Metal fiber brushes can achieve not only low dimensionless wear rates, measured in wear length of brush shortening per length of sliding path, but they can also be constructed with very long, and in some cases nearly unlimited, permissible wear lengths. This translates to extremely long brush life and greatly lengthened service intervals. For example, metal fiber brushes have demonstrated a dimensionless wear rate of 2×10
−11
, and at this rate a brush will wear by 5 cm of wear length over 2.5×10
9
meters of sliding path, or over 1.5 million miles. Obviously, continuously operated equipment would greatly benefit from this feature of metal fiber brushes.
High Sliding Speeds
Many applications such as high speed motors and generators require electrical brushes which can operate at high sliding speeds. Metal fiber brushes have been successfully operated at speeds in excess of 70 m/s and their theoretical limit certainly lies considerably higher than that.
Compact Size
Electronic systems which need close proximity to a moving power or signal coupling, or spacecritical sliding contacts could be further miniaturized by the use of this new generation of metal fiber brushes because these brushes can be made in sizes down to fractions of millimeters in thickness or diameter. This has a particular application relating to signal power, and control-line pickups from rotating shafts such as are found in satellites, aircraft, periscopes, or many kinds of rotor testing systems.
Low Heat Dissipation
Because they operate at low loads and have very low resistance, metal fiber brushes dissipate much less heat than typical brushes in high-current or high-sliding-speed applications. This could be of great benefit in insulated or temperature sensitive equipment such as refrigeration systems or devices that incorporate compact rotating electronics.
Clean Operating
Unlike graphite-based brushes, metal fiber brushes do not generate fine carbon dust, which can cause problems not only with appearance and clean-up but also with long-term fouling and shorting. Metal fiber brush wear debris is heavy enough to be easily trapped or filtered making it therefore much easier to keep the system clean.
In addition, an advantage of metal fiber brushes is the smaller production of presumably more benign wear debris as compared to that of graphite-based brushes. At anticipated similar dimensionless wear rates of conventional and metal fiber brushes, reduction of wear debris volume from the latter is due to smaller running areas on account of increased current densities in combination with the fact that typically 80% to 90% of the brush is voidage, (1−f) with f the “packing fraction” of the volume occupied by fibers, which does not produce wear debris. The extreme limits of packing fraction range between 1% and 90%.
DESCRIPTION OF THE INVENTION
a. General Considerations
The previous metal fiber brushes suffered from the following problems;
difficulty of manufacture
limitations on the achievable relationship between macroscopic brush stiffness and microscopic fiber compliance
problems associated with the necessity of using a removable constituent during manufacturing
limitations on the types of metals usable as conductors in the brushes on account of the need for differential etchability or dissolution of the matrix material.
The ideal, therefore, are fibers assembled into t
Gillies George T.
Kuhlmann-Wilsdorf Doris
Makel David D.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
University of Virginia
Zimmerman John J.
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