Airflyte Electronics Co. PO Box 231 Bayonne, NJ 07002 (201) 436-2230

The electrical requirements generally determine the cross sectional area of the conductor (lead wire) which is normally assumed to be 10 milliamperes per circular mil. This will insure that no undue heating will occur to affect the dielectric properties of the slip ring insulation. Because the slip ring must work in conjunction with a brush (wire or block) the ring width is a function of the size of the brush. The thickness of the ring is usually much greater than is required to carry the current and is related to the diameter of the ring for mechanical reasons. As a rule of thumb, noble metal wire brushes will carry approximately 40% of the current of a copper wire of equivalent cross section and silver graphite material will carry approximately 300 amperes per square inch. The silver graphite block is usually mounted at the end of a BeCu spring member which has a conductivity of 45% IAS. If the current becomes very high (above 25 amperes) shunt wires will be required since the cross-sectional area of the arm will become too heavy and cease to act as a spring. The voltage problem is relatively easy to contend with, as most insulating materials have properties that will allow them to withstand 200 to 400 volts per .001 (mil). The major problem is the dielectric strength which is about 80 volts per mil clean air. (For normal use this figure should be derated to about 50 volts per mil at sea level). To increase the creepage path, raised barriers are necessary, especially in the presence of condensation.

Although noise is an electrical problem, the generation of noise is mechanical and measured as an AC signal generated by the change in dynamic contact resistance. Most slip rings exhibit changes in contact resistance of approximately .005 ohms. This value will be reduced after proper run-in procedures, which seat the brush against the slip ring. From the wear theory, it is known that all surfaces are composed of hills (asperities) and valleys distributed in random fashion. Through the mechanism of sliding friction, as more and more of the peaks come into contact with one another through shear or plastic deformation, there will be more and more surface area for the current to pass from one member to the other, thereby decreasing the contact resistance. Because of the difficulty of making statistical computations, physical measurements can be made that will provide empirical data of the performance. The noise voltage is approximately proportional to the change of contact resistance when small currents are being applied, but this does not hold true for values higher than 100 milliamperes. Eccentricity will also create noise because of the cyclic variation in circuit resistance due to the motion of the brush arm. This last condition is also encountered in vibration testing when the resonant point is approached. A method of minimizing noise is to start with brush and ring surfaces which are closely finished to provide a maximum of asperities of low amplitude so that there is a minimum of distortion before seating is accomplished. High pressure or large forces are not beneficial to this condition because rapid wear will ensue and shorten the useful life of the slip ring.