In high vacuum (under 10-6 Torr), the major problem encountered is seizing which occurs when two clean metals are held in contact in the absence of water vapor and atmospheric gases. The absence of water vapor and oxygen makes it impossible for the ring to film and consequently reduces lubricity to a point of non-existence. Sliding friction under these phenomena produces galling, seizing, and dusting with consequential problems in the transmission of an acceptable electrical signal. To avoid this major problem encountered in high vacuum, lubrication of some type must be introduced either through artificial means such as controlled evaporation or actual inclusion into the material of some lubricant.

In the case of lubrication by wicking or similar means, the end result is a reduction in vacuum (From 10-8 to 10-4 Torr for instance)which will only last as long as he lubrication supply lasts. Additives such as MoS2 are being used successfully and have provided a partial answer. The contact material must also be chosen carefully in order to provide a surface that can be plastically deformed. There is a need for much more investigation before phenomena of high vacuum are completely understood, so that satisfactory design can be specified.

In critical applications it is often necessary to transmit current in the micro-volt region over a relatively narrow band pass (10KHz). This can be successfully achieved by choosing two compatible alloys and increasing the brush pressure to assure complete penetration of the surface film, and at the same time allowing for proper burnishing of the contact area. This will result in a low dynamic noise but unfortunately it will require some compromise regarding life, since forces will create greater torque and wear. Pick-up voltages of the order of one microvolt can then be transmitted. The most difficult problem is the measurement techniques required to monitor the ultra sensitive circuitry.

Since slip rings are often used as rotary joints to transmit currents and voltages in the RF or IF range, it has become necessary to consider a co-axial design. Because of the complexity of designing a rotary coupling with a relatively wide band width, other means are utilized and it has been found that slip rings can operate successfully up to 100 MHz. The basic problem is relatively simple: the slip ring must look like part of the transmission line and its characteristic impedance Zo must match that of the co-ax line. This impedance is usually 50 or 75 ohms.

The solution is not simple and requires a custom design which will be based on the requirements. These requirements are: VSWR, cross-talk, attenuation between circuits, insertion loss, and WOW. All of these requirements can be met with some compromise by using the proper type of shielding (electrostatic, electromagnetic, or both), adequate spacing between conductors and a dielectric material having a low K. The diameter of the slip ring is also very important because of its relationship to the wave length. The closer one approaches this value the more difficult it becomes to avoid an impedance mismatch which results in a high VSWR. Therefore, in order to obtain the best results the diameter of the slip ring must be kept as small as possible especially in the high frequency ranges. If a limited range is required, it is possible to obtain excellent results by tuning the circuits over this range using an LC network.

The use of the high speed slip rings is generally confined to strain gages and thermo-coupling measurements. Because of the low level signals it is necessary to provide a slip ring that will be able to transmit these signals with an absolute minimum of dynamic noise. To avoid these results certain steps must be incorporated at the design stage.