Introduction To Slip Ring Trades
Slip rings seem to be one of the last considerations for systems designers, and for good reasons. Before serious work can begin on a slip ring design, the circuit requirements for the system must be finalized. In an ideal world the system design should allow sufficient space of a convenient shape to accommodate the slip ring and cable harnesses to the rotor and stator. Somewhere along the path to a final design concept the systems design team and the slip ring specialists should confer to establish the slip ring design concept to meet the circuit requirements within the space available. Here is a list of trades and other considerations that should help advance this process to a happy conclusion:
- Power, Voltage and Current. Designers should recognize that current requires copper and voltage requires space. When amps go up there is no alternative to increasing the cross section of copper in the wire and beefing up other components to control heat generation. At the same time when voltage increases there is a need for increased isolation, and this usually means more distance between circuits and more space for the slip ring.
- Shape. In general additional length is more useful to a slip ring designer than additional diameter. Length can often be shortened by using an inner and outer slip ring (an approach pioneered by Electro-Miniatures), but this is more costly than a single long slip ring.
- Compactness vs Manufacturability and Cost. There is often pressure on the systems people, especially for airborne systems, to drive size and weight as small and light as possible. But this comes at the expense of manufacturability. The effect of making slip rings more difficult to manufacture is to make them more expensive.
- Isolation. The more compact the slip ring, the tougher it is to achieve circuit isolation goals.
- Connector Selection. It seems there should be potential for cost and lead time savings by selecting common connectors used in industry rather than trying to optimize space utilization with a custom or rarely used connector. Lead times tend to be less for commonly used connectors, too.
- Sealing vs. Torque. Some applications must operate in harsh environments that require the rotating seal to be tight. Other applications, especially where the slip ring is part of an accurate aiming device, demand that the slip ring offer low resistance torque. Systems designers should be aware that there is a trade between torque and the degree of sealing. It is tough to optimize the seal and provide the lowest torque in the same unit.
- Housing Material. Most slip ring assemblies for defense applications use aluminum housings. Aluminum is strong, inexpensive, easy to machine, relatively light, conducts electricity and provides the basis for a Faraday Cage, if required. If weight is critical, then titanium can be used. Titanium is expensive and harder to machine. For applications in corrosive environments the housing can be made from stainless steel. This adds weight and cost.
Since the introduction of the Sequestration budget and DoD’s “Better Buying Power” guidelines, there has been increased demands from defense customers for suppliers to offer alternatives to the RFP specification that could save the Government money and have negligible impact on performance. This new era of procurement will be a challenge, requiring engineers to consider alternative designs and understand cost impacts of their choices.
We hope this summary of slip ring trades will help you to shape your thoughts on your next slip ring design project. Contact us for help with your slip ring design challenges.