What is a slip ring?
A slip ring is an electromechanical device which in combination with brushes that allows the transmission of power and electrical signals from a stationary to a rotating structure. Also called a rotary electrical joint, collector or electric swivel, a slip ring can be used in any electromechanical system that requires unrestrained, intermittent or continuous rotation while transmitting power, analog, digital, or RF signals and/or data. It can improve mechanical performance, simplify system operation and eliminate damage-prone wires dangling from movable joints.
While the primary goal of the slip ring is to transmit power and electrical signals, the physical dimensions, operating environment, rotating speeds and economic constraints often affect the type of packaging that must be employed.
The customer’s requirements and cost objectives are critical elements in driving the decisions that lead to the development of a successful slip ring design. The four key elements are:
■ electrical specifications
■ mechanical packaging
■ operating environment
Slip rings are used to transmit power, analog, RF signals and data through a rotating unit. The number of circuits, types of signals, and the electrical noise immunity requirements of the system play an important role in the determination of the physical design constraints imposed upon the slip ring design. High power circuits, for example, require larger conductive paths and greater spacing between the paths to increase dielectric strength. Analog and data circuits, while physically narrower than power circuits, also require care in their design to minimize the effects of cross-talk or interference between signal paths. For low speed, low current applications a gold-on-gold brush/ring contact system may be employed. This combination produces the smallest packaging configurations as shown in AOOD compact capsule slip rings. For higher speed and current needs the incorporation of composite silver graphite brushes and silver rings are used. These assemblies normally require larger package sizes and are shown under through bore slip rings. Using either method most slip ring circuits exhibit changes in dynamic contact resistance of approximately 10 milliohms.
The packaging considerations in designing a slip ring are often not as straightforward as the electrical requirements. Many slip ring designs require cabling and installation shaft or media to pass through the slip ring. These requirements often dictate the unit's inner diameter dimensions. AOOD offer a variety of through bore slip ring assemblies. Other designs require a slip ring to be extremely small from a diameter stand-point, or from a height standpoint. In other cases, the space available for the slip ring is limited, requiring the slip ring components be provided as separate, or that the slip ring be integrated with a motor, position sensor, fiber optic rotary joint or RF rotary joint in an integrated package. Based on sophisticated slip ring technologies, AOOD enable all these complex requirements can be met in one complete compact slip ring system.
The environment that the slip ring is required to operate under has an influence on the slip ring design in many ways. Rotational speed, temperature, pressure, humidity, shock & vibration and exposure to corrosive materials impact the bearing selection, exterior material selection, flange mounts and even cabling choices. As standard practice, AOOD utilizes lightweight aluminum housing for its packaged slip ring. Stainless steel housing is heavier, but it is necessary for marine, underwater, corrosive and other harsh environment.
How to Specify a Slip Ring
Slip rings are always part of a larger mechanism with a need to pass specific electrical power and signal circuits through a rotating surface. The mechanism the slip ring is part of operates in an environment such as an aircraft or a radar antenna system. Therefore, to create a slip ring design that will succeed in its application three criteria must be satisfied:
1. Physical dimensions, including attachment arrangement and de-rotating features
2. Description of circuits required, including maximum current and voltage
3. Operating environment, including temperature, humidity, salt fog requirements, shock, vibration
More detailed slip ring requirements include:
■ Maximum resistance between rotor and stator
■ Isolation between circuits
■ Isolation from EMI sources outside the slip ring housing
■ Starting and running torque
■ Data circuit descriptions
Common extra features that can be incorporated in a slip ring assembly include:
■ Fluid rotary unions
■ Coax rotary unions
■ Fiber optic rotary joints
AOOD will help you to specify your slip ring need and select the optimum model for your design requirements.