What is a Slip Ring?

A slip ring is a device which allows the transfer of electrical current or a signal between a static source and a rotating object without the use of a physical wired connection between the two. A slip ring arrangement achieves this connection with a conductive ring attached around the rotating component and a spring loaded brush or contact on the static component. This brush bears against the revolving ring and maintains a constant electrical contact with it while it remains static. Slip ring devices have various applications.

Passing a current or signal between revolving and stationery objects poses an engineering conundrum which is best solved by using a low friction, sliding contact between the two. In applications involving a rotor such as those found in generators and motors, the best way to achieving this is to use a solid slip ring. This consists of a conductive ring, typically made from high grade, polished copper which runs around the shaft of the rotor. This ring is insulated from the shaft and wired to the windings of the rotor. A spring loaded brush carrying the electrical current is positioned so that it maintains a constant pressure on the ring throughout its rotational travel in this way completing the circuit.

Brushes are usually made of a conductive material such as carbon which is softer than the ring. This material is used to ensure that the brush, rather than the ring, wears first as the brush is easier and cheaper to replace when it becomes worn down. In applications which require a low resistance connection, mercury may be used to replace carbon brushes. Due to its toxicity though, it may not be used in pharmaceutical or food related industries where mercury contamination poses a health risk.

The use of a slip ring system is not limited to the passage of electric current. This transfer is also not limited to a static to rotor passage either; current, signal, or data may be transferred from the rotating component out to the static element. As long as the surfaces of the brush and the ring are kept clean and free of corrosion and chips, the slip ring connection can be kept very “clean” with a minimum of sparking, data interference, and audio noise. Slip ring applications include rotating signal or warning beacons, alternators, generators, and electric slip ring motors and make use of several designs and materials for the rings and contacts.