Slip rings and rotating connections on winches

Most winches need no electrical connection to the drum itself; the motor turns the drum and that is all. But some do, and there the problem is obvious as soon as you think about it: a cable run to a turning drum would simply wind up and break as the drum rotated. The device that solves this is the slip ring, which passes power or signal between the fixed frame and the rotating drum so the connection stays live however far the drum turns. Slip rings appear on specialised winches, and understanding them is part of understanding how a winch can power or monitor something on a part that never stops turning, as our note on subsea winches touches on for ROV umbilicals.

The problem of a turning connection

If something on a rotating drum needs electrical power or sends a signal, a fixed wire cannot simply be run to it. As the drum turns, the wire winds around the axis, twisting and tightening until it snaps, and even before it breaks the winding limits the drum to a part turn. So a winch that must power or read something on the rotating part cannot use an ordinary fixed connection; it needs a way to carry the electricity across the rotating joint without the wire twisting. This is a real constraint on any winch where the drum or its load must stay electrically connected while it spins, and the slip ring is the answer to it.

What a slip ring does

A slip ring carries electrical power or signal between a stationary part and a rotating part through sliding contacts. Conducting rings on the rotating side are contacted by fixed brushes on the stationary side, so as the rings turn with the drum, the brushes stay in contact and the electricity passes across the joint continuously. The drum can rotate freely, as many turns as it likes in either direction, while the power or signal flows without interruption and without any wire twisting. So the slip ring turns an impossible fixed connection into a continuous rotating one, which is exactly what a winch with a live drum needs.

Powering an umbilical

The clearest use of a winch slip ring is a powered umbilical, where the cable on the drum carries power and signal to equipment on its end, such as an ROV or a subsea tool. The umbilical winds onto the drum as the winch hauls in, so the connection to it must pass through the rotating drum, and a slip ring carries the power and the data across that rotation. Without it the umbilical could only be wound a part turn before the connection twisted. So on a winch handling a powered umbilical, the slip ring is essential, letting the drum spool the cable freely while keeping the equipment on the end powered and in communication, the subject of our note on subsea and ROV winches.

Signals from the rotating part

A slip ring is also used to carry signals from sensors on the rotating drum or load back to the fixed side. A sensor measuring something on the drum, or an instrument on the rotating part, would have its wires wind up without a slip ring, so the signal is passed across the rotation through the slip ring just as power is. This lets a winch monitor or measure something on the turning part continuously, which would otherwise be impossible. So the slip ring carries information as well as power, and a winch that must both feed and read a rotating part may use a slip ring with conductors for both.

Heated and instrumented drums

Some specialised winches power equipment built into the rotating drum, such as a heated drum or instrumentation, and these too need a slip ring. A heater or a device on the drum cannot be fed by a fixed wire that would wind up, so the power crosses to the rotating drum through the slip ring, letting the drum function be powered while it turns. These are specialist applications, but they share the same need: an electrical function on a part that rotates continuously, which only a slip ring or a similar rotating connection can serve. So wherever the drum itself carries something electrical, the slip ring is what makes it possible.

Looking after the slip ring

A slip ring uses sliding contacts, brushes on rings, and like any contact it wears and must be maintained. The brushes wear with use and are replaced in time, the contacts are kept clean for a good connection, and the whole unit is protected from dirt and moisture that would foul the contacts, especially on a winch working in the wet. A neglected slip ring gives a poor or intermittent connection, which on a powered umbilical or an instrumented drum means lost power or data. So the slip ring is part of the winch's maintenance, watched and serviced like any wearing part, so the rotating connection stays sound through the winch's life.

When a winch needs a slip ring

A slip ring is needed only where something on the rotating drum or load must be electrically connected while it turns, which is a minority of winches but an essential feature where it applies. A powered umbilical, a sensor on the drum, a heated or instrumented drum, all need it; an ordinary winch where only the motor drives the drum does not. The honest approach is to identify whether any electrical function must cross the rotating joint, and fit a slip ring rated for the power, the signal and the conditions where it does. We specify the slip ring with the winch where the duty needs it, so the rotating connection is reliable rather than an afterthought.

A winch with a rotating connection from us

We supply winches with slip rings where the duty needs a live connection to a turning drum, such as powered umbilicals and instrumented drums. See the range in our winch catalogue, read our overview of subsea and ROV winches and how encoders and positioning use signals from the winch. Tell us what must be powered or monitored on the rotating part, and the conditions, and we will specify a slip ring that keeps the connection live through the rotation.

Frequently asked questions

What is a slip ring on a winch?

A device that passes electrical power or signal between the fixed frame and the rotating drum through sliding contacts, so the drum can turn freely while the connection stays live. It lets a winch power or monitor something on the rotating part without a wire winding up and breaking.

Why can a fixed cable not feed a turning drum?

Because as the drum turns, the cable winds around the axis, twisting and tightening until it snaps, and even before breaking the winding limits the drum to a part turn. A slip ring carries the electricity across the rotating joint without any wire twisting, allowing continuous rotation.

When does a winch need a slip ring?

When something on the rotating drum or load must be electrically connected while it turns, such as a powered umbilical, a sensor on the drum, or a heated or instrumented drum. An ordinary winch where only the motor drives the drum does not need one; it is specialist but essential where it applies.

Does a slip ring need maintenance?

Yes. It uses sliding contacts, brushes on rings, that wear and must be replaced in time, with the contacts kept clean and the unit protected from dirt and moisture. A neglected slip ring gives a poor or intermittent connection, so it is serviced like any wearing part to keep the rotating connection sound.