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What is a Slip Ring?

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A slip ring is a vital electromechanical device that enables the transfer of power and electrical signals from a stationary platform to a rotating structure. A slip ring can be used in any electromechanical system or machine/equipment that requires rotation while transmitting the power or signals, such as those found in radar antenna, gun turrets, periscopes, electro-optic sensor gimbals, space satellites, and etc. Slip ring can excellently improve inside the system the mechanical performance, simplify system operation and eliminate damage-prone wires dangling from movable joints.

The slip ring is also called rotating electrical connectors, rotary electrical interfaces, swivels, collectors, or electrical rotary joints, these rings are commonly found in slip ring motors, electrical generators for alternating current (AC) systems and alternators and in packaging machinery, cable reels, and wind turbines. It has become a commonplace component in many of today’s industrial, military and even everyday equipment/machine, where they play a crucial role. They can be used on any rotating object to transmit power, control circuits, or analog or digital signals including data such as those found on aerodrome beacons, rotating tanks, power shovels, radio telescopes or heliostats.
 

Diagram of a cross-section of slip rings for an electric motor

Sketch of a cross-section of slip rings for an electric motor. In this example, the slip rings have a brush-lifting device and a sliding contact bar, allowing the slip-rings to be short-circuited when no longer required. This can be used in starting a slip-ring induction motor, for example.

A slip ring (in electrical engineering terms) is a method of making an electrical connection through a rotating assembly. Formally, it is an electric transmission device that allows energy flow between two electrical rotating parts, such as in a motor. Slip rings handle either power, data or both across a number of rings on which a spring or other loaded contact finger, ring or brush is held in place to pass power as the rings rotate.  The power levels range from milliwatts to hundreds of kilowatts dependent on the real application. In all applications, frictional wearing is the main determinant of the lifecycle of the slip ring.

Slip ring products can be designed in a variety of configurations. For example they can provide an open through-bore to accommodate other components such as a fluid joint or a wave guide. Instead of a drum the rings can be configured in a platter or pancake. This is appropriate if the space available for the slip ring is short and wide instead of tall and narrow.

Typically, slip ring is constituted by a stationary graphite or metal contact (brush) which rubs on the outside diameter of a rotating metal ring. At a very elementary level, a slip ring is comprised of two major components – a metal brush, and a metal ring. The “brush” can look like a brush made from metal fibers, but more often it looks like a single wire or a spring with a pad of material affixed to the end that contacts the ring. As the metal ring turns, the electric current or signal is conducted through the stationary brush to the metal ring making the connection. Additional ring/brush assemblies are stacked along the rotating axis if more than one electrical circuit is needed. Either the brushes or the rings are stationary and the other component rotates. This simple design has been used for decades as a rudimentary method of passing current into a rotating device.

slip ring, rotor leads, shaft, brush, full contact with core

The ring itself is a band of electrically conductive material, mounted on a shaft. Although it’s insulated from the shaft itself, the slip ring is connected to the rotor or rotating assembly through windings or other electrical connections. The outer part of this slip ring remains in continuous sliding contact with the stationary brushes – or other stationary conductors – which provides either intermittent or continuous rotating devices with unbroken contact between the rotating assembly and external circuit. This ensures power and data are able to be transmitted at all times. Both this ring and the brush are contained within a single housing, designed to protect the working parts from harmful environmental factors such as dust and moisture.
 

Slip rings on a hydroelectric generator;

Slip rings on a hydroelectric generator; A - stationary spring-loaded graphite brushes, B - rotating steel contact ring, C - insulated connections to generator field winding, D - top end of generator shaft.

A critical aspect of slip ring design is the materials used to make the rings and the brushes. After World War II slip ring suppliers and manufacturers responded to requirements from the emerging electronics industry with gold plated rings using gold wire brushes. Up until the 1970’s many slip rings used copper rings and graphite brushes. Copper is smooth and conducts electricity well. Graphite conducts well, it is soft, and it does not abrade the copper rings. But for advanced applications such as space satellites and missiles, graphite brushes were inadequate. There was a burst of research and invention in the early years of the space program to address emerging needs, mostly in space and missile programs, that produced advanced gold on gold alloy systems and systems that employed silver alloy rings with brush tips made from silver graphite mixtures. Today these two systems, gold-on-gold and silver/silver-graphite, are the mainstays of the slip ring industry. Gold-on-gold systems are often used for low current signal circuits because they are compact. Silver/Silver-graphite systems are often selected by manufacturer of slip rings for higher current power circuits because in these applications they are more economical, and have long service life.

Some other names used for slip ring are collector ring, rotary electrical contact and electrical slip ring. Some people also use the term commutator; however, commutators are somewhat different and are specialized for use on DC motors and generators. While commutators are segmented, slip rings are continuous, and the terms are not interchangeable. Rotary transformers are often used instead of slip rings in high-speed or low-friction environments.

A slip ring can be used within a rotary union to function concurrently with the device, commonly referred to as a rotary joint. Slip rings do the same for electrical power and signal that rotary unions do for fluid media. They are often integrated into rotary unions to send power and data to and from rotating machinery in conjunction with the media that the rotary union provides.

Slip rings are made in various types and sizes by slip ring designer and product developer; one device made for theatrical stage lighting, for example, had 100 conductors.  The slip ring allows for unlimited rotations of the connected object, whereas a slack cable can only be twisted a few times before it fails.

Wireless and contactless slip rings do not rely on the typical friction based metal and carbon brush contact methods that have been employed by slip rings since their invention, such as those explored above. Instead, they transfer both power and data wirelessly via an electromagnetic field, which is created by the coils that are placed in the rotating receiver, and the stationary transmitter. Wireless slip rings are considered an upgrade from - or alternative to - traditional slip rings, as their lack of standard mechanical rotating parts means they're typically more resilient in harsh operating environments, and require less maintenance and upkeep. Contactless, wireless slip rings are the next generation in inductive power transfer solutions, specifically designed to replace existing, aging mechanical slip ring technology. Moving on from a reliance on friction based metal and carbon brush contact to enable the transfer of power and data, wireless slip rings utilize IPT to transfer power via an electromagnetic field created by coils placed in the stationary transmitter and rotating receiver. By removing the inherent need for friction or contact to generate an electrical current, wireless slip rings significantly reduce the maintenance costs associated with worn mechanical slip rings, while also offering a simple, clean design that’s easily integrated into any application or device.

Today’s slip rings are reliable, have long service life and can be designed for extraordinary environments, including the vacuum of space, the shock of a tank gun turret or the vibration of a helicopter rotor.

Where are slip rings most commonly found?

Slip rings are utilized in a variety of devices across a broad range of industries. Some of the slip ring applications include:

TRUCKS & MAN LIFT EQUIPMENT:
Aerial Work Platforms
Bridge Cranes
Platform Cranes

INDUSTRIAL/COMMERCIAL IN-PLANT EQUIPMENT:
Automated Welding Equipment
Centrifuges
Conveyor Systems
Custom Machinery
Fabrication Equipment

DRILLING & OTHER BORING EQUIPMENT:
Off-Shore Drilling Rigs
Down hole Drill and Geotechnical Testing
Horizontal Earth Boring & Drilling

SIGNAGE RELATED EQUIPMENT:
Advertising Displays
Exhibits and Displays

MISC. OFF-HIGHWAY HEAVY EQUIPMENT:
Forklifts
Utility Trucks
Truck Body / Service Cranes

OTHER:
Amusement Rides
Arcade Machines
Car Wash Equipment
Wind Turbines

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