Slip rings safely send power and data across rotary interfaces
What do you need to transmit electric power and/or information between continuously rotating structures and stationary equipment? Slip rings are your most likely solution. These electromechanical devices —also known as rotary joints and rotary electrical interfaces—fall into two basic types.
What do you need to transmit electric power and/or information between continuously rotating structures and stationary equipment? Slip rings are your most likely solution.
These electromechanical devices —also known as rotary joints and rotary electrical interfaces—fall into two basic types. Self-contained or capsule slip-ring assemblies consist of a rotor, stator, integral ball bearings, and stationary brushes that ride on the rotating rings. Separate slip rings supply the ring structure (rotor) and brush block (stator) with brushes as individual components, which users can incorporate in their systems.
Among specific slip-ring designs available, two further configurations emerge. The more common drum (or axial) layout is actually a series of adjacent rings arranged along an axial centerline. It favors applications where linear mounting space is less restricted. In a pancake (radial) layout the rings form a concentric pattern. This suits applications with more diametric space available than axial length. In either case, adequate space for the slip-ring unit must be allotted on the machine or system.
Physical size of units varies greatly. Users have choices from slip rings as large as several feet in diameter down to separate units with rotors under 0.1-in. diameter.
Signal rates, electrical factors
Passing signals is more complex than transmitting power. Most standard slip rings handle digital signals up to 50 MHz frequency. This makes them compatible with a variety of today's data bus networks that work in the 500 kbit/sec to 10 Mbit/sec range. Special slip rings handle higher data rates, for example: 100 Mbit/sec (for Ethernet 100Base T) and 400 Mbit/sec (for Firewire).
A variation of electrical slip rings, known as fiber-optic rotary joints, is available to handle optical signal transmission. On-axis units flange mount on their rotational centerline to a machine, while more complex off-axis joints offer a through-bore design (similar to that in the photo). Most optical units provide a choice of single- or multi-mode fiber links.
Current flow magnitude determines the size (or cross section) of rings, brushes, and lead wires; voltage rating sets the distance between adjacent rings and brushes. Overall size of an assembly depends on the number of circuits (or individual conducting paths) needed and their current rating. As many as 72 signal circuits and 24 power circuits are not unusual in one slip-ring assembly. Some manufacturers supply slip rings with several hundred contacts or connections for extreme data-transfer requirements.
A central through-bore is often part of the design, allowing passage of auxiliary wiring, pneumatic and hydraulic lines, waveguides, or other connections. Electrical connections to the slip ring can be 1) integral to the rotor and stator or 2) the "flying leads" variety. These leads can have an axial exit (see photo) or radial exit from the rotor or stator—or combine the two.
How fast? What environment?
Speed requirements for slip rings vary widely. Just a few revolutions per minute satisfy many applications, while 20,000 rpm or higher is not unusual for cases such as turbomachinery test stands. Speeds in the 100-200 rpm range are considered routine. Surface speed—a combination of ring diameter and rotational speed—is the key design criterion for material selection and brush life. Typical operating life of slip rings is measured in the millions of revolutions. Slip-ring design also must limit electrical noise, especially for applications that transmit instrumentation signals from rotating machinery to data monitoring equipment.
Harsh operating environments impose more attention to details. Exposure to fluids, dust, or contaminants requires use of seals. Standard brushes used at higher operating temperatures need the proper lubricant at the contact surfaces. However, advanced brush contacts without the need for lubrication are available for clean room, vacuum, or special industrial applications.
Slip ring to machine mounting also requires care. Ability to flex must be incorporated to prevent overloading slip-ring bearings due to misalignment. This usually means hard mounting only one side of the assembly and soft mounting the other.
Frank J. Bartos, executive editor firstname.lastname@example.org
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