Through-bore slip rings

What a through-bore slip ring is

A through-bore slip ring is a rotary electrical joint built around a hollow centre, so a drive shaft, pneumatic or hydraulic line, fibre or cable bundle can pass straight through it. The rings are mounted on an insulated sleeve that fits over or around the machine shaft, and the brush blocks sit outside them in a stationary housing. Power, signal and data circuits are combined in one unit, while the bore keeps the rotation axis available for mechanics or services. Bore sizes range from a few millimetres to hundreds of millimetres.

When to choose a through-bore slip ring

Engineers choose a through-bore design when the axis of rotation is already occupied, typically by a shaft, a rotary union or a cable that must stay continuous. It is the standard choice for indexing tables, wrapping machines and turntables built around a central column. The trade-off is that larger rings mean higher surface speed, more brush wear and more drag torque at the same rpm, and the unit is bigger than a capsule. Where axial length is the constraint rather than diameter, a pancake design may suit better.

How to specify through-bore slip rings

• Bore diameter and shaft tolerance (mm)
• Outer diameter and length limits (mm)
• Circuit count and current per circuit (A)
• Working voltage and any high-voltage isolation (V)
• Speed, continuous and peak (rpm)
• Data or bus protocols to pass, such as Ethernet, CAN or PROFINET
• Sealing (IP code), temperature (°C) and mounting method

Common mistakes

• Sizing the bore to the shaft only, with no allowance for a rotary union or cables that must also pass
• Mounting the rotor hard on a shaft with radial runout and a separate stator fixing, so the unit is forced to align two axes
• Forgetting that surface speed rises with diameter, so the rated rpm is lower for larger bores
• Not stating which circuits need screening or twisted pairs for encoder and fieldbus signals

Series compared

Maximum figures across each series, summarised from manufacturers' published data on the date shown on each series page. Always confirm against the current datasheet.

Where through-bore slip rings are used

Through-bore slip rings: frequently asked questions

How do I choose the bore size for a through-bore slip ring?

Start with the shaft diameter and its tolerance, then add anything else that must pass through the centre, such as hoses, a rotary union stem or a fibre. Allow clearance for assembly and for the insulating sleeve. Larger bores raise surface speed, cost and drag torque, so pick the smallest bore that genuinely clears what has to pass through.

Can a through-bore slip ring carry Ethernet as well as power?

Yes. Many through-bore units combine power rings with signal rings designed for industrial Ethernet, using controlled impedance, twisted-pair routing and screening between groups. State the protocol and speed, typically 100 Mbit/s or gigabit, so the supplier can allocate suitable rings and keep them away from motor power. For very high data rates, a contactless or fibre channel may be added.