Through-bore slip rings: series compared and how to specify

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

Brand	Series	Circuits	Current	Speed	Bore / OD
Motion Control Products	ER Series Through Bore Slip Ring	24	15 A	300 rpm	bore up to 100 mm
Moog	SRA-73683	24	2 A	250 rpm	bore up to 12.7 mm
Moog	SRA-73830	22	8 A	250 rpm	bore up to 12.7 mm
Moog	AC6438	24	5 A	250 rpm	bore up to 12.7 mm
Moog	AC6349 / AC6875	24	15 A	250 rpm	bore up to 25.4 mm
Moog	AC4598 (AC6419 sealed)	24	10 A	250 rpm	bore up to 38.1 mm
Moog	AC6200	48	10 A	250 rpm	bore up to 38.1 mm
Moog	AC6815	128	10 A	250 rpm	bore up to 38.1 mm
Moog	AC6428	96	2 A	250 rpm	bore up to 34.9 mm
Moog	AC6429	60	10 A	250 rpm	bore up to 34.9 mm
Moog	AC6275 (AC6793 sealed)	144	50 A	1000 rpm	bore up to 69.9 mm
Moog	AC6098	72	50 A	250 rpm	bore up to 101.6 mm
Moog	AC7296	72	25 A	500 rpm	bore up to 25.4 mm
Moog	Endura-Trac Standard W Series	36	30 A	60 rpm	bore up to 76.2 mm
Moog	Endura-Trac Fiber W Series	76	30 A	250 rpm	bore up to 228.6 mm
Moog	Flow Pack F 7270	—	20 A	500 rpm	bore up to 25.4 mm
Moog	F 7154 B	5	12 A	20 rpm	bore up to 50 mm
Servotecnica	SVTS C (C 01-C 11)	96	20 A	500 rpm	bore up to 100 mm
Kübler	SR085	—	25 A	800 rpm	bore up to 30 mm
MOFLON	MT Series through-bore slip rings	144	—	—	bore up to 300 mm
Senring	H Series through-hole slip rings	144	20 A	—	bore up to 300 mm
Deublin	Through-bore slip rings	—	—	—	bore up to 300 mm
Morgan Advanced Materials	Wind turbine generator slip ring assemblies	—	—	—	OD up to 450 mm
Rotary Systems	Through-bore capsules and sealed rings (SR013–SR018)	—	10 A	600 rpm	bore up to 38.1 mm
Meridian Laboratory	ROTOCON MXT series (through-bore)	40	35 A	—	bore up to 101.6 mm
Fabricast	Self-contained through-bore slip rings	24	10 A	10000 rpm	bore up to 50.8 mm

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

Wind turbines (pitch & yaw)Robotics & robot wrists Packaging machines & rotary indexing tables CCTV & pan-tilt cameras Radar & antenna positioners ROVs & subsea winches Cable reels & reelers Cranes & slewing machinery Defence turrets & weapon stations Rotating test rigs & dynamometers Bottling & filling machines Stretch-wrapping & flow-wrapping machines Amusement rides Stage revolves & entertainment Welding positioners & turntables Mixers & process vessels Satellite communication antennas (SATCOM on the move)

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.

Through-bore slip rings