Comparison · Overload protection

Friction vs. ball-detent torque limiters: which overload protection?

When a conveyor jams or a robot end-effector hits a fixture, the motor's stall torque can break gear teeth and shafts in milliseconds. A torque limiter sits between motor and load and slips or disengages above a set value. The two principles have different strengths: a friction limiter slips smoothly while the motor keeps turning; a ball-detent limiter disengages instantly and signals the machine to stop. This guide compares them and says which duty gets which.

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Comparison · Published 2026-09-12 · Wolfer Transmission

How the two principles work

Friction type: disc springs press friction plates together; when torque exceeds the value set by the spring pre-load, the plates slip, the motor side keeps turning and the load side is held back. Wolfer's RTL series (multi-plate friction faces, RTL50 / 65 / 89 adjusted by one nut, RTL127 / 178 by three bolts, 2.94–1087.8 N·m), the CTL sprocket type (limiter and sprocket in one body, 2.9–1080 N·m, up to 1200 rpm, rated to 280 °C) and the CTX flexible-coupling type (damping coupling plus limiter, 2.9–1080 N·m, 3000 rpm) all belong to this family.

Ball-detent type: spring-loaded balls sit in pockets and carry the torque; above the set value the balls ride out of the pockets, the two halves separate within milliseconds, residual torque drops to almost nothing, and a signal ring trips a proximity switch so the controller can stop the machine. Once the overload is gone the balls drop back into their pockets and the limiter re-engages by itself. Wolfer's WFSC series has 8 sizes with L / M / H spring stages covering 4–2400 N·m, up to 4000 rpm, low inertia and a compact envelope.

How they behave during an overload

A friction limiter keeps slipping for as long as the overload lasts, holding torque near the set value, so the machine never loses drive abruptly — on a jammed conveyor the material ahead is still being pushed, and drive resumes automatically once the obstruction clears. The price is heat: prolonged slipping wears the friction plates and lets the setting drift. A ball-detent limiter is switch-like: torque collapses to near zero in milliseconds, the machine stops, and an electrical signal tells the PLC a collision has happened — right for any case where one more revolution would break something. It is not meant to run for long in the disengaged state.

Both designs repeat to ± 5 % of the set value and are pre-set to the order before shipping. Where a one-off protection needs to be more precise — a test stand, for example — a shear-pin unit repeats to ± 1 %, but the pin must be replaced after every trip.

Friction and ball-detent torque limiters compared
Criterion	Friction type (RTL / CTL / CTX)	Ball-detent type (WFSC)
Behaviour on overload	Continuous slip, torque held near the set value	Instant disengagement, residual torque near zero
Motor side	Keeps turning	Runs free; machine must stop
Signal output	None (external sensor possible)	Signal ring + proximity switch
Response	Smooth	Milliseconds
Repeatability	± 5 %	± 5 %
Reset	Resumes automatically when the overload clears; check plates after long slips	Balls re-seat automatically once torque falls
Heat / wear	Slip heat, friction-plate wear	Brief disengagement, little wear
Wolfer torque range	RTL 2.94–1087.8 N·m; CTL / CTX 2.9–1080 N·m	4–2400 N·m (L / M / H stages)
Max. speed	CTL 1200 rpm; CTX 3000 rpm	4000 rpm
Temperature	CTL up to 280 °C	Standard conditions
Typical use	Conveying, packaging, machine tools, heavy drives that jam often	Collision protection on robots, index tables, gantries, assembly equipment
Integrated bodies	Sprocket (CTL), flexible coupling (CTX), through-bore (RTL)	Through-bore; sprocket / pulley / coupling on request

Torque ranges and speeds are taken from the size tables on the selector pages; slip torque is pre-set to the order before shipping and can be re-set on site within the size's range.

Choosing by duty

Conveyor lines, packaging machines and any drive that jams often and should keep the motor turning: friction type — CTL sprocket type for a chain drive, CTX where a coupling is needed between motor and load, RTL for heavy loads and frequent overloads. Robots, index tables, gantry axes and assembly equipment, where a collision must stop the machine: the WFSC ball-detent type with its proximity-switch signal wired into the PLC. Both families can be built into a sprocket, pulley or coupling body so the limiter replaces the existing transmission element without adding shaft length; for a quotation give the slip torque (or motor power and speed), the shaft diameters and keyways on both sides, speed, how often overloads occur, the space available and the operating temperature.

FAQ

Common questions on this topic

Friction or ball-detent — how do I choose?

When overload is continuous and the motor should keep turning while the load is held back (conveying, packaging): friction. When overload means "something just crashed" and the machine must stop at once with a signal to the controller (robots, index tables): ball-detent. If in doubt, ask whether the machine may turn another half revolution.

How is the slip torque set, and is it set at the factory?

Friction types change the disc-spring pre-load with an adjusting nut (RTL50 / 65 / 89) or three bolts (RTL127 / 178); ball-detent types use the spring stage and an adjusting nut. Every unit is pre-set to the order and run through a multi-cycle slip repeatability test before shipping, and can be re-set on site within the range of its size.

How is a ball-detent limiter reset after it trips?

Once the overload is removed and torque falls below the set value, the balls drop back into their pockets by themselves — no manual reset. The controller should stop the machine on the signal-ring input, clear the fault and then restart.

Can the limiter be built into a sprocket, pulley or coupling?

Yes — about half of Wolfer's torque-limiter business is exactly that. The limiter sits inside the sprocket or pulley hub or is combined with a flexible coupling, replacing the existing transmission element without adding shaft length; we need the drawing of that sprocket or pulley and the slip torque.

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