Every drivetrain has a weakest link. A torque limiter is a deliberate weak link you engineer into the system — a device that disengages, slips, or breaks when torque exceeds a set threshold, sacrificing itself (or just briefly slipping) to save the expensive components downstream. Without one, a jammed conveyor can twist a motor shaft, strip a gearbox, or bend a linkage in the milliseconds before the overload relay trips.

The main types:

• Friction (slip) clutches — spring-loaded friction discs squeezed together. When torque exceeds the friction capacity, the discs slip. Reusable, resettable, and self-limiting: they'll slip continuously without damage (until heat becomes a problem). Common on agricultural PTO shafts and packaging machinery.
• Ball-detent limiters — spring-loaded balls sit in detents in a driven plate. Above the trip torque, the balls climb out of their pockets and the coupling disengages with a distinctive click. Very precise trip point (±5%), zero backlash before trip, and re-engages automatically on the next revolution. Standard on servo-driven indexing tables.
• Shear pin couplings — a hardened pin sized to fail at a calculated torque. Cheapest option, but requires stopping the machine and replacing the pin. Used on snowblower augers, boat propeller hubs, and mine conveyors.
• Magnetic couplings — permanent magnets transmit torque across an air gap. Slip is smooth and non-contact, but power is lost as heat in the induced currents.

Sizing rule of thumb: set the trip torque to 1.25 to 1.5× the maximum expected running torque, but well below the yield torque of the weakest downstream component. Too low and you get nuisance trips on startup inrush; too high and the limiter never protects anything.

Quick calculation: A 3 kW motor at 1750 RPM produces T = 9550 × P(kW) / N(RPM) = 9550 × 3 / 1750 ≈ 16.4 N·m of continuous torque. Size the limiter for roughly 20–25 N·m. If the gearbox behind it has a rated input torque of 40 N·m, you have comfortable protection margin.

Real-world example: On a case packer, a servo-driven flight bar pushes cartons at 200 cycles per minute. If a carton jams, the servo can produce 5× rated torque in under 10 ms — enough to bend the flight bar or crack the gearbox housing. A ball-detent limiter set at 30 N·m disengages within a fraction of a revolution, the servo fault-stops, and after clearing the jam the coupling re-engages by hand. Downtime: 30 seconds instead of a gearbox rebuild.