Holding brakes for robot joints: types, sizing and placement

By Tolerance Labs engineering team · Updated 18 October 2026

A backdrivable joint drops its load when the power goes off. A holding brake stops that. Choosing one means picking a type, sizing it to the gravity load, deciding which side of the reducer it goes on, and wiring it so it releases and engages at the right moments.

Any joint that carries a gravity load and can be back-driven needs a brake: most arm shoulders and elbows, lift axes, and the legs of a robot that must not collapse when switched off. A high ratio alone is not enough. A harmonic or multi-stage planetary joint is hard to backdrive but not self-locking, so it creeps or falls under a steady load; backdrivability in robot joints explains why. A brake also holds a pose with no holding current, which saves heat.

Spring-applied and permanent-magnet brakes

Spring-applied (power-off) electromagnetic brakes use springs to press an armature plate against a friction disc keyed to the shaft. Energizing the coil pulls the armature back and frees the disc. With no power the springs clamp, so the brake fails safe. The disc slides on a splined or hexagonal hub, which can leave a small amount of rotational play.

Permanent-magnet brakes use a magnet to pull the armature onto the friction face. Current in the coil cancels the magnet’s field to release it. The armature is usually carried on a spring plate, which gives zero backlash, and torque per unit size is high. They need the correct polarity and a controlled current: too little leaves the brake dragging, and too much can pull the armature back in.

A third type, a solenoid that drops a pin into a notched disc, holds only at the notches.

Holding torque and dynamic torque

Holding torque, also called static torque, is the torque an engaged brake holds without slipping. Dynamic torque is the torque it produces while slipping, as it does when it stops a turning shaft. Sliding friction is lower than static friction, so dynamic torque is lower. Some makers give only the holding figure; ask for both.

Most joint brakes are holding brakes, meant to clamp a stopped shaft and to stop a moving joint only in an emergency, a limited number of times. Each emergency stop turns kinetic energy into heat and wear at the friction face, so check the rated friction energy per stop. Some brakes need running in, or burnishing, to reach rated torque, and grease on the friction face lowers torque sharply.

Sizing to the gravity load: worked arm example

Take a two-link arm held horizontal, the worst case for its shoulder. Treat each part as a point mass at its center of mass.

Gravity torque at the shoulder: 9.81 × 2.535 ≈ 24.9 N·m. A safety factor of 1.5 to 2 is a common starting point, to cover wear, temperature, burnishing and a heavier payload than planned. With a factor of 2, the brake must hold about 2 × 24.9 ≈ 50 N·m at the joint output.

Then check the engagement delay. If power fails with the arm moving at 1 rad/s and the brake takes 20 ms to clamp, the arm turns another 1 × 0.020 = 0.020 rad, about 1.1°. At the payload, 0.6 m out, that is 0.020 × 600 = 12 mm before the brake grips, more as gravity speeds the arm up.

The joint torque calculator and sizing robot arm joints cover gravity torque for other poses and arms.

Motor side or output side

A brake on the motor side of the reducer works through the ratio. The output torque it holds is roughly its own holding torque times the ratio N. With a 100:1 reducer in the example, a motor-side brake needs about 50 ÷ 100 = 0.5 N·m. Ignoring back-driving efficiency, as here, is conservative, because friction in the reducer helps the brake hold. That is why most joint modules put a small brake on the motor shaft.

The motor side has two costs. First, the brake holds the output only while the gear train is intact; a broken tooth or a ratcheting harmonic flexspline lets the load fall. Second, when the brake stops a turning rotor, the reducer carries the brake torque times N. A motor-side brake with 2 N·m of dynamic torque on a 100:1 reducer can put up to 2 × 100 = 200 N·m into the gears, above many reducers’ momentary peak rating. Check brake torque times ratio against that rating. An output-side brake avoids both problems but must be about N times stronger, so it is used mainly where the load must stay held even if the reducer fails.

Release voltage, overexcitation and timing

Brake coils are commonly rated for 24 V DC. Each brake has a minimum release voltage: below it the armature does not pull clear, and the brake drags, heats and wears. A 24 V battery of six lithium-ion cells in series runs from about 6 × 4.2 = 25.2 V full to about 6 × 3.0 = 18 V empty, which is 75% of the 24 V coil rating. Feed the coil from a regulated supply if the bus can sag below the release minimum; see power supplies and batteries.

Once released, the armature needs less force to stay clear than to pull in, so many brake drivers lower the coil voltage after release. Coil power goes as V² ÷ R, so a 10 W coil held at half voltage draws a quarter of the power, 2.5 W. Overexcitation is the reverse: a short pulse above rated voltage for a faster release, then a drop to the holding level, within the maker’s limits.

Release and engage times are typically tens of milliseconds. Engage time depends on how fast the coil current dies away. A plain flyback diode keeps current circulating and slows engagement; a Zener diode or varistor clamp lets it fall faster, so the brake bites sooner. Sequence the brake with the drive:

• On enable: power the drive and hold position with motor torque, then release the brake, and wait the release time before moving.
• On disable: command the brake to engage, wait the engage time, then remove motor torque.
• On power loss: the brake engages by itself, so size it for the engagement delay above.

Self-locking drives and safety limits

A worm gear or fine-pitch lead screw self-locks when its forward efficiency is below about 50%, so it holds a load with no brake. The price is efficiency, heat and a joint that cannot be back-driven, and vibration can make a marginal one creep. A brake keeps an efficient transmission and adds holding only when needed. For fingers and other small mechanisms, a self-locking screw is often the better choice; see choosing a micro linear servo actuator.

A holding brake on its own does not make a joint safe. Friction faces wear and get contaminated, and a coil driver can fail in the on state. Where a falling load could hurt someone, treat the brake, its control circuit and a periodic brake test as one safety function, and rate the whole chain, not the brake datasheet. The brake also does nothing while the joint is powered and moving; collision handling and speed limits belong to the drive and controller.

Checklist

• Compute gravity torque at each joint in its worst pose, with a safety factor of 1.5 to 2.
• Pick spring-applied for tolerance of supply variation, or permanent-magnet for zero backlash and compact size.
• For a motor-side brake, divide output holding torque by the ratio, then check brake torque times ratio against the reducer’s momentary peak rating.
• Confirm the minimum release voltage at the lowest bus voltage, or feed the coil from a regulated supply.
• Get release and engage times, and compute how far the load moves before the brake grips.
• Sequence brake and drive on enable, disable and power loss.
• Plan periodic brake tests where a falling load is a hazard.

Browse motor brakes and robot arm and cobot joints, and check which joint modules offer a built-in brake.