Backdrivability in robot joints

By Tolerance Labs engineering team · Updated 13 October 2026

A backdrivable joint lets the load turn the motor. That property shapes how a joint behaves on impact, in contact and next to people, and the reducer sets most of it.

What backdrivability means

A joint is backdrivable when torque applied at the output can turn the motor through the transmission. It is a matter of degree. One measure is the output torque needed to start the joint moving with the motor unpowered. A second is how heavy the joint feels when you move it quickly, which depends on inertia rather than friction.

Low backdrive torque and low reflected inertia let the joint yield to external forces without help from a sensor. High values mean the outside world has to fight the transmission before the controller knows contact has happened.

Four quantities set it

Gear ratio. Drag on the motor side, such as cogging, bearing friction and seal drag, appears at the output multiplied by the ratio N. A motor with 0.05 N·m of drag behind a 6:1 gearbox needs about 0.05 × 6 = 0.3 N·m at the output to turn. Behind a 100:1 gearbox it needs about 0.05 × 100 = 5 N·m, before gearbox losses.

Back-driving efficiency. Datasheets usually give forward efficiency, from motor to output. Efficiency with the output driving the motor is lower. For a lead screw or worm gear, a standard first-order model is η_back ≈ 2 − 1/η_forward. A forward efficiency of 70% gives about 2 − 1/0.7 ≈ 57% backward. At 50% forward the backward efficiency reaches zero, and the drive self-locks. Other reducers follow the same trend: the lower the forward efficiency, the wider the gap.

Friction. Coulomb friction from seals, bearings and gear mesh sets the breakaway torque. Preloaded bearings and contact seals add drag even at low ratio. Grease-lubricated joints are usually harder to backdrive when cold.

Reflected inertia. Rotor inertia appears at the output multiplied by N². A rotor of 1 × 10⁻⁴ kg·m² behind 6:1 reflects 36 × 10⁻⁴ = 0.0036 kg·m². Behind 100:1 it reflects 10,000 × 10⁻⁴ = 1 kg·m², about 280 times more. Friction resists slow motion. Reflected inertia resists fast motion, and it dominates during an impact because the controller has no time to react.

Why it matters

• Legged locomotion. At touchdown the foot stops in a few milliseconds. A low-inertia, backdrivable joint absorbs the impact through the motor, which the controller can then run as a virtual spring and damper. A high-ratio joint passes the impulse into the gear teeth.
• Impact and gear life. Tooth force in a collision rises with the reflected inertia that must be stopped. High-ratio joints need compliance, a slip clutch or larger gears to survive falls.
• Force control. In a backdrivable joint, motor current is a usable estimate of output torque, so you can run torque control without a torque sensor. With high friction, the current-to-torque relation has a dead band and hysteresis, and you usually need a joint torque sensor.
• Human interaction. A person can push a backdrivable arm aside, powered or not. In exoskeleton joints, backdrive torque is felt by the wearer as resistance on every step.

How the joint types compare

Compare datasheets for quasi-direct-drive joints, high-ratio and cycloidal joints and harmonic joint modules. QDD vs high-ratio joints covers the trade-off in more depth.

Reading a datasheet for backdrivability

• Ratio. The first filter. Below about 10:1, most joints backdrive easily. At high ratios, expect to need a torque sensor or a series elastic element for compliant behavior.
• Rotor inertia. Multiply by the ratio squared and compare the result with the inertia of the link the joint drives. If the reflected inertia is larger than the link, the joint will feel heavy and impacts will load the gears.
• Efficiency. Check the conditions it was measured at. Efficiency in most reducers drops at light load and low temperature, which is where backdriving usually happens.
• No-load backdrive or starting torque. Some makers list the torque needed at the output to start the joint turning. It is the most direct number. If it is missing, ask for it, or measure it with a spring scale on a known lever arm, warm and cold.

When backdrivability is not needed

Backdrivability costs holding power. A backdrivable joint must draw current to hold a load against gravity, and that current heats the motor even when nothing moves. Joints that hold static loads for long periods, such as a lift axis or an arm held outstretched, often do better with a high ratio, a self-locking screw or a brake.

A spring-applied brake holds the joint when power is off and lets you keep a backdrivable transmission for motion. Many micro linear servos use lead screws that self-lock, which suits fingers, grippers and adjustment axes that must hold position without power.

Checklist

• Compute reflected inertia: rotor plus input-side gear inertia, times ratio squared. Compare it with the link inertia.
• Estimate backdrive torque: motor-side drag times ratio, divided by back-driving efficiency.
• Ask for or measure no-load starting torque at the output, warm and cold.
• Decide if you will estimate torque from current or need a joint torque sensor.
• For joints that hold loads for long periods, compare holding-current heat with a brake or a self-locking drive.
• Favor low ratios for legs and exoskeletons. For static holds, ratio and stiffness usually matter more.