Torque is turning force — the twist a motor applies to move a joint. It's the number that decides whether a robot can lift the load, hold a pose, or push off the ground, and it's why gearboxes exist.
Torque is rotational force — how hard a motor twists. A long wrench needs less push to loosen a bolt than a short one because it makes more torque. Robots need enough torque at each joint to move and hold their loads.
Ask why one robot can lift a 5 kg payload at arm's length and another sags under 1 kg, and the answer is almost always torque — the turning force at its joints.
What it is
Torque is the rotational cousin of force. Push on a door far from the hinge and it swings easily; push near the hinge and it barely moves. Same force, different torque, because torque is force times the distance from the axis:
τ = r × F (newton-metres)
For a robot, the load's weight acting at the end of a link creates a torque the joint motor must fight just to hold still — and beat to move.
Why reach multiplies the torque a joint must supply
The farther out the load sits, the more torque the joint must produce to hold or lift it — which is why arms are strongest close to the body.
Speed vs torque: the motor's dilemma
Electric motors naturally make low torque at high speed — the opposite of what a joint needs (high torque, low speed). A gearbox resolves this, trading spin for twist: a 100:1 reduction turns a fast, weak motor into a slow, strong joint. Sizing that chain — motor torque × gear ratio × efficiency ≥ the worst-case joint torque, with margin — is one of the first calculations in any robot design.
Holding torque and thermal limits
Two subtleties bite in practice. First, holding a heavy arm still can demand as much torque as moving it, and a motor stalled at high torque dumps heat — so continuous torque is limited by cooling, not just peak numbers. Second, collaborative robots deliberately limit torque so a collision with a person stays gentle; torque sensing is how they feel contact.
Why it matters
Torque budgets decide what a robot can physically do. Under-spec it and joints sag, overheat, or stall; understand it and you can size actuators, pick gear ratios, and predict a robot's real-world strength.
