The gear ratio is the biggest design choice in a joint. It decides how the joint feels to the load, not just how strong it is.
What the ratio does
A reduction multiplies the motor’s torque by the ratio and divides its speed by the same amount. It also multiplies the motor’s inertia, as seen from the output, by the square of the ratio. A rotor behind a 9:1 stage feels 81 times heavier at the output; behind a 64:1 stage it feels about 4,100 times heavier. That is why high-ratio joints resist being pushed and low-ratio joints give way.
Quasi-direct drive (about 6:1 to 12:1)
- Backdrivable. The load can turn the motor, so the joint absorbs impacts instead of passing them into the gears.
- Torque from current. With little friction in one planetary stage, motor current is a good estimate of output torque, so the robot can sense contact without a torque sensor.
- Fast. Output speeds of a few hundred rpm suit swinging legs.
- Trade-off: less torque for a given motor size, so the motor itself must be large (a wide, flat outrunner is typical).
This is the layout used by most quadrupeds and humanoid legs. See quasi-direct-drive joints.
High ratio (above 12:1)
- Multi-stage planetary (about 20:1 to 64:1): compact and efficient, with some backlash.
- Cycloidal: high ratio in one stage, very shock tolerant, low backlash.
- Harmonic (strain wave): near-zero backlash and high precision, but less tolerant of shock loads.
High-ratio joints hold a pose with little current, resist disturbances and position precisely, which suits arms, wrists, pan-tilt heads and slow, strong axes. They are hard or impossible to backdrive, so contact has to be sensed another way. See high-ratio and cycloidal joints.
Rules of thumb
- Legged robots, hips and knees: quasi-direct drive, 6:1 to 10:1.
- Exoskeletons and wearables: quasi-direct drive, so the wearer can move the joint when it is off.
- Arm shoulders and elbows: 30:1 to 100:1, with a brake if the arm must hold a load when powered down.
- Wrists and grippers: high ratio for holding torque in a small package.
Unsure? Size the torque first (see sizing torque for a legged robot), then pick the lowest ratio whose rated torque still covers the continuous load.