Knob

Gear ratio

You might say: reduction

Gear ratio is motor turns per output turn. Increasing it raises ideal output torque and lowers ideal output speed, while increasing reflected inertia and usually reducing backdrivability.

What it does

It places the task operating point on the motor's torque-speed curve.

Ask

Which ratio keeps output torque, speed, heat and backdrivability inside their limits?

Typical

Quasi-direct drives use ratios below about 10:1; Harmonic Drive CSG component sets offer reduction ratios from 50:1 to 160:1.

Catalogue: Ratio

• 1:1 (direct drive): Direct drive is the most backdrivable option in the QDD comparison.
• 10:1: A 10:1 ratio is at the boundary of the QDD example range; it trades some backdrivability for torque multiplication.
• 50:1: A 50:1 reducer is a high-reduction joint choice; Harmonic Drive lists 50:1 CSG options. A reducer where the motor makes fifty turns per output turn defines it; Harmonic Drive CSG is one example.
• 100:1: A 100:1 reducer is a high-ratio arm-joint example; Modern Robotics describes the UR5 with 100:1 harmonic gearing. A reducer where the motor makes one hundred turns per output turn defines it; UR5 is one example.

Connections

• See also: Electric motor

Sources

1. Kevin M. Lynch and Frank C. Park, Modern Robotics: Mechanics, Planning, and Control, Ch. 8 §§8.9.1–8.9.2For a gear ratio G, the output speed of the gearhead is w gear = w motor G.
2. Harmonic Drive LLC, CSG-2A Component Set, Product overviewIdeal for high torque applications, CSG 2A is available in 10 sizes with gear reduction ratios from 50:1 to 160:1.

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