Statics, Materials and Motion
Gears, Torque & Advantage
13 min · 2 worked examplesFree
Trading speed for force, why a motor alone rarely does the job, and how to pick a ratio.
After this lesson you can
- Work out output torque and speed from a gear ratio
- Pick a ratio from what the output actually needs
- Say what backlash costs you in a positioning system
Torque and gear ratio
Torque is a turning force: force times radius, in newton-metres. A motor produces torque, and how much it produces depends on the current through it.
A gear train trades speed for torque. A 10:1 reduction turns the output ten times slower and, ignoring losses, with ten times the torque.
Power is conserved, because power is torque times angular speed. Gears cannot create it. What they do is convert a motor's natural output, which is usually fast and weak, into what a machine actually needs, which is usually slow and strong.
That is why almost every robot drive has a gearbox: a bare DC motor spinning at 10,000 rpm with a few millinewton-metres of torque is useless for driving a wheel directly.
In the full lesson
- Torque and gear ratio
- Picking a ratio
- Backlash and efficiency
Free with an account. Read it, work the examples, answer the questions, then build it in the simulator.