Speed, torque, and gearing

Understand the idea

Torque is turning effort; angular speed is how fast the shaft rotates. A gearbox trades output speed for output torque. Ideally, a reduction ratio multiplies torque by the ratio and divides speed by the same ratio; real gears lose energy to friction.

A motor's speed falls as load torque rises. At stall, speed is zero and current can be very high, so stall is a limit condition rather than a normal operating point. A design must consider startup and jam conditions.

The load is part of the motor system. Wheel radius, robot mass, incline, friction, and acceleration all affect the torque needed. Change one variable at a time and measure current, speed, and temperature.

Worked example

A 20:1 reduction ideally turns 3000 rpm at the motor into 150 rpm at the output and multiplies shaft torque by about 20 before losses. The output cannot deliver that torque without the motor drawing the corresponding current.

Try it at the bench

1. Mark a point on a motor shaft or wheel and count rotations over ten seconds with no load.
2. Repeat with a small, safe load and note speed and supply current.
3. Compare a geared output if available. Explain the speed/torque tradeoff using your observations.

Check your understanding

Does a gearbox create energy?