Physics, visually · Work, Energy & Power · Concept 07
Gears: Speed vs Torque
Why does a bicycle have gears, and why is a lift motor always connected through a big gearbox?
Gears let a motor swap speed for turning force (torque). Change the teeth and watch both numbers move.
- Make the driven gear bigger than the driver (for example 20 and 40 teeth). It turns slower but pushes harder.
- Make it smaller than the driver. Now it spins fast but is weak. This is what a bicycle does in a high gear.
- Give both gears the same teeth. Same speed, same torque: only the direction flips.
Big gear is slow and strong, small gear is fast and weak.
Meshed gears share the same tooth speed at the contact point. So a gear with more teeth turns more slowly but with more torque. The ratio of teeth is the ratio of speed (inverted) and the ratio of torque.
Gearbox on a conveyor motor
A 1400 rpm motor must drive a belt at 60 rpm. A 23:1 gearbox makes the speed right and raises torque 23 times.
Servo and robot joints
Harmonic and planetary gearboxes give ratios of 50 to 160 so a small motor can hold a heavy robot arm.
Lifts and winches
A winch drum turns slowly with huge torque: motor speed is traded down through the gears.