Gear Ratio
The input speed of a gearbox divided by its output speed, usually written as 1:20 or 1:100. Output speed is the motor speed divided by the ratio, and output torque is the motor torque multiplied by the ratio. Because of losses the real torque gain stays below this theoretical value.
Example: if a motor running at 3000 rpm is connected to a 1:50 gearbox, the output is 60 rpm. If the motor shaft produces 0.2 Nm the theoretical output is 10 Nm, and at 70 percent gear efficiency it is about 7 Nm in reality.
When choosing a ratio, weigh three things together:
- Travel time: The higher the ratio, the slower the work. If a gate must cover 4 metres in 20 seconds, the required output speed follows from that.
- Current: Doing the same work with a higher ratio lowers the motor current; the cheapest way to relieve a driver at its 15 A limit is usually to increase the ratio.
- Back-drivability: Worm gearboxes above roughly 1:50 largely prevent the load from turning the motor backwards.
Increasing the ratio also increases the stall torque; make sure the mechanical parts can withstand that higher torque, otherwise they can be damaged before the protection ever trips.
Where this term is used: DC Motor Driver Selection Guide