What parameter defines how much input speed is transmitted to the output in a gear drive?

• A. Torque
• B. Gear ratio (speed ratio)
• C. Efficiency
• D. Load

Answer: (B)

The speed relationship in a gear train is governed by the gear ratio. This parameter tells you how the input speed is scaled when it appears at the output, and it’s determined by the sizes of the gears (the number of teeth).

In a simple two-gear set, the output speed equals the input speed multiplied by the ratio of the driving gear’s teeth to the driven gear’s teeth. If the driving gear has fewer teeth than the driven gear, the output will turn more slowly; if it has more teeth, the output will turn faster. For example, a driving gear with 20 teeth meshing with a driven gear of 40 teeth produces a gear ratio of 2:1, so the output speed is half the input speed.

Torque, efficiency, and load describe related aspects of performance but do not define how much input speed is transmitted. The gear ratio is the parameter that sets that speed scaling in a gear drive.