The basic calculation for determining gear ratios is:
# of teeth on driven gear divided by # of teeth on driving gear
The quotient will be expressed as a ratio (indicating the number of times the first value contains or is contained by the second), reduced to its lowest equivalent.
The Basics: What is a Gear Ratio?
The gear ratio is the relationship of the number of teeth between a driving gear and a driven gear. In terms of performance, the gear ratio describes the number of times the driving gear must turn to complete a full rotation of the driven gear. Gear ratios are presented as a quotient (e.g. 4:1) in which the first number is an expression of the number of teeth of the driven gear, and the second represents the number of teeth of the driving gear.
So, if a driven gear has 64 teeth, and the driving gear has 8 teeth, the ratio would be 8:1. This means the driving gear must make 8 complete revolutions in order to make the driven gear rotate once.
Finding the Gear Ratio in Four Steps
Follow these steps to find the gear ratio of a set of mated gears:
- Determine which is the driving gear and the driven gear
- The driving gear will be connected to a power source
- Count the number of teeth on both gears
- Refer to schematics for an accurate count
- Divide the number of teeth on the driven gear by the number of teeth on the driving gear
- If driven gear teeth = 20 and drive gear = 5, the calculation is 20 5 or 20/5
- Simplify the ratio
- In the above example, 20/5 becomes 4:1
Why Gear Ratio Matters
Understanding gear ratios is key for engineers to get the best performance from a gear train. Gear ratio is a determining factor in adjusting torque, changing gear rotation speed, and altering a gear’s direction. By knowing the gear ratio, mechanical engineers can properly adjust gear sizes and teeth amounts to get their desired output. The gear ratio gives crucial information to engineers allowing them to make informed alterations to a gear train, including:
- Finding rotational speed of the drive gear
- Balancing speed & torque
- Increasing gear accuracy
- Maximizing motor efficiency
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