Table of Contents
What Is Gear Ratio?
Gear ratio is the ratio of the number of teeth on the driven gear to the driving gear. It determines how speed and torque are transformed between input and output shafts. A gear ratio greater than 1:1 means the output rotates slower but with more torque (speed reduction), while a ratio less than 1:1 provides speed increase at the cost of torque.
Gear ratios are fundamental to mechanical engineering, appearing in everything from watches and bicycles to automobile transmissions and industrial machinery. The total ratio of a gear train is the product of individual stage ratios.
Gear Ratio Formula
Common Gear Ratios
| Application | Typical Ratio | Purpose |
|---|---|---|
| Bicycle low gear | 3:1 | Climbing hills |
| Car 1st gear | 3.5-4.5:1 | Maximum torque |
| Car 5th gear | 0.7-0.9:1 | Highway cruising |
| Clock minute-hour | 12:1 | Time keeping |
Frequently Asked Questions
How do multi-stage gear trains work?
In a multi-stage gear train, the total ratio is the product of each stage's ratio. For example, two stages of 3:1 give an overall 9:1 ratio. This allows very high ratios in compact packages, which is why gearboxes use multiple stages rather than a single pair of very different-sized gears.
What is gear efficiency?
Real gears lose energy to friction, typically 1-3% per mesh for spur gears. Worm gears can lose 20-50%. The efficiency reduces the output torque and power compared to ideal calculations. For multi-stage gearboxes, efficiencies multiply, so a 4-stage gearbox at 97% per stage gives 88.5% overall efficiency.