Gear Trains
A simple gear train is used to transmit rotary motion
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A simple gear can change both the magnitude and the line of action of an effort force.
The effort force is applied to the "driver" and the load is applied to the "follower".
The movement ratio can be expressed as
μM = ND / NF = TF / TD (1)
where
μM = movement ratio
ND = speed of driver
NF = speed of follower
TF = number of teeth on follower
TD = number of teeth on driver
When the same direction of rotation is required for both the driver and the follower, an idler wheel is used.
The movement ratio for a gear with an idler wheel can be expressed as
μM = ND / NF = TI / TD TF/ TI = TF/TD (2)
where
TI = number of teeth on idler
Typical Gear Ratios
Typical gear ratios for different types of gearsets are indicated below.
| Type of Gearset | Typical Gear Ratios | |
| Min | Max | |
| Spur gear, external | 1 : 1 | 5 : 1 |
| Spur gear, internal | 1.5 : 1 | 7 : 1 |
| Helical gear, external | 1 : 1 | 10 : 1 |
| Helical gear, internal | 1.5 : 1 | 10 : 1 |
| Straight bevel gear | 1 : 1 | 8 : 1 |
| Spiral bevel gear | 1 : 1 | 8 : 1 |
| Epicyclic planetary gear | 3 : 1 | 12 : 1 |
| Epicyclic star gear | 2 : 1 | 11 : 1 |
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