Learn about Gear Train and Variable Gear Train

What are Gear Trains?

As the name indicates it is the train of gears. When more than one gear is arranged together for transmitting torque or power from one system to another system then the arrangement is called a gear train. The size of the gear trains can be very small (in a wristwatch) to very large (in an industrial gear box).The arrangement of the gear train depends upon the amount of torque to be transmitted, the orientation of the input and the output shafts, and the size of the gear box.

Types of Gear Trains

○      Simple Gear Trains: In this type of gear train, only a pair of gears is engaged with each other. The input and the output shaft are necessarily being parallel to each other.

As the gear reduction ratio increases, the size of the gear train also increases, and this is one of the limitations of using simple gear trains for large reduction ratio.

○      Compound Gear Train: More than one gear is fixed to one shaft for the compound gear train.

You can see from the above picture that a compound gear train is actually combination of more than one simple gear train. For large reduction ratios, compound gear trains are preferred over simple gear trains.

○      Reverted Gear Train: This is also a kind of compound gear train. Here, the input shaft and output shaft are collinear to each other.

○      Variable Gear Train: Variable gear train is a compound gear train, which has a variable gear reduction ratio. These types of gear trains are used widely in automobile manual transmission systems.

Gear Box Automobile1
Gear Box Automobile2

See the above two pictures to understand how the gear reduction ratio can vary from 1:2 to 1:5. The variable gear train is often coupled with a clutch. The clutch is activated while shifting the gear from one ratio to another one. The reason why most automobiles use this gear train is because of its ability to achieve variable gear ratio without increasing complexity or decreasing efficiency.

○      Sun and Planet Gear Trains: Here, different gears are arranged in a cyclic manner instead of the linear manner. That’s why this arrangement is also known as epicyclic gear train. The sun and planet gear train is constructed using a big gear at center (sun) and few small surrounding gears (planet) typically. However, more complex types of epicyclic gears are also used, where the compound gears and ring gears are used in the gear train.

Planetary Gear : 8 Steps - Instructables

Some of the advantages of the epicyclic gear train are its ability to achieve higher gear reduction ratio without increasing its size and the possibility of rotation direction reversal.


Different gear trains have different applications and the level of complexity. The variable gear train is a gear train in which gear ratio can be varied without replacing the gear train with a new one.

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