GEAR TRAINS

    A gear train is a combination of gears used to transmit motion from one shaft to another. This is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage. It becomes necessary when when it is required to obtain large speed reduction within a small space. The main purpose of a gear train is to increase torque or speed.

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The following are the main types of gear trains-

• Simple gear train
• Compound gear train
• Reverted gear train
• Planetary or epicyclic gear train

a] Simple gear train- A series of gears, capable of receiving and transmitting motion from one gear to another is called a simple gear train. In it, all the gear axes remain fixed relative to the frame and each gear is on a separate shaft. In this gear train each shaft supports one gear.

FIG. Simple gear train

In a simple gear train we can observe the following:

• Two external gears of a pair always move in opposite directions.
• All odd numbered gears move in one direction and all even numbered gears in the opposite direction.
• Speed ratio, the ratio of the speed of the driving to that of the driven shaft, is negative when the input and the output gears rotate in the opposite direction and it is positive when the two rotate in the same direction. 
• The reverse of the speed ratio is known as the train value of the gear train.
• All the gears can be in a straight line or arranged in a zig-zag manner. A simple gear train can also have bevel gears.

b] Compound gear train- When a series of gears are connected in such a way that two or more gears rotate about an axis with the same angular velocity, it is known as compound gear train. In this type, some of the intermediate shafts, i.e., other than the input and the output shafts, carry more than one gear as shown in fig. In a compound gear train , each shaft supports two gear wheel except the first and the last.

FIG. Compound gear train

c] Reverted gear train- If the axes of the first and the last wheels of a compound gear coincide, it is called a reverted gear train. Such an arrangement is used in clocks and in simple lathes where back gear is used to give a slow speed to the chuck. In a reverted gear train the driving and the driven gears are coaxial or coincident.

FIG. Reverted gear train

In all these three types, the action of rotation of the wheels are fixed in position and gears rotate about their respective axes.

d] Planetary or epicyclic gear train- It is also possible that, in a gear train, the axes of some of the wheels are not fixed but rotate around the axes of the other wheels with which they mesh. Such trains are known as planetary or epicyclic gear trains. Epicyclic gear trains are useful to transmit very high velocity ratios with gears of smaller size in a lesser space. In this gear train, the axis of at least one of the gears also moves relative to the frame.

FIG. Epicyclic gear train

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