Gear Train – Definition & its Types

By BYJU'S Exam Prep

Updated on: September 25th, 2023

Let us first grasp knowledge about gear before going into detail about the gear train. A gear is a spinning circular machine part with cut teeth (called cogs) that mesh with another (suitable) toothed part to transmit (convert) torque and speed. Geared devices can adjust a power source’s speed, torque, and direction. Stirring, measuring, cutting, and differential mechanisms all require gears.

A gear train can contain any or all gear, such as spur, bevel, spiral, etc. The axes of none of the gears move relative to the frame in an ordinary gear train. Almost every equipment that works with mechanical power has a gear train. Engines, lathes, clocks, gearboxes, and differentials of automobiles are all particular areas where gear trains are used.

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What is a Gear Train?

A gear train is a collection of gear wheels that transmit motion from one shaft to another. Ordinary gear trains consist of simple and compound gear trains. The epicyclic gear trains, which allow relative motion between gear axes, are the other types of gear trains. The gear trains are necessary when-

  • A significant reduction in velocity or mechanical advantage is desired.
  • The distance between two shafts is not excessively long but not short enough to allow for the use of a single big gear.
  • When a particular velocity ratio is desired

Types of Gear Trains

Depending on how the wheels are arranged, there are several different types of gear trains. Below we have mentioned four types of gear trains where the axes of the shafts on which the gears are mounted are fixed relative to each other in the first three types of gear trains. Knowing the types of gear trains is helpful in designing gears as well. It is essential to know gear terminologies as well before going further. The axes of the shafts on which the gears are mounted may, however, move relative to a fixed axis in epicyclic gear trains. The types of gear trains are:

  • Simple gear train
  • Compound gear train
  • Reverted gear train
  • Epicyclic gear train

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Simple Gear Trains

This is the simplest type of gear train for conveying motion from one shaft to the other, as the name implies. All of the gear axes stay locked in position with respect to the frame, and each gear is mounted on its own shaft, which is a distinct feature of this type of train. A simple gear train is one that has only one gear on each shaft, as shown in Figure. Pitch circles are used to depict the gears.


The two gears 1 and 2 mesh with each other to transmit motion from one shaft to the other when the distance between the two shafts is minimal, as shown in Figure. Because gear 1 drives gear 2, it is referred to as the driver, while gear 2 is referred to as the driven or follower. The motion of the driving gear is the opposite of the motion of the driving gear.
Because the gear train’s speed ratio (or velocity ratio) is the driver’s speed to the driver’s or follower’s speed. The train value of a gear train is defined as the ratio of the driver’s or follower’s speed to the driver’s speed.

Compound Gear Train

A compound train of gears is when there are multiple gears on a shaft, as seen in Figure. In a simple train of gears, we know that idle gears have no effect on the system’s speed ratio. On the other hand, these gears are useful for bridging the gap between the driver and the driven. When the distance between the driver and the driven or follower must be bridged by intermediate gears while still requiring a large (or much smaller) speed ratio, the advantage of intermediate gears is amplified by using compound gears on intermediate shafts. Each intermediate shaft has two gears rigidly attached to it in this scenario, allowing them to rotate at the same speed. As seen in Figure, one of these two gears meshes with the driver, while the other meshes with the driven or follower linked to the next shaft.

The driving gear is positioned on shaft A in a compound train of gears, as shown in Figure, while the compound gears 2 and 3 are mounted on shaft B. Gears 4 and 5 are compound gears that are mounted on shaft C, whereas gear 6 is the driven gear that is located on shaft D.


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Reverted Gear Train

A reversed gear train is one in which the first and last gear axes are co-axial, as seen in Figure.


Gear 1 appears to be driving gear 2 in the other direction. Because gears 2 and 3 are attached to the same shaft, they create a compound gear, which means that gear 3 rotates in the same direction as gear 2. The third gear pushes the fourth gear in the same direction as the first. As a result, we can see that the motion of the first and last gears are similar in a reversed gear train.

Epicyclic Gear Train

The axes of the shafts over which the gears are mounted can move relative to a fixed axis in an epicyclic gear train. The figure shows a simple epicyclic gear train with a common axis at O1 around which the gear A and the arm C can rotate. The gear B meshes with the gear A and rotates about its axis on the arm at O2. The gear train is simple if the arm is stationary, and gear A can drive gear B or vice versa; however, if the arm is fixed and rotated around the axis of gear A, the gear B is forced to spin on and around gear A. Epicyclic motion is defined as gear trains constructed in such a way that one or more of their members travel over and around another member. The epicyclic gear trains may be simple or compound.


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