The gear train is made up of several gears that are meshing together to attain the required gear ratio. Based on the arrangement it is classified as a simple gear train, compound gear train, epicyclic gear train, etc. In this article, we are discussing the simple gear train in detail.
Contents:
What is a Simple gear train?
Simple gear train is a type of gear train in which a single gear is mounted on each shaft. The speed ratio of a simple gear train depends only on the driven and driven gear.

The simple gear train consists of the following components:-
1] Driver gear:- It is the input gear mounted on the input shaft of a simple gear train to which the power is supplied.
2] Driven gear:- It is the output gear mounted on the output shaft of the simple gear train.
3] Idlers/intermediate gears:- The gears which do not affects the speed ratio of the gear train are known as idlers.
In a simple gear train, the idlers are used for the following purposes:-
1] To connect gears located at a large central distance.
2] To change the direction of rotation at the output shaft.
Simple gear train equation:
Speed ratio/ Gear ratio:-
The formula for the speed ratio (i) of the simple gear train is given by,
i = `\mathbf{\frac{N_{\text{Driver}}}{N_{\text{Driven}}}}` = `\mathbf{\frac{t_{\text{Driven}}}{t_{\text{Driver}}}}` |
Where,
`N_{\text{Driver}}` = Speed of driver gear
`N_{\text{Driven}}` = Speed of driven gear
`t_{\text{Driven}}` = Number of teeths on driven gear
`t_{\text{Driver}}` = Number of teeths on driver gear
The speed ratio of the simple gear train never depends on idler gears.
Direction of motion:-
The relation between the direction of driving and driven gear is given by,
`\mathbf{Direction_{\text{Driven}}}` = `\mathbf{(-1)^{\text{(Number of idler +1)}}. Direction_{\text{Driver}}}` |
Simple gear train advantages and disadvantages:
Advantages:-
- The simple gear train can transfer large amounts of power without causing a slip.
- It can transfer power with less central distance.
- The desired direction of rotation can be achieved with the help of idlers.
Disadvantages:-
- As, like a compound gear train, the simple gear train can’t give high gear ratios within less space.
- It is more expensive than the chain drive or belt drive.
- It requires proper lubrication.
Simple gear train examples:
1] Find the speed ratio and the direction of the driven shaft for the below simple gear train. |

Given:-
`t_{\text{Driven}}` = 20
`t_{\text{Driver}}` = 60
Number of idlers = 2
Speed ratio:-
i = `\frac{t_{\text{Driver}}}{t_{\text{Driven}}}` = `\frac{60}{20}`
i = 3 |
This is the speed ratio for the above gear train.
Direction of motion:-
The relation between the direction of driving and driven gear is given by,
`Direction_{\text{driven}}` = `(-1)^{\text{(Number of idler +1)}}. Direction_{\text{driver}}`
`Direction_{\text{driven}}` = `(-1)^{2+1}. Direction_{\text{driver}}`
`Direction_{\text{driven}}` = `- Direction_{\text{driver}}`
It means that the driven gear rotates in the opposite direction of the driver gear.
As the driver gear rotates in a clockwise manner, thus the driven gear rotates in an anticlockwise direction.
2] A simple gear train consists of a driver gear, driven gear, with two idlers having a gear ratio of 3.5. Find the number of teeths on the driven gear if the driver gear has 30 teeth. |
Given:-
i = 3.5
Number of idlers = 2
`t_{Driver}` = 30
The number of teeths on driven shaft is given by,
i = `\frac{t_{\text{Driven}}}{t_{\text{Driver}}}`
3.5 = `\frac{t_{\text{Driven}}}{30}`
`\mathbf{t_{\text{Driven}}}` = 105 |
Thus the driven gear has 105 teeth.
FAQs:
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Is a simple gear train reversible?
Yes, the simple gear train is reversible. It means that we can rotate the input shaft by supplying power to the output shaft.
-
What is the difference between the simple gear train and the compound gear train?
In a simple gear train, a single gear is mounted on each shaft while in a compound gear train more than one gear is mounted on a single shaft.
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What is the name of the first gear in a simple gear train?
The first gear in a simple gear train is known as driver gear or input gear.
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