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Gear Train.Combination of two or more gears when they made to mesh each other to transmit power from one shaft to another.

Types of Gear Train.Simple Gear TrainCompound Gear TrainRiveted Gear trainEpicycle Gear Train

Simple Gear Train

When there is only one gear on each shaft.

Why we use idler gear ? The only function of the idler gear is to change the direction of rotation.

It has no affect on the gear ratio.

Modulet=Number of teeth on the gearD=Pitch circle diameter,N=speed in rpmm=module=D/t Module must be same for all gears otherwise they would not mesh.

The velocity v of any point on the circle must be the same for all the gears, otherwise they would be slipping.

Power TransmissionThe power transmitted by a torque is given byIn an ideal gear box, the input and output powers are the same so;

It follows that if the speed is reduced, the torque is increased and vice versa. In a real gear box, power is lost through friction and the power output is smaller than the power input. The efficiency is defined as:

Compound Gear TrainWhen there are more than one gear on a shaft,it is called a compound gear train

Compound Gear trainFor large velocities ratios, compound gear train arrangement is preferred.The velocity of each tooth on A and B are the same so:

A tA = B tB -as they are simple gears.

Likewise for C and D, C tC = D tD.

Reverted Gear TrainWhen the axes of the first driver and the last driven are co-axial, then the gear train is known as reverted gear train.

In a reverted gear train, the motion of the first gear and the last gear is same.

Epicyclic Gear Train

Epicyclic means one gear revolving upon and around another. The design involves planet and sun gears as one orbits the other like a planet around the sun.

Epicyclic Gear TrainA small gear at the center called the sun, several medium sized gears called the planets and a large external gear called the ring gear.

Velocity ratio of epicyclic gear train velocity ratio; velocity ratio of epicyclic gear train is the ratio of the speed of the driver to the speed of the driven or follower.

The following two methods may be used for finding out the velocity ratio of an epicyclic gear train.

Tabular method2. Algebraic method

1. Tabular methodTA = Number of teeth on gear A TB = Number of teeth on gear B.Suppose that the arm is fixed. Therefore, the axes of both the gears are also fixed relative to each other. When the gear A makes one revolution anticlockwise, the gear B will make TA / TB NB / NA = TA / TB Since NA = 1 revolution, therefore NB = TA / TB Assuming the anticlockwise rotation as positive and clockwise as negative.

2. Algebraic methodIn this method, the motion of each element of the epicyclic train relative to the arm is set down in the form of equations. The number of equations depends upon the number of elements in the gear train. Let the arm C be fixed in an epicyclic gear train as shown in Fig Therefore, Seed of the gear A relative to the arm C= N(A)-N(C) Speed of the gear B relative to the arm C= N(B) N(C)

Compound Epicyclic Gear Train(Sun and Planet Gear)

ADVANTAGES of Simple Gear Train to connect gears where a large center distance is required

to obtain desired direction of motion of the driven gear ( CW or CCW)

to obtain high speed ratio

ADVANTAGES of Compound Gear Train A much larger speed reduction from the first shaft to the last shaft can be obtained with small gear. If a simple gear trains used to give a large speed reduction, the last gear has to be very large.

Advantages of Riverted Gear Train The reverted gear trains are used in automotive transmissions, lathe back gears, industrial speed reducers, and in clocks (where the minute and hour hand shafts are co-axial).

ADVANTAGES of Epicyclic Gear TrainThey have higher gear ratios. They are popular for automatic transmissions in automobiles. They are also used in bicycles for controlling power of pedaling automatically or manually. They are also used for power transmission between internal combustion engine and an electric motor.

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