# gear trains

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Theory of Machinery

GEAR TRAINS

INTRODUCTIONA gear train is a combination of gear Used to transmit motion from one shaft to another. It becomes necessary when it is reqired to obtain large speed reduction within a small space. The following are the main types of gear trains. a)Simple gear train b)Compound gear train c)Reverted gear train d)Epicyclic gear train

INTRODUCTION (Contd.)In simple gear trains, each shaft supports one gear. In a compound gear train, each shaft supports two gear wheels except the last. In a reverted gear train, the driving and the driven gear are coaxial or coincident. In all these three types, the axes of rotation of the wheels are fixed in position and the gear rotate about their respective axes. However, 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 other wheels with which they mesh. Such trains are known as planetary or Epicyclic gear trains. Epicycle gear trains are useful to transmit very high velocity ratios with gear of smaller sizes in a lesser size.

SIMPLE GEAR TRAIN:-

A series of gear, capable of receiving and transmitting motion from one gear to another is called a simple gear train. In it, all the axes remain fixed relative to the frame and each gear is on a separate shaft (fig. 11.1).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. For example, gears 1,3,5, etc, move in the counter-clockwise direction.Speed ratio, the ratio of the of 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.Let T= number of teeth on a gearN= speed of a gear in rpm.

SIMPLE GEAR TRAIN (Contd.)

2 3 32.T T NNratioSpeed==

Simple gear train in which each shaft carries only one gear

2 4 423 4 234332..T TNNratiospeedT TTTNNNNratioSpeed====

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 input and the output shafts, carry more than one gear shown in figure.

45 342352443345 3423544332 =======T T TTTTNNNNiNNNNTT TTTTNNNNNNioutputinput

34 23423334 234332 ======T T TTNNNNiNNT T TTNNNNioutputinput

3 4 12413 23 4 124321T T TTNNNNiN NT T TTNNNNioutputinput======

Reverted Gear Train If the axis of the first and the last wheels of the compound gear coincide, it is called a reverted gear train. Such an arrangement is used in clocks and simple lathes where back gear is used to give a slow speed to the chuck.

EPICYCLIC GEAR TRAINA gear train having a relative motion of axes is called a planetary or an epicyclic gear train. In an epicyclic gear train, the axis of atleast one of the gears also moves relative to the frame.Consider two gear wheels S and P, the axes of which are connected by an arm a. if the arm a is fixed, the wheels s and p constitute a simple train. However, if the wheel S fixed so that the arm can roatate about the axis of S, the wheel P would move around S. Therefore, it is an epicyclic train.

The End