Gear Train:
Sometimes, two or more gears are made to mesh with each other to transmit power from one shaft to another. Such a combination is called gear train or train of toothed wheels. The nature of the train used depends upon the velocity ratio required and the relative position of the axes of shafts. A gear train may consist of spur, bevel or spiral gears.
Types of Gear Train:
Following are the different types of gear train, depending upon the arrangement of wheels:
1. Simple Gear Train: When there is only one gear on each shaft, as shown in fig, it is known as simple gear train. The gears are represented by their pitch circles.
When the distance between the two shafts is small, the two gears 1 and 2 are made to mesh with each other to transmit motion from one shaft to the other. Since the gear 1 drives the gear 2, therefore gear 1 is called driver and the gear 2 is called follower. It may be noted that the motion of the driven gear is opposite to the motion of driving gear.
2. Compound Gear Train: When there are more than one gear on a shaft as shown in fig, it is called a compound gear train.
We have seen that the idle gears in a simple train of gears don’t effect the speed ratio of the system. But these gears are useful in bridging over the space between the driver and the driven.
3. Reverted Gear Train: When the axes of the first gear and the last gear are co-axial, then the gear train is known as reverted gear train.
We see that gear 1 drives the gear 2 in the opposite direction. Since the gears 2 and 3 are mounted on the same shaft. Therefore, they form a compound gear and the gear 3 will rotate in the same direction as that of gear 2. The gear 3 drives the gear 4 in the same direction as that of gear 1.
4. Epicycle Gear Train: In an epi-cyclic gear train, the axes of the shafts over which the gears are mounted. It may more relative to a fixed axis. A single epi-cyclic gear train in shown fig, where a gear A and the arm C have a common axis at O1 about which they can rotate. The gear B meshes with gear A and has its axis on the arm at O2, about which the hear B can rotate.
If the arm is fixed, the gear train is simple and gear A can drive gear B of vice-versa. But if gear A is fixed and the arm is rotated about the axis of gear A then the gear B is forced to rotate upon and around gear A. Such a motion is called epicyclic. The gear train arranged in such a manner that one or more of their members move upon and around member are known as epicyclic gear trains. The epicyclic gear trains are useful for transmitting high velocity ratios with gears of moderate size in a comparatively lesser space.