Gear trains are widely used in all kinds of mechanisms and machines, from can openers to aircraft carriers. Whenever a change in the speed or torque of a rotating device is needed, a gear train or one of its cousins, the belt or chain drive mechanism, will usually be used. This chapter will explore the theory of gear tooth action and the design
of these ubiquitous devices for motion control. The calculations involved are trivial compared to those for cams or linkages. The shape of gear teeth has become quite standardized for good kinematic reasons which we will explore.
Gears of various sizes and styles are readily available from many manufacturers. Assembled gearboxes for particular ratios are also stock items. The kinematic design of gear trains is principally involved with the selection of appropriate ratios and gear diameters. A complete gear train design will necessarily involve considerations of strength of materials and the complicated stress states to which gear teeth are subjected. This text will not deal with the stress analysis aspects of gear design. There are many texts which do. This chapter will discuss the kinematics of gear tooth theory, gear types, and the kinematic design of gear sets and gear trains of simple, compound, reverted, and epicyclic types. Chain and belt drives will also be discussed. Examples of the use of these devices will be presented as well.
following are the different types of gear trains,depending upon the arrangement of wheels:-
- simple gear train
- compound gear train
- reverted gear train
- epicyclic gear train
Simple Gear Train
The simple gear train is used where there is a large distance to be covered between the input shaft and the output shaft. Each gear in a simple gear train is mounted on its own shaft.
When examining simple gear trains, it is necessaryto decide whether the output gear will turn faster, slower, or the same speed as the input gear. The circumference (distance around the outside edge) of these two gears will determine their relative speeds.
Suppose the input gear’s circumference is larger than the output gear’s circumference. The output gear will turn faster than the input gear. On the other hand, the input gear’s circumference could be smaller than the output gear’s circumference. In this case the output gear would turn more slowly than the input gear. If the input and output gears are exactly the same size, they will turn at the same speed.
In many simple gear trains there are several gears between the input gear and the output gear.
These middle gears are called idler gears. Idler gears do not affect the speed of the output gear.
Compound gear train
In a compound gear train at least one of the shafts in the train must hold two gears.
Compound gear trains are used when large changes in speed or power output are needed and there is only a small space between the input and output shafts.
The number of shafts and direction of rotation of the input gear determine the direction of rotation of the output gear in a compound gear train. The train in Figure has two gears in between the input and output gears. These two gears are on one shaft. They rotate in the same direction and act like one gear. There are an odd number of gear shafts in this example. As a result, the input gear and output gear rotate in the same direction.
Since two pairs of gears are involved, their ratios are “compounded”, or multiplied together.
Reverted gear train
A reverted gear train is very similar to a compound gear train. They are both used when there is only a small space between the input and output shafts and large changes in speed or power are needed.
There are two major differences between compound and reverted gear trains. First, the input and output shafts of a reverted train must be on the same axis (in a straight line with one another). Second, the distance between the centers of the two gears in each pair must be the same.
Epicyclic gear train
Epicyclic gear train is the one in which the axes of some of the gears have motion. The said gear(s) would be revolving about external axis or axes. Whereas in other gear trains, the axes of all the gears do not have motion, only the gears rotate on their axes. Planetary gear trains are often employed to make more compact gear reducer (large speed reduction in a small volume) compared to other gear trains. Multiple kinematic combinations (multiple inputs) are possible with planetary gear trains. Since few gears are revolving around, the bearings are subjected to high loads; requiring constant lubrication. Hence, planetary gears are placed in box with lubricants, sometimes in a sealed box inaccessible to maintenance crew. Their design and manufacturing is complex and require a very high degree of balance.
An epicyclic gear train with one degree of freedom is shown in Fig The sun gear A is grounded. In other words, it is held stationary. The arm/lever is pivoted on the axis of gear A and on its other end it carries a planetary gear B. The gear B is meshing with the sun gear A. As the arm rotates, the planetary gear B revolves around the periphery of the gear A and also rotates on its axis since it is meshing with the sun gear A. The gear B is the output gear. Since the sun gear is grounded, the gear B gets its input only from the rotation of arm. This is called ‘one degree of freedom’.