24-09-2013, 04:51 PM
GEAR TRAINS
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INTRODUCTION
In this unit, the analysis of gears was presented to you. Involute profile is universally
used for gears but they have problem of interference which can be avoided by using any
one of the several methods proposed in the last unit. Here, you will be explained gear
trains. A gear train is a combination of gears that is used for transmitting motion from
one shaft to another shaft. It may include various types of gears depending on the
location of input and output shafts. There are various types of gear trains which are used
in practice. A gear train can be designed to provide smaller gear ratio to larger gear ratios
depending on the requirement.
TYPES OF GEAR TRAINS
There are several types of gear trains. In some cases, the axes of rotation of the gears are
fixed in space. In one case, gears revolve about axes which are not fixed in space.
Therefore, the gear trains can be classified into the following three classes :
(a) Simple gear train,
(b) Compond gear train, and
© Epicyclic or Planetory gear train.
SIMPLE GEAR TRAIN
In this gear train, there are series of gears which are capable of receiving and transmitting
motion from one gear to another. They may mesh externally or internally. Each gear
rotates about separate axis fixed to the frame. Figure 10.1 shows two gears in external
meshing and internal meshing.
EPICYCLIC GEAR TRAIN
In an epicyclic gear train or planetory gear train, the axis of at least one of the gears also
moves relative to the frame. Figure 10.6 shows an epicyclic gear train. It consists of two
gears S and P. The axes of these gears are connected with each other by an arm a. If arm
is fixed, these two gears constitute a simple gear train. If gear S is fixed and arm rotates
about axis of gear S, it constitute an epicyclic gear train. It is, however, not necessary that
S is fixed, it may have some known rotational speed. Since gear P moves around gear S
like a planet moving around sun, it is also called planetary gear and gear S is called sun
gear.
As the arm rotates, planetory gear P moves around gear S. Epicyclic gear trains are used
in automotive transmission, computing devices, vehicle differential, etc. An epicyclic
gear train can be designed to provide large gear ratio, also which is not possible in other
gear trains in smaller space.