A gear train is combination of two or more gears which are mesh together and used to transmit motion/power from one shaft to another. The gear trains are mostly used in high speed reduction where area is constraint like in watches. A gear train is combination of spur gear, bevel gears and helical gears etc as per requirements. We use different types of gear trains according to their requirements. The selection of gear trains is depends upon the various factors like velocity ratio, gear ratio, space available and the relative position of the axes of shafts.
Following are the main types of gear trains which are generally used in our machinery.
- Simple gear train
- Compound gear train
- Reverted gear train
- Planetary or Epicyclic gear train
SIMPLE GEAR TRAIN:
In simple gear train every gear is supported by the one shaft. All gears have their own shaft. The axes of the shafts on which gears are mounted are fixed and rotate relative to each other and transmit power. Number of gears are attached in series for transmitting power from driver gear to driven gear. This transmission is used where the distance between two shafts is small. The gears used in between the driver and driven gear which have no effect on the speed ratio are known as idlers. The two gears having external teeth always move in opposite direction. If the gear train has odd number of gears then the direction of driver and driven gear is same whereas if the number of gears is even then input and output gear has motion in opposite direction.
The speed ratio is negative when the input and the output gears rotate in the opposite direction where as speed ratio is positive when input and output gears rotate in same direction. The speed ratio is defined as the ratio of the speed of the driving shaft to the driven shaft. The main advantage of simple gear train is that gears can be arranged in straight line or may be in zigzag manner according to the space available and requirement. The gears used in simple gear train are spur gear, helical gear and sometimes bevel gears.
COMPOUND GEAR TRAIN:
In compound gear train the gears are connected in series but the two or more gears connected in such a way that both gears rotate about the same axes of rotation with the same angular velocity. The compound gear train almost same as the simple gear train but the only difference is two or more gears compounded on same shaft. The intermediate shafts have more than one gear but the input and output shafts have only single gear for transmit power. It is used in gear boxes of various machinery like lathe machines, automobile gear box etc. The compound gears have much larger speed reduction ratio as compared to simple gear train.
REVERTED GEAR TRAIN:
Reverted gear train is also a type of compound gear train. In this the axis of the first gear i.e. driver and the last gear i.e. driven gear are co-axial. In Reverted gear the motion of the first and the last gear is same. If we take four gears, gear 1 is driving gear which drives gear 2 in opposite direction. Gear 2 is compound with gear 3 on same shaft and gear 3 drives the gear 4. 1 and 4 rotates in same direction and also they are co-axial gears. This type of gear train is used in clocks where minute hand and hour hand shafts are co-axial, lathes where back gear gives the slow speed to the chuck and in some other power transmissions industry.
PLANETARY OR EPICYCLIC GEAR TRAIN:
Planetary gear train is the most important and little bit complex type of gear train. In this two or more gears are rotate about the axes of the other gear by means of arm. The axes of the some gears are not fixed so they rotate around the axes of some other gear. These types of gear trains are used to transmit very high velocity ratios with small size of gear when area is constrained. Here sun and planet gear terminology used for better understanding. The gears rotate about the some other gears are known as planetary gears and the fixed gear is known as sun gear. This gear train is used in transmission system, computing devices etc.
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