01-06-2012, 04:17 PM
GEARS
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HISTORY OF GEARS
Indian history as per our mythological stories is more than 12,000 years old. Since then
people living here have been striving to improve the living conditions. We also know that
earlier people were living in the caves and the doors of the caves were made of granite.
How were these heavy doors opened and closed? They were opened and closed by
none other than a system with gear mechanism, wheel, lever and rope drives. However,
the documented evidence has been lost due to destruction by the invaders and
improper storing of palm leaf literature. The guru Kula method of teaching and passing
of the information from mouth to ear procedure and keeping some of the advances as
closely guarded secret have resulted in poor dissemination of the knowledge and
documentation. But, the knowledge of gears has gone from India to east through some
of the globe trotters from China as back as 2600 years BC.
DEFINITION OF GEARS
Gears are toothed members which transmit power / motion between two shafts by
meshing without any slip. Hence, gear drives are also called positive drives. In any pair
of gears, the smaller one is called pinion and the larger one is called gear immaterial of
which is driving the other.
When pinion is the driver, it results in step down drive in which the output speed
decreases and the torque increases. On the other hand, when the gear is the driver, it
results in step up drive in which the output speed increases and the torque decreases.
CLASSIFICATION OF GEARS
Gears are classified according to the shape of the tooth pair and disposition into spur,
helical, double helical, straight bevel, spiral bevel and hypoid bevel, worm and spiral
gears
DOUBLE HELICAL GEAR OR HERRINGBONE GEAR
Double helical or Herringbone gears used for transmitting power between two parallel
shafts. They have opposing helical teeth with or without a gap depending on the
manufacturing method adopted, Fig. 1.11. Two axial thrusts oppose each other and
nullify. Hence the shaft is free from any axial force. Though their load capacity is very
high, manufacturing difficulty makes them costlier than single helical gear.
INTERNAL GEAR
Internal gears are used for transmitting power between two parallel shafts. In these
gears, annular wheels are having teeth on the inner periphery. This makes the drive
very compact