Sunday, 29 March 2015

Polygonal Effect - POLYGONAL EFFECT IN CHAIN DRIVES

The number of teeth on the sprocket affects the
velocity ratio during its rotation through the pitch
angle. The chain passes around the sprocket as
a series of chordal-links. This action is similar to
a non-slipping belt wrapped around a rotating
polygon. For the sack of clarity, a chain drive
with sprocket having four teeth is shown in Fig.



It is evident that linear speed of the chain is not
uniform but varies during every cycle of tooth
engagement. This results in a pulsating and jerky
motion to the chain and the driven sprocket. This
is called “Chordal speed variation”, and is plotted
in Fig.






For smooth operation of chain drives at
moderate to high speeds, it is considered a good
practice to use a driving sprocket with at least
17 teeth. Of course, higher number of teeth 19 or
21 gives better life expectancy to the chain with
less noise during operation. It is preferable to
use an odd number of teeth for the driving
sprocket in combination with an even number of
chain links for uniform wear and tear on the
teeth and rollers. In this case, a particular tooth
of the sprocket wheel does not come in contact
with a particular link of the chain for every
rotation. For space consideration or for very low
speeds, sprocket with less number of teeth may
be used, by sacrificing the life expectancy of the
chain.
The failure of roller chains is mainly due to wear
on the rollers and pins. The chain is elongated
due to wear in the joints and as a result the
rollers shift with respect to the teeth on the
sprocket. When the sprocket is made of large
number of teeth (with shorter pitch), even a
small elongation of the chain displaces the chain
with respect to the teeth on sprocket to a
greater extent, leading to improper meshing. This
limits the maximum number of teeth on the
sprocket. For roller chains, the recommended
maximum number of teeth for the sprocket wheel
is about 100 to 120.

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