Hand Chain Hoist
A differential pulley, sometimes called a "hand chain hoist,"
or sometimes colloquially called a "chain fall," is used to manually lift very
heavy objects like car engines. It is operated by pulling upon the slack section
of a continuous chain that wraps around pulleys. The relative sizes of two top
pulleys determine the maximum weight that can be lifted by hand.
A differential pulley can lift very large masses a short distance. The two fixed
pulleys at the top (which are attached to each other and rotate together) are of
unequal radii, R and r and an endless chain passes around the pulleys and hangs
in a loop. The fixed pulleys are toothed to avoid slippage of the chain.
To illustrate the way it works: if both pulleys at the top are of the same size,
a long pull on the chain will raise the section of chain extending down to the
lowermost pulley by a similar length to the amount of chain lowered by the other
top pulley. The weight will therefore go nowhere.
If the top pulleys are of different sizes, however, the
ascending section of chain and the descending section will travel by different
amounts and the weight will travel also. When the top pulley playing out the
descending section of chain is the smaller of the two, more chain rises than
falls and the object is lifted.
When the sizes of the top pulleys are such that a long pull on
the slack portion of the chain raises the weight only by a very small amount,
the weight can be considerably larger than a man can lift by himself; an effect
that utilizes an engineering phenomenon known as "mechanical advantage".
The greater the mechanical advantage, the easier it is to lift
an object and the heavier the object that can be lifted.
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