and in this case,
(R - r)
If A is a large wheel and B is a little smaller, the
value of 2R becomes large and then (R r) becomes
small. Then you have a large number for
(R - r)
which is the theoretical mechanical advantage.
You can lift heavy loads with chain hoists. To give
you an idea of the mechanical advantage of a chain hoist,
suppose the large wheel has a radius (R) of 6 inches and
the smaller wheel a radius (r) of 5 3/4 inches. What
theoretical mechanical advantage would you get? Use
Then substitute the numbers in their proper places, and
Since the friction in this type of machine is
considerable, the actual mechanical advantage is not as
high as the theoretical mechanical advantage. For
example, that theoretical mechanical advantage of 48
tells you that with a 1-pound pull you can lift a 48-pound
load. However, actually your 1-pound pull might only
lift a 20-pound load. You will use the rest of your effort
in overcoming the friction.
The most important point to remember about block
and tackle is that they are simple machines. And simple
machines multiply effort or change its direction. You
should also remember the following points:
A pulley is a grooved wheel that turns by the action
of a rope in the groove.
There are different types of pulleys. Pulleys are
either fixed or movable.
You attach a fixed pulley to one place. The fixed
pulley helps make work easier by changing the
direction of the effort.
You hook a movable pulley to the object you are
lifting. As you pull, the object and the pulley
move together. This pulley does not change the
direction of the effort, but it does multiply the
You can use fixed and movable pulleys together to
get a large mechanical advantage (M.A.).