Figure 4-23.Moving a heavy object horizontally along a floor with limited access using snatch blocks and fairleads.
The mechanical advantage of a tackle is the term
applied to the relationship between the load being
lifted and the power required to lift it. If the load and
the power required to lift it are the same, the
mechanical advantage is 1. However, if a load of 50
pounds requires only 10 pounds to lift it, then you
have a mechanical advantage of 5 to 1, or 5 units of
weight are lifted for each unit of power applied.
The easiest way to determine the mechanical
advantage of a tackle is by counting the number of
parts of the falls at the running block. If there are two
parts, the mechanical advantage is two times the
power applied (disregarding friction). A gun tackle,
for instance, has a mechanical advantage of 2.
Therefore, lifting a 200-pound load with a gun tackle
requires 100 pounds of power, disregarding friction.
To determine the amount of power required to lift
a given load by means of a tackle, determine the
weight of the load to be lifted and divide that by the
mechanical advantage. For example, if it is necessary
to lift a 600-pound load by means of a single luff
tackle, first determine the mechanical advantage
gained by the tackle. By counting the parts of the falls
at the movable block, you determine a mechanical
advantage of 3. By dividing the weight to be lifted,
600 pounds, by the mechanical advantage in this
tackle, 3, we find that 200 pounds of power is required
to lift a weight of 600 pounds using a single luff
Remember though, a certain amount of the force
applied to a tackle is lost through friction. Friction
develops in a tackle by the lines rubbing against each
other, or against the shell of a block. Therefore, an
adequate allowance for the loss from friction must be
added. Roughly, 10 percent of the load must be
allowed for each sheave in the tackle.