A THREEFOLD PURCHASE consists of two
triple blocks and has a mechanical advantage of 6 (fig.
A COMPOUND TACKLE is a rigging system
using more than one line with two or more blocks.
Compound systems are made up of two or more
simple systems. The fall line from one simple
system is secured to the hook on the traveling block
of another simple system, which may have one or
To determine the mechanical advantage of a
compound tackle system, you must determine the
mechanical advantage of each simple system in the
compound system. Next, multiply the individual
advantages to get the overall mechanical advantage. As an
example, two inverted luff tackles, each has a mechanical
advantage of 4. Therefore, the mechanical advantage of
this particular compound system is 4 x 4 = 16.
Figure 6-13.A threefold purchase.
ALLOWANCE FOR FRICTION
Because of friction, some of the force applied to
tackle is lost. Friction develops in tackle by the lines
rubbing against each other or the shell of the block. It
is also caused by the line passing over the sheaves or
by the rubbing of the pin against the sheaves. Each
sheave in the tackle system is expected to create a
resistance equal to 10 percent of the weight of the load.
Because of fiction, a sufficient allowance for loss
must be added to the weight being moved in
determining the power required to move the load.
As an example, you have to lift a 1,000-pound load
with a twofold purchase. To determine the total force
needed to lift the load, you take 10 percent of 1,000
pounds, which is 100 pounds. This figure is multiplied
by 4 (the number of sheaves), which gives you 400
pounds. This value is added to the load; therefore, the
total load is 1,400 pounds. This figure is divided by 4,
the mechanical advantage of a twofold purchase,
which results in 350 pounds being the force required
to move the load.
Safety rules you should follow when using
blocks and tackle are as follows:
Always stress safety when hoisting and moving
heavy objects around personnel with block and tackle.
Always check the condition of blocks and
sheaves before using them on a job to make sure they
are in safe working order. See that the blocks are
properly greased. Also, make sure that the line and
sheave are the right size for the job.
Remember that sheaves or drums which have
become worn, chipped, or corrugated must not be used
because they will injure the line. Always find out
whether you have enough mechanical advantage in the
amount of blocks to make the load as easy to handle as
You must NOT use wire rope in sheaves and
blocks designed for fiber line. They are not strong
enough for that type of service, and the wire rope will
not properly fit the sheaves grooves. Likewise, sheaves
and blocks built for wire rope should NEVER be used
for fiber line.
Slings are widely used for hoisting and moving
heavy loads. Some types of slings come already made.
Slings may be made of wire rope, fiber line, or chain.