block, as shown in figure 4-28, The mechanical
advantage of the single is 3, whereas the mechanical
advantage of the double is 5.
A twofold purchase consists of two double
blocks, as shown in figure 4-29, whereas a threefold
purchase consists of two triple blocks, as shown in
The mechanical advantage of the
twofold purchase is 4; the advantage of the threefold
In reeving a simple tackle, lay the blocks a few
feet apart. The blocks should be placed down with the
sheaves at right angles to each other and the becket
ends pointing toward each other.
To begin reeving, lead the standing part of the
falls through one sheave of the block that has the
greatest number of sheaves. If both blocks have the
same number of sheaves, begin at the block fitted with
the becket. Then, pass the standing part around the
sheaves from one block to the other, making sure no
lines are crossed, until all sheaves have a line passing
over them. Now, secure the standing part of the falls
at the becket of the block containing the least number
of sheaves, using a becket hitch for a temporary
securing or an eye splice for a permanent securing.
With blocks of more than two sheaves, the
standing part of the falls should be led through the
sheave nearest the center of the block. This method
places the strain on the center of the block and
prevents the block from toppling and the lines from
being cut by rubbing against the edges of the block.
Falls are generally reeved through 8- or 10-inch
wood or metal blocks in such a reamer as to have the
lower block at right angles to the upper block. Two,
three-sheave blocks are the usual arrangement, and
the method of reeving these is shown in figure 4-31.
The hauling part must go through the middle sheave
of the upper block, or the block will tilt to the side and
the falls jam when a strain is taken.
If a three- and two-sheave block rig is used, the
method of reeving is about the same (figure 4-32),
but, in this case, the becket for the dead end must be
on the lower, rather than the upper, block.
Naturally, you must reeve the blocks before you
splice in the becket thimble, or you will have to reeve
the entire fall through from the opposite end.
SAFE WORKING LOAD OF A TACKLE
You know that the force applied at the hauling
part of a tackle is multiplied as many times as there
Figure 4-31.Reeving a threefold purchase.
are parts of the fall on the movable block. Also, an
allowance for friction must be made, which adds
roughly 10 percent to the weight to be lifted for every
sheave in the system. For example, if you are lifting a
weight of 100 pounds with a tackle containing five
sheaves, you must add 10 percent times 5, or 50
percent, of 100 pounds to the weight in your
calculations. In other words, you determine that this
tackle is going to lift 150 pounds instead of 100
Disregarding friction, the safe working load of a
tackle should be equal to the safe working load of the
line or wire used, multiplied by the number of parts of
the fall on the movable block. To make the necessary
Figure 4-32.Reeving a double-luff tackle.