two metal framed, heavy-duty blocks. Block A is
designed for manila line, and block B is for wire rope.
BLOCK TO LINE RATIO
The size of a fiber line block is designated by the
length in inches of the shell or cheek. The size of
standard wire rope block is controlled by the diameter
of the rope. With nonstandard and special-purpose
wire rope blocks, the size is found by measuring the
diameter of one of its sheaves in inches.
Use care in selecting the proper size line or wire
for the block to be used. If a fiber line is reeved onto
a tackle whose sheaves are below a certain minimum
diameter, the line becomes distorted which causes
unnecessary wear. A wire rope too large for a sheave
tends to be pinched which damages the sheave. Also,
the wire will be damaged because the radius of bend
is too short. A wire rope too small for a sheave lacks
the necessary bearing surface, puts the strain on only
a few strands, and shortens the life of the wire.
With fiber line, the length of the block used should
be about three times the circumference of the line.
However, an inch or so either way does not matter too
much; for example, a 3-inch line may be reeved onto
an 8-inch block with no ill effects. Normally, you are
more likely to know the block size than the sheave
diameter; however, the sheave diameter should be
about twice the size of the circumference of the line
Wire rope manufacturers issue tables that give the
proper sheave diameters used with the various types
and sizes of wire rope they manufacture. In the
absence of these, a rough rule of thumb is that the
sheave diameter should be about 20 times the diameter
of the wire. Remember, with wire rope, it is the
diameter, rather than circumference, and this rule
refers to the diameter of the sheave, rather than to the
size of the block, as with line.
TYPES OF BLOCKS
A STANDING BLOCK is a block that is
connected to a fixed object.
A TRAVELING BLOCK is a block that is
connected to the load that is being lifted. It also moves
with the load as the load is moved.
A SNATCH BLOCK (fig. 6-5) is a single sheave
block fabricated so the shell opens on one side at the
base of the hook to allow a rope to slip over the sheave
without threading the end through the block. Snatch
Figure 6-5.Snatch blocks.
blocks are used when it is necessary to change the
direction of pull on the line.
To reeve blocks in simple tackle, you must first
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 bends pointing toward each other.
To start reeving, lead the standing part of the falls
through one sheave of the block that has the greatest
number of sheaves. Begin at the block fitted with the
becket. Next 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 having the fewest number of sheaves,
using a becket hitch for temporary securing or an eye
splice for permanent securing.
When blocks have two or more sheaves, the
standing part of the fall should be led through the
sheave closest to the center of the block. This places
the strain on the center of the block and prevents the
block from toppling and the lines from being chafed
and cut through by rubbing against the edges of the
Falls are normally reeved through 8-inch or
10-inch wood or metal blocks, in such away as to have
the lower block at right angles to the upper. Two
3-sheave blocks are the traditional arrangement, and
the method of reeving is shown in figure 6-6. The
hauling part has to go through the middle sheave of
the upper block or the block will tilt to the side and the
falls will jam under load.