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Builder 3&2 Volume 01 - Construction manual for building structures | Next
Mechanical Advantage |
line. However, an inch or so either way doesn't
matter too much; for example, a 3-inch line may be
reeved onto an 8-inch block with no ill effects. As a
rule, 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 used.
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 that with wire rope,
it is diameter rather than circumference that is
important. Also, remember that this rule refers to the
diameter of the sheave rather than to the size of the
block.
SNATCH BLOCKS AND FAIRLEADS
A snatch block (figure 4-22) is a single-sheave
block made so that the shell opens on one side at the
base of the hook to permit a rope or line to be slipped
over a sheave without threading the end of it through
the block. Snatch blocks ordinarily are used where it
is necessary to change the direction of the pull on a
line.
Figure 4-23 shows a system of moving a heavy
object horizontally away from the power source using
snatch blocks. This is an ideal way to move objects in
limited spaces. Note that the weight is pulled by a
single luff tackle, which has a mechanical advantage
of 3 (mechanical advantage is discussed below).
Adding snatch blocks to a rigging changes the
direction of pull, but the mechanical advantage is not
affected. It is, therefore, wise to select the proper
rigging system to be used based upon the weight of
the object and the type and capacity of the power that
is available.
The snatch block that is used as the last block in
the direction of pull to the power source is called the
leading block.
This block can be placed in any
convenient location provided it is within 20 drum
widths of the power source. This is required because
the fairlead angle, or fleet angle, cannot exceed 2°
from the center line of the drum; therefore, the
20-drum width distance from the power source to the
leading block will assure the fairlead angle. If the
fairlead angle is not maintained, the line could jump
the sheave of the leading block and cause the line on
the reel to jump a riding turn.
Figure 4-22.—Top dead end snatch blocks.
4-14
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