to raise loads weighing several tons. The slow lifting
travel of a chain hoist permits small movements,
accurate adjustment of height, and gentle handling of
loads. A ratchet handle pull hoist is used for short,
horizontal pulls on heavy objects. Chain hoists differ
widely in their mechanical advantage, depending upon
their rated capacity.
Three general types of chain hoists for vertical
operation are the spur gear hoist, the differential chain
hoist, and the screw gear hoist.
The spur gear hoist (fig. 13-46, view A) is the most
satisfactory for ordinary operations. This type of hoist
is about 85 percent efficient. The differential chain hoist
(fig. 13-46, view B) is only about 35 percent efficient
and is satisfactory for occasional use and light loads. The
screw gear hoist is about 50 percent efficient and is
satisfactory where less frequent use of the hoist is
Chain hoists arc usually stamped with their load
capacities on the shell of the upper block. Chain hoists
Figure 13-46.-A. Spur gear chain hoist; B. Differential chain
are constructed with their lower hook as the weakest part
of the assembly. This is done as a precaution, so the
lower hook will be overloaded before the chain hoist is
overloaded. The lower hook will start to spread under
load, indicating the approaching overload limit. Under
ordinary circumstances the pull, exerted on a chain hoist
by one or two people, will not overload the hoist.
Chain hoists should be inspected before each use.
Any evidence of spreading of the hook or excessive
wear is sufficient cause to require replacement of the
hook. If the links of the chain are distorted, it indicates
that the chain hoist has been heavily overloaded and
probably unsafe for further use. Under such
circumstances the chain hoist should be condemned.
Before using any permanently mounted chain hoists,
you should ensure that the annual certification is current.
Vehicular-mounted winches and engine-driven
winches are sometimes used in conjunction with tackles
for hoisting. When placing a power winch to operate
hoisting equipment, you must consider two points. First,
you must consider the angle with the ground that the
hoisting line makes at the drum of the hoist. This angle
is sometimes referred to as ground angle, as shown in
figure 13-47. The second point to consider is the fleet
angle of the hoisting line winding on the drum, as shown
Figure 13-47.Vehicle winch used for hoisting.