The unit shown in figure 7-23 is an adaptation of the
equipment and its purpose so that as various situations
overhead type of crane applied to outdoor service where
arise, you can select the proper piece of equipment.
no permanent elevated structure exists on which to install
Hoists
a crane. The crane bridge is mounted on trestles having
legs that are generally constructed with wheel trucks for
Chain hoists or chain falls provide a convenient
operating on tracks. Such cranes are referred to as
and efficient method for hoisting loads by hand. The
portable gantry cranes.
advantages of chain hoists are that one person can raise
a load of several tons. Also, without securing the load,
If the trestles rest directly on the ground or footings,
it can remain stationary. One person can carry and
the term "fixed gantry crane" is applied. This type of
operate the manually operated chain hoists (figure
crane is built especially for particular locations. It has
7-24, View A). They are particularly useful in loading
been constructed with a span of 200 or more feet. The
and unloading cargo trucks. They also are convenient
gantry may have a trolley running on the bridge carrying
for working in small storerooms aboard ship when
a hoist. This is the most common form and is what is
other mechanized equipment is not available.
meant by gantry crane. However, the gantry may have a
stiff-legged derrick, a rotating pillar, a job crane, or a
Some larger storerooms have electrically operated
hammerhead crane mounted on its bridge as auxiliary
hoists that move along overhead tracks (figure 7-24,
equipment.
View B). Electric hoists lift their loads by either chain
Because this type of crane spans the area over which
or cable. Other models are pneumatic or air hoists that
it operates, it has been particularly useful in
operate by compressed air. These hoists have the
shipbuilding, in storage yards, and at docks for handling
advantages of speed and ease of operation.
bulk material.
Block and Tackle
A wharf crane is located on and generally is a part of
A block and tackle is the arrangement of one or
the wharf or pier structure. It is particularly adapted to
more pulleys with rope or cable for pulling or hoisting
the transfer of cargo between the wharf or pier and a
large or heavy objects. The block and tackle (also
vessel.
called tackle or pulley) is used in the same situations as
HOISTS, PULLEYS, AND DOLLIES
the chain hoist, except for smaller loads. Figure 7-25
shows the different types of blocks and tackles. It also
Various types of hoists, pulleys, and dollies are
provides the formula for figuring the amount of power
available ashore and afloat for moving equipment and
needed to move the weight of the load.
supplies. You should familiarize yourself with this
Dollies
The dolly or dolly truck is a frame mounted on
wheels or rollers. Dolly trucks are used for moving or
shifting heavy loads for short distances. Figure 7-26
shows the three common types of dollies used by the
Navy.
The general-purpose dolly (figure 7-26, View A) is
used to move unpalletized large, bulky, or heavy
material over short distances.
The pallet-rollers dolly (figure 7-26, View B) has a
capacity of 4,000 pounds. The purpose of the
pallet-rollers dolly is to move palletized loads in and
out of boxcars, trucks, trailers, and storerooms.
The reefer-car dolly (figure 7-26, View C) is easy
to maneuver, and is suitable for use on truck and reefer
floors. The 24 wheels in the central position are
slightly lower than the wheels at the ends. The springs
SKf07023
hold the end wheels in position to allow the wheels to
Figure 7-23.--Crane, gantry.
7-11
