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Page Title: Hoists
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Figure 13-23.-Crane, gantry. to  install  a  crane.  The  crane  bridge  is  mounted  on trestles having legs which are generally constructed with wheel trucks for operating on tracks. Such cranes are referred to as portable gantry cranes. If the trestles rest directly on the ground or footings, the term “fixed gantry crane” is applied. This type of crane is built especially for particular locations. It has been constructed with a span of 200 or more feet. The gantry may have a trolley running on the bridge carrying a hoist. This is the most common form and is what is meant by gantry crane. However, the gantry may have a stiff-legged derrick, a rotating pillar, a job crane, or a hammerhead crane mounted on its bridge as auxiliary equipment. Because this type of crane spans the area over which it   operates,   it   has   been   particularly   useful   in shipbuilding, in storage yards, and at docks for handling bulk  material. A  wharf  crane  (figure  13-24)  is  located  on  and generally is a part of the wharf or pier structure. It is particularly adapted to the transfer of Cargo between the wharf or pier and a vessel. HOISTS, PULLEYS, AND DOLLIES Various types of hoists, pulleys, and dollies are available  aboard  air  stations  and  ships  for  moving equipment and supplies. The SK should be acquainted Figure 13-24.-Crane, wharf. with this equipment and its purpose so that as various situations arise, the SK will be able to select and use the necessary  piece  of  equipment. Hoists Chain hoists or chain falls provide a convenient and efficient  method  for  hoisting  loads  by  hand.  The  chief advantages of chain hoists are that one person can raise a load of several tons, and the load can remain stationary without being secured. Manually operated chain hoists of the type illustrated in figure 13-25 can be carried and operated by one person. They are particularly useful in trucks and small storerooms aboard ship and when other more mechanized equipment is not available. Some   larger   storerooms   are   equipped   with electrically  operated  hoists  which  move  along  overhead trucks. These hoists have the advantages of speed and ease  of  operation. Block  and  Tackle A block and tackle (figure 13-26) is an arrangement of one or more pulleys with rope or cable for pulling or hoisting  large,  heavy  objects.  The  block  and  tackle (also  called  tackle  or  pulley)  is  used  in  the  same situations as the chain hoist for smaller loads. 13-11

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