to raise loads weighing several tons. The slow liftingtravel of a chain hoist permits small movements,accurate adjustment of height, and gentle handling ofloads. A ratchet handle pull hoist is used for short,horizontal pulls on heavy objects. Chain hoists differwidely in their mechanical advantage, depending upontheir rated capacity.Three general types of chain hoists for verticaloperation are the spur gear hoist, the differential chainhoist, and the screw gear hoist.The spur gear hoist (fig. 13-46, view A) is the mostsatisfactory for ordinary operations. This type of hoistis about 85 percent efficient. The differential chain hoist(fig. 13-46, view B) is only about 35 percent efficientand is satisfactory for occasional use and light loads. Thescrew gear hoist is about 50 percent efficient and issatisfactory where less frequent use of the hoist isrequired.Chain hoists arc usually stamped with their loadcapacities on the shell of the upper block. Chain hoistsFigure 13-46.-A. Spur gear chain hoist; B. Differential chainhoist.are constructed with their lower hook as the weakest partof the assembly. This is done as a precaution, so thelower hook will be overloaded before the chain hoist isoverloaded. The lower hook will start to spread underload, indicating the approaching overload limit. Underordinary circumstances the pull, exerted on a chain hoistby 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 excessivewear is sufficient cause to require replacement of thehook. If the links of the chain are distorted, it indicatesthat the chain hoist has been heavily overloaded andprobably unsafe for further use. Under suchcircumstances the chain hoist should be condemned.Before using any permanently mounted chain hoists,you should ensure that the annual certification is current.WINCHESVehicular-mounted winches and engine-drivenwinches are sometimes used in conjunction with tacklesfor hoisting. When placing a power winch to operatehoisting equipment, you must consider two points. First,you must consider the angle with the ground that thehoisting line makes at the drum of the hoist. This angleis sometimes referred to as ground angle, as shown infigure 13-47. The second point to consider is the fleetangle of the hoisting line winding on the drum, as shownFigure 13-47.—Vehicle winch used for hoisting.13-26
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