CHAIN HOISTS

Chain hoists provide a convenient means for hoisting heavy objects. When a chain is used, the load can remain stationary without requiring attention. The slow lifting travel of a chain hoist is also advantageous in that it permits small movements, accurate adjustments of height, and cautious handling of loads.

Chain hoists differ widely in their mechanical advantage, depending upon their rated capacity. The mechanical advantage may vary from 5 to 250; that is, the ratio 5:1 to 250:1. Two types of chain hoists generally used for vertical hoisting operations are the spur gear hoist and the differential chain hoist.

The SPUR GEAR HOIST (fig. 6-14) is best for ordinary operations that require frequent use of a hoist and that have a minimum number of crew members available to operate it. The spur gear hoist is about 85-percent efficient. In other words, about 85 percent of the energy exerted by the operator is converted into useful work for lifting the load. The remaining 15 percent of the energy is spent in overcoming friction in the gears, bearings, chains, and so on.

Figure 6-14.\Spur gear hoist.

The DIFFERENTIAL CHAIN HOIST (fig. 6-15) is suitable for light loads and in situations in which only occasional use of the hoist is involved. This hoist is only about 35-percent efficient.

A ratchet-handle pull hoist, commonly called a COME-ALONG, can be obtained and will prove beneficial for making short, horizontal pulls on heavy objects. A typical come-along, having a rated capacity of 1 1/2 tons, is shown in figure 6-16. You will find the come-along to be one of the most useful hoisting devices available. The chain will not foul up because it is flexible and cannot kink. The chain is kept in place in the sheave by a hardened steel-load chain guide.

The load capacity of a chain hoist usually is stamped on the shell of the upper block. The rated load capacity of hoists runs from 1/2 ton (0.45 metric tons) upward to 40 tons (36 metric tons).

The lower hook is usually the weakest part in the assembly of a chain hoist. This is intended as a safety measure so that the hook will start to

Figure 6-16.\Come-along.

spread open if overloaded. Spreading in a hook is a signal to the operator, warning that the chain hoist is nearing the overload point. Thus, close observance on the part of the operator is necessary to detect any sign of overloading in time to prevent damage to the chain hoist. Under ordinary circumstances, pull exerted on a chain hoist by one or two crew members in NOT enough to overload the hoist.

Frequent inspection of chain hoists is necessary to ensure safe operation. A hook that shows signs of spreading or excessive wear should be replaced. If links in the chain are distorted, the chain hoist has probably been overloaded. In such a case, see that the chain hoist is condemned and removed from service immediately.