ADDITIONAL LIFTING EQUIPMENT
In addition to block and tackle, slings, and chains, hooks, shackles, and beam clamps are also used for lifting objects and material.
There are two types of hooks available: the slip hook and the grab hook (fig. 6-24).
Slip hooks are made so the inside curve of the hook is an arc of a circle. They are used with wire rope, chains, and fiber line. Chain links can slip through a slip hook so that the loop formed in the chain can tighten under a load.
Figure 6-23.-Chain sting.
Grab hooks have an inside curve that is almost U-shaped so that the hook will slip over a link edgeways and not allow the next link to slip past. Grab hooks have a much more limited range of use than slip hooks. They are used exclusively when the loop formed in the chain is not intended to close around the load.
Mousing a Hook
As a rule, a hook should always be moused as a safety measure to prevent slings or line from coming off. Mousing also helps prevent the straightening of a hook but does not add to the strength of the hook. To mouse a hook (fig. 6-25) after the sling is on the hook you should wrap the wire or small stuff 8 or 10 turns around the two sides of the hook. Mousing is then
Figure 6-25.-Mousing a hook.
completed by winding several turns around the wire or small stuff and tying the ends securely. Inspection of Hooks
Hooks should be inspected at least once a month, but those used for heavy and continuous loading should be inspected more frequently. Attention must be given to the small radius fillets at the neck of the hooks for any deviation for the original inner arc. Additionally, each hook must be examined for small dents, cracks, sharp nicks, worn surfaces, or distortions. If any of these defects are present, the hook must be discarded.
Hooks normally fail by straightening. If any deviation of the inner are of a hook is evident, it indicates that the hook has been overloaded. Evidence of overloading a hook is easy to detect, so it is customary to use a hook that is weaker than the chain it is attached to. Using this system, distortion of the hook will occur before the hook is overloaded. Any distorted, cracked, or badly worn hook is dangerous and should be discarded immediately.
The safe working load of a hook can be formulated by using the following rule of thumb:
SWL = 2/3 x D2x 1 ton. D is the diameter (in inches) of the hook where the inside of the hook starts to are (fig. 6-26).
Below is an example of the safe working capacity of a hook with a diameter of 5/8 inch:
Figure 6-26.-Hook diameter.
D'= 5/8 X 5/8= 25/64
SWL = 2/3 x 25/64 x 1 ton= 25/96= 0.2604 ton 0.2604 ton x 2,000 pounds/ton= 520.8 pounds In the metric system, the formula for the safe
working load for hooks is as follows: SWL = .46 x D2x 1 tonne
Below is an example of the safe working capacity of a hook having a diameter of 1.59 cm.
D = 1.59 cm D 2 = 2.52 em'
SWL= .046 x 2.52 cm'x 1 tome = .116 tonne
Shackles (fig. 6-27) should be used for loads too heavy for hooks to handle. They provide a useful way of attaching, hauling, and lifting a load without tying directly to the object with a line, wire rope, or chain. Additionally, they can be attached to wire rope, line, or chain.
Safe Working Load of Shackles
The formula for computing the safe working load for a shackle is as follows:
SWL = 3D2x 1 ton Example:
Figure 6-27.-Two types of shackles: A. Anchor; B. Chain.
D = 5/8 (See fig. 6-28) D2= 5/8 X 5/8= 25/64 SWL = 3 X 25/64 x 1 ton = 75/64 x 1 ton=1.1719
The SWL in pounds = 1.1719 x 2,000 pounds = 2,343.8 pounds
In the metric system, the formula for the safe working load
for shackles is as follows: SWL = .417 X D2 X 1 tonne Example:
D2= 1.59 X 1.59 = 2.52
SWL = .417 x 2.52 x 1 tonne SWL = 1.05 tomes
NOTE: A hook or a shackle can actually lift more than these formulas allow. These formulas give you the safe working load UNDER ANY CONDITIONS. Mousing Shackles
Mouse shackles whenever there is danger of the shackles pin working loose or coming out due to vibration. To mouse a shackle properly, you take several turns with seizing wire through the eye of the pin and around the bow of the shackle. Figure 6-29 shows what a properly moused shackle looks like.
Figure 6-28.-Shackle diameter.