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This information sheet introduces the machine shop student to basic, acceptable lifting and rigging techniques. It explains the equipment and operations necessary for safe rigging and lifting. This document is not intended to show the student how to perform rigging and lifting, but to help him/her recognize safe and unsafe rigging and lifting practices.
Rigging is the preparation of a load prior to being moved. In this unit we will concentrate on cable sling rigging, chain sling rigging, and synthetic web slings.
A sling is a synthetic webbing which is designed in a configuration for hoisting, lifting, and lowering applications (Figure 1).
Cable slings or wire rope slings are made up of individual strands of wire. The number of strands, and wires, the type of material, and the nature of the core will depend on the intended purpose of the wire rope. We must always determine the weight of the load before we can begin selecting the sling type.
As with any rigging and lifting efforts, we visually inspect all cable slings before use. Remove from service if the following conditions are present: See also Figure 2.
When using a wire rope sling always follow these safety precautions:
5. Hang the sling up after every use. This will help to keep it clean, undamaged, and ready for use for the next job.
Try to avoid using chains when it is possible to use wire rope. The failure of a single link of a chain results in the complete failure of the chain, whereas a cable is made up of many wires and strands, and they must all fail before the rope breaks. Chains give no warning as to when they are going to fail. A wire rope will show visible signs of impending failure. Chains are better suited for certain jobs as they will withstand rougher handling and they wonít kink. Chains are much more resistant to abrasion and corrosion than wire ropes are. Chains are well suited as slings in the machine shop for lifting heavy castings. Use the following chain safety tips as a guide.
Synthetic web slings, because of their flexibility and elasticity, are a very popular rigging tool. The material with which these synthetic slings are made allows them to be very flexible, but it also makes them susceptible to heat and sharp edges. Synthetic web slings must be well taken care of, and the user must always be aware of sharp corners, welding sparks, and metal chips.
Slings used in environments where they are subject to continuous exposure to ultra-violet light should be proof tested to two times the rated capacity annually, or more frequently depending on the severity of exposure.
Double Choker Hitch - The double choker hitch seems to be preferred by many good riggers because it is twice as strong as a single choker hitch in the same sling type (Figure 6).
When this hitch is made in the right way, both legs will automatically equalize over the crane hook. However, when it is made wrong, there is usually no equalization and one of the legs will support most of the load.
Figure 6 Double choker hitch correctly and incorrectly performed
Basket Hitches - Basket hitches, whether single or double, may be used successfully in a variety of applications. However, they have inherent limitations as indicated in Figure 7.
Adjusting Hitch Ė One of the useful hitches available to riggers is the adjusting hitch (Figure 10). It is particularly useful when lifting an object that is heavier on one end than the other. The adjusting hitch makes it fairly easy to adjust the length of the legs of the bridle to maintain the load level. This hitch should not be loaded any more than a single basket hitch.
Figure 10 Adjusting Hitch used correctly and incorrectly on rolls
Lifting should only be done by a certified lift or crane operator. Before a lift can be used the operator must know all of the functions of the lift. Although this unit of instruction is not intended to show you how to operate a crane or hoist, here are some safety tips to adhere to while around lifting devices.
Signals for Crane Movement
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