A sling is a piece of hardware that connects an object being lifted with the equipment performing the lifting. Slings are made from a variety of materials, such as chain, wire rope, natural or synthetic fiber rope, or synthetic web. Slings typically include hooks, rings or shackles to assist in the lifting process. The size, weight, and shape of the object being lifted, as well as environmental conditions, are things to consider when choosing a sling as each material has its own advantages, disadvantages and lifting capabilities.
Chain slings are commonly used because of their strength and toughness. They can be used in rugged conditions and for lifting hot materials. They can typically be repaired and recertified.
Wire rope is composed of twisted strands of individual steel wires. They are resistant to abrasion and the least expensive steel sling. They are available with wire cores to aid in strength or fiber cores for increased flexibility.
Fiber rope is used primarily for lightweight or short-term applications. Fiber rope slings are light, flexible and do not damage the load surfaces, so they are suitable for fragile or polished parts.
The most commonly used synthetic web slings are made of nylon or polyester. They are lightweight and easy to use and they have a wider-bearing surface than rope for stability of the load.
All sling types must be visually inspected prior to use each day and must be inspected by trained personnel. The sling shall be thoroughly inspected for cuts, cracks, worn surface areas and many other defects to safeguard against injury, death and property damage. Some materials contain red warning threads inside the core of the sling. If those threads become exposed by being worn or cut, the sling should not be used. Most slings also have permanently affixed safety and identification tags and must not be used if the tags have been removed or are illegible. Inspection records are not required for these ‘frequent’ inspections.
Every sling must also undergo ‘periodic’ inspections. The interval of the inspection is based upon the frequency and severity of use of the sling and shall not exceed one year. This inspection should be performed by someone other than the person who performs the daily inspection. Written records of periodic inspections must be maintained.
To ensure the safety of slings, OSHA requires that all new, repaired, or reconditioned slings, be proof tested before putting them into use.
The proof test is where a load is applied to a sling, usually twice the working load limit or rated load. All components of the sling are tested together as a system to assure that each part of the sling is capable of performing at the working load limit.
After proof testing, the sling and components are removed from the test fixture and inspected for any signs of destructive failure such as broken welds, cracks or any other visual damage. Documentation of the proof test will need to be saved.
If damage is identified in any inspection or testing process, or the safety of the sling is in question, the sling should be removed from service immediately and a determination will need to be made whether the sling should be repaired or destroyed.
The ultimate guide to understanding aircraft jack proof load testing.
Read it here.