Lewis (lifting appliance)

A lewis (sometimes called a lewisson) is one of a category of lifting devices used by stonemasons to lift large stones into place with a crane, chain block, or winch. It is inserted into a specially prepared hole, or seating, in the top of a stone, directly above its centre of mass. It works by applying principles of the lever and utilises the weight of the stone to act on the long lever-arms which in turn results in a very high reaction force and friction where the short lever-arms make contact with the stone inside the hole and thereby prevents slipping.

The name lewis may come from the Latin levo -avi, -atum meaning to levitate or lift, but the Oxford English Dictionary Online states, "the formation and the phonology are not easily explained on this hypothesis", preferring "origin obscure", and speculating that the term may derive from a personal name. The Romans used the lewis. The specially shaped hole that is shaped to fit the device is known as a lewis hole. Lewis holes in the uppermost masonry coursings are neatly repaired with matching indented plugs after the stone has been set in place.

A lewis is most useful when it is not possible to lift the stone with chains or slings, because of either the location or shape of the stone, or delicate projections. Examples include the closing stone in a string course, cylindrical column drums, decorated column capitals, and coping stones in a pediment. Heavy ashlar stones are also bedded using a lewis.

The lewis is liable to slip out of the seating if some of the weight of the stone is subtracted from the appliance, such as when the stone bumps on the scaffolding on its way up to its final location. For this reason, a safety sling should always be used together with the lewis until the stone is reasonably close to its final position. Lifting the stone a small distance from the ground before hoisting is the best way to test a lewis. Any sign of looseness or damage should be corrected by adjusting the lewis hole or packing the lewis with metal shims.

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Wikipedia