Lewis (Lifting appliance)
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A Lewis is a steel lifting device used by Stonemasons to lift large stones into place with a crane, chain block or winch. It is inserted into a specially prepared seating in the top of a stone, directly above its centre of mass. It works by transferring the stone's own weight into leverage on the seating.
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 e.g. the closing stone in a string course; cylindrical column drums; decorated column capitals; coping stones in a pediment. Heavy ashlar stones are also bedded using a Lewis.
The name Lewis probably came from the Latin leuis meaning ‘to levitate’. 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.
There are a number of different types of Lewis used in the Stonemasonry trade:
Safety caution 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 put right by adjusting the Lewis hole or packing the Lewis with metal shims.
Bedding a stone using a Lewis: The stone is placed on dunnage laid flat with enough clearance for a mortar bed to be placed beneath it. The safety straps are removed, the stone is lifted using the Lewis alone, and the dunnage removed with fingers clear. The stone is then lowered onto the mortar bed, and positioned with sharp taps from a rubber mallet.
[edit] External links
- The Postgraduate Forum Ted Morgan investigates connections between ancient Roman technology and recent British engineering practices.
