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A lewis (sometimes called a lewisson) is one of a category of lifting devices used by
A three-legged lewis, also known as a dovetailed lewis, St Peter's keys, or a Wilson bolt, fits into a dovetailed seating in the top of a building stone. It is made from three pieces of rectangular-section -thick steel legs held together with a shackle, allowing connection to a lifting hook. The middle leg is square throughout its length, while the outer legs are thinner at the top, flaring towards the bottom. Held together, the three legs thus form a dovetail shape. The lewis hole seating is undercut (similar to a chain-linked lewis hole) to match its profile.
The first outer leg is inserted into the lewis hole, followed by the second outer leg. The inner (parallel) leg is inserted last, pushing the outer legs into contact with the inside of the lewis hole. The shackle is unbolted, placed over the legs, and the bolt fastened through both the shackle eyes and the eye in the top of each leg. (See gallery below for diagram.)
This type of lewis is the safest to use because it relies on its dovetailed shape for security instead of friction alone, but the seating is time-consuming to prepare.
Their resemblance, once assembled, to a bunch of keys gave rise to an alternative name for them of "St Peter's keys". This has frequently been represented allegorically, drawing the name of "St. Peter" as "the Rock on which I shall found my Church" into an allegory between the fabric of a church building and the community of the church itself. Some illustrations of
A manual kerb lifter is a large, adjustable pair of tongs, made with a pair of handles so that two people can manoeuvre heavy blocks of stone into position. A mechanical kerb lifter can also be made to fit mechanical lifters like forklifts or crane-trucks so that larger stones can be placed.
In stone yards, a slab lifter is hung from a shed gantry or forklift to transport slabs of stone between storage racks and stone processing machines. It consists of two hinged, weighted friction pads that close astride the top of a slab, and are pulled tightly together by the weight of the slab itself.
The slab lifter uses two safety devices. Safety chains and a support bracket allow safe lifting of large slabs. This lifting appliance also has a safety locking device that is engaged when the gripping pads are activated by the weight of the stone. This prevents any jerking movement from releasing the stone. It is easily disengaged once the slab is secured at its destination.
File:Lewis Hole Indents.JPG, Indented lewis holes
File:CWO (3).JPG , A split-pin lewis placed in the top of an altar stone
File:Louve de carrier.JPG, A French-type chain-linked lewis
File:Wilson bolt.jpg, St Peter's keys in the Duomo, Florence
File:Three-legged lewis diagram - 02.png, The principle of the three-legged lewis. The sections are inserted 1, 2, then 3.
File:Slab Lifter1.JPG, External lewis: gravity-clamp slab lifter
File:Lewis slab lifter.jpg , Slab lifter
File:Lewis slab lifter safety lock.jpg , Slab lifter safety lock
File:Maulbronn - Steinzange.jpg, A German kerb lifter and St Peter's keys
File:Lewis Lifter ACOR Jordan0869.jpg, Roman three-legged lewis
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Ted Morgan investigates connections between ancient Roman technology and recent British engineering practices. {{Stonemasonry Stonemasonry tools Cranes (machines)
A lewis (sometimes called a lewisson) is one of a category of lifting devices used by stonemason
Stonemasonry or stonecraft is the creation of buildings, structures, and sculpture using stone as the primary material. Stonemasonry is the craft of shaping and arranging stones, often together with mortar and even the ancient lime mortar ...
s to lift large stones into place with a crane, chain block
A hoist is a device used for lifting or lowering a load by means of a drum or lift-wheel around which rope or chain wraps. It may be manually operated, electrically or pneumatically driven and may use chain, fiber or wire rope as its lifting m ...
, or winch
A winch is a mechanical device that is used to pull in (wind up) or let out (wind out) or otherwise adjust the tension (physics), tension of a rope or wire rope (also called "cable" or "wire cable").
In its simplest form, it consists of a Bobb ...
. It is inserted into a specially prepared hole, or ''seating'', in the top of a stone, preferably above its centre of mass
In physics, the center of mass of a distribution of mass in space (sometimes referred to as the barycenter or balance point) is the unique point at any given time where the weighted relative position of the distributed mass sums to zero. For a ...
. It works by applying principles of the lever
A lever is a simple machine consisting of a beam (structure), beam or rigid rod pivoted at a fixed hinge, or '':wikt:fulcrum, fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, l ...
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
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...
where the short lever-arms make contact with the stone inside the hole and thereby prevents slipping.
Etymology
The name ''lewis'' may come from theLatin
Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
' meaning ''to levitate or lift'', but the ''Oxford English Dictionary Online
The ''Oxford English Dictionary'' (''OED'') is the principal historical dictionary of the English language, published by Oxford University Press (OUP), a University of Oxford publishing house. The dictionary, which published its first editio ...
'' 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
Roman or Romans most often refers to:
*Rome, the capital city of Italy
*Ancient Rome, Roman civilization from 8th century BC to 5th century AD
*Roman people, the people of Roman civilization
*Epistle to the Romans, shortened to Romans, a letter w ...
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
Masonry is the craft of building a structure with brick, stone, or similar material, including mortar plastering which are often laid in, bound, and pasted together by mortar (masonry), mortar. The term ''masonry'' can also refer to the buildin ...
coursings are neatly repaired with matching indented plugs after the stone has been set in place.
Use
A lewis is most useful when it is not possible to lift the stone withchain
A chain is a serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a rope in that it is flexible and curved in compression but linear, rigid, and load-bearing in tension. A ...
s or slings, because of either the location or shape of the stone, or delicate projections. Examples include the closing stone in a string course
A belt course, also called a string course or sill course, is a continuous row or layer of stones or brick set in a wall. Set in line with window sills, it helps to make the horizontal line of the sills visually more prominent. Set between the ...
, cylindrical column drums, decorated column capitals, and coping stones in a pediment
Pediments are a form of gable in classical architecture, usually of a triangular shape. Pediments are placed above the horizontal structure of the cornice (an elaborated lintel), or entablature if supported by columns.Summerson, 130 In an ...
. Heavy ashlar
Ashlar () is a cut and dressed rock (geology), stone, worked using a chisel to achieve a specific form, typically rectangular in shape. The term can also refer to a structure built from such stones.
Ashlar is the finest stone masonry unit, a ...
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
Scaffolding, also called scaffold or staging, is a temporary structure used to support a work crew and materials to aid in the construction, maintenance and repair of buildings, bridges and all other human-made structures. Scaffolds are widely u ...
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.
To bed a stone using a lewis, the stone is placed on dunnage
Dunnage is inexpensive or waste material used to load and secure cargo during transportation; more loosely, it refers to miscellaneous baggage, brought along during travel. The term can also refer to low-priority cargo used to fill out transport ...
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.
Types of lewis
There are a number of different types of lewis used in the stonemasonry trade:Chain-linked lewis
A chain-linked lewis or chain lewis is made from two curvedsteel
Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...
legs, linked by three steel rings. The legs fit into a seating cut in the top of the stone, above the centre of mass. When the top of the curved legs are pulled together by the rings, the bottom portions are forced into the lower part of the seating, thereby providing enough friction to lift the stone.
Split-pin lewis
The split-pin lewis is similar to the chain-linked lewis in that it uses a scissor-like action to produce friction against the inside of the lewis hole. The two legs, semicircular in section, lie side-by-side, and fit inside a hole drilled in the stone. This type of lewis seating is the simplest to prepare, requiring a single drilled hole.Two-pinned lewis
A two-pinned lewis consists of two pins, linked by a short chain. The pins are inserted into opposing holes that are drilled into the top of the stone at about 15° from vertical. It operates by gripping the stone (like two fingers lifting a tenpin bowling ball) as the weight of the stone is taken up by a crane or winch. The advantage of using this type of lewis is that it is simple to prepare: two angled drill holes are all that is necessary. Like other types of lewis, it is susceptible to pulling out as the stone is lifted. It should always be tested before hoisting, and used in conjunction with safety slings.Three-legged lewis
A three-legged lewis, also known as a dovetailed lewis, St Peter's keys, or a Wilson bolt, fits into a dovetailed seating in the top of a building stone. It is made from three pieces of rectangular-section -thick steel legs held together with a shackle, allowing connection to a lifting hook. The middle leg is square throughout its length, while the outer legs are thinner at the top, flaring towards the bottom. Held together, the three legs thus form a dovetail shape. The lewis hole seating is undercut (similar to a chain-linked lewis hole) to match its profile.
The first outer leg is inserted into the lewis hole, followed by the second outer leg. The inner (parallel) leg is inserted last, pushing the outer legs into contact with the inside of the lewis hole. The shackle is unbolted, placed over the legs, and the bolt fastened through both the shackle eyes and the eye in the top of each leg. (See gallery below for diagram.)
This type of lewis is the safest to use because it relies on its dovetailed shape for security instead of friction alone, but the seating is time-consuming to prepare.
Their resemblance, once assembled, to a bunch of keys gave rise to an alternative name for them of "St Peter's keys". This has frequently been represented allegorically, drawing the name of "St. Peter" as "the Rock on which I shall found my Church" into an allegory between the fabric of a church building and the community of the church itself. Some illustrations of St Peter
Saint Peter (born Shimon Bar Yonah; 1 BC – AD 64/68), also known as Peter the Apostle, Simon Peter, Simeon, Simon, or Cephas, was one of the Twelve Apostles of Jesus and one of the first leaders of the early Christian Church. He appears repe ...
even show him carrying a bunch of keys, which appear to have no wards. These are not keys in the lock-making sense, but in this sense of stonemasonry.
External lewis
The external lewis, kerb lifter or slab lifter is a type of lifting device used in thestonemasonry
Stonemasonry or stonecraft is the creation of buildings, structures, and sculpture using rock (geology), stone as the primary material. Stonemasonry is the craft of shaping and arranging stones, often together with Mortar (masonry), mortar ...
trade since Medieval
In the history of Europe, the Middle Ages or medieval period lasted approximately from the 5th to the late 15th centuries, similarly to the post-classical period of World history (field), global history. It began with the fall of the West ...
times. The external lewis was originally shaped like a pair of scissor-tongs
Tongs are a type of tool used to grip and lift objects instead of holding them directly with hands. There are many forms of tongs adapted to their specific use. Design variations include resting points so that the working end of the tongs d ...
, and swung from a treadwheel
A treadwheel, or treadmill, is a form of engine typically powered by humans. It may resemble a water wheel in appearance, and can be worked either by a human treading paddles set into its circumference (treadmill), or by a human or animal standing ...
crane.
This type of tong device has been known as dogs and the holes in the stone as dog holes for many centuries. Many old bridges and walls in the UK still have dog holes to reveal how the stones were lifted, particularly onto bridge parapet
A parapet is a barrier that is an upward extension of a wall at the edge of a roof, terrace, balcony, walkway or other structure. The word comes ultimately from the Italian ''parapetto'' (''parare'' 'to cover/defend' and ''petto'' 'chest/brea ...
s.
The external lewis has been modified to handle kerbstones and large slabs of polished stone in contemporary stone yards.
A manual kerb lifter is a large, adjustable pair of tongs, made with a pair of handles so that two people can manoeuvre heavy blocks of stone into position. A mechanical kerb lifter can also be made to fit mechanical lifters like forklifts or crane-trucks so that larger stones can be placed.
In stone yards, a slab lifter is hung from a shed gantry or forklift to transport slabs of stone between storage racks and stone processing machines. It consists of two hinged, weighted friction pads that close astride the top of a slab, and are pulled tightly together by the weight of the slab itself.
The slab lifter uses two safety devices. Safety chains and a support bracket allow safe lifting of large slabs. This lifting appliance also has a safety locking device that is engaged when the gripping pads are activated by the weight of the stone. This prevents any jerking movement from releasing the stone. It is easily disengaged once the slab is secured at its destination.
Gallery
References
External links
The Postgraduate Forum
Ted Morgan investigates connections between ancient Roman technology and recent British engineering practices. {{Stonemasonry Stonemasonry tools Cranes (machines)