{{short description|Device used to grip stones for lifting}} {{Other uses|Lewis (disambiguation)}} Lewises in a stonemasonry workshop|200px|thumb Chain lewis|200px|thumb thumb|200px|Split-pin lewis thumb|200px|Three-legged lewis 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, preferably 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.
==Etymology== The name ''lewis'' may come from the Latin ''{{Lang|la|levo -avi, -atum}}'' meaning ''to levitate or lift'',<ref>{{Cite web | url=https://www.perseus.tufts.edu/ |title = Perseus Digital Library}}</ref> but the ''Oxford English Dictionary Online''<ref>{{Cite web | url=http://www.oed.com/view/Entry/107743?rskey |title = Home : Oxford English Dictionary}}</ref> 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.
==Use== 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.
To bed 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.
==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 curved steel 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=== thumb|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 {{convert|13|mm|adj=on|abbr=on}}-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 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.{{citation needed|date=January 2020}}
===External lewis=== The '''external lewis''', '''kerb lifter''' or '''slab lifter''' is a type of lifting device used in the stonemasonry trade since Medieval times. The external lewis was originally shaped like a pair of scissor-tongs, and swung from a treadwheel crane.<ref>Günther Binding published this example of an external lewis hanging from a tripod in "Medieval Building Techniques" (Tempus Publishing).</ref> 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 parapets.<ref>{{Cite book | url=https://books.google.com/books?id=GiLICQAAQBAJ&q=masons+lifting+dogs.&pg=PA72 |title = Modern Practical Masonry|isbn = 9781317742647|last1 = Warland|first1 = Edmund George|date = 2015-06-03| publisher=Routledge }}</ref>
The external lewis has been modified to handle kerbstones and large slabs of polished stone in contemporary stone yards. [[File:A lewis in Hainan - 01.jpg|thumb|A kerb lifter lifting kerb stones in Hainan, China]] 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.<ref>{{Cite web |last=Stutt |first=Donald E. |title=Annual Grand Masonic Day |url=https://www.freemasonry.bcy.ca/texts/gmd2007/vgmd26.pdf |page=5}}</ref>
==Gallery==
<gallery widths="220px" heights="150px"> 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. <!-- Deleted image removed: File:External Lewis.jpg|External lewis --> 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, found in Amman, Jordan </gallery>
==References== {{reflist}}
==External links==
{{Commons category|Stone lifting tools}} *[https://web.archive.org/web/20140327200126/http://www.societies.ncl.ac.uk/shspgf/ed_1/ed_1_morgan.pdf The Postgraduate Forum] Ted Morgan investigates connections between ancient Roman technology and recent British engineering practices.
{{Stonemasonry}}
Category:Stonemasonry tools Category:Cranes (machines)