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{{Short description|Undriven wheel that is designed to be attached to the bottom of a larger object}}
{{Other uses}}
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==Types==
[[File:Löffelholz-Codex Ms-Berol-Germ-Qu-132 Fol 010v.png|thumb|Drawing of a swivelling caster from Codex Löffelholz, Nuremberg 1505]]
Casters may be fixed to roll along a straight line path, or mounted on a [[wiktionary:Pivot|pivot]] or [[pintle]] such that the wheel will automatically align itself to the direction of travel.
===Rigid casters===
A basic, rigid caster consists of a wheel mounted to a stationary fork. The orientation of the fork, which is fixed relative to the vehicle, is determined when the caster is mounted to the vehicle.<ref>{{Cite book |last=Wagner |first=Markus |url=https://books.google.com/books?id=HkoRC4_vR1IC&pg=PA557 |title=VERZEICHNIS MECHATRONIK: Grundlagen-BEGRIFFE werden erklaert - in German language: alphabetical index of technical terms mechatronics |date=2015-05-19 |publisher=LEHRMITTEL WAGNER / Autor |isbn=978-3-86479-138-3 |language=de}}</ref> An example of this is the wheels found at the rear of a shopping cart in North America. Rigid casters tend to restrict vehicle motion so that the vehicle travels along a straight line.
===Swivel casters===
[[Image:Caster-wheels-chair.jpg|thumb|left|upright|Two wheeled swivel casters on a [[desk chair]] (no fork)]]
Like the simpler rigid caster, a swivel caster incorporates a wheel mounted to a fork, but an additional [[swivel joint]] above the fork allows the fork to freely rotate about 360°, thus enabling the wheel to roll in any direction. This makes it possible to easily move the vehicle in any direction without changing its orientation. Swivel casters are sometimes attached to handles so that an operator can manually set their orientation. The improved swivel caster was invented in 1920 by Seibert Chesnutt, US Patent 1341630, which was easily manufactured by stamping, and incorporated ball bearings for longer life. Basic swivel casters were in evidence in Charles Darwin's famous "office chair" as early as the 1840s.
Additionally, a swivel caster typically must include a small amount of offset distance between the center axis of the vertical shaft and the center axis of the caster wheel. When the caster is moved and the wheel is not facing the correct direction, the offset will cause the wheel assembly to rotate around the axis of the vertical shaft to follow behind the direction of movement. If there is no offset, the wheel will not rotate if not facing the correct direction, either preventing motion or dragging across the ground.
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=== Kingpinless casters ===
A Kingpinless caster has an inner raceway, an outer raceway which is attached to the vehicle, and ball bearings between the raceways. This mechanism has no [[kingpin (automotive part)|kingpin]], hence the name kingpinless. The absence of a kingpin eliminates most causes of swivel caster failure{{citation needed|date=February 2016}} and reduces or eliminates shimmy
== Caster flutter ==
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Many parameters play a role in how well the caster performs. Parameters such as tire hardness, tread width and shape, the length of the trailing offset (the 'caster') and wheel diameter all affect the effort required to start the platform moving. Harder wheels will make the caster easier to roll by reducing deformation resistance. A less inflated tire offers more deformation resistance and thus more effort is required to move the attached platform. Turning effort is affected by the amount of caster and by the wheel diameter.<ref>[http://www.casterconcepts.com/solutions/by-application/focus-on-ergonomic-design-improvement/ Focus on Ergonomic Design Improvements] Caster Concepts' Solutions</ref>
Enhancements to traditional caster design include toe guards, track wipers, reinforced legs, steering tubes, swivel locks and brakes, all implemented in an effort to reduce operator injuries in the workplace.
== Wheel diameter, wheel width, and tandem wheels ==
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* [[Caster board]]
* [[Bogie]]
== References ==
{{Reflist}}
{{Authority control}}
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