Editing Caster (section)

==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.

When in motion along a straight line, a swivel caster will tend to automatically align to, and rotate parallel to the direction of travel. This can be seen on a [[shopping cart]] when the front casters align parallel to the rear casters when traveling down an aisle. A consequence of this is that the vehicle naturally tends to travel in a straight direction. Precise steering is not required because the casters tend to maintain straight motion. This is also true during vehicle turns. The caster rotates perpendicular to the turning radius and provides a smooth turn. This can be seen on a shopping cart as the front wheels rotate at different velocities, with different turning radius depending on how tight a turn is made.

The angle of, and distance between the wheel axles and swivel joint can be adjusted for different types of caster performance.<ref>Siegwart, R. and Nourbakhsh, I. "Introduction to Autonomous Mobile Robots", MIT Press, Cambridge, MA, 2004. 321 p. {{ISBN|0-262-19502-X}}</ref>

===Industrial casters===
[[File:Subwoofer driver and enclosure.jpg|thumb|Four heavy duty casters on a large [[subwoofer]] whose weight can exceed {{convert|500|lb|kg}} depending on driver and top loading, and must be able to withstand rough handling.]]
Industrial casters are heavy duty casters that are designed to carry heavy loads, in some cases up to thirty thousand pounds. An Industrial caster may have either a swivel or rigid caster design. Industrial casters typically have a flat top plate that has four bolt holes to ensure a sturdy connection between the top plate and the load. They are used in a variety of applications including dolly carts, assembly turntables, heavy duty storage racks, holding bins, tow lines, maintenance equipment, and material handling mechanisms.

In early manufacturing, industrial caster bodies were typically fabricated from three separate, stamped metal parts, which were welded to the top plate. Today, many industrial caster bodies are made by laser cutting the body from a single metal blank and then using a press brake to shape the legs to the required ninety degree angle, thus producing a mechanically stronger device.

Various factors affect industrial caster performance. For example, larger wheel diameters and widths provide higher weight capacity by distributing the load's weight across a larger wheel surface area. Also, harder wheel materials (e.g., cast iron, high profile polyurethane) are less sensitive to and tend to not track dirt and debris on floors.

=== Braking and locking casters ===
[[File:Swivel caster with wheel lock.JPG|thumb|A swivel caster with a wheel lock. The vertical swivel on this type of caster cannot be locked in position.]]
[[File:Caster - Central locking mechanism - Stryker hydraulic stretcher.JPG|thumb|Caster central locking mechanism on an old [[Stryker Corporation|Stryker]] hydraulic [[stretcher]]. Shown in the locked position with the center cam rotated downward.]]

Common inexpensive casters may include a [[brake]] feature, which prevents the wheel from turning. This is commonly achieved using a lever that presses a brake cam against the wheel. However a swivel caster is still able to move around slightly, in a small circle rotating around offset distance between the vertical shaft and the center of the locked wheel.

A more complex type of swivel caster, sometimes called a total lock caster, has an additional rotational lock on the vertical shaft so that neither shaft swiveling nor wheel rotation can occur, thus providing very rigid support. It is possible to use these two locks together or separately. If the vertical shaft is locked but the wheel can still turn, the caster becomes a directional caster, but one which may be locked to roll in one direction along any horizontal axis.

In some cases it is useful to be able to brake or lock all casters at the same time, without having to walk around to individually engage a mechanism on each one. This may be accomplished using a central lock mechanism engaged by a rigid ring encircling each swivel caster, slightly above the wheel, that lowers and presses down on the wheel, preventing both wheel and swivel rotation. An alternative method is the central lock caster, which has a rotating cam in the center of each vertical caster shaft, leading down to a braking mechanism in the bottom of each caster.

=== 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 {{clarify span|after use|date=February 2016}}. They offer capacity and durability comparable to units having sealed precision ball or tapered bearings{{citation needed|date=February 2016}}, and are a practical alternative to traditional swivel casters in high-impact situations{{why|date=February 2016}}.