TheInfoList

]] File:Roue primitive.png|thumb|An early wheel made of a solid piece of wood In its primitive form, a wheel is a circular block of a hard and durable material at whose center has been bored a hole through which is placed an axle bearing about which the wheel rotates when torque is applied to the wheel about its axis. The wheel and axle assembly can be considered one of the six simple machines. When placed vertically under a load-bearing platform or case, the wheel turning on the horizontal axle makes it possible to transport heavy loads. This arrangement is the main topic of this article, but there are many other applications of a wheel addressed in the corresponding articles: when placed horizontally, the wheel turning on its vertical axle provides the spinning motion used to shape materials (e.g. a potter's wheel); when mounted on a column connected to a rudder or to the steering mechanism of a wheeled vehicle, it can be used to control the direction of a vessel or vehicle (e.g. a ship's wheel or steering wheel); when connected to a crank or engine, a wheel can store, release, or transmit energy (e.g. the flywheel). A wheel and axle with force applied to create torque at one radius can translate this to a different force at a different radius, also with a different linear velocity.

Etymology

The English word ''wheel'' comes from the Old English word , from Proto-Germanic , from Proto-Indo-European , an extended form of the root "to revolve, move around". Cognates within Indo-European include Icelandic "wheel, tyre", Greek , and Sanskrit , the latter two both meaning "circle" or "wheel".

History

Mechanics and function

A wheeled vehicle requires much less work to move than simply dragging the same weight. The low resistance to motion is explained by the fact that the frictional work done is no longer at the surface that the vehicle is traversing, but in the bearings. In the simplest and oldest case the bearing is just a round hole through which the axle passes (a "plain bearing"). Even with a plain bearing, the frictional work is greatly reduced because: * The normal force at the sliding interface is same as with simple dragging. * The sliding distance is reduced for a given distance of travel. * The coefficient of friction at the interface is usually lower. Example: * If a 100 kg object is dragged for 10 m along a surface with the coefficient of friction ''μ'' = 0.5, the normal force is 981 N and the work done (required energy) is (work=force x distance) 981 × 0.5 × 10 = 4905 joules. * Now give the object 4 wheels. The normal force between the 4 wheels and axles is the same (in total) 981 N. Assume, for wood, ''μ'' = 0.25, and say the wheel diameter is 1000 mm and axle diameter is 50 mm. So while the object still moves 10 m the sliding frictional surfaces only slide over each other a distance of 0.5 m. The work done is 981 × 0.25 × 0.5 = 123 joules; the work done has reduced to 1/40 of that of dragging. Additional energy is lost from the wheel-to-road interface. This is termed rolling resistance which is predominantly a deformation loss. It depends on the nature of the ground, of the material of the wheel, its inflation in the case of a tire, the net torque exerted by the eventual engine, and many other factors. A wheel can also offer advantages in traversing irregular surfaces if the wheel radius is sufficiently large compared to the irregularities. The wheel alone is not a machine, but when attached to an axle in conjunction with bearing, it forms the wheel and axle, one of the simple machines. A driven wheel is an example of a wheel and axle. Wheels pre-date driven wheels by about 6000 years, themselves an evolution of using round logs as rollers to move a heavy load—a practice going back in pre-history so far that it has not been dated.

Construction

Rim

The rim is the "outer edge of a wheel, holding the tire." It makes up the outer circular design of the wheel on which the inside edge of the tire is mounted on vehicles such as automobiles. For example, on a bicycle wheel the rim is a large hoop attached to the outer ends of the spokes of the wheel that holds the tire and tube. In the 1st millennium BC an iron rim was introduced around the wooden wheels of chariots.

Hub

The hub is the center of the wheel, and typically houses a bearing, and is where the spokes meet. A hubless wheel (also known as a rim-rider or centerless wheel) is a type of wheel with no center hub. More specifically, the hub is actually almost as big as the wheel itself. The axle is hollow, following the wheel at very close tolerances.

Spokes

A ''spoke'' is one of some number of rods radiating from the center of a wheel (the hub where the axle connects), connecting the hub with the round traction surface. The term originally referred to portions of a log which had been split lengthwise into four or six sections. The radial members of a wagon wheel were made by carving a spoke (from a log) into their finished shape. A spokeshave is a tool originally developed for this purpose. Eventually, the term spoke was more commonly applied to the finished product of the wheelwright's work, than to the materials used.

Wire

The rims of ''wire wheels'' (or "wire spoked wheels") are connected to their hubs by wire spokes. Although these wires are generally stiffer than a typical wire rope, they function mechanically the same as tensioned flexible wires, keeping the rim true while supporting applied loads. Wire wheels are used on most bicycles and still used on many motorcycles. They were invented by aeronautical engineer George Cayley and first used in bicycles by James Starley. A process of assembling wire wheels is described as wheelbuilding. right|thumb|A 1957 MGA automobile with wire wheels

Tire/Tyre

A tire (in American English and Canadian English) or tyre (in some Commonwealth Nations such as UK, India, South Africa, Australia and New Zealand) is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground. The word itself may be derived from the word "tie," which refers to the outer steel ring part of a wooden cart wheel that ties the wood segments together (see Etymology below). The fundamental materials of modern tires are synthetic rubber, natural rubber, fabric and wire, along with other compound chemicals. They consist of a tread and a body. The tread provides traction while the body ensures support. Before rubber was invented, the first versions of tires were simply bands of metal that fitted around wooden wheels to prevent wear and tear. Today, the vast majority of tires are pneumatic inflatable structures, comprising a doughnut-shaped body of cords and wires encased in rubber and generally filled with compressed air to form an inflatable cushion. Pneumatic tires are used on many types of vehicles, such as cars, bicycles, motorcycles, trucks, earthmovers, and aircraft.

Protruding or covering attachments

Extreme off-road conditions have resulted in the invention of several types of wheel covers, which may be constructed as removable attachments, or as permanent covers. Wheels like this are nolonger necessarily round, or have panels that make the ground contact area flat. Examples: *Snow chains - Specially designed chain assemblies that wrap around the tire to provide increased grip, designed for deep snow. *Dreadnaught wheel - A type of permanently attached hinged panels for general extreme off-road use. These are not connected directly to the wheels, but the eachother. *Pedrail wheel - A system of rails that holds panels that hold the vehicle. These do not necessarily have to be built as a circle (wheel) and are thus also a form of Continuous track. *A version of the above examples (name unknown to the writer) was commonly used on heavy artillery during World War I. Specific examples: Cannone da 149/35 A and the Big Bertha. These were panels that were connected to eachother by multiple hinges and could be installed over a contemporary wheel. *Continuous track - A system of linked and hinged chains/panels that cover multiple wheels in a way that allows the vehicles mass to be distributed across the space between wheels that are positioned in front of / behind other wheels. *"Tire totes" - A bag designed to cover a tire to improve traction in deep snow.

Alternatives

While wheels are very widely used for ground transport, there are alternatives, some of which are suitable for terrain where wheels are ineffective. Alternative methods for ground transport without wheels include: *Maglev *Sled or travois *Hovercraft *A walking machine *Caterpillar tracks (operated by wheels) *Pedrail wheels, using aspects of both wheel and caterpillar track *Spheres, as used by Dyson vacuum cleaners and hamster balls *Screw-propelled vehicle *wagon wheel

Symbolism

The wheel has also become a strong cultural and spiritual metaphor for a cycle or regular repetition (see chakra, reincarnation, Yin and Yang among others). As such and because of the difficult terrain, wheeled vehicles were forbidden in old Tibet. The wheel in ancient China is seen as a symbol of health and strength and utilized by some villages as a tool to predict future health and success. The diameter of the wheel is indicator of one's future health. The winged wheel is a symbol of progress, seen in many contexts including the coat of arms of Panama, the logo of the Ohio State Highway Patrol and the State Railway of Thailand. The wheel is also the prominent figure on the flag of India. The wheel in this case represents law (dharma). It also appears in the flag of the Romani people, hinting to their nomadic history and their Indian origins. The introduction of spoked (chariot) wheels in the Middle Bronze Age appears to have carried somewhat of a prestige. The sun cross appears to have a significance in Bronze Age religion, replacing the earlier concept of a Solar barge with the more 'modern' and technologically advanced solar chariot. The wheel was also a solar symbol for the Ancient Egyptians.