Wheel

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For other uses, see Wheel (disambiguation).

The force bearing on the axle has an eccentricity e with the point of contact to the rolling surface and exerts a moment about the contact point

A wheel is a circular device capable of rotating on its axis, facilitating movement or transportation or performing labour in machines. A wheel together with an axle overcomes friction by facilitating motion by rolling. Common examples are found in transport applications. More generally the term is also used for other circular objects that rotate or turn, such as a Ship's wheel and flywheel.

1 History of the wheel and axle
2 Mechanics and function
3 Wheeled vehicles
4 Alternatives to wheels
5 Wheels as symbols
6 See also
7 References

History of the wheel and axle

The English word wheel comes from the Proto-Indo-European *k^wek^wlo-,^[1] which was an extended form of the root *k^wel- meaning "to revolve, move around". This is also the root of the Greek kuklos, the Sanskrit chakra and Persian Charkh, all meaning "circle" or "wheel",^[2] and also in Lithuanian, sukti means "to rotate". The Latin word rota is from the Proto-Indo-European *rotā-, the extended o-grade form of the root *ret- meaning "to roll, revolve".^[3]

Notably there are no macroscopic wheels in macroscopic animals or plants, though some animals can roll. Whether this is a case of human ingenuity topping nature's "blind ingenuity" is a continuing source of debate. It should, however, be noted that microscopic wheels do exist in nature such as in ATP synthase^[4] and bacterial flagellum.^[5]

A depiction of onager-drawn carts on the Sumerian "battle standard of Ur" (circa 2600 BC)

Most authorities regard the wheel as one of the oldest and most important inventions, which originated in ancient Sumer in Mesopotamia (modern Iraq) in the 5th millennium BC, originally in the function of potter's wheels. The wheel reached India and Pakistan with the Indus Valley Civilization in the 3rd millennium BC. Near the northern side of the Caucasus several graves were found, in which since 3700 BC people had been buried on wagons or carts (both types). The earliest depiction of what may be a wheeled vehicle (here a wagon -- four wheels, two axles), is on the Bronocice pot, a ca. 3500 BC clay pot excavated in southern Poland.^[6]

The wheel reached Europe and India (the Indus Valley civilization) in the 4th millennium BC. In China, the wheel is certainly present with the adoption of the chariot in ca. 1200 BC, and Barbieri-Low (2000) argues for earlier Chinese wheeled vehicles, circa 2000 BC. Whether there was an independent "invention of the wheel" in East Asia or whether the concept made its way there after jumping the Himalayan barrier remains an open question.

Some archeologists argue for the European origin of the wheel on an axle.^[7] It is claimed that natural conditions of Europe (hard-surface plains making it difficult to drag heavy loads as well as plenty of timber used by skilled craftsmen in every aspect of life) provided the best background for the invention of a wheel used for transport. The chronology of the oldest known specimens and representations of wheels and/or wheeled vehicles suggest that the invention may have been made in Europe no later than in the first half of the 4th millennium BC. The prototype for the wheeled wagon was most probably "traga na szpuli" which was a primitive sledge with wooden logs put underneath. Through contacts and migration the invention was spread to the peoples from the regions of the Caspian Sea and the Black Sea. From there, it reached Mesopotamia in the late 4th millennium BC. This could explain why in the Near East even the oldest wagons were already relatively highly diverse and sophisticated and no evidence of evolutionary stages of their development there have been found yet.

Although they did not develop the wheel proper, the Olmec and certain other western hemisphere cultures seem to have approached it, as wheel-like worked stones have been found on objects identified as children's toys dating to about 1500 BC.

A spoked wheel on display at The National Museum of Iran, in Tehran. The wheel is dated late second millennium BCE and was excavated at Choqa Zanbil.

The invention of the wheel thus falls in the late Neolithic and may be seen in conjunction with the other technological advances that gave rise to the early Bronze Age. Note that this implies the passage of several wheel-less millennia, even after the invention of agriculture. Looking back even further, it is of some interest that although paleoanthropologists now date the emergence of anatomically modern humans to ca. 150,000 years ago, 143,000 of those years were "wheel-less". That people with capacities fully equal to our own walked the earth for so long before conceiving of the wheel may be initially surprising, but populations were extremely small through most of this period and the wheel, which requires an axle and socket to actually be useful, is not as simple a device as it may seem.

Early wheels too were simple wooden disks with a hole for the axle. Because of the structure of wood a horizontal slice of a trunk is not suitable, as it does not have the structural strength to support weight without collapsing; rounded pieces of longitudinal boards are required. The oldest such wheel, believed to have been made by the Alekern tribe, was found by the Slovenian archaeologist Janez Dirjec in 2002 at the Ljubljana Marshes (Ljubljansko barje), some 20 kilometres southeast of Ljubljana, Slovenia.[1] According to the experts in Vienna, Austria, the specimen was manufactured somewhere between 3350 and 3100 BC and is even older than others of similar construction found in Switzerland and Germany.

The spoked wheel was invented more recently, and allowed the construction of lighter and swifter vehicles. The earliest known examples are in the context of the Andronovo culture, dating to ca 2000 BC. Shortly later, horse cultures of the Caucasus region used horse-drawn spoked-wheel war chariots for the greater part of three centuries. They moved deep into the Greek peninsula where they joined with the existing Mediterranean peoples to give rise, eventually, to classical Greece after the breaking of Minoan dominance and consolidations led by pre-classical Sparta and Athens. Celtic chariots introduced an iron rim around the wheel in the 1st millennium BC. The spoked wheel had been in continued use without major modification until the 1870s, when wire wheels and pneumatic tires were invented [2].

The invention of the wheel has also been important for technology in general, important applications including the water wheel, the cogwheel (see also antikythera mechanism), the spinning wheel, and the astrolabe or torquetum. More modern descendants of the wheel include the propeller, the jet engine, the flywheel (gyroscope) and the turbine.

In July 2001, the wheel was the object of an Australian "innovation patent" as a "circular transportation facilitation device".^[8] The innovation patent was obtained by John Keogh, a lawyer from Melbourne, Australia, with the declared intention of demonstrating flaws in the recently introduced innovation patent system. Innovation patents are intended for minor innovations that do not qualify as patentable inventions, and an innovation patent is not the same as a patent. Applications for innovation patents, like Mr. Keogh's wheel application, are not examined by IP Australia, the Australian Patent Office, before they are registered.

Mechanics and function

Modern motorcycle wheel with disc brake.

The wheel (with axle) is considered one of the simple machines and lies near the starting point of advanced human technology (advanced, that is, in comparison with even earlier mechanical innovations such as stone/bone knives and axes, tension-sprung projectiles, scoops and shovels).

When wheels are used in conjunction with axles, either the wheel turns on the axle or the axle turns in a vehicle (as in a cart) or a housing (as in a mill). The mechanics are the same in either case.

The low resistance to motion (compared to dragging) is explained as follows (refer to friction):

the normal force at the sliding interface is the same.
the sliding distance is reduced for a given distance of travel.
the coefficient of friction at the interface is usually lower.

Bearings are used to reduce friction at the interface.

Example:

If dragging a 100 kg object for 10 m along a surface with 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 μ = 0.1, 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 x 0.1 x 0.5 = 49 joules.

Additional energy is lost at the wheel to road interface. This is termed rolling resistance which is predominantly a deformation loss.

Wheeled vehicles

Vehicles can be classified according to number of wheels:

Alternatives to wheels

While wheels are used for ground transport very widely, there are alternatives, some of which are suitable for terrain where wheels are ineffective. Alternative methods for ground transport without wheels include:

Dragging with runners (sled) or without (travois)
Being carried (litter/sedan chair or stretcher)
Being raised by air pressure (hovercraft)
Being raised by electromagnetic energy (maglev train)
Riding an animal such as a horse
Walking on one's own legs
A walking machine
Caterpillar tracks (although it is still operated by wheels)

Wheels as symbols

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).

The winged wheel is a symbol of progress, seen in many contexts including the coat of arms of Panama.

The introduction of spoked (chariot) wheels in the Middle Bronze Age appear to have carried somewhat of a prestige. The solar wheel 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.

In recent times, the custom aftermarket wheel has become a status symbol. These wheels are often incorrectly referred to as "rims." The term "rim" is incorrect because the rim is only a portion of a wheel, just as with a coffee cup or meteor crater. These "rims" have a great deal of variation, and are often very shiny. Some custom "rims" include a bearing-mounted, free-spinning disc which continues to rotate by inertia after the automobile is stopped. In slang, these are referred to as "Spinners" [3].