Wheel

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A wheel is a circular object that, together with an axle, allows low friction in 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.

History of the wheel and axle

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.

According to most authorities, the wheel-and-axle combination originated in ancient Mesopotamia during the 5th millennium BC, probably originally in the function of potter's wheels. The wheel's efficient use of input energy must have been quickly understood by its inventors because it was almost immediately set to work in other contexts, most importantly in transport (vehicles) and in foodstuff processing (mill wheels).

The earliest undisputed depiction of a wheeled vehicle (here a wagon -- four wheels, two axles), is on the Bronocice pot, a ca. 4000 BC clay pot excavated in southern Poland.

The wheel reached Europe in the 4th millennium, and India with the Indus Valley Civilization in the 3rd millennium. 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.

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. The wheel was apparently unknown in sub-Saharan Africa, Australia, the Pacific Islands and North America until relatively recent contacts with Eurasians.

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 be actually useful, is not so simple a device as it may seem.

Early wheels were simple wooden disks with a hole for the axle. Note that 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 was found by the Slovenian archeologist Dr. Anton Velušček and his team in 2002 at the Ljubljana Marshes (Ljubljansko barje), some 20 kilometres southeast of Ljubljana, Slovenia.^* 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 (see chariot). Shortly later, horse cultures of the Caucasus region were given a tremendous politico-military boost by their state-of-the-art horse-drawn spoked-wheel war chariots which enabled them to range far and wide against suddenly enfeebled opposition for the greater part of three centuries, most notably 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 was in continued use without major modification until the early 20th century.

The invention of the wheel turned out to be of great importance not only as a transportation device, but for the development of technology in general, important applications including the water wheel, the cogwheel (see also antikythera mechanism), the spinning wheel, the astrolabe or torquetum. More modern descendants of the wheel include the propeller, the jet engine, the flywheel (gyroscope) and the turbine.

The central importance of the wheel also resulted in its becoming a strong cultural and spiritual metaphor for a cycle or regular repetition (see chakra, reincarnation, Yin and Yang among others). In the coat of arms of Panama a winged wheel is a symbol of progress.

In July 2001, the wheel was the object of an innovative, but non-inventive, patent as a "circular transportation facilitation device".http://news.bbc.co.uk/hi/english/world/asia-pacific/newsid_1418000/1418165.stm The patent was obtained by John Keogh, a lawyer from Melbourne, Australia, with the declared intention of demonstrating the unfairness and inaccuracy of the recent innovations in the Australian patent system.

Mechanics and function

The wheel (with axle) is considered one of the simple machines and lies near the practical 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, etc.,.). Since a wheel is a rigid object, it will only be non-rotating when all the torques on it are balanced. Since forces produce larger torques when they are closer to the axis, a wheel can be used to transform between large and small forces applied by friction with belts or other wheels. The combination of the wheel with the wedge produced the toothed gear, itself fundamental to the advent of industrial class complex machines. Other variations on wheels produce the pulley and the windlass.

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.

References

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 ones own legs
A walking machine
Matty Wheels
Tracks (like a Train)

External links

Wheeled Vehicles in the Chinese Bronze Age (c. 2000-741 b.c.) by Anthony Barbieri-Low, Sino-Platonic Papers, 99 (February 2000)
Car Wheels - explains the different types of wheels available for cars

Mechanical engineering | Auto parts