A wheel is a circular component that is intended to rotate on an axial bearing. The wheel is
one of the main components of the wheel and axle which is one of the six simple machines.
Wheels, in conjunction with axles, allow heavy objects to be moved easily facilitating movement
or transportation while supporting a load, or performing labor in machines. Wheels are
also used for other purposes, such as a ship's wheel, steering wheel, potter's wheel and
flywheel. Common examples are found in transport applications.
A wheel greatly reduces friction by facilitating motion by rolling together with the use of
axles. In order for wheels to rotate, a moment needs to be applied to the wheel about its
axis, either by way of gravity, or by the application of another external force or torque.
Etymology The English word wheel comes from the Old
English word hweol, hweogol, from Proto-Germanic *hwehwlan, *hwegwlan, from Proto-Indo-European
*kwekwlo-, an extended form of the root *kwel- "to revolve, move around". Cognates within
Indo-European include Icelandic hjól "wheel, tyre", Greek κύκλος kúklos, and Sanskrit
chakra, the latter both meaning "circle" or "wheel".
History
Evidence of wheeled vehicles appears from the second half of the 4th millennium BC,
near-simultaneously in Mesopotamia, the Northern Caucasus and Central Europe, so that the question
of which culture originally invented the wheeled vehicle remains unresolved and under debate.
The oldest securely dated wheel-axle combination, that from Stare Gmajne near Ljubljana in Slovenia
is now dated in 2σ-limits to 3340-3030 cal BC, the axle to 3360-3045 cal BC
The earliest well-dated depiction of a wheeled vehicle is on the Bronocice pot, a c. 3500
– 3350 BC clay pot excavated in a Funnelbeaker culture settlement in southern Poland.
The wheeled vehicle spread from the area of its first occurrence across Eurasia, reaching
the Indus Valley by the 3rd millennium BC. During the 2nd millennium BC, the spoke-wheeled
chariot spread at an increased pace, reaching both China and Scandinavia by 1200 BC.
In China, the wheel was certainly present with the adoption of the chariot in c. 1200
BC, although Barbieri-Low argues for earlier Chinese wheeled vehicles, c. 2000 BC.
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. It is thought that the primary obstacle to
large-scale development of the wheel in the Western hemisphere was the absence of domesticated
large animals which could be used to pull wheeled carriages. The closest relative of
cattle present in Americas in pre-Columbian times, the American Bison, is difficult to
domesticate and was never domesticated by Native Americans; several horse species existed
until about 12,000 years ago, but ultimately went extinct. The only large animal that was
domesticated in the Western hemisphere, the llama, did not spread far beyond the Andes
by the time of the arrival of Columbus. Nubians from after about 400 BC used wheels
for spinning pottery and as water wheels. It is thought that Nubian waterwheels may
have been ox-driven It is also known that Nubians used horse-driven chariots imported
from Egypt. The wheel was barely used in Sub-Saharan Africa
into the 19th century, only arriving with Europeans after they explored the region and
then moved to exploit it. The invention of the wheel thus falls in the
late Neolithic, and may be seen in conjunction with 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 and of pottery:
9500–6500 BC: Aceramic Neolithic 6500–4500 BC: Ceramic Neolithic, earliest
wooden wheels c. 4500 BC: invention of the potter's wheel,
beginning of the Chalcolithic 4500–3300 BC: Chalcolithic, earliest wheeled
vehicles, domestication of the horse 3300–2200 BC: Early Bronze Age
2200–1550 BC: Middle Bronze Age, invention of the spoked wheel and the chariot
Early wheels were simple wooden disks with a hole for the axle. Because of the structure
of wood, a horizontal slice of a tree trunk is not suitable, as it does not have the structural
strength to support relevant stresses without failing; rounded pieces of longitudinal boards
are required. The spoked wheel was invented more recently,
and allowed the construction of lighter and swifter vehicles. In the Harappan civilization
of the Indus Valley and Northwestern India, we find toy-cart wheels made of clay with
lines which have been interpreted as spokes painted or in relief, and a symbol interpreted
as a spoked wheel in the script of the seals, already in the second half of the 3rd millennium
BC. The earliest known examples of wooden spoked wheels are in the context of the Andronovo
culture, dating to c. 2000 BC. Soon after this, 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 was in continued use
without major modification until the 1870s, when wire wheels and pneumatic tires were
invented. The invention of the wheel has also been important
for technology in general, important applications including the water wheel, the cogwheel, the
spinning wheel, and the astrolabe or torquetum. More modern descendants of the wheel include
the propeller, the jet engine, the flywheel and the turbine.
Mechanics and function
The wheel is a device that enables efficient movement of an object across a surface where
there is a force pressing the object to the surface. Common examples are a cart pulled
by a horse, and the rollers on an aircraft flap mechanism.
Wheels are used in conjunction with axles; either the wheel turns on the axle, or the
axle turns in the object body. The mechanics are the same in either case.
The low resistance to motion is explained as follows:
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 help reduce friction at the interface. In the simplest and oldest
case the bearing is just a round hole through which the axle passes.
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 is 981 × 0.5 × 10 = 4905 joules. Now give the object 4 wheels. The normal force
between the 4 wheels and axles is the same 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.
This energy is also lowered by the use of a wheel because the net force on the contact
point between the road and the wheel is almost perpendicular to the ground, and hence, generates
an almost zero net work. This 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. Note that 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, 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 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, 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 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 he used.
Wire
The rims of wire 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.
Tyre
A tyre or tire 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. The fundamental materials of modern tyres
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 tyres were simply bands of metal that fitted
around wooden wheels to prevent wear and tear. Today, the vast majority of tyres 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 tyres
are used on many types of vehicles, such as cars, bicycles, motorcycles, trucks, earthmovers,
and aircraft. 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: Electromagnetic maglev trains
Sled or travois Hovercraft
A walking machine Caterpillar tracks
Pedrail wheels, using aspects of both wheel and caterpillar track
Spheres, as used by Dyson vacuum cleaners and hamster balls
Screw-propelled vehicle Symbolism
The wheel has also become a strong cultural and spiritual metaphor for a cycle or regular
repetition. 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 and the logo of the Ohio State
Highway Patrol. The introduction of spoked 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.
The wheel is also the prominent figure on the flag of India. The wheel in this case
represents law. It also appears in the flag of the Romani people, hinting to their nomadic
history and their Indian origins. See also
References
External links The Oldest Wooden Wheel. Virtual exhibition.
City Museum of Ljubljana. Accessed 31 January 2012.