Wheel and Axle

The wheel and axle is one of the six simple machines and one of the most important inventions in human history. It consists of a wheel (a large round disc or circle) firmly attached to a smaller shaft running through its centre, called the axle. Turn the wheel and the axle turns with it (and vice versa). The combination multiplies either force or speed, depending on whether you turn the wheel to drive the axle or the axle to drive the wheel. From the earliest pottery wheels to the engines of modern cars, the wheel and axle has been the backbone of mechanical engineering for over 5,500 years.

  • What it isWheel attached to a shaftBoth turn together
  • First knownPottery wheel around 3500 BCCart wheels soon after
  • Big wheel turns small axleForce multipliedEasier turning at greater torque
  • Big axle turns small wheelSpeed multipliedFast spinning at lower force
  • Modern examplesCars, doorknobs, fans, gearsEverywhere in machines
  • Without itNo modern transportOr industrial machines

How it works

The trick is that the wheel and axle turn together, but they have very different radii. Whichever one you push, the force you apply gets multiplied (or divided) by the ratio of the radii.

  • Turn a big wheel (large radius) and the axle gets a bigger force over a shorter turn. Used to multiply force.
  • Turn the axle (small radius) and the wheel turns much faster. Used to multiply speed.

Why it makes work easier

Imagine trying to turn a thick metal shaft with your bare hands. Hard work. Now imagine that same shaft has a big steering wheel attached. Easy. The big wheel lets you apply force over a longer distance, multiplying your turning force (called torque) onto the small axle.

A doorknob is a small wheel-and-axle: you grip the knob (wheel) and turn it to twist the small shaft (axle) that releases the door latch. Without the knob, you would have to grip the bare shaft and twist with much more effort.

Fact The first known wheel was not a transport wheel: it was a pottery wheel. Ancient potters in Mesopotamia (modern Iraq) invented it around 3500 BC to spin clay smoothly while shaping pots. Wheels for transport (cart and wagon wheels) seem to have appeared a few centuries later.

Everyday wheels and axles

  • Car wheels: the wheel turns on the axle (or vice versa, depending on the design); engines deliver power through the axles.
  • Doorknobs: big knob (wheel) makes it easy to turn the latch shaft (axle).
  • Screwdrivers: the handle is a wheel; the shaft is an axle; together they multiply the small torque your hand can produce.
  • Steering wheels: a big wheel makes it easy to turn the relatively stiff steering shaft of a car.
  • Bicycle gears: combinations of wheels and axles let you trade speed for hill-climbing force.
  • Roller skates and skateboards: small wheels on tiny axles, optimised for low friction.
  • Ferris wheels and water wheels: enormous wheels on a central axle.
  • Electric fans: the central axle spins the blades (wheel) at high speed.
  • Hand drills: a turning handle (wheel) drives a smaller shaft (axle), giving high torque at the drill bit.

Why wheels for transport are special

Wheels for transport solve a problem that pure sledges and rollers do not. A sledge dragged along the ground has to overcome friction along its whole bottom surface. A roller works briefly but has to be moved every time the sledge slides off it. A wheel on an axle keeps rolling forever, and the only friction is between the small bearing surfaces at the axle joints.

A bullock cart with wheels could haul loads up to 10 times heavier than a sledge with the same animals pulling. That extra carrying power transformed early agriculture, trade and warfare. Cities could be supplied with food from further away. Armies could move faster. Goods could be traded over greater distances. The wheel changed the world.

Did you know? Native American civilisations of South and Central America never used the wheel for transport, even though they had wheeled toys and pottery wheels. The likely reason: there were no large draft animals (no horses, oxen or donkeys) to pull wheeled vehicles, and the mountainous landscape of the Andes made wheels of limited use compared to llamas as pack animals. The story shows that even a powerful technology needs the right setting to take hold.

Gears: connected wheels and axles

A gear is a wheel with teeth around its rim that interlocks with another gear. Gears are essentially pairs of wheels and axles that pass force between them.

  • A small gear driving a big gear: speed is reduced, but force (torque) is multiplied. Used in low gears for climbing hills.
  • A big gear driving a small gear: force is reduced, but speed is multiplied. Used in high gears for going fast on flat roads.

The gear systems in a bicycle let you change between climbing power and high speed. Car gearboxes do the same thing for engines: low gears at start-up for power, high gears at speed for fuel efficiency.

Try this Find a screwdriver and look at the handle. Notice how the wide rubber or plastic grip lets you turn it easily, even when the shaft is hard to twist. Compare this to twisting the shaft of the screwdriver itself with your fingers: much harder. The big handle is a wheel, multiplying the turning force you can apply to the small axle (the shaft) and through it to the screw. Without the handle, screwdrivers would be far less useful.
Deeper dive: how wheels changed history

The invention of the wheel and axle around 5,500 years ago was one of the biggest turning points in human history. Before wheels, people moved heavy goods by carrying them, dragging them on sledges, or floating them on rivers and seas. None of those methods could move goods far overland in a reasonable time.

The first cart wheels appeared in Mesopotamia and Eastern Europe around 3200 BC. Within a few centuries, wheeled vehicles had spread across the ancient world. Their effects rippled through every part of life:

  • Trade: wheeled wagons could carry goods 10 times further than human porters. Long-distance trade networks grew, connecting regions and spreading ideas.
  • Farming: wheelbarrows and animal-drawn carts let farmers move crops, manure and tools easily.
  • Cities: bigger cities became possible because food and water could be transported in from farms further away.
  • War: war chariots (Egyptian, Hittite, Greek) and later cavalry-supported wagons gave armies new advantages.
  • Travel: chariots, carts and (eventually) carriages let people travel faster and more comfortably.
  • Roads: civilisations like Rome built thousands of kilometres of well-paved roads to take advantage of wheeled vehicles.

The wheel kept transforming the world. Steam engines in the 1700s drove wheels in factories. Trains and cars in the 1800s and 1900s let humans travel faster than ever. Today, even the most advanced electric cars and bullet trains rely on wheels turning on axles, exactly the same simple machine that Mesopotamian potters invented over 5,500 years ago.

For more, see lever and pulley.