Wheel & Axle

A wheel turning on an axle lets a load roll instead of scraping along the ground, so the rubbing moves to a small, greased bearing. Wheeled carts appeared in Mesopotamia and Europe around 3500 BCE, and the same idea later turned winches, mills and waterwheels.

What it is

A wheel and axle is a disc fixed to, or turning on, a shaft. Under a cart it carries a load over the ground. On a well winch or a capstan it multiplies force.

Rollers under a sledge have to be carried round to the front again and again, while a wheel stays under the load. The hard part, which took people a long time, was making the wheel turn smoothly around a fixed axis.

The plate is a reconstruction of an early solid wheel built from three planks, a type found in many excavations in Europe and the Near East, under an ox cart of around 3000 BCE.

How it works

On rollers a load moves easily, but each roller left behind has to be carried round to the front again.

A dragged sledge presses its whole weight on its runners and scrapes over every bump. On wheels the rubbing happens in one small place, between the axle and the hole it turns in, while the rim rolls over the ground.

The gain comes from leverage, because the axle’s surface is much closer to the centre than the rim. On a wheel 70 centimetres across turning on an axle 7 centimetres thick, the rim has ten times the leverage, so the pull needed to overcome the axle’s friction is about a tenth of that friction. The rim still has to roll over the ground, which adds its own resistance on soft or rough roads. Grease, usually animal fat or tar, lowers the axle friction further.

An axle that is too thin breaks, and one that is too thick adds friction. The hole must be round and smooth, the axle snug without binding. On some early carts the wheels were locked onto the squared ends of the axle and both turned together, the rounded middle of the axle running in supports under the body. Most later carts had a fixed axle, with the wheels turning on its ends and held on by linchpins.

How to build

A solid wheel for a light handcart can be made with hand tools. The steps below are an outline, without measured drawings; a cart for heavy loads or draught animals needs a wheelwright’s experience.

Choose the wood. Use a hard, tough, well-seasoned timber such as oak, ash or elm. You need three planks about 5 centimetres thick, the middle one wider than the two side ones.

Cut the disc. Lay the planks edge to edge and mark a circle with a cord and a peg. Cut grooves across the grain and drive in two hardwood battens to lock the planks together, with wooden dowels between the planks. Then saw the outline to the circle.

Make the hub. The centre needs extra thickness to stop the wheel wobbling, so fix a turned block, the nave, on each face with glued hardwood pegs driven right through the disc. Bore the axle hole at right angles to the disc, then smooth it with a rasp or a heated iron rod.

Shape the axle. Hew a straight, knot-free piece of hardwood. Taper the ends to fit the holes with a little play and bore a hole through each tip, just beyond where the hub and washer will sit, for the linchpin. Bolt the axle under the cart bed with wooden blocks or iron straps.

Fit and test. Grease the ends, slide the wheels on, add washers and drive the linchpins home. Each wheel should spin freely and run true without rocking. Load the cart a little at a time and check the joints after each trial.

Safety. Adzes, axes and augers cause deep cuts: keep both hands behind the edge. A loaded cart on a slope can run away and crush whoever stands in front of it, so test on level ground and chock the wheels when you stop. If you burn out the axle hole with hot iron, keep water at hand.

History and evolution

A Bronze Age chariot wheel: six spokes and a rim of bent felloes, far lighter than a solid disc.

Between about 3600 and 3300 BCE, signs of the wheel appear over a wide area. Clay tablets from Uruk in Mesopotamia use a sign for a wagon. A pot from Bronocice in southern Poland is incised with what looks like a four-wheeled cart. Other finds come from the northern Caucasus and the steppes north of the Black Sea. Archaeologists still argue over where the idea began.

The oldest wheel that survives was found in 2002 in the Ljubljana Marshes in Slovenia. It is about 70 centimetres across, made of ash planks held by oak battens, and radiocarbon dates put it between about 3350 and 3100 BCE. Its oak axle, about 1.2 metres long, has square ends that locked into the wheel.

The anthropologist David Anthony has argued that the wheel came late because of tools. Shaping round holes and true axles needed cast copper chisels and gouges, which became common in the Near East only after about 4000 BCE.

Fitting a tyre: the iron ring is put on hot and shrinks as it cools, binding the wheel tight.

Solid wheels were sturdy but heavy, and suited slow ox carts. Around 2000 BCE people of the Sintashta culture, east of the Ural Mountains, built light wheels with spokes and fitted them to two-wheeled chariots pulled by horses. Spoked wheels and chariots spread to Egypt, Greece and China over the following centuries.

In the first millennium BCE, Celtic wheelwrights began to bind the rim with a single iron tyre, fitted hot so that it shrank and pulled the joints tight, a method used into the twentieth century.

The nineteenth century brought wire-spoked wheels in tension, light enough for the bicycle, and then rubber tyres filled with air. A railway axle still works the same way: the load rests on a small, lubricated bearing while the wheel rolls along the rail.