How do belts transfer power between wheels?

1. The motor is spinning, but the fan is too far away to touch it. We need something to carry the turning from one wheel to the other.
2. Loop a rubber belt tightly around both wheels. The motor's wheel grips the belt and drags it along. The belt grips the fan's wheel and pulls it round.
3. When both wheels are the same size, each turn of one gives one turn of the other. They spin at exactly the same speed.
4. Now a big wheel drives a small one. One turn of the big wheel pulls a lot of belt, enough to spin the small wheel twice. The fan goes faster!
5. Swap them around. A small wheel driving a big one turns it slowly, but with a stronger twist. That is good for heavy work.
6. A belt only works if it grips. Too loose, and it slips and squeals while the fan slows down. Slide the motor back to tighten the belt, and it grips again.

Tap each part. Follow the turning from the motor to the fan!

• Motor. The electric motor supplies the turning. It spins its shaft at a steady speed.
• Driver pulley. The wheel on the motor is called the driver pulley. It grips the belt and drags it along.
• Belt. The belt is a loop of tough, slightly stretchy rubber. It carries the pull from one pulley to the other.
• Driven pulley. The wheel that the belt turns is called the driven pulley. Its size, compared with the driver, sets how fast it spins.
• Sliding base. The motor sits on a sliding base. Sliding it away from the fan stretches the belt tighter, so it grips better.
• Fan. The fan is the machine being driven. The faster the driven pulley turns, the faster the fan spins.

Change the size of each pulley and watch the fan's speed. Then slacken the belt, add a load, and see when it slips.

Experiment: A big wheel drives a small wheel. How does the small wheel turn?

1. Make the motor's pulley big and the fan's pulley small. Watch the fan.
2. Now make the motor's pulley small and the fan's pulley big.
3. Slacken the belt until it slips. What happens to the fan?

What we found: A big wheel driving a small one makes it spin faster, because each turn of the big wheel pulls enough belt to turn the small wheel more than once. And a belt that is too slack slips, so the fan slows down.

Turn on JavaScript to try it yourself.

Make the fan turn at 600 turns a minute with the load on, and without the belt slipping.

A belt drive is two wheels, called pulleys, joined by a loop of belt. The belt is stretched tight, so it presses on the pulleys and grips them by friction. When the motor turns the first pulley, the belt is dragged along, and it pulls the second pulley round.

The clever part is the size of the pulleys. The belt moves at one speed all the way round. A big pulley has a long rim, so one turn pulls a lot of belt. If that belt goes round a small pulley, the small pulley has to turn several times to use it all. Big driving small means faster.

Turn it around and a small pulley driving a big one makes it turn slowly. But you get something in return: a stronger twist. Machines use this to trade speed for strength, or strength for speed.

Belts are quiet and cheap, and they can slip a little if a machine jams, which protects it. But they must be tight enough to grip. A bicycle chain is a good comparison, with one difference: a chain has teeth, so it never slips, while a belt holds on only by rubbing.

Key words

Pulley

A wheel with a groove for a belt.

Driver and driven

The driver pulley is turned by the motor. The driven pulley is turned by the belt.

Speed ratio

How many times the driven wheel turns for one turn of the driver.

Tension

How tightly the belt is stretched.

Slip

When the belt slides over a pulley instead of gripping it.

Check your thinking

1. How does a belt make the second pulley turn?

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   It grips the pulley by friction and pulls it round. The stretched belt presses on the pulley, and friction lets it drag the pulley round.

2. A big pulley drives a small one. What happens?

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   The small pulley turns faster. One turn of the big pulley pulls enough belt to turn the small one more than once.

3. A 10 cm driver turns at 300 rpm and drives a 15 cm pulley without slip. How fast does the driven pulley turn?

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   200 rpm. Driven speed = 300 × 10 ÷ 15 = 200 rpm. A larger driven pulley turns more slowly.

4. The belt is slipping and squealing. What should you do?

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   Tighten the belt so it grips. A tighter belt presses harder on the pulleys, so friction can grip.

5. A small pulley drives a big one. The big one turns slowly. What do you gain?

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   A stronger twist for heavy work. Slower turning comes with a stronger twist, which is useful for heavy jobs.