Wheels, pulleys and gears

1. Wheels, pulleys and gears

2. The Wheel • Possibly the most important mechanical invention ever made. • From the tiny gears in a watch to cars and computers, nearly every machine constructed since the Industrial Revolution (in the early nineteenth century) uses a wheel in its design. • Earliest known application was a potter’s wheel, used in Mesopotamia (modern day Iraq) around 3500 BC. • Probably first used for transportation on chariots in this region around 3200 BC.

3. Wheels can be levers • A wheel on an axle is actually a special kind of lever. • The wheel acts as a lever and the pivot is the centre of the axle. • Wheels can act as force multipliers or speed multipliers. • Steering wheels are force multipliers, the larger the steering wheel, the easier the axle will be to turn. • Car wheels operate as speed multipliers in the opposite manner to the steering wheel. A large force is supplied to the axle of the wheel by the motor which makes the wheel move faster.

4. Pulleys • A pulley is like a wheel with a groove around it into which a cable or rope can fit. • It also acts like a lever, with its axle as the pivot. • Simple pulleys allow us to lift upwards by pulling downwards. • They do not alter the size of the force applied, just the direction.

5. Pulleys • To increase the size of the force, we need to use a system involving more than one pulley. • We need to apply a force over a greater distance, but multiple pulleys allow us to lift large loads with minimal effort. They act as a force multiplier. • The multiple pulley system shown in the picture is called a ‘block and tackle’. • The ‘block’ is the outer casing around the pulley wheels, and the ‘tackle’ is the cable or rope connecting them.

6. Using pulleys • To calculate the effort required to lift the load we divide the load by the number of ropes (do not count the rope that goes to the effort). • The image on the right shows a four pulley system. • The person lifting the 200kg load experiences a pull equal to only 50kg (200kg/4).

7. Pulley question 1 • What is the maximum load that can be lifted with this system? • For every 2 metres the rope is pulled through what height does the load rise off the ground? • What is the mechanical advantage?

8. Pulley question 2 • What is the minimum effort that must be applied to lift the load? • For every 2 metres the rope is pulled through what height does the load rise off the ground? • What is the mechanical advantage?

9. Pulley question 3 • What is the maximum load that can be lifted with this system? • For every 2 metres the rope is pulled through what height does the load rise off the ground? • What is the mechanical advantage?

10. Gears • Gears or cogs are like wheels with teeth that mesh together. • When one gear turns, the one it interlocks with also turns, but in the opposite direction. • Sometimes gears are joined by a chain, such as in a bicycle or the overhead cam shaft in some car engines.

11. Driving and Driven Gears • The cog that turns first is called the driving gear. • Gears that are made to turn by this are called driven gears. • If the driving gear is smaller than the driven gear, the system acts as a force multiplier. • If the driving gear is larger than the driven gear, then the system acts as a speed multiplier.

12. Idler Gears • An idler gear can be used in a gear combination to make the driving and driven gears rotate in the same direction.

13. Bevelled Gears • Bevelled gear wheels are positioned at right angles to each other • This changes the plane of rotation. • Used in hand-drills and in many other machines.

14. Rack and pinion gears • Rack and pinion gears are made up of a row of teeth (called the rack), and a gear wheel that rolls on top of this. • One use of this type of system is to stop mountain trains slipping on steep slopes.

15. Bicycle Gears • A bike with twenty-one gears gives us a choice of twenty-one possible gearing combinations. • The front chain wheel has three sprockets and the rear wheel has seven. • To select the lowest gear, we use the largest rear sprocket and the smallest chain wheel sprocket. • This combination of gears makes it easier to go up hills. • To select the highest gear, we use the largest chain wheel sprocket and smallest rear wheel sprocket. • The combination of a large front sprocket and a small rear sprocket makes the bike go faster.

16. Bicycle Gears • The gears used in a bicycle are called sprockets. • The pedal and crank are attached to a gear sprocket called the chain wheel. • A chain connects this to the rear sprocket. • Pedalling turns the chain wheel, which turns the rear wheel, via the chain.

17. Gear Ratios • Are calculated by dividing the number of teeth on the driving sprocket by the number of teeth on the driven sprocket. GR = teeth on driver teeth on driven gear Example: • If the driving gear has 9 teeth and the driven gear has 3 • GR=9/3=3

18. Gear ratio question 1 • Gear "B" has 36 teeth and gear "A" has 12. • Calculate the gear ratio of this gear train if "B" is the driver.

19. Gear ratio question 2 • Look at the image on the right. Gear "B" has 12 teeth and gear "A" has 24. • Calculate the gear ratio of this gear train if "B" is the driver.