Pulleys & Belts

Mr BILLINGTON

Mechanical Devices - Pulleys and Belts

Pulleys are often used to gain a mechanical advantage against gravity to lift forces larger than those applied. They equally can be used to transfer motion into different directions within a system. A pulley is made up of a 'Block and Tackle' whereby the block is the pulleys and tackle often rope or cable

The use of a single pulley will require the same effort force as the load lifted (100N) and gain no mechanical advantage other than the users body weight. As further pulleys are added to the system an advantage is achieved over the load lifted with 4) being the most efficient (25N)

The trade off in a pulley system is distance. The advantage is gained through increasing the length of rope over the pulleys to allow load height to be increased. Each additional pulley gives a mechanical advantage which is equal to the total number. As shown above;

1) 1 100><100 2) 1/2 50><100 3) 1/3 33><100 4) 1/4 25><100

The strength of ceiling, as shown in the horizontal arrangement, can be used as the fourth pulley with the 100N load shared across the rope. In an ideal world a greater advantage could be increased by the addition of further pulleys, however, friction and pulley weight in fact increase resistance and reduce the efficiency of the system.

Using pulleys the 3 classes of lever can be recreated as shown by replacing the fulcrum and a using vertical 'block' fixing point

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Pulleys can be combined with belts to transfer motion between each other simply without the need for meshed toothed gearing. The principal remains the same that there is a driver (powered) and driven (free). Equally, the use of gear ratios is applicable as well by comparing pulley size rather than tooth number.

Velocity Ratio = Driven diameter / Driver diameter

Output Speed = input speed ÷ velocity ratio

Output Torque = input torque × velocity ratio

The advantage of using belts, which are often manufactured from synthetic fibres, is that they require no lubrication. Chains are, however, less likely to slip on the pulleys but belts can be twisted to reverse the direction of motion from input to output. To improve grip, grooves can be added to the belt & pulleys, or slots that mesh with with geared pulleys can be used

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Within a cars engine system belts and pulleys are used to transfer kinetic energy from the engine to various parts of the car. The timing belt is responsible for transferring rotary motion from the crankshaft to the cam shaft which open and closes valves sync with the engines pistons. If slipped or snapped it could incorrectly open valves which can be damaged by the engines pistons

The Serpentine belt, or drive belt, transfers power to; the compressor, air conditioning, power steering pump, cooling fan , injection pump and more. Failure of this can result in the engine over heating and a total loss of power