Machines: Pulleys

• A pulley is a basic device or machine made of a wheel with a rim that a chain/cord/rope fits around
• A single fixed pulley as shown below is used to change the direction of effort

Single pulley

• The pulley makes it easier to lift a load by changing the direction in which an effort is applied to a more convenient direction
• In this case pulling instead of lifting
• The single fixed pulley has a velocity ratio of 1
• It would also have a mechanical advantage of 1
• However because of friction the effort is always bigger than the load
• A movable pulley can also be used instead of a fixed pulley
• A single movable pulley has a velocity ratio of 2
• This means for example the effort has to move 2m to lift the load 1m
• However the pulley requires an effort to be applied in an inconvenient direction

Block and tackle pulley.

• If the two pulleys (the movable and fixed pulley) are combined
• The required effort is reduced and the effort can be applied in a more convenient direction
• This type of an arrangement is known as a block and tackle

Block and Tackle pulley

• More and more of these pulleys can be arranged in blocks of fixed and movable pulleys
• Pulleys can also be arranged side by side
• Each of the fixed pulleys in the system merely changes the direction of the applied force
• Each of the movable pulleys increases the mechanical advantage of the system by two
• This is when friction and the weight of the blocks below are not taken into account
• Each movable pulley also increases the velocity ratio by 2.

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