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Pulleys A pulley is a simple mechanical machine and consists of a wheel that turns. The wheel is usually grooved for a rope or a wire cable. There are three types of pulleys:  Single fixed pulley.  Single movable pulley.  Compound pulley system.

Diagrams of Pulleys Single fixed pulley: 2.htm Single movable Pulley: 3.htm

How does a pulley work? A pulley works two ways. It can change a)the direction of a force or b)the amount of force.

1. How does a fixed pulley work? -Wheels attached to surface -Changes the direction of the applied force -No mechanical advantage (same amount of force is required) Example: Raising a flag

2. How does a moveable pulley work? -Pulley moves along the rope -Wheel supports the load -Effort is the same direction as movement -Reduces the forces needed to move an object

3. How does a compound pulley work? -It has at least two wheels (one movable and one fixed) -The more complex the pulley, the effort needed to move the object decreases. -Meaning, as you increase the number of supporting ropes, mechanical advantage also increases.

Stop here If you want to read more about the Pulley system, please continue to the next slide.

1. Single Fixed Pulley This pulley provides the user Directional Advantage, allowing someone to pull down to lift the load up. Is Mechanical Advantage provided with this system? No, the effort needed to lift this load is equal to the weight of the load. This is because the load is supported by only 1 rope arm. You must also pull an amount of rope equal to the height you wish to lift the load

2. Single Moveable Pulley This pulley system provides Mechanical Advantage, requiring only ½ the effort needed to lift the weight of the load. If the load weighs 20 lbs, only 10 lbs of effort is needed to lift it. Can you tell why? The load is supported by 2 rope arms under the pulley. Each load arm supports half the weight of the load, so the arm pulling up uses only 10 lbs of effort to lift this load. You must pull twice as much rope as desired to lift the load a certain height. Example: Pull 2 ft of rope for each 1 foot of height that you want to lift the load.

3. Compound Pulley System – a block and tackle (Single Fixed/Single Moveable Pulley) This pulley system combines the properties of their single components into a system that provides both Mechanical and Directional Advantage. Do you see how this happens? How much rope must you pull? This pulley provides Directional Advantage by allowing the user to pull down to lift the load. It also provides Mechanical Advantage by using 2 rope arms to support the load, reducing the effort needed to lift the load by ½. In this system you pull 2 times as much rope for every unit of height that you want to lift the load.

 A single fixed pulley: It changes the direction of the force acting on it and its magnitude remains the same on either sides of the pulley rope irrespectively to the angle of pull of the force. It’s mechanical advantage is (1).  A single movable pulley: It acts as a second-class lever. It’s mechanical advantage is (2) as it magnifies force.  Pulley combination: There are many combinations and the simplest one is a combination of one fixed and one movable pulley as used in the Guthrie-Smith suspension frame. The mechanical advantage of pulley combination will be equal to the number of strands supporting the movable pulley.

Pulley Power! Single-Fixed Pulleys provide or Directional Advantage. You can make use of Counter-Weights in order to reduce your effort needed to lift the load. However, your effort PLUS the the counter-weight together must be equal to or be more than the weight of the load being lifted. Single-Moveable Pulleys provide Mechanical Advantage. You must pull 2 times the distance in rope that you wish to lift the load because the load is supported by 2 rope arms. You gain a Mechanical Advantage of 2 because you are using only ½ as much effort to lift the load. Combining Single-Fixed and Single-Moveable into Compound Pulley Systems allows the user to gain both Mechanical and Directional Advantage. The more rope arms supporting the load the greater the Mechanical Advantage.