- A fixed pulley is attached to a stationary support, whereas a movable pulley is attached to the load. The latter type affords some mechanical advantage. The former just changes the direction of the tension force. Fixed and movable pulleys don't necessarily constitute pulley systems on their own, but in combination, they can make interesting and useful configurations.
- Mechanical Advantages of Block and Tackle Hoists
Pulleys get especially interesting when fixed and movable pulleys are combined to form compound pulleys. Sometimes fixed and movable pulleys are combined into blocks of pulleys that share the same shaft. The movable block moves with the load, while the fixed block is attached to a support.
The distance between the two blocks shortens the same distance the load is lifted. To see this, suppose the load rises one foot. The attached movable block must rise one foot. Therefore, the lengths of line between the (load-bound) movable and (support-bound) fixed blocks must each shorten by one foot. The puller must pull out a lot more than the one foot that the load rose. Like a lever, gaining mechanical advantage is a matter of inputting a smaller force over a greater distance than the load itself experiences. - A pulley can be either loose or fast, i.e., affixed to the shaft or not. If a fast pulley and a loose pulley are next to each other on the same shaft (but not affixed to each other), then switching a continuous belt back and forth between them allows stopping a machine without turning the motor off. In other words, the shaft is connected to some motor, which drives the fast pulley continuously. The wheel to which the fast and loose pulley are attached is rotating or not rotating, depending on whether it's attached to the fast or loose pulley.
- Differential Pulley
The differential pulley is a hoist commonly found in car mechanic garages. It's a block and tackle hoist with two pulleys in the fixed block fixed to each other. A chain line from the load is taken up on either side of the two-pulley block. Since they are fixed to each other, and the line pulls them from both sides, the load doesn't drop. A small difference in the radii of the two pulleys creates mechanical advantage. - Serpentine Belt
Car engines must distribute their power to other systems in the car--e.g., the alternator, the a/c compressor, the air compressor, the water compressor and the alternator. A serpentine, continuous belt can connect several pulleys so that the engine drives them all. An additional pulley may be thrown in, attached to a spring, to maintain tension and avoid slippage of the belt. A serpentine belt may even be disconnected from the a/c compressor in cooler weather to reduce drag and save fuel.
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