Pulleys
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[edit] So what is a pulley exactly?
A Pulley is a simple machine in which a rope or belt is passed through a grooved wheel or set of wheels in order to lift a load. Pulleys are usually mounted in either metal or wooden frames which are called blocks. The blocks together with the ropes are called tackles, which are used to lessen the effort it takes to move a load. The more pulleys used in a system, the less effort will be required to complete the job and the shorter the distance the load will have to travel. Pulleys are used in many industries where lifting is required such as shipping, farming, and manufacturing, as well as in automobile engines.
[edit] History of a pulley
As in news the case with all the simple mchines, the origin of the pulley is known. When early people lifted heavy objects by throwing vines or other crude ropes over tree limbs, they used the idea of a single fixed pulley to change the direction of a force. But since there was no wheel to turn, this use resulted in considerable friction. It is believed that by 1500 bc people in Mesopotamia used rope pulleys for hoisting water. Legend has it that the Greek inentor Archimdes (287-212 bc) used a block and tackle system to pull ships onto dry land.
Modern pulley systems are often combine with motors to create hoists for lifing heavy loads. By using a motor, the user only has to push a button to lift or lower the load. Costruction cranes and cranes used at shipyrds move heavy loads using block and tackle systems connected to powerful motors. An elvator in a building uses a pulley system to raise and lower the elevator cars.
[edit] There are three basic types of pulleys
[[Media:
[['===='Simple (fixed) pulley:==
A simple pulley is designed as a grooved wheel on a fixed axle. When using a simple pulley to a load it is simply a rope passing over the grooved wheel, with the other end of the rope tied to the load. A pull on the other end of the rope is then needed to actually lift the weight. The simple pulley does not offer a mechanical advantage, but it does aid in changing the direction of the applied force on the load. A simple pulley is illustrated below.
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[edit] Movable Pulley:
In this pulley, the lifted load is joined to the axle. It is designed in a way that one end of the rope is passed directly through the pulley; this end is attached to a stationary support which is located above the load. When using this pulley, the pull is directed on the unattached end of the rope, continuing in the same direction where the load is to be moved. There is a mechanical benefit for this type of pulley. The pull that is used on the unattached/free end of the rope needs to be only half the weight of the actual load. The rope that is joined to the stationary support only bears half the weight of the load as well. A movable pulley is illustrated below.
[edit] Compound Pulley:
A compound pulley is a combination of the movable and the simple pulley. It is comprised of the block and tackle apparatus. A compound pulley is illustrated below.
[edit] The Pulleys Within!!!!:
One of the most interesting appplications of a pulley system is within our own bodies. Our muscles are designed to move when stimulated by a motor nerve impulse. But the direction and control of the movement of particular muscles actually gets determined by a pulley system, much like the ropes and wheels described above, where there are fixed tendon connections to bony elements of the skeleton. This is very apparent in the hand, especially the muscle that flexes or bend our fingers to make a fist. The muscle is actually located quite a distance up in the forearm, and the tendons come down ( acting as the ropes), and get attached by sheaths over four joints ( the wrist, and three district finger joints) which function as the grooved wheels. Contracting the muscle will cause the hand pulley system to work to effectively flex and bend each individual joint until you can make a full fist.
Below is a color coded chart of the hand - for the hand pulley system to work to flex the hand to make a fist , tendons (ropes) have to pass through the all the joints labeled on the right side of the image ( CMC, MCP, PIP, DIP).
Below on the left you can now see the orange tendon as it attaches along various parts of the finger - if the pulley system is working once you contract your muscle your finger will flex starting at the C3 joint, then the C2 and C1 joint until you have a full fist. Anyone who has suffered a hand injury, and severs a tendon can tell you what happens when part of the pulley system is destroyed - this often leads to an inability to make a full fist. The image on the right shows a dissection of the pulley system of the finger.
[edit] SOURCES:
www.pt.ntu.edu.tw/.../ Kines04/KINupper/Hand.htm
www.vandemarkortho.com/ patient/pated/hand/han
Barron's Review Course Series: Physics- The Physical Setting by Miriam A. Lazar Barron's Educational Series, Inc. 2004
Brunnstrom's Clinical Kinesiology by Sydney Brunnstrom. Davis F A, Inc. 1996
Physical Setting: Physics Stareview by Nancy A. Moreau. N & N Publishing, Inc. 2002
Concise Encyclopedia Science by Neil Ardley. DK Publishing, Inc. 1994
