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A pulley is a wheel with two raised edges so that a rope or a string will run along the wheel without coming off. It's often also called a block and tackle. Pulleys may have been invented by Archimedes in ancient Sicily, about 250 BC. A pulley can be used in several different ways to make it easier to pull a rope, to change the direction of a force, or to get more mechanical advantage and lift something heavier than you can lift by yourself. Pulleys are examples of what scientists call simple machines. That doesn't mean they're packed with engines and gears; it just means they help us multiply forces. If you want to lift a really heavy weight, there's only so much force your muscles can supply. But use a simple machine such as a pulley and you can effectively multiply the force your body produces. The simplest theory of operation for a pulley system assumes that the pulleys and lines are weightless, and that there is no energy loss due to friction. It is also assumed that the lines do not stretch. In equilibrium, the forces on the moving block must sum to zero.
In addition the tension in the rope must be the same for each of its parts. This means that the two parts of the rope supporting the moving block must each support half the load. Their main function is to change the direction of the tension force in a rope. The pulley systems almost always consist of idealized, massless and frictionless pulleys, and idealized ropes that are massless and that don’t stretch. To lift mass m at a constant velocity without a pulley, you would have to apply a force equal to the mass’s weight, or a force of mg upward. Using a pulley, the mass must still be lifted with a force of mg upward, but this force is distributed between the tension of the rope attached to the ceiling, T, and the tension of the rope gripped in your hand.
Because there are two ropes pulling the block, and hence the mass, upward, there are two equal upward forces, F and T. We know that the sum of these forces is equal to the gravitational force pulling the mass down, so F + T = 2F = mg or F = mg/2. Therefore, you need to pull with only one half the force you would have to use to lift mass m if there were no pulley.
There are 3 types of pulley:
Fixed Pulley: A fixed pulley is the only pulley that when used individually, uses more effort than the load to lift the load from the ground. The fixed pulley when attached to an unmovable object e.g. a ceiling or wall, acts as a first class lever with the fulcrum being located at the axis but with a minor change, the bar becomes a rope
Movable Pulley: A movable pulley is a pulley that moves with the load. The movable pulley allows the effort to be less than the weight of the load. The movable pulley also acts as a second class lever. The load is between the fulcrum and the effort.
Combined Pulley: A combined pulley makes life easier as the effort needed to lift the load isless than half the weight of the load. The main advantage of this pulley is that the amount of effort is less than half of the load.
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Connective tissue bands connect the horizontal rectus muscles at the level of the posterior pole to the orbital wall. These bands, referred to as pulley bands or faisseaux tendineux, purportedly act like springs to keep the rectus muscle bellies in place during eye movement out of the plane of the muscle. We examined the mechanical properties of these bands in human specimens obtained during surgery. In addition, we examined eye motility and stability of rectus muscles in a patient who had no functional pulley bands.
The pulley bands showed leash-like mechanical behaviour: they were slack over approximately 10mm and became taut when stretched further. In the patient with Crouzon's syndrome, both CT and observation of the muscle during surgery showed little sideways displacement of the muscle bellies in eye movement out of the plane of the muscle, despite the lack of functional pulley bands.
Japan is in the lead in terms of labor productivity (defined as value added per employee that can be taken as an indication for price competitiveness), closely followed by the US after China. Third in this ranking is the EU-27. This could be caused by heterogeneity within the EU-zone, which includes member states with substandard economic performance. However, intra-EU regional differentiation discloses that none of the Member States comes close to the US or Japan. For the EU-countries under investigation, The US ME has the highest wages per employee, with about 20% above the EU average. Despite much higher labor productivity, Japan’s wages are only close to those in the EU. China lags far behind, with wages of 11% of the EU average.
2nd Biomedical Engineering Conference
November 30-December 02, 2015 | San Antonio, USA
List of major Associations and Societies:
National Society of Professional Engineers
Indian Society of Mechanical Engineers
Institute of Industrial Engineers (IIE)
The Japan Society of Mechanical Engineers
Institution of Mechanical Engineers
The Association of Mechanical and Aeronautical Engineers (AMAE)
Mechanical Engineering Honor Society
Mechanical Engineers Australia: MEA
List of Companies:
Aetna Bearing Company
Sparks Belting Company
GG Manufacturing Company
Clark Pulley Industries
Reeves Pulley Company
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This page was last updated on 14th Sep, 2015