A pulley is a simple machine that uses a wheel and rope or cable to change the direction of applied force, making it easier to lift heavy objects. There are three types of pulley systems, and the more wheels in a system, the more efficient it is. Pulleys are used in many modern settings, including elevators, car engines, sailboats, and cranes.
A pulley is a simple machine that was designed in ancient times but is still the basis of many modern machinery. It is essentially a wheel through which a rope or other cable is wound. One end of the rope is usually anchored to a weight or other load, and when the loose end is pulled—usually by a person, but possibly also by a machine—the wheel changes the direction of the applied force and makes it much more simple to lift the attached weight. This type of tool comes in different sizes and specific varieties, but in almost all cases the goal is the same – to make lifting heavy objects easier and to channel energy more efficiently.
Basic components
Engineers generally talk about “simple machines” in six different categories: inclined planes, levers, screws, wedges, wheels and axles, and pulleys. All are made of basic, everyday components and all make human work easier in one way or another. Pulleys are usually more concerned with lifting and moving large objects.
In its most basic form, this tool consists of little more than a wheel with a groove. A rope, belt or cable runs inside the groove. That mechanism can be used alone or connected to others in a pulley system. The more pulleys in the system, the less force is required to lift the object.
System types
There are three basic types of system: (1) fixed, with a fixed axis; (2) movable, with a free axis; and (3) compound, with a combination of fixed and moving axes. Moving and compound, force multiplying axes tend to be more efficient than fixed systems.
Wheel and rope tools can also be used in conjunction with other tools for more specific purposes. The so-called “belt and pulley system” is one example. These tend to be ‘closed’ systems in which several grooved wheels are connected by a connected circular belt and are designed for energy transmission and amplification rather than straight-line lifting force.
Lifting advantage
One of the biggest advantages of this type of machine is the ability to move heavy objects and channel lifting energy without exerting much force. In general, the heavier the object, the more force a person must exert to move it. Channeling that energy onto a string and around a wheel — or set of wheels, depending on the weight of the object &madash; it can make the overall effort, commonly known as the “lifting advantage,” much more efficient.
For example, a person will typically need to exert about 10 pounds (4.5 kilograms) of force to lift a 10-pound (4.5 kg) weight, and in most cases that force will need to be applied in an upward motion. If the weight is attached to a pulley, the person will still have to exert the same measured amount of force, but the direction in which it must be exerted changes. Once attached to the rope system, the person must pull down rather than lift up. This makes the most efficient use of the individual’s body weight and effort.
The more wheels there are in a given system, the more efficient it is likely to be. If the weight is attached to two systems, for example, the person would need to exert only 5 pounds (2.26 kg) of force. Four will require 2.5 (1.13 kg) of force and so on.
Common examples
These tools can be found in many modern settings. Elevators are perhaps one of the most common: While not seen from the outside, large systems of caster wheels and cables help cars move up and down. In most cases these are computer controlled and are lifted by mechanical means rather than manpower, but the efficiency is the same. Many car engines also depend on belt systems to help transfer energy from the engine to wheel shafts and other moving parts. Sailboats often use them to control the tension of the sails and they are also common in cranes and other machinery.
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