Simple pulley . This is the name given to the mechanical pulling or lifting device formed by a wheel or pulley mounted on an axle, with a rope that surrounds the circumference of the wheel. Both the pulley and the wheel and axle can be considered simple machines that constitute special cases of the lever .
Summary
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- 1 Definition
- 2 Features
- 3 History
- 4 types
- 1 Fixed pulleys
- 2 Moving pulley
- 3 Composite pulleys or hoist
- 5 Sources
Definition
To drag some heavy objects, a wheel is used that turns freely on an axis, and that is provided with a rim with an appropriate shape to pass a rope or similar object. This mechanism is what is known as a pulley, which constitutes one of the special cases of the lever , and belongs to the set known as simple machines. The advantage it provides us is to facilitate the application of force. From their different types, different combinations are achieved, depending on the activity for which they are intended.
characteristics
One of the simple machines is a pulley, garrucha, carrucha, trocla, trocó or carrillo, a wheel, generally solid and grooved, that with the course of a rope is used as a transmission element in machines and mechanisms, to change the direction of movement or its speed and forming sets (rig or hoist); to also reduce the magnitude of the force necessary to move a weight. According to Hatón de la Goupillière’s definition “the pulley is the point of support of a rope, which, moving, coils over it without giving a complete turn”, acting at one end of the resistance and at the other the power.
The pulley is mainly used to transmit movements or to lift loads. The shape of the wheel grooves changes depending on the type of object that will pass through them. For this reason, they can be semicircular in section, for the passage of cables or ropes; trapezoidal, in the case of belts with this shape; and honeycomb, for the passage of chains.
History
It is not known who invented the pulley or when; The only historical note on its use is due to Plutarch who in his work “Parallel Lives” ( 100 BC ) relates that Archimedes , in a letter to King Hieron of Syracuse -who was united by great friendship-, stated that with a given force he could move any weight and even bragged that if there was another Earth going to it it could move this one. Astonished, Hieron asked Archimedes to hold a demonstration, both agreeing that the object to be moved would be a ship of the king’s army, since Hieron believed that it could not be removed from the dock and taken to a dry dock, without the use of a great effort and numerous men.
According to Plutarch, after loading the ship with many passengers and with the hold full, Archimedes sat at a certain distance, and pulling the rope dragged the ship without great effort, pulling it out of the water as straight and stable as if it were still at sea.
Types
According to their displacement, the pulleys are classified into:
- Fixed: those that are suspended from a fixed point and, therefore, do not undergo any translation movement, when used.
- Mobiles; which are those in which one end of the rope is suspended from a fixed point and which, during its operation, generally move vertically.
- Compound or hoist: when the pulley is joined with others – forming a system – it is called combined or composite.
Fixed pulleys
This type of machine hangs from a fixed point and although it does not decrease the force exerted, which is equal to the resistance , it facilitates many jobs.
The fixed pulley simply allows a better position to pull the rope, as it changes the direction and direction of the forces. For example, in a well a bucket full of water can be raised more comfortably by pulling down instead of lifting it.
Mobile pulley
In this mode, the pulley is attached to the object and can move vertically along the rope. In this way, the force is multiplied, since the load is supported by both rope segments; The more movable pulleys a set has, the less effort is needed to lift a weight. The driving force used to lift a load is half the resistance, even if it requires twice the distance to pull the rope.
Composite pulleys or hoist
This type of machine is also called rigging and is used to lift heavy weights with moderate effort. The hoist is made up of a system of fixed and mobile pulleys, thereby achieving the effects of both. In each group, an arbitrary number of pulleys are installed. The load is attached to the mobile group.
