The constituent elements of a pulley are the wheel or pulley itself, in whose circumference (rim) there is usually a groove called "throat" or "pocket" whose shape adjusts to that of the rope in order to guide it; the "arms", an inverted U-shaped or rectangular frame that completely surrounds it and at the upper end of which mounts a hook by which the assembly is suspended, and the "axis", which can be fixed if it is attached to the arms with the pulley crossed through it ("eye pulleys"), or mobile if it is integral with the pulley ("axis pulleys"). When, as part of a transmission system, the pulley rotates freely on its axis, it is called "idle".

According to their displacement, the pulleys are classified as "fixed", those whose arms are suspended from a fixed point (the structure of the building) and therefore, do not suffer any translational movement when used with "mobile", which are those in which one end of the rope is suspended from a fixed point and which during their operation generally move vertically.[9]​.

When the pulley works independently it is called "simple", while when it is joined with others forming a system it is called "combined" or "compound".

Contenido

Existen sistemas múltiples de poleas que pretenden obtener una gran ventaja mecánica, es decir, elevar grandes pesos con un bajo esfuerzo. Estos sistemas de poleas son diversos, aunque tienen algo en común, en cualquier caso, se agrupan en grupos de poleas fijas y móviles: destacan los polipastos:.

Hoists or rigging

The hoist (from Latin polyspaston, and this from Greek πολύσπαστον), is the most common configuration of compound pulley. In a hoist, the pulleys are distributed in two groups, one fixed and one mobile. An arbitrary number of pulleys are installed in each group. The load joins the mobile group.

Un sistema de cuerda y polea, es decir, un bloque y aparejo, se caracteriza por el uso de una sola cuerda continua para transmitir una fuerza de tensión alrededor de una o más poleas para levantar o mover una carga; la cuerda puede ser un línea ligera o un cable fuerte. Este sistema está incluido en la lista de máquinas simples identificadas por los científicos del Renacimiento.[10]​[11]​.

Si el sistema de cuerda y polea no disipa ni almacena energía, entonces su ventaja mecánica es el número de partes de la cuerda que actúan sobre la carga. Esto se puede demostrar de la siguiente manera.

Considere el conjunto de poleas que forman el bloque móvil y las partes de la cuerda que soportan este bloque. Si hay p de estas partes de la cuerda que soportan la carga W, entonces un balance de fuerzas en el bloque móvil muestra que la tensión en cada una de las partes de la cuerda debe ser W/p. Esto significa que la fuerza de entrada en la cuerda es T=W/p. Por lo tanto, el bloque y el aparejo reducen la fuerza de entrada por el factor p..

Operation method

The simplest theory of operation for a pulley system assumes that the pulleys and lines are weightless. And that there is no loss of energy due to friction. The lines are also not supposed to stretch.

In equilibrium, the forces on the moving block must sum to zero. Furthermore, 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.

There are various types of pulley systems:

The mechanical advantage of the gun rig can be increased by interchanging the fixed and moving blocks so that the rope is attached to the moving block and the rope is pulled in the direction of the lifted load. In this case, blocking and tackling is said to be "advantage shooting".[12]​ Diagram 3 shows that three parts of the rope now carry the load "W", which means that the tension in the rope is "W/3". Therefore, the mechanical advantage is three.

By adding a pulley to the fixed block of a gun rig, the direction of the pulling force is reversed, although the mechanical advantage remains the same, Diagram 3a. This is an example of the Luff tackle.

Free body diagrams

The mechanical advantage of a pulley system can be analyzed using free-body diagrams in which the tension force in the rope is offset by the force of gravity acting on the load. In an ideal system, massless, frictionless pulleys dissipate no energy and allow a change of direction of a rope that does not stretch or wear out. In this case, a balance of forces on a free body that includes the load, W and n supporting sections of a rope with tension T, gives:.

The relationship between the load and the tension force exerted is the mechanical advantage MA of the pulley system,[13]​.

Therefore, the mechanical advantage of the system is equal to the number of rope sections supporting the load.

A belt and pulley system is characterized by having two or more pulleys in common with one belt. This allows mechanical power "Power (physical)"), torque and speed to be transmitted through the axles. If the pulleys have different diameters, a mechanical advantage is obtained.

A belt drive is analogous to a chain drive; However, a belt pulley may be smooth (devoid of discrete interlocking elements such as would be found in a chain sprocket, spur gear, or timing belt), so the mechanical advantage is given approximately by the pitch diameter ratio of the pulleys only, not exactly fixed by the tooth ratio as in the case of gears and sprockets.

In the case of a drum-type pulley, without a groove or flanges, the pulley is usually slightly convex to keep the flat belt centered. Sometimes called a crown pulley. Although once widely used on factory line axles, this type of pulley is still found driving the rotating brush on upright vacuum cleaners, belt sanders, and band saws. Farm tractors built until the early 1950s typically had a pulley for a flat belt (which is what Belt Pulley magazine is named after). It has been replaced by other mechanisms with more flexibility in methods of use, such as power take-off") and hydraulics").

Just as the diameters of gears (and, consequently, their number of teeth) determine a transmission ratio and therefore the increases or reductions in speed and the mechanical advantage they can provide, the diameters of pulleys determine those same factors. Conical pulleys and stepped pulleys (which work on the same principle, although the names tend to be applied to flat belt versions and V-belt versions, respectively) are a way of providing multiple gear ratios in a belt and pulley system that can be changed as needed, just like a transmission&action=edit&redlink=1 "Transmission (mechanical) (not yet written)") provides this function with a gear train") that can be changed. Stepped pulleys with V-belts are the most common way that drill presses provide a range of spindle speeds.

With belts and pulleys, friction is one of the most important forces. Some uses of belts and pulleys involve peculiar angles (leading to poor belt tracking and possibly belt slipping off the pulley) or low belt tension environments, causing unnecessary belt slippage and thus additional belt wear. To solve this, the pulleys are sometimes set back. The term "lag" is used to describe the application of a coating, cover or wear surface with various texture patterns that is sometimes applied to pulley housings. Coating is often applied to extend the life of the casing by providing a replaceable wear surface or to improve friction between the belt and pulley. In particular, drive pulleys are often rubber coated (with a rubber friction layer) for precisely this reason.[15]​.

It is made up of three parts:

The constituent elements of a pulley are the wheel or pulley itself, in whose circumference (rim) there is usually a groove called "throat" or "pocket" whose shape adjusts to that of the rope in order to guide it; the "arms", an inverted U-shaped or rectangular frame that completely surrounds it and at the upper end of which mounts a hook by which the assembly is suspended, and the "axis", which can be fixed if it is attached to the arms with the pulley crossed through it ("eye pulleys"), or mobile if it is integral with the pulley ("axis pulleys"). When, as part of a transmission system, the pulley rotates freely on its axis, it is called "idle".

According to their displacement, the pulleys are classified as "fixed", those whose arms are suspended from a fixed point (the structure of the building) and therefore, do not suffer any translational movement when used with "mobile", which are those in which one end of the rope is suspended from a fixed point and which during their operation generally move vertically.[9]​.

When the pulley works independently it is called "simple", while when it is joined with others forming a system it is called "combined" or "compound".

Contenido

Existen sistemas múltiples de poleas que pretenden obtener una gran ventaja mecánica, es decir, elevar grandes pesos con un bajo esfuerzo. Estos sistemas de poleas son diversos, aunque tienen algo en común, en cualquier caso, se agrupan en grupos de poleas fijas y móviles: destacan los polipastos:.

Hoists or rigging

The hoist (from Latin polyspaston, and this from Greek πολύσπαστον), is the most common configuration of compound pulley. In a hoist, the pulleys are distributed in two groups, one fixed and one mobile. An arbitrary number of pulleys are installed in each group. The load joins the mobile group.

Un sistema de cuerda y polea, es decir, un bloque y aparejo, se caracteriza por el uso de una sola cuerda continua para transmitir una fuerza de tensión alrededor de una o más poleas para levantar o mover una carga; la cuerda puede ser un línea ligera o un cable fuerte. Este sistema está incluido en la lista de máquinas simples identificadas por los científicos del Renacimiento.[10]​[11]​.

Si el sistema de cuerda y polea no disipa ni almacena energía, entonces su ventaja mecánica es el número de partes de la cuerda que actúan sobre la carga. Esto se puede demostrar de la siguiente manera.

Considere el conjunto de poleas que forman el bloque móvil y las partes de la cuerda que soportan este bloque. Si hay p de estas partes de la cuerda que soportan la carga W, entonces un balance de fuerzas en el bloque móvil muestra que la tensión en cada una de las partes de la cuerda debe ser W/p. Esto significa que la fuerza de entrada en la cuerda es T=W/p. Por lo tanto, el bloque y el aparejo reducen la fuerza de entrada por el factor p..

Operation method

The simplest theory of operation for a pulley system assumes that the pulleys and lines are weightless. And that there is no loss of energy due to friction. The lines are also not supposed to stretch.

In equilibrium, the forces on the moving block must sum to zero. Furthermore, 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.

There are various types of pulley systems:

The mechanical advantage of the gun rig can be increased by interchanging the fixed and moving blocks so that the rope is attached to the moving block and the rope is pulled in the direction of the lifted load. In this case, blocking and tackling is said to be "advantage shooting".[12]​ Diagram 3 shows that three parts of the rope now carry the load "W", which means that the tension in the rope is "W/3". Therefore, the mechanical advantage is three.

By adding a pulley to the fixed block of a gun rig, the direction of the pulling force is reversed, although the mechanical advantage remains the same, Diagram 3a. This is an example of the Luff tackle.

Free body diagrams

The mechanical advantage of a pulley system can be analyzed using free-body diagrams in which the tension force in the rope is offset by the force of gravity acting on the load. In an ideal system, massless, frictionless pulleys dissipate no energy and allow a change of direction of a rope that does not stretch or wear out. In this case, a balance of forces on a free body that includes the load, W and n supporting sections of a rope with tension T, gives:.

The relationship between the load and the tension force exerted is the mechanical advantage MA of the pulley system,[13]​.

Therefore, the mechanical advantage of the system is equal to the number of rope sections supporting the load.

A belt and pulley system is characterized by having two or more pulleys in common with one belt. This allows mechanical power "Power (physical)"), torque and speed to be transmitted through the axles. If the pulleys have different diameters, a mechanical advantage is obtained.

A belt drive is analogous to a chain drive; However, a belt pulley may be smooth (devoid of discrete interlocking elements such as would be found in a chain sprocket, spur gear, or timing belt), so the mechanical advantage is given approximately by the pitch diameter ratio of the pulleys only, not exactly fixed by the tooth ratio as in the case of gears and sprockets.

In the case of a drum-type pulley, without a groove or flanges, the pulley is usually slightly convex to keep the flat belt centered. Sometimes called a crown pulley. Although once widely used on factory line axles, this type of pulley is still found driving the rotating brush on upright vacuum cleaners, belt sanders, and band saws. Farm tractors built until the early 1950s typically had a pulley for a flat belt (which is what Belt Pulley magazine is named after). It has been replaced by other mechanisms with more flexibility in methods of use, such as power take-off") and hydraulics").

Just as the diameters of gears (and, consequently, their number of teeth) determine a transmission ratio and therefore the increases or reductions in speed and the mechanical advantage they can provide, the diameters of pulleys determine those same factors. Conical pulleys and stepped pulleys (which work on the same principle, although the names tend to be applied to flat belt versions and V-belt versions, respectively) are a way of providing multiple gear ratios in a belt and pulley system that can be changed as needed, just like a transmission&action=edit&redlink=1 "Transmission (mechanical) (not yet written)") provides this function with a gear train") that can be changed. Stepped pulleys with V-belts are the most common way that drill presses provide a range of spindle speeds.

With belts and pulleys, friction is one of the most important forces. Some uses of belts and pulleys involve peculiar angles (leading to poor belt tracking and possibly belt slipping off the pulley) or low belt tension environments, causing unnecessary belt slippage and thus additional belt wear. To solve this, the pulleys are sometimes set back. The term "lag" is used to describe the application of a coating, cover or wear surface with various texture patterns that is sometimes applied to pulley housings. Coating is often applied to extend the life of the casing by providing a replaceable wear surface or to improve friction between the belt and pulley. In particular, drive pulleys are often rubber coated (with a rubber friction layer) for precisely this reason.[15]​.