Sheave

A role (also: guide pulley ) is a power converter and machine element consisting of a wheel or a circular disk, which is mounted on an axle friction as possible. You used to guide a rope or a chain. At the boundary of the disk can usually find a bar which prevents slippage of the rope. Roles are to change the direction of a pulling force ( the force vector ) is used (without changing the amount of force), to guide the rope or as a compensation element in cable systems. Large pulleys, for example to derricks or cableways are called sheaves. In the maritime field, a circular disk is referred to with its housing as a block. Typical applications include cable and transmission equipment such as lifts, hoists and cranes.

Festivals and loose pulleys

  • A fixed pulley is mounted so that it does not change its position during use. By means of fixed rollers are diverted forces ( for example, pulling up a load on a deflected traction rope). The tensile force remains unchanged, only the direction of pull is affected. Is called a fixed role pulley, since it serves to deflect the force.
  • Loose Wheels are the cable guide and are supported by the rope. Each of the two parts of the rope, which include the loose pulley, absorbs 50 % of the force. In this way, a load with half the effort lift. The length of the role to be drawn rope is twice as long as the stroke, so that and apply.

Example illustrating

Three people want to draw a box in the direction of the cliff without being able to leave the cliff. It can be a certain amount of force be applied F3Männer transmitted to the box with virtually no loss through the cable. (Represented in the figure by a red vector that runs with the rope. ) Without pulley ( Figure 1) is a subset of the available force used in this case to lift the box. This point is illustrated by the decomposition of the vector F3Männer into two components. The vertical force component removes the box, the horizontal moving the box towards the cliff (green vector). The vertical component is greater, the closer the box to the cliff. Figure 2 shows the use of two fixed rollers. The applied force is fully implemented to move the crate ( The length of the green corresponds to the length of the red vector ).

Analysis

  • However, it must be remembered that in purely waagrechtem train the friction surface is unfavorable large.
  • Second, even taking into account that the first man near the cliff edge can bring less horizontal train into the rope when the rope is horizontal, but more ( purely horizontal train! ) When the rope proceeds obliquely because his legs then with more than body weight the ground are compressed before it slips.
  • Third, the box is easier to draw ( = less horizontal tension component ) when the pull rope is inclined, the box is so relieved some of the dead weight, especially with the uneven ground feel free to verhakelnde lower front edge.
  • Fourth, it is dangerous to boot to deal with a pull rope with several people close to a cliff edge. Because the jump from static friction to sliding friction on the soles makes slipping the edge neighbor with his legs over the cliff edge in addition possible and he would then probably delivered some time the accelerating effect of gravity.
  • Graphically documented - - increase the tension - in cable direction - Fifth appears by the merely slightly Z-shaped guiding the rope around two ideal frictionless functioning roles but noteworthy.
  • Most recently, due to the special value of the example in the rarity, in pulling a crate can be observed to a cliff in the reality of everyday life.

The mere idler pulley on a small boom in the scaffolding above, by the means of a handy, non-slip hemp rope from a person below 1-4 Baukübel be pulled up with mortar, on the other hand is just banal.

Friction and efficiency

For the losses caused by the tension of the cables and the friction of the bearing in practice, the following values ​​are assumed, where the tension of the ropes is:

  • Cable tension (depending on design and load):
  • Friction storage: and wherein the coefficient of friction is /

For the total losses, there is therefore

The efficiency is found to be from

In practice the efficiency of a roller is at the following values ​​:

  • In a rolling bearing:
  • In a sliding bearing:
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