Block and tackle

A pulley is a machine which reduces the amount of force to be applied, for example, for moving loads. The pulley is composed of fixed and / or loose pulleys and a rope. A chain pursues the same principle, except that a chain instead of a rope is used here. For complicated hoists the roles are combined using " scissors " to the block.

3.1 Factors pulley 3.1.1 Change of tensile direction and point of application

3.5.1 Swiss pulley
3.5.2 Ellington Pulley
3.5.3 lacings

Invention, history, application

Even in ancient times the force reduction was known by applying the laws of leverage. The inventor of the pulley is not known, but the invention of the composite pulley is attributed to Archimedes. A first pictorial representation of the combination of rope and simple role is, however, already on an Assyrian relief around 970 BC.

With ropes strained Einbaumkräne that ran over three ( " Trispastos " ) or five roles ( " Pentaspastos " ), have been known since 750 BC.

Vitruvius, a Roman architect, engineer and architectural theorist of the 1st century BC, described in his 10th book on the engineering detail the Trispastos, Pentaspastos and Polyspastos. But at that time it was hardly possible to make ropes for pulleys with very many roles in the required length and carrying capacity, or they were almost priceless in the required quantity.

The Greek mathematician and engineer Hero of Alexandria probably lived in the 1st century and dealt also with the Pulley. Under the construction activity of the Caesars in ancient Rome the construction crane was indispensable for the construction of the arenas. Thanks to various guide rollers, the crews were able to lift up to seven tons of heavy stone blocks. Even Leonardo da Vinci made in his inventions ( for example in the Codex Atlanticus, Codex Forster II & III) advantage of the pulley.

During the Renaissance, of the tackle found its most spectacular application in the transport and erection of the obelisk in St. Peter's Square in Rome from April 30 to September 16, 1586 by the engineer Domenico Fontana. 907 workers and 75 horses turned to the capstans 40 and thus attracted over pulleys on up to 220 meters long ropes hanging on a gigantic wooden skeleton 40 tackles to bring the 330 -ton and 25.31 meter long obelisk in the vertical.

Until 1861, the pulley system and its application remained largely unchanged. Only with the differential pulley block, used for the first time in London, an increase in efficiency could be achieved. Today Hoists are primarily used in cranes or fixtures ( Radspannwerke ) for contact wires.

Parts of the pulley

Bottles

Bottles were called the holders of the roles and were mostly as a block ( MHG plock, Ploch " big" or " continuous piece " ) of a piece of hardwood (ash, elm ) worked. Today it is called the flat parts on both sides at the edge ( jaw, cheek) and between the rollers ( dam ) total shears. In the sailor language of the pulley is called a tackle.

As a bridesmaid round or semicircular stable wooden disks are usually called with several holes that do not contain any roles and served, for example, for tensioning the shrouds.

Roll

The roles of a pulley block used to be also called bottles or "slices". The term originated around the 18th century: the case of weaving, especially in narrow fabric looms, the tension rollers which hold the warp threads always curious called bottles. Preferably, ball bearing rollers are to be used to achieve a more favorable ratio with little muscle power. Climbers use in emergency solutions ( crevasse rescue ) even simple carbine, but the more difficult by the sliding friction, the traction work.

Rope

The rope should be a static rope to get any unnecessary strain or stretch, which would lengthen the stroke. Slacklines have preferred instead of the rope a flat or tubular tape. Named after its inventor Ellington pulley is exceptional and elegant in its simplicity. For extreme conditions steel cables.

The way, as the rope is passed over the rollers and threading are called reeving.

Designs

Factors pulley

The roles in a pulley can be arranged very differently. But crucial for the tension is always the number of supporting ropes, on the the load is distributed. In the basic form shown on the right of the pulley is equal to the voltage at any point of the rope. The weight of the mass is therefore equally among all connections between the lower and the upper rollers, the supporting ropes, distributed. The tensile force at the end of the cable is proportional to the tension in the cable and thus the following applies:

According to the golden rule of mechanics, that means, however, that the hook has to be moved a long distance to reach the same height change:

It is easy to show that the energy required for this is independent of:

The load on the ceiling hook (FH ) results from the sum of traction and weight.

The load on the ceiling hook decreases with each additional role.

Change of the direction of pull and point

How important is the arrangement of the attack Punke ( fixed point, tow point and pull direction ) shows the accompanying picture.

Zugrichtungsänderung

Point of change

Power pulley

Compared to the power block and tackle: Two identical, combined tackles 1:16 ( FH = tractive force on the hook )

The same two hoists modified by installation but now 1:12 (with a different direction of pull )

The pulley, the force power saving is achieved solely by means of loose rollers: each roller is fixed to the rope on the support and of the next roll. While still affects the full weight on the lower idler roller, this is just a half of the bottom rope, so that only half the attacking force on the upper loose pulley. In this role, the force is halved again. Finally, the last roller on the cable affects the tensile force is deflected by a fixed roller downward. Thus, the effect of the number of idle rollers is potentiated:

Differential pulley

The differential pulley consists of two fixed rollers which are fixed to each other and have different diameters. The load depends on a loose pulley. In this type of pulley, a continuous (i.e., at the ends connected ) cable is used in which the voltage is not the same everywhere. The rope is fed back from the larger roller and the load on the other side on the smaller roll. By the arrangement of the torque of the small roller counteracts the large roll, so that only the difference has to be applied as a pulling force; added to halve the load force by the loose pulley. The smaller the difference in diameter between the two rollers, the more efficiently the differential pulley:

An important advantage of this type of tackle is the material and weight savings. By rotating cable whose length from the ratio of the pulley is almost independent. Furthermore, only three roles are always required. Since fixed contact of the two cables to the rollers is required for the function of a ( non - ), wherein a differential hoist rope instead of the a chain is often used, the members run through related gears.

Münchhausenplatz technology

The Münchhausenplatz technique is suitable for self- rescue, for example, the previous guides crashes in one column and the partner does not speak any rescue techniques. A variant of the Münchhausenplatz technique is the use of a pulley as self-winding. The distribution of forces here is different from calculate the ordinary tackle. The toppled and hanging on the rope climbers (FL = load ) runs over two pulleys (alternatively with two Umlenkkarabinern ) on the hawser itself into the air. The simple pulley now bears the climbers rope with three strands, with the draw weight (FC) reduces the pulling rope to 1/3 of the weight. Thus, neglecting friction is the ratio 1: 3

Pulley without bottles or roles

Swiss pulley

A Swiss pulley is a method of crevasse rescue in case of accident on the glacier using a combined power and factors pulley. The cable guide is made directly over the carabiners, as this is a relief effort and mostly no roles are available.

Ellington - pulley

The Ellington - pulley (named after its inventor ) usually consists of two carbines, is looped several times through the tape. The band is the overlapping wraps to " Replacement Wheel ". When clamping band position is at the bottom (inner) pulled and the Overlying move in parallel with. Due to the high friction of the tape on the carbine, however, only a poor efficiency to conditions.

Lacings

Lacings, for example, to corsets and shoes also use a loose roll train technology. Here, the eyelets, hooks or holes take the place of the rollers. The large number of penetrations increases friction and degrades the efficiency, but can still be a train reinforcing or tensioning effect on a broad basis to be achieved.

Reversion of hoists

Specialised Hoists also use a reversal of the sliding function of a pulley.

The aircraft carrier USS Hornet (1943-1970) used a steam-powered catapult, which is expressed by a pressure cylinder, a factor pulley system apart. About the guide rollers so driven pull rope pulled a carriage which accelerated the aircraft.
Elevators indirectly -acting hydraulic lift use the same function, to move with a short stroke a lift.
A reverse power pulley accelerates the pull rope.
Drawing by Leonardo Da Vinci with 20 rolls Mechatronic presses utilize advantageously the pulley principle, inspired by Leonardo Da Vinci's drawing 500 years ago.

Lifting height of different designs

The arrangement of the rollers, and the pulley - type also plays a critical role in the maximum possible stroke position. While the superposed rollers is the longest type ( proper length ), the arrangement on an axis adjacent to each other can shorten the overall length. One of the shortest versions of a pulley, the pulley Ellington dar.

Backflow prevention

When loads it may be necessary to secure the load to a height near future or to fix. The required backflow protection is available with a variety of tools. Firstly, there are some flat trains already grown back Lifelines, which block the rope with eccentric clamps. In addition, on or be incorporated attachments are useful and are either manually or automatically nachgeschoben. Examples: Garda terminal, Kara -eight loop, Prusikschlinge, ascender, Tibloc and tube just to name a few. Some of these clamping devices have a blocking effect in one direction and can not automatically loosen to allow, for example, a lowering of the load. By different combinations of these, however, can be achieved. The Bachmann terminal node on the towed carbine (pictured left: by the blue "rip cord " ) are relaxed. The Blake node leaves you with a durchgeschlauften "rip cord " screen (Picture right: blue line → to bottom right) also loosen. Thus can be from a distance (eg, from a stand on the ground) fix a pulley, loosen and thereby serve again absenkend.

Friction losses / efficiency

All above equations are only valid under the condition that there are no losses due to friction. In practice, however, the friction losses of the guide rollers themselves, and the rope at the deflection rollers are not to be underestimated. Also, the rope, the movable pulleys and the lower suspension possess a considerable mass, which increases with each new roll on. The tensile force is calculated in practice, therefore, in any case greater than theoretically. For the efficiency of a pulley system applies.