Scheme of a chain hoist with a multiplicity of 4. Polyspast. Purpose and device. Simple and complex pulley blocks - we understand their design

4. POLYSPASTS

Polyspastom is called a device, which is a system of blocks and cables, designed to gain strength or speed. In hoisting mechanisms, power pulleys are used to reduce the force in the cable and reduce the gear ratio of the gearbox.

In marine practice, chain hoists that are used to lift cargo, booms and other equipment are called hoists. These include cargo hoists, topping hoists, top-hoists, sloop hoists, guy-hoists, etc.

The running end of the chain hoist (hoist), which is wound on the drum, is called a lopar.

The main parameter of the chain hoist is its multiplicity u (gear ratio) the multiplicity of the chain hoist is the ratio of the number of cable branches that escape from the moving blocks to the number of blades.

A rope designed to raise and lower a load is called a pendant. The cable, designed to hold the boom and change its reach, is called a toe-tenant.

The multiplicity of the cargo chain hoist is the ratio of the number of cable branches on which the load hangs to the number of lappets

where
- the number of rope branches on which the load hangs;

- the number of Lapps.

According to the number of lappets, the pulley blocks are subdivided into single (Figure 4.1 a)) ( = 1) and doubled (Figure 4.1 b)) ( =2).

Figure 4.1. Single pulley block with multiplicityu G =2

Figure 4.2. Double chain hoist with multiplicityu G =2

Let us determine the efficiency pulley block on the example of a single pulley block shown in Fig. 4.2 with multiplicity u G ... In a motionless pulley block, the pulling force is the same in all

, (4.2)

where F Q - the force of the weight of the load, N.

u G - the frequency of the cargo chain hoist.

If the chain hoist begins to lift the load, then the tension forces in its branches are unevenly distributed. This is due to the loss of efficiency. in blocks and from the rigidity of the cable. Efforts are distributed as follows:

,
,
,
….
,
,

where - efficiency, taking into account the friction losses in the block and from the rigidity of the rope.

The system of forces is in balance

Here in brackets is the sum of a geometric progression

, taking this into account, expression (4.3) is reduced to the form
... Where do we get the formula for determining the tractive effort in the rope lopar

(4.4)

Efficiency d. the pulley block represents the ratio of useful work

Figure 4.3. Distribution of efforts in the branches of the chain hoist

when lifting a load F Q to the height h to the work spent

. (4.5)

Between the speed of lifting (lowering) the load V under and the speed of picking out (etching) the lappet pendant V lsh. there is a dependency

(4.6)

The disadvantage of single chain hoists is that when the load is lifted, it also moves horizontally. This makes it difficult to stop the load accurately and causes uneven reactions in the drum bearings.

    It makes sense to use such a chain hoist when lifting loads weighing up to 700 kg. Highly good quality bearings and large pulleys at a relatively low cost of blocks make this chain hoist one of the most reasonable and reasonable options. The only drawback of such a kit is its rather high weight.

    The working length of the chain hoist is 20 meters, but it can be changed at your request.

    Kit Contents

    • Block Promalp double: 2 pcs.
    • Carabiner Height 513: 3 pcs.
    • Rope Promtex 10 mm: 100 m
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Polyspast(from the Greek polyspastos - pulled by many ropes) is a lifting device operating on the principle of a lever, and is designed to move various loads, both vertically and horizontally, or to pull a bowstring, for example, a ferry.

Any pulley block gives a certain gain in the effort to lift the load.

The degree of increase in the applied tractive effort in the chain hoist is called the gear ratio or the multiplicity of the chain hoist and is denoted by the Latin letter "I".

Structurally, the chain hoist consists of two types of blocks:

  • motionless attached to an anchor station (beam, mast, tripod, etc.);
  • and mobile that are attached to the load being lifted.

The blocks are connected to each other by a rope (rope, cable, chain, etc.), which, sequentially bending around all the rollers of the blocks, with one end usually is attached to the stationary block (when lifting loads - the upper one), and its other end (traction) through the branch blocks is directed to the place of application of force (physical or mechanical).

The roller of the stationary block, from which the end of the rope runs, is considered to be diverting. The lower block of the chain hoist, as a rule, has a load hook (or a connecting element: carabiner, shackle, etc.), to which the load being lifted is suspended. And the upper one - for which the chain hoist itself is suspended (see Fig. 1).

Rice. 1. Polyspast in general view(a), in a tightened state with a hook on the blocks (b) and in a tightened state with earrings on the blocks (c)

Symbols:

1 - movable block;
2 - rope;
3 - stationary block;
4 - traction rope;
5 - earring;
6 - hook.

The main advantage of the chain hoist is that it allows you to get a significant gain in strength. Therefore, the use of pulley blocks is advisable in cases where the tractive effort of the available traction means is less than the effort required to lift or move the object.

As for the gain in effort, it is only given by movable blocks fixed directly to the load or to a rope coming from the load. Fixed (stationary) blocks serve only to change the direction of movement of the rope and do not give a gain in effort.

For example, 3: 1 and 4: 1 pulley blocks can be obtained with the following options (see Fig. 2):

Rice. 2. Schemes of pulley blocks

The speed and distance traveled by the object being moved decrease (in relation to the speed and distance traveled by the traction end of the cable) in direct proportion to the increase in force, that is, how many times you win in effort, how many times you lose in distance, and the "stronger" pulley block - so it is "slower".

Based on this pulley blocks are divided into power (direct) and high-speed (reverse)... The first ones are the most frequently used, therefore, we will talk about them below.

As for the design features, the chain hoist can be configured in a wide variety of combinations of the corresponding constituent elements (movable and fixed blocks, ropes, anchors, clamps, auxiliary clamps, brakes for smooth removal of loaded systems). Due to this feature chain hoists are differentiated into simple and complex by design:

  • in simple pulley block all blocks are bent around with one cable;
  • in a complex- several simple chain hoists are connected in series with each other, that is, one simple chain hoist pulls a simple chain hoist after another, etc. In them, the last block loads not the load being lifted, but the rope passing through the block. That is, it happens serial connection pulley blocks (for example, the connection of pulley blocks 2: 1 and 3: 1 - theoretically provides an increase in effort 6 times).

Depending on where the end of the cargo rope is fixed (at the station or on the cargo), simple pulley blocks are divided into even and odd. If the end of the rope is fixed at the station, then all subsequent pulleys will be even (2: 1, 4: 1, 6: 1, etc.). If the end of the load rope is fixed to the load, then odd pulleys will be obtained (3: 1, 5: 1, etc.).

V separate species highlighted complex pulley blocks, which are neither simple nor complex. Their distinctive feature is the presence in the system of block rollers moving towards the load. This is the main advantage of complex chain hoists in cases where the station is located higher and it is necessary to pull the chain hoist down. There are other schemes as well.

The process of calculating the theoretical gain in effort in pulley blocks is quite simple and is based on following rules, according to which:

  • each movable roller (attached to the load) added to the system gives a 2x theoretical strength gain;
  • with a simple pulley block, the additional effort is added to the previous one;
  • when using a complex pulley block, it is necessary to multiply the values ​​of the simple pulley blocks that make up it.

In this case, the calculation of the effort of each of the simple pulley blocks that are part of the complex one is carried out according to the rule of simple pulley blocks. The number of strands is calculated from the point of attachment of the chain hoist to the load or cargo rope coming out of another chain hoist.

Consider the most commonly used chain hoist systems, emphasizing that there are other, more complex, chain hoist systems, but rarely used.

The figure below shows the most popular schemes of simple and complex pulley blocks, shown in a natural and schematic form (see Fig. 3, 4 and 5).

Rice. 3. Schemes of simple pulley blocks of various configurations

Rice. 4. Schemes of complex pulley blocks of various configurations

Rice. 5. Conditional schemes of simple and complex pulley blocks of various configurations

The process of selecting a chain hoist has certain features. Initially, the chain hoist scheme is selected based on the parameters of the moving object (mainly, its gravity) and the traction capabilities of the subject of this process (the physical capabilities of a person or the parameters of the equipment used).

Friction losses are inevitable in any moving system consisting of rope and blocks. Theoretically, you can endlessly add stationary blocks to the station and moving blocks to the load in order to obtain simple pulley blocks of various efforts. In practice, however, frictional force interferes with this. Due to the frictional force, a pure transfer of force is impossible and due to frictional losses, the efficiency of the chain hoist can be significantly lower than theoretically possible. And the increase in the number of blocks has a certain limit, beyond which the opposite effect is possible. Thus, from the above, we can conclude that the main problem of the chain hoist is to overcome the friction force. The main solution to the problem of friction is the use of high-quality block rollers on ball bearings with large diameters of streams and soft ropes. It is known that the efficiency for blocks supported on plain bearings is η = 0.95 ÷ 0.96; for blocks on rolling bearings η = 0.97 ÷ 0.98.

And when using a chain hoist design without rollers, for example, put two carabiners side by side instead of one. This increases the bend radius of the rope and reduces the friction force.

TM "KROK",
August 2015

The polyspast is a lifting structure that was invented during the time of the great thinker Archimedes. Now it is impossible to establish exactly who was that genius, but the already mentioned philosopher also had his hand in the development of this construction. It is also called a block system, because of the main purpose and the counterweights available at that time, in the form of limestone blocks.

Blocks and pulley blocks, the purpose and device of which is now for an ordinary person accustomed to high technology looks rather primitive. But it is worth considering the fact that it was thanks to this mechanism that great historical structures were built, such as the pyramids, the Pantheon, the Colosseum and the like. But the technology did not remain on the pages of textbooks, but continued its development, adapting to the emerging technology and the needs of people.

Description and device of chain hoists

The design itself is a device for lifting loads using special connection blocks and ropes between them. Using the rule of leverage and frictional force, the structure is brought into action increasing the force or speed of lifting an object... There are different types chain hoists, which differ in the number of blocks, rope connections, load capacity and other configurations.

The system, in turn, consists of movable and fixed elements, along which ropes are drawn, creating tension and ensuring the transportation of the load. A fixed element is a basic structure that is attached to a vehicle or a static bar, and a movable element is attached to the load. Therefore, the first must be able to withstand a lot of pressure, and the second must evenly distribute it.

The lower or movable block is usually equipped with a special mount, in the form of a hook, a powerful magnet, a carabiner, and so on. The upper block has special rollers along which the rope is guided and the pressure exerted on each rope separately depends on the number of rollers. This means that lifting large, heavy loads requires the appropriate number of rollers and working branches.

The video tells and demonstrates how the chain hoist works, and also reveals its advantages

Appointment

Given the fact that this invention is more than two thousand years old, it has been used to perform an unimaginable amount of work and tasks. Often this is the construction industry, where pulley blocks are used in cranes, winches, and the like. Also, the mechanism was used on ships, for launching and lifting lifeboats. It was used for some time in the first prototypes of the elevator, before the advent of hydraulic and electric drives.

Polyspasts, purpose and device, their frequency varied and found application in sports, namely, in rock climbing and other extreme activities at high altitudes. Also, for a long time, the rescue team in mountainous areas were equipped with devices to pull the victims out of hard-to-reach places. You can also often find the use of blocks in electrical wiring, or rather, to create tension in the cable network.

Varieties of pulley blocks

All pulley blocks can be divided into two categories:

  • power;
  • high-speed.

Based on the name, the purpose of each type is determined and based. The first is the most common and is used for lifting loads, exactly as it was invented. The high-speed version is a modified design, where great efforts are directed to increasing the transport speed. According to this principle, cable cars are created in ski resorts.

In addition, the difference lies in the number of rollers and working branches, as well as other modifications. An electric drive and a stopper can be connected to the structure. Another difference lies in the material of the rope, after all, it can be represented in the form:

  • ropes;
  • metal rope;
  • iron chain;
  • electrical cable.

In construction equipment, the second option is most often used, due to the strength of the material. Rope ropes are used most often in tourism, rescue operations, and so on. The use of an iron chain is very rare; these are narrowly targeted varieties for certain jobs.

Lift with at home

Sometimes in everyday life there is a need to lift a heavy load, but not all people have the opportunity to fit a construction crane to the threshold, from which they have to get out. And here the block system can just come to the rescue. Polystyrene, the purpose and device of which may seem rather complicated in design, but with proper preparation, the creation of such a structure at home will not be a problem. Everything is done in four stages:

  • Calculations. They are made taking into account your goals and objectives, namely, the parameters of the working room, the presence of restrictions in it, the weight of the cargo and the distance over which you need to transport. It is necessary to fix all this data for drawing up a drawing and choosing a design.
  • Drawing creation. If there is no experience in this matter, then it is better to turn to a person with experience and engineering education who can reduce the time for creating a model on paper. If you can't get help from anywhere, it is better to turn to the Internet and see the working drawings of the basic structures. Each of the types will be effective under certain conditions, which you will measure earlier.
  • Selection of materials. The selection should start already at the first stages, namely, starting from the things that already have and those that can be purchased. It depends on your drawing and calculations what parts will be needed and what material is better to use. Buy to fit your budget and keep in mind. It is not worth saving too much, otherwise the design can fail at the most crucial moment.
  • Construction. This step is the easiest of all, because here you just need to follow the plan and do it conscientiously.

With a certain skill and preparedness, a very solid working model can be obtained, which will not be inferior to construction analogues. But if the tasks are not too ambitious, then it will not require large expenditures. For reliability, you can use ready-made parts from construction chain hoists in the creation.

Conclusion

Polyspasts are simple in the device, but their purpose is important, because thanks to them, you can hassle the most complex lifting manipulations. Construction of buildings, installation of electric cable lines, installation of a funicular or a rescue operation, in any of these situations, the reliability of the performance can be guaranteed by the block system.


For lifting large loads, a person is not very strong, but he came up with many mechanisms that simplify this process, and in this article we will discuss the pulley blocks: the purpose and structure of such systems, and also try to make the simplest version of such a device with our own hands.

The hoist chain hoist is a rope-and-block system that allows you to gain effective strength with a loss in length. The principle is pretty simple. In length, we lose exactly as much as the gain in strength turned out to be. Thanks to this golden rule of mechanics, a large mass is possible without much effort. Which, in principle, is not so critical. Let's give an example. Here you won in strength 8 times, while you have to stretch a rope 8 meters long to lift the object to a height of 1 meter.

The use of such devices will cost you less than renting a crane, moreover, you can control the gain in power yourself. The chain hoist has two different sides: one of them is fixed, which is attached to the support, and the other is movable, which clings to the load itself... The gain in strength comes from the movable blocks that are attached to the movable side of the chain hoist. The fixed part serves only to change the trajectory of the rope itself.

Types of pulley blocks are distinguished by complexity, parity and multiplicity. In terms of complexity, there are simple and complex mechanisms, and the multiplication means the multiplication of strength, that is, if the multiplicity is 4, then theoretically you will win in strength by 4 times. Also rarely, but nevertheless, a high-speed chain hoist is used, this type gives a gain in the speed of movement of goods at a very low speed of the drive elements.

Let's start with a simple assembly chain hoist. It can be obtained by adding blocks to the support and weight. To get an odd mechanism, it is necessary to fix the end of the rope to the movable point of the load, and to get an even one, we attach the rope to the support. When adding a block, we get +2 to strength, and a movable point gives +1, respectively. For example, to get a chain hoist for a winch with a multiplicity of 2, it is necessary to fix the end of the rope on the support and use one block that is attached to the load. And we will have an even kind of fixture.

The principle of operation of a chain hoist with a multiplicity of 3 looks different. Here the end of the rope is attached to the load and two rollers are used, one of which we attach to the support and the other to the load. This type of mechanism gives a 3-fold gain in strength, this is an odd option. To understand what the gain in strength will be, you can use simple rule: how many ropes go from the load, this is our gain in strength. Usually, pulley blocks with a hook are used, on which, in fact, the load is attached, it is a mistake to think that this is only a block and a rope.

Now we will find out how a complex type chain hoist works. This name means a mechanism where several simple versions of this cargo device are connected into one system, they pull each other. The strength gain of such structures is calculated by multiplying their multiplicities. For example, we pull one mechanism with a multiplicity of 4, and another with a multiplicity of 2, then the theoretical gain in strength will be 8. All the above calculations take place only for ideal systems that do not have a friction force, in practice things are different ...

In each of the blocks, there is a small loss in power due to friction, since it is still spent just to overcome the frictional force. In order to reduce friction, it is necessary to remember: the larger the bend radius of the rope, the less the friction force will be. It is best to use rollers with a large radius where possible. When using carbines, you should make a block of the same options, but the rollers are much more effective than carbines, since we have a loss of 5-30% on them, but on carbines up to 50%. Also, it will not be superfluous to know that the most effective block must be located closer to the load in order to obtain the maximum effect.

How do we calculate the actual strength gain? To do this, we need to know the efficiency of the units used. Efficiency is expressed in numbers from 0 to 1, and if we use a rope with a large diameter or too stiff, then the efficiency from the blocks will be significantly lower than indicated by the manufacturer. This means that it is necessary to take this into account and adjust the efficiency of the units. To calculate the real strength gain of a simple type of hoisting device, it is necessary to calculate the load on each rope branch and fold them. To calculate the gain in the strength of complex types, it is necessary to multiply the real strengths of the simple ones, of which it consists.

Do not forget about the friction of the rope, since its branches can twist among themselves, and the rollers from heavy loads can converge and clamp the rope. To prevent this from happening, the blocks should be spaced relative to each other, for example, you can use a circuit board between them. You should also purchase only static ropes that do not stretch, since dynamic ones give a serious loss in strength. To collect the mechanism, both a separate and a load rope can be used, attached to the load independently of the lifting device.

The advantage of using a separate rope is that you can quickly assemble or prepare the lifting structure in advance. You can also use its full length, this also makes it easier to pass the knots. Of the minuses, we can mention the fact that there is no possibility of automatic fixation of the load being lifted. The advantages of a load rope are that the object being lifted can be auto-locked, and there is no need for a separate rope. Of the minuses, it is important that when working it is difficult to pass the knots, and you also have to spend a load rope on the mechanism itself.

Let's talk about the reverse course, which is inevitable, since it can occur when the rope is grabbed, or at the moment of removing the load, or when stopping for rest. To prevent reverse movement, it is necessary to use blocks that pass the rope in only one direction. At the same time, we organize the structure so that the blocking roller is attached first from the object being lifted. Thanks to this, we not only avoid reverse movement, but also allow the load to be secured during unloading or simply rearranging the blocks.

If you are using a separate rope, the locking roller is attached last from the load being lifted, and the locking roller must be of high efficiency.

Now a little about attaching the lifting mechanism to the load rope. It is rare that we have a rope of the right length at hand to secure the movable part of the block. There are several types of mechanism fastening. The first method is with the help of grasping knots, which are knitted from cords with a diameter of 7-8 mm, in 3-5 turns. This method, as practice has shown, is the most effective, since the grasping knot of 8 mm cord on a rope with a diameter of 11 mm begins to slide only at a load of 10-13 kN. At the same time, at first, it does not deform the rope, but after some time, it melts the braid and sticks to it, starting to play the role of a fuse.

Another way is to use a general purpose clamp. Time has shown that it can be used on icy and wet ropes. It begins to crawl only at a load of 6-7 kN and slightly injures the rope. Another method is to use a personal clamp, but it is not recommended, as it starts to crawl at a force of 4 kN and at the same time tears the sheath, or may even bite the rope. These are all industrial designs and their application, but we will try to create a homemade chain hoist.

 

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