Fastening the chain hoist. Polyspast - what is it? Operating principle. Application. How conventional block construction works

Blocks can be classified both in appearance and in functional content. In addition, the elements of the block can be part of a more complex device.

By the number of pulleys, blocks can be divided into single, double, triple, etc. Blocks that have a rope locking mechanism in their construction are called fixing blocks. A block with consecutive pulleys is called a tandem and is used on horizontal or oblique lines. Depending on the purpose, the pulley on the block can be mounted on a bearing, brass or plastic bushing. This feature affects the efficiency of the system where they are used and, of course, the price.

A polyspast can be any system or device that contains a rope (rope) and uses blocks to provide gains in strength or speed. In industrial mountaineering and rope technology, power pulleys are used for handling loads, tensioning lines or rescue tasks. The efficiency of a chain hoist is determined by its multiplicity and depends on the configuration of the chain hoist system and the number of branches in the system.

Polystyles are often used as elements of a system for organizing evacuation measures and rescue and allow you to quickly release an employee from hovering and move him to a safe area.

To buy blocks and pulleys in bulk in Moscow, contact our sales department or place an order on the website for further agreement on the delivery with our specialists.

Polyspast- a system of movable and fixed blocks connected by a flexible connection (rope or chain), used to increase the force - power pulley or speed - high-speed chain hoist... Usually, power pulley blocks are used to reduce the tension of the flexible load body, the moment from the weight of the load on the drum and the gear ratio of the mechanism. High-speed pulley blocks, which make it possible to obtain an increase in the speed of movement of the load at low speeds of the drive element, are used much less often, for example, in hydraulic or pneumatic lifts. The pulley block includes movable blocks, the axis of which moves in space, and fixed blocks.

Polyspast schemes

Rice. 1: a - single double; b - single threefold; c, d - double double; g - double threefold

In single pulley blocks (Fig. 1, a, b), one end is fixed on the drum, and the other end is fixed at an even multiplicity (a) on a fixed element of the structure, and at an odd multiplicity (b) - on a hook cage. When winding or unwinding the rope from the drum, if there are no bypass blocks, that is, the rope from the hook block goes directly to the drum, the load moves not only vertically, but also horizontally.

To ensure a strictly vertical lifting of the load, double chain hoists are used (Fig. 1, c-d), consisting of two single chain hoists. In this case, both ends of the rope are fixed on the drum. To ensure a normal position with an uneven extension of the rope branches of both pulleys, use the installation of a balancer or, more often, an equalizing block C (Fig. 1, c). When installing an equalizing block, you can use the whole rope without additional fastenings on the balancers. However, inspection and control of the condition of the rope on this block is difficult due to the small angle of rotation. Therefore, in cranes with heavy and very heavy duty, it is preferable to use equalizing balancers A (Fig. 73, e).

When lifting and lowering a load, the equalizing block C usually does not rotate and serves only to equalize the length of the branches of both pulleys with an uneven rope extension, therefore, according to the rules of Gosgortekhnadzor, its diameter is allowed to be equal to 0.8 of the diameter determined by the formula, and for electric hoists and self-propelled jib cranes - equal to 0.6 of this diameter. With an even multiplicity of the chain hoist, it is located among the fixed blocks, and with an odd one - among the movable blocks of the hook suspension.

The calculation of a double chain hoist is carried out in a similar way, and each chain hoist is considered separately when half of the total load is applied to it. If h is the lifting height of the load, then the length of the rope of a single chain hoist wound on a drum, L = ah, where a is the multiplicity of the chain hoist. The multiplicity of a double chain hoist is equal to the multiplicity of the single chain hoists that make up it. For a double pulley block, the L value corresponds to the length of the rope wound on one half of the drum.

The speed of lifting the load υ gr and the speed of the rope wound on the drum are interconnected by the ratio υ = aυ gr, where υ = πD 2 n bar / 60, m / s; D 2 - drum diameter, measured in the center of the rope; n bar - drum rotation frequency, rpm.

Power pulley

In the power pulley blocks of hoisting machines, small diameter ropes can be used and, therefore, to reduce the diameters of the drum and blocks, to reduce the weight and dimensions. An increase in the frequency of the chain hoist allows to reduce the gear ratio of the gearbox, but at the same time requires a greater length of the rope and the rope capacity of the drum. An increase in the number of blocks with an increase in the frequency of the chain hoist causes an increase in losses and an increase in the power spent on lifting the load, as well as an increase in the number of bends of the rope, which causes a slight decrease in its service life. At the same time, as already mentioned, the rope with a large multiplicity of the chain hoist has a small diameter and, therefore, great flexibility, which contributes to an increase in durability. The choice of the rope, the type and frequency of the chain hoist is associated with the problem of the general arrangement of the mechanism and its parameters, in particular with the gear ratio of the mechanism, dimensions and weight, which in turn affects the dimensions of the entire hoisting machine and the dimensions of the building where this machine is installed.

So, if to lift a load of the same weight G gr with the same given lifting speed υ gr use pulley blocks of different multiplicity, then the parameters of the lifting mechanisms will be different. The static power of these mechanisms N st = G gr υ gr / 1000η p, required for lifting the load, will be different only because of the difference in the values of efficiency, and at rates that differ slightly (for example, mechanisms with a multiplicity of two and four), the required power the engine can be considered the same. Since the maximum forces in the ropes of the chain hoists change almost inversely with the multiplicity of the chain hoist, with an increase in the multiplicity, the load in the rope and its diameter, as well as the diameter of the drum, decrease. The speed of winding the rope onto the drum changes in direct proportion to the multiplicity, and in a chain hoist with a higher multiplicity it is more important. Then, at the same preset lifting speed and the same rotor speed, the gear ratio of the gearbox connecting the engine to the drum turns out to be less with a chain hoist of higher magnification due to the higher speed of winding the rope onto the drum and its smaller diameter.

High-speed pulley block

High-speed pulley block(Fig. 2) differs from a power chain hoist in that in it the work force F, usually developed by a hydraulic or pneumatic cylinder, is applied to the movable cage, and the load is suspended from the free end of the rope.

High-speed pulley block diagram

Rice. 2

The calculation of high-speed chain hoists does not fundamentally differ from the calculation of a power chain hoist. When moving the chain hoist clip (point A in Fig. 2) at a distance h, the load travels the path H = ah, where a is the multiplicity of the high-speed chain hoist and, consequently, the speed of movement of the load υgr = aυA, where υA is the speed of movement of the chain hoist clip.

The force F required to lift a load of weight Ggr is determined by the formula.

Hoisting machines are designed to help a person lift something heavy to a height. At the heart of most lifting mechanisms is a simple block system - a chain hoist. He was already familiar to Archimedes, but now many do not know about this ingenious invention. Remembering the physics course, find out how such a mechanism works, its structure and field of application. Having understood the classification, you can proceed to the calculation. To make it work - your attention is the instructions for constructing a simple model.

The invention of the chain hoist gave a huge impetus to the development of civilizations. The block system helped build huge structures, many of which have survived to this day and are bewildering to modern builders. Shipbuilding was also improved, people were able to travel great distances. It's time to figure out what it is - a chain hoist and find out where you can find a use for it today.

Simplicity and efficiency of the mechanism

The structure of the lifting mechanism

The classic pulley block is a mechanism that consists of two main elements:

pulley;
flexible communication.

The simplest scheme: 1 - movable block, 2 - fixed, 3 - rope

The pulley is a metal wheel that has a special groove for the cable along the outer edge. A conventional cable or rope can be used as a flexible connection. If the load is heavy enough, use synthetic fiber ropes or steel ropes and even chains. In order for the pulley to rotate easily, without jumps and jamming, roller bearings are used. All elements that move are lubricated.

One pulley is called a block. Polyspast is a system of blocks for lifting loads. The blocks as part of the lifting mechanism can be fixed (rigidly fixed) and movable (when the axis changes position during operation). One part of the chain hoist is attached to a fixed support, the other to the load. Movable rollers are located on the side of the load.

Fixed block

The role of the fixed block is to change the direction of movement of the rope and the action of the applied force. The role of the mobile is to gain strength.

Movable block

How it works - what's the secret

The principle of operation of a chain hoist is similar to a lever: the effort that must be applied becomes several times less, while the work is performed in the same volume. The role of the lever is played by the cable. In the work of the chain hoist, the gain in strength is important, so the resulting loss in distance is not taken into account.

Depending on the construction of the chain hoist, the power gain may be different. The simplest mechanism of two pulleys gives approximately two-fold gain, from three - three-fold, and so on. The increase in distance is calculated according to the same principle. For the operation of a simple chain hoist, a cable is needed twice as long as the lifting height, and if a complex of four blocks is used, then the length of the cable increases in direct proportion to four times.

The principle of operation of the block system

In which areas is the block system used

Polyspast is a faithful assistant in a warehouse, in production, in the transport sector. It is used wherever it is necessary to apply force to move all kinds of goods. The system is widely used in construction.

Despite the fact that most of the heavy work is performed by construction equipment (crane), the chain hoist has found a place in the design of load-gripping mechanisms. The block system (pulley block) is a component of such lifting mechanisms as a winch, a hoist, construction equipment (cranes of various types, a bulldozer, an excavator).

In addition to the construction industry, pulley blocks are widely used in the organization of rescue operations. The principle of operation remains the same, but the design is slightly modified. The rescue equipment is made of a strong rope, carabiners are used. For devices of this purpose, it is important that the entire system is quickly assembled and does not require additional mechanisms.

Polyspast as part of a crane hook

Classification of models according to different characteristics

There are many versions of one idea - a system of blocks connected by a rope. They are differentiated depending on the method of application and design features. Get to know the different types of lifts, find out what their purpose is and how the device differs.

Classification depending on the complexity of the mechanism

Depending on the complexity of the mechanism, there are

simple;
complex;
complex pulley blocks.

An example of even models

A simple pulley block is a system of rollers connected in series. All movable and fixed blocks, as well as the load itself, are united by one cable. Differentiate even and odd simple pulleys.

Even-numbered are those lifting mechanisms whose end of the cable is attached to a fixed support - the station. All combinations in this case will be considered even. And if the end of the rope is attached directly to the load or to the place where the force is applied, this structure and all its derivatives will be called odd.

Odd pulley scheme

A complex chain hoist can be called a chain hoist system. In this case, not individual blocks are connected in series, but whole combinations, which may well be used by themselves. Roughly speaking, in this case one mechanism sets in motion another similar one.

The complex pulley block does not apply to one or the other type. Its distinctive feature is rollers moving towards the load. The complex model can include both simple and complex pulley blocks.

Combining a double and six-fold simple chain hoist gives a complex six-fold option

Classification by purpose of the lift

Depending on what they want to get when using a chain hoist, they are divided into:

power;
high-speed.

A - power option, B - high-speed

The power option is used more often. As the name suggests, its job is to provide a power win. Since significant gains require equally significant losses in distance, losses in speed are inevitable. For example, for a 4: 1 system, when lifting a load one meter, you need to pull 4 meters of rope, which slows down the work.

The high-speed pulley block by its principle is a reverse power structure. It does not give a gain in strength, its goal is speed. It is used to speed up work at the expense of the applied effort.

Multiplicity is the main characteristic

The main indicator that is paid attention to when organizing the lifting of loads is the multiple of the chain hoist. This parameter conventionally indicates how many times the mechanism allows you to win in strength. In fact, the multiplicity shows how many branches of the rope the weight of the load is distributed.

Kinematic multiplicity

The multiplicity is divided into kinematic (equal to the number of bends in the rope) and power, which is calculated taking into account the overcoming of the friction force by the cable and imperfect efficiency of the rollers. The reference books contain tables that show the dependence of the power ratio on the kinematic ratio at different efficiency of the units.

As can be seen from the table, the power ratio differs significantly from the kinematic one. With a low roller efficiency (94%), the actual gain in the power of a 7: 1 chain hoist will be less than the gain of a six-fold chain hoist with a block efficiency of 96%.

Polyspast schemes of different multiplicity

How to make calculations for a chain hoist

Despite the fact that theoretically the design of the chain hoist is extremely simple, in practice it is not always clear how to lift the load using blocks. How to understand what multiplicity is needed, how to find out the efficiency of the lift and each block separately. In order to find answers to these questions, you need to perform calculations.

Calculation of a separate block

The calculation of the chain hoist must be performed due to the fact that the working conditions are far from ideal. Friction forces act on the mechanism as a result of the movement of the cable along the pulley, as a result of the rotation of the roller itself, no matter what bearings are used.

In addition, flexible and pliable rope is rarely used on the construction site and in construction equipment. A steel rope or chain is much more rigid. Since additional effort is required to bend such a cable when running onto the block, it must also be taken into account.

For the calculation, the equation of moments for the pulley relative to the axis is derived:

S run R = S run R + q S run R + Nfr (1)

Formula 1 shows the moments of such forces:

Sbeg - effort from the side of the running rope;
S run - the effort from the side of the oncoming rope;
q S run - the effort for bending / unbending the rope, taking into account its rigidity q;
Nf is the friction force in the block, taking into account the friction coefficient f.

To determine the moment, all forces are multiplied by the shoulder - the radius of the block R or the radius of the sleeve r.

The force of the incoming and outgoing rope arises as a result of the interaction and friction of the rope threads. Since the force for bending / unbending the cable is significantly less than the others, when calculating the effect on the axis of the block, this value is often neglected:

N = 2 S run × sinα (2)

In this equation:

N - impact on the pulley axis;
S run - the effort from the side of the oncoming rope (taken approximately equal to S run;
α is the angle of deviation from the axis.

Pulley block

Calculation of the efficiency of the block

As you know, efficiency is the coefficient of efficiency, that is, how effective the work performed was. It is calculated as the ratio of completed and expended work. In the case of a pulley block, the formula is applied:

ηb = S run / S run = 1 / (1 + q + 2fsinα × d / D) (3)

In the equation:

3 ηb - unit efficiency;
d and D - respectively, the diameter of the sleeve and the pulley itself;
q - coefficient of rigidity of flexible connection (rope);
f is the coefficient of friction;
α is the angle of deviation from the axis.

It can be seen from this formula that the efficiency is influenced by the structure of the block (through the coefficient f), its size (through the ratio d / D) and the material of the rope (coefficient q). The maximum efficiency can be obtained using bronze bushings and rolling bearings (up to 98%). Plain bearings provide up to 96% efficiency.

The diagram shows all the forces S on different branches of the rope

How to calculate the efficiency of the entire system

The lifting mechanism consists of several blocks. The total efficiency of the chain hoist is not equal to the arithmetic sum of all the individual components. For the calculation, a much more complex formula is used, or rather, a system of equations, where all forces are expressed through the value of the primary S0 and the efficiency of the mechanism:

S1 = ηп S0;
S2 = (ηп) 2 S0; (4)
S3 = (ηп) 3 S0;

Sn = (ηп) n S0.

Efficiency of the chain hoist at different multiplicity

Since the efficiency value is always less than 1, with each new block and equation in the system, the value of Sn will rapidly decrease. The total efficiency of the chain hoist will depend not only on ηb, but also on the number of these blocks - the multiplicity of the system. According to the table, ηп can be found for systems with a different number of units at different values of the efficiency of each.

How to make a lift with your own hands

In construction, during installation work, it is far from always possible to fit a crane. Then the question arises of how to lift the load with the rope. And here a simple chain hoist finds its application. For its manufacture and full-fledged work, you need to make calculations, drawings, choose the right rope and blocks.

Different schemes of simple and complex lifts

Base preparation - diagram and drawing

Before proceeding with the construction of a chain hoist with your own hands, you need to carefully study the drawings and choose a scheme suitable for yourself. You should rely on how it will be more convenient for you to place the structure, what blocks and cable are available.

It happens that the lifting capacity of the pulley blocks is not enough, and there is no time and opportunity to build a complex multiple lifting mechanism. Then they use double pulley blocks, which are a combination of two single ones. This device can also lift the load in such a way that it moves strictly vertically, without distortions.

Twin model drawings in different variations

How to find a rope and a block

The most important role in building a chain hoist with your own hands is played by a rope. It is important that it does not stretch. Such ropes are called static ropes. Stretching and deformation of the flexible connection results in a serious loss of work efficiency. For a homemade mechanism, a synthetic cable is suitable, the thickness depends on the weight of the load.

The material and quality of the blocks are indicators that will provide the calculated lifting capacity to the self-made lifting devices. Depending on the bearings that are installed in the block, its efficiency changes and this is already taken into account in the calculations.

But how to raise a load to a height with your own hands and not drop it? To secure the load against possible reverse movement, you can install a special fixing block that allows the rope to move in only one direction - the desired direction.

The roller along which the rope moves

Step-by-step instructions for lifting a load through a block

When the rope and blocks are ready, the scheme is selected, and the calculation is made, you can start assembling. For a simple double chain hoist you will need:

roller - 2 pcs.;
bearings;
bushing - 2 pcs.;
holder for the block - 2 pcs.;
rope;
hook for hanging the load;
slings - if they are needed for installation.

For quick connection, use carabiners

The step-by-step lifting of the load to a height is carried out as follows:

Connect the rollers, bush and bearings. Combine it all into a cage. Get a block.
The rope is launched into the first block;
The holder with this block is rigidly attached to a fixed support (reinforced concrete beam, pillar, wall, specially mounted stem, etc.);
Then the end of the rope is passed through the second block (movable).
A hook is attached to the clip.
The free end of the rope is fixed.
They strap the load to be lifted and connect it to the chain hoist.

The homemade hoist is ready to use and will give you double the power wins. Now, to raise the load to a height, it is enough to pull on the end of the rope. By looping around both rollers, the rope will lift the load effortlessly.

Is it possible to combine a chain hoist and a winch

If you attach an electric winch to the homemade mechanism that you build according to this instruction, you get a real do-it-yourself crane. Now, to lift the load, you do not have to strain at all, the winch will do everything for you.

Even a hand winch will make lifting a load more comfortable - no need to wash your hands on the rope and worry about the rope slipping out of your hands. In any case, turning the winch handle is much easier.

Winch chain

In principle, even outside the construction site, the ability to build an elementary chain hoist for a winch in field conditions with a minimum of tools and materials is a very useful skill. It will be especially appreciated by motorists who are lucky enough to get stuck in a car somewhere in an impassable place. A whipped-up chain hoist will significantly increase the performance of the winch.

It is difficult to overestimate the importance of the chain hoist in the development of modern construction and mechanical engineering. Everyone should understand the principle of operation and visually imagine its design. Now you are not afraid of situations when you need to lift a load, but there is no special equipment. Multiple pulleys, rope, and ingenuity make it unnecessary to use a crane.

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, thanks to which you can benefit from effective force 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 the rope 8 meters long in order to lift the object to a height of 1 meter.

The use of such devices will cost you less than renting a crane, besides, 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 occurs thanks to 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 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 load. 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 a 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 have no 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 up to 50% on carbines. 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. The 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 lifting 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 the 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 during operation 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 caught, or at the moment of removing the load, or when stopping for rest. To prevent reverse movement, it is necessary to use blocks that let the rope pass 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, then the locking roller is attached last from the load being lifted, and the locking roller should 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 way 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.

Lecture number 6 POLYSPASTY

Lecture plan:

6.1 Purpose of pulley blocks.

6.2 Types of chain hoists.

6.1 Purpose of pulley blocks.

In hoisting machines, the load can be suspended from one or more branches of the flexible traction body.

If the load is suspended from several branches, the flexible body bends around a system of blocks connected to fixed and movable clips, forming a chain hoist. In pulley blocks, all blocks are bent around by one flexible body, the end of which can be attached to a movable or fixed frame. The blocks in the cages must rotate independently of each other, since the traction element, passing unequal distances over the blocks, rotates them at different speeds.

6.2 Types of chain hoists.

The simplest scheme for suspending a load is shown in Fig. 10, a, when one end of the rope is fixed on the drum, and on the other there is a load of mass Q.

A force S arises in the rope. With a large mass of the load, the force also increases. Since it determines the choice of the diameters of the rope, blocks and drum, their sizes also grow. Since the moment on the drum is equal to the product of the force in the rope and the radius of the drum (M b = SR b), it will turn out to be significant. In general, the mechanism will become cumbersome, heavy and economically unprofitable. Therefore, it makes sense to reduce the force in the rope. For this purpose, the load is suspended on two or four ropes or with the help of pulley blocks.

Polyspastom is called a system of several movable and fixed blocks bent around by a flexible body (rope or chain). Polyspasts are used for lifting and pulling loads. The introduction of chain hoists into the lifting or boom mechanisms of the crane allows you to reduce the tension of the traction unit and the load moment on the drum. Polystyles also allow reducing the gear ratio of the mechanisms in which they are installed, and, consequently, the weight and their cost.

As an independent device for lifting loads, the chain hoist is rarely used due to the lack of self-braking.

^ Fixed guide blocks are fixed on fixed axles and serve only to change the direction of the traction element, without giving a gain in strength and speed.

Due to friction in the block bearings and additional efforts from overcoming the rigidity of the ropes during bending, the ratio between the required tractive effort S and the weight Q of the load being lifted will be expressed by the formula:

^ S = Q / ή block

Where ή block- efficiency taking into account the friction losses in the block bearings and the rigidity of the traction element. When bending around a single block with a steel rope and installing the block on plain bearings ή block = 0.95; when installing the unit on rolling bearings ή block = 0,98.

Movable guide blocks

If the load Q is suspended from the movable block and one end of the rope is fixed at a fixed point, then the traction force at the other end of the rope will be approximately 2 times less than the weight of the load, and the speed of movement of the block and the load is approximately 2 times less than the speed of the pulling end.

^ S = Q / 2ή block

In this case, the result is a gain in strength and a loss in speed.

If a traction force S is applied to the movable block, and the load Q is suspended from the traction element, then the traction force will be approximately 2 times the weight of the load, and the speed of lifting the load will be approximately 2 times higher than the speed of the block;

^ S = 2Q / ή block

In this case, there is a loss in strength and a gain in the speed of lifting the load.

That. in machines for lifting the load, chain hoists of direct and reverse action are used.

Direct action polystyles are used to gain strength. In the lifting mechanism, they are designed to reduce the tension of the flexible body, the end of which runs onto the drum.

6.3 Frequency rate and types of chain hoists.

The main parameter of the chain hoist is its multiplicity (gear ratio), which is understood as the ratio of the speed of the leading end of the flexible traction element to the driven one or the number of rope (chain) branches on which the load is suspended to the number of rope branches wound on the drum.

The multiplicity of the chain hoists of direct action is equal to or more than one.

i = V S / V Q

Where V S , V Q – respectively, the speed of the leading and trailing ends of the flexible body.

In the general case, chain hoists of any multiplicity can be formed by introducing additional blocks into the storage circuit of a flexible body, and for odd multiplicity chain hoists the end of the traction element must be fixed on a movable cage, for even multiplicity chain hoists it is fastened on a cage of fixed blocks.

NS
reverse acting olyspasts provide

bake gains in the way and speed

driving body and are widely used in

lifts and forklifts.

For reverse-acting chain hoists

the multiplicity of the chain hoist is less than one because

V S
High-speed pulley blocks differ from

power pulley blocks in that they have a working

the force usually generated by the hydraulic

or pneumatically actuated, applies -

Xia to the movable holder, and the load is suspended from the free end of the rope. Therefore, they are, as it were, inverse in relation to the power pulley blocks.

V
single pulley blocks with nama-

pulling or unwinding the rope from the drum,

due to the movement of the rope

along the axis of the drum, an unwanted

new load change on the drum supports

In addition, if in a single chain hoist

no bypass blocks and rope from the hook block

which clip goes directly

on the drum, then when moving the rope

along the axis of the drum there is a displacement

lifting the load not only vertically, but also along

horizontally. To ensure strictly

vertical lifting of the load and constant

loads on the drum supports are used

double pulley blocks, consisting of

two single pulley blocks.

To ensure the normal position of the load cage with a possible uneven extension of the rope branches of both pulleys, an equalizing block is most often used. When lifting and lowering the load, this block does not rotate and serves only to equalize the length of the branches of both pulleys with an uneven rope extension. With an even multiplicity, it is located among the fixed blocks, and with an odd multiplicity, among the moving blocks of the lightning clip. Since the rope moves on the equalizing block only occasionally, its diameter is allowed to be equal to 0.8 of the block diameter, and for electric hoists and self-propelled boom cranes it is equal to 0.6 of this diameter.

From a comparison of single chain hoists with double ones, the latter have some advantages:

Provide vertical lifting of the load in the absence of a guide block at the drum;

The load is in the air in a more stable position i.e. sways less, because hangs on two widely spaced branches;

With the same decrease in the force in one branch of the rope, its wear will be less, because it has fewer kinks on the blocks. For example, to reduce the force by about four times in a single four-time pulley block, the rope is bent four times on the blocks, and in a two-time double chain - twice, since the bend on the equalizing block is not taken into account. At the same time, the effort on the drum doubles in comparison with single chain hoists, because two branches of the rope are fed to the drum;

When lifting the same load at the same speed, it turns out that in a double chain hoist, due to the suspension of the load on a twice as many branches, the diameters of the rope, drum and blocks are smaller and, therefore, the number of drum revolutions is higher, and the gear ratio of the gearbox is less. However, due to twice the length of the rope being wound on the drum, the drum length is much longer than with a single chain hoist.
2 What is a chain hoist?

3 What does a direct-action chain hoist give?

4 What does the reverse-acting chain hoist give?

5 What is the frequency of the chain hoist?

6 What are the disadvantages of single chain hoists?

7 Describe the double pulley blocks?