Polystyrene: purpose and device, their multiplicity. How the pulley block works. Polystyrene structure of the lifting mechanism

Part B

2.5. The choice of the optimal design of the chain hoist.

2.5.1 ... Each design of chain hoists, in addition to gain in effort, has other important indicators that affect the overall efficiency of its work.

General design features that improve the efficiency of the chain hoists:

The greater the working length of the chain hoist, the greater its working stroke and the distance to which the load is lifted in one working stroke.

With the same working length, a chain hoist with a long working stroke works faster.

With the same working length and working stroke, the chain hoist works faster, requiring fewer permutations.

4 . Simple 2: 1 and 3: 1 pulleys give the fastest lift with a minimum of system shuffling.

Before moving on to heavy-duty chain hoists, you need to make sure that all measures are taken to combat friction in a simple chain hoist.

Often, by reducing friction losses, it is possible to continue working with a simpler chain hoist and maintain a high lifting speed.

But in general, it all depends on the specific situation in which one or another type of chain hoist should be used. Therefore, it is impossible to give unequivocal recommendations.

In order to choose the optimal chain hoist for work in each specific situation, rescuers should know the main pros and cons of each system.

2.5.2. General performance characteristics of simple chain hoists

Pros of simple pulley blocks:

* Simple and straightforward to assemble and operate.

* In simple chain hoists, the working stroke is close to the working length of the chain hoist, since they "fold" quite fully in operation - the 1st cargo roller is pulled up close to the station. This is a serious plus, especially in cases where the total working length of the chain hoist is limited (for example, a short working shelf on a rock, etc.)

* It is required to move only one grasping (clamp).

* With enough people picking the rope, simple 2: 1 and 3: 1 pulleys give the fastest ascent speed.

Cons of simple pulley blocks:

* More (in comparison with complex pulley blocks of similar efforts) the number of rollers. Consequently, the overall friction loss is large.

For this reason, simple pulley blocks are not used in rescue practice anymore.than 5: 1.And when using carbines, it makes no sense to make a simple chain hoist more than 4: 1

* For the same total working length, simple pulleys use more rope than complex pulleys of similar forces. Fig. 18

2.5.3. General performance characteristics of complex chain hoists.

Pros of complex pulley blocks:

* With an equal number of rollers and gripping units (clamps), they can create high-effort chain hoists. For example:

3 rollers are required for complex 6: 1 and simple 4: 1.

4 rollers for complex chain hoist 9: 1 and simple 5: 1. Figure: 19, 20.

* Requires less rope compared to similar simple chain hoists. Fig 16.

* Compared to similar simple ones, complex pulleys give a greater actual gain in effort, since fewer rollers are involved.

For example: in a complex 4: 1 chain hoist 2 rollers work, and in a simple 4: 1 - 3 rollers.

Accordingly, in a complex pulley block, friction losses will be less, and the PV will be greater.

An example in fig. 21:

In a complex chain hoist 4: 1 (2 rollers) using rollers with a friction loss of 20% PV will be -3.24:1. In a simple chain hoist 4: 1 (3 rollers) - FV \u003d2.95:1

Cons of complex pulley blocks:

* Harder to organize.

* Some designs of complex chain hoists require more permutations, since in order to stretch the chain hoist over the entire working length again, it is necessary to move 2 grasping knots (clamps)

* With the same working length, the working stroke of complex chain hoists is less than that ofsimple,since they do not fold completely at each working stroke (the roller closest to the pulling one is pulled to the station, and the 1st cargo roller stops before reaching the station). This significantly reduces work efficiency, especially in cases where the total working length of the chain hoist is limited (for example, a short working shelf on a rock, etc.) It can also complicate the work in the last stages of the lifting, when the load must be lifted to the working platform.

* In general, they significantly lose to simple pulley blocks in lifting speed.

Practical tips for working with complex pulleys:

* In order for a complex chain hoist to fold more fully at each working stroke, and less permutations are required, it is necessary to space the stations of simple chain hoists that are part of a complex one. Fig. 22

* The system of a complex chain hoist requires fewer permutations in work, if a simplepulley block withbig pulls the pulley with an effortsmaller effort.

Example on fig. 22A

A -pulley block 6: 1 (2: 1 pulls for 3: 1) In this case, it is required to rearrange 2 grasping knots.

B -another chain of pulley 6: 1 - 3: 1 pulls for 2: 1. Only one grasping unit (clamp) must be repositioned. Accordingly, the system works faster.

2.5.4. In all the above structures of the chain hoists, the rope must be pulled towards the cargo station. In the mountains, in a confined area or on a wall, pulling from the bottom up can be very difficult and inconvenient. In order to pull down and turn on your weight, and also not to tear your backs, an additional stationary roller (carabiner) is often fastened. Figure: 23.

However, according to the Rule of pulley blocks No. 1 - stationary rollers do not give a gain in effort.Friction losses in such a setup, especially when using a karabiner, can negate all the benefits of pulling down.

b.Use complex pulley block.

Complex pulley blocks are neither simple nor complex - they are separateview.

A distinctive feature of complex pulley blocks is the presence of rollers in the system moving towards the load.

This is the main advantage of complex chain hoists in cases where the station is located above the rescuers and it is necessary to pull the chain hoist down.

On Fig 25.shows two schemes of complex pulley blocks used in rescue operations.

There are other schemes, but they do not find application in rescue practice and are not considered in this article.

Note:

Diagram shown on Figure: 25 complex chain hoist 5: 1 is given in the book "School of Mountaineering. Initial training ", 1989 edition, p. 442.

The main disadvantages of complex pulley blocks are similar to those of complex pulley blocks:

Complex pulley blocks do not fold completely, have a small working stroke and require many permutations at each working cycle. For example, a 5: 1 pattern requires swapping two grasping knots.

2.5.5. In cases where the efforts of the assembled chain hoist are not enough, and the length of the pulling rope is not enough to assemble a more powerful circuit, an additional 2: 1 chain hoist attached to the end of the rope with a grasping knot or clamp can help.

To do this, it is enough to have a short end of the rope or a re-cord folded 2-3 times, 1 roller (carabiner) and 1 grasping (clamp). Example on Figure: 26.

Also, for an additional 2: 1 chain hoist, the slack of the cargo rope can be used, as shown in the figure from the book by F. Kropf. "Rescue work in the mountains" 1975 Figure: 26A

This is one of the fastest and easiest-to-organize ways to increase the effort of the chain hoist - a kind of "magic wand". By adding a 2: 1 scheme to any chain hoist, you will automatically get 2x theoretical gain in effort.What will be actual winnings,depends on the situation.

The disadvantages of this scheme have already been mentioned above - this is a short working stroke and many permutations (it is necessary to permute two grasping ones).

However, there are situations when this method can help. For example, this method is often used in cases where some of the rescuers pulling the chain hoist are forced to switch to other tasks, and the efforts of those who remain to work on the chain hoist are not enough and the effort must be quickly increased.

2.5.6. Figure 27 shows a diagram of the so-called "built-in two".

Simple chain hoist 2: 1 is “built in” into the system of simple chain hoist 3: 1. The result is a chain hoist with TV 5: 1. This pulley block is neither simple nor complex. I was unable to find its exact name. The name "compound" in fig. 27 and 27A were invented by me.

Despite the small loss in TV compared to the circuit in Fig. 26 (5: 1 versus 6: 1) this system has a number of practical advantages:

* This is an even more economical way, since in addition to the rope, only one additional roller (carabiner) is required.

* In work, this method requires the permutation of only one grasping (clamp) and therefore more effective in work.

* Another example of this “built-in two” system is shown in fig. 27A.

A complex tackle 10: 1 works here - the tackle 2: 1 is "built" into the tackle 6: 1.

A similar system can be used when pulling out the victim alone. In such a scheme, large friction losses are inevitable and lifting is slow. But overall the system is quite practical, works well and allows one rescuer to work without strain.

Part C

2.6. Ways to optimize the location of the chain hoist on the ground.

Here it is important not only to reduce friction on the relief of the entire chain hoist system or its individual parts. It is also important to create the necessary working space for the effective operation of the chain hoist.

2.6.1. The main method is the use of guide rollers (hereinafter HP). Figure: 28

The guide rollers are placed in a separate station directly above the ascent (descent) point.

The station can be placed on a rock, on a tree, on a special or improvised tripod, etc. see fig. 30-37.

When climbing and descending with rope augmentation, guide rollers of the largest diameter are used, through which the rope with knots freely passes.

The station for the idler roller must be designed for heavy loads.
fig. 29.

Benefits of using guide rollers *

In short, the competent use of HP allows rescuers to work more efficiently and safely.

Below are examples of the main benefits of using idler rollers:

* Sliding of the rope under load to the side along the edge of the working platform during the work of rescuers (it does not matter whether it is an ascent or descent, a rock or a building) extremely undesirable and dangerous by rubbing the rope!

Optimally, the rope should approach the edge at an angle of 90 °. Otherwise, the load rope will inevitably slip to the side.

HP allows the load rope to be guided at the correct angle to the edge of the pad. Figure: 31

* When there is no suitable work platform directly above the lift or descent point, the HP allows you to position the load station for launching and retrieving away from the lift line in a more convenient location for work.

In addition, the location of the station away from the ascent (descent) line reduces the likelihood of hitting the rescuer, the victim, the load and safety ropes, stones, etc., which can be thrown by the rescuers working above.

* НР makes it possible to fully or partially raise the chain hoist system over the terrain. This significantly increases the efficiency of work by reducing the friction losses of the chain hoist and its components against the terrain. Due to this, the overall safety of work also increases, since the likelihood of chafing, jamming or jamming of any component of the chain hoist is reduced.

* НР allows to reduce or completely eliminate the friction of the load rope on the edge (bend) of the working platform. This is also a very big plus from a security point of view.

* HP can significantly facilitate the transition over the edge of the rescuer and the victim, both on the ascent and on the descent. This is one of the most difficult and time consuming moments in transportation, especially for an accompanying rescuer.

Guide rollers are extremely widely used by professionals in a variety of situations, both in the mountains and in technogenic conditions. Therefore, I want to illustrate this way of optimizing the location of the chain hoists on the ground in more detail. Figure: 30-37.

HP allows you to:

* Raise the ferry higher.

* Convenient to position the chain hoist system.

* Pull the pulley down.

* Adjust the tension of the crossing during operation.

Important! With a strong tension of the crossing, very large loads arise onextreme attachment points of the crossing. Figure: 38.

The conclusions from the above diagram are as follows:

* Over-tensioning of the crossings should be avoided - this is dangerous!

For example:
When two people are simultaneously crossing a very stretched crossing (the victim and the accompanying person. Total weight ~ 200kg), due to the inevitable swinging of the crossing, peak loads on the extreme points can reach 20 KN (2000kg)and higher! This load is close to the limit of strength characteristics.climbing carabiners, guy wires and ropes (taking into account the loss of strength of the rope innodes).

* All ferry attachment points including the idler roller attachment station andall its components must be extremely reliable!

To be continued…

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 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 pulley block 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 puzzling 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 its use 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 - stationary, 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 load side.

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.

How the block system works

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 you need to use 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-handling mechanisms. The block system (pulley block) is a component of such lifting mechanisms as a winch, 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 from 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. In this case, all combinations 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 the rollers moving towards the load. The complex model can include both simple and complex pulley blocks.

Combining a double and sixfold simple chain hoist gives a complex sixfold 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 cable, 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 ratio

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 the 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 is significantly different from the kinematic. 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 \u003d 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 - 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 \u003d 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 unit efficiency

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

ηb \u003d S run / S run \u003d 1 / (1 + q + 2fsinα × d / D) (3)

In the equation:

• 3 ηb - unit efficiency;
• d and D - respectively, the diameter of the bushing and the pulley itself;
• q is the coefficient of rigidity of the 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 rope material (coefficient q). The maximum value of 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 pulley block 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 \u003d ηп S0;
• S2 \u003d (ηп) 2 S0; (4)
• S3 \u003d (ηп) 3 S0;

• Sn \u003d (ηп) n S0.

Efficiency of the chain hoist at different multiplicity

Since the efficiency 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 \u200b\u200bof 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 how to lift the load with the rope. And here a simple pulley block 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 carrying 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. These ropes are called static ropes. Stretching and deformation of the flexible connection results in serious losses in 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 home-made lifting devices. Depending on the bearings 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 - 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;
• load suspension hook;
• slings - if they are needed for installation.

For quick connection, use carbines

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

1. Connect the rollers, bush and bearings. Combine it all into a cage. Get a block.
2. The rope is launched into the first block;
3. The holder with this block is rigidly attached to a fixed support (reinforced concrete beam, pillar, wall, specially mounted stem, etc.);
4. Then the end of the rope is passed through the second block (movable).
5. A hook is attached to the clip.
6. The free end of the rope is fixed.
7. 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 bending 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 you don't have to strain at all to lift the load, 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 polyspast

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 were 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. A few pulleys, rope and ingenuity make it unnecessary to use a crane.

Polystyles are called a system formed by movable and stationary blocks, which are interconnected by cable (less often - chain) transmissions. Known even in ancient times, pulley blocks are still a device without which lifting and transport equipment cannot function. In fact, the components of this mechanism have not changed much over the millennia. Polystyles, their purpose and structure are issues that are important for the effective use of all designs of lifting mechanisms.

The device of the chain hoist and its working conditions

The main area of \u200b\u200bapplication of chain hoists is boom mechanisms of cranes. The whole variety of pulley blocks can be reduced to two requirements: either increase the strength (power pulley blocks), or increase the speed (high-speed pulley blocks). In cranes, the former are more often used, and the latter are used in hoists. Thus, the schemes of high-speed and power pulleys are mutually inverse.

The pulley block includes the following components:

1. Blocks with fixed axles
2. Blocks with movable axles.
3. Bypass blocks.
4. Bypass drums.

All of the above elements are located mainly in a vertical arrangement, and the location of the drum depends on the presence of bypass blocks: from above, if such blocks are absent, and from below - if present.

The number of blocks with fixed axles is always one less than with movable ones. In this case, the total number of blocks determines (for power pulley blocks) the multiplicity of the increase in the total effort on the mechanism. The number of bypass blocks is determined by the dimensions of the node: with an increase in the number of such blocks, the force also increases.

Power pulley blocks, the purpose and design of which is characterized by several parameters, the most important of which is the load developed in the lifting mechanism. It increases with an increase in the estimated lifting capacity of the crane, the multiplicity of the device (the number of rope branches on which the load is suspended) and the efficiency of the unit. The efficiency takes into account the friction losses in the axial supports, as well as the losses determined by the rigidity of the rope or chain.

There can be several polyspasts, then the total load on the block decreases proportionally. Single pulley blocks are structurally simpler, but also the least effective. In them, one end is fixedly fixed on a stationary element, and the other on a drum. In this case, the deflection angle is very limited due to the danger of the rope coming off the block. The presence of a bypass block significantly improves the operating conditions of the mechanism: the load becomes symmetrical, which reduces rope wear, and increases the permissible rotation speed of the blocks. The stability of the chain hoist action also depends on the distance between the bypass and main blocks. With an increase in this parameter, the reliability of the chain hoist as a functional unit increases, although at the same time its complexity increases (due to the presence of a connecting axis).
Other chain hoist schemes used in practice are:

• Double triple, when there are three working blocks and two bypass blocks in the circuit;
• Double three-fold, equipped with an equalizing traverse. The variant is used in lifting equipment, which is operated in difficult and especially difficult conditions.

Performance characteristics of chain hoists and their choice

The following factors influence the effectiveness of the pulley blocks, their purpose and device in a particular mechanism:

1. The carrying capacity of the main mechanism, which includes these units.
2. Number of bypass blocks: with an increase in their number, friction losses increase.
3. Angles of deflection of the ropes from the middle plane of the drum.
4. Block diameters.
5. Rope diameter / chain height.
6. Rope material.
7. The nature of the supports (in rolling or sliding bearings).
8. Lubrication conditions for all chain hoist axles.
9. The speed of rotation of blocks or movement of traction ropes (depending on the purpose of the device).

The greatest losses in chain hoists are associated with friction conditions. In particular, the efficiency of the considered mechanisms, which operate in plain bearings, depending on the conditions of their operation, is:

• In case of unsatisfactory lubrication and at elevated temperatures - 0.94 ... 0.54;
• With rare lubrication - 0.95 ... 0.60;
• With periodic lubrication - 0.96 ... 0.67;
• With automatic lubrication - 0.97 ... 0.74.

Smaller values \u200b\u200bcorrespond to pulley blocks with the highest possible multiplicity. Friction losses for units that operate in rolling bearings are much lower and amount to:

• With insufficient lubrication and high operating temperatures - 0.99 ... 0.83;
• At normal operating temperatures and lubrication - 1.0 ... 0.92.

Thus, using modern antifriction coatings on the contact surface of blocks, friction losses can be practically excluded.

The deflection angles of the rope located on the block / blocks of the chain hoist determine not only the wear of the ropes and blocks, but also the safety of the production personnel of the lifting device. This is explained by the fact that if the permissible indicators are exceeded, the rope pulling off the block is fraught with an industrial accident. This parameter is influenced by the material of the ropes, the profile of the groove of the drum, as well as the direction of winding.
Rope materials are most often types TLK-O in accordance with GOST 3079, LK-R in accordance with GOST 2688 and TC in accordance with GOST 3071. The third type has the lowest rigidity (no more than 1.7), which has a positive effect on the maximum permissible angle of deflection of the rope on the chain hoist. Accordingly, for the first two types of ropes, the rigidity reaches 2.

Normal angles of deviation from the chain hoist axis are considered angles 7.5 ... 2.5 0 (smaller values \u200b\u200bare taken for maximum ratios of the block diameter to the rope diameter). In general, when designing these devices, they always try to choose this ratio in the range of values \u200b\u200b12 ... 40. The permissible angle of deflection of ropes made of low-stiff materials is less: up to 6.5 ... 2 0.

GOST allows an increase in the maximum deviation, in comparison with the recommended one, by no more than 10 ... 20% (depending on the operating mode of the lifting equipment). On the equalizing block, the permissible deviation angles can increase, but not more than 1.5 times.

To reduce the angles of deflection, profile grooves are made on the pulley blocks, and the angle of their direction depends on the direction of winding. Therefore, drums in mechanisms of modern design are always made with a cross-section, suitable for both types of winding.

Reeving the pulley blocks

Stocking is a technological operation of changing the location of the main cargo blocks of the chain hoist, as well as the distances between them. The purpose of the reserve is to change the speed or height of the lifting of loads by means of a certain pattern of the passage of the ropes through the blocks of the device.

Reeving schemes are determined by the type of lifting equipment. It is known, in particular, that the mechanisms for changing the boom reach are different for a manual or electric hoist - on the one hand, and for cranes - on the other. Therefore, for winches, reeving is performed by changing the position of the axis of the guide block, and is intended only to change the length of the boom. In cargo cranes, a reserve is used to correct the possible curvature of the cargo movement. In addition to cargo ropes, the reserve is also used for rope devices for moving the working trolley.

There are the following stock schemes:

1. Single entry, which is used for boom-type hoisting mechanisms with a jib. At the same time, the hook is suspended on one thread of the rope, sequentially passed through all fixed blocks, and then wound on a drum. This way of stocking is the least efficient.
2. Double, which can be used on cranes, both with a luffing jib and a beam boom. In the first case, fixed blocks are located on the boom head, and the opposite end of the rope is fixed in the cargo winch. In the second case, one of the ends of the rope is fixed at the root of the boom, and the second is sequentially passed through the bypass drum, hook suspension blocks, boom blocks, tower head blocks and then brought to the cargo winch.
3. Fourfoldused for heavy duty mechanisms. Here one of the schemes described above is implemented, but separately for each of the blocks of the hook suspension. At the same time, two working branches of the rope are directed to the blocks of the working boom. The connection of adjacent pulleys is made through an additional fixed block, which is installed on the stand of the crane swing platform.
4. Variable, the essence of which is to change the lifting capacity of the crane. With this type of storage (it can be two- or four-fold), a corresponding increase in the mass of the lifted load is possible. For this, one or two movable clips are additionally installed in the movable blocks. The retention of the clips is produced by the load rope itself due to the difference in the forces that are created by the presence of the hook suspension. Changing the re-ordering frequency is performed by lowering the hook suspension onto the support while continuing to rewind the rope.

Two - and especially - four - fold reserving allows safe lifting of the load, which is almost twice the pulling force developed by the winch. In this case, the rotation of the ropes under load is excluded, which significantly reduces their wear.

The article is based on the work "Polystyrene for rescue operations" by Fedor Farberov. The main focus of this article is the lifting and movement of loads weighing up to 100 kg. Above this mass, it is necessary to use other special equipment and other equipment and systems. The article uses technical materials from PETZL.
The material is not exhaustive and does not claim to be the truth in a single instance. These are just practical recommendations for the use of chain hoist systems when performing various works at height.

TERMINOLOGY

What is a pulley block

This is a system consisting of several movable and stationary blocks connected by a rope or cable, which allows losing in distance to obtain a significant gain in the applied force, several times less than the weight of the load. Designed for lifting, lowering, moving cargo, as well as for organizing anchor lines. Polyspast - from the Greek "poly", which means "a lot", and "spao" - "pull")
Theoretically winning - the theoretical value of the possible effort developed by the pulley block without taking into account the loss from friction on various parts of the system. It is taken as a basis for the simplicity of calculating the size of the chain hoist.
Actual winnings - the magnitude of the effort developed by the chain hoist system with the deduction of all obstructing forces affecting its efficiency.

Types of pulley blocks

Complex (reverse) pulley block - a system of sequentially located blocks or their combination (simple and complex). It is characterized by the obligatory presence of a block moving towards the load.
Simple pulley block- a system with a sequential arrangement of movable and fixed blocks.
Complex pulley block Is a system in which one simple chain hoist pulls a simple chain hoist after another.

Design features of pulley blocks

Anchor - the place of attachment of the beginning of the chain hoist and fixed blocks.
- a block located on the load or built into the chain hoist system, but always moves towards or away from the load. Always doubles strength.
- a block fixed motionlessly at the anchor point is necessary to change the direction of the applied force. Doesn't gain in effort.
The working length of the chain hoist Is the distance from the anchor to the element closest to the load (grasping knot,). The longer this value, the greater the distance the load can travel in one working stroke of the chain hoist.
The working stroke of the chain hoist - the distance that all elements of the system travel to any contact with other elements. The working stroke depends on the type of chain hoist, on its working length and because of how tightly the chain hoist "folds" - that is, how close the first element to the load is pulled to the anchor when the rope is fully selected.
Rearrange the system - the necessary manipulations to return the chain hoist to its working length after it has "folded". This can be a rearrangement of the grasping knots (clamps) and other actions.

TYPES OF POLYSPASTES IN DETAILS
Simple pulley blocks
The basis of the chain hoist: if you fix the rope at the anchor point and pass it through the block on the load, then lifting the load requires an effort 2 times less than its mass. The roller moves up with the load. In order to lift the load by 1 meter, it is necessary to stretch 2 meters of rope through the roller. then the scheme of the simplest chain hoist is 2: 1.

If you fix the rope on the load, throw it over the block attached to the anchor point and pull it down, then to lift the load, you must apply an effort equal to the weight of the load, and in order to lift the load by 1 meter, you need to stretch 1 meter of rope through the block.
How many times we win in effort - the same amount of times we lose in distance.

Calculation of effort in a simple pulley block
For simplicity of calculating the theoretical gain of the chain hoist, it is customary to use the "T - method" (from the English Tension - tension).

The theoretical gain in a simple pulley block is equal to the number of strands going up from the load. If the movable blocks are fixed not on the load itself, but on a rope coming from the load, then the strands are counted from the point of fixing the blocks.
In simple pulley blocks, each movable roller (fixed to the load) added to the system gives a two-fold theoretical gain. The additional effort is added to the previous one.

Types of simple pulley blocks
Continuing to add movable and fixed blocks, we get the so-called simple pulley blocks of different efforts. Depending on where the end of the working rope is fixed (on an anchor or on a load), simple pulley blocks are divided into even and odd.

• • If the end of the rope is attached to the anchor point, then all subsequent pulleys will be even: 2: 1, 4: 1, etc.
  • If the end of the load rope is attached to the load, then odd pulleys will be obtained: 3: 1, 5: 1, etc.

Benefits of simple pulley blocks	Disadvantages of simple pulley blocks
Simple and straightforward to assemble and operate.	A lot of equipment is required to organize chain hoists with large TVs
The working stroke is close to the working length of the chain hoist.	Difficult transition from ascent to descent.
With enough people, simple 2: 1 and 3: 1 pulleys give the fastest lifting speed.	It is difficult to pass nodes through the system.
Automatic rope fixing system can be arranged	A large number of blocks and ropes used with patterns greater than 4: 1, and therefore large overall friction losses.
No additional rope required.
Convenient to use in small work areas

It is impractical because of friction, in a simple pulley block to use schemes more than 5: 1.

Polypast made from extra rope.
In practice, most often there is a situation when a chain hoist made of a separate rope is attached to a working rope. This is primarily due to the saving of equipment. In such a scheme, fixing of the reverse stroke is required. The chain hoist is attached to the working rope with a grasping knot or clamp.

Complex pulley blocks
When creating a complex chain hoist, 2, 3 or more simple chain hoists can be connected. To calculate the theoretical gain in effort when using a complex pulley block, it is necessary to multiply the values \u200b\u200bof the simple pulley blocks that make up it.

Calculation of effort in complex pulley blocks
The calculation of the effort of each of the simple pulley blocks that are part of the complex is made according to the rule of simple pulley blocks. The 6: 1 scheme adds up so 2: 1 pulls for 3: 1 it turns out 6: 1. And 3: 1 pulls for 3: 1 and it turns out 9: 1.

Practical tips for working with complex pulleys:
In order for the complex chain hoist to fold more fully at each working stroke, and less permutations are required, it is necessary to space the stations of the simple chain hoists that are part of the complex.

Complex pulley blocks
In all the above construction of chain hoists, the rope must be pulled towards the anchor point. In practice, it is always more convenient to pull from the anchor point because a counterweight can be used. In order to pull down, an additional fixed block is attached. But it does not provide a gain in strength, and frictional losses in such a setup can negate all the benefits of pulling down. A distinctive feature of complex pulley blocks is the presence of rollers in the system moving towards the load. Complex pulley blocks are also simple and complex.
The disadvantages are the same as for the main complex pulley blocks:

• • The polystyles do not fold completely,
  • They have a small working stroke and require a lot of permutations.

Calculation of effort in complex pulley blocks
The calculation of the theoretical gain in complex pulley blocks differs from the basic ones. 3: 1 (simple) \u003d 1T + 2T
5: 1 (hard) \u003d 1Т + 1Т + ЗТ (or as it is commonly considered 5: 1 \u003d 2Т * ЗТ-1Т)
7: 1 (hard) \u003d 2T * 3T + 1T

Compound pulley blocks
In cases where the efforts of the assembled chain hoist are not enough, and the length of the pulling rope is not enough to assemble a more powerful circuit, an additional 2: 1 chain hoist attached to the cargo rope with a grasping knot or clamp can help.
By adding a 2: 1 pattern to any chain hoist you will automatically get 2x the theoretical gain in effort.

The calculation of the theoretical gain from them is made on the basis of complex or complex, depending on the design of the chain hoist.

To be continued…

It makes sense to use such a chain hoist when lifting loads weighing up to 700 kg. Very 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

• Double Promalp block: 2 pcs.
• Carabiner Height 513: 3 pcs.
• Rope Promtex 10 mm: 100 m
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