Load lifting system 4 letters. Double block rollers and chain hoists

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A pulley block is a system of movable and fixed blocks connected by a flexible connection (ropes, chains) used to increase the force or speed of lifting loads. A chain hoist is used in cases where it is necessary to lift or move a heavy load with minimal effort, provide tension, etc. The simplest pulley system consists of just one block and a rope, and at the same time it allows you to halve the traction force required to lift a load.

Typically, lifting mechanisms use power pulleys to reduce the tension of the rope, the moment from the weight of the load on the drum and the gear ratio of the mechanism (hoist, winch). High-speed pulleys that allow you to gain a gain in the speed of movement of the load at low speeds of the drive element. They are used much less frequently and are used in hydraulic or pneumatic lifts, loaders, and mechanisms for extending telescopic booms of cranes.

The main characteristic of the pulley is the multiplicity. This is the ratio of the number of branches of the flexible body on which the load is suspended to the number of branches wound on the drum (for power pulleys), or the ratio of the speed of the leading end of the flexible body to the driven end (for high-speed pulleys). Relatively speaking, the multiplicity is a theoretically calculated coefficient of gain in strength or speed when using a chain hoist. Changing the multiplicity of the pulley system occurs by introducing or removing additional blocks from the system, while the end of the rope with an even multiplicity is attached to a fixed structural element, and with an odd multiplicity - on the hook clip.

Depending on the number of rope branches attached to the drum of the lifting mechanism, single (simple) and double chain hoists can be distinguished. IN single pulley hoists, when winding or unwinding a flexible element due to its movement along the axis of the drum, an undesirable change in the load on the drum supports is created. Also, if there are no free blocks in the system (the rope from the hook suspension block directly passes to the drum), the load moves not only in the vertical, but also in the horizontal plane.

To ensure strictly vertical lifting of the load, double pulleys (consisting of two single ones) are used; in this case, both ends of the rope are fixed to the drum. To ensure the normal position of the hook suspension in case of uneven stretching of the flexible element of both pulleys, a balancer or equalizing blocks are used. Such pulleys are used mainly in overhead and gantry cranes, as well as in heavy tower cranes, so that two standard cargo winches can be used instead of one large, high-power one, and also to obtain two or three speeds for lifting loads.

IN power pulleys when increasing the multiplicity, it is possible to use ropes of reduced diameter, and as a result, reduce the diameter of the drum and blocks, reduce the weight and dimensions of the system as a whole. Increasing the multiplicity allows you to reduce the gear ratio, but at the same time requires a larger rope length and rope capacity of the drum.

High-speed pulleys differ from power pulleys in that in them the working force, usually developed by a hydraulic or pneumatic cylinder, is applied to a movable cage, and the load is suspended from the free end of a rope or chain. The gain in speed when using such a pulley is obtained as a result of increasing the height of the load.

When using pulleys, it should be taken into account that the elements used in the system are not absolutely flexible bodies, but have a certain rigidity, so the oncoming branch does not immediately fall into the stream of the block, and the running branch does not straighten out immediately. This is most noticeable when using steel ropes.

Blocks are used to change the direction of traction when lifting and moving small weights or when tightening gear, as well as for founding hoists. The block consists of a wooden, metal or molded plastic housing, inside of which one or more metal pulleys are loosely mounted on an axis called a dowel. The blocks come in one-, two-, three- and multi-pulley types. The block body has partitions that separate one pulley from the other. The outer surfaces of the outermost partitions are called cheeks.

Rice. 1. Gorden.

The simplest design is the single-pulley block. A cable passed through such a block, fixed motionless, is called a gorden (Fig. 1). The arbor allows you to change the direction of thrust when lifting and moving a load, but does not provide any gain in strength, so it is used for lifting small weights. Single-pulley blocks with halyards passed through them are used to hoist flags and pennants, signal lights and signs.

Wooden and plastic blocks are used only when working with vegetable and synthetic cables. Most marine equipment uses metal blocks.

Rice. 2. Metal blocks.

Double-pulley metal block (Fig. 2, A) consists of a body 3, two steel or cast iron pulleys 4, bushings 5 with lubrication groove or with bearing, dowel 6, shackles 7 , mounting bolts 1 and pendants 2.

To equip the block, the cable must be passed between the cheeks of the block and placed in the pulley bale. Equipment simple block inconvenient, since you have to thread the cable from the end. Therefore, on ships they use single-pulley blocks with a folding cheek - rosin blocks (Fig. 2, b). The folding jaw allows you to insert the middle of the cable into such a block.

To prevent excessive bending of the cable passing through the pulley of the block, the dimensions of the block must correspond to the thickness of the cable. Pulley diameter metal block should be at least 10 - 15 diameters of a steel cable, and a wooden one - 2 times the circumference of a vegetable or synthetic cable.

The blocks must be periodically disassembled, cleaned of dirt and rust, and lubricated rubbing parts. If cracks or significant wear of the dowel or pulley are detected, the block should be replaced. Units that are not in use must be thoroughly lubricated and stored in a dry place in a suspended state.

Hoists are devices that allow you not only to change the direction of traction, but also to gain strength when lifting and moving heavy objects, when tightening gear and in other cases. According to their design, hoists are divided into ordinary and mechanical.

Ordinary hoists consist of two blocks, through the pulleys of which a cable called a rope is passed. One end of the shovel, attached to the block, is called the main end, the other, coming out of the block, to which an external traction force is applied, is called the running end. One block of hoists, fixed, is secured in place through a suspension. The other block is called movable, since during operation it rises along with the load or moves in the direction of tightening the gear. According to the number of pulleys in both blocks, hoists are divided into two-, three-, four- and multi-pulley.

Rice. 3. Ordinary two-pulley hoists.

The simplest are double-pulley hoists, based on a lopar between two single-pulley blocks. Such hoists can be based in two ways: the running end of the lopar comes off the stationary one (Fig. 3, A) or from a mobile one (Fig. 3, b) block. Let's consider the gain in force when lifting a load with a mass T will be in both cases.

In the first case, the mass of the load is distributed over two branches of the lopar coming out of the lower, movable block, and in the second - over all three branches. Therefore, to hold a load weighing T in the first and second cases, efforts must be made to the running ends of the lopars F 1 And F2, equal to 1/2 respectively T and 1/3 T. This means that the gain in strength is equal to the number of loaded branches of the Lapp or the total number of pulleys in both blocks in the first case and the total number of pulleys plus one in the second. Thus, indicating the total number of pulleys in both blocks P, we obtain formulas expressing the dependence of the force applied to the running end of the lopar to hold the load suspended, and the total number of pulleys in both blocks:

F 1 =m/n; F 2 =m / (n+1)

To lift the load to the running end of the lopar, additional effort is required to overcome the friction forces arising in the hoists. It is practically believed that the effort to overcome frictional forces in each pulley of vegetable or flexible hoists steel cable, constitutes 10 and 5% of the mass of the lifted load, respectively.

Rice. 4. Ordinary multi-pulley hoists.

Ordinary hoists are used on ships various designs and load capacity. To tighten gear, three-pulley grip hoists are used (Fig. 4, A). Along with them, hoists are used, based between two blocks with the same number of pulleys - gintsy (Fig. 4, b). The armament of heavy booms includes multi-pulley hoists with blocks with pulleys on ball bearings - gini (Fig. 4, V).

The methods for founding hoists depend on the number of pulleys in the blocks (Fig. 5). They are always founded with the root end of the lapar clockwise for right-hand descent cables and counterclockwise for left-hand descent cables. The hoists are based on the deck, placing one block opposite the other at some distance with the pendants outward. For the base of double-pulley hoists (Fig. 5, A) the one that has a device for attaching the root end of the lapar is taken as a fixed block. The root end is passed through the pulley of the stationary block, then through the pulley of the movable one and attached to the stationary block.

Rice. 5. Methods for founding hoists.

When founding three-pulley hoists (Fig. 5, b) a two-pulley block is taken to be a fixed block, and a single-pulley block is taken to be a movable block. The root end is passed through the lower (closest to the deck) pulley of the two-pulley block, through the pulley of the single-pulley block, then through the upper pulley of the double-pulley block and is attached to the single-pulley block.

When founding four-pulley hoists (Fig. 5, V), consisting of two two-pulley blocks, the root end is passed sequentially, first through the lower pulleys of the fixed and movable blocks, then through the upper pulleys of these blocks, after which the root end is brought to the fixed block and secured to it.

The base between two three-pulley blocks of six-pulley guineas (Fig. 5, G) is carried out with the root end of the paddle according to the scheme: middle pulley of the stationary block - lower pulley of the movable - lower pulley of the stationary - middle pulley of the movable - upper pulley of the stationary - upper pulley of the movable - to the attachment point on the stationary block. This wiring diagram for the root end of the paddle prevents skewing of the blocks during lifting of the load.

In all cases, after passing the root end of the lopar through all the pulleys of both blocks, it is sealed with a fire and thimble, with which it is attached to the butt on the corresponding block.

Mechanical hoists allow you to obtain multiple gains in strength, the ability to smoothly lift the load and keep it automatically locked in any position.

Rice. 6. Mechanical differential hoists.

Mechanical differential hoists are widely used on ships (Fig. 8). The suspension of such hoists contains a fixed block cage, which consists of two rigidly connected pulleys of different diameters with a diameter ratio of 7:8 or 11:12. The suspension with the block is attached to a fixed support or to the traverse of a trolley moving along a suspended rail. The lower (movable) single-pulley block is also placed in a cage that has a hook for hanging the load. The closed operating chain sequentially covers the small pulley of the fixed block, the pulley of the movable and the large pulley of the fixed block. Lifting of the load is ensured by turning the large pulley of the stationary block by applying a traction force to the branch of the working chain running from this pulley.

When lifting heavy loads with differential hoists, a 16-fold (with a ratio of the diameters of the fixed block pulleys of 7:8) and 24-fold (with a ratio of these diameters of 11:12) theoretical (without taking into account friction) gain in strength is obtained.

Ordinary hoists that are not in use are stored in a dry, ventilated area in a suspended state. All rubbing parts of the blocks are well lubricated. After finishing work with portable hoists, they are carefully folded, preventing the hoist from becoming tangled. When working with ordinary hoists, try to avoid sudden jerks, which can lead to breakage of the paddle or damage to the blocks. If, upon inspection of the blocks, significant wear of the dowels, hooks, brackets or butts is discovered, such blocks are replaced and the hoists are re-founded.

Mechanical hoists are kept clean, rubbing parts are regularly lubricated, and their serviceability is monitored.

Lifting machines are designed to help a person lift something heavy to a height. Most lifting mechanisms are based on simple system blocks - chain hoist. He was already familiar to Archimedes, but now many people do not know about this brilliant invention. Remembering your physics course, find out how such a mechanism works, its structure and scope. Having understood the classification, you can begin to calculate. For everything to work out, here are instructions for constructing a simple model.

The invention of the pulley 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 puzzle modern builders. Shipbuilding also improved, and people were able to travel great distances. It's time to figure out what it is - a chain hoist and find out where it can be used today.

Simplicity and efficiency of the mechanism

Structure of the lifting mechanism

A classic chain hoist is a mechanism that consists of two main elements:

pulley;
flexible connection.

The simplest diagram: 1 – moving block, 2 – fixed, 3 – rope

A pulley is a metal wheel that has a special groove for a cable along its outer edge. An ordinary cable or rope can be used as a flexible connection. If the load is heavy enough, ropes made of synthetic fibers or steel ropes and even chains are used. To ensure that the pulley rotates easily, without jumping or jamming, roller bearings are used. All elements that move are lubricated.

One pulley is called a block. A pulley block is a system of blocks for lifting loads. The blocks in the lifting mechanism can be stationary (rigidly fixed) and movable (when the axis changes position during operation). One part of the pulley 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 stationary 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 operating principle of a pulley block is similar to a lever: the force that needs to be applied becomes several times smaller, while the work is performed in the same volume. The role of the lever is played by the cable. In the operation of a chain hoist, the gain in strength is important, so the resulting loss in distance is not taken into account.

Depending on the design of the pulley, the gain in strength may vary. The simplest mechanism of two pulleys gives approximately a twofold gain, of three - threefold, and so on. The increase in distance is calculated using the same principle. To operate a simple pulley, you need a cable twice as long as the lifting height, and if you use a set of four blocks, then the length of the cable increases in direct proportion to four times.

Operating principle of the block system

In what areas is the block system used?

Pulley hoist – faithful assistant in the warehouse, in production, in the transport sector. It is used wherever force needs to be used to move all kinds of loads. The system is widely used in construction.

Despite the fact that most of the heavy work is performed by construction equipment (cranes), 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, and construction equipment (various types of cranes, bulldozer, excavator).

In addition to the construction industry, pulley hoists received wide application in organizing rescue operations. The principle of operation remains the same, but the design is slightly modified. Rescue equipment is made of durable rope and carabiners are used. For devices of this purpose, it is important that the entire system is quickly assembled and does not require additional mechanisms.

Pulley hoist as part of a crane hook

Classification of models according to different characteristics

There are many executions of one idea - a system of blocks connected by rope. They are differentiated depending on the method of application and design features. Meet different types 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 chain hoists.

Example of even models

A simple chain hoist is a system of series-connected rollers. All movable and fixed blocks, as well as the load itself, are combined by one cable. Even and odd simple pulleys are differentiated.

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

Odd chain hoist diagram

A complex pulley system can be called a pulley system. In this case, not individual blocks are connected in series, but entire combinations that can be used on their own. Roughly speaking, in this case one mechanism sets in motion another similar one.

The complex chain hoist does not belong to one or the other type. His distinguishing feature– rollers moving towards the load. The complex model can include both simple and complex chain hoists.

Combining a two-fold and six-fold simple pulley gives a complex six-fold version

Classification according to the 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 version, B – high-speed

The power option is used more often. As the name suggests, its task is to ensure a gain in strength. Since significant gains require equally significant losses in distance, losses in speed are also 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.

By its principle, a high-speed pulley is a reverse power design. It does not give a gain in strength, its goal is speed. Used to speed up work at the expense of the applied effort.

Multiplicity is the main characteristic

The main indicator that people pay attention to when organizing cargo lifting is the multiplicity of the pulley. 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 over.

Kinematic ratio

The multiplicity is divided into kinematic (equal to the number of kinks in the rope) and force, which is calculated taking into account the cable’s overcoming the friction force and the non-ideal efficiency of the rollers. The reference books contain tables that display the dependence of the power factor on the kinematic factor at different block efficiencies.

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

Schemes of pulleys of different multiplicities

How to make calculations for a chain hoist

Despite the fact that theoretically the design of a pulley hoist is extremely simple, in practice it is not always clear how to lift a 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. The mechanism is subject to frictional forces 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 a construction site or as part of construction equipment. Steel rope or chain has much greater rigidity. Since bending such a cable when running against a block requires additional force, it must also be taken into account.

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

SrunR = SrunR + q SrunR + Nfr (1)

Formula 1 shows the moments of such forces:

Srun – force from the side of the escape rope;
Srun – force from the oncoming rope;
q Srun – force 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 arm - the radius of the block R or the radius of the sleeve r.

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

N = 2 Srun×sinα (2)

In this equation:

N – impact on the pulley axis;
Srun - force from the oncoming rope (taken to be approximately equal to Srun;
α is the angle of deviation from the axis.

Pull block block

Calculation of the useful action of the block

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

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

In the equation:

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

From this formula it can be seen that the efficiency is affected by the structure of the block (through the f coefficient), its size (through the d/D ratio) and the rope material (q coefficient). The maximum efficiency value can be achieved using bronze bushings and rolling bearings (up to 98%). Sliding bearings will 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. Total Efficiency of pulley block not equal arithmetic sum all individual components. For the calculation, they use a much more complex formula, 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 a chain hoist at different magnifications

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 pulley will depend not only on ηb, but also on the number of these blocks - the multiplicity of the system. Using the table, you can find ηп for systems with different numbers of blocks at different meanings Efficiency of each.

How to make a lift with your own hands

In construction during installation work It is not always possible to fit a crane. Then the question arises of how to lift the load with a rope. And here a simple chain hoist finds its application. To make it and fully operate, you need to make calculations, drawings, and choose the right rope and blocks.

Different schemes simple and complex lifts

Preparation of the base - diagram and drawing

Before you start building a chain hoist with your own hands, you need to carefully study the drawings and choose a suitable scheme 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 load-carrying capacity of the pulley blocks is not enough, and constructing a complex multiple lifting mechanism there is no time and opportunity. Then double chain hoists are used, which are a combination of two single ones. This device can also lift the load so that it moves strictly vertically, without distortions.

Drawings of the twin model in different variations

How to choose a rope and block

The most important role in building a chain hoist with your own hands is played by the rope. It is important that it does not stretch. Such ropes are called static. Stretching and deformation of a flexible connection causes 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 homemade lifting devices with the calculated load capacity. 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 can you lift a load to a height with your own hands and not drop it? To protect the load from possible reverse movement, you can install a special locking block that allows the rope to move only in one direction - the desired direction.

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 diagram has been selected, and the calculations have been made, you can begin assembly. For a simple double pulley you will need:

roller – 2 pcs.;
bearings;
bushing – 2 pcs.;
clip for block – 2 pcs.;
rope;
hook for hanging cargo;
slings - if they are needed for installation.

Carabiners are used for quick connection

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

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

The homemade lifting mechanism is ready to use and will provide double the strength benefits. Now, to raise the load to a height, just pull the end of the rope. By going around both rollers, the rope will lift the load without special effort.

Is it possible to combine a chain hoist and a winch?

If to homemade mechanism, which you will build according to these instructions, attach an electric winch, you will get a real crane, made by yourself. Now you don’t have to strain at all to lift the load; the winch will do everything for you.

Even hand winch will make lifting the load more comfortable - no need to rub 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.

Pulley hoist for winch

In principle, even outside the construction site, the ability to hiking conditions With a minimum of tools and materials, building a basic pulley for a winch is a very useful skill. It will be especially appreciated by motorists who are lucky enough to get their car stuck somewhere in an impassable place. A quick-made chain hoist will significantly increase the performance of the winch.

Overestimate the importance of the chain hoist in development modern construction and mechanical engineering is difficult. 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, a rope and ingenuity will allow you to do this without using a crane.

A person is not very strong for lifting large loads, but he came up with a lot of mechanisms that facilitate this process, and in this article we will discuss pulley hoists: the direction and design of systems of this type, and we will also try to implement the simplest version of such a device with my own hands.

1 How do we make lifting easier?

A cargo pulley is a system that consists of ropes and blocks, thanks to which you can gain effective strength while losing length. The principle is very simple. In length we lose exactly as many times as we win in strength. Due to this golden rule of mechanics, it is possible to lift large loads without exerting considerable effort. Which is usually not very critical. Let's give an example. Now you have gained 8 times the strength, and you will need to take out a rope 8 meters long to lift the object to a height of 1 meter.

The use of such devices will be more affordable for you than renting a crane for lifting work; moreover, you can control the gain in strength yourself. The pulley has two opposite sides: one of them is fixed, which is fixed on the support, and the rest is movable, which clings to the load itself. The gain in strength is achieved thanks to the movable blocks, which are fixed on the movable side of the pulley. The fixed part serves solely to change the path of movement of the rope itself.

Types of pulleys are distinguished by problem, parity and multiplicity. According to the problem, there are ordinary and difficult mechanisms, and the multiplicity means multiplying the force, in other words, if the multiplicity is 4, then in theory you gain 4 times in strength. Also, infrequently, but still, a high-speed pulley is used; this option provides a gain in the speed of moving loads at a very low speed of the drive components.

2 How does conventional block construction work?

Let's first consider a simple assembly pulley. It can be obtained by adding blocks to a support and a load. In order to get an odd mechanism, you should strengthen the end of the rope on a moving point of the load, and in order to get an even one, we fix the rope to a support. When adding a block, we gain +2 to strength, and the moving point provides +1, based on this. For example, in order to get a pulley for a winch with a multiplicity of 2, you should strengthen the end of the rope on a support and use one block, which is fixed to the load. And we will have an even view of the device.

The working principle of a chain hoist with a multiplicity of 3 looks different. Here the end of the rope is fixed to the load, and two rollers are used, one of which we fix to the support, and the other to the load. This type of mechanism provides 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 come from the load, such is our gain in strength. In most cases, pulleys with a hook are used, on which, essentially speaking, the load is fixed; it is wrong to think that it is just a block and a rope.

3 Difficult block system - how to calculate the gain in strength?

Now let’s find out how a difficult type chain hoist works. This name refers to a mechanism where several conventional versions of this cargo device are connected into one system; they pull each other. The gain in strength 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 our theoretical gain in force will be equal to 8. All of the above calculations take place only for wonderful systems that do not have friction force, as practice has shown things are different.

In any of the blocks there is a small loss in power due to friction, since it is still spent on overcoming the friction force. In order to reduce friction, we must remember: the larger the bend radius of the rope, the less frictional force will be. It is advisable to use rollers with a large radius where this is feasible. When using carabiners, it is necessary to make a block of similar options, but rollers are much more effective than carabiners, since the loss on them is 5-30%, but on carabiners it is up to 50%. It is also useful to know that the most effective block should be placed closer to the load to obtain the greatest effect.

How can we calculate the real gain in power? For this, it is important for us to know the efficiency of the blocks used. Efficiency is expressed by numbers from 0 to 1, and if we use a rope of large diameter or too stiff, then the effectiveness of the blocks will be much less than indicated by the manufacturer. This means that it is necessary to provide for this and adjust the efficiency of the blocks. To calculate the actual gain in regular type strength lifting mechanism, you need to calculate the load on each branch of the rope and fold them. To calculate the gain in strength of difficult types, you need to multiply the real strengths of the ordinary ones that it consists of.

It is also necessary to remember about the friction of the rope, since its branches can twist among themselves, and the rollers from high loads can gather and pinch the rope. To prevent this from happening, the blocks should be spaced apart in relation to each other, for example, you can use a circuit board between them. It is also necessary to buy only static ropes that do not stretch, since dynamic ones give a serious loss in strength. To assemble the mechanism, either a separate or a cargo rope can be used, attached to the load regardless of the lifting device.

The advantage of using individual rope is that you can quickly assemble or prepare a load-lifting structure in advance. You can also use its entire length, this also makes it easier to pass the knots. One of the disadvantages is that there is no possibility of automatic fixation of the lifted load. The advantages of a cargo rope are that automatic fixation of the lifted object is possible, and there is no need for an individual rope. Among the disadvantages, the main thing is that it is difficult to move the knots during operation, and you also have to waste a cargo rope on the mechanism itself.

Let's talk about the reverse motion, which is inevitable, since it can appear when the rope is caught, or at the moment of removing the load, or when stopping to rest. To prevent reverse motion, you need to use blocks that allow the rope to pass only in one direction. At the same time, we organize the structure so that the locking roller is fixed first from the object being lifted. Due to this, we not only avoid reverse movement, but also allow us to fix the load for a certain period of time during unloading or simply rearranging the blocks.

If you are using a separate rope, then blocking roller is fixed at the end of the load being lifted, while the fixing roller must be very effective.

5 Options for fastening the rope to the lifting mechanism

Now a little about fastening lifting mechanism to the cargo rope. It’s not often that we have a rope of the required length nearby to fix the moving part of the block. Here are a couple of types of mechanism mounting. The first method is using grasping knots, which are knitted from cords with a diameter of 7-8 mm, in 3-5 turns. This option, in practice, is considered the best, since a gripping knot made of 8 mm cord on a rope with a diameter of 11 mm begins to slide only under a load of 10-13 kN. At the same time, at first it does not deform the rope, but after a while, it melts the braiding and sticks to it, beginning to play the role of a fuse.

Another option is to use a general direction clamp. Time has shown that it can be used on icy and wet ropes. It begins to crawl only with a load of 6-7 kN and slightly damages the rope. The next method is to use a personal clamp, but it is considered not recommended, since it begins to crawl with a force of 4 kN and at the same time tears the braid, or can even eat the rope. All these are industrial designs and their use, but we will try to make a handmade chain hoist.

6 We create the simplest lift with our own hands

But if a mechanism for loads is needed immediately or for one time, but you don’t have enough time to pick it up in stores and don’t have enough cash, we’ll tell you how to make a chain hoist with your own hands. It’s great if you have threaded rods, bearings, a block, a cable, a hook, and a gear in your workshop. It will take a little time: you need to place the bearings on the stud. It would be a good idea to secure the nut from the stud so as not to waste a certain amount of energy down the drain on turning the peculiar shaft. The end of the pin can be equipped with a gear, thus making a much more convenient manual drive.

We throw the cable over the block and secure it to the support, but we attach a hook to the other end, on which we will hang the load. You can also attach a sling system to the end of the cable if the nature of the load does not allow it to be attached to the hook. As a rule, the simplest version of the chain hoist is ready. All that remains is to start working, following safety precautions, which are the same for absolutely all mechanisms, both purchased and homemade. Carefully check all parts for integrity before work, and during the working period there is no need to make sudden movements, the load should be lifted slowly, and, of course, you should not stand under a suspended load.

The block consists of one or more wheels (rollers) encircled by a chain, belt or cable. Just like a lever, a pulley reduces the force required to lift a load, but it can also change the direction of the force applied.

The gain in strength comes at the cost of distance: the less effort required to lift a load, the longer the distance that the point of application of this effort must travel. The pulley system increases the strength gain by using more load-carrying chains. Such power-saving devices have a very wide range of applications - from moving massive steel beams to heights to construction sites before the flags are raised.

As with others simple mechanisms, the inventors of the block are unknown. Although blocks may have existed before, the first mention of them in literature dates back to the fifth century BC and relates to the use of blocks by the ancient Greeks on ships and in theaters.

Movable block systems mounted on a suspended rail (picture above) widely used on assembly lines because they greatly facilitate the movement of heavy parts. The applied force (F) is equal to the weight of the load (W) divided by the number of chains used to support it (n).

Single fixed blocks

This simplest type block does not reduce the force required to lift the load, but it changes the direction of the applied force, as shown in the figures above and at the top right. Fixed block on the top of the flagpole makes it easier to lift the flag by allowing the cord to which the flag is attached to be pulled down.

Single moving blocks

The single pulley, which can be moved, reduces the force required to lift the load by half. However, halving the applied force means that the point of application must travel twice as far. IN in this case force is equal to half the weight (F=1/2W).

Block systems

When using a combination of a fixed block and a moving one, the applied force is a multiple of the total number of load-carrying chains. In this case, the force is equal to half the weight (F=1/2W).

Cargo, suspended vertically through the block, allows horizontal electrical wires to be pulled taut.

Suspended lift(picture above) consists of a chain wrapped around one movable and two fixed blocks. Lifting a load requires a force that is only half of its weight.

Pulley hoist, usually used in large cranes(picture on the right), consists of a set of movable blocks from which the load is suspended, and a set of fixed blocks attached to the crane boom. Receiving a gain in strength from such large quantity blocks, the crane can lift very heavy loads, such as steel beams. In this case, the force (F) is equal to the quotient of the weight of the load (W) divided by the number of supporting cables (n).

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