The crane drive has its limit. Or rather, the cost of the engine is growing much faster than the weight of the load that it can lift. Of course, nothing prevents you from installing a very expensive engine, but there is a better way - using a chain hoist.
In fact, it was with the chain hoist that the development of the GPM as complex mechanisms began. In its scheme, the chain hoist uses more ancient inventions, such as a block and a flexible joint. The rope instead of the lever began to be used far from immediately.
In the future, the chain hoist began to be used everywhere. No sailing ship can do without such a simple but indispensable rigging. Of course, the modern design of the chain hoist has changed a lot, but the essence remains the same.
Pulley scheme
Here is the simplest scheme of the chain hoist.
Circles are blocks. A large circle drive, or rather a drum,. The end of the cable is not fixed on the crane hook, but on a surface that is stationary relative to the crane. Such a surface can be a crane boom or, if we talk about tower cranes, a carriage. The lower block is not fixed in any way on the crane and is movable relative to it. These are the two simplest schemes for a pulley block device.
What kind of loads arise in this case?
Pulley block calculation
It would be more correct to ask how the load on the engine and on the rope itself will change. In our case, it will be doubled. Of course, you can give formulas and school examples known since the time of Archimedes, but you can take my word for it. But this is a relatively simple example. more complex I will tell in another article. And now consider what kind of pulley blocks are.
The device and types of chain hoists
To begin with, it is worth noting that all chain hoists are divided into two types:
- power chain hoist
- high-speed chain hoist
Of course, we, as practitioners, are more interested in a power pulley block, but it’s worth understanding the device of another type of pulley block.
In the example above, the device is a power chain hoist. In it, the effort is halved, but there is also a significant drawback. Look carefully at the drawing. The speed of changing the position of the load will be two times lower than the speed of the “winding” of the cable on the motor bobbin.
The high-speed chain hoist is the opposite picture. Just imagine that the engine and the hook are swapped. The speed relative to the basic blockless version will double. But the effort required to lift the load will also increase.
Polyspast multiplicity
We complicate the scheme. Nobody prevents us from using not two blocks, but three, four or more.
The figure shows a double chain hoist. The load on the engine is reduced by about four times. “Approximately” because we lose part of the effort on the friction of the rope on the block. The block efficiency is typically 0.97.
The multiplicity of the chain hoist is just the ratio of the efforts of the cable on the drum and near the load. In the example above, the pulley multiplicity is four.
Appointment and use of a chain hoist
In modern construction, chain hoists are used very widely. complex design with cheeks are immediately designed for them.
The pulley structure can be blocked if it is not needed. The use of a chain hoist as an independent GPM is limited by only one factor - the absence of a brake, which is vital in hoisting machines.
Many specialized companies are engaged in the sale of chain hoists. Before buying a chain hoist, make sure that the selected characteristics are correct for your needs, and if in doubt, contact the professionals.
Blocks and chain hoists- simple mechanisms used to lift loads either with little effort or with effort in a position convenient for the user.
Blocks and chain hoists consist of two parts: a wheel with a circumferential groove (pulley) and a rope or cable. A block, as a rule, is a device consisting of one pulley in a frame with a suspension and one cable. Polyspast - a combination of pulleys and cables. The principle of its operation is similar to the operation of a lever - the gain in strength affects the increase in distance with the theoretical equality of the work performed.
These mechanisms can be used independently of other lifting units, such as: winches, hoists, cranes, and also as parts of them.
The figures show the principle of operation block and chain hoist:
In Fig. 1a, a load with a weight W1 is lifted using a single block with a force P1 equal to the weight. In Fig.1,b, the load W2 is lifted by the simplest multiple chain hoist, consisting of two blocks, with a force P2 equal to only half the weight W2. The effect of this weight is divided equally between the rope strands on which pulley B2 is suspended from pulley A2 by means of hook C2. Therefore, in order to lift the load W2, it is sufficient to apply a force P2 equal to half the weight W2 to the cable branch passing through the groove of the pulley A2; thus, the simplest chain hoist gives a double gain in strength. Fig. 1, c explains the operation of a chain hoist with two pulleys, each of which has two grooves. Here, the force P3 required to lift the load W3 is only a quarter of its weight. This is achieved by distributing the entire weight of W3 between the four suspension cables of the B3 unit. Note that the multiplicity of the gain in strength when lifting weights is always equal to the number of cables on which the movable block B3 hangs.
Rice. 2
In the past as a rope for blocks and chain hoists flexible and durable hemp rope was used. It was woven with a braid of three strands, each of which consisted of many small strands. Polyspasts with such ropes were used wherever it was necessary to lift loads: sea vessels, in agriculture, at construction sites. The most complex of them (Fig. 2) were often used on sailing ships. There they were needed to work with sails, spars and other moving equipment.
Over time, hemp piping was replaced by steel cables and cables made of synthetic and mineral fibers. They are more durable and wear resistant. chain hoists with steel cables and multi-groove pulleys are integral parts of the lifting mechanisms of all modern lifting equipment. Pulleys blocks usually rotate on roller bearings and all their moving surfaces are forcibly lubricated.
Lifting machines are designed to help a person lift something heavy to a height. Most lifting mechanisms are based on a simple system of blocks - a chain hoist. He was already familiar to Archimedes, but now many do not know about this brilliant invention. Remembering the physics course, find out how such a mechanism works, its structure and scope. Having understood the classification, you can proceed to the calculation. To make it work - your attention to the instructions for constructing a simple model.
Block system - theory
The invention of the chain hoist gave a huge impetus to the development of civilizations. The block system helped to 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 application for it today.
Simplicity and efficiency of the mechanism The structure of the lifting mechanism
The classic chain hoist is a mechanism that consists of two main elements: a pulley; flexible connection
The simplest scheme: 1 - movable block, 2 - fixed, 3 - rope A pulley is a metal wheel that has a special groove for the cable along the outer edge. As a flexible connection, a conventional cable or rope can be used. If the load is heavy enough, synthetic fiber cables or steel ropes and even chains are used. 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. Blocks in 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. The movable rollers are located on the side of the load.
Fixed block The role of the fixed block is to change the direction of the rope movement and the action of the applied force. The role of mobile is to gain a gain in strength.
Movable block The principle of operation - what is the secret
The principle of operation of a chain hoist is similar to a lever: the force to be applied becomes several times less, while the work is performed in the same volume. The rope plays the role of a lever. 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 design of the chain hoist, the gain in strength can be different. The simplest mechanism out of two pulleys it gives approximately a double gain, out of three pulleys it gives a threefold gain, 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 what 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 hard work is done by construction equipment (crane), the chain hoist has found a place in the design of load-handling mechanisms. The block system (polyspast) is a component of such lifting mechanisms as a winch, a hoist, construction equipment (cranes of various types, a bulldozer, an excavator).
Apart from construction industry, chain hoists are widely used in the organization of rescue operations. The principle of operation remains the same, but the design is slightly modified. Rescue equipment is made of durable 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.
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Classification of models according to different characteristics
There are many versions of one idea - a system of blocks, united by a rope. They are differentiated depending on the method of application and design features. Get to know 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, simple ones are distinguished; complex; complex polyspasts.
An example of even models A simple chain hoist is a system of rollers connected in series. All movable and fixed blocks, as well as the load itself, are connected by one cable. Differentiate even and odd simple chain hoists.
Even called 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 the place where the force is applied, this construction and all derivatives from it will be called odd.
Scheme of an odd chain hoist rc="//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js">
A complex chain hoist can be called a chain hoist system. In this case, not individual blocks are connected in series, but whole combinations that can be used on their own. Roughly speaking, in this case, one mechanism sets in motion another similar one.
Combining a two-fold and six-fold simple chain hoist gives a complex six-fold option The complex chain hoist does not belong to either one or the other type. His distinguishing feature- rollers moving towards the load. The composition of the complex model can include both simple and complex chain hoists.
Classification according to the purpose of the lift
Depending on what they want to get when using the 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 implies, its task is to ensure a gain in strength. Since a significant gain requires an equally significant loss in distance, a loss in speed is 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 chain hoist, 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 to the detriment of the applied effort.
Multiplicity is the main characteristic.
The main indicator that is paid attention to when organizing the lifting of goods is the multiplicity of the chain hoist. This parameter conditionally 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 of the rope) and power, which is calculated taking into account the overcoming of the friction force by the cable and the non-ideal efficiency of the rollers. The reference books contain tables that display the dependence of the power multiplicity on the kinematic for different block efficiency.
As can be seen from the table, the force multiplicity differs significantly from the kinematic one. With a low efficiency of the roller (94%), the actual gain in the pulley block strength of 7: 1 will be less than the gain of a six-fold pulley block with a block efficiency of 96%.
Schemes of chain hoists 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.
Single block calculation
The calculation of the chain hoist must be performed due to the fact that 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, a flexible and pliable rope is rarely used on a construction site and as part of construction equipment. A steel rope or chain is much more rigid. Since additional force is required to bend such a cable when running on a block, it must also be taken into account.
For calculation, the moment equation for the pulley about the axis is derived:
SrunR = SrunR + q SrunR + Nfr (1)
Formula 1 shows the moments of such forces:
- Srun - the force from the side of the escaping rope;
– Sraid – effort from the side of the oncoming rope;
- q Sraid - 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 shoulder - the radius of the block R or the radius of the sleeve r.
The force of the incoming and outgoing cable 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 impact on the axis of the block, this value is often neglected:
N = 2 Sraid×sinα (2)
In this equation:
– N – impact on the pulley axis;
- Srun - force from the side of the oncoming rope (assumed to be approximately equal to Srun;
– α is the angle of deviation from the axis.
Pulley block Calculation useful action block
As you know, efficiency is a coefficient of performance, that is, how effective the work was done. It is calculated as the ratio of the work performed and the 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 - coefficient of rigidity of the flexible connection (rope); f is the coefficient of friction;
– 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 coefficient f), its size (through the ratio d / D) and the material of the rope (factor q). The maximum efficiency value can be obtained using bronze bushings and rolling bearings (up to 98%). Plain bearings will give up to 96% efficiency.
The diagram shows all the forces S on different branches of the rope 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; (- four)
S3=(ηп)3 S0; ….
– Sn=(ηп)n S0.
The efficiency of the chain hoist at different multiplicitySince 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, you can find ηп for systems with a different number of blocks for different values of the efficiency of each.
How to make a do-it-yourself lift
In construction during installation work it is not always possible to adjust the crane. Then the question arises, how to lift the load with a 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 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 carrying capacity of the chain hoist blocks is not enough, and there is no time and opportunity to build a complex multiple lifting mechanism. Then double chain hoists are used, 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.
Drawings of a dual model in different variations How to choose rope and 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. Stretching and deformation of the flexible connection gives 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 home-made lifting devices with an estimated 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 to lift the load to a height with your own hands and not drop it? To protect the load from a possible reverse motion, you can install a special locking block that allows the rope to move in only one direction - the desired direction.
Roller on which the rope moves Step-by-step instructions for lifting a load through the 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 cargo suspension;
- 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:
1. Connect the rollers, bushing and bearings. Combine it all in a cage. Get a block.
2. The rope is launched into the first block;
3. The clip with this block is rigidly attached to a fixed support (reinforced concrete beam, pole, wall, specially mounted extension, 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. Sling the load to be lifted and connect it to the chain hoist.
A homemade lifting mechanism is ready to use and will provide a double gain in strength. Now, to lift the load to a height, it is enough to pull the end of the rope. By bending around both rollers, the rope will lift the load without much effort.
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 the 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.
Chain hoist for winch 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 chain hoist made in haste 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 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, a rope and ingenuity will allow you to do without involving a crane.
Chain hoist - 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, to provide tension, etc. The simplest chain hoist consists of only one block and a rope, while it allows you to halve the pulling force required to lift the load.
Usually, power chain hoists are used in lifting mechanisms, which make it possible 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 (hoists, winches). High-speed chain hoists, allowing you to get 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, telescopic crane extension mechanisms.
The main characteristic of the chain hoist 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 chain hoists), or the ratio of the speed of the leading end of the flexible body to the driven one (for high-speed chain hoists). Relatively speaking, the multiplicity is a theoretically calculated gain in strength or speed when using a chain hoist. The change in the multiplicity of the chain hoist 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 a hook clip.
Depending on the number of rope branches attached to the drum of the hoisting mechanism, single (simple) and double chain hoists can be distinguished. In single chain hoists, when winding or winding 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, in the absence of free blocks in the system (the rope from the hook block goes directly to the drum), the load moves not only in the vertical, but also in the horizontal plane.
To ensure a strictly vertical lifting of the load, double chain hoists are used (consisting of two single ones), in this case both ends of the rope are fixed on the drum. To ensure the normal position of the hook suspension with uneven stretching of the flexible element of both chain hoists, a balancer or leveling blocks are used. Such chain hoists are mainly used in overhead and gantry cranes, as well as in heavy tower cranes in order to be able to use two standard cargo winches instead of one large-sized large power, as well as to obtain two or three lifting speeds.
In power chain hoists, with an increase in 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. An increase in the multiplicity allows to reduce the gear ratio of the gearbox, but at the same time requires a longer rope length and rope capacity of the drum.
High-speed chain hoists differ from power ones in that they work force, usually developed by a hydraulic or pneumatic cylinder, is applied to the movable clip, and the load is suspended from the free end of the rope or chain. The gain in speed when using such a chain hoist is obtained as a result of an increase in the height of the load.
When using chain hoists, it should be borne in mind 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 escaping branch does not straighten immediately. This is most noticeable when using steel ropes.
Has always been a priority. In this regard, quite a long time ago a device was invented that contributed to a significant relief physical labor workers involved in carrying out operations to raise or lower various objects. The name of this device is a pulley block. What is it, we will consider in this article.
History of creation and definition
No one knows for sure when exactly the mechanisms for moving weights in space appeared and began to be used. First of all, we note: a chain hoist (what it is, technical literature can also suggest) is a system of blocks and ropes, which makes it possible to significantly simplify and speed up the planned work with heavy objects.
The study of such architectural monuments as the pyramids of Cheops in Egypt, the Great Wall of China and other ancient structures unequivocally confirms that pulley blocks, the purpose and construction of which will be discussed below, were invented several thousand years ago. It is quite obvious that initially they were characterized by primitiveness.
general information
Let's try to study the chain hoist in as much detail as possible. What is it from a technical point of view? At its core, it is a group of blocks assembled into special clips through which either a chain or a rope passes. The simplest chain hoist is one block with a traction element stretched through it. This version of the scheme allows you to halve the traction force required to move the load.
Classification
Polyspasts are divided into two large groups: power and high-speed. Often in practice, power analogues are used, which can significantly reduce the tension of the cable. By the way, this effort can be calculated quite simply. To do this, you will need to divide the mass of the load by the available multiplicity of the chain hoist. The question arises: what is multiplicity? Answer: multiplicity - the ratio of the number of branches of the organ on which the load is located to the numerical value of the branches that are already wound on the drum. This definition is applicable to power chain hoists. As for high-speed chain hoists, here the multiplicity is the value obtained from dividing the speed of the leading end of the rope by the speed of the follower.
In high-speed chain hoists, the working force is applied to the movable clip, while the load, in turn, is fixed at the free end of the cable. The gain in speed during the operation of this type of chain hoist arises due to the increase in the lifting distance of the object.
Changing the multiplicity
Polyspasts (their purpose and arrangement have basically remained unchanged over the years of existence) allow the introduction or removal of additional blocks from the system. Due to this, it is ensured that the required multiplicity is obtained. If the multiplicity is even, then in this case the free end of the rope is fixed on a stationary structural element. If the multiplicity is odd, then the same end is attached to the clip with a hook.
In power chain hoists, an increase in the multiplicity allows you to reduce the diameter of the rope and, accordingly, the dimensions of the drum and blocks. All this ultimately leads to a decrease in the total mass of the entire system, a decrease in But at the same time, a large length of the rope is required.
Branching by number of branches
The chain hoist (what it is, now you obviously already understood) can be single or double, depending on the number of branches. In the first case, the flexible element moves along the axis of the drum. This option has a drawback, which is an undesirable change in the load on the drum supports, and in the absence of free blocks (that is, the rope from the suspension is immediately wound onto the drum), the object will move not only vertically, but also horizontally.
The double chain hoist provides for fixing both ends of the rope on the drum. An example is a chain hoist for a winch. In order to avoid distortion, equalizing blocks or balancers are used. Most often, such a system is operated in gantry or overhead or heavy tower cranes.
Peculiarities
Absolutely any chain hoist, the principle of operation of which is generally similar to the operation of a lever, is good because it does not require any special skills from the user, but it requires you to be extremely careful, because, like any other, it is fraught with danger and can injure. In addition, the operation of chain hoists shows that the traction elements used in their systems do not have ideal flexibility and are endowed with some rigidity. That is why the running branch of the rope is not able to instantly lie down in the stream of the block or drum, and the running branch cannot immediately straighten up. This is best seen when using steel ropes.
Pulley rules
Each manual chain hoist operates on the basis of the laws of physics, and therefore its functioning corresponds to several sufficiently simple rules which you would like to know.
A complex chain hoist deserves special attention. By itself, it is a collection of simple chain hoists, each of which pulls the other. Thus, several chain hoists can be mounted among themselves. This type is most often used at the time of rescue operations.
In conclusion, it would be correct to say the following: the chain hoist (its principle of operation is quite easy to understand with a careful study of the issue) was, remains and, most likely, will still be very for a long time a faithful assistant to a person in solving many pressing issues related to construction, installation, loading, unloading and other operations that are quite laborious. The main problem, the elimination of which is not completely possible today due to, again, ideally working physical laws, is the presence of a friction force in the system.
