The ancient Egyptians and Archimedes, without wondering what a chain hoist is, already used it to move heavy objects. It is still widely used in all lifting mechanisms, in sports, at home, and is also used by rescuers. Since then, the design of this device has undergone significant changes, but the principle of operation has not changed.

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Device

A simple chain hoist consists of two pulleys connected by a rope, cable, or chain. The pulley is made in the form of a metal wheel rotating on an axis. A groove is made along the outer edge for laying the cable. The pulleys that make up the structure are called blocks. Some of them are fixed, others change position as the load moves. Movable blocks are placed on the gravity side. The fixed block changes the direction of movement of the cable and the vector of force application, while the moving ones increase the force applied to the load. The movement of the load occurs due to the fact that it is pulled through a system of blocks by a rope to the fixed part of the pulley.

In essence, a chain hoist is a system of levers, the role of which is played by parts of the rope located between the blocks. As you know, the law of leverage states that while you gain in strength, you lose in distance, and therefore in speed, and vice versa. Means, for moving cargo for 1 meter with a double winning mechanism you will have to choose 2 meters of rope, that is, spend 2 times more time. The applied force will be 2 times less than the mass of the load, however, the amount of energy expended will not change.

In the same way, the gain in distance is calculated if the attachment points of the pulley and the load are swapped.

Multiplicity

This is the main characteristic showing how many times the pulley theoretically increases force or speed. The multiplicity is determined by the number of cable branches between which the load is distributed and can be even or odd. In the first case, the free end of the cable is fixed to the stationary part of the lifting mechanism, and in the second it is attached to the hook clip.

It may seem that by increasing the number of blocks you can multiply the effort endlessly.

However, no one has canceled friction, which requires at least 10% of effort to overcome even in the best pulley models. That's why if you calculate the real winnings taking into account friction for a chain hoist with a multiple of 5:1 (5*0.9*0.9*0.9*0.9 = 3.28), the result will be more modest. And if you use carabiners instead of blocks (for example, in mountaineering), which have much greater friction losses, the gain will be even more modest.

Classification

Pulley hoists, the purpose and design of which have not changed over the past centuries, can be power and speed. The former are used on lifting mechanisms, and the latter are equipped with lifts. According to execution they are manufactured:

1. Simple circuits, consisting of a linear sequence of blocks. They are connected to each other and to the load by a common rope.
2. Complex. This is a system in which not individual blocks are connected in series, but several independent mechanisms. This solution allows you to create chain hoist schemes with a large multiplicity with a small number of blocks. For example, connecting pulleys with a multiplicity of 2:1 and 3:1 will give a gain of 6 times when using only three blocks. Due to lower friction losses, the actual result will be higher than that of a simple design with similar parameters.
3. Complex chain hoists occupy a separate place. This is a system of pulley blocks made of simple and complex mechanisms, connected in such a way that the blocks move towards the load when lifted.

What does polysplast look like, photo

How to make a chain hoist yourself

In the household there is no need to lift weights every day, so for one-time work you can make a lifting device with your own hands. Everything you need for this can be found in the thrifty owner’s workshop:

• threaded steel studs;
• bearings;
• rollers;
• rope;
• hook.

The bearing is inserted into the roller and placed on the stud. Screw on the nut and lock it so as not to waste effort on turning the resulting shaft. A hook is attached to the stud or slings. One end of the rope, passed through the made block, is secured to a fixed support, and the other is pulled upward when lifting the load. The result was a simple chain hoist with a multiplicity of 2:1.

Since it is inconvenient to work with such a mechanism, you should make another block and, having secured it, pass a rope through it. Now it can be pulled down and even connected to a winch. In addition to improving working conditions, this will provide the ability, if necessary, to secure the load in any intermediate position.

For a DIY chain hoist, it is better to use a rope rather than a steel cable. Its advantage is that it allows quickly assemble or disassemble design. You should choose static types that do not stretch. Dynamic types “eat up” part of the gain in strength.

Lecture No. 6 PULLEYS

Lecture outline:

6.1 Purpose of chain hoists.

6.2 Types of pulleys.

6.1 Purpose of chain hoists.

In lifting machines, the load can be suspended from one or more branches of a flexible traction member.

In the case of suspending a load from several branches, the flexible organ bends around a system of blocks connected into fixed and movable clips, forming pulleys. In pulley hoists, all blocks are wrapped around one flexible body, the end of which can be attached to a movable or fixed holder. The blocks in the cages must rotate independently of one another, since the traction element, passing unequal distances along the blocks, rotates them at different speeds.

6.2 Types of pulleys.

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

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

Pulley hoist is a system of several movable and fixed blocks surrounded by a flexible body (rope or chain). Pull blocks are used for lifting and pulling loads. The introduction of pulley blocks into the lifting or jib mechanisms of the crane allows you to reduce the tension of the traction element and the load moment on the drum. Pulley hoists also make it possible to reduce the gear ratio of the mechanisms in which they are installed, and, consequently, their weight and cost.

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

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

Due to friction in the block bearings and additional efforts from overcoming the rigidity of the ropes when bending, the relationship between the required traction force S and the weight Q of the lifted load will be expressed by the formula:

^S = Q/ή block

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

Movable guide blocks

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

^S = Q/2ή block

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

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

^S = 2Q/ή block

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

That. in machines for lifting loads, direct and reverse action pulleys are used.

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

6.3 Multiplicity and types of pulleys.

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

The multiplicity of direct action pulleys is equal to or greater than one.

i = V S /V Q

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

In the general case, pulleys of any multiplicity can be formed by introducing additional blocks into the reeving scheme of the flexible body, and for pulleys of odd multiplicity, the end of the traction element must be fixed to a movable holder; for pulleys of even multiplicity, it is fastened to the holder of fixed blocks.

P
Reverse action pulleys provide

bake the gain in travel and speed

driving organ and are widely used in

lifts and loaders.

For reverse action pulleys

The multiplicity of the pulley is less than one because

V S
High-speed pulleys differ from

power pulleys because they contain working

force, usually developed by hydraulic

or pneumatic drive, applies -

to the movable clip, and the load is suspended from the free end of the rope. Therefore, they are, as it were, the opposite of power pulleys.

IN
single pulley hoists when nam-

spinning or unwinding the rope from the drum,

due to rope movement

along the axis of the drum, unwanted

new change in the load on the drum supports

In addition, if in a single pulley

there are no bypass blocks and a rope from the hook block

the forged holder goes directly into

on the drum, then when moving the rope

movement occurs along the axis of the drum

load not only vertically, but also vertically

horizontal. To ensure strictly

vertical lifting of the load and constant

loads on the drum supports are applied

There are double pulleys consisting of

two single chain hoists.

To ensure the normal position of the load cage in the event of possible uneven stretching of the rope branches of both pulleys, an equalizing block is most often used. This block does not rotate when lifting and lowering the load and serves only to equalize the length of the branches of both pulleys in case of uneven rope pulling. If the multiplicity is even, it is located among the stationary blocks, and if the multiplicity is odd, it is located among the moving blocks of the thunderstorm cage. Since the rope moves only occasionally on the leveling block, its diameter can be taken equal to 0.8 of the block diameter, and for electric hoists and self-propelled jib cranes equal to 0.6 of this diameter.

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

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

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

With the same reduction in force in one branch of the rope, its wear will be less, because it has fewer kinks on the blocks. For example, to reduce the force by approximately four times in a single four-fold pulley, the rope is bent four times on the blocks, and in a double-double pulley - twice, since the bend on the equalizing block is not taken into account. At the same time, the force on the drum doubles compared to single pulleys, because two branches of rope enter the drum;

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

3 What does a direct action chain hoist provide?

4 What does a reverse action chain hoist do?

5 What is the multiplicity of a chain hoist?

6 What are the disadvantages of single chain hoists?

7 Describe double chain hoists?

Lifting a heavy load even to a small height without the use of special tools is not always possible. We're not just talking about cranes, truck cranes and forklifts - there are other devices to solve this problem.

One of the mechanisms for lifting loads is a chain hoist.

Polyspast is a block system with chain or rope transmission. Its task is to simplify and speed up the lifting of any heavy load using human power. Such schemes (or their close analogues) were used even before our era - during the construction of the Egyptian pyramids and the Great Wall of China.

Stationary lifts are used in warehouses and production facilities where it is necessary to lift various weights. Portable block systems are used in construction, logistics, and rescue work.

Design and principle of operation

The chain hoist allows you to lift weights using less human effort. The principle is similar to the action of a lever to lift a load, but instead of a lever, a cable is used.

Structurally, the simplest chain hoist consists of 1 block and a rope. The roller is fixed above the load (on the ceiling, beam, or a movable special support). One end of the rope with a hook goes down to the load. The person holds the second end of the rope in his hands and pulls on it, lifting the weight.

The following factors influence the gain in strength:

1. Number of rollers.
2. Rope length.

1 block increases the force by about 2 times (approximately - because some losses will be written off due to friction). That is, if a person without a lift can lift 30 kg to a height of 1 meter, then with a chain hoist it will be 60 kg. If there are more rollers, then more weight can be lifted.

As for the length of the rope: the longer it is, the more weight a person can lift, but also the more time it will have to be spent on it.

Types of pulleys

Pulley hoists are divided according to several criteria:

1. By appointment. There are power schemes, and there are speed schemes. Power lifts allow you to lift more weight, but more slowly. High-speed ones allow you to lift weights faster, but will “handle” less weight.
2. By the number of blocks. The simplest option is 1 video. But there can be 2, or 3, or 4, or more. The more of them, the more weight you can lift.
3. According to the complexity of the scheme. There are simple schemes (when the rollers are connected in series by 1 rope) and complex ones (when 2 or more separate pulleys are used). Complex systems are more productive, producing more results with fewer blocks. For example, if you combine 2 chain hoists (from 1 and from 2 blocks), you will get a 6-fold gain in strength. Whereas a simple scheme will give a 6-fold win only when using 6 rollers.

What affects the efficiency of a lift?

The multiplicity mentioned above (gain in strength) is very approximate, rounded up. In practice it is less.

The effectiveness of the lift (what exact gain in strength it will give) is influenced by the following factors:

• number of blocks;
• cable material;
• bearing type;
• quality of lubrication of all axes;
• rope diameter and length;
• the angle between the rope and the middle plane of the roller.

How is the rope attached to the mechanism?

You can attach the lifting mechanism to the cable in the following ways:

1. Knots connected from cords. Number of revolutions - 3-5.
2. General purpose clamp.

What is a refill, how is it made and what is it like?

Refilling is a change in the position of blocks and the distance between them. It is carried out to change the speed or height of lifting weights.

There are storage schemes:

1. Single: the hook is hung by 1 rope, which is then passed sequentially through each fixed block and wound onto a drum.
2. Double. For beam cranes, 1 end of the rope is attached to the boom root, and the second end is passed through the bypass drum, all the blocks, and then attached to the winch. For cranes, the rope is attached to the winch and the stationary blocks are located on the boom head.
3. Quadruple. A combination of the schemes listed above is used for each hook suspension unit.
4. Variable. The movable rollers are complemented by 1 or 2 movable cages.

How to make a chain hoist with your own hands?

Let's consider a scheme for creating a double chain hoist.

You will need:

• 2 bushings.
• 2 videos.
• 2 clips.
• Bearings.
• Hook (to hook the load).
• Rope.

Step by step design:

1. Bushings, rollers and bearings are connected and inserted into the cage. The result is 2 rotating blocks.
2. The cable is passed through the block.
3. The clip with the missing rope is attached to the support under which the load will be located.
4. The second end of the rope is passed through the second block.
5. A hook is attached to the second clip.
6. The end of the rope that remains hanging is fixed (you will need to pull on it to lift the load).

After this, all that remains is to secure the load (pick it up with a hook), and you can begin lifting.

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

Schemes of chain hoists

Rice. 1: a – single double; b – single triple; c, d – double double; g – double triple

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

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

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

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

The speed of lifting the load υ gr and the speed of the rope wound onto the drum are related by the relation υ = aυ gr, where υ = πD 2 n bar /60, m/s; D 2 - drum diameter measured at the center of the rope; n bar - drum rotation speed, rpm.

Power chain hoist

In power pulleys of lifting machines, it is possible to use ropes of small diameter and, therefore, reduce the diameters of the drum and blocks, reduce weight and dimensions. Increasing the multiplicity of the pulley allows you to reduce the gear ratio, but at the same time requires a larger rope length and rope capacity of the drum. An increase in the number of blocks with an increase in the multiplicity of the pulley causes an increase in losses and an increase in the power spent on lifting the load, and also increases the number of kinks in the rope, which causes a slight decrease in its service life. At the same time, as already indicated, the rope with a large pulley multiplicity has a small diameter and, therefore, greater flexibility, which helps to increase durability. The choice of rope, type and multiplicity of pulley is associated with the problem of the general layout of the mechanism and its parameters, in particular the gear ratio of the mechanism, dimensions and weight, which in turn affects the dimensions of the entire lifting machine and the size of the building where this machine is installed.

Thus, if pulley blocks of different multiplicities are used to lift a load of the same weight G gr with the same specified lifting speed υ gr, then the parameters of the lifting mechanisms will be different. The static power of these mechanisms N st = G gr υ gr /1000η p, required for lifting a load, will be different only because of the difference in efficiency values, and for multiplicities that differ slightly (for example, mechanisms with a multiplicity of two and four), the required power engine can be considered the same. Since the maximum forces in the pulley ropes change almost inversely with the multiplicity of the pulley, with increasing multiplicity the load in the rope and its diameter, as well as the diameter of the drum, decrease. The speed of winding the rope onto the drum changes in direct proportion to the multiplicity, and in a pulley with a higher multiplicity it has a greater significance. Then, at the same specified lifting speed and the same rotor speed, the gear ratio of the gearbox connecting the engine to the drum turns out to be lower with a chain hoist of higher multiplicity due to the higher speed of winding the rope onto the drum and its smaller diameter.

High-speed chain hoist

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

High-speed pulley scheme

Rice. 2

The calculation of high-speed pulleys is not fundamentally different from the calculation of a power pulley. When the pulley cage (point A in Fig. 2) moves over a distance h, the load travels the path H = ah, where a is the multiplicity of the high-speed pulley and, therefore, the speed of movement of the load υgr = aυA, where υA is the speed of movement of the pulley clip.

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

Always a priority. In this regard, a device was invented quite a long time ago that significantly facilitated the physical labor of workers involved in carrying out operations to lift or lower various objects. The name of this device is chain hoist. We will look at what this is in this article.

History of creation and definition

No one knows for sure when exactly mechanisms for moving heavy objects in space appeared and began to be used. First of all, we note: a pulley system (technical literature can also tell you what it is) is a system of blocks and ropes that can significantly simplify and speed up planned work with heavy objects.

The study of such architectural monuments as the Cheops pyramids in Egypt, the Great Wall of China and other ancient structures clearly confirms that pulleys, the purpose and design of which will be discussed below, were invented several thousand years ago. It is quite obvious that they were initially characterized by primitiveness.

general information

Let's try to study the chain hoist in as much detail as possible. What is this from a technical point of view? At its core, it is a group of blocks assembled into special cages through which either a chain or a rope passes. The simplest pulley 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

Pulley hoists are divided into two large groups: power and speed. Often in practice, power analogues are used to significantly reduce the cable tension. 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 pulley. The question arises: what is multiplicity? Answer: multiplicity is 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 applies to power pulleys. As for high-speed pulleys, the multiplicity here is the value obtained by dividing the speed of the leading end of the rope by the speed of the driven end.

In high-speed pulleys, the working force is applied to the movable cage, and the load, in turn, is secured to the free end of the cable. The gain in speed during operation of this type of pulley arises due to the increase in the distance of lifting the object.

Changing the ratio

Pulley hoists (their purpose and design have remained essentially unchanged over the years of existence) allow the introduction or removal of additional blocks from the system. Due to this, the required multiplicity is obtained. If the multiplicity is even, then the free end of the rope is fixed to a stationary structural element. If the multiplicity is odd, then the same end is attached to the clip with a hook.

In power pulleys, increasing 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 larger length of rope will be required.

Division by number of branches

The pulley (what it is, now you obviously already understand) 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 the disadvantage that it causes 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 securing both ends of the rope to the drum. An example would be a pulley for a winch. In order to avoid distortion, equalizing blocks or balancers are used. Most often, such a system is used in gantry or bridge 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 extreme caution, since, like any other, it is fraught with danger and can cause injury. In addition, the operation of pulleys shows that the traction elements used in their systems do not have ideal flexibility and are endowed with some rigidity. That is why the advancing branch of the rope is not able to immediately fall into the stream of a block or drum, and the running branch cannot immediately straighten out. This is best seen when using steel ropes.

Rules for chain hoists

Each manual pulley operates on the basis of the laws of physics, and therefore its operation complies with several fairly simple rules that it is advisable to familiarize yourself with.

The complex chain hoist deserves special attention. By itself, it represents a collection of simple pulley hoists, each of which pulls the other. In this way, several pulleys can be mounted together. This type is most often used during rescue operations.

In conclusion, it would be correct to say the following: the pulley system (the principle of its operation is quite easy to understand upon careful study of the issue) was, remains and, most likely, will be a faithful assistant to a person for a very long time in solving many pressing issues related to construction, installation, loading, unloading and other operations that are quite labor-intensive. The main problem, the complete elimination of which is not possible today due to, again, ideally working physical laws, is the presence of friction force in the system.