Scheme of a chain hoist with a multiplicity of 4. How a chain hoist works. Polyspasty. lift with at home

chain hoists

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Construction machines and their operation

chain hoists

A chain hoist is a system consisting of several movable and fixed blocks and a rope that sequentially envelops all the blocks. One end of the chain hoist is fixed on the cage of movable or fixed blocks, and the other - on the winch drum.

Rice. 1. Schemes of rope pulley blocks a - three-fold pulley block; b, c, d - four-, five- and six-fold chain hoists

Rice. 2. Scheme of a double chain hoist

The number of working branches (multiplicity of the chain hoist) is equal to the number of blocks when the rope runs off the fixed block of the chain hoist, and the number of blocks of the chain hoist plus one, when the rope runs off the movable block.

Rice. 3. Scheme of a reverse action chain hoist

The chain hoist is the simplest lifting device, consisting of blocks interconnected by a rope. With the help of a chain hoist, you can lift the load or move it horizontally. The chain hoist gives a gain in strength due to a loss in speed: how many times it is won in strength, how many times it is lost in speed.

The chain hoist consists of two blocks: a fixed one, attached to a lifting device (beam, mast, tripod), and a movable one, which is attached to the load being lifted. Both blocks are interconnected by a rope. The rope, sequentially bending around all the rollers of the blocks, is attached at one end to the upper fixed block. Its other end is attached to the winch drum through the outlet blocks. If the number of working threads of the chain hoist going to the movable block is even, then the end of the rope is fixed to the upper fixed block, and if it is odd, to the lower movable one.

If the chain hoist thread runs not from the lower block, but from the upper one, then the upper block of the fixed block is considered to be a branch block. This condition must be taken into account when calculating pulley blocks.

The chain hoist is stocked in two ways. According to the first method, used when equipping heavy-duty multi-strand chain hoists, a fixed block without ropes is lifted into working position and fix; the lower movable block is at the bottom. Then, through the streams (grooves) of the rollers of the upper and lower blocks, the rope is sequentially passed. The end of the rope is fixed to the upper or lower block, depending on the accepted chain hoist reeving scheme. Through the streams of the rollers, the rope is often passed using manual lever winches, which greatly facilitates the work of reeving the chain hoist.

Recently, when equipping a multi-strand chain hoist, an auxiliary thin light steel rope with a diameter of 5-6 mm is used, which is passed through the rollers of the blocks manually. The end of the working rope is attached to one end of the thin rope, its second end is fixed on the winch drum. During the operation of the winch, the working rope is pulled through the pulley block rollers.

During the reeving of the chain hoist, it is necessary to ensure that the junction of the thin and thick ropes, when moving, freely passes through the rollers of the blocks.

In the second method, the chain hoist is equipped below (on a boardwalk or concrete floor), and then, in finished form, is lifted and fixed in necessary place. The blocks are laid flat at a distance of 3-4 m from each other and fixed.

The rope begins to be pulled from the roller from which the running thread comes off, leading to the winch. When the rope goes around the last roller of the block, its end is fixed to one of the blocks. After fixing the dead thread, the chain hoist is set to its original position.

In some cases, one upper fixed block or the entire chain hoist is lifted with the help of an auxiliary single-roll block or a small-capacity chain hoist. First, an auxiliary block is fixed, a rope is passed through it, to which the main pulley block is attached. The second end of the rope is fixed on a winch, with which the chain hoist will be raised. The main block of the chain hoist is fixed from the cradle or from the scaffold.

On fig. 4 shows schemes for reeving chain hoists with two-, four-, five- and six-roll blocks.

When performing rigging work, there are often cases when blocks of various carrying capacities and ropes are available. In order to choose the right rope for equipping the chain hoist, as well as a winch with the necessary traction force, the rigger needs to know the calculation of the chain hoists.

The calculation of chain hoists comes down to determining the forces in the threads of the chain hoists. Usually, the blocks themselves do not have to be calculated, since they are calculated during design, and each of them has a certain load capacity.

During rigging, the calculation begins with finding out the carrying capacity of the existing blocks, which must correspond to the weight of the load being lifted. For example, according to the scheme (Fig. 22, a), blocks with a carrying capacity of 20 tons are needed to lift a load weighing 20 tons.

Rice. 4. Schemes for reeving pulley blocks with the number of working threads: a - six with three single-roll pull-off blocks, b - three, c - four, d - five, d - six, e - seven, g - eight, h - ten, and - eleven , k - twelve, S0, 1, 2, 3, 4, 5.6.7 - chain hoist threads

The suspension, on which the upper block of the chain hoist is suspended, is calculated for the entire load that the chain hoist lifts: the weight of two blocks, the weight of the rope, and also the force in the running thread of the cargo chain hoist.

When calculating the chain hoists, the fastening of the upper block of the chain hoist to the mechanism or fixture is calculated.

If we assume that both threads run vertically, then the first take-off roller is fixed to a force equal to the sum of the forces in the 5th and 6th threads: 3.68+3.82=7.5 tf. The fastening of the second outlet block is calculated for the efforts in the 6th and 7th threads.

Since the forces in both threads and the angle between them can be different, the force for which the block is calculated is determined by the parallelogram rule.

Example. Choose a chain hoist for lifting a load weighing 10 tons and a rope of the required section for hanging the chain hoist at a height of 18 m.

We select two blocks for chain hoists. According to the table 11 we select for the lower movable block a two-roll block with a load capacity of 10 tf, for the upper fixed block - a three-roll block with a load capacity of 15 tf.

According to the maximum force in the 6th thread Se, we select the rope section. The smallest permissible safety factor of ropes k for a machine-driven cargo chain hoist in light duty is 5.

Since there can only be an even number of threads, we accept eight threads for suspension.

In the absence of blocks of the required carrying capacity, double chain hoists are used, for example, a double chain hoist with a leveling roller and one or two drive winches is shown in fig. 5.

A double chain hoist with one drive winch is calculated as a single one with the corresponding number of working threads.

A chain hoist with two drive winches is calculated as two independently operating chain hoists,

Rice. 5. Schemes of reeving of double chain hoists with one (a) and two (b) drive winches: 1 - leveling block, 2 - fixed block, 3 - movable block, 4 - traverse, 5 - suspension

The chain hoist is the simplest lifting device, consisting of a system of movable and fixed blocks (rollers) that are wrapped around by a flexible body (usually a rope). Polyspasts are used as independent mechanisms in combination with winches and as elements of complex lifting machines (cranes).

Blocks (rollers) of the chain hoist are placed in two clips - movable and fixed - and are sequentially bent around by one rope, to the free end or both ends of which a pulling force is applied. The fixed cage of blocks (rollers) is attached to the supporting structure (mast, boom, etc.), the movable one is supplied with a load-gripping body (hook, loop, bracket).

Rice. 6. Schemes of chain hoists a - in four threads; b - in six threads; 1 - fixed blocks; 2 - moving blocks; 3 - outlet block; 4 - rope

Polyspasts are used to gain strength (rarely speed). The gain in strength is the greater, the greater the multiplicity of the chain hoist, equal to the number of working branches of the rope, on which the movable holder of the chain hoist blocks is suspended.

Rice. 7. Calculation schemes of chain hoists

1. Determine the force 5L in the rope going to the winch when lifting a load weighing Q = 20 tons with a chain hoist made according to scheme I. The blocks (rollers) of the chain hoist are mounted on rolling bearings (/j = 1.02), the outlet rollers are on bronze bushings (= 1.04).

2. Determine the force 5L in the rope going to the winch when lifting a load weighing 20 tons with a chain hoist, made according to scheme II. Blocks (rollers) are adopted on bronze bushings (= 1.04).

3. Determine what load Q can be lifted by a winch with a pulling force of 5L = 1.5 tf and a chain hoist made according to scheme III. Blocks (rollers) are adopted on bronze bushings.

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

The whole variety of chain hoists can be reduced to two requirements: either increase strength (power chain hoists), or increase speed (high-speed chain hoists). In cranes, the former are more often used, and the latter are used in hoists. Thus, the schemes of high-speed and power chain hoists are mutually inverse.

The polyspast consists of the following components:

Blocks with fixed axles
Blocks with moving axes.
Bypass blocks.
Stroke drums.

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

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

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

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

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

Operational characteristics of chain hoists and their choice

The following factors influence the efficiency possessed by chain hoists, their purpose and arrangement in a particular mechanism:

The carrying capacity of the main mechanism in which these nodes operate.
The number of bypass blocks: with an increase in their number, friction losses increase.
Deviation angles of ropes from the median plane of the drum.
block diameters.
Rope diameter/chain height.
Rope material.
The nature of the supports (in rolling or sliding bearings).
Conditions for lubrication of all chain hoist axles.
The speed of rotation of blocks or movement of traction ropes (depending on the purpose of the device).

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

With poor lubrication and at elevated temperatures - 0.94 ... 0.54;
With rare lubrication - 0.95 ... 0.60;
With periodic lubrication - 0.96 ... 0.67;
With automatic lubrication - 0.97 ... 0.74.

Smaller values correspond to chain hoists with the highest possible multiplicity. Friction losses for units that operate in rolling bearings are much lower and are:

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

Thus, using modern anti-friction coatings of the contact surface of the blocks, friction losses can be practically eliminated.

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

The normal angles of deviation from the chain hoist axis are considered to be angles of 7.5 ... 2.50 (smaller values are accepted for the maximum ratios of the block diameter to the rope diameter). In general, when designing these devices, they always try to choose this ratio in the range of 12 ... 40. The permissible deflection angle of ropes made of low-rigid materials is less: up to 6.5 ... 20 .

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

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

Reeving of chain hoists

Reserving is a technological operation of changing the location of the main cargo blocks of the chain hoist, as well as the distances between them. The purpose of the reeving is to change the speed or height of the lifting of goods by means of a certain scheme for the passage of ropes through the blocks of the device.

Reeving schemes are determined by the type of lifting equipment. It is known, in particular, that the mechanisms for changing the reach of an arrow are different for a manual or electric hoist, on the one hand, and for cranes, on the other. Therefore, for winches, reeving is carried out by changing the location of the axis of the guide block, and is intended only for changing the length of the boom. In cargo cranes, the possible curvilinearity of the movement of the cargo is corrected with a reeving. In addition to cargo ropes, the reeving is also used for rope devices for moving the working trolley.

There are the following storage schemes:

Single, which is used for hoisting mechanisms of boom type with a jib. At the same time, the hook is suspended on one thread of the rope, sequentially passed through all the fixed blocks, after which it is wound on the drum. This method of stocking is the least efficient.
double, which can be used on cranes, both with a lifting and beam boom. In the first case, the fixed blocks are located on the boom head, and the opposite end of the rope is fixed in the cargo winch. In the second case, one of the ends of the rope is fixed on the root of the boom, and the second one is sequentially passed through the bypass drum, hook suspension blocks, boom blocks, tower head blocks and then brought to the cargo winch.
quadruple used for heavy duty applications. Here one of the schemes described above is implemented, but separately for each of the hook suspension blocks. Two working branches of the rope are directed to the blocks of the working boom. The connection of adjacent chain hoists is made through an additional fixed block, which is installed on the rack of the crane turning platform.
Variable, the essence of which is to change the lifting capacity of the crane. With this type of reeving (it can be both two- and four-fold), a corresponding increase in the mass of the lifted load is possible. To do this, one or two movable clips are additionally installed in the movable blocks. The holding of the clips is produced by the load rope itself due to the difference in forces that are created by the presence of a hook suspension. Changing the reeving ratio is performed by lowering the hook suspension onto the support while continuing to wind the rope.

Two- and especially four-fold reeving allows you to safely lift the load, which is almost twice the traction force developed by the winch. At the same time, the rotation of the ropes under load is excluded, which significantly reduces their wear.

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PRACTICE:

Simple lifting mechanism consists of a block and a cable (rope or chain).

The blocks of this lifting mechanism are divided into:

by design into simple and complex;

according to the method of lifting the load on mobile and stationary.

Let's start our acquaintance with the construction of blocks with simple block, which is a wheel rotating around its axis, with a groove around the circumference for a cable (rope, chain) Fig. 1 and it can be considered as an equal-arm lever, in which the arms of forces are equal to the radius of the wheel: OA \u003d OB \u003d r. Such a block does not give a gain in strength, but allows you to change the direction of movement of the cable (rope, chain).

double block consists of two blocks of different radii, rigidly fastened together and mounted on a common axis Fig.2. The radii of the blocks r1 and r2 are different and when lifting the load they act as a lever with unequal arms, and the gain in force will be equal to the ratio of the lengths of the radii of a block of a larger diameter to a block of a smaller diameter F = Р·r1/r2.

gate consists of a cylinder (drum) and a handle attached to it, which acts as a block of large diameter. The gain in strength given by the collar is determined by the ratio of the radius of the circle R described by the handle to the radius of the cylinder r, on which the rope is wound F = Р r / R.

Let's move on to the method of lifting the load in blocks. From the design description, all blocks have an axis around which they rotate. If the axis of the block is fixed and does not rise or fall when lifting loads, then such a block is called fixed block, simple block, double block, gate.

At rolling block the axle rises and falls together with the load (Fig. 10) and it is intended mainly to eliminate the kink of the cable at the place of suspension of the load.

Let's get acquainted with the device and method of lifting the load. The second part of a simple lifting mechanism is a cable, rope or chain. The cable is made of steel wires, the rope is made of threads or strands, and the chain consists of links connected to each other.

Ways of suspension of the load and obtaining a gain in strength, when lifting the load, with a cable:

On fig. 4 the load is fixed at one end of the cable and if you lift the load at the other end of the cable, then a little force is required to lift this load more weight cargo, since a simple block of gain in strength does not give F = P.

In Fig. 5, the load is lifted by the worker himself by the cable, which goes around a simple block from above, at one end of the first part of the cable there is a seat on which the worker sits, and by the second part of the cable the worker lifts himself with a force 2 times less than his weight, because the weight of the worker was distributed into two parts of the cable, the first - from the seat to the block, and the second - from the block to the hands of the worker F \u003d P / 2.

In Fig. 6, the load is lifted by two workers for two cables and the weight of the load is distributed equally between the cables and therefore each worker will lift the load with the force of half the weight of the load F = P / 2.

In Fig. 7, workers lift a load that hangs on two parts of one cable and the weight of the load is distributed equally between the parts of this cable (as between two cables) and each worker will lift the load with a force equal to half the weight of the load F = P / 2.

In Fig. 8, the end of the cable, for which one of the workers lifted the load, was fixed on a fixed suspension, and the weight of the load was distributed into two parts of the cable, and when the worker lifts the load by the second end of the cable, the force with which the worker will lift the load is doubled less than the weight of the load F = P / 2 and the lifting of the load will be 2 times slower.

In Fig. 9, the load hangs on 3 parts of one cable, one end of which is fixed and the gain in strength, when lifting the load, will be equal to 3, since the weight of the load will be distributed over three parts of the cable F = P / 3.

To eliminate the inflection and reduce the friction force, a simple block is installed at the place of suspension of the load and the force required to lift the load has not changed, since a simple block does not give a gain in strength Fig. 10 and Fig. 11, and the block itself will be called moving block, since the axis of this block rises and falls along with the load.

Theoretically, the load can be hung on an unlimited number of parts of one cable, but in practice they are limited to six parts and such a lifting mechanism is called chain hoist, which consists of a fixed and movable clips with simple blocks, which are alternately bent around by a cable, fixed at one end to a fixed clip, and the load is lifted by the second end of the cable. The gain in strength depends on the number of parts of the rope between the fixed and movable clips, as a rule it is 6 parts of the rope and the gain in strength is 6 times.

Literature:

Peryshkin, A. V. Physics, 7th grade: textbook / A. V. Peryshkin. - 3rd ed., add. - M .: Bustard, 2014, - 224 s,: ill. ISBN 978-5-358-14436-1. § 61. Application of the lever balance rule to the block, pp. 181-183.
Gendenstein, L. E. Physics. 7th grade. At 2 pm Part 1. Textbook for educational institutions/ L. E. Gendenshten, A. B. Kaydalov, V. B. Kozhevnikov; ed. V. A. Orlova, I. I. Roizen. - 2nd ed., corrected. - M.: Mnemosyne, 2010.-254 p.: ill. ISBN 978-5-346-01453-9. § 24. Simple mechanisms, pp.188-196.
Elementary textbook of physics, edited by Academician G. S. Landsberg Volume 1. Mechanics. Heat. Molecular physics. - 10th ed. - M.: Nauka, 1985. § 84. Simple machines, pp. 168-175.
Gromov, S. V. Physics: Proc. for 7 cells. general education institutions / S. V. Gromov, N. A. Rodina. - 3rd ed. - M.: Enlightenment, 2001.-158 s,: ill. ISBN-5-09-010349-6. §22. Block, pp. 55-57.

A chain hoist is a mechanism by which loads are lifted. It consists of one or more groups of blocks wrapped around by a rope. The word "polyspast" comes from the Greek polyspastion. This term is translated as "stretched by several ropes." The main function of the chain hoist is to increase the carrying capacity of the main mechanism.

In other words, this device gives a gain in strength. However, the reverse effect of using a chain hoist is to reduce the rate of ascent. You can also gain speed at the expense of strength. However, such pulley blocks are used much less frequently. In any case, the principle of operation of the device is the action of the lever.

Mechanism device

A chain hoist is one that allows you to get a force that exceeds the lifting force of the winch several times. In other words, this mechanism increases the load capacity of the device. The use of a chain hoist allows you to lift a heavy load with a winch that has a small carrying capacity. It is important to remember that the lifting speed of heavy structures will decrease as much as the gain in lifting capacity is achieved.

Purpose of the mechanism

The chain hoist is necessary for lifting heavy loads with a minimum of effort. The simplest design of the chain hoist is designed so that one end of the rope is fixed on the drum, and a suspended load is located at the opposite end of the rope. Devices with a more complex design include several fixed and movable rollers.

For each weight, the dimensions, blocks and diameter of the rope should be taken into account. A load having a large mass, when suspended on a rope, increases the load. Such a mechanism is characterized by rapid wear. In this case, a reduction in the tension in the rope is required. Therefore, two or four ropes are used to suspend a large mass. It is also possible to use a chain hoist of complex design.

Principle of operation

To a person who has nothing to do with loading, the name of this mechanism will seem incomprehensible. However, in reality, a chain hoist is a very simple lifting mechanism, which almost everyone can build. The principle of operation of this device is extremely simple and it is studied at school in physics lessons. And the scheme of operation of such a small "crane" is very simple.

The design of the chain hoist includes several groups of blocks assembled in special clips. And they alternately bend around with a rope or a rope. Even such a simple design can be used quite effectively to increase the force applied to lower or raise loads. Also, the design of a simple chain hoist contains cargo blocks. They can be of the following types:

multi-roller or single-roller;
immobile or mobile.

The traction force of the rope in this case depends entirely on the number of rope threads in the structure used.

In what areas is the device used?

The chain hoist is used to lift and move cargo in cases where only the physical strength of a person and the least number of auxiliary mechanisms can be used. Also, the chain hoist is the most important component of winches, cranes and other means of mechanization.

For this reason, these devices are used in almost all areas where lifting and transport mechanisms are somehow used: from household tasks to heavy industry.

So, on what principle does the chain hoist operate? The operation of this device is based on the law of the lever: with a gain in strength, you get a loss in distance. Since this principle is very simple, it will not be difficult to make a chain hoist with your own hands. To do this, you need only two single-roller blocks.

To lift a load of a certain mass with the help of a chain hoist, you need to make efforts, half of its mass. Do not forget about the length of the rope used. It should be twice the height to which the load will be lifted. It should be noted that chain hoists with the simplest device are called "two-to-one chain hoists", since they double the applied force. The design with three blocks, respectively, gives an increase in strength three times.

Polyspast multiplicity

It should be noted that the calculation of the chain hoist plays very important role. After all, the mechanism works far from ideal conditions. It is affected by the friction forces that arise when the cable moves along the pulley. Also, friction forces arise when the roller rotates, regardless of which bearings are used in it.

To determine the tension force of the rope used, without taking into account friction losses, it is necessary to divide the weight of the load by the multiplicity of the chain hoist. It should be understood as the number of rope threads holding the load. Also, friction should not be neglected. The efficiency of the chain hoist also depends on it.

It can be reduced by using blocks and ropes of high quality, as well as by means of high-quality execution, which excludes unnecessary overlaps and kinks.

Today, chain hoist schemes are studied even in a school physics course. With their help, making this design will not be difficult. You also need to purchase the following items:

fitting;
rope;
winch.

What device models are there?

To create the simplest model, only one block is needed. The use of such a mechanism gives a twofold gain in strength. This means that to lift the load you need to make half the effort. However, the rope in this case should be twice as long. Such a chain hoist has a ratio of two to one. Such a design may not contain pulley blocks at all, since an ordinary carbine can be used instead.

When using two blocks at once in a chain hoist, you can triple the advantage in the applied effort. There is also a safety feature that works when the rope is lowered. In this case, two are tightened and block the load.

If two more blocks are added to the previous mechanism, then a pulley block device will be obtained, giving a fourfold gain in strength. Such a mechanism has a relationship of four to one. In this mechanism, a quarter of the weight goes to the end of the rope, and the rest of the load goes to the rope itself.

Complex chain hoists

It should be noted that the transmission of force in its pure form cannot be achieved due to the occurrence of frictional force. When the rope rubs against the block, ten to twenty percent of the applied force is lost. Therefore, in a simple chain hoist, in fact, the ratio will be approximately 1.8 kilograms per kilogram of the load being lifted. And a 5-fold chain hoist will give a gain in strength a little more than 3 times.

The above ratio indicates that it is possible to increase the number of pulley blocks to a certain limit, after which the opposite effect may occur. However, in order to increase the maximum ratio, complex chain hoists can be used.

This chain hoist is designed in such a way that the weight being lifted does not create a load on the last block. Instead, he loads the rope that passes through the block. As a result, when using 3 blocks, pulley blocks 2:1 and 3:1 are alternately connected. In theory, this gives a sixfold gain in strength, but in practice - 4.3 times.

How to reduce friction?

The main problem of the chain hoist is that in the process of work he has to overcome the emerging friction forces. This task can be partially solved if high-quality ropes, pulley blocks with smooth streams, as well as thick grease are used.

Additional opportunities also appear with the proper use of the chain hoist design. For example, if you use not one carbine, but two. Due to this, the friction force is reduced, and the radius of the inflection also increases.

4. POLYSPATS

Polyspastom called a device, which is a system of blocks and cables, designed to win in strength or speed. In lifting mechanisms, power chain hoists are used to reduce the force in the cable and reduce the gear ratio of the gearbox.

In marine practice, chain hoists, which are used to lift cargo, booms and other equipment, are called hoists. These include cargo hoists, topenant hoists, toprik hoists, sloop hoists, guy hoists, etc.

The running end of the chain hoist (hoist), which is wound around the drum, is called a fall.

The main parameter of the chain hoist is its multiplicity u (gear ratio) the multiplicity of the chain hoist is the ratio of the number of cable branches that run from the movable blocks to the number of falls.

A cable designed to lift and lower a load is called a pendant. A cable designed to hold an arrow and change its reach is called a topenant.

The multiplicity of the cargo chain hoist is the ratio of the number of cable branches on which the load hangs to the number of falls

- the number of branches of the cable on which the load hangs;

- the number of lopars.

According to the number of falls, pulley blocks are divided into single ones (Fig. 4.1 a)) ( \u003d 1) and double (Fig. 4.1 b)) ( =2).

Fig.4.1. Single chain hoist with multiplicityu G =2

Fig.4.2. Double chain hoist with multiplicityu G =2

Let us determine the efficiency chain hoist on the example of a single chain hoist shown in fig. 4.2, which has multiplicity u G . In a fixed chain hoist, the tension force is the same in all

, (4.2)

where F Q - load weight force, N.

u G - the multiplicity of the cargo chain hoist.

If the chain hoist begins to lift the load, then the tension forces in its branches are distributed unevenly. This is due to efficiency losses. in blocks and on the stiffness of the cable. Efforts are distributed as follows:

….

where  - efficiency, taking into account friction losses in the block and from the stiffness of the rope.

The system of forces is in balance

Here in parentheses is the sum of the geometric progression

, taking this into account, expression (4.3) will be reduced to the form

. Where do we get the formula for determining the traction force in the cable lopar

(4.4)

efficiency chain hoist is the ratio of useful work

Fig.4.3. Distribution of efforts in the branches of the chain hoist

when lifting a load F Q to the height h to the work done

. (4.5)

Between the speed of lifting (lowering) the load V under and speed of selection (etching) of the pendant fall V l.sh. there is a dependency

(4.6)

The disadvantage of single chain hoists is that when lifting a load, it also moves horizontally. This makes it difficult to accurately stop the load and causes uneven reactions in the drum supports.

When choosing a chain hoist, friction losses should also be taken into account. The best blocks used in practice lead to friction losses of at least 10% of the applied force. Thus, putting effort into 1 kg to a simple double chain hoist, you can lift the load in 2 × 0.9 = 1.8 kg, and when using a simple fourfold chain hoist not 4 kg, as expected, and 4 x 0.9 x 0.9 x 0.9 = 2.92 kg, that is, the gain in strength will be less than 3 times, with a loss in speed of 4 times. A simple fivefold chain hoist gives real amplification slightly more than 3 times. When used instead of carabiner blocks, the friction is even greater.

Link List

Aleksandrov M.P. Hoisting and transport machines: A textbook for engineering specialties of universities. - 6th edition, revised. - M.: graduate School, 1985. - 520 p., ill.
Shestopalov A. How the chain hoist works // Internet project "How things work". – http://howitworks.iknowit.ru/paper1144.html .

Questions to control

What is the purpose of the polyspast?
How to determine the multiplicity of the chain hoist?
What is the reason for the inexpediency of using large multiplicity chain hoists?

Polyspast - this is a lifting device, consisting of several movable and fixed blocks enveloped by a rope, rope or cable, which allows lifting loads with an effort several times less than the weight of the load being lifted.

Any chain hoist gives a certain gain in effort to lift the load. In any mobile system consisting of a rope and blocks, friction losses are inevitable. In this part, for ease of calculation, inevitable frictional losses are not taken into account and the theoretically possible gain in effort, or abbreviated TB theoretical gain, is taken as the basis.

Note: Of course, in real work with chain hoists, friction cannot be neglected. More about this and about the main ways to reduce friction losses will be discussed in the next part " Practical Tips for work with chain hoists "

Basics of building chain hoists

If you fix the rope (cable) on the load, throw it over the block fixed at the station (hereinafter referred to as the stationary or fixed block) and pull it down, then to lift the load, you must apply a force equal to the mass of the load. There is no gain in effort In order to lift the load by 1 meter, it is necessary to stretch 1 meter of rope through the block.

This is the so-called 1:1 scheme.

The rope (cable) is fixed at the station and passed through the block on the load. With this scheme, to lift the load, an effort is needed 2 times less than its mass. Effort win 2:1. The roller moves up with the load. In order to lift the load by 1 meter, it is necessary to stretch 2 meters of rope through the roller.

This is a diagram of the simplest chain hoist 2: 1

Figures No. 1 and 2 illustrate the following Basic Rules of Polyspasts:

Rule number 1.

The gain in effort is given only by MOVING rollers fixed directly on the load or on a rope coming from the load. STATIONARY rollers serve only to change the direction of movement of the rope and WIN IN EFFORT DO NOT GIVE.

Rule number 2.

How many times we win in effort - the same number of times we lose in distance. For example: if in the one shown in Fig. 2 chain hoist 2:1 for each meter of lifting the load up, 2 meters of rope must be pulled through the system, then in the chain hoist 6: 1 - respectively 6 meters. The practical conclusion is that the “stronger” the chain hoist, the slower the load rises.

Continuing to add stationary rollers to the station and movable rollers to the load, we get the so-called simple chain hoists of different forces:

Examples of simple chain hoists Fig. 3, 4.

Rule #3

Calculation of the theoretical gain in effort in simple chain hoists. Everything here is quite simple and clear.

If it is necessary to determine the TV of an already finished chain hoist, then you need to count the number of strands of rope going up from the load. If the movable rollers are fixed not on the load itself, but on the rope coming from the load (as in Fig. 6), then the strands are counted from the point of attachment of the rollers. Figures 5, 6.

Bold

Calculation of TV when assembling a simple chain hoist

In simple chain hoists, each movable roller (fixed on the load) added to the system additionally gives a double TV. The incremental effort DOES add up to the previous one.

Example: if we started with a chain hoist 2:1, then by adding another movable roller, we get 2:1 + 2:1 = 4:1; By adding one more roller, we get 2:1 + 2:1+2:1= 6:1, etc.

Figures 7.8.

Depending on where the end of the cargo rope is fixed (at the station or on the load), simple chain hoists are divided into even and odd.

If the end of the rope is fixed at the station, then all subsequent chain hoists will be EVEN: 2:1, 4:1, 6:1, etc. Figure 7

If the end of the load rope is fixed on the load, then ODD chain hoists will be obtained: 3:1, 5:1, etc. Figure 8

In addition to simple chain hoists, the so-called COMPLEX chain hoists are also widely used in rescue work.

Complex chain hoist

A complex chain hoist is a system in which one simple chain hoist pulls another simple chain hoist. Thus, 2, 3 or more chain hoists can be connected.

Figure 9 shows the designs of the most commonly used complex chain hoists in rescue practice.

Rule number 4. Calculation of TV complex chain hoist.

To calculate the theoretical gain in effort when using a complex chain hoist, it is necessary to multiply the values \u200b\u200bof the simple chain hoists that it consists of. An example in fig. 10. 2:1 pulls for 3:1=6:1. An example in fig. 11. 3:1 pulls for 3:1 = 9:1.

The calculation of the effort of each of the simple pulley blocks that make up the complex one is carried out according to the rule of simple pulley blocks. The number of strands is counted from the point of attachment of the chain hoist to the load or cargo rope coming out of another chain hoist. Examples in fig. 10 and 11.

Figure 9 shows almost all the main types of chain hoists used in rescue operations. As practice shows, in most cases, these structures are quite enough to perform any tasks. Further in the text, several more options will be shown.

Of course, there are other, more complex, chain hoist systems. But they are rarely used in rescue practice and are not considered in this article.

All the pulley blocks shown above can be very easily learned at home by hanging some kind of load, say, on a horizontal bar. To do this, it is quite enough to have a piece of rope or cord, several carabiners (with or without rollers) and grasping (clamps). I highly recommend it to all those who are going to work with real chain hoists. From my own experience and the experience of my students, I know that after such training, there are much fewer errors and confusion in real conditions.

Complex chain hoists

Complex chain hoists are neither simple nor complex - this is a separate type.

A distinctive feature of complex chain hoists is the presence in the system of rollers moving towards the load. This is the main advantage of complex chain hoists in cases where the station is located above the rescuers and it is necessary to pull the chain hoist down.

Figure 12. shows two schemes of complex chain hoists used in rescue work. There are other schemes, but they are not used in rescue practice and are not considered in this article.

Part B

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

2.5.1 . Each design of chain hoists, in addition to winning in effort, has others important indicators affecting its overall performance.

General design features that improve the efficiency of chain hoists:

The greater the working length of the chain hoist, the greater its working stroke and the distance that the load rises in one working stroke.

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

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

4 . Simple chain hoists 2:1 and 3:1 give the fastest lift with a minimum of system rearrangements.

Before moving on to chain hoists with great effort, you need to make sure that all measures have been taken to combat friction in a simple chain hoist.

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

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

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

2.5.2. General performance characteristics of simple chain hoists

Advantages of simple chain hoists:

* Simple and easy to assemble and operate.

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

* Only one gripper (clamp) needs to be moved.

* With enough people picking up the rope, simple 2:1 and 3:1 chain hoists give the fastest climbing speed.

Cons of simple chain hoists:

* Larger (compared to complex chain hoists of similar efforts) number of rollers. Consequently, large total friction losses.

For this reason, simple chain hoists are no longer used in rescue practice.than 5:1.And when using carbines, it makes no sense to make a simple chain hoist more than 4: 1

* For the same total working length, simple chain hoists use more rope than complex chain hoists of similar strength. Fig.18

2.5.3. General performance characteristics of complex chain hoists.

Advantages of complex chain hoists:

* With an equal number of rollers and gripping units (clamps), they make it possible to create chain hoists of great effort. For instance:

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

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

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

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

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

Accordingly, in a complex chain hoist, friction losses will be less, and PV will be greater.

An example in fig. 21:

In a complex chain hoist 4:1 (2 rollers) when using rollers with a friction loss of 20% PV will be -3.24:1. In a simple chain hoist 4:1 (3 rollers) – FV =2.95:1

Cons of complex chain hoists:

* Harder to organize.

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

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

* In general, they significantly lose to simple chain hoists in lifting speed.

Practical tips for working with complex chain hoists:

* In order to complex chain hoist folded more fully with each working stroke, and fewer rearrangements were required, it is necessary to separate the stations of simple chain hoists that are part of the complex one. Fig.22

* A complex chain hoist system requires fewer shifts in work, if a simple onechain hoist with big forcefully pulls the chain hoist with smaller effort.

Example on fig.22A

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

B - another chain hoist scheme 6:1 - 3:1 pulls for 2:1. Only one gripping knot (clamp) needs to be changed. Accordingly, the system works faster.

2.5.4. In all the above designs of chain hoists, the rope must be pulled towards the loading station. In the mountains, on a limited area or on a wall, pulling from below - up can be very difficult and inconvenient. In order to pull down and put their weight into work, and also, in order not to tear their backs, an additional stationary roller (carbine) is often fastened. Rice. 23.

But, according to the Pulley Block Rule No. 1 - stationary rollers do not give a gain in effort. Friction losses in this arrangement, especially when using a carabiner, can negate all the benefits of pulling down.

b. Use complex polyspast.

Complex chain hoists are neither simple nor complex - it is a separateview.

A distinctive feature of complex chain hoists is the presence in the system of rollers moving towards the load.

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

On the Figure 25. two schemes of complex chain hoists used in rescue work are given.

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

Note:

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

The main disadvantages of complex chain hoists are similar to the disadvantages of complex chain hoists:

Complex chain hoists do not fold completely, have a small working stroke and require many rearrangements with each working cycle. For example, a 5:1 scheme requires a swap of two grasping knots.

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

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

Also, for an additional pulley block 2: 1, the slack of the cargo rope can be used, as shown in the figure from F. Kropf's book. "Rescue work in the mountains" 1975 Rice. 26A

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

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

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

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

A simple chain hoist 2:1 is "built into" a simple chain hoist 3:1. The result is a chain hoist with TV 5:1. This chain hoist is neither simple nor complex. I have not been able to find its exact name. The name "composite" in fig. 27 and 27A invented by me.

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

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

* In operation, this method requires the rearrangement of only one grasping (clamp) and therefore is more efficient in operation.

*Another example of this "built-in two" system is shown in rice. 27A.

A complex 10:1 pulley block works here - a 2:1 pulley block is “built-in” into a 6:1 pulley block.

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

Part C

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

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

2.6.1. The main method is the use of guide rollers (hereinafter referred to as HP). Rice. 28

Guide rollers are placed at a separate station directly above the place of ascent (descent).

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

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

The guide roller station must be designed for heavy loads.
rice. 29.

Benefits of using guide rollers*

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

Below are examples of the main advantages of using guide rollers:

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

Ideally, the rope should approach the edge at an angle of 90 0. Otherwise, the cargo rope will inevitably slip to the side.

HP allows you to direct the load rope at the right angle to the edge of the site. Rice. 31

* In cases where there is no suitable work platform directly above the place of ascent or descent, HP allows you to position the cargo station for descent and ascent away from the ascent line, in a more convenient place for work.

In addition, the location of the station away from the line of ascent (descent) reduces the likelihood of hitting the rescuer, victim, cargo and safety ropes with stones, etc., which can be dropped by rescuers working on top.

* HP makes it possible to fully or partially raise the chain hoist system above the terrain. This significantly increases the efficiency of work by reducing friction losses of the chain hoist and its components on the terrain. This also increases the overall safety of work, as it reduces the likelihood of chafing, jamming or jamming of any component of the chain hoist.

* HP allows you to reduce or completely eliminate the friction of the cargo rope on the edge (kink) of the working platform. This is also a very big plus in terms of security.

* HP can make it much easier for the rescuer and the victim to go over the edge, both on the ascent and on the descent. This is one of the most difficult and time-consuming moments in transportation, especially for the accompanying rescuer.

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

HP allows:

* Raise the ferry higher.

* It is convenient to arrange the chain hoist system.

* Pull the chain hoist down.

* Adjust the tension of the ferry in the process.

Important! With a strong tension of the crossing, there are very large loads onextreme points of attachment of the crossing. Rice. 38.

The conclusions from the diagram above are as follows:

* Excessive tension of crossings should be avoided - this is dangerous!

For instance:
With the simultaneous crossing of a heavily stretched crossing of two people (Injured and accompanying. Total weight ~ 200 kg), due to the inevitable rocking of the crossing, peak loads at the extreme points can reach 20 KN (2000kg)and higher! Such a load is close to the limit of strength characteristicsclimbing carabiners, quickdraws and ropes (taking into account the loss of strength of the rope innodes).

* All anchorage points of the crossing, including the anchorage station of the guide roller andall its components must be exceptionally reliable!

To be continued…

The article was based on the work "Polyspasty for rescue operations" by Fedor Farberov. The main emphasis in this article is the lifting and moving of loads weighing up to 100 kg. Above this mass, it is necessary to use other special equipment and other equipment and systems. The article involved technical materials PETZL.
The material is not exhaustive and does not claim to be the truth in a single instance. It's just practical advice on the use of chain hoist systems when performing various works at height.

TERMINOLOGY

What is a polyspast

This is a system consisting of several movable and fixed blocks connected by a rope or cable, which allows, playing in the distance, to get a significant gain in the applied effort, several times less than the weight of the load. Designed for lifting, lowering, moving cargo, as well as for organizing anchor lines. Polispast - from the Greek "poly", which means "a lot", and "spao" - "I pull")
Theoretically winning- the theoretical value of the possible effort developed by the chain hoist without taking into account the loss from friction on various parts of the system. It is taken as a basis for the simplicity of calculating the size of the chain hoist.
Actual win- the magnitude of the effort developed by the chain hoist system when subtracting all the obstructing forces that affect its effectiveness.

Types of chain hoists

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

Design features of chain hoists

Anchor- the place of attachment of the beginning of the chain hoist and fixed blocks.
- a block located on the load or built into the chain hoist system, but always moves towards or away from the load. Always gives a double win in strength.
- a block fixed motionlessly at the anchor point is necessary to change the direction of the applied force. Gives no gain in effort.
Operating length of the chain hoist- the distance from the anchor to the element closest to the load (grasping node, ). The longer this value, the greater the distance the load can travel in one working stroke of the chain hoist.
The working stroke of the chain hoist- the distance that all elements of the system travel before any contact with other elements. The working stroke depends on the type of chain hoist, on its working length and on how tightly the chain hoist “folds” - that is, how close the first element to the load is pulled to the anchor with the rope fully selected.
Rearranging the system- the necessary manipulations to return the chain hoist to its working length after it has "folded". This may be a permutation of grasping knots (clamps) and other actions.

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

If you fix the rope on the load, throw it over the block fixed on the anchor point and pull it down, then to lift the load, you need to apply a force equal to the mass of the load, and in order to raise the load by 1 meter, you need to stretch 1 meter of rope through the block.
How many times we win in effort - the same number of times we lose in distance.

Calculation of effort in a simple chain hoist
To simplify the calculation of the theoretical gain of the chain hoist, it is customary to use the "T - method" (from the English. Tension - tension).

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

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

- If the end of the rope is fixed on the anchor point, then all subsequent chain hoists will be even: 2:1, 4:1, etc.
- If the end of the cargo rope is fixed on the load, then odd chain hoists will be obtained: 3:1, 5:1, etc.

The advantages of simple chain hoists	Disadvantages of simple chain hoists
Simple and easy to assemble and operate.	To organize chain hoists with large TVs, a lot of equipment is required
The working stroke is close to the working length of the chain hoist.	Difficult transition from ascent to descent.
With a sufficient number of people, simple chain hoists 2:1 and 3:1 give the highest lifting speed.	It is difficult to pass nodes through the system.
Can arrange automatic system rope fixation	A large number of blocks and rope used in schemes greater than 4:1, and consequently, large total friction losses.
No extra rope required.
Convenient for small workspace

Due to friction, it is impractical to use schemes greater than 5:1 in a simple chain hoist.

Polyspasts made from an additional rope.
In practice, most often there is a situation when a pulley block made from a separate rope is attached to a working rope. First of all, this is due to the savings in equipment. In such a scheme, a reverse motion is required. The chain hoist is attached to the working rope with a grasping knot or clamp.

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

Calculation of effort in complex chain hoists
The calculation of the effort of each of the simple pulley blocks that make up the complex one is carried out according to the rule of simple pulley blocks. The 6:1 scheme adds up so 2:1 pulls for 3:1, it turns out 6:1. And 3:1 pulls for 3:1 and it turns out 9:1.

Practical tips for working with complex chain hoists:
In order for a complex chain hoist to fold more fully with each working stroke, and fewer rearrangements are required, it is necessary to separate the stations of simple chain hoists that are part of the complex one.

Complex chain hoists
In all the above designs of chain hoists, the rope must be pulled towards the anchor point. In practice, it is always more convenient to pull from the anchor point, because a counterweight can be used. In order to pull down, an additional fixed block is fastened. But it doesn't provide a power gain, and the friction losses in such a setup can negate all the benefits of pulling down. A distinctive feature of complex chain hoists is the presence in the system of rollers moving towards the load. Complex chain hoists are also simple and complex.
The disadvantages are the same as those of the main complex chain hoists:

- Pulley blocks do not fold completely,
- They have a small working stroke and require many permutations.

Calculation of effort in complex chain hoists
The calculation of the theoretical gain in complex chain hoists differs from the main ones. 3:1(simple)= 1T+2T
5:1(hard)= 1T+1T+ST (or as it is commonly believed 5:1= 2T*ST-1T)
7:1(hard)= 2T*ST+1T

Composite chain hoists
In cases where the force of the assembled chain hoist is not enough, and the length of the pulling rope is not enough to assemble a more powerful scheme, an additional 2: 1 chain hoist attached to the load rope with a grasping knot or clamp can help.
By adding a 2:1 scheme to any chain hoist, you will automatically receive a 2-fold theoretical gain in effort.

The calculation of the theoretical gain for them is carried out according to the principle of complex or complex, depending on the design of the chain hoist.

To be continued…

A person is not very strong for lifting large loads, but he came up with many mechanisms that simplify this process, and in this article we will discuss chain hoists: the purpose and arrangement of such systems, and we will also try to make the simplest version of such a device with our own hands.

The cargo chain hoist is a system consisting of ropes and blocks, thanks to which it is possible to gain in effective strength with a loss in length. The principle is pretty simple. In length, we lose exactly as much as how many times the gain turned out to be in strength. Thanks to this golden rule of mechanics, large masses can be made without much effort. Which, in principle, is not so critical. Let's take an example. Here you have won in strength 8 times, while you have to pull out a rope 8 meters long in order to raise an object to a height of 1 meter.

The use of such devices will cost you less than renting a crane, and besides, you can control the gain in strength yourself. The chain hoist has two different sides: one of them is fixed, which is mounted on a support, and the other is movable, which clings to the load itself. The gain in strength is due to the movable blocks that are mounted on the movable side of the chain hoist. The fixed part serves only to change the trajectory of the rope itself.

Types of chain hoists are distinguished by complexity, parity and multiplicity. In terms of complexity, there are simple and complex mechanisms, and the multiplicity means the multiplication of force, that is, if the multiplicity is equal to 4, then theoretically you will win 4 times in strength. Also rarely, but still, a high-speed chain hoist is used, this type gives a gain in the speed of movement of goods at a very low speed of the drive elements.

Let's start with a simple mounting chain hoist. It can be obtained by adding blocks to the support and load. To get an odd mechanism, it is necessary to fix the end of the rope on the moving point of the load, and to get an even one, we fasten the rope to a support. When adding a block, we get +2 to strength, and a moving point gives +1, respectively. For example, to get a chain hoist for a winch with a multiplicity of 2, you need to fix the end of the rope on a support and use one block that is attached to the load. And we will have an even type of fixture.

The principle of operation of a chain hoist with a multiplicity of 3 looks different. Here the end of the rope is attached to the load, and two rollers are used, one of them we attach to the support, and the other to the load. This type of mechanism gives a gain in strength of 3 times, this is an odd option. To understand what the gain in strength will be, you can use simple rule: how many ropes come from the load, this is our gain in strength. Chain hoists are usually used with a hook, on which, in fact, the load is attached, it is a mistake to think that this is only a block and a rope.

Now we will find out how a complex type pulley block works. This name means a mechanism where several simple variants of a given cargo device are connected into one system, they pull each other. The gain in strength of such constructions is calculated by multiplying their multiplicities. For example, we pull one mechanism with a multiplicity of 4, and the other with a multiplicity of 2, then our theoretical gain in strength will be 8. All the above calculations take place only for ideal systems that have no friction force, but in practice things are different .

In each of the blocks, there is a small loss in power due to friction, since it is still spent just to overcome the friction force. In order to reduce friction, it must be remembered: the larger the bending radius of the rope, the less the friction force will be. It is best to use large radius rollers where possible. When using carbines, you should make a block of the same options, but rollers are much more effective than carbines, since we have a loss of 5-30% on them, but up to 50% on carbines. It is also useful to know that the most efficient block must be placed closer to the load for maximum effect.

How can we calculate the real gain in strength? To do this, we need to know the efficiency of the blocks used. Efficiency is expressed in numbers from 0 to 1, and if we use a rope of large diameter or too stiff, then the efficiency of the blocks will be much lower than indicated by the manufacturer. So, it is necessary to take this into account and adjust the efficiency of the blocks. To calculate the real gain in strength of a simple type of hoist, you need to calculate the load on each branch of the rope and add them up. To calculate the gain in strength of complex types, it is necessary to multiply the real strengths of the simple ones of which it consists.

Do not forget also about the friction of the rope, since its branches can twist among themselves, and the rollers from heavy loads can converge and pinch the rope. To prevent this from happening, you should space the blocks relative to each other, for example, you can use a circuit board between them. You should also purchase only static ropes that do not stretch, as dynamic ones give a serious loss in strength. To collect the mechanism, both a separate rope and a cargo rope attached to the load, regardless of the lifting device, can be used.

The advantage of using a separate rope is that you can quickly assemble or prepare a lifting structure in advance. You can also use its entire length, this also makes it easier to pass knots. Of the minuses, we can mention that there is no possibility of automatic fixation of the lifted load. The advantages of a load rope are that the object being lifted can be auto-fixed and there is no need for a separate rope. Of the minuses, it is important that it is difficult to pass knots during operation, and you also have to spend a cargo rope on the mechanism itself.

Let's talk about the reverse move, which is inevitable, since it can occur when grabbing the rope, or at the time of removing the load, or when stopping for a rest. To prevent reverse movement, it is necessary to use blocks that allow the rope to pass only in one direction. At the same time, we organize the structure in such a way that the blocking roller is attached first from the object being lifted. Thanks to this, we not only avoid reverse movement, but also allow us to secure the load during unloading or simply rearranging the blocks.

If you use a separate rope, then the blocking roller is attached last from the load being lifted, while the fixing roller must be highly effective.

Now a little about attaching the lifting mechanism to the cargo rope. It is rare that we have the right length of rope on hand to secure the moving part of the block. Here are several types of mounting mechanism. The first method is with the help of grasping knots, which are knitted from cords with a diameter of 7-8 mm, in 3-5 turns. This method, as practice has shown, is the most effective, since the grasping knot of 8 mm cord on a rope with a diameter of 11 mm begins to slip only at a load of 10-13 kN. At the same time, at first it does not deform the rope, but after some time, it melts the braid and sticks to it, starting to play the role of a fuse.

Another way is to use a general purpose clamp. Time has shown that it can be used on icy and wet ropes. It starts to crawl only at a load of 6-7 kN and slightly injures the rope. Another way is to use a personal clamp, but this is not recommended, as it starts to crawl at a force of 4 kN and at the same time breaks the sheath, or can even bite the rope. These are all industrial designs and their application, but we will try to create a home-made chain hoist.

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