The chain of a chain hoist with a multiplicity of 4. How a chain hoist works. Polypast. Lift with at home

Polypast

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Construction machines and their operation

Polypast

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

Fig. 1. Schemes of rope pulley blocks a - three-time chain hoist; b, c, d - four-, five- and six-fold pulley blocks

Fig. 2. Scheme of a double pulley block

The number of working branches (the frequency of the chain hoist) is equal to the number of blocks when the rope escapes from the stationary block of the chain hoist, and the number of blocks of the chain hoist plus one when the rope escapes from the movable block.

Fig. 3. Reverse action chain hoist diagram

Polyspast is the simplest lifting device, consisting of blocks connected by a rope. With the help of a chain hoist, you can lift the load or move it horizontally. The polyspast 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 polyspast consists of two blocks: fixed, attached to the lifting device (beam, mast, tripod), and movable, which is attached to the load being lifted. Both blocks are connected with 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 branch 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 of the chain hoist runs not from the lower block, but from the upper one, then the upper block of the fixed block is considered to be diverting. This condition must be taken into account when calculating the pulley blocks.

The polyspast is stored in two ways. According to the first method, used when equipping multi-line pulleys with large carrying capacity, the stationary block without ropes is lifted into the working position and fixed; the lower movable block is at the bottom. Then, a rope is sequentially passed through the grooves (grooves) of the rollers of the upper and lower blocks. The end of the rope is fixed to the upper or lower block, depending on the adopted chain hoist reeving scheme. The rope is often passed through the streams of rollers using manual lever winches, which greatly facilitates the work of reserving the chain hoist.

Recently, when equipping a multi-line pulley block, 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 other end is fixed to the winch drum. During the operation of the winch, the working rope is pulled through the rollers of the pulley blocks.

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

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

The rope begins to stretch from the roller from which the runaway 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 raised using an auxiliary single-roll block or a low-carrying capacity chain hoist. First, the auxiliary block is fixed, a rope is passed through it, to which the main block of the chain hoist is attached. The second end of the rope is fixed on a winch, with which the chain hoist will be lifted. The main block of the chain hoist is fixed from the cradle or from the scaffold.

In fig. 4 shows the reeving diagrams of pulley blocks with two-, four-, five- and six-wheel blocks.

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

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

When rigging work, the calculation begins with finding out the carrying capacity of the available blocks, which must correspond to the weight of the load being lifted. For example, according to the scheme (Fig. 22, a), to lift a load weighing 20 tons, blocks with a lifting capacity of 20 tons are required. In the diagram, the upper block is three-roll, but in order to highlight the diverter, it is conventionally shown as two-roll.

Fig. 4. Schemes of reserving chain hoists with the number of working threads: a - six with three branching single-roll blocks, b - three, c - four, d - five, e - six, f - 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, as well as the effort in the runaway thread of the cargo chain hoist.

When calculating the pulley blocks, the fastening of the upper block of the pulley block to the mechanism or device is calculated.

If we assume that both threads run vertically, then the first diverter roller is fixed on a force equal to the sum of the efforts in the 5th and 6th threads: 3.68 + 3.82 \u003d 7.5 tf. The securing of the second branch block is calculated on the forces 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 according to the parallelogram rule.

Example. Pick up 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 the chain hoists. According to the table. 11 we choose a double-roll block with a lifting capacity of 10 tf for the lower movable block, and a three-roll block with a lifting capacity of 15 tf for the upper fixed block.

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

Since there can be only an even number of threads, we take 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 an equalizing roller and one or two drive winches is shown in Fig. five.

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,

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

A polyspast is a simple lifting device consisting of a system of movable and stationary blocks (rollers), bent around by a flexible body (usually a rope). Polypasts are used as independent mechanisms in combination with winches and as elements of complex hoisting machines (cranes).

The 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 traction is applied. The fixed frame 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).

Fig. 6. Schemes of pulley blocks a - in four lines; b - in six threads; 1 - fixed blocks; 2 - movable blocks; 3 - branch block; 4 - rope

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

Fig. 7. Design schemes of pulley blocks

1. Determine the force 5L in the rope going to the winch when lifting a load weighing Q \u003d 20 t with a chain hoist made according to scheme I. The blocks (rollers) of the chain hoist are installed on rolling bearings (/ j \u003d 1.02), diverter rollers - on bronze bushings (\u003d 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 taken on bronze bushings (\u003d 1.04).

3. Determine which load Q can be lifted by a winch with a pulling force of 5L \u003d 1.5 tf and a chain hoist made according to scheme III. Blocks (rollers) are adopted on bronze bushings.

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

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

The pulley block includes the following components:

Blocks with fixed axles
Blocks with movable axles.
Bypass blocks.
Bypass drums.

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

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

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

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

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

Performance characteristics of chain hoists and their choice

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

The carrying capacity of the main mechanism, which includes these units.
Number of bypass blocks: with an increase in their number, friction losses increase.
Angles of deflection of the ropes from the middle plane of the drum.
Block diameters.
Rope diameter / chain height.
Rope material.
The nature of the supports (in rolling or sliding bearings).
Lubrication conditions for 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, which operate in plain bearings, depending on the conditions of their operation, is:

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

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

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

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

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

Normal angles of deviation from the chain hoist axis are considered angles 7.5 ... 2.50 (smaller values \u200b\u200bare taken for maximum ratios of the block diameter to the rope diameter). In general, when designing these devices, they always try to choose this ratio in the range of values \u200b\u200b12 ... 40. The permissible angle of deflection of ropes made of light-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 operating mode of the lifting equipment). On the equalizing block, the permissible deviation angles can increase, but not more than 1.5 times.

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

Reeving the pulley blocks

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

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

There are the following stock schemes:

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

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

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PRACTICE:

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

The blocks of this lifting mechanism are subdivided:

by design to simple and complex;

by the method of lifting the load to movable and fixed.

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, whose arms are equal to the radius of the wheel: ОА \u003d ОВ \u003d r. Such a block does not give a gain in strength, but it allows you to change the direction of movement of the cable (rope, chain).

Double block consists of two blocks of different radii, rigidly fastened to each other and mounted on a common axis in 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 strength 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 \u003d P · 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 force given by the gate 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 \u003d P r / R.

Let's move on to the method of lifting the load in blocks. From the description of the structure, 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.

Have rolling blockthe axle rises and falls together with the load fig. 10 and it is mainly intended to eliminate the kink of the cable at the place of the load suspension.

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

Methods for suspending the load and obtaining a gain in strength when lifting the load with a rope:

In fig. 4, the load is fixed at one end of the cable and if the load is lifted by the other end of the cable, then to lift this load, a force slightly greater than the weight of the load is required, since a simple block of gain in strength does not give F \u003d P.

In Fig. 5, the worker lifts himself by the cable, which bends 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 for 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 divided 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 using two ropes and the weight of the load is distributed equally between the ropes, and therefore each worker will lift the load with a force of half the weight of the load F \u003d 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 \u003d 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 lifted the load by the other end of the cable, the force with which the worker would lift the load doubled less weight of the load F \u003d P / 2 and lifting 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 is distributed into three parts of the cable F \u003d P / 3.

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

Theoretically, the load can be suspended on an unlimited number of parts of one rope, but practically limited to six parts and such a lifting mechanism is called pulley block, which consists of fixed and movable clips with simple blocks, which are alternately bent around by a cable, one end fixed to a fixed clip, and the load is lifted at the other end of the cable. The strength gain depends on the number of cable parts between the fixed and movable clips, usually 6 cable parts and the strength gain 6 times.

Literature:

Peryshkin, A. V. Physics, grade 7: textbook / A. V. Peryshkin. - 3rd ed., Additional - M .: Drofa, 2014, - 224 p,: ill. ISBN 978-5-358-14436-1. § 61. Application of the balance rule of the lever 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. Kaidalov, V. B. Kozhevnikov; ed. V.A. Orlova, I. I. Roisen, 2nd ed., Rev. - M.: Mnemosyne, 2010. ISBN 978-5-346-01453-9. § 24. Simple mechanisms, pp. 188-196.
An elementary textbook of physics, edited by Academician GS Landsberg Volume 1. Mechanics. Heat. Molecular Physics. - 10th ed. - M .: Nauka, 1985. § 84. Simple machines, pp. 168-175.
Gromov S.V. Physics: Textbook. for 7 cl. general education. institutions / S. V. Gromov, N. A. Rodina. - 3rd ed. - M .: Education, 2001.-158 s,: ill. ISBN-5-09-010349-6. §22. Block, pp. 55 -57.

Polyspast is a mechanism by which the lifting of goods is carried out. It includes one or more groups of blocks, bend by a rope. The word "polyspast" comes from the Greek polyspastion. This term translates as "being pulled by multiple ropes". The main function of the chain hoist is to increase the load capacity of the main mechanism.

In other words, this device gives a gain in strength. However, the inverse effect of using the chain hoist is to reduce the lifting speed. It is also possible to gain in speed by losing in strength. However, such pulley blocks are used much less often. In any case, the principle of operation of the device is the action of the lever.

Mechanism device

Polyspast - this allows you to get a force that exceeds the lifting force of the winch several times. In other words, this mechanism increases the carrying capacity of the device. The use of a chain hoist allows you to lift a heavy load using a winch with a small lifting 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.

The purpose of the mechanism

The pulley is necessary for lifting heavy loads with a minimum of effort. The simplest construction of the chain hoist is arranged so that one edge of the rope is fixed to the drum, and a suspended load is located at the opposite end of the rope. More complex devices include several fixed and movable rollers.

For each weight, the dimensions, blocks and diameter of the rope should be taken into account. A load with a large mass increases the load when suspended from the rope. This mechanism is characterized by rapid wear. In this case, a decrease in the voltage in the rope is required. Therefore, to suspend a load of large mass, two or four ropes are used. It is also possible to use a chain hoist of a 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 fact, the chain hoist is a very simple lifting mechanism, which almost anyone 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 construction of the chain hoist includes several groups of blocks that are assembled into special clips. And they alternately bend around with a rope or rope. Even such a simple structure can be used quite effectively to increase the force applied to lower or raise loads. Also, the construction of a simple pulley block contains cargo blocks. They can be of the following types:

multi-roll or single-roll;
fixed or movable.

The pulling 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?

Polyspast is used to lift and move cargo in cases where it is possible to use only the physical strength of a person and the least number of auxiliary mechanisms. Also, the pulley block 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 at least somehow used: from everyday tasks to heavy industry.

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

To lift a load of a certain mass using a chain hoist, you need to apply an effort that is half 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 the pulley blocks, which have the simplest device, are called "two-to-one pulley blocks", since they double the applied force. The design with three blocks, respectively, gives a three-fold increase in strength.

Pulley rate

It should be noted that the calculation of the chain hoist plays a very important role. After all, the mechanism does not work in ideal conditions. It is affected by the frictional forces that occur when the cable moves along the pulley. Also, frictional 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 frequency 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 pulley block also depends on it.

It can be reduced by using high quality blocks and ropes, as well as through high-quality performance that excludes unnecessary overlaps and kinks.

Today, pulley blocks are studied even in the school physics course. With their help, it will not be difficult to make this structure. You must also purchase the following items:

fitting;
rope;
winch.

What device models are there?

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

When using two blocks in a chain hoist at once, the advantage in the applied effort can be tripled. There is also a safety function that is triggered when the rope is lowered. In this case, two are tightened and the load is blocked.

If you add two more blocks to the previous mechanism, you will get a tackle device that gives 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 tackles

It should be noted that the transfer of force in a 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 force is lost. Therefore, in a simple tackle, in fact, the ratio will be approximately 1.8 kilograms per kilogram of load lifted. And a 5-fold pulley block 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 pulley blocks can be used.

This chain hoist is designed so that the lifting weight does not create a load on the last block. Instead, it loads the rope that passes through the block. As a result, when 3 blocks are used, the 2: 1 and 3: 1 tackles 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 tackle is that in the process he has to overcome the arising friction forces. This problem can be partially solved if you use high-quality ropes, pulley blocks with smooth streams, as well as thick lubrication.

There are also additional opportunities with the competent use of the construction of the chain hoist. For example, if you use not one carbine, but two. Due to this, the friction force is reduced, as well as an increase in the bend radius.

4. POLYSPASTS

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

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

The running end of a chain hoist (hoist), which is wound on a drum, is called a lopar.

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

A rope designed to raise and lower a load is called a pendant. A cable designed to hold the boom and change its reach is called a topenant.

The multiplicity of the tackle is the ratio of the number of branches of the cable on which the load hangs to the number of lopars

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

–Number of Lapps.

By the number of lopars, tackles are divided into single (Fig.4.1 a)) ( \u003d 1) and doubled (Figure 4.1 b)) ( =2).

Fig. 4.1. Single polyspast with multiplicityu g =2

Figure 4.2. Double chain hoist multiplicityu g =2

Let us determine the efficiency pulley block on the example of a single pulley block shown in Fig. 4.2 with multiplicity u g ... In a fixed pulley, the tension force is the same in all

, (4.2)

where F Q - load weight force, N.

u g - the multiplicity of the cargo block.

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

….

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

The system of forces is in equilibrium

Here in brackets is the sum of a geometric progression

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

... Where do we get the formula for determining the tractive effort in the rope lopar

(4.4)

C.p.d. the pulley block represents the ratio of useful work

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

when lifting cargo F Q to height h to the work spent

. (4.5)

Between the speed of lifting (lowering) the load V under and the speed of picking out (etching) the lappet pendant V l.h. there is dependence

(4.6)

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

When choosing a chain hoist, friction losses should also be considered. The best blocks used in practice result in a friction loss of at least 10% of the applied force. Thus, making an effort to 1 kg to a simple double pulley block, you can lift the load in 2 × 0.9 \u003d 1.8 kg, and when using a simple four-time pulley, not 4 kgexpected as well 4 x 0.9 x 0.9 x 0.9 \u003d 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 five-fold pulley block gives a real gain a little more than 3 times. When used instead of carabiner blocks, the friction is even greater.

List of links

Alexandrov M.P. Hoisting-and-transport machines: Textbook for mechanical engineering specialties of universities. - 6th edition, revised. - M .: Higher school, 1985. - 520 p., Ill.
Shestopalov A. How the pulley block works // Internet project "How things work." - http://howitworks.iknowit.ru/paper1144.html.

Questions for control

What is the purpose of the chain hoist?
How to determine the ratio of the chain hoist?
What is the reason for the inexpediency of using high frequency pulley blocks?

Polyspast is a lifting device consisting of several movable and stationary blocks bent around by a rope, rope or cable, allowing to lift loads with an effort several times less than the weight of the load being lifted.

Any pulley block gives a certain gain in the effort to lift the load. Friction losses are inevitable in any moving system consisting of rope and blocks. In this part, in order to facilitate calculations, the inevitable friction losses are not taken into account and the Theoretically Possible Win in Force or abbreviated TV theoretical gain is taken as the basis).

Note: Of course, in real work with pulley blocks, 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 working with chain hoists"

The basics of building pulley blocks

If you fasten the rope (cable) on the load, throw it over the block fixed at the station (hereinafter stationary or fixed block) and pull it down, then to lift the load you need to apply a force equal to the mass of the load. There is no gain in effort. In order to raise 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 such a scheme, lifting the load requires an effort 2 times less than its mass. The gain in effort is 2: 1. The roller moves up with the load. In order to raise 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 1 and 2 illustrate the following Basic Rules of Polyspasts:

Rule # 1.

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

Rule # 2.

How many times we win in the effort - as many times we lose in the distance. For example: if in the one shown in fig. 2 chain hoist 2: 1 for each meter of lifting of the load upwards, 2 meters of rope must be pulled through the system, then in a 6: 1 chain hoist - 6 meters, respectively. The practical conclusion - the "stronger" the chain-link - 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 pulley blocks of different efforts:

Examples of simple pulleys Fig. 3, 4.

Rule No. 3

Calculation of theoretical gain in effort in simple pulley blocks. Everything here is quite simple and clear.

If it is necessary to determine the TV of the already prepared pulley block, then you need to calculate the number of strands of rope coming from the load up. If the movable rollers are not fixed on the load itself, but on a rope coming from the load (as in Fig. 6), then the strands are counted from the point where the rollers are fixed. Figures 5, 6.

Bold

Calculation of TV when assembling a simple chain hoist

In simple pulley blocks, each movable roller (fixed to the load) added to the system additionally gives a double TV. The additional effort COLLAPSES with the previous one.

Example: if we started with a 2: 1 chain hoist, then by adding one more movable roller, we get 2: 1 + 2: 1 \u003d 4: 1; Adding one more video - we get 2: 1 + 2: 1 + 2: 1 \u003d 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 tackles are divided into even and odd.

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

If the end of the cargo rope is secured to the cargo, then Odd tackles will be obtained: 3: 1, 5: 1, etc. Figure 8.

In addition to simple pulley blocks, so-called COMPLEX POLYSPASTS are also widely used in rescue operations.

Complex pulley

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

Figure 9 shows the designs of the most common complex pulley blocks in rescue practice.

Rule number 4. Calculation of TV complex tackle.

To calculate the theoretical gain in effort when using a complex chain-link, it is necessary to multiply the values \u200b\u200bof the simple chain-link that it consists of. An example in fig. 10.2: 1 pulls for 3: 1 \u003d 6: 1. An example in fig. 11.3: 1 pulls for 3: 1 \u003d 9: 1.

The calculation of the effort of each of the simple pulley blocks that are part of the complex is made according to the rule of simple pulley blocks. The 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 block. Examples in Fig. 10 and 11.

Figure 9 shows almost all the main types of pulleys used in rescue operations. As practice shows, in most cases, these structures are quite enough to carry out 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 designs of chain hoists 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 a cord, several carabiners (with or without rollers) and grasping (clamps). I highly recommend it to all those who are going to work with real pulley blocks. From my own experience and the experience of my students I know that after such a practice there are much fewer mistakes and confusion in real conditions.

Complex pulley blocks

Complex pulley blocks are neither simple nor complex - they are a separate type.

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

Figure 12. shows two schemes of complex pulley blocks used in rescue operations. There are other schemes, but they are not used in rescue practice and are not considered in this article.

Part B

2.5. Selection of the optimal construction of the chain hoist.

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

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

The longer the working length of the chain hoist - the greater its working stroke and the distance by which the load rises in one working stroke.

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

With the same working length and working stroke, the chain hoist is faster, requiring fewer shifts.

4 . Simple tackles 2: 1 and 3: 1 give the fastest rise with a minimum of system permutations.

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

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

But in general, it all depends on the specific situation in which this or that type of tackle should be used. Therefore, it is impossible to give unequivocal recommendations.

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

2.5.2. General performance characteristics of simple chain hoists

Pros of simple pulley blocks:

* Simple and understandable in assembly and operation.

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

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

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

Cons of simple pulleys:

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

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

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

2.5.3. General performance characteristics of complex pulley blocks.

Pros of complex pulley blocks:

* With an equal number of rollers and gripping units (clamps), they allow the creation of high-effort chain hoists For example:

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

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

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

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

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

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

An example in fig. 21:

In a complex pulley 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) - PV \u003d2.95:1

Cons of complex pulley blocks:

* More difficult in the organization.

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

* With the same working length, the working stroke of complex pulleys is less than that ofsimple,since they do not add up completely at each working stroke (the roller closest to the pulling ones is pulled to the station, and the 1st cargo roller stops before reaching the station). This significantly reduces the efficiency of the work, 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 work in the last stages of the lifting, when the load must be lifted to the jobsite.

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

Practical tips for working with complex pulley blocks:

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

* A complex tackle system requires fewer shifts in operation, if simplepulley block withbig pulls the chain hoist with an effortsmaller effort.

Example on fig.22A

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

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

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

However, according to the Polyspast 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 pulley block.

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

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

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

On the Fig 25.there are two schemes of complex pulley blocks used in rescue operations.

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

Note:

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

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

Complex pulley blocks do not add up completely, have a small working stroke and require many permutations with each working cycle. For example, a 5: 1 scheme requires the rearrangement of two gripping nodes.

2.5.5. In cases where the effort 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 re-cord folded 2-3 times, 1 roller (carabiner) and 1 grasping (clamp). Example on Fig. 26.

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

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

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

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

2.5.6. Figure 27 shows a diagram of the so-called “built-in deuce”.

Simple chain hoist 2: 1 is “built in” into the system of simple chain hoist 3: 1. The result was a tackle with TV 5: 1. This chain hoist is neither simple nor complex. I could not find its exact name. The name "compound" in Fig. 27 and 27A were invented by me.

Despite the small loss in TV compared to the circuit in Fig. 26 (5: 1 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 work, this method requires the rearrangement of only one grasping (clamp) and therefore more efficient in work.

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

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

A similar system can be used when pulling out the victim alone. In such a scheme, large friction losses are inevitable and the lift is slow. But in general, 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.

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

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

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

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

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

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

Benefits of using guide rollers *

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

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

* Sliding the rope under load to the side along the edge of the working platform during the work of rescuers (it doesn’t matter whether it is climbing or descending, a rock or a building) extremely undesirable and dangerous by rubbing the rope!

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

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

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

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

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

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

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

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

HP allows you to:

* Raise the ferry higher.

* Conveniently position the tackle system.

* Pull the chain hoist down.

* Adjust the tension of the ferry during operation.

Important! With a strong crossing tension, very large loadsextreme points of fastening of the crossing. Fig. 38.

The conclusions from the above diagram are as follows:

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

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

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

To be continued…

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

TERMINOLOGY

What is a chain hoist

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

Types of pulley blocks

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

Design features of chain hoists

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

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

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

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

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

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

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

The advantages of simple chain hoists	Disadvantages of simple pulley blocks
Simple and straightforward to assemble and operate.	A lot of equipment is required to organize chain hoists with large TVs
The working stroke is close to the working length of the chain hoist.	Difficult transition from ascent to descent.
With a sufficient number of people, simple tackles 2: 1 and 3: 1 give the highest lifting speed.	It is difficult to pass nodes through the system.
You can organize an automatic rope fixation system	A large number of blocks and the rope used in the schemes is more than 4: 1, and consequently, large total friction losses.
No additional rope required.
It is convenient to use with a small work area

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

Polypast made from extra rope.
In practice, the most common situation is when a chain hoist made of a separate rope is attached to the working rope. This is primarily due to the saving of equipment. In such a circuit, a backstop is required. The chain hoist is attached to the working rope with a grasping knot or clamp.

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

Calculation of effort in complex pulley blocks
The calculation of the effort of each of the simple pulley blocks that are part of the complex is made according to the rule of simple pulley blocks. Scheme 6: 1 develops 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 pulley blocks:
In order for the complex chain hoist to fold more fully with each working stroke, and less permutations are required, it is necessary to space the stations of the simple chain hoists that are part of the complex.

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

- Polyspasta do not add up completely,
- They have a small working stroke and require many permutations.

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

Compound pulley blocks
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 circuit, an additional 2: 1 chain hoist attached to the load rope with a grasping unit or clamp can help.
By adding a 2: 1 pattern to any chain hoist you will automatically get 2x the theoretical gain in effort.

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

To be continued…

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

The hoist pulley is a ropes and pulley system that can benefit from effective force when lost in length. The principle is pretty simple. In length, we lose exactly as much as the gain in strength. Thanks to this golden rule, mechanics can be of great mass, without making great efforts. Which, in principle, is not so critical. Let's give an example. You have won 8 times in strength, and you will have to stretch a rope 8 meters long to raise the object to a height of 1 meter.

The use of such devices will cost you less than renting a crane, in addition, you can control the gain in power yourself. The chain hoist has two different sides: one of them is fixed, which is attached to the 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 moving side of the chain hoist. The fixed part serves only to change the trajectory of the rope itself.

The types of pulley blocks are distinguished by complexity, parity and multiplicity. In terms of complexity, there are simple and complex mechanisms, and the multiplication means the multiplication of strength, that is, if the multiplicity is 4, then theoretically you win in strength by 4 times. Also rarely, but nevertheless, high-speed pulley 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 tackle. 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 to the movable point of the load, and to get an even one, we attach the rope to the support. When adding a block, we get +2 to strength, and a movable point gives +1, respectively. For example, to get a chain hoist for a winch with a multiplicity of 2, it is necessary to fix the end of the rope on the support and use one block that is attached to the load. And we'll have an even kind of fixture.

The principle of operation of the pulley 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 fasten on the support, and the other on the load. This type of mechanism gives a 3-fold gain in strength, this is an odd option. To understand what the gain in power will turn out, you can use a simple rule: how many ropes comes from the load, this is our gain in strength. Usually, pulley blocks with a hook are used, 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 learn how the pulley block of a complex type works. This name refers to a mechanism where several simple variants of a given cargo device are connected into one system, they pull each other. The gain in the strength of such structures is calculated by multiplying their multiplicities. For example, we pull one mechanism with a magnification of 4, and another with a magnification of 2, then the theoretical gain in force will be 8. We will have all the above calculations only for ideal systems that do not have friction, 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 is necessary to remember: the larger the bend radius of the rope, the less the friction force will be. It is best to use rollers with a large radius where possible. When using carabiners, you should make a block of the same options, but the rollers are much more effective than carabiners, since on them we have a loss of 5-30%, but on carbines, up to 50%. Also, it will not be superfluous to know that the most efficient block must be located closer to the load to obtain the maximum effect.

How do we calculate the actual strength gain? To do this, we need to know the efficiency of the units used.Efficiency is expressed in numbers from 0 to 1, and if we use a rope with a large diameter or too stiff, then the efficiency from the blocks will be significantly lower than indicated by the manufacturer. This means that it is necessary to take this into account and adjust the efficiency of the units. To calculate the real gain in strength of a simple type of lifting mechanism, it is necessary to calculate the load on each branch of the rope and add them. To calculate the gain in the 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 clamp the rope. To prevent this from happening, the blocks should be spaced 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, since dynamic ones give a serious loss in strength. To collect the mechanism, both a separate and a load rope can be used, attached to the load independently of the lifting device.

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

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

If you are using a separate rope, then the locking roller is attached last from the load being lifted, and the locking roller should be of high efficiency.

Now a little about fastening the load-lifting mechanism to the cargo rope. It’s rare when we have at hand a rope of the right length to secure the movable part of the block. Here are some 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 a grasping knot of 8 mm cord on a rope with a diameter of 11 mm begins to slide 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 begins to crawl only at a load of 6-7 kN and slightly injures the rope. Another way is to use a personal clamp, but it is not recommended, since it starts to creep with a force of 4 kN and at the same time breaks the braid, or can even bite the rope. These are all industrial designs and their application, but we will try to create a homemade chain hoist.

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