The simplest mechanisms for lifting loads. Double block rollers and chain hoists

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The use of a movable block gives a double gain in force, the use of a stationary block allows you to change the direction of the applied force. In practice, combinations of movable and fixed blocks are used. Moreover, each moving block allows you to halve the applied force or double the speed of moving the load. Fixed blocks are used to connect movable blocks into a single system. Such a system of movable and fixed blocks is called a pulley block.

A chain hoist is used in cases where it is necessary to lift or move a heavy load with minimal effort, provide tension, etc. The simplest chain hoist consists of just one block and a rope, while allowing you to halve the traction force required to lift the load.

Figure 1. Each moving block in the pulley gives a double gain in strength or speed

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

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

Figure 2. Rope fastening with even and odd multiplicity of pulley system

The gain in force when using a pulley with $n$ movable and $n$ fixed blocks is determined by the formula: $P=2Fn$, where $P$ is the weight of the load, $F$ is the force applied at the input of the pulley, $n$ - number of moving blocks.

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

Figure 3. Single and double pulleys

To ensure strictly vertical lifting of the load, double pulleys (consisting of two single ones) are used; in this case, both ends of the rope are fixed to the drum. To ensure the normal position of the hook suspension in case of uneven stretching of the flexible element of both pulleys, a balancer or equalizing blocks are used.

Figure 4. Methods to ensure vertical lifting of the load

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

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

Question: why do construction cranes have a hook that carries the load, not attached to the end of the cable, but to the holder of the moving block?

Answer: to ensure vertical lifting of the load.

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How do blocks work?

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

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

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

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

Single fixed blocks

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

Single moving blocks

Single block, which has the ability to move, reduces by half the effort required to lift the load. However, halving the applied force means that the point of application must travel twice as far. IN in this case force is equal to half the weight (F=1/2W).

Block systems

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

A weight suspended vertically through the block allows the horizontal electric wires.

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

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

information-technology.ru

Levers and blocks | Strength and movement

Levers and blocks are simple mechanisms. A lever consists of a rod and a point of fulcrum or rotation. The pulleys use a rope secured in the wheel groove.

If you apply a force to one end of a lever, then a force also arises at the other end. A lever can be used to gain strength - the force acting on the load can be significantly greater than that applied to the other end of the lever. In particular, with the help of a crowbar, used as a lever, you can move huge boulders that cannot be lifted manually.

To increase the force, a system with two or more blocks is sometimes used. Not only do pulley systems provide gains in strength, but they also make it possible to change the direction of the force applied to the rope.

The lever rotates around a fixed point called the fulcrum or axis of rotation. The distance between the fulcrum and the place where the force is applied and where the load is placed determines how many times you can gain in force. Using a coin as a lever, you can open a can of paint. To do this, one end of the coin must be pushed into the gap between the lid and the jar, and the rim of the jar will act as a fulcrum. Now, by pressing the coin on the other side, you can lift the tightly inserted lid. If you don’t have enough strength to open the jar with a coin, you can use the handle of a spoon. Its free end is placed further from the fulcrum, and a significantly greater force will be applied to the lid from the second end.

SINGLE BLOCKS

The block is a wheel with a groove that rotates around an axis. A rope is fixed in the gutter. By pulling one end of the rope, they lift the load attached to the other end. When using a single immovable block, the traction force is equal to the gravity of the load. So, a single stationary block does not provide a gain in strength. The direction of the force that moves the load is different from the direction of the traction force.

CHALLENGES

You can make two blocks work simultaneously if the axis of one of them is secured to a high support, such as a beam, and a load is suspended from the axis of the second. One end of the rope is attached to the axis of the immovable block. If you pull the free end of the rope, the load will begin to rise. Systems of several movable and fixed blocks are called pulley blocks.

Using a double block reduces the force required to lift a load by half, that is, applying a force of 100 N to the rope, you can lift a load with a gravity force of 200 N. Thus, a double block gives a double gain in strength; in this case, to move the load 1 m, you have to choose 2 m of rope. Since the work is equal to the product of the acting force and the distance, the work done is 100 N x 2 m = 200 J, and it is equal to the work done in lifting the load 200 N x 1 m = 200 J. In general, the gain in force is equal to the number of blocks.

techis.ru

Simple mechanisms. Block:: Cool Physics

IN modern technology For the transfer of goods at construction sites and enterprises, lifting mechanisms are widely used, which are indispensable components which can be called simple mechanisms. Among them are the most ancient inventions of mankind: the block and the lever. The ancient Greek scientist Archimedes made man's work easier by giving him a gain in strength when using his invention, and taught him to change the direction of force.

A block is a wheel with a groove around its circumference for a rope or chain, the axis of which is rigidly attached to a wall or ceiling beam. Lifting devices usually use not one, but several blocks. A system of blocks and cables designed to increase load capacity is called a chain hoist.

The movable and fixed block are the same ancient simple mechanisms as the lever. Already in 212 BC, with the help of hooks and grapples connected to blocks, the Syracusans captured siege equipment from the Romans. The construction of military vehicles and the defense of the city was led by Archimedes.

Archimedes considered a fixed block as an equal-armed lever. The moment of force acting on one side of the block is equal to the moment of force applied on the other side of the block. The forces that create these moments are also the same. There is no gain in strength, but such a block allows you to change the direction of the force, which is sometimes necessary.

Archimedes took the movable block as an unequal-armed lever, which gives a 2-fold gain in force. Relative to the center of rotation, moments of forces act, which in equilibrium must be equal.

Archimedes studied mechanical properties moving block and applied it in practice. According to Athenaeus, “many methods were invented to launch the gigantic ship built by the Syracusan tyrant Hieron, but the mechanic Archimedes, using simple mechanisms, alone managed to move the ship with the help of a few people. Archimedes came up with a block and with the help of it launched a huge ship.” .

The block does not give a gain in work, confirming " Golden Rule" mechanics. This is easy to verify by paying attention to the distances traveled by the hand and the weight.

Sports sailing ships, like the sailboats of the past, cannot do without blocks when setting and controlling the sails. Modern ships need blocks for lifting signals and boats.

This combination of moving and fixed units on an electrified line railway to adjust wire tension.

Such a system of blocks can be used by glider pilots to lift their devices into the air.

Can you figure it out?

1. A rope is thrown over a fixed block. One end of it is attached to the installer’s belt, and he pulls the other down with some force. What is this force if the worker's weight is 700 N? Neglect friction in the block and the mass of the rope.

2. When checked with a dynamometer, it turns out that the force holding the load on a stationary block is slightly less than the force of gravity of the load, and with a uniform rise it is greater than it. What explains this?

3. Why do construction cranes have a hook that carries the load, not attached to the end of the cable, but to the holder of the moving block?

Answers./ But first think for yourself!/

Other pages on physics topics for grade 7:

Measurement of time Units of measurement Conversion of units of measurement Structure of matter Density Friction force Static friction Friction in nature and technology. Bearings Solid pressure Gas pressure Transmission of pressure by liquids and gases. Pascal's law Pressure in liquid and gas Atmospheric pressure Measuring instruments atmospheric pressure Archimedes power Aeronautics Block. Simple gate mechanisms. Winch. Gear transmission Inclined plane. Wedge. Screw Using simple mechanisms Mechanical work Mechanical power

class-fizika.narod.ru

Calculation of chain hoist, calculation of efficiency

It's time to continue the series of articles about crane equipment. In the previous article about chain hoists, the schemes and methods of their use were discussed in detail. But the mathematical basis of the pulley was touched upon very little. Judging by the reaction of our readers, this is an omission. Therefore, let's take a closer look at the calculation of the chain hoist in this article.

Let's start in order.

Let's consider a separate pulley block and the loads arising in it.

Calculation of the pulley block

Picture 1

Sn is the force with which the load acts on the pulley block;
Sc is the force with which the crane motor acts on the pulley block;
a (alpha) - potential angle of deviation from the axis;
d - diameter of the block sleeve of the pulley block;
D is the diameter of the pulley block stream.

Based on this chain hoist diagram, we will create an equation for the moments of forces.

Sн*R - moment of load impact force;
q*Sн*R - moment of force required to bend and straighten the cable;
f is the friction coefficient of the pulley bushing on the block.

The coefficient q is determined experimentally and means the rigidity of a given cable when bending around a given pulley roller. The forces that arise when the cable runs up and down are determined by the structure of the cable itself, or more precisely by the frictional forces of the threads inside the cable.

As you yourself understand, in comparison with the frictional forces of the bushing of the pulley block, the required force for bending and unbending the cable is extremely small. Therefore, I recommend not to think too much about this coefficient for now.

Now let’s find the load on the axis of the pulley block. We neglect the difference in loads on the approaching and descending branches.

Putting it all together, we get:

Formula for calculating the efficiency of a pulley block

As always, efficiency shows the ratio of work performed to work expended. For further calculations, let's turn a little to practice.

First.
When reading, you most likely immediately had a question about what angles of deflection in general we're talking about? Indeed, modern chain hoists simply do not have them. These angles make no practical sense. You can safely replace the sine from the formula with one.
Second.
As mentioned earlier, the value of q is extremely small relative to f. In real conditions it is omitted. Also, the diameter of the chain hoist thread is of very little importance.

Well, actually, we only have the friction force of the pulley block on its bushing. Thus, the main importance when choosing a pulley is the quality of the materials from which it is made, or rather the materials of the bushing.

The following values of the efficiency of the pulley block are used in the calculations:

100% is an unattainable ideal;
97% is the average value when using bronze bushings in rolling bearings;
95% - average value when using plain bearings;
93% and less - very dusty places, very high temperatures or aggressive environments of use.

Don't forget that we are still looking at one single video, and we have more than one or two.

Calculation of the power pulley system

Figure 2

As can be seen from the figure, the entire weight of the load will be distributed evenly over all branches of the pulley and the branch of the cable going to the drum. But this is only static, i.e. in the absence of movement. In dynamics the picture is completely different.

When lifting a load, the force of the crane motor, passing through each pulley block, will decrease due to losses in overcoming the friction forces inside the block. The amount of losses on each block is our efficiency found a couple of paragraphs above. Let's express all the loads inside the pulley through the load So.

Adding up all these efforts and applying the geometric progression transformation formulas, we obtain the weight of the load depending only on So. Now, knowing the weight of the load, it is easy to find the load So, and therefore the parameters (quality) of the cable required to lift a given load using a given pulley.

But that is not all. Between the pulley block and the crane drum there will definitely be several bypass rollers, and the heaviest load will fall on the branch running from the last bypass block to the drum. Therefore, we need to refine the formula for a more accurate result.

k is the total number of bypass blocks;
(n+1) - the total number of threads on which the load hangs.

That's all, actually. Knowing the quantity and quality of all the rollers in the pulley, you can easily calculate the parameters of the cable you need.

Attention! The website has added a service for calculating the main characteristics of the pulley and the parameters of the rope for reeving.

spctex.ru

THE HEAVY IS NOT THE HEAVY THING WITH THE LEVER AND BLOCK

We live in an age of technology, and for some reason many people think that this technology is mostly electronic. Well, in extreme cases, electric, or at worst, simply motorized, that is, equipped with some kind of engine. Indeed, wherever you look, you are sure to see a computer, radio, television, car or crane. Meanwhile, there is simpler technology, but it was this technology that allowed people to erect grandiose architectural structures and dig canals, build sailboats and plow the seas and oceans on them, and simply get water from a well every day and do many other useful things.

Almost any “classical” mechanism is based on a lever or block. The simplest lever is an ordinary stick lying on a support. The lever has two arms - long and short. The arm is the distance from the fulcrum of the lever to the point of application of force. To be very precise - up to the line of application of force (Fig. 1).

A remarkable property of a lever is that if a force is applied to its short arm, for example, a load is hung, then in order to lift it or keep the lever in balance, a force will have to be applied to the long arm as many times less than the weight of the load as the long arm more short. The product of the magnitude of the force applied to the lever and the length of the arm of this force in mechanics is called the moment of force. Its dimension is Newton×meter (Nm). The lever is in equilibrium when the moment of force applied to the long arm is equal to the moment on the short arm and is directed in the opposite direction.

People learned to use this property of leverage a very long time ago. If you need to lift a heavy stone, just put a long strong stick or a metal crowbar under it, place a smaller pebble or log under this lever and press on the long end. You can lift a cabinet or refrigerator in the same way (Fig. 2). By the way, it is with the use of the properties of the lever that carts are made for transporting furniture in stores.

The block is no less interesting. This is just a small roller mounted on a strong axle with a rope thrown over it. If the roller is mounted on a high support, it will be convenient to lift small loads on a rope, such as a bucket of paint. But you won’t get any noticeable relief in your work this way. But if you take two videos, you can gain a lot in strength.

Let's look at the diagram (Fig. 3A). Let's imagine that the rope is fixedly fixed at both ends to the ceiling, and at the bottom of it hangs a roller with a hook attached to its axis. There is a weight hanging on a hook. Rope is a flexible thing and can only resist stretching. As they say about such objects, they do not work for bending and compression. And in fact, you can bend, knit and crumple a rope as much as you like, but breaking it (if it is good, of course) is difficult. So, the rope transfers the force from the weight of the load to the suspension points, and each of its branches is exactly half.

Now let’s replace one of the rope fasteners with a block, attach its axis firmly to the ceiling, and allow the roller to rotate freely (Fig. 3B). If you don’t hold the rope, then the load, of course, will crash to the floor. What if you hold it? And if you hold it, you will be surprised to notice that the load does not fall, although the force applied to the rope is half the weight of the load! Of course, look at the diagram: to keep the system in balance, exactly this kind of force is required! Now try to lift the load. It will turn out that this is much easier to do than with a rope simply thrown over a block. True, the ascent will take more time. And why? Yes, because, winning twice in strength, we lose by the same amount in distance. And this is also clearly visible in the diagram.

If instead of one pair of blocks you take two, then the gain in strength will double again, and if there are three, then the gain will increase six times (Fig. 4). This design of several blocks is called a chain hoist. They are used in cranes, elevators, and on sailboats to raise sails. From several blocks you can make a small, but very convenient and useful hand winch.

Captions for illustrations

Rice. 1. The lever arm is the distance from the fulcrum (marked by the letter A) to the line of application of force F. The size of the arm in the figure can be easily determined by constructing a perpendicular from the fulcrum to the line of action of the force and measuring its length. This will be the shoulder. Note: the length of the lever (indicated by the letter L) is always greater (or, in extreme cases, equal) to the size of the shoulder. The lever is in equilibrium if the moments of forces applied to its shoulders are equal to each other and directed in opposite directions.

Rice. 2. The design of the furniture transport trolley is based on the properties of the lever. Moment from the weight of the load: M1 = F1h2. To lift a load, you need to apply a force in the horizontal direction. When the cart tilts, strength will allow it to be held. Note that F"2>F2 due to the fact that as the cart “overwhelms,” the arm h2 increases, and the arm h3, on the contrary, decreases.

Rice. 3. The weight of the load suspended on a rope thrown over a block is transferred equally to the suspension points by each of its branches. In our case, the tension of each branch is 1 kg (the weight of the load and block is 2 kg). To balance a load suspended on a block, you need a counterweight with half the mass.

Rice. 4. A pair of blocks, one of which is fixed on a fixed axis, and the other on a free one, is called a chain hoist. If there are two blocks, it’s a simple pulley, if there are four (that is, two pairs), it’s a double pulley, if there are eight, it’s a quadruple pulley. How many blocks are in the chain hoist, the same number of times when lifting you can win in strength and lose exactly the same number of times in distance.

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Double block rollers and pulleys | Product data sheets | Documentation | KROK™

PASSPORT BLOCK-ROLLERS double and pulleys

1. General information

Block rollers and pulley blocks (blocks) are used to reduce the friction force in load lifting systems and pulley systems.

Used on ropes or steel cables with a diameter of 8 mm to 12 mm.

2. Specifications

The block is a device with independent rollers located on the same axis, which have a groove (stream) for the rope along the outer perimeter.

The axis of the block is fixed on a bearing between the movable cheeks.

Block rollers with rollers and axles are produced various diameters.

The rollers and cheeks of the block rollers are made of steel or duralumin alloy.

Double block rollers

Pulley hoists

Only pulleys with heat-treated alloy steel rollers are used on steel cable!

Working and maximum loads, diameters of rollers and axles, material of cheeks and rollers are indicated in the table of the test report, which is an integral part of this passport.

To prepare the device for operation you must:

1. Thread the rope along the groove in the roller(s), hang the block through the carabiner and secure the load.

DO NOT use as individual remedy protection against falls from heights!

The simplest set for lifting loads, sufficient for lifting light loads, is a carabiner with a block (block roller). Using a swivel prevents the rope from twisting.

To lift heavy loads, a pulley system is used, which is assembled from two or more single, double and triple blocks.

For safe operation Before using the unit, it is necessary to inspect it for mechanical defects, corrosion, cracks, deformation and damage, and check the fixation of the fastening nuts.

If there are defects or wear of more than 10% of the initial size of the block, operation of the device is PROHIBITED!!!

The gap between the rollers and cheeks is adjusted by the number of spacers between them. If for some reason the roller has shifted towards one of the cheeks, it can be restored by installing an additional spacer washer.

The unit can be used both indoors and outdoors.

4. Maintenance and storage conditions

After use, the unit must be thoroughly cleaned, dried, and the roller bearings lubricated with industrial oil.

If necessary, carry out maintenance of the product.

When conducting preventive work Be sure to replace the self-locking nuts with new ones!

Store in a dry place at a temperature from +5 to +25 C.

It is prohibited to store the block together with aggressive chemicals.

It is allowed to transport provided that the unit is protected from mechanical damage, precipitation and exposure to aggressive environments.

The unit is subject to static load testing corresponding to the working load from the test report at least once every 6 months.

Devices that have withstood shock and are stored in a warehouse for more than 1 year are subject to the same tests.

5. Manufacturer's warranty

The manufacturer guarantees the preservation of the basic characteristics and operation of the unit in the absence of mechanical wear and proper storage throughout the entire service life.

Therefore, a specific service life is not established.

Service life depends on intensity of use. Warranty obligations do not apply to devices modified by the consumer or used in violation of the rules of operation, transportation or storage.

The manufacturer is not responsible for inappropriate or improper use of the product.

The block roller has been checked for compliance with regulatory and technical documentation and found suitable for use.

Download/open passport in PDF format

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

The invention of the chain hoist gave a huge impetus to the development of civilizations. The block system helped build huge structures, many of which have survived to this day and puzzle modern builders. Shipbuilding also improved, and people were able to travel great distances. It's time to figure out what it is - a chain hoist and find out where it can be used today.

Simplicity and efficiency of the mechanism

Structure of the lifting mechanism

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

pulley;
flexible connection.

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

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

One pulley is called a block. A pulley block is a system of blocks for lifting loads. The blocks in the lifting mechanism can be stationary (rigidly fixed) and movable (when the axis changes position during operation). One part of the pulley is attached to a fixed support, the other to the load. Movable rollers are located on the load side.

Fixed block

The role of the stationary block is to change the direction of movement of the rope and the action of the applied force. The role of the mobile is to gain strength.

Movable block

How it works - what's the secret?

The operating principle of a pulley block is similar to a lever: the force that needs to be applied becomes several times smaller, while the work is performed in the same volume. The role of the lever is played by the cable. In the operation of a chain hoist, the gain in strength is important, so the resulting loss in distance is not taken into account.

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

Operating principle of the block system

In what areas is the block system used?

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

Although construction equipment does most of the heavy lifting ( crane), the chain hoist found a place in the design of load-handling mechanisms. The block system (pulley block) is a component of such lifting mechanisms as a winch, hoist, and construction equipment (various types of cranes, bulldozer, excavator).

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

Pulley hoist as part of a crane hook

Classification of models according to different characteristics

There are many executions of one idea - a system of blocks connected by rope. They are differentiated depending on the method of application and design features. Get to know different types lifts, find out what their purpose is and how the device differs.

Classification depending on the complexity of the mechanism

Depending on the complexity of the mechanism, there are

simple;
complex;
complex chain hoists.

Example of even models

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

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

Odd chain hoist diagram

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

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

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

Classification according to the purpose of the lift

Depending on what they want to get when using a chain hoist, they are divided into:

power;
high-speed.

A – power version, B – high-speed

The power option is used more often. As the name suggests, its task is to ensure a gain in strength. Since significant gains require equally significant losses in distance, losses in speed are also inevitable. For example, for a 4:1 system, when lifting a load one meter, you need to pull 4 meters of cable, which slows down the work.

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

Multiplicity is the main characteristic

The main indicator that people pay attention to when organizing cargo lifting is the multiplicity of the pulley. This parameter conventionally indicates how many times the mechanism allows you to win in strength. In fact, the multiplicity shows how many branches of the rope the weight of the load is distributed over.

Kinematic ratio

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

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

Schemes of pulleys of different multiplicities

How to make calculations for a chain hoist

Despite the fact that theoretically the design of a pulley hoist is extremely simple, in practice it is not always clear how to lift a load using blocks. How to understand what multiplicity is needed, how to find out the efficiency of the lift and each block separately. In order to find answers to these questions, you need to perform calculations.

Calculation of a separate block

The calculation of the chain hoist must be performed due to the fact that the working conditions are far from ideal. The mechanism is subject to frictional forces as a result of the movement of the cable along the pulley, as a result of the rotation of the roller itself, no matter what bearings are used.

In addition, flexible and pliable rope is rarely used on a construction site or as part of construction equipment. Steel rope or chain has much greater rigidity. Since bending such a cable when running against a block requires additional force, it must also be taken into account.

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

SrunR = SrunR + q SrunR + Nfr (1)

Formula 1 shows the moments of such forces:

Srun – force from the side of the escape rope;
Srun – force from the oncoming rope;
q Srun – force for bending/unbending the rope, taking into account its rigidity q;
Nf is the friction force in the block, taking into account the friction coefficient f.

To determine the moment, all forces are multiplied by the arm - the radius of the block R or the radius of the sleeve r.

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

N = 2 Srun×sinα (2)

In this equation:

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

Pull block block

Calculation of the useful action of the block

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

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

In the equation:

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

From this formula it can be seen that the efficiency is affected by the structure of the block (through the f coefficient), its size (through the d/D ratio) and the rope material (q coefficient). The maximum efficiency value can be achieved using bronze bushings and rolling bearings (up to 98%). Sliding bearings will provide up to 96% efficiency.

The diagram shows all the forces S on different branches of the rope

How to calculate the efficiency of the entire system

The lifting mechanism consists of several blocks. Total Efficiency of pulley block is not equal to the arithmetic sum of all individual components. For the calculation, they use a much more complex formula, or rather, a system of equations, where all forces are expressed through the value of the primary S0 and the efficiency of the mechanism:

S1=ηп S0;
S2=(ηп)2 S0; (4)
S3=(ηп)3 S0;

Sn=(ηп)n S0.

Efficiency of a chain hoist at different magnifications

Since the efficiency value is always less than 1, with each new block and equation in the system, the value of Sn will rapidly decrease. The total efficiency of the pulley will depend not only on ηb, but also on the number of these blocks - the multiplicity of the system. Using the table, you can find ηп for systems with different numbers of blocks at different meanings Efficiency of each.

How to make a lift with your own hands

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

Different schemes of simple and complex lifts

Preparation of the base - diagram and drawing

Before you start building a chain hoist with your own hands, you need to carefully study the drawings and choose a suitable scheme for yourself. You should rely on how it will be more convenient for you to place the structure, what blocks and cable are available.

It happens that the lifting capacity of the pulley blocks is not enough, and there is no time or opportunity to build a complex multiple lifting mechanism. Then double chain hoists are used, which are a combination of two single ones. This device can also lift the load so that it moves strictly vertically, without distortions.

Drawings of the twin model in different variations

How to choose a rope and block

The most important role in building a chain hoist with your own hands is played by the rope. It is important that it does not stretch. Such ropes are called static. Stretching and deformation of a flexible connection causes serious losses in work efficiency. For a homemade mechanism, a synthetic cable is suitable; the thickness depends on the weight of the load.

The material and quality of the blocks are indicators that will provide homemade lifting devices with the calculated load capacity. Depending on the bearings that are installed in the block, its efficiency changes and this is already taken into account in the calculations.

But how can you lift a load to a height with your own hands and not drop it? To protect the load from possible reverse movement, you can install a special locking block that allows the rope to move only in one direction - the desired direction.

Roller along which the rope moves

Step-by-step instructions for lifting a load through a block

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

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

Carabiners are used for quick connection

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

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

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

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

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

Even a manual winch will make lifting the load more comfortable - you don’t need to rub your hands on the rope and worry about the rope slipping out of your hands. In any case, turning the winch handle is much easier.

Pulley hoist for winch

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

It is difficult to overestimate the importance of pulley hoists in the development of modern construction and mechanical engineering. Everyone should understand the principle of operation and visually imagine its design. Now you are not afraid of situations when you need to lift a load, but there is no special equipment. A few pulleys, a rope and some ingenuity will allow you to do this without using a crane.

Gorden - a single-pulley block with a cable passed through it; to gain strength in rigging, they use a grab-tali and a gini (Fig. 137).

Rice. 137. The simplest mechanisms for lifting loads:
a - gorden, b - hvat-tali, c - gini

A hoist is a chain hoist, i.e. a system of single-pulley blocks (two or more) with one cable, designed to work together. Most often, hoists are used in the form of two blocks with one to three pulleys in each. The most widely used are grip hoists that have one movable block and the other (upper) block in the form of double hoists.

Hoists are called gins that have two blocks with three or more pulleys in each. Multi-pulley blocks (more than three) are rarely used; they have a special design and are used only in special devices. Ginis are the largest hoists, used for lifting heavy weights; they are different from regular hoists large sizes blocks and the thickness of the rope-rope.

The cable that connects two blocks to work together is called a tackle hoist. The end with which the lopar is tightly embedded in the butt of the upper or lower block is called the main lopar, and the end coming out of the upper block, which is pulled when lifting the load or etched when lowering it, is called the running lopar; the remaining branches of the hoist cable are called lopar branches, the number of which is equal to the number of pulleys of both blocks.

Hoists come with two single-pulley blocks, one single-pulley and one double-pulley; with two two-pulley blocks, with one two-pulley and one three-pulley and, finally, with two three-pulley blocks (gini). Consequently, there can be from three to seven branches of the Lapp.

Plant ropes are used for hoists and steel cables, as well as rigging chains.

Mechanical hoists are hoists that are called differential hoists. There are also differential hoist systems with screw drive and gear drive hoists.

To lift loads to a small height, manual hoists are used; According to the load capacity, hoists are produced from 1-10 tons; they are made geared with gear and worm drives.

Manual hoists with a worm drive consist of a hook on which they are suspended from structures, an upper steel fixed block, on the rim of which teeth are cut to engage the elements of the chain drive; this drive unit is connected to the worm. A welded calibrated chain, made as a closed endless chain, is thrown over a drive block that rotates when the chain is handled by hand. As the drive unit with the worm rotates, the worm gear connected to the sprocket also rotates. If you manually move the rotation chain of the drive block, the worm will rotate and transmit rotation to the upper block along with the load chain located on the sprocket slots. The load chain passes through the lower block (small diameter) of the hoists and the upper sprocket. When the worm gear and sprocket rotate, the load chain shortens in length and lifts the load. For lifting loads manual hoists It is necessary to apply a traction force of 33-68 kgf to the chain (depending on the load being lifted).

Lifting a load using mechanical hoists with a gear drive occurs in the same way as lifting a load using hoists with a worm drive. However, in the first case, the load is lifted in parallel to the plane, in which the drive unit rotates, and with a worm gear in mutually perpendicular planes. To reduce the lifting effort, two gear transmissions are made (Fig. 138).

Rice. 138. Differential (mechanical) hoists

Manual mechanical hoists have a limited range of action; they can only lift the load at the point where it is secured.

To expand the range of action of hoists, they are suspended from a trolley, which moves along tracks made of I-beams, suspended from the floors of the workshop.

A more advanced lifting device is a hoist - an electric hoist with a trolley moving on a monorail. Lifting mechanism The hoist has an electric motor connected to a drum that replaces the upper block of hoists. The lifting and movement of the hoist is controlled via a remote control with buttons on a flexible wire. Telphers can be moved over considerable distances with the help of a trolley - a current-carrying wire located on the side of the monorails or above them.

In shipbuilding and ship repair, capstans and winches are also used. They are manual and electric.

A hand winch has a strong and massive base, a frame, a main drum (with a horizontal axis), shafts with gears for changing speeds, a brake and handles for applying muscle force. Hand winches manufactured with a load capacity of 0.5; 1.0; 3.0; 5 tons. When working with such winches, rosin blocks and hoists are used. Rosin blocks are used to divert the cable going to the drum, and hoists are used to obtain a greater gain in strength.

The capstan, unlike the winch, has a vertical axis of rotation. Spiers and winches usually operate at low speeds with high traction forces. When lifting light loads, use one branch of the cable (pendant), and when lifting heavy loads, use hoists.

Electric capstans (Fig. 139) and winches operate on shore from a power plant or plant substation, and on a ship from a generator. The shaft with the drum on them is driven into rotation by an electric motor. To control them, controllers and starting rheostats are used. By turning the starting rheostat lever in one direction or another, the mechanisms are given the desired speed.

Rice. 139. Spiers and winches:
a - diagram of the operation of the capstan, b - diagram of the operation of the winch, c - manual rigging winch; 1 - drum, 2 - handle, 3 - adjustable handle shaft, 4, 5 - spur gear, the drive wheel of which can be engaged and disengaged, 6, 7 - drum gear, 8 - locking mechanism for stopping the shaft, 9 - ratchet brake , 10 - sheet steel panels, 11 - spacer bolts

Before lifting loads, it is necessary to check the correct rotation of the winch (or capstan) and determine its suitability for the job. Particular attention should be paid to the serviceability of the stopper. If the stopper and brake are faulty, the winch cannot operate.

Jacks are used to lift heavy machines and units to a small height and move them over short distances, as well as perform various rigging works. Their advantages: low weight, high load capacity, simplicity of design, ease of braking and ease of handling.

Jacks are available: screw, hydraulic, air and rack-and-pinion; their common disadvantage is the relatively low efficiency. The lifting capacity of jacks reaches 20-25 tons. The average lifting height of loads is 400 mm, the weight of rack and pinion screw jacks ranges from 5 to 120 kg.

Rope products and rigging chains are widely used in the operation of mechanisms.

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

A cargo pulley is a system consisting of ropes and blocks, thanks to which you can gain effective strength while losing in length. The principle is quite simple. In length we lose exactly as many times as we win in strength. Thanks to this golden rule of mechanics, large masses can be built without much effort. Which, in principle, is not so critical. Let's give an example. Now you have won 8 times in strength, and you will have to stretch a rope 8 meters long to lift the 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 pulley 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 occurs thanks to the movable blocks, which are mounted on the movable side of the pulley. The fixed part serves only to change the trajectory of the rope itself.

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

Let's first consider a simple assembly chain hoist. It can be obtained by adding blocks to a support and a load. To get an odd mechanism, you need to secure the end of the rope to a 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 pulley for a winch with a multiplicity of 2, you need to secure the end of the rope to a support and use one block that is attached to the load. And we will have an even type of device.

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 which 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, such is our gain in strength. Typically, pulleys with a hook are used, on which, in fact, the load is attached; it is a mistake to think that it is just a block and a rope.

Now we will find out how a complex type chain hoist works. This name refers to a mechanism where several simple options of this cargo device, 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 another with a multiplicity of 2, then the theoretical gain in force will be equal to 8. All of 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 on overcoming the friction force. In order to reduce friction, it is necessary to remember: the larger the bend radius of the rope, the less frictional force will be. It is best to use rollers with a larger radius where possible. When using carabiners, you should make a block of identical options, but rollers are much more effective than carabiners, since the loss on them is 5-30%, but on carabiners it is up to 50%. It is also useful to know that the most effective block must be located closer to the load to obtain maximum effect.

How do we calculate the real gain in strength? To do this, we need to know the efficiency of the units used. Efficiency is expressed by numbers from 0 to 1, and if we use a rope large diameter or too hard, then the efficiency of 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 blocks. To calculate the actual gain in strength simple type lifting mechanism, it is necessary to calculate the load on each branch of the rope and fold them. To calculate the gain in strength complex types it is necessary to multiply the real forces of the simple ones of which it consists.

You should also not forget about the friction of the rope, since its branches can twist among themselves, and the rollers under heavy loads can converge and pinch 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 assemble the mechanism, either a separate or a cargo 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 a lifting structure in advance. You can also use its entire length, which also makes it easier to pass knots. One of the disadvantages is that there is no possibility of automatic fixation of the lifted load. The advantages of a cargo rope are that automatic fixation of the lifted object is possible, and there is no need for a separate rope. The important thing about the disadvantages is that it is difficult to pass through the knots during operation, and you also have to spend a cargo rope on the mechanism itself.

Let's talk about the reverse motion, which is inevitable, since it can occur when the rope is caught, or at the moment of removing the load, or when stopping to rest. To prevent backlash from occurring, it is necessary to use blocks that allow the rope to pass 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 backtracking, but also allow us to secure the load while unloading or simply rearranging the blocks.

If you are using a separate rope, the locking roller is attached last from the load being lifted, and the locking roller should be highly effective.

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

Another way is to use a clamp general purpose. Time has shown that it can be used on icy and wet ropes. It begins to crawl only with a load of 6-7 kN and slightly injures the rope. Another method is to use a personal clamp, but it is not recommended, since it begins to creep with a force of 4 kN and at the same time tears 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.

Definition

A pulley block is a system of movable and fixed blocks connected by a flexible connection (ropes, chains) used to increase the force or speed of lifting loads.

A chain hoist is used in cases where it is necessary to lift or move a heavy load with minimal effort, provide tension, etc. The simplest pulley system consists of just one block and a rope, and at the same time it allows you to halve the traction force required to lift a load.

Figure 1. Each moving block in the pulley gives a double gain in strength or speed

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

Figure 2. Rope fastening with even and odd multiplicity of pulley system

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

Figure 3. Single and double pulleys

Figure 4. Methods to ensure vertical lifting of the load

Question: why do construction cranes have a hook that carries the load, not attached to the end of the cable, but to the holder of the moving block?

Answer: to ensure vertical lifting of the load.

Figure 5 shows a power chain hoist, in which there are several moving blocks, and only one fixed one. Determine how much weight can be lifted by applying fixed block force $F$ = 200 N?

Figure 5

Each of the moving blocks of the power pulley doubles the applied force. The weight that a power polypaste of the third degree can lift (without taking into account corrections for friction forces and cable rigidity) is determined by the formula:

Answer: The chain hoist can lift a load weighing 800 N.

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