Polyspast is a mechanism by which loads are lifted. It consists of one or more groups of blocks wrapped around by a rope. The word "polyspast" comes from the Greek polyspastion. This term is translated as "stretched by several ropes." The main function of the chain hoist is to increase the carrying capacity of the main mechanism.
In other words, this device gives a gain in strength. However, the reverse effect of using a chain hoist is to reduce the rate of ascent. You can also gain speed at the expense of strength. However, such pulley blocks are used much less frequently. In any case, the principle of operation of the device is the action of the lever.
Mechanism device
Polyspast is a lifting mechanism that allows you to get a force that exceeds the lifting force of the winch several times. In other words, this mechanism increases the load capacity of the device. The use of a chain hoist allows you to lift a heavy load with a winch that has a small carrying capacity. It is important to remember that the lifting speed of heavy structures will decrease as much as the gain incapacity.
Purpose of mechanism
Polyspast is needed to lift heavy loads with a minimum of effort. The simplest design of the chain hoist is designed so that one end of the rope is fixed on the drum, and a suspended load is located at the opposite end of the rope. Devices with a more complex design include several fixed and movable rollers.
For each weight, the dimensions, blocks and diameter of the rope should be taken into account. A load having a large mass, when suspended on a rope, increases the load. Such a mechanism is characterized by rapid wear. In this case, a reduction in the tension in the rope is required. Therefore, two or four ropes are used to suspend a large mass. It is also possible to use a complex chain hoist.
Working principle
To a person who has nothing to do with loading, the name of this mechanism will seem incomprehensible. However, in reality, a chain hoist is a very simple lifting mechanism that 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 assembled in special holders. And they alternately bend around with a rope or a rope. Even such a simple design can be used quite effectively to increase the force applied to lower or raise loads. Also, the design of a simple chain hoist contains cargo blocks. They can be of the following types:
- multi-roller or single-roller;
- fixed or movable.
The traction force of the rope in this case depends entirely on the number of strands of the rope in the construction used.
In what areas is the device used?
Pulley block is used to lift and move cargo in cases where only the physical strength of a person and the least number of auxiliary mechanisms can be used. Also, the chain hoist is the most important component of winches, cranes and other mechanization equipment.
For this reason, these devices are used in almost all areas where lifting and transport mechanisms are somehow used: from household tasks to heavy industry.
So, on what principle does the chain hoist operate? The operation of this device is based on the law of the lever: with a gain in strength, you get a loss in distance. Since this principle is very simple, it will not be difficult to make a chain hoist with your own hands. To do this, you need only two single-roller blocks.
To lift a load of a certain mass with the help of a chain hoist, you need to make an effort, half as much as its mass. Do not forget about the length of the rope used. It should be twice the height to which the load will be lifted. It should be noted that chain hoists with the simplest device are called "two-to-one chain hoists", as they increase the applied force.twice. Three blocks design, respectively, gives a threefold increase in strength.
Polyspast multiplicity
It should be noted that the calculation of the chain hoist plays a very important role. After all, the mechanism works far from ideal conditions. It is affected by the friction forces that arise when the cable moves along the pulley. Also, friction forces arise when the roller rotates, regardless of which bearings are used in it.
To determine the tension force of the rope used without taking into account friction losses, it is necessary to divide the weight of the load by the multiplicity of the chain hoist. It should be understood as the number of rope threads holding the load. Also, friction should not be neglected. The efficiency of the operation of the chain hoist also depends on it.
It can be reduced by using high quality blocks and ropes, as well as through quality workmanship that eliminates unnecessary overlaps and kinks.
Today, chain hoist schemes are studied even in the school physics course. With their help, making this design will not be difficult. You also need to purchase the following items:
- fitting;
- rope;
- winch.
What device models are there?
To create the simplest model, only one block is needed. The use of such a mechanism gives a twofold gain in strength. This means that to lift the load you need to make half the effort. However, the rope in this case should be twice as long. Such a chain hoist has a ratio of two to one. Such a design may not contain blocks at all.chain hoist, as you can use a regular carabiner instead.
When using two blocks at once in the chain hoist, you can triple the advantage in the applied effort. There is also a safety feature that works when the rope is lowered. In this case, two grappling knots are tightened and block the load.
If you add two more blocks to the previous mechanism, you get a pulley block 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 chain hoists
Note that pure force transmission cannot be achieved due to friction. When the rope rubs against the block, ten to twenty percent of the applied force is lost. Therefore, in a simple chain hoist, in fact, the ratio will be approximately 1.8 kilograms per kilogram of the load being lifted. A 5-fold chain hoist will give a gain in strength a little more than 3 times.
The above ratio indicates that it is possible to increase the number of pulley blocks to a certain limit, after which the opposite effect may occur. However, in order to increase the maximum ratio, complex chain hoists can be used.
This chain hoist is designed in such a way that the lifted weight does not create a load on the last block. Instead, he loads the rope that passes through the block. As a result, atusing 3 blocks, chain hoists 2:1 and 3:1 are alternately connected. In theory, this gives a sixfold gain in strength, but in practice - 4.3 times.
How to reduce friction?
The main problem of the chain hoist is that in the process of work he has to overcome the emerging friction forces. This problem can be partially solved if high-quality ropes, pulley blocks with smooth streams, as well as thick grease are used.
Additional opportunities also appear with proper application of the chain hoist design. For example, if you use not one carbine, but two. Due to this, the friction force is reduced, and the radius of the bend also increases.
