Polyspast is a mechanism by which the lifting of goods is carried out. It includes one or more groups of blocks enveloped by a rope. The word "polyspast" comes from the Greek polyspastion. This term translates as "pulled by several 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 a decrease in the lifting speed. You can also win in speed by losing strength. However, such tackles 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 load-lifting mechanism that allows you to get a force several times higher than the lifting force of the winch. In other words, this mechanism increases the carrying capacity of the device. The use of the tackle allows you to lift a heavy load with a winch having a small load capacity. It is important to remember that the lifting speed of heavy structures will decrease as much as the payload gain is achieved.
The purpose of the mechanism
The polyspast is necessary for lifting heavy loads with a minimum of effort. The simplest construction of the tackle is designed 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. Devices having a more complex structure 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 from a rope, increases the load. Such a 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 pulley of complex construction.
Principle of operation
To a person who has nothing to do with loading, the name of this mechanism will seem incomprehensible. However, in reality, 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 operation scheme of such a small “crane” is very simple.
The construction of the tackle includes several groups of blocks assembled in special holders. And they alternately bend around with a rope or rope. Even such a simple design can be used quite effectively to increase the force exerted to lower or raise loads. Also, the construction of a simple pulley block contains cargo blocks. They can be of the following types:
- multi-roller or single-roller;
- motionless or mobile.
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 tackle 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 used at least somehow: from domestic tasks to heavy industry.
So, by what principle does the chain block operate? 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 with the help of a chain hoist, efforts must be made, half its weight. Do not forget about the length of the rope used. It should be twice as high as the load will be lifted. It should be noted that the tackles having the simplest device are called “two-to-one tackles”, as 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, friction forces occur during rotation of the roller, regardless of which bearings are used in it.
To determine the pulling force of the rope used without taking into account friction losses, it is necessary to divide the weight of the load by the multiplicity of the chain hoist. It should be understood as the number of rope threads holding the load. Also, friction should not be neglected. The efficiency of the chain hoist also depends on it.
It can be reduced using blocks and ropes of high quality, as well as through high-quality performance, eliminating unnecessary overlaps and excesses.
To date, the chain hoists are studied even in the school physics course. Using them to make this design will not be difficult. You must also purchase the following items:
What device models exist?
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 to lift the load you need to make half as much effort. However, the rope in this case should be two times longer. Such a pulley has a ratio of two to one. Such a design may not contain pulley blocks at all, since instead of it you can use a regular carbine.
When using two blocks at once in a chain hoist, you can increase the triple advantage in the applied force. There is also a safety net function that works when the rope is lowered. In this case, two gripping units are tightened and block the load.
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 is to the rope itself.
Complex tackles
It should be noted that the transmission of force in its pure form cannot be achieved due to the occurrence of friction. When a rope rubs against a block, ten to twenty percent of the applied force is lost. Therefore, in a simple tackle, in fact, the ratio will be approximately 1.8 kilograms per kilogram of load lifted. A 5-fold pulley block will give a gain in power 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 tackles can be used.
This chain hoist is designed in such a way 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 six-fold 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, tackle blocks with smooth streams, as well as thick grease.
There are also additional opportunities with the competent use of the construction of the tackle. For example, if you use not one carbine, but two. Due to this, the friction force is reduced, and also there is an increase in the radius of bending.