Lifting mechanization has always been a priority. In this regard, a device was invented quite a long time ago, which contributed to a significant facilitation of the physical labor of workers involved in carrying out operations to raise or lower various objects. The name of this device is polyspast. What is this, we will consider in this article.
History of Creation and Definition
Nobody reliably knows when exactly the mechanisms for moving in the space of heavy things appeared and began to be operated. First of all, we note: the pulley block (what it is, technical literature can also suggest) is a system of blocks and ropes, which can significantly simplify and speed up the planned work with heavy objects.
The study of such architectural monuments as the Pyramids of Cheops in Egypt, the Great Wall of China and other ancient structures unequivocally confirms that tackles, the purpose and structure of which will be discussed below, were invented several thousand years ago. It is quite obvious that initially they were characterized by primitiveness.
general information
Let's try to study the chain hoist as detailed as possible. What is it from a technical point of view? At its core, it is a group of blocks assembled into special holders through which either a chain or a rope passes. The simplest chain hoist is one block with a pulling element stretched through it. This version of the scheme allows to halve the traction force that is required to move the load.
Classification
Polyspasta are divided into two large groups: power and speed. Often, power analogs are used in practice, which can significantly reduce the cable tension. By the way, this effort can be calculated quite simply. To do this, you will need to divide the mass of the cargo by the available chain hoist. The question arises: what is multiplicity? Answer: the multiplicity is the ratio of the number of branches of the body on which the load is located to the numerical value of the branches that are already wound on the drum. This definition applies to power pulleys. As for high-speed pulley blocks, here the multiplicity is the value obtained from dividing the speed of the leading end of the rope by the speed of the follower.
In high-speed chain hoists, the working force is applied to the movable holder, while the cargo, in turn, is fixed to the free end of the cable. The gain in speed during the operation of this type of chain hoist arises due to the increase in the lifting distance of the object.
Multiplicity Change
The polyspasts (the purpose and arrangement of them over the years of existence have basically remained unchanged) allow the introduction or removal of additional blocks from the system. Due to this, the required multiplicity is obtained. If the multiplicity is even, then in this case the free end of the rope is fixed on a stationary structural element. If the multiplicity is odd, then the same end is attached to the clip with a hook.
In power pulley blocks, an increase in the multiplicity allows to reduce the diameter of the rope and, accordingly, the dimensions of the drum and blocks. All this ultimately leads to a decrease in the total mass of the entire system, a reduction in the gear ratio of the gearbox. But at the same time, a large rope length will be required.
Branch Number Separation
The polyspast (what it is, now you obviously already understood) can be single or double depending on the number of branches. In the first case, the flexible element moves along the axis of the drum. This option has a drawback, which consists in an undesirable change in the load on the drum supports, and in the absence of free blocks (that is, the rope from the suspension is immediately wound on the drum), the object will move not only vertically, but also horizontally.
The double pulley block provides for fixing both ends of the rope on the drum. An example is a chain hoist for a winch. In order to avoid distortion of the hook suspension, leveling blocks or balancers are used. Most often, such a system is operated in gantry or bridge cranes or heavy tower cranes.
Features
Absolutely any pulley block, the principle of which is generally similar to the operation of the lever, is good because it does not require any special skills from the user, however, it requires you to be extremely careful, because, like any other lifting mechanism, it is fraught with danger and can injure . In addition, the operation of the chain hoists shows that the traction elements used in their systems do not have ideal flexibility and are endowed with some rigidity. That is why the oncoming branch of the rope is not able to instantly lie in the stream of a block or drum, and the runaway branch cannot immediately straighten. This is best seen when using steel ropes.
Polyspast rules
Each chain hoist is manual It works on the basis of the laws of physics, and therefore its functioning complies with several fairly simple rules, which it is advisable to familiarize yourself with.
- Rule number one. The gain in effort is provided exclusively by moving blocks or rollers that are fixed on the load or on a rope coming from the load. Stationary winning units in force never provide.
- Rule number two. Victory in power is a multiple of losing in the distance. In other words, for example, two-time assembly pulley for each meter of lifting the item requires pulling through its entire system of two meters of rope. At the same time, the 6: 1 tackle obliges you to stretch already 6 meters of cable to move up the load by one meter. We summarize: the greater the "strength" of the chain hoist, the slower (as much) the load will rise.
Special attention deserves a complex pulley block. By itself, it is a collection of simple chain-links, each of which pulls another along with it. Thus, several chain hoists can be mounted between themselves. This variety is most often used at the time of rescue operations.
In conclusion, it will be correct to say the following: the chain hoist (the principle of its operation is quite easy to understand with careful study of the issue) was, remains, and most likely will be for a very long time a faithful assistant to a person in solving many pressing issues related to construction, installation, loading, unloading and other operations that are quite time-consuming. The main problem, the elimination of which is completely impossible today due to again perfectly working physical laws, is the presence of friction in the system.