Defective steel ropes are often violated by workers. And this becomes the cause of undesirable consequences. For example, the operation of a heavily damaged sling can lead to an accident and personal injury (and possibly death). On the other hand, if the value of the signs of rejection of steel ropes was greatly exaggerated, then this will turn out to be quite significant unreasonable expenses for the enterprise. This article will be useful to engineers and workers, as well as sailors who deal with cables. Knowledge of the standards for the rejection of steel ropes will improve the efficiency and safety of work.
Key Points
The principles of inspection and the reject rate of steel ropes are specified in a special document under the number 12-97. This will seem strange to many engineers, because usually state standards are developed for such things. However, this is true. And this document establishes not only the standards for the rejection of steel ropes, but also the procedure for their production and operation, installation and maintenance.
According to this manual, metal cables and ropes should be carefully inspected for damage and defects once every ten days. In addition to periodic maintenance, products of this type must be inspected before operation (this does not apply only to elevators and other systems in which the cables are hidden).
Cables that work in especially dangerous conditions and under heavy loads, with a certain period, must be tested using a flaw detector. However, such a device is quite expensive, and only highly qualified engineers who have special courses behind them can work with it.
Where are the cables exposed to wear?
Many years of experience and research into the patterns of wear of steel cables reveal areas that are most susceptible to wear and tear. These observations formed the basis for the rejection standards of steel ropes of lifting structures. In the course of numerous experiments and experiments, it was proved that the cables begin to lose their performance and break down in the places of fastening of the ends, at the ends (overlock), in places of friction with fixed parts of the structure (guides, leveling and so on).
Cable requirements
Defective steel ropes are only valid if the latter comply with industry and production standards.
The cables that are used when carrying out rigging, cargo, traction and other works must have a quality certificate from the manufacturer of the corresponding sample. This document contains information about the tests for strength and wear resistance of steel ropes. It is for this reason that in our country almost never use ropes that were produced abroad. Indeed, in Europe and the USA, completely different standards apply. Theoretically, this is possible, but if you intend to use steel ropes of foreign manufacture, it is necessary to obtain documentary evidence of the conformity of product quality to domestic standards. Such a document is issued by the parent organization for standardization and certification.
If there is no documentation on a steel rope confirming compliance with quality standards, then such products cannot be operated.
Cables and ropes come from plants to the end consumer in batches. For each such delivery there must be a so-called passport. In one batch there can be no steel ropes of different sizes.
What are the ways to detect marriage?
The most affordable, simple and cheap way to determine the reject rate of slings from steel ropes is the visual method. An external examination conducted by an experienced specialist using simple auxiliary equipment will allow timely detection of critical damage to the wire strands and take measures to prevent an emergency or accident.
In second place in popularity and effectiveness is the so-called instrumental method of establishing compliance with the standards for the rejection of slings from steel ropes. As research and experimental work have shown, by the nature of straightness and violation of the geometric shape in the cross section of the cable, it is possible to fairly accurately predict the useful life of the cable. Not every engineer can do such a job. It is necessary to have a powerful mathematical apparatus and appropriate instrumental support. Currently, highly specialized software packages have appeared on the market that allow partially automating routine mathematical calculations. However, the cost of the license is very high, and its acquisition is unlikely to be economically feasible.
The role of safety factor
This indicator is very important. And its value is numerically equal to the ratio of the tensile force to the cable tension index. In accordance with the requirements of the state labor protection inspectorate, norms of strength coefficient values for various ropes are established. So, for ropes that do not experience significant loads during operation (easy operation), this value should not fall below 5. For those cables that work in difficult conditions and experience enormous loads, the value of the coefficient of strength should not fall below 6. Stand out also an intermediate type of cables designed to work in the middle mode. The value of the strength coefficient of such ropes cannot be lower than 5.5.
The verification of the strength coefficient can be attributed to destructive quality control and rejection, if not of a single steel rope, then of a whole batch. To conduct such tests, it is necessary to have sophisticated and expensive equipment.
Determination of the rope winding pitch
Such a parameter as the step of twisting a rope or rope is very important when considering the issue of rejection. The fact is that the number of breaks of thin fibers, as a rule, is calculated on a rope length equal to this step.
If there is no experience in carrying out such work, then difficulties may arise. Although in fact, everything is quite simple. The step will depend on how many strands are in the rope. Then, on the surface of one of the strands, marking is made with a marker or punch. It remains only to count in one right or left the number of strands in the cable and put a second mark. The distance between the two marks will be equal to the pitch. And it is precisely at this interval that it is necessary to count the number of breaks.
Rope waviness
This is a very important sign, and the rules for the rejection of steel ropes can not be silent about it. During the active operation of the cable, when it is wound on a drum and unwound, residual curvatures are formed in it. If the cable is not replaced in time, then these curvatures increase, and dangerous destructive processes begin in the rope itself.
Therefore, it is important to replace the cable in time. In order to prevent industrial accidents, recommendations have been developed. The cable must be replaced if the cross-sectional diameter in the spiral area is greater than or equal to 1.08 of the nominal outer diameter of the cable.
Criteria for assessing the degree of wear of steel cables
When deciding whether to continue using or decommissioning metal ropes, a lot of indicators are taken into account. Thus, the wear of steel cables is a multifactorial concept. The decision is influenced by the degree of mechanical wear on the surface, the nature of the destruction of the wire strands, a decrease in cross-sectional area, the presence of foci of corrosion damage, a change in the winding pitch. When making a decision, all of these factors should be considered.
Unconditional reject standards for steel ropes of hoisting machines, elevators and other mechanisms
A cable is recognized as defective if the reduction in the cross section of the outer wires is forty or more percent. Moreover, the length of the active wear section is not taken into account. Such a cable may not have any external manifestations of wear, however, it may break during stress.
In addition to reducing the cross-sectional area, concern should also be caused by its abnormal increase. The cause of such "bloating" is the swelling of the core. In accordance with the rejection rate of steel ropes of hoisting cranes, the critical value of increasing the cross-sectional area is 7% of the nominal area.
Tensile steel ropes is a natural process that occurs during the active use of the cable. However, it should not exceed the established norm. This figure is 5%. Thus, if the inspection revealed an increase in the length of the cable by five percent or more, then the product is recognized as defective, despite the absence of visible signs of destruction. Moreover, the rate of rejection of steel small-wire ropes does not differ from a similar rate for heavy-duty cables.
Special reject standards
If a metal rope is used to drive vehicles with people or to transport potentially dangerous and dangerous goods (acids, straightened metal, etc.), then the safety requirements, of course, are tightened, and the reject rate is reduced by half. That is, if a rope with forty fiber breaks per stride length is usually considered defective, then in this case a cable with 20 breaks is already decommissioned.
Special techniques are also applied when deciding on the rejection of ropes twisted from thick wire. The fact is that domestic enterprises for the production of steel ropes produce two varieties of products: from thin wire and from thicker. It is clear that the cliffs of thick and thin rolled can not be equivalent. Therefore, when calculating the number of breaks of a thick wire per dyne of the twisting step, one break of a thick wire is equivalent to 1.7 breaks of a thin wire. The total number of defects should not exceed 40.
How is the state of the core of the cable evaluated?
In accordance with the standards for the rejection of steel ropes of elevators and other hoisting-and-transport machines and mechanisms, in the absence of visible signs on the outer surface, the cable can be declared defective on the basis of damage to its core. Such an analysis requires a special instrument - a flaw detector.
If the total loss of the cross section of the core wires is more than 18%, then such a rope is considered defective.
Not every company has a flaw detector and specialists with certain training. But the need to control the geometric parameters of the core does not always arise. In accordance with the requirements, this kind of control is necessary only for cables with special strict safety requirements.