Failure analysis of steel wire rope for the hottes

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Failure analysis of steel wire rope for crane

(author longhuaming) steel wire rope is one of the most widely used lifting parts. It has the advantages of large bearing capacity, good windability, smooth movement and no noise. Almost all lifting equipment use steel wire rope to bear the load. If sudden fracture occurs due to failure, it will cause serious consequences. However, the failure analysis of steel wire rope is of great significance in both safety and economy. In this paper, the factors causing the failure of steel wire rope are analyzed in order to improve the service life of steel wire rope

I. strength and elongation

according to the design, the maximum breaking strength of the steel wire rope is less than the bunching strength of all steel wires, and is related to the structure of the rope and the performance level of all steel wires. When designing the properties of steel wire rope, all load factors (including additional load, acceleration, deceleration, rope speed, etc.), the number of pulleys and drums, structural installation mode, corrosion and wear conditions, rope length, etc. shall be considered

the steel wire in the steel rope is usually made of high-quality carbon structural steel with carbon content of 0.5~0.8%, and the elastic modulus of the steel rope is about 1.6 × 105N/mm2。 This is a measure of the possible elongation of the wire under load. When the steel wire rope is under the action of relay, in order to adjust its position to achieve the stability corresponding to the loaded load, each steel wire will move relatively. The resulting elongation has two forms. When the steel wire rope bears the load for the first time, the steel wire will be rearranged slightly to produce a permanent extension, that is, structural elongation; At the same time, it also produces a recoverable elongation, namely elastic elongation. The structural elongation depends on the load to some extent

the smaller the diameter of the steel wire in the steel wire rope, the smaller the moment required for bending, that is, the greater the toughness. Generally, the steel wire rope with more steel wires and fiber core steel wire rope have better toughness, while the all metal steel wire rope composed of less steel wires has poorer toughness, and the former has greater elongation than the latter. The greater the toughness, the better the failure resistance

II. Pulley

the minimum winding diameter of the main dimensions of the pulley is expressed by the pulley diameter at the bottom of the rope groove plus the diameter of the steel wire rope. As the pulley size decreases, the stress caused by bending and the contact pressure between the wire rope and the pulley increases. The higher the bending stress, the faster the fatigue of the steel wire of the rope. The increase of contact stress also accelerates the damage of rope and the wear of pulley. As the size of pulley increases, the pressure between rope and pulley decreases, and the bending degree also decreases. If only bending stress is considered, for 6 × 19 point contact wire rope can increase the pulley diameter to 90 times the limit value of the rope diameter, so as to improve the service life of the rope. However, in addition to the necessary equipment, such a large-diameter pulley is rarely used, because (1) for most lifting equipment, it is impractical to use such a large pulley. (2) In all cases, there is rarely a single factor, bending. In fact, in addition to bending, there are many factors that affect the life of the rope, such as repeatedly applied stress, wear, knocking, impact, vibration, torsion, speed, drum winding error, corrosion and lack of maintenance. One or more of these factors affect the life of the rope more than the size of the pulley

some factors of safe and economic operation are adopted to comprehensively determine the optimal pulley size. However, it should be noted that the combination of the huge pressure between the steel wires at each contact point in the steel wire rope and the large bending stress caused by working on the small pulley will play a certain role in the damage and failure of the steel wire rope. Under such adverse conditions, the breaking failure of steel wire often occurs in the parts between the contact points of each strand. The broken steel wire in these parts is often difficult to detect, or even impossible to find from the appearance

therefore, for equipment requiring high safety for manned or other reasons, the pulley size shall be larger, so as to more effectively ensure that the damage process of the steel wire rope is gradually carried out and occurs on the rope surface, so that it is easy to find that the string slogans are accurately set in the serial port setting window and the damage degree is estimated

III. pulley rope groove

in order to maximize the effect of the steel wire rope, the pulley rope groove must have a large enough diameter to keep the steel wire rope in a proper gap. The diameter of pulley rope groove cannot be kept fixed, and its change is mainly affected by steel wire rope, and is largely related to pulley material and pressure. As the rope groove is constantly worn, when a steel wire rope is close to being unavailable, the rope groove has been worn to a smaller size. Thus, there is a problem, that is, each time a new steel wire rope is replaced, the rope groove shall be re opened or the pulley shall be replaced. However, in practical application, it is not economical to re slot or replace the pulley. When the pulley is re grooved, it is generally necessary to trim the flange first, and then cut the bottom of the rope groove into an appropriate diameter. If only the groove bottom is cut, the throat angle adjacent to the groove bottom may be very small, which can not provide proper clearance, seriously causing the wear of steel wire rope

IV. wire rope pressure

in addition to the bending stress caused by movement on the pulley, the wire rope also bears the radial pressure caused by its contact with the pulley. This pressure will cause shear stress in the wire rope, change the structure of the wire rope, and affect the wear rate of the pulley groove. Therefore, when selecting the most appropriate steel wire rope structure, the magnitude of this pressure and the wear resistance of pulley materials should be considered

v. drum

drum size, rope groove profile and pressure are similar to those discussed in the previous pulley

when the smooth surface drum is used, the actual diameter will increase, and the monofilament diameter pressure will be higher. This is because only one side of the steel wire rope is supported, and its peripheral parts cannot be supported. For this kind of drum, the wear of steel wire rope and drum is more serious than that of drum with rope groove, and the rope is easier to be damaged due to pressure. When the bending stress is the main factor affecting the service life of the steel wire rope, the diameter of the drum can be slightly smaller than the diameter of the pulley in the system, because when running in each direction, the steel wire rope will be bent twice on the pulley and only once on the drum. The operation of the steel wire rope on each pulley is as follows: when the rope approaches the pulley from a straight line to the same curvature as the pulley, it begins to bend for the first time; When the rope straightens out again and leaves the pulley, it bends for the second time. In some cases, the stroke of the rope is not large. When running back and forth on the drum, part of the rope does not move through the pulley, the bending times of the rope will be reduced and the failure will be slowed down

the grooved drum is used. If the drum rope groove contour is appropriate, it will help to keep the winding rope in a proper position. It is very important to maintain a proper pitch between rope grooves, so as to obtain sufficient but not too large clearance between two consecutive rope turns, which plays a great role in preventing crowding and friction between the unwound and wound steel ropes on the drum. In order to provide proper conditions for multi-layer winding ropes, the pitch between the center lines of rope grooves shall be sufficient to prevent the steel ropes from contacting each other at the maximum inclination angle

VI. other failure factors

① corrosion is also a common factor for wire rope failure. The corrosion atmosphere formed by the service environment has a great impact on the service life of wire rope

② impact load and vibration are applied to the steel wire rope, resulting in high-frequency and high bending stress, especially at the connection point at the end of the steel wire rope, which can cause fatigue failure

③ if the steel wire rope is at too high temperature during operation, it will also fail due to the reduction of tensile strength

from the above failure analysis, it can be seen that the failure of crane steel wire rope is often caused by the comprehensive accumulation of many factors. In the analysis of actual failure cases, comprehensive analysis should be made to distinguish the primary and secondary, and find out the main failure causes, so as to improve the service life of steel wire rope

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