Reasons for wire rope shaking and analysis of wire rope tension testing

The friction hoist relies on friction to lift heavy objects. In terms of its working principle, the biggest difference between it and the winding hoist is that its steel wire rope is not wound on the drum, but is placed on the friction wheel. At both ends of the rope, a lifting container is suspended, and the lifting power is transmitted through the friction between the rope groove installed on the friction wheel and the steel wire rope, causing the lifting container to move up and down, thereby completing the material lifting Personnel promotion or demotion.

From the operating principle of the friction hoist, it can be seen that the stable operation of the steel wire rope plays a crucial role in the entire lifting system. Once the steel wire rope has problems, it often poses a threat to the safe operation of the entire lifting system. For example, when the steel wire rope shakes, it will directly lead to the instability of friction. When the shaking amplitude increases or the shaking time is extended, it is easy to cause rope slipping accidents, damage the friction pad, and the damage to the friction pad will exacerbate the shaking of the steel wire rope, resulting in a vicious cycle and ultimately leading to serious consequences.

However, the current management department of mining equipment does not attach enough importance to the issue of wire rope shaking, and ignores the existence of the problem after it occurs, resulting in equipment operating with faults and frequent accidents. Therefore, it is urgent to conduct in-depth analysis of the causes of wire rope vibration.

1. Improve system operation and fault description

The model of the friction hoist for the auxiliary shaft of a certain mine is JKMD-3.25 × 6 (III) E is driven by a low-speed direct current motor with a power of 800 kW and a speed of 42 r/min. It adopts a rigid track single cage lifting method with a balance hammer, and uses steel wire ropes produced by the same manufacturer, model, and batch.

During operation, there is a phenomenon of wire rope shaking, which is manifested by the obvious up and down oscillation of the lifting wire rope at the outlet of the lifting machine room. Whether it is lifting or lowering the counterweight, shaking will exist.

2. Design of fault diagnosis scheme

Conduct a preliminary analysis of the wire rope shaking problem and determine that the cause of the malfunction may be deformation of the wellhead or inconsistent diameter of the wire rope groove. When the foundation of a certain corner of the mast sinks, it will cause the crown wheel axis to tilt, resulting in a travel difference of the steel wire rope during operation, which will cause the tension of the steel wire rope to be unbalanced and cause it to shake. However, after actual measurement of the parallelism between the crown wheel shaft and the main shaft device, it was found that there was no deformation of the wellhead, and the parallelism between the centerline of the hoist main shaft and the centerline of the crown wheel shaft was good, ruling out the deformation of the wellhead. For the issue of the rope groove, through communication and exchange with the mining party, it was confirmed that the mine regularly performs turning on the rope groove, and the actual measurement results show that the effective diameter of the rope groove is consistent, which also ruled out the problem of the rope groove.

During the operation of the hoist, it was accidentally discovered that the system speed map on the driver's console display screen had slight ups and downs, but due to poor resolution, details could not be displayed. Is it a malfunction in the electronic control system or spindle that causes periodic fluctuations in the speed chart? With these questions in mind, the author focuses the research on the following three aspects: first, whether the tension of the steel wire rope is balanced, which is the direct cause of the wire rope vibration; Secondly, whether there is a malfunction in the electrical control of the hoist; The third issue is whether there is a malfunction in the main bearing of the hoist. The following testing plan has been designed for the above three aspects.

2.1 Steel wire rope tension testing plan

The tension of the steel wire rope can be calculated by measuring the vibration frequency. According to the theory of string vibration, ignoring the vertical effect and bending stiffness, the tension of steel wire rope

When m and L remain constant, the tension T of the steel wire rope is directly proportional to the square of the natural frequency fn of the steel wire rope. By using vibration measuring instruments and software analysis, the natural frequency fn can be obtained. Using equation (1), the tension values of 6 steel wire ropes can be obtained. Finally, the deviation of the tension of the steel wire ropes can be determined through calculation.

2.2 Elevator Electric Control Test Plan

The most commonly used testing method to determine the stability of a hoist operation is to compare the operating speed of the hoist with the current of the driving motor. Real time monitoring of two items is carried out through an intelligent tester, collecting the speed operation curve and motor current curve of the lifting machine's counterweight during lifting, analyzing the change law of the curve, and thus determining whether the lifting machine operates smoothly.

2.3 Main bearing vibration testing plan

Under normal operation of the hoist, a vibration measuring instrument is used to detect the vibration of the main bearing connecting the motor and drum. After spectrum analysis using the instrument's built-in software, the vibration spectrum is obtained, which is then compared with the fault frequency of the bearing to determine whether there is a fault frequency of the bearing in the spectrum, and to determine whether there is a fault in the main bearing. The vibration testing points and directions of the main bearing are shown in Figure 1.

During normal lifting of the hoist, vibration monitoring is conducted on the horizontal, vertical, and axial directions of the main bearing at the driving end and non driving end, respectively.

3. Fault diagnosis analysis

3.1 Analysis of tension testing for steel wire ropes

The tension test of steel wire rope is conducted under two working conditions, namely when the counterweight is at the wellhead and when the counterweight is at the bottom of the well. During testing, the hoist should be kept stationary, and the steel wire rope should be struck sequentially with a small hammer, while the data should be recorded using a vibration measuring instrument. The wire ropes from the non driving side to the driving side are numbered 1-6, as shown in Figure 2. The vibration test results of the steel wire rope are listed in Table 1.

Table 1 Vibration Test Results of Steel Wire Rope

According to Table 1, it can be calculated that when the counterweight is at the wellhead, the maximum deviation of the tension of the steel wire rope is 8.3%; When the counterweight is at the bottom of the well, the maximum deviation of the tension of the steel wire rope reaches 13.6%. At the bottom of the well, the maximum deviation of the tension of the steel wire rope exceeds the requirement in Article 411 of the Coal Mine Safety Regulations that "the difference between the tension of any lifting steel wire rope and the average tension shall not exceed ± 10%", indicating that the tension borne by these six steel wire ropes is unbalanced. Therefore, it is determined that the cause of wire rope vibration is tension imbalance.

3.2 Testing and Analysis of Elevator Electric Control System

The operation of the hoist is controlled by the electronic control system, and an intelligent tester is used to collect the speed curve and motor current curve of the hoist counterweight during lifting, as shown in Figures 3 and 4. In order to analyze the detailed changes in lifting speed and motor current during the maximum speed operation of the hoist, local enlarged images were obtained through processing, as shown in Figures 5 and 6.

Figure 3 Running speed curve of the hoist when lifting the counterweight

Figure 4 Current curve of the motor when lifting the counterweight

Figure 5 Partial Enlarged View of Running Speed during Counterweight Lifting

Figure 6 Partial Enlargement of Motor Current during Weighting Up

From Figure 5, it can be seen that when the counterweight of the hoist is lifted, there is a significant periodic change in operating speed, with a period of 14.7/10=1.47 seconds, and the frequency is the reciprocal of the period. Through calculation, the speed change frequency f1=1/1.47 ≈ 0.68 Hz is obtained.

As shown in Figure 6, when the counterweight of the hoist is lifted, the motor current also shows significant periodic changes, with a period of 14.6/10=1.46 s and a frequency of f2=1/1.46 ≈ 0.68 Hz.

Based on the above calculations, the operating speed of the hoist and the motor current exhibit periodic changes at the same frequency. It is preliminarily determined that this may be due to a malfunction in the control system of the hoist, resulting in periodic changes in the motor current and ultimately causing periodic changes in speed.

3.3 Spectral analysis of vibration results of the main bearing seat

According to the plan formulated in 2.3, vibration testing is conducted on the driving end of the main bearing during normal operation of the hoist. The horizontal and vertical vibration spectra are shown in Figures 7 and 8.

Figure 7 Horizontal vibration spectrum diagram of the driving end of the main bearing seat

Figure 8 Vertical vibration spectrum diagram of the driving end of the main bearing seat

From Figures 7 and 8, it can be seen that there is no bearing fault characteristic frequency in the main frequency of vibration, indicating that the bearing is in good operating condition and no faults have occurred. However, in both horizontal and vertical spectrograms, it is found that the frequency of 600 Hz and its harmonics have the highest amplitude, corresponding to twice the SCR (characteristic frequency of full wave rectifier thyristor fault). The occurrence of this frequency in the spectrum indicates a malfunction in the electronic control system, which has led to unstable operating speed of the hoist. This is consistent with the speed chart tested earlier.

Compare the single peak amplitude of the frequency component at the same measuring point and direction with the bearings used in the main shaft of the mine (the same as the auxiliary shaft), and the results are listed in Table 2.

Table 2 Comparison of vibration amplitudes at the driving end of the main and auxiliary shaft main bearing seats at a frequency of 600 Hz

From Table 2, it can be seen that the single peak amplitude of the 600 Hz frequency component in the vertical direction of the auxiliary shaft is significantly larger than that of the main shaft. According to the relevant theory of DC motors, the occurrence of this frequency component (600 Hz) indicates an electrical fault. By combining the operating speed chart of the hoist and the periodic changes in the motor current curve, the problem was pointed towards the electric motor control. After contacting the electronic control manufacturer in the later stage, eliminating the faults of the electronic control system and ensuring smooth operation, the problem of wire rope vibration has been significantly improved.

4. Conclusion

The tension test of the steel wire rope shows that the natural vibration frequencies of the six ropes are different. Based on the fact that the tension of the steel wire rope is proportional to the square of its natural vibration frequency, it is determined that the tension borne by the steel wire rope is unbalanced. The test results of the electronic control system and the frequency spectrum analysis of the main bearing vibration clearly indicate that the reason for the wire rope vibration is the abnormality of the control module of the electronic control system, which makes the speed control of the hoist unstable. In this unstable state, coupled with the unbalanced tension of the steel wire rope, it leads to vibration of the steel wire rope during operation.