Fault diagnosis and treatment of cold acid rolling mill bearings

Liu Aijun1 Xiao Fuying2

(1. School of Mechanical and Electrical Engineering, Hunan University of Science and Technology; 2. School of Mechanical and Electrical Engineering, Central South University)

Abstract: By measuring the vibration speed of the bearings on the steel strip production line of the cold acid rolling mill under abnormal conditions, the spectrum analysis of the vibration signals was performed when the cause of the abnormal vibration was unknown. At the same time, it is combined with the calculation results of the characteristic frequencies of the bearing outer ring, inner ring and rolling elements to determine the cause and location of the failure. After shutdown inspection, it was confirmed that the outer ring of the bearing at measuring point 4 was severely pitted. This shows the correctness of the spectrum analysis results.

1 Introduction

At a major Chinese steel producer, the primary production process is hot rolling, and large rolling mills are critical equipment on the steel production line. However, most rolling mills operate in harsh environments that hinder machinery monitoring and maintenance, making it extremely difficult to establish an online monitoring system. Production line system failures therefore occur from time to time. During routine maintenance, installation, and commissioning, severe vibration often prevents normal production. The causes of vibration are complex and interconnected, and basic methods relying solely on listening and touch make it difficult to analyze and determine the causes comprehensively and accurately.

Using advanced equipment status detection and fault diagnosis technology, we can grasp the operating status of various types of equipment in a certain period of time through vibration detection, and provide a complete set of equipment operating status data for engineering and technical personnel engaged in equipment maintenance, installation, and debugging. Based on data analysis based on these data, we can accurately analyze and judge the cause of the failure, and reasonably arrange maintenance plans, thereby avoiding economic losses due to sudden failures and ensuring smooth production.

2 test device and data collection

2.1 Motor Bearing Vibration Sensor Arrangement

6 vibration speed sensors are installed at the 1 and 2 measuring points of the motor bearing. Each measuring point uses three sensors to measure the speed in the horizontal, vertical and axial directions. The specific layout diagram is shown in Figure 1.

After the motor rotor was replaced, the operating status of the unit remained stable for a short period of time, showing a rapid deterioration trend. During the inspection on 9 month 27 day, it was found that the vibration of the unit had changed significantly, as shown in the table.

The bearing vibration at measurement point 2 remained greatest in the axial direction, far exceeding the allowable limit, while vertical vibration was greater than horizontal vibration. Based solely on vibration magnitude and direction, conventional diagnostic analysis would suggest serious coupling misalignment: the original coupling alignment accuracy may have been disturbed during rotor replacement, and the base may also be loose. However, analysis of the vibration spectrum showed characteristics clearly different from before. The dominant axial vibration frequency was 82.1 Hz, with a prominent single peak and concentrated energy. It was neither the rotational frequency nor a harmonic excitation frequency.

It can be seen that compared with before the rotor replacement, the fault cause of the strong vibration of the bearing bush at the measuring point 2 is obviously different. It can be inferred that the direct cause of motor vibration degradation may be related to the transmission of vibration excited by the failure of reducer components. Considering that the shaft rotation frequency and gear meshing frequency at all levels of the transmission part have strong certainty in spectrum identification, bearing fault excitation has become the focus of the analysis.

Analysis from the vibration spectra of measuring points 3 and 4 (see Figures 2 and 3) shows that 82.1Hz and its multiple frequency components are rich in components. Although the vibration amplitude is relatively small, the spectra contain obvious fault information. According to the vibration characteristics of the rolling bearing, the bearing vibration has strong attenuation on the interface, and the vibration speed value of the fault-excited vibration in the casing is not very obvious. The bearing fault and type can be determined through spectrum analysis.

According to the theoretical calculation method of bearing fault characteristic frequency, the bearing fault characteristic frequency can be calculated. According to the formula, feed bearing model and motion parameters, the bearing fault characteristic frequency is calculated as follows (see the bearing diagram 4):

Calculation of fault characteristic frequency of high-speed shaft bearing model 3538 (double row radial spherical roller bearing):

d=37.5mm, D=225mm, n=19(grain), fr=9.8Hz, ∂=10.25’. It can be concluded that: f outside = 82.16Hz; f inside = 106.1Hz; f roll = 35.8

Comparing the calculated values of the bearing fault characteristic frequency, it can be seen that the fault characteristic frequency of the outer ring of the high-speed shaft bearing is very close to the excitation frequency. It can be inferred that there is a fault in the high-speed shaft bearing.

The analysis shows that there are obvious differences in the response of the unit to high-speed shaft bearing failure depending on the rigidity and transmission mode. The load end of the motor is the weak link of the unit in terms of stiffness. It is sensitive to excitation and responds strongly. The direct cause of strong vibration at the load end of the motor is the vibration excitation due to bearing failure, and the indirect cause is the possible looseness and poor rigidity of the machine base.

3 processing measures and effects

Based on the above diagnosis and analysis, the unit was shut down for inspection beginning on October 19, 2005. Severe pitting was found on the bearing outer ring at measurement point 4, along with wear on the high-speed gear, wear on the coupling balls and raceways, and looseness in the motor's load-end base. During this overhaul, the faulty bearing, high-speed gear shaft, and ball coupling were all replaced, and the foundation bolts were tightened. Vibration decreased significantly after the work. Axial vibration at measurement point 2 fell from 23.2 mm/s to 4.0 mm/s, and the bearing fault frequency and gear-meshing fault frequency disappeared.

Source: "Mechanical Design and Manufacturing"

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