2022 September the Fourth Week KYOCM Technical Knowledge: Diagnosis of rolling bearing faults with portable vibration tester

Abstract: The vibration of rolling bearing is measured with a portable vibrometer, and the damage degree of rolling bearing is judged through three numerical analysis of displacement, velocity and acceleration on the instrument. It is convenient for the staff to master the equipment maintenance time and cycle, and improve the maintenance staff's ability to judge faults and work efficiency.

Key words: vibration; Frequency; Displacement; Speed; Acceleration




0 Preface

More than 80% of the rotating equipment in thermal power plants use rolling bearings. The judgment and prevention of rolling bearing faults are very important to ensure the safe operation of rotating equipment. Now the power plants are equipped with hand-held vibration meters, such as Zhentong 908, Nihon Riyin Vm - 63A, etc. These vibration meters have displacement, velocity and acceleration measurement functions.

Through many years of field measurements and tests, it is concluded that rolling bearing faults can be judged by measuring displacement, velocity and acceleration of rotating equipment and combining traditional diagnosis methods.


1 Fault mechanism

1.1 Failure frequency of rolling bearing components

Rolling bearing is generally composed of inner ring, outer ring, rolling element and cage. The defects on the surface of each element, such as peeling pits, cracks or scuffing marks, cause the bearing to vibrate during operation. The characteristic frequency of its vibration is obtained from the corresponding formulas.


The larger the defect size, the stronger the signal. In practice, each part of the bearing will have multiple peeling pits at the same time, and the frequency is several times of the above single frequency.


1.2 Relationship among displacement, velocity and acceleration

According to the theory of simple harmonic vibration:

Vibration displacement: d (t)=Dsin ω t

Vibration speed: V (t)=- D ω cos ω t

Vibration acceleration: a (t)=- D ω 2 sin ω t

ω= 2π f


Here, D is the displacement amplitude, V is the velocity amplitude, and a is the acceleration amplitude, that is, the vibration displacement, vibration velocity and vibration acceleration measured by the vibration meter. In the process of measuring the bearing vibration of rotating equipment, various frequencies are included. The main concern is which frequency causes the large bearing vibration of the equipment.


1.3 For the vibration caused by imbalance, misalignment, rotor bending and other reasons, the vibration energy is concentrated in the low frequency band. For the vibration caused by bearing pitting and peeling pits (rolling bearings running on site are all multiple defects), the vibration energy is displayed in the high frequency band. The relationship among vibration velocity, acceleration and vibration displacement is based on the function of frequency f. When the vibration displacement is constant, the higher the frequency, the greater the vibration acceleration; In high-frequency vibration, although the vibration amplitude may be very small, since the vibration acceleration is times of the vibration displacement, a slight increase in the vibration frequency will lead to a large increase in the system acceleration, which often causes damage to the inertia force of parts in high-frequency vibration. However, the hidden dangers of high-frequency vibration are mostly hidden and sudden, which can be achieved when the vibration displacement and speed increase are not obvious, And sudden fracture occurs. The initial fatigue of rolling bearings is generally manifested by the increase of acceleration. As the acceleration continues to increase, the fatigue becomes more severe. In the middle and late stages, the speed and displacement will generally increase. After fatigue failure, the speed and displacement will increase significantly.


2 Steps for judging faults

2.1 When the bearing of the equipment is replaced for 3 to 4 days, use a vibration meter to measure the displacement, velocity and acceleration of each bearing pad in vertical, horizontal and axial directions (both LO and HI are measured). As the newly replaced bearing equipment operates normally, the measured values are relatively small, so auscultate the bearing with a listening needle to ensure it operates smoothly. The measurement data shall be recorded every month for future comparison.


2.2 With the increase of operation time, the bearing sound starts to increase gradually. When measuring the vibration of the bearing pedestal, it is found that the acceleration value rises fastest, but the vibration speed and displacement do not change much. Add some lubricating oil (grease) to the bearing chamber first, and measure the vibration acceleration after 6 hours of operation. If there is no change before oil (grease) filling, it means that the bearing has started to be damaged and the monitoring should be strengthened. The vibration shall be measured every week and compared with the previous measurements.


2.3 When the vibration acceleration (HI gear position) reaches the above (empirical value); The vibration speed is more than 8mm/s; The displacement value exceeds the acceptable range at the corresponding speed; Auscultate the squeal of the bearing sound "click, click". At this time, the equipment must be stopped to replace the rolling bearing.


3 Fault examples

Vibration tracking measurement after replacing a new rolling bearing (SKF6312) with a 11kW fan (rotating speed 3000r/min). See Figure 1 for fan structure.



Figure 1


From the measured values, it can be seen that the vibration displacement, speed and vibration acceleration of the newly installed bearing are very small; After more than half a year, the three parameters all increased, and the vibration acceleration increased significantly (especially the HI value), indicating that the pitting corrosion of the inner and outer rings of the bearing began to increase, and the sound of the auscultation bearing also became louder. This process may be accompanied by the following situations: the vibration value will suddenly decrease, pay attention! This is by no means an improvement in the condition of the bearing. The reason is that after the fatigue failure of the bearing components, due to the continuous operation, the fatigue damaged surface is rolled by the rolling element, which produces a similar "running in" process, so that the original "sharp" surface peels off and becomes relatively flat again. However, with the extension of running time, more serious peeling will occur again at uncertain times; The vibration displacement also starts to increase when the number of accelerations (HI values) reaches 150 m/s2. It shows that the bearing clearance becomes larger after wear, the inner and outer rings are severely corroded, and the ball has begun to deform. The bearing cage was also seriously worn. The auscultation of the rolling bearing made a squeal of "click, click", which indicates that the rolling bearing has been seriously worn and must be shut down for maintenance.


More about KYOCM Spherical Roller Bearing

A spherical roller bearing is a rolling-element bearing that permits rotation with low friction, and permits angular misalignment. Typically these bearings support a rotating shaft in the bore of the inner ring that may be misaligned in respect to the outer ring. The misalignment is possible due to the spherical internal shape of the outer ring and spherical rollers.[1] Despite what their name may imply, spherical roller bearings are not truly spherical in shape. The rolling elements of spherical roller bearings are mainly cylindrical in shape, but have a (barrel like) profile that makes them appear like cylinders that have been slightly over-inflated.




Spherical Roller Bearing 






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