2023 February the Fourth Week KYOCM Technical Knowledge: Analysis on Correlation Between Vibration and Noise of Rolling Bearings

Abstract:Through in -depth analysis of relationship be tween vibration and noise of rolling bearings, the essential correlation and main difference be tween vibration and noise of rolling bearings are expounded.The bearing noise can be controlled by control of bearing vibration, but the current bearing vibration measuring method can not completely reflect bearing noise.Meanwhile, the estimating method for bearing noise life is also introduced, and the main technology development direction of vibration and noise of rolling bearings is pointed out.

Key words:rolling bearing;vibration;noise;correlation

The vibration and noise of rolling bearings have become one of the basic performance requirements of many types of bearings from one of the special performance requirements of motor bearings in the past. The research on bearing vibration and noise has attracted much attention for a long time. From theoretical analysis to experimental research, there are many achievements. But so far, the relationship between bearing vibration and the theory of noise are still recognized as immature; There are still many problems to be explored in the test of bearing vibration and noise. In the specific research and analysis, sometimes the bearing vibration and noise are generally classified into one problem, sometimes they are regarded as two aspects of one problem, and sometimes they are completely divided into two independent problems. Whether the bearing noise can be controlled by controlling the bearing vibration has formed a speculative problem that has been debated for a long time.

1. The essential relationship between vibration and noise

Vibration is the reciprocating motion of the object, that is, the motion state of the object changes alternately between the maximum and the minimum with time. Sound is produced by object vibration. All objects that make sound are vibrating, but not all object vibrations produce sound. Only when the vibration frequency is within a certain range and is perceived by human auditory organs through the medium, it is called audible sound, which is called sound for short. Among them, all irregular or random noises that are not expected from people's subjective and psychological feelings are called noise. So in a nutshell, the causal logic relationship between vibration and noise is that the motion of objects under certain conditions is called vibration, and the vibration under certain conditions produces sound with certain conditions belongs to noise.

In addition to specific applications, vibration and noise are harmful and must be controlled. For example, the harm of vibration to equipment and instruments-will affect the realization of functions, reduce the working accuracy, intensify the wear of parts, and even cause structural fatigue damage; Harm to human body - excessive vibration will make people uncomfortable, tired, and even cause pathological damage to human body. The noise generated by vibration will mainly cause environmental pollution and endanger human health. Because of the causal relationship between vibration and noise, for the control of vibration, it is usually to directly control the vibration itself; For noise control, it is generally necessary to trace back to the source and control its cause - vibration (of course, other measures such as sound absorption, sound insulation or noise reduction can also be adopted). Since the audio frequency range is 20~20 000 Hz, infrasonic waves below 20 Hz and ultrasonic waves above 20 000 Hz do not emit when they act on human auditory organs. The sense of sound is that the human ear cannot hear it, so the noise control only needs to control the vibration within the audio range, especially the frequency of 300~6000 Hz (the most sensitive frequency is 3000~4000 Hz, of which the most harmful is 1000~4000 Hz), which is particularly sensitive to the human ear.

2 Characteristics of bearing vibration and noise

Bearing vibration and noise have both the commonness and characteristics of general mechanical vibration and noise. In addition to the vibration and noise caused during lubrication, installation and use, the bearing itself has the following vibration and noise characteristics.

2.1 Vibration characteristics of bearing

(1) The causes of bearing vibration are very complex. The vibration forms include radial vibration, axial vibration and many coupling vibrations.

(2) Due to the bearing structure, it has inherent vibration that cannot be avoided: ① the rolling body vibrates through the bearing area; ② The ferrule is subject to bending deformation and vibration under load.

(3) At the current manufacturing level, bearing vibration is mainly related to the waviness of the ring raceway and rolling body, but not significantly related to roundness and surface roughness.

(4) Bearing vibration includes vibration of various frequency components from low frequency to high frequency, that is, its vibration frequency is dense everywhere.

(5) The full frequency range of bearing vibration measurement is 50~10 000 Hz. In order to facilitate analysis, the full frequency band is also divided into three frequency bands: 50~300 Hz; 300 ～ 1 800 Hz ; 1 800 ～ 10 000Hz.

2.2 Bearing noise characteristics

(1) Bearing noise is caused by many factors that produce bearing vibration, of which the major influence is the surface roughness and waviness of the race and rolling body.

(2) The important sound sources of bearing noise also include the impact sound of rolling body and cage, the squealing sound of cage due to whirling, and the contact friction sound of rolling body and raceway (when the lubrication condition is not good).

(3) The background noise of bearing sound frequency has white noise characteristics, but the main frequency components of abnormal noise are concentrated in 1000~10000 Hz, that is, bearing noise generally shows as medium and high frequency noise.

(4) The full frequency range of bearing noise measurement is 100~15000 Hz (also 125~16000 Hz).According to the above frequency component characteristics of vibration and noise 3 The purpose of bearing vibration control is to control bearing noise.

According to the above frequency component characteristics of vibration and noise, it can also be distinguished as bearing vibration or noise according to the vibration frequency range. For example, Japan's practice is that when the frequency is below 1000 Hz, it is attributed to vibration, and when the frequency is above 1000 Hz, it is attributed to noise.

3 The purpose of bearing vibration control is to control bearing noise

As long as the bearing is running, it will definitely produce vibration and noise. Due to the structural characteristics and precision of the bearing, the vibration generated by the bearing itself is very small, and the possibility of damage caused by the bearing vibration is also very small. In fact, the main purpose of bearing vibration control is not to control vibration, but to control noise. For example, Japan has carried out research on vibration reduction of motor bearings since 1933, because the motor running noise was too high at that time; During World War II, the United States began to concentrate on reducing the vibration of submarine bearings, also because the German sonar can capture the noise of the submarine engine when it works. Therefore, in order to control bearing noise, bearing vibration has been indirectly controlled. Now, the control of bearing vibration or noise has developed from the deep groove ball bearing mainly used for motors in the past to many types of angular contact ball bearing, cylindrical roller bearing, tapered roller bearing, and even self-aligning roller bearing. Vibration and noise have become one of the most important dynamic properties of these bearings in some applications. Especially for bearings used in household appliances, office machines and tools, the requirements are more strict, and "silent bearings" need to be used, and even "super silent bearings" need to be used for air conditioner bearings. Therefore, in the final inspection of the bearing production process for this purpose, 100% of the vibration or noise shall be measured and controlled system.

Of course, in monitoring and diagnosing bearing faults, it is much easier and more reliable to use vibration than noise. Therefore, vibration is undoubtedly the main research direction. However, this is not a problem of the bearing itself, but a problem of the bearing application, which should be another matter.

Since the direct correlation between bearing vibration and noise can be said to be synonymous under certain conditions, "vibration reduction and noise reduction" is always connected in production and use, and "low vibration bearing" is often habitually referred to as "low noise bearing", which is why.

Controlling bearing noise by controlling bearing vibration not only directly captures the source and cause of noise, but also because it is relatively simple to realize, such as measuring vibration is less affected by environmental conditions, foundation vibration is easy to separate, and is easy to apply in production practice. However, the measurement noise requires high environmental conditions and requires a very low noise level. Therefore, it must be carried out in an anechoic room with high construction cost.

4 Measurement of bearing vibration cannot completely replace measurement of bearing noise

Although vibration is the cause and noise is the result, the noise can be controlled by controlling vibration, the current method of controlling bearing vibration cannot completely replace the control of bearing noise. The current conventional method for measuring bearing vibration is to rotate the inner ring and measure the radial vibration on the stationary outer ring. Although the bearing vibration evaluated based on this measurement result can also evaluate the bearing noise, it is obviously only the noise related to the radial vibration. However, the bearing vibration is not only the radial vibration of the ring, but also the axial vibration of the ring; The free movement of the rolling element and cage, the friction sound between the rolling element and the raceway, etc., except for the components that have affected the radial vibration of the ring, other parts will cause bearing noise, but they are not included. In addition, some bearing noises are not caused by bearing vibration, such as "airborne noise" formed by moving parts in the bearing and surrounding air, which must also be included. Therefore, the current method of measuring bearing vibration alone cannot completely replace the measurement of bearing noise. Some research results show that the correlation between bearing vibration and noise is 0.7. Although this conclusion shows that it is feasible and effective to replace the control of bearing noise with the control of bearing vibration, it also proves that the other 30% influence is not included. Therefore, to control the bearing noise in a complete sense, it is obvious that only by directly controlling the bearing itself is the most effective solution.

5 Methods for measuring bearing vibration and noise

5.1 Method of measuring bearing vibration

There are many methods to measure vibration, including electrical method, mechanical method and optical method, among which the most commonly used is electrical method. According to the different physical quantities of vibration measurement, it can be divided into displacement, velocity and acceleration. Among them, displacement is suitable for measuring low-frequency vibration; The speed is suitable for measuring medium frequency vibration; Acceleration is suitable for measuring high-frequency vibration. The three parameters of displacement, velocity and acceleration can be derived from each other by mathematical differentiation or integration, but the error of velocity and displacement obtained by integration of acceleration is small, while the error of differential of displacement or velocity is large.

Since the bearing vibration usually presents as medium and high frequency vibration, the physical quantities applicable to the measurement of bearing vibration are speed and acceleration. Among them, when measuring acceleration, the full frequency band is generally used, that is, 50~10 000 Hz, and the 3 frequency band can also be used; When measuring speed, the 3-band or full-band is generally used. Production corresponding to 3-band.

The main factors affecting the manufacturing accuracy of bearing vibration are: 50~300 Hz: the roundness of the raceway of the ring; 300 ～ 1 800 Hz: waviness of ring raceway, rolling body roundness of; 1 800 ～ 10 000 Hz: surface roughness of the ring raceway and rolling body. Although the measurement of bearing vibration acceleration has many advantages, such as frequency bandwidth, high sensitivity, small system error, good reliability, small volume and light weight, and the most widely used piezoelectric accelerometer, the measured vibration is basically determined by the medium and high frequency components, and it is not easy to reflect even if the vibration value changes greatly in the low frequency range.

Although there are some shortcomings in measuring the bearing vibration speed, such as the commonly used electric speedometer with limited amplitude and frequency, easy to be disturbed by strong current and strong magnetic field, large and heavy volume, etc., the measured vibration value is relatively stable and can reflect the change of vibration level in any frequency band in the total vibration level, so the vibration quality can be more objectively evaluated, and the information content of vibration measurement can be improved by using 3 frequency bands. In view of the advantages and disadvantages of the above measurement of vibration speed and acceleration, the measurement of bearing vibration has become more and more inclined to use speed. In particular, ISO/TC4, the Fourth Technical Committee of the International Organization for Standardization (Technical Committee of Rolling Bearings), issued ISO15424 Rolling Bearing Vibration Measurement Method in 2004, which clearly stipulates that the measurement of bearing vibration should use speed, which has established its priority position.

5.2 Method of measuring bearing noise

Measuring noise is more difficult than measuring vibration and many other physical quantities, and it is also difficult to measure accurately, that is, the measurement error is large.

The ideal sound field for measuring noise is free sound field. Free sound field refers to the sound field in homogeneous and isotropic media with negligible boundary effect. In the free sound field, sound waves can transmit the radiation characteristics of sound sources in all directions without obstruction or interference. However, it is difficult to realize free sound field. Generally, only approximate free sound field meeting certain measurement error requirements can be obtained, such as the sound field in anechoic chamber. The anechoic chamber can effectively absorb incident sound waves, and the impact of reflected sound waves on the sound field can be basically ignored. Therefore, within a certain frequency range, the sound field in the anechoic chamber can be basically considered as a free sound field.

The method of measuring bearing noise is to use A-weighted network sound pressure level (A-weighted sound level is applicable to simulate the frequency characteristics of human ear to low intensity noise below 55 dB, and the attenuation of low frequency components is large) in the anechoic room. The background noise requirement should generally be less than 15 dB (A) or even 12 dB (A). During the measurement, the difference between the bearing noise and the background noise should preferably be more than 10 dB (A), and the minimum must be more than 4 dB (A), otherwise it is difficult to accurately measure the bearing noise. For micro and small bearings, due to their low noise level, it is generally difficult to meet the requirement that the difference with the background noise is more than 10 dB (A). When the difference is lower than the specified requirements, it should be corrected according to the values listed in Table 1.

The frequency range of bearing noise is wide, but when the frequency exceeds 8000 Hz, it is difficult to measure the noise intensity, and the noise exceeding 8000 Hz has little effect on the bearing noise intensity. Therefore, as long as the sound pressure level of 500~8000 Hz is measured, the noise quality level of the bearing can be basically evaluated. In other words, it is also feasible not to measure the bearing noise in the full frequency range. The measurement of bearing noise in the anechoic chamber belongs to non-contact measurement. There is also one kind of contact measurement commonly used in the factory site. The method is to convert the vibration signal of the bearing into sound signal, amplify it through the loudspeaker, and then listen to the sound by the human ear, and check the noise quality of the bearing based on experience. Although this method has no special requirements for the measurement environment, it is essentially not noise measurement, but vibration measurement.

6 Definition and estimation of bearing noise life

The definition of bearing noise life is the total time that the bearing operates at a certain noise limit level. After a period of operation, the noise of the bearing passing the noise detection will increase due to the wear of the raceway surface and the deterioration of the lubricating grease. When the noise exceeds the allowable range, the bearing noise life is considered to be over and cannot be used again. According to the above definition of bearing noise life, for low-noise bearings, there are actually two requirements, that is, not only low noise is required, but also good maintenance of noise level is required, that is, certain noise life.

Tests and experience have proved that the estimation of bearing noise life can be related to some operating conditions or factors. The following is an example of household appliance bearing with strict noise requirements.

6.1 Bearing noise life is related to contact stress between rolling body and raceway

The contact stress between the rolling body and the raceway will directly affect the friction and wear of the bearing, and then affect the noise life of the bearing. Taking the deep groove ball bearing as an example, when the noise life is required to exceed 10000 h (in the case of long noise life), the contact stress should not be greater than 800 MPa; The contact stress shall not be greater than 1000 MPa at 5000~10000 h (general purpose); When it is less than 5000 h (the rigidity requirement is strict), the contact stress shall not be greater than 1500 MPa.

6.2 Bearing noise life is related to preload

When bearing bears radial load, due to the existence of radial clearance, only part of the rolling element in the bearing area bears the load. When the rolling body moves from the non-bearing area to the bearing area, the size and direction of the resultant force of the dead weight of the rolling body and the centrifugal force will change constantly, and the rolling body will collide with the ring raceway alternately, resulting in noise. This noise can be eliminated by preloading the bearing.

Taking the deep groove ball bearing as an example, when the noise life is required to exceed 10000 h, the preload is taken as (0.005~0.01) Cr (Cr is the radial rated dynamic load of the bearing); When 5 000~10 000 h, the preload is (0.01~0.015) Cr; When the preload is less than 5000 h, the set value of preload is (0.015 ～ 0.02) Cr. For common types of deep groove ball bearings, the recommended preload is: 608 is 30 N; 6200-6202 is 40 N.

6.3 Bearing noise life is related to grease life

Lubricating grease has damping effect on bearing vibration, so it has silencing effect on bearing noise. The continuous deterioration of lubricating grease quality will seriously affect the noise of bearings. According to the size and working conditions of the bearing, the noise life is about 1/3~1/2 of the grease life.

7 Conclusion

In essence, vibration and noise are two aspects of one thing and highly correlated; However, in terms of the form of expression, because they are two physical phenomena, each has its significant differences. The vibration and noise of bearings are no exception, but they also have their particularity.

The main purpose of bearing vibration control is to control noise. With the continuous improvement of the quality of bearing products, the sound of bearing operation is becoming smaller and smaller, and the environmental conditions and instruments required to measure the bearing noise are more strict and complex, which is not only costly, but also technically difficult to achieve. The measurement of vibration is more convenient than the measurement of noise, and it has proved to be very effective. Therefore, ISO/TC4 decided to cancel the international standard work item on bearing noise measurement methods in 2007. Before that, a series of standards on bearing vibration measurement methods had been issued in 2004. It can be seen from this trend that vibration will become the main factor in the control of bearing vibration and noise. To control the object, the vibration speed will become the main measurement and evaluation index, which in fact also defines the development direction of the mainstream technology related to bearing vibration and noise in the future.

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