1 Abnormal rotation sound analysis and diagnosis

The detection and analysis of abnormal rotation sound is an analytical method to monitor the working state of bearings by auscultation. The common tool is a long screw screwdriver with a wooden handle, or a hard plastic pipe with an outer diameter of about 20mm can be used. Relatively speaking, the use of electronic stethoscope for monitoring is more conducive to improving the reliability of monitoring. When the bearing is in normal working state, the operation is smooth and brisk, there is no stagnation phenomenon, the sound is harmonious and no noise, and the uniform and continuous "splash" sound can be heard, or the lower "boom" sound. The abnormal sound reflects the bearing fault as follows.

(1) The bearing emits a uniform and continuous "sizzle" sound, which is generated by the rotation of the rolling body in the inner and outer rings, and contains irregular metal vibration sounds independent of the speed. It is generally shown that the amount of grease in the bearing is insufficient and should be supplemented. If the equipment downtime is too long, especially in the case of low temperatures in winter, the bearing operation will sometimes emit a "sizzling rustle" sound, which is related to the bearing radial clearance becoming smaller and the grease working needle penetration becoming smaller. The bearing clearance should be properly adjusted and the new grease with a larger needle penetration should be replaced.

(2) The bearing emits a uniform periodic "ho luo" sound in the continuous "splash" sound, which is caused by scars, grooves and rust spots in the rolling body and the inner and outer ring raceways. The period of the sound is proportional to the speed of the bearing. The bearing should be replaced.

(3) The bearing emits an irregular and uneven "scrape" sound, which is caused by impurities such as iron filings and sand particles falling into the bearing. The sound intensity is small, and there is no connection with the revolution. Bearings should be cleaned, refat or oil changed.

(4) The bearing emits a continuous and irregular "rustling" sound, which is generally related to the inner ring of the bearing and the shaft being too loose or the outer ring being too loose with the bearing hole. When the sound intensity is large, the matching relationship of the bearing should be checked and the problem should be repaired in time.

2 Vibration signal analysis and diagnosis

Bearing vibration is very sensitive to bearing damage, such as spalling, indentation, rust, cracks, wear, etc., will be reflected in the bearing and vibration measurement. Therefore, by using a special bearing vibration measuring device (frequency analyzer, etc.), the size of the vibration can be measured, and the specific situation of the anomaly can be inferred through the frequency distribution. The measured value varies according to the use conditions of the bearing or the installation position of the sensor, so it is necessary to analyze and compare the measured value of each machine in advance to determine the judgment standard.

There are many kinds of detection and diagnosis technologies for rolling bearing faults, such as vibration signal detection, lubricating oil analysis and detection, temperature detection, acoustic emission detection and so on. Among the various diagnostic methods, the diagnostic technology based on vibration signal is widely used, which is divided into two kinds: simple diagnosis method and precision diagnosis method.

The simple diagnosis uses various parameters of the vibration signal waveform, such as amplitude, waveform factor, crest factor, probability density, kurtosis coefficient, etc., as well as various demodulation techniques to make a preliminary judgment of the bearing to confirm whether there is a fault.

Precision diagnosis uses a variety of modern signal processing methods to determine the fault categories and causes of bearings that are considered to be faulty in simple diagnosis.

2.1 Simple diagnosis method

In the process of using vibration for simple diagnosis of rolling bearings, it is usually necessary to compare the measured vibration value (peak value, effective value, etc.) with some predetermined criterion, and determine whether the bearing has a fault according to whether the measured vibration value exceeds the limit given by the standard, so as to determine whether further precision diagnosis is needed.

The criteria for simple diagnosis of rolling bearings can be roughly divided into three types:

(1) Absolute criterion: it is an absolute value used to judge whether the measured vibration value exceeds the limit;

(2) Relative criteria: The same part of the bearing is regularly tested for vibration, and compared according to time, the vibration value of the bearing without fault is the standard, and the ratio of the measured vibration value to the reference vibration value is the standard for diagnosis;

(3) The analogy criterion: it is a standard for vibration detection of several bearings of the same model in the same part under the same conditions, and the vibration value is compared with each other to judge.

The absolute criterion is a standard developed on the basis of the specified detection method, so attention must be paid to its applicable frequency range, and vibration detection must be carried out in accordance with the specified method. The absolute criteria applicable to all bearings do not exist, so it is generally a combination of absolute criteria, relative criteria and analog criteria, so as to obtain accurate and reliable diagnostic results.

There are several methods for simple diagnosis:

(1) amplitude value diagnosis method.

The amplitude value here refers to the peak value XP, the mean value X (for simple harmonic vibration is the average value within half a cycle, for bearing shock vibration is the average value after absolute value treatment) and the root mean square value (effective value) Xrms.

This is the most simple and most commonly used diagnostic method, which is diagnosed by comparing the measured amplitude value with the given value in the criterion.

The peak value reflects the maximum value of the amplitude at a certain time, so it is suitable for fault diagnosis with instantaneous impact such as surface pitting damage.

The effect of the mean value for diagnosis is basically the same as that of the peak value, and its advantage is that the detection value is more stable than the peak value, but it is generally used for high speed (such as 300r/min or more).

The RMS value is averaged over time, so it is suitable for fault diagnosis where the amplitude value changes slowly over time, such as wear.

(2) Probability density diagnosis method.

The probability density curve of the amplitude of fault-free rolling bearing is a typical normal distribution curve. Once a fault occurs, the probability density curve may be skewed or dispersed.

(3) Kurtosis coefficient diagnosis method.

The kurtosis value of fault-free bearings with normal amplitude distribution is about 3. With the occurrence and development of faults, kurtosis value has a similar variation trend as crest factor. The advantage of this method is that it is independent of the bearing speed, size and load, and it is mainly suitable for the diagnosis of pitting faults.

(4) Waveform factor diagnosis method.

The waveform factor is defined as the ratio of peak to mean (XP/X). This value is also one of the effective indexes for simple diagnosis of rolling bearings.

(5) Wave crest factor diagnosis method.

The crest factor is defined as the ratio of the peak to the root-mean-square value (XP/Xrms). The advantage of this value for simple diagnosis of rolling bearings is that it is not affected by bearing size, speed and load, and is not affected by sensor, amplifier and other primary and secondary instrument sensitivity changes. This value is suitable for the diagnosis of pitting faults. By monitoring the change trend of XP/Xrms value with time, it can effectively forecast the fault of rolling bearing early and reflect the development trend of fault.

When the rolling bearing has no fault, XP/Xrms is a small stable value;

When the bearing is damaged, the impact signal will be generated, and the vibration peak value increases significantly, but the root-mean-square value does not increase significantly at this time, so XP/Xrms increases.

When the fault continues to expand and the peak value gradually reaches the limit value, the root mean square value begins to increase, and XP/Xrms gradually decreases until it recovers to the size without fault.

2.2 Precision diagnosis

The vibration frequency components of rolling bearings are very rich, containing both low frequency components and high frequency components, and each specific fault should have a specific frequency component. The task of precision diagnosis is to separate specific frequency components through appropriate signal processing methods, so as to indicate the existence of specific faults. There are several kinds of precision diagnosis commonly used.

(1) Low frequency signal analysis

A low frequency signal is a vibration with a frequency below 8kHz. Generally, acceleration sensors are used to measure the vibration of rolling bearings, but the vibration speed is analyzed for low-frequency signals. Therefore, the acceleration signal is converted to the speed signal by the integrator after the charge amplifier, and then the high-frequency signal is removed by a low-pass filter with an upper cutoff frequency of 8kHz, and the frequency component analysis is performed after the acceleration signal to find the characteristic frequency of the signal for diagnosis.

(2) Demodulation analysis of medium and high frequency signals

The frequency range of the intermediate frequency signal is 8kHz-20kHz, and the frequency range of the high frequency signal is 20kHz-80kHz. Since the acceleration of the medium and high frequency signals can be directly analyzed, the sensor signal passes through the charge amplifier, the low-frequency signal is removed directly through the high-pass filter, and then the demodulation is performed, and the frequency analysis is carried out to find out the characteristic frequency of the signal.

3  Bearing temperature analysis and diagnosis

The temperature of the bearing, generally the temperature of the bearing outside can be inferred, if the oil hole can be used to directly measure the temperature of the bearing outer ring, it is more appropriate. Usually, the temperature of the bearing begins to rise slowly as the bearing runs, and reaches a stable state after 1-2 hours. The normal temperature of the bearing varies according to the heat capacity, heat dissipation, speed and load of the machine. If the lubrication and installation are not suitable, the bearing temperature will rise sharply, and abnormal high temperature will occur, and the operation must be stopped and necessary preventive measures taken.

The high temperature often indicates that the bearing is in abnormal condition. High temperatures are also harmful to bearing lubricants. Sometimes the bearing overheating can be attributed to the bearing lubricant. If the bearing is continuously transferred at temperatures exceeding 125 ° C for a long time, the bearing life is reduced. Causes of high temperature bearings include: insufficient or excessive lubrication, impurities in the lubricant, excessive load, bearing damage, insufficient clearance and high friction generated by the oil seal and so on.

Therefore, continuous monitoring of bearing temperature is necessary, whether it is to measure the bearing itself or other important parts. If the operating conditions are unchanged, any temperature change can indicate that a fault has occurred. The bearing temperature can be measured regularly with the help of thermometers, such as SKF digital thermometers, which can accurately measure the bearing temperature and display it in units of ° C or Fahrenheit. The importance of the bearing means that when it is damaged, it will cause the shutdown of the equipment, so this kind of bearing should be equipped with a temperature detector. Under normal circumstances, the bearing will have a natural temperature rise after just lubricating or re-lubricating and last for one or two days.

4  Lubricant analysis and diagnosis

The method of lubricant analysis is based on ferrography, which is particularly suitable for identifying and predicting rolling fatigue.

A part of the lubricating oil of the rolling bearing is extracted as an oil sample, and the solid foreign matter contained in the oil sample flowing through the magnetic field is deposited on the glass sheet in proportion to its size by using a high gradient magnetic field. The shape, size, color and material of the foreign matter particles can be observed, so that the type of wear can be clearly determined, the running state of the machine can be predicted, and hidden dangers can be discovered in time. In principle, ferrography technology is mainly aimed at the identification of strong magnets such as steel, but it also has excellent identification ability for non-ferrous metals such as copper, sand, organic matter and sealing debris.

When the steel spherical particles with a diameter of 1-5μm appear in the oil sample, it is certain that the bearing has begun to appear fatigue micro-cracks. When there are 10:1 fatigue spalling particles in the oil sample, and the length is greater than 10μm, the abnormal fatigue wear of the bearing has begun, and when the length is greater than 100μm, the bearing has failed.

The third type of fatigue debris is a fatigue sheet with a length to thickness ratio of 30:1, which is between 20 and 50μm in length and often has cavities. At the onset of fatigue, the number of such flakes will increase significantly, which can be used together with spherical particles as a sign of fatigue.

5 Acoustic emission detection

Principle of acoustic emission detection technology, when the material is deformed by external force or internal force or crack expansion, the phenomenon of releasing strain energy in the form of elastic waves is called acoustic emission.

The technology of detecting and analyzing acoustic emission signals with instruments and using acoustic emission signals to infer acoustic emission sources is called acoustic emission detection technology, which uses the phenomenon that particles inside a substance release strain energy in the form of elastic waves due to relative motion to identify and understand the internal state of a substance or structure.

Acoustic emission signal includes burst type and continuous type. The burst acoustic emission signal is composed of pulses which are different from the background noise and can be separated in time. A single pulse of a continuous acoustic emission signal is indistinguishable. In fact, the continuous acoustic emission signal is also composed of a large number of small burst signals, but too dense to be distinguishable.

In the case of poor operation of rolling bearings, burst and continuous acoustic emission signals may be generated. Hertzian contact stress caused by relative movement and friction between contact surfaces of bearing components (inner ring, outer ring, rolling element and cage), As well as due to failure, overload, such as surface cracks, wear, indentation, grooving, biting, poor lubrication caused by surface roughness, lubrication pollution particles caused by the surface hard edge and pitting caused by the current through the bearing and other failures, will produce a burst of acoustic emission signals.

Continuous acoustic emission signals mainly come from poor lubrication (such as the failure of lubricating oil film, the immersion of pollutants in grease) resulting in the bearing surface oxidation wear and resulting in global failures, too high temperature and multiple local bearing failures, etc. These factors cause a large number of sudden acoustic emission events in a short period of time, resulting in continuous acoustic emission signals.

In the running process of rolling bearings, their faults (whether surface damage, cracks or wear faults) will cause elastic impact on the contact surface and generate acoustic emission signals, which contain rich friction information, so acoustic emission can be used to monitor and diagnose rolling bearing faults.