Rolling elements diagnosis.

Nicodim, Mariana ; Tudose, Virgil ; Gheorghiu, Horia 等

1. INTRODUCTION

To detect defects on the rolling bearing, we can use the shock impulse method. The need of using this method is given by the high accuracy information regarding the mechanical state of the bearing area and lubrication conditions, throughout the life of the bearings. Shock pulse method consists of identifying noise in operation and in grouping them into two categories (LR--Low Rate and HR--High Rate). HR shocks are the most common shocks, normally functioning shocks, and LR shocks are abnormal shocks indicating a fault or an incorrect operation (no- lubrication, roll jam, pitting, fretting, etc.) and the difference between LR and HR shows the nature of the defect. A small value of HR and LR indicate a normal functioning, as both values increase the highlight the development of the defect. When the difference between LR and HR is high enough it already indicates that the defect has occurred and the replacement of the bearing is required.

2. ROLLING ELEMENT DIAGNOSTIS

When a ball hits a damaged area on the rolling bearing, it produces a shock wave. Each shock is a unique event repeated at regular intervals, an interval is the gap between the time a ball passes over the defect and the next ball passage.

To identify the sources of shock impulses, the article presents a test using a mechanism that can through steel balls at the end of a hard steel bar.

An essential step to achieve this is to use a system for generating Dirac signals (Fig. 1). The Dirac signal is a relatively "sharp" signal and the most important feature of this signal is that the Fourier transform contains all the frequencies.

A real signal that is very "sharp", found in real applications, contains a wide range of frequencies, while a "low" signal is more limited in the spectrum.

The steel ball speed of the bal mechanism of "throwing with balls" can be controlled and measured. For registration of the elastic wave (shock impulse) that is propagated through the steel bar (5 000 m/s speed) we used a Doppler laser vibrometer

The vibrometer is very sensitive and shows the vibration results as velocity (mm/s). The entire system is controlled from a Lab View program.

[FIGURE 1 OMITTED]

The first figure 2 presents the signal which is relatively "sharp" with a duration of about 40 us as a response of the surface at the end of the bar when the ball hits the opposite side (the delay time between the impact and the bar movement is 0.57 ms).

The second figure 2 shows Fourier transformed of the same signal and it can be seen that there is a wide spectrum with a peak around 10 KHz frequencies; there is an increase of energy in the frequency spectrum. Even a small impact (the steel ball barely reaches the steel bar) produces a similar spectrum but with a lower amplitude. The idea is that if the transducer (SPM transducer) which is mounted on the steel bar of the "ball shooting" mechanism is very sensitive to a certain high frequency (32 KHz), it will select sharp elastic waves (shock pulse), because shock impulse is normally a broadband signal.

The third figure 2 shows the signal received from the Doppler laser vibrometer over a long period of time. Shock impulse "jumps" practically between the ends of the steel bar due to reflections. Repetition frequency is nearly 900 Hz.

1/(2,85 * 2/5000 M/S) = 877 (Hz] (1)

[FIGURE 2 OMITTED]

3. SPM TRANSDUCER FOR SHOCK

The main damage of the bearings can be detected using transducers, therefore the article notes the utility and importance of an SPM transducer. Following the tests illustrated in the article we present both the output signal for an SPM transducer mounted on a steel bar (Fig. 3) and the adjacent data (tab.1.).

SPM transducers are the essence of true monitoring of shock pulse; they are build and calibrated only for shock measurements work only at their resonance frequency of 32 KHz and they are seven times more sensitive than the conventional vibration transducers.

TMU unit contains an inductive component (a transformer) which together with the SPM transducer capacitance form a band-pass filter which further increases the pass-band system characteristics. Anyway, the main purpose of the TMU is to convert impedance in order to allow more cables.

If the SPM transducer's answer is superimposed over the output frequency of the steel bar then it is clear that the SPM transducer will choose the content with high frequency from the steel bar and will produce a 32 KHz signal. Low-frequency vibrations have no influence on the signal. Fig 4. presents the frequency spectrum.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

4. TRANSITIONS DUE TO SHOCK IN TIME REGISTRATION

Shock waves are transitions, short-term waves starting with relatively high amplitude which is quickly absorbed. When the distance between transitions is constant and corresponds to the crossing ball frequency, there is the proof of a damaged bearing. In the area around the resonance frequency, we can record a signal over time showing clear transitions from damaged bearing (fig.5.). The signal from the transducer is treated by rectification (which cuts negative amplitudes) and enveloping (which produces well-defined peaks). After enveloping the peaks can be counted on time, thereby achieving their frequency of occurrence and we can compare it to the ball pass frequency, run on the inside or the outside race.

[FIGURE 5 OMITTED]

5. CONCLUSIONS

An elastic wave (shock pulse) can be seen as compression and decompression of material in a structure, while a vibration can be seen as material moving. A "real" vibration is a mixture of these signals.

It was found that the amplitude of the shock impulse is proportional to the square of the object speed which hits the mass of steel, while mass does not affect the amplitude (as long as the object is smaller than the steel bar). In the spectrum, the peak amplitude is determined by the vibration's energy content at any frequency. Compared to the energy of the spindle frequency, the shock energy caused by damaged bearing is negligible. Thus, the line of ball crossing frequency has low amplitude and is easily lost among the "noise".

It was also notified that in case of a bearing, the speed of steel balls can be calculated using the spindle diameter and the velocity, the dBi calculated amount is used, both to remove the dependence on velocity and to extract "normal" shock pulse that is proportional to the squared ball speed.

The secret behind the true monitoring of the shock impulse is the SPM transducer for shock pulse. It can be shaken without getting any response, but when gently struck by a hard object or a shock wave that is transmitted through a metal element of the machine, it will deliver a powerful electric pulse.

6. ACKNOWLEDGEMENTS

The paper was conceived in the framework P.O.S.D.R.U. program at University POLITEHNICA of Bucharest

7. REFERENCES

Butler, D.E.. The shock pulse method for the detection of damaged rolling bearing, NDT Int. v6. 92-95. 2000

Cheng, J., Yu, D. and Yang, Y. Application of an impulse response wavelet to fault diagnosis of rolling bearing, Mech. Syst. Signal Process. v21. 920-929. 1998-2001

Li, L. and Qu, L., Cyclic statistics in rolling bearing diagnosis. J. Sound Vib. v267. 253-265. 1998

Tandon, N.; Choudhury, A. A review of the vibration and acoustic measurement methods for detection of defects in rolling element bearing, Tribol. Int. v32. 469-480. 2001

*** (1999) http://www.spminstrument.com/methods/spectrum Accesed on:1999-08-13

Tab. 1. Data for output signal of a SPM transducer

 Ball
 velocity Amplitude raw signal dBsv

0,157 m/s 0,018 Vp-p 40
0,3 m/s 0,094 Vp-p 54
0,7 m/s 0,680 Vp-p 72
0,98 m/s 0,120 Vp-p 76
1,1 m/s 1,450 Vp-p 78