Shock Pulse Method

Shock pulse technology was developed by SKF AB, Gothenburg, in the early 1970s [8.35]. It was prompted by difficulties encountered by techniques based on the analysis of the repetitive components of the vibration signals from rolling element bearings.

The method involves the analysis of the high-frequency (ultrasonic) shock waves generated by metal-to-metal impacts in a rotating bearing, where most of the information about bearing damage can be found.

Fig. 8.21 (from [8.14])

Empirical relationships were developed that provided both a measure of the theoretical lubricant film thickness between the bearing surfaces, as well as the overall condition of the bearing surface.

The shock pulse analyzer detects impacts of very short duration arising from the presence of pits and spalls. Unlike conventional vibration analysis, that monitors a broad vibration band with the objective of detecting discrete frequencies, the shock pulse method (SPM) measures and evaluates the ultrasonic frequency band centered around 36 kHz.

Shock (or stress) waves that result from metal-to-metal contact are short duration bursts of energy that travel at the speed of sound through the material. As the wave travels, it dissipates energy through the structure, thereby reducing the wave pulse. The SPM is designed to detect the weak shock pulse signals using an accelerometer with a natural frequency of about 36 kHz, ideally placed very closely to the subject bearing. In fact, a patented design called Tandem-Piezo is used, which enables the accelerometer to accurately measure both shock pulse and vibration. To distinguish the shock pulses from vibration, a band pass filter around de 36 kHz shock pulse signal is used. This helps isolate the shock pulse from other interference created by machinery vibrations.

The last stage of signal processing is the conversion from a waveform to analog pulses. This process provides a signal that then can be processed to determine bearing condition.

a b c Fig. 8.22 (from [8.36])

Figure 8.21 shows the block diagram of an early shock pulse meter [8.14].

The accelerometer output (Fig. 8.22, a) is passed through a high-gain amplifier tuned at the resonant frequency of the accelerometer, the amplifier acting as a very sharp band filter. The filtered and amplified shock pulse is shown in Fig. 8.22, b.

Fig. 8.23 (from [8.37])

The signal is rectified, averaged and then passed through a peak-sample-and-held circuit. This measures the information and displays it on a counter which records the number of peaks occurring above a defined peak amplitude;

alternatively, it presents the signal r.m.s. value. The amplitudes of analog shock pulses are displayed as function of time in Fig. 8.22, c.

The bearing condition is defined by a string of pulses with varying magnitudes (Fig. 8.23). A shock pulse analyzer measures the shock pulse magnitude on a decibel scale, in dBsv (decibel shock value). It takes a sample count of the shock pulses occurring over a period of time and displays: LR (Low Rate of occurrence), the value for the relatively small number of strong shock pulses, and HR (High Rate of occurrence), the value for the large number of weak shock pulses in the pattern. The difference between LR and HR is called the delta value, Δ.

a b Fig. 8.24 (after [8.38])

The strength of the individual pulses, and the ratio between stronger and weaker pulses in the overall pattern, provide the row data for bearing condition analysis. The magnitude of these pulses is dependent on the bearing surface condition and the peripheral velocity of the bearing.

In undamaged bearings, the shock level varies with the thickness of the lubricant film between the rolling elements and raceway. The relationship between stronger and weaker pulses, however, is only slightly affected (Fig. 8.24, a).

Surface damage causes an increase of up to 1000 times (60 dB) in shock pulse strength, combined with a distinct change in the ratio between stronger and weaker pulses (Fig. 8.24, b).

Fig. 8.25 (from [8.37])

The shock pulse readings are evaluated and a code is displayed describing the general bearing condition.

Code A is for a bearing in good condition. There is no detectable damage to the surfaces of the load carrying parts, and no extreme lack of lubricant in the rolling interface. Figure 8.25, a shows a typical shock pulse pattern from a good bearing: a low shock level and a normal delta value.

Code B indicates a dry running condition, causing a high HR value and a low delta value (Fig. 8.25, b). Code C is for reduced condition defined by an increased shock pulse level with a large delta value (Fig. 8.25, c). This denotes incipient surface damage. Code D is for bearing damage characterized by a high shock level with a large delta value (Fig. 8.25, d). A contamination of the lubricant by hard particles causes a similar pattern.

Fig. 8.26 (from [8.37])

Output data are displayed as in Fig. 8.26. The delta value Δ=LR−HR is plotted as a function of HR. The fields marked A, B, C, D correspond to the condition codes. The black point marks a shock pulse reading. For a bearing in good condition it is in the field A.

Developing surface damage causes a marked increase of the delta value, HR remains low while LR increases. The marker point moves upwards, from A through field C towards D.

For poor lubrication, the condition code changes from A to B then to D as damage develops and increases. The marker point moves to the right.

Shock pulse is not limited to determining the condition of rolling element bearings. Any machine element with continuous metal-to-metal contact gives off shock pulse signals. Equipment such as gearboxes, lobe compressors, screw compressors and centrifuges can be monitored using SPM.