Condition monitoring of hydraulic cylinder seals using acoustic emissions

3.1

Leakage

Figure 4 presents the images of the piston rod surfaces from the experiments conducted using unworn, semi-worn, and worn seals. In this study, the leakage was defined when the water glycol was visible on the piston rod. For the unworn seal condition, leakage was not observed (see Fig. 4a). For the semi-worn seal, the leakage was visible on the rod as indicated in Fig. 4b. Whereas, for the worn seal, the leakage was visible on the piston rod surface and was also observed from the leakage port (see Fig. 4c). As each test was performed for a short duration (5 strokes), quantification of the leakage was not performed in this study. This explanation of the leakage condition will be used later to qualitatively correlate and define the AE signal and AE features for the unworn, semi-worn, and worn piston rod seals.

Experiments conducted using a unworn seal, b semi-worn seal, c worn seal. (Note: leakage (in red) on the rod in panels b and c)

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3.2

AE signal from the rod

From Fig. 5, we can observe that the continuous AE signal was observed for each stroke. Figure 5a represents the AE signal obtained for five consecutive strokes. As observed in Fig. 5b, each stroke (extension and retraction) lasted 25 s in total. From Fig. 5c and d, we can observe that the extension retraction strokes lasted for 12 s each. The AE amplitude range for extension and retraction strokes was nearly the same (≈ 0.2 V). From Fig. 5a, a similar AE amplitude range was observed for extension and retraction for all the strokes for the experiments conducted with unworn seal. Therefore, only the AE signal analysed from the extension stroke is presented in the remaining analysis.

AE signal from experiment conducted using unworn seal, 20 bar pressure. a Five strokes. b One stroke. c Extension. d Retraction

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3.3

AE signal from different seal conditions

To understand the behaviour of the AE signal for unworn, semi-worn, and worn seals, the AE signal from the extension stroke was analysed. Figure 6 represents the AE signal obtained from extension stroke for the experiments conducted using unworn, semi-worn, and worn seals. The AE amplitude for the unworn seal is observed in the range of 0.1–0.2 V (see Fig. 6a). For the semi-worn and worn seals, the AE amplitude is observed in the range of 0.4–0.6 V (see Fig. 6b, c). From Figs. 4b and c and 6b and c, a good qualitative correlation can be observed between the leakage conditions and the AE signal behaviour. By comparing the AE amplitude from the unworn and worn seals, it is possible to identify non-leakage and leakage conditions of the seal flange. However, the AE signal due to leakage from the semi-worn seal and worn seal is not clear. Therefore, the AE signal is further analysed using different techniques.

AE signal from extension cylinder stroke for experiments conducted at 40 bar using a unworn seal, b semi-worn seal, and c worn seal

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3.4

AE analysis

3.4.1

AE time-domain features

Figure 7 represents the AE statistical features used to identify non-leakage and leakage conditions of the seal flange in the test rig. These AE statistical features were calculated using an average of three strokes. From the AE time-domain features such as mean, RMS, peak, and skewness, it was possible to separate non-leakage and leakage conditions of the seal flange. The behaviour of the AE features such as mean and RMS for the semi-worn and worn seals are nearly the same. From the AE statistical features for the unworn, semi-worn, and worn seal conditions (Fig.7), irrespective of the pressure conditions, the AE features such as mean, RMS, peak, and skewness are able to separate the non-leakage (unworn seal) and leakage (semi-worn and worn seals) conditions in the seal flange. The standard deviation from Fig. 7 is minimal for all the AE time-domain features. Therefore, the reliability of the AE features such as mean, RMS, peak, and skewness is high and can be used to identify non-leakage and leakage conditions of the seal flange.

AE statistical features. a Mean. b RMS. c Peak. d Kurtosis. e Skewness

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3.4.2

AE time-frequency analysis

The window size for the time-frequency analysis using the STFT technique depends on the type of application [14, 15]. Therefore, an attempt was made to determine the appropriate window size for the STFT analysis of the AE signal. Figure 8 represents the STFT analysis of the AE signal obtained from the semi-worn seal at the pressure condition of 30 bar, for the different window size of 64, 100, 128, 256, and 512. From Fig. 8a–e, it is evident that, with the increase in window size, the time resolution deteriorates as longer window size tends to average more compared with the shorter windows [16]. In a comparison of STFT plots for window sizes 100 and 128, the time resolution of the AE signal is good; however, the low-intensity peaks highlighted in Fig. 8b are not clearly visible in Fig. 8c. Also, with a further decrease in window size (Fig. 8a), the time resolution becomes coarse and computation time increases. Therefore, the window size of 100 is selected for the STFT analysis in this study.

Time-frequency analysis using STFT technique at window size of a 64, b 100, c 128, d 256, e 512. (Note: Window type for STFT analysis: Kaiser, seal: semi-worn, pressure: 30 bar)

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Figure 9 represents the time-frequency representation of the AE signal obtained from the test rig with unworn, semi-worn, and worn seals for the pressure conditions of 10, 20, 30, and 40 bar. From the Fig. 9a and d, for the unworn seal, we can observe that there are two frequency bands in the AE frequency range of 0–30 kHz and 50–100 kHz. These AE frequency bands are likely due to the events that occur due to bearing strips and piston rod seals in the cylinder head, as shown in Fig. 1c. Similarly, from the time-frequency plot for the experiments conducted using semi-worn seal (Fig. 9e–h) and worn seal (Fig. 9i–l), there are two AE frequency bands (0–30 kHz and 50–200 kHz). It is important to note that the power intensity in the AE frequency range of 0–30 kHz nearly remains the same for the tests conducted with unworn, semi-worn, and worn seal. The power intensity in the AE frequency range of 50–100 kHz in Fig. 9e–h and i–l is higher when compared with that in Fig. 9a–d. This indicates that, due to the wear in the piston rod seal, the power intensity increases. Therefore, it is possible to identify the non-leakage (unworn seal) and leakage (semi-worn and worn seal) conditions in the hydraulic test rig. However, from the qualitative observation of time-frequency plots in Fig. 9e–h and i–l, the difference is power intensity in the AE frequency range of 50–100 kHz of the leakage due to semi-worn seal and worn seal being not clear. Therefore, further AE analysis is required to understand the quantitative difference between unworn seal in test rig and leakage due to semi-worn and worn seal in the test rig.

Time-frequency analysis using STFT technique at pressure condition of 10, 20, 30, and 40 bar for a–d unworn seal, e–h semi-worn seal, and i–l worn seal

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3.4.3

AE frequency features

In Fig. 9, from the STFT plot, it is difficult to quantify the difference due to leakage from the semi-worn and worn seal. Therefore, AE power spectral density is calculated to quantify the difference between the non-leakage condition in the test rig and to quantify the difference between the leakage due to semi-worn seal and worn seal in the test rig. Figure 10 represents the AE power spectral density plot for the unworn, semi-worn, and worn seals. The magnitude of the AE frequency plot for the experiments conducted using unworn, semi-worn, and worn seals is nearly the same in the AE frequency range of 0–30 kHz for all the pressure conditions. Therefore, similar to Fig. 9, we can reconfirm that the AE frequency range of (0–30 kHz) is due to bearing event in the cylinder head and not due to the event occurring due to piston rod seal interaction. For all the pressure conditions, the magnitude of the frequency plot of worn seal > semi-worn seal > unworn seal is in the AE frequency range of 50–100 kHz. For the non-leakage condition (unworn seal), the maximum magnitude of the peak is ≈ 0.2e−6, and for the leakage condition with the semi-worn and worn seals, the maximum magnitude of the peak is ≈ 1e−6 and ≈ 1.2e−6 respectively. However, with increasing pressure, a small drop occurs in the value of the maximum magnitude of the peak for the leakage conditions with the semi-worn (≈ 0.8e−6) and worn seals (≈ 1e−6). This minor drop in the maximum magnitude of the peak may be due to variation in friction conditions at the piston rod seal and piston rod interface due to leakage. By using AE power spectral density feature, it is possible to identify the unworn seal condition, leakage due to semi-worn seal, and leakage due to worn seal. As it is time consuming to calculate and analyse the AE power spectral density for a large number of strokes that is typically observed in the industries, an attempt is made to further analyse using other AE frequency features (mean frequency, median frequency, and bandpower) that can be used for continuous monitoring of seal wear condition.

Power spectral density (PSD) plot for unworn, semi-worn, and worn seal at pressure condition of a 10 bar, b 20 bar, c 30 bar, and d 40 bar. (Note: For clarity the AE frequency range has been capped to 0–200 kHz)

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Figure 11 represents the AE frequency features calculated for each stroke for all the pressure conditions. The behaviour of AE mean frequency and median frequency feature is similar to that of AE time-domain features where it is possible to identify the non-leakage (unworn) and leakage conditions (semi-worn and worn). In Fig. 11b, the AE median frequency feature of the semi-worn and worn seal is nearly the same. In Fig. 11c, the AE bandpower of worn seal > semi-worn seal > unworn seals for all the pressure conditions. For the bandpower feature, the standard deviation is also minimal for the unworn, semi-worn, and worn seals and at each pressure conditions. Therefore, AE bandpower can be used for continuous monitoring of the piston rod seals in the hydraulic test rig.

Frequency domain features. a Mean frequency. b Median frequency. c Bandpower. (Note: Bandpower feature is calculated at frequency range of 0–200 kHz based on AE frequency distribution observed

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