Case History – Acceleration Enveloping for Early Malfunction Diagnosis of an Electrical Motor Bearing

Can Uslu │ Principal, PdMechanics

Introduction

Acceleration enveloping is an analysis method for the early detection of malfunctions that generate repetitive impacts due to metal-to-metal contact. The method is commonly used to diagnose developing bearing and gear faults. This text aims to demonstrate the benefits of acceleration enveloping by using real-life machinery data. Interested readers may refer to the white papers and articles available online to get more details about the technical background of acceleration enveloping. 

An important point that needs to be highlighted is that the vibration velocity and enveloped acceleration data are not alternative to each other but complementary for successful machinery diagnostics. As the enveloped acceleration data is adopted for the early detection of bearings and gear faults, vibration velocity is useful to detect various malfunctions over a wide range of machinery operating speeds. 

A Short Explanation of Acceleration Enveloping

Regarding a bearing with an early stage outer race surface defect, a low magnitude impact force is generated each time a rolling element rolls over the defect. This low magnitude impact force is repetitive and occurs at the bearing’s defect frequency. However, it is difficult to detect the signal generated by this repetitive impact with overall vibration velocity readings since the rotor’s running vibrations and the environment noise mask it. At this point, the acceleration enveloping method manages to diagnose the early stage malfunction. 

Acceleration enveloping uses the physical phenomena of a bearing’s vibration at its natural frequency (ringing) upon the rolling element’s pass over the surface defect. The ringing follows a particular pattern and repeats with each rolling element’s pass over the defect. The acceleration enveloping method searches for this repetitive high-frequency ringing in the vibration signal to diagnose a malfunction. The ringing is monitored in a high-frequency band, typically from 0.5 kHz to 10 kHz for roller bearings. The monitoring unit is acceleration, which is more suitable than velocity for measuring the high frequency vibration signals.

The Case 

This case took place in a cement plant that PdMechanics provides remote diagnostics services. It is a good example to compare the outputs of acceleration enveloping with overall vibration velocity readings. The vibration data belongs to a 1000 kW electrical motor of a grate cooler exhaust fan. Alarm thresholds of the electrical motor were generated by PdMechanics using the motor’s historical data and statistical tools. The plant’s existing standard condition monitoring system monitors the electrical motor’s overall vibration velocities. PdMechanics continuously measures both the raw vibration velocity and enveloped acceleration data for detailed diagnostics analysis. 

During the cement plant’s operation, the acceleration enveloping trends (gE-peak) monitored by PdMechanics recorded high accelerations and pointed an anomaly at the motor’s free end side (Figure 1). In the meantime, the plant operation team received no warning flag from their local standard condition monitoring system; the overall vibration velocities (mm/s rms) of the electrical motor bearing housings were stable and below the alarm threshold (Figure 2).  

Figure 1 – Drive End (—) and Free End (—) enveloped acceleration trends (gE-peak)

Figure 2 – Drive End (—) and Free End (—) vibration velocity trends (mm/s rms)

Upon detecting high acceleration enveloping data, PdMechanics engineers investigated the electrical motor’s enveloped frequency spectrum to see whether the recorded high acceleration data was repetitive and had a particular pattern. The enveloped spectrum in Figure 3 clearly showed peaks of bearing outer race defect frequency, which indicated that the bearing’s ringing was related to a surface defect at the outer race.  

Figure 3 – Free End (—) enveloped acceleration frequency spectrum (gE-peak)

Conclusion

The acceleration enveloping method is benefical for detecting early stage malfunctions of bearings and gears. PdMechanics reported the enveloped acceleration data to the customer with the diagnosis of a bearing defect. In this condition, the electrical motor was unsatisfactory for long-term continuous operation. PdMechanics recommended the bearing’s inspection and replacement in the soonest maintenance interval. With PdMechanics’ early malfunction diagnosis, the plant operation gained time to organize and perform a smooth maintenance activity and avoided any machinery damage. 

  • Post published:December 16, 2020
  • Post last modified:March 21, 2021