Introduction
This case study describes a failure in a gearbox driving a cement mill in the Natal Portland Cement plant in South Africa.
According to the customer, cement particles were found in the gearbox lubricant.
In December 2023, the DMGH 22 gearbox was removed from the mill for cleaning and replacing all bearings.
Conclusion and summary
The gearbox damage was the result of contaminated oil. The cement particles adversely affected bearing lubrication, hence damaging the bearings. Consequently, the gears did not work properly and were also damaged.
The results of this case study show that HD technologies combined with traditional vibration measurements are ideal for detecting gearbox problems such as gear mesh and bearing element wear at early stages.
For successful gearbox monitoring, SPM recommendations are to use the Intellinova online monitoring system with one sensor per shaft, with the following measuring assignments set up on each of them:
Bearing condition assignments:
- VEL
- HD ENV, Filter 3
- HD ENV, Filter 4, or SPM HD
Gear condition assignments:
- VEL TSA
- HD ENV, Filter 3 with TSA (Time Synchronous Averaging)
Application description
The mill is a crucial part of the cement manufacturing process. In most cases, the entire process could be severely limited or even completely stopped if the mill stops operating. Therefore, close observa- tion of the mill’s condition is essential. It is imperative to avoid unplanned stops at almost any cost.
Image 1 A typical cement mill.
Image 2 The DMGH 22 gear unit on the cement mill.
System setup
Measuring equipment
Measurements were taken using a Leonova Diamond handheld analyzer with a magnet-mounted ac- celerometer and a shock pulse transducer with a probe. This report is intended to show the power of SPM’s patented measurement technologies, SPM HD and HD ENV, and the value of long pre- warning times (in terms of corrective maintenance practices) when these methods are used with the Intellinova online condition monitoring system.
Figure 1 Section of the gearbox and location of shock pulse and vibration sensors at critical points.
Measurement technologies
The following measurement techniques were used:
- SPM HD
- HD ENV Filter 4 (5000 – 40000 Hz)
- HD ENV Filter 3 (500 – 10000 Hz)
- Acceleration
- Velocity
Condmaster setup
The following measurement configurations were used for the respective measuring techniques.
Image 3 SPM HD measuring assignment.
Image 4 HD ENV Filter 4 measurement.
Image 5 HD ENV Filter 3 measurement.
Image 6 Acceleration measurement.
Image 7 Velocity measurement.
Gear damage on input and intermediate shafts
The root cause of this damage was cement particles entering the gearbox.
During a planned machine stop, the gearbox was opened and inspected. The internal parts showed signs of wear, and the output shaft bearings had significant wear. The girth gear seal had a visible oil leak. The cause of the wear on the internals and bearings was contaminated oil.
The acceleration spectrum analysis revealed signs of eccentricity and backlash. The 2nd, 3rd, and 5th gear mesh frequency harmonics dominate the spectrum.
Image 8 The acceleration spectrum from the input shaft shows symptoms of eccentricity and backlash.
Gear backlash also generates shaft speed sidebands around the gear mesh frequency. Signs of gear wear were identified using the HD ENV Filter 3 spectrum; see Image 9 below.
Image 9 The HD ENV Filter 3 spectrum from the input shaft shows symptoms of gear wear.
In the envelope spectrum, the sidebands correspond to the rotational speed of the worn gear on the intermediate shaft (8,5 Hz).
This problem is clearly identified in the SPM HD spectrum; see screenshot below.
Image 10 The SPM HD spectrum showing symptoms of gear wear on the input shaft.
Image 11 Time signal clearly showing a modulated gear mesh period on the input shaft.
Image 12 Contamination in the base of the unit.
Image 13 Wear on the high-speed pinion tooth flank.
Signs of wear on the intermediate shaft gear were detected in the velocity spectrum; see below.
Image 14 The velocity spectrum from the intermediate shaft showing symptoms of gear wear.
Gear mesh frequency with sidebands at intermediate shaft speed (8,5 Hz) apart.
Image 15 Wear found on 1st stage gear teeth.
Economic justification
The combination of vibration and SPM HD measurements is ideal for this type of unit. Shock pulse technology is well-suited for detecting bearing damage in “noisy” environments, such as the gearbox in this case. Vibration technology, on the other hand, is optimal for troubleshooting lower-frequency issues, such as imbalance, loose gears, and misalignment.
Thanks to the sensitivity of SPM’s HD technologies and the long pre-warning times, maintenance actions could be planned well in advance.
