When the curiosity of a Vibration Issue Pays Off

 

When the curiosity of a Vibration Issue Pays Off

Case History by Robert Smith 

 

Primary Regrind Mill Broken Gear

 

Introduction

 

I had only been in my Reliability Superintendents role for a few months and I was in the process of setting up a Vibration Analysis Program. This involves looking at all plant locations and the critical of equipment.

 

As I was inspecting the exterior of this vertical mill gearbox I could hear what I would describe as a clicking sound. The sound did not sound like a high-energy impact but I was curious. I asked a maintenance person working in the area who explained the clicking noise had been there for a while and they did not think it was an issue. The vibration database was not set up at this time for this area of the plant.

 

To get a reasonable idea of the frequency of the impacts I used a stopwatch. I would count 50 low energy impacts and stop the timing. The timing would vary and remember I was only doing this for basic information. The 50 impacts would be counted in 19.3 seconds. 19.6 seconds etc. So 2.6 to 2.5 impacts per second. So we are looking at the rotating part in question at approximately 158 CPM or maybe slightly higher. This information was used to assist in setting the frequency range for vibration measurement to determine the cause.

REGRIND MILL GEARBOX

The next step was to look through the OEM manual. I calculate the second reduction of this gearbox was rotating at 160 CPM. I set up this machine in the vibration database to cover velocity and acceleration in both spectrum and time waveform. There is one reading, demodulation, or enveloping that is sensitive to low energy impacts. I set the demodulation /enveloping to take both spectrum and Time Wave Form

 

Findings

The vibration data analyzed in velocity and acceleration appeared normal. Taking demodulation Time Waveform at the second reduction stage indicated low energy impacts were evident at approximately 160 CPM The impacting energy levels were low.

From the drawings of the gearbox there appeared to be a reasonable distance between the second reduction gearing rotating at 160 CPM and the casing. Then there was the thickness of the casing itself. Were these factors masking the true energy of the impacts?

 

Inspection

As can be seen from the picture above there is an inspection port at the second gear reduction. A works order was submitted to inspect the gearbox's internal gearing at the next scheduled shutdown.

The Mechanical Supervisor called me when the hatch was opened and said “You have to see this” There was one gear tooth completely sheared off. By luck when the Mill has shut down the view of the broken tooth was directly in line with the inspection port.

The teeth on the rest of the gear looked in immaculate condition.

View 1 broken gear tooth

View 2 broken gear tooth

                                                                                                     

Conclusion

Vibration analysis is a great tool in any predictive maintenance program. If vibration data indicates an issue and the

impact energy is low, additional investigation is required. Simply any impacting no matter what the amplitude, should not be present.

It should mention that demodulation /enveloping is very sensitive to low energy impacts. In relation to bearings, subsurface defects can be picked up. All other vibration data should be analyzed before making an assessment or recommendation.

In this instance demodulation / enveloping proved to be the best parameter.

I hope this article will assist people new to vibration analysis.

Contacts

Email: robbieccs@gmail.com