Vibrating Screen Exciter Gearbox – Bearing defects

Home / Vibrating Screen Exciter Gearbox – Bearing defects

Hi all,

How do you monitor vibrating screen exciter gearboxes for deterioration and reliability risks? Do any of you monitor vibrating screens exciters for bearing defects using routine vibration analysis, how do you cope with the harsh environment? Or do you just use oil analysis?

The screens in question are the in line type with two gearboxes with weights at the ends of the gearbox shafts, direct driven by a motor via carden shaft. These screens vibrate around 298mm/s RMS in the highest direction of motion.

I found a beauty of a defect when I was conducting a vibrating screen structure survey, I decided to collect the usual routine vibration data from the exciter gearboxes via a flat magnet mount and found an inner race defect! Site actually pulled it 2 days later due to increase in noise and temperature. This gearbox was only installed two weeks prior.

The unit was removed from service with the bearing in the early stages of failure, prior to catastrophic failure and secondary damage. I would be interested in others thinking for the root cause of this infant failure?

 

VA Data:

No historical data as this was a one off survey. The top plot is the PeakVue spectrum and this displays one order and harmonics together with a match for the bearing inner raceway defect frequency (BPFI).

The bottom plot is the PeakVue acceleration time waveform, and this displays dominant one order activity. In PeakVue this means that something is modulating at 1 Order i.e. Inner Race defect.

 

Bearing Images:

The long and short shaft fixed bearing had an inner race localised spalled area at the inner ring centre shoulder, on one side of the raceway. Failure due to flaking of the inner raceway. I think the most likely cause is ISO 15243:2004 – 5.1.2 subsurface initiated fatigue due to overloading (Axial shock load).

The above is the short shaft fixed bearing.

The above is the long shaft fixed bearing.

I believe these images show rolling fatigue flaking that may be caused early by over-load, excessive load due to improper handling, poor shaft or housing accuracy, installation error, ingress of foreign objects, rusting, etc.

Root Cause:

As for the cause of subsurface initiated fatigue is, among other things, caused by surface distress. Under the influence of loads in rolling contacts, described by the Hertzian Theory, structural changes will occur and micro-cracks will be initiated at a certain depth under the surface i.e. subsurface.

There are another two major causes of bearing flaking; (1) Fatigue Life and (2) Improper Handling. (1) Fatigue Life: This is discounted as the cause as the bearing has only been running for two weeks before being removed from service. (2) Improper Handling: There are no signs of any ‘True Brinelling’ with marks on the inner raceway equal to the distances between the rolling elements.

So what is your opinion of the root cause?

3 thoughts on “Vibrating Screen Exciter Gearbox – Bearing defects”

  1. The first thought when I saw the spall, it is across the raceway. That is unusual, spalls usually are inside the raceway. That it occurred within two weeks of installation leads me to believe it is an installation error of some type. Wrong installation tools or wrong application of the tools. And I would take a look at how and where the bearings are stored as part of my investigation.

  2. What are the shaft diameters? If the bearings were not creeping, can you id the spall positions in relation to counter weights? The origins of both spalls seem to be at the edges of rolling elements, are they on outboard or inboard raceway? What is the condition of rolling elements edges which were in contact with spall onset?

  3. I agree with the possible commissioning issue as well as how long since it was assembled until it was used? Meaning the bearing did not turn and may have had some fare amount of external forces and a dry spot.
    Bearing race fatique can also be due to old bearing being re-used.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: