Screen Exciter Gearbox Bearing BPFI

Background:
On 4th September 2015 vibration analysis was conducted on a screen structure and during this survey routine data was collected from the exciter gearbox. This routine data revealed a defect with the bearing inner raceway. The gearbox was changed on the 7th September 2015 after it was noticed on site that the unit had become noisy and hot.
Executive Summary:
The unit was removed from service with the bearing in the early stages of failure, prior to catastrophic failure and secondary damage.
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. The most likely cause is ISO 15243:2004 – 5.1.2 subsurface initiated fatigue due to overloading (Axial shock load).
Assessment:
The inspection and disassembly of the bearings revealed the following findings on the inner raceway:
• Single area of flaking on both the fixed bearings
• Flaking on one side of the spherical roller bearing, on the outside face of the gearbox
• Initiation site of the flaking is in the highest loaded section of the inner raceway
Taking into account the findings and the background information the most probable cause of bearing failure is subsurface initiated fatigue due to ‘Axial shock overloading and/or axial misalignment’ and ‘Point loading exceeding the designed load capability of the bearing’.

Further Information:
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 microcracks 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.

Findings:
Images and observations of the FAG X-Lite 22326-E1-T41D bearing.

Short shaft fixed bearing.

Short shaft fixed bearing defect.

Long shaft fixed bearing.

Long shaft fixed bearing defect.
Recommendations:
Rolling fatigue flaking 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.
Solution options are:
• Find the cause of the heavy load and resolve.
• Examine operating conditions and adopt bearings with larger capacity as necessary
• Increase viscosity of oil and improve lubrication system to form an adequate lubricating film.
• Eliminate installation errors.

VA Data.
The top plot is the PeakVue spectrum and this shows 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 shows dominant one order activity.