Dirt analysis case study: Gear box

Aug. 1, 2007

Location: Coal-fired power station
Description of sample: Gear oil from pulverizer gear box

Sample preparation: 20 ml of thoroughly mixed oil was diluted with heptane and drawn through a 0.45-micron nitrocellulose filter, followed by a thorough rinsing with heptane.

Analysis parameters: 100% of the exposed filter area was searched to find particles of 4 micron and larger using 15-keV beam energy, vacuum and a backscatter element detector.

Summary of findings: A total of 2,083 particles in eight elemental composition categories were identified on the filter (Table 5).

Mining the data

Table 6 shows only the key findings for a typical analysis. With 285 particles counted in 41 different categories of particle characteristics, there is far too much data to present it all here,. However, this table illustrates that the analyst can sort by variables of interest. In this case, the data is sorted on Dmax (descending). It’s clear the particles are:

  • Slightly oblong or rectangular (average aspect value around 2).
  • The particles are primarily iron and silicates.
  • The average size is 85 micron.

Summary of results: This analysis showed that gear wear was caused by three-body abrasion from contaminating silicates.

The SEM/AFA analysis for size and elemental composition is superior to conventional particle analysis methods. The method described can save you dollars and needless speculation.