Page 216 - An Introduction To Predictive Maintenance

206 An Introduction to Predictive Maintenance

Rubbing Wear. Rubbing wear is the result of normal sliding wear in a machine. During
a normal break-in of a wear surface, a unique layer is formed at the surface. As long
as this layer is stable, the surface wears normally. If the layer is removed faster than
it is generated, the wear rate increases and the maximum particle size increases. Exces-
sive quantities of contaminant in a lubrication system can increase rubbing wear by
more than an order of magnitude without completely removing the shear mixed layer.
Although catastrophic failure is unlikely, these machines can wear out rapidly.
Impending trouble is indicated by a dramatic increase in wear particles.

Cutting Wear Particles. Cutting wear particles are generated when one surface pene-
trates another. These particles are produced when a misaligned or fractured hard surface
produces an edge that cuts into a softer surface, or when abrasive contaminant becomes
embedded in a soft surface and cuts an opposing surface. Cutting wear particles are
abnormal and are always worthy of attention. If they are only a few microns long and
a fraction of a micron wide, the cause is probably contamination. Increasing quantities
of longer particles signals a potentially imminent component failure.

Rolling Fatigue. Rolling fatigue is associated primarily with rolling contact bearings
and may produce three distinct particle types: fatigue spall particles, spherical particles,
and laminar particles. Fatigue spall particles are the actual material removed when a
pit or spall opens up on a bearing surface. An increase in the quantity or size of these
particles is the first indication of an abnormality. Rolling fatigue does not always gen-
erate spherical particles, and they may be generated by other sources. Their presence
is important in that they are detectable before any actual spalling occurs. Laminar par-
ticles are very thin and are formed by the passage of a wear particle through a rolling
contact. They often have holes in them. Laminar particles may be generated through-
out the life of a bearing, but at the onset of fatigue spalling the quantity increases.

Combined Rolling and Sliding Wear. Combined rolling and sliding wear results from
the moving contact of surfaces in gear systems. These larger particles result from
tensile stresses on the gear surface, causing the fatigue cracks to spread deeper into
the gear tooth before pitting. Gear fatigue cracks do not generate spheres. Scuffing of
gears is caused by too high a load or speed. The excessive heat generated by this con-
dition breaks down the lubricating film and causes adhesion of the mating gear teeth.
As the wear surfaces become rougher, the wear rate increases. Once started, scuffing
usually affects each gear tooth.

Severe Sliding Wear. Excessive loads or heat causes severe sliding wear in a gear
system. Under these conditions, large particles break away from the wear surfaces,
causing an increase in the wear rate. If the stresses applied to the surface are increased
further, a second transition point is reached. The surface breaks down, and catastrophic
wear enses.

Normal spectrographic analysis is limited to particulate contamination with a size of
10 microns or less. Larger contaminants are ignored. This fact can limit the benefits
derived from the technique.