Machinery Lubrication July August 2008

oughened surfaces cause cutting and
damage to a mating surface that is in relative
motion to the first.
Three-body abrasion occurs when a relatively
hard contaminant (particle of dirt or
wear debris) of roughly the same size as the
dynamic clearances (oil film thickness)
becomes imbedded in one metal surface and
is squeezed between the two surfaces, which
are in relative motion. When the particle size
is greater than the fluid film thickness,
scratching, ploughing or gouging can occur.
This creates parallel furrows in the direction
of motion, like rough sanding. Mild abrasion
by fine particles may cause polishing with a
satiny, matte or lapped-in appearance. This
can be prevented with improved filtration,
flushing and sealing out small particles.
Two-body abrasion occurs when metal
asperities (surface roughness, peaks) on one
surface cut directly into a second metal
surface. A contaminant particle is not directly
involved. The contact occurs in the boundary
lubrication regime due to inadequate lubrication
or excessive surface roughness which
could have been caused by some other form
of wear. Higher oil viscosity, increased metal
hardness and even demagnetizing bearings
after induction heating during installation
may help to reduce two-body abrasion.
Adhesive Wear
Adhesive wear is the transfer of material from
one contacting surface to another. It occurs
when high loads, temperatures or pressures
cause the asperities on two contacting metal
surfaces, in relative motion, to spot-weld
together then immediately tear apart, shearing
the metal in small, discrete areas.
The surface may be left rough and jagged or
relatively smooth due to smearing/deformation
of the metal. Metal is transferred from one
surface to the other. Adhesion occurs in equipment
operating in the mixed and boundary
lubrication regimes due to insufficient lube
supply, inadequate viscosity, incorrect internal
clearances, incorrect installation or misalignment.
This can occur in rings and cylinders,
bearings and gears.
Normal break-in is a form of mild adhesive
wear, as is frosting. Scuffing usually refers to
moderate adhesive wear, while galling,
smearing and seizing result from severe adhesion.
Adhesion can be prevented by lower
loads, avoiding shock loading and ensuring
that the correct oil viscosity grade is being
used. If necessary, extreme pressure (EP) and
antiwear (AW) additives are used to reduce
the damage.
Corrosion
Moisture corrosion involves material
removal or loss by oxidative chemical reaction
of the metal surface in the presence of moisture
(water). It is the dissolution of a metal in
an electrically conductive liquid by low
amperage and may involve hydrogen embrittlement.
It is accelerated, like all chemical
reactions, by increased temperatures. No
metal-to-metal contact is needed. It will
occur with a full oil fluid film.
Corrosion is often caused by the contamination
or degradation of lubricants in service.
Most lubricants contain corrosion inhibitors
that protect against this type of attack. When
the lubricant additives become depleted due
to extended service or excessive contamination
by moisture, combustion or other gases
or process fluids, the corrosion inhibitors are
no longer capable of protecting against the
acidic (or caustic) corrosive fluid and corrosion-induced
pitting can occur. The pits will
appear on the metal surface that was exposed
to the corrosive environment.
This may be the entire metal surface or just
the lower portion of the metal that may have
been submerged in water not drained from the
oil sump or at the roller/race contact points.
Generally, an even and uniform pattern of pits
will result from this form of attack. Mild forms
of moisture corrosion result in surface staining
or etching. More severe forms are referred to as
corrosive pitting, electro-corrosion, corrosive
spalling or rust.
Frictional corrosion is a general form of
wear caused by loaded micromovements or
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