Machinery Lubrication July August 2008

in the area and inadequate grounding or insulation.
It may also be referred to as electrical
pitting, arcing or sparking.
Current leakage (electrical fluting) is a less
severe form of damage caused by a lower
continuous electrical current. The damage
may be shallow craters that are closely positioned
and appear dark gray in color. If the
electrical discharge occurs while the bearing is
in motion, with a full fluid film, a washboard
effect or grooves appear on the entire bearing
raceway and is called fluting or corduroying.
Plastic Deformation
This is the denting, indentations or depressions
in the race or rollers caused by impact or
overloading. The surface metal flows, causing
irreversible deformation (not wear). The
machining marks are still visible in the bottom
of the dent. The dents often have a raised lip
which increases stresses and leads to surfaceinitiated
fatigue (surface cracks) and eventual
pit formation or adhesive wear. Plastic deformation
consists of three subcategories.
Overload or true brinelling is characterized
by static or shock loading, or impact from
operational abuse, causing a permanent dent
in the metal without cutting or welding of the
metal. An example occurs in roller bearings
when impact causes the rollers to create a
series of dents in the bearing race surface at
intervals that match the roller spacing exactly.
Some people consider denting from the
impact of hammering on a bearing as overload;
others may consider it as an indentation
from handling.
Indentation from debris is a form of plastic
deformation but it is caused by a particle
trapped within the dynamic clearances
between two machine elements and being
over-rolled. The force causes a round-bottom
dent to form in the race or rolling element.
Cracks may propagate down into the metal.
Indentation from handling is similar to
that from debris, but results from a bearing
being dropped or hammered, causing localized
overloading. It can also be due to nicks
from hard or sharp objects.
It is common to encounter erosion from
particles in the oil and cavitation, although
this is not included in the ISO standard for
rolling bearings.
Erosion
Erosion could be considered a form of abrasive
wear. It occurs principally in high-velocity,
fluid streams where solid particle debris,
entrained in the fluid (oil), impinges on a
surface and erodes it away. Hydraulic systems
are an example where this type of wear may
occur. Flow rates have a significant influence
on these wear rates, which are proportional to
at least the square of the fluid velocity. Erosion
typically occurs in pumps, valves and nozzles.
Metal-to-metal contact does not occur. The
mechanism of erosion is used to an advantage
in water-jet cutting.
Cavitation
This is a special form of erosion in which
vapor bubbles in the fluid form in low-pressure
regions and are then collapsed
(imploded) in the higher-pressure regions of
the oil system. The implosion can be powerful
enough to create holes or pits, even in hardened
metal if the implosion occurs at the
metal surface. This type of wear is most
common in hydraulic pumps, especially those
which have restricted suction inlets or are
operating at high elevations.
Restricting the oil from entering the pump
suction reduces the pressure on the oil and,
thus, tends to create more vapor bubbles.
Cavitation can also occur in journal bearings
where the fluid pressure increases in the load
zone of the bearing. No metal-to-metal
contact is needed to create cavitation.
Just to be clear, pitting is a general term
used in failure analysis to describe almost any
small, rough-bottomed, circular potholes in
the metal surface. Pits can be caused by
mechanical pitting (fatigue or cavitation),
chemical pitting (corrosion) or by electrical
pitting (stray arcing), all of which are
described above.
Machinery Lubrication machinerylubrication.com July - August 2008 39