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Anthony Beeman
HW 6
Wear Category
(Major/subcategories)
Fatigue
Subsurface
Subsurface Initiated
Wear
Abrasive
Three-Body Abrasion
Two-Body Abrasion
Erosion
Cavitation
Problem 1
Description1
Subsurface fatigue is a form of wear that occurs after many cycles of high-stress flexing of the metal. This causes
cracks in the subsurface of the metal, which then propagate to the surface, resulting in a piece of surface metal
being removed.
This begins with reduced lubrication regime and a loss of the normal lubricant film. The oil film is reduced to
boundary or a mixed regime. Some metal-to-metal contact and sliding motion occurs. Surface damage occurs. The
high points of the metal surface asperities are removed, which initially appear as a matted or frosted surface. This is
not smearing, as in adhesion (discussed below). This type of surface damage is usually visible with a magnification
of three to five times.
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.
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.
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.
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Anthony Beeman
HW 6
Adhesive
Corrosion
Moisture
Frictional
Fretting Corrosion
Problem 1
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.
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 corrosioninduced 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 vibration between contacting
parts without any water contaminant being present, although humidity may be necessary. It may also be referred to
as fretting wear. It includes both fretting corrosion and false brinelling, which in the past were often considered to
be the same mechanism.
Fretting corrosion is the mechanical fretting wear damage of surface asperities accompanied and escalated by
corrosion, mostly oxidation in air with some humidity present. It occurs due to many oscillating micromovements
at contacting interfaces between loaded and mating parts in which the lubricant has not been replenished (an
unlubricated contact). Adhesion is occurring and it is generally considered more severe than false brinelling.
It usually appears as a reddish-brown oxide color (rust without water being present) on steel and black on
aluminum. Metal wear debris flakes are created or shed off.
Fretting corrosion occurs on many mechanical devices such as gear teeth and splines, not just rolling element
bearings, and can occur on surfaces other than the rolling contact. In bearings, it is also associated with bearing fit
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Anthony Beeman
HW 6
False Brinelling
Electrical Erosion
Excessive Voltage
Current Leakage
Plastic Deformation
Overload (True Brinelling)
Indents from Debris
Indents from Handling
Problem 1
on the shaft and in the housing. It occurs where there is not any large relative motion between the mating parts such
as between the shaft and the inner race and between the housing and the outer race. Fretting corrosion can occur on
materials that do not oxidize.
False brinelling occurs due to micromovements under cyclic vibrations in either static or rotating boundary
lubrication contacts. Mild adhesion of the metal asperities is occurring. Shallow depressions or dents are created in
which the original machining marks are worn off and no longer visible due to the wearing damage of the metal.
False brinelling occurs on the rolling elements and raceway, similar to small-scale plastic deformation or brinelling
(see below) and hence the name "false brinelling".
False brinelling is usually associated with static nonrotating equipment and, thus, the wear appears at the roller
contacts with the exact same spacing as the rollers. The depressions in the metal can appear shiny with black wear
debris around the edges. If the equipment is rotating, the wear appears as a gray, wavy washboard pattern on the
raceway. Reduced bearing life or failure ultimately occurs, sometimes in a catastrophic fashion, through surface
fatigue initiating in these damaged surface layers.
Excessive voltage (electrical pitting) is caused by a high electrical current or amperage passing through only a few
asperities on the metal. Voltage builds up and then arcs, causing localized heating/melting and vaporization of the
metal surface. This causes deep, large craters or pits in the metal surfaces, which may correspond to the spacing
between the rolling elements of the bearing. It is possibly due to welding 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.
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.
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Anthony Beeman
HW 6
Problem 1
References
1. Scott, Robert. Basic Wear Modes in Lubricated Systems. Machinery Lubrication. Accessed 4/18/2015.
http://www.machinerylubrication.com/Read/1375/wear-modes-lubricated
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