basic wear modes in lubricated systems

8/4/2019 Basic Wear Modes in Lubricated Systems

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Basic Wear Modes in

Lubricated SystemsRobert ScottTags:industrial lubricants

This article provides a basic definition and

understanding of the major wear modes or

mechanisms based around the ISO

15243.2004 rolling bearing failure mode

classification. Several other modes of wearthat occur in gears, journal bearings,

hydraulic pumps and pistons - but don't

occur in rolling bearings - will be

discussed.

The ISO system discusses wear in six

major categories with 15 subcategories.

Not contained in the ISO classification isErosion from particles and Cavitation.

Wear mechanisms can also be thought of

as occurring in two separate categories:

contact and noncontact modes. Contact

wear requires the components to have
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direct metal-to-metal contact for wear to

occur. Noncontact modes do not require

the surfaces to come into direct contact for


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them to wear; in other words, a full fluid

lubricant film may exist.

Subsurface Fatigue

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.

It begins with inclusions or faults in thebearing metal below the surface.

Subsurface microcracks form due to long-

term repeated load cycles and stress

(500,000 psi), causing elastic deformation

(flexing) of the metal. This is typical in all

rolling bearing elements and races and

gear teeth, all of which operate in the

elastohydrodynamic (EHD) lubrication

regime. The contact stress is concentratedat a point below the metal surface.

These microcracks normally propagate to

the surface, which eventually results in a

piece of the surface material beingremoved or delaminated. They appear as


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surface damage or wear (large pits)

referred to as spalling. Other terms for

subsurface fatigue include flaking, peeling

and mechanical pitting. A full oil film exists

and no metal-to-metal contact or surface

damage is needed. Subsurface fatigue is

not a common issue if better quality metals

are used in bearing manufacture. Mostbearings will fail by another mechanism

first.

Subsurface fatigue failure is the result of a

bearing living out its normal life span

based on the load, speed and lubricant film

thickness that it is exposed to. The L10

fatigue life of a bearing is the average time

(in hours or cycles) to fail 10 percent of aset of identical bearings under certain


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conditions. An estimate of the L10 life can

be calculated, providing a rating life of a

bearing.

Surface-initiated Fatigue

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 metalsurface asperities are removed, which


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initially appear as a matted or frosted

surface. This is not smearing, as in

adhesion (discussed below). This type ofsurface damage is usually visible with a

magnification of three to five times.

The surface damage is coupled with the

cyclic loading of the rollers rolling over the

race. This creates asperity microcracks and

microspalling. The cracks start at the

surface and migrate down into the metal.

An edge of metal is created at the surfacewhich flexes at the edge of the surface

crack. This creates a cold worked edge

which is lighter in color. The cracks

propagate and may intersect within the

metal, and a piece of surface material is

then removed. Flaking, mechanical pitting

and micropitting are other names used to

describe spalling.

Surface fatigue can also occur as a result

of plastic deformation (described below).

Contaminant particles in the oil enter the

high-load rolling contact area between

rollers and the race, or between gear

teeth, and cause some form of surface


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damage - a dent. Improper handling of

bearings can cause similar surface

damage.

These round-bottomed dents often have a

raised berm around their edges. The raised

berm of metal acts as a point of increased

load or stress, or creates a reduced

lubrication regime (mixed or boundary),

and leads to a lower surface fatigue life.

Improved filtration reduces plastic

deformation, and therefore indirectlyreduces the occurrence of surface fatigue.

Notice that the term "contact fatigue" is

not used by ISO. This is a vague term

sometimes used to describe both forms of

fatigue. It does not specify whether metal

flexing damage started in the subsurface

or from some initial surface damage. It

encompasses any change in the metalstructure caused by repeated stresses

concentrated at a microscopic scale in the

contact zone between the rolling elements

and raceways, and between gear teeth.


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Abrasive Wear

Abrasive wear is estimated to be the most

common form of wear in lubricatedmachinery. Particle contamination and


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roughened 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 filmthickness, 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


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inadequate lubrication or excessive surface

roughness which could have been caused

by some other form of wear. Higher oilviscosity, 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


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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, incorrectinternal 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 bylower 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.


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Corrosion

Moisture corrosion involves material

removal or loss by oxidative chemical

reaction of the metal surface in thepresence 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.


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Corrosion is often caused by the

contamination or degradation of lubricants

in service. Most lubricants containcorrosion 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-inducedpitting 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, aneven 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.


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Frictional corrosion is a general form of

wear caused by loaded micromovements or

vibration between contacting parts withoutany 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 asperitiesaccompanied 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.


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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 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 donot 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 offand 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".


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False brinelling is usually associated with

static nonrotating equipment and, thus,

the wear appears at the roller contactswith 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 surfacelayers.

An example of false brinelling occurs in

standby electric motors and pumps (and

others) which sit idle for periods of time,

but are subjected to vibration from the

plant floor up through the load-bearing

rolling elements of the bearings. Antiwear

additives may be beneficial in reducing thewear damage.


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Electrical Erosion

This type of wear occurs when electriccurrent passes between two metal surfaces

(for example, bearing roller and race)

through the oil or grease film. It is

subdivided based on the severity of the

damage. Electrical erosion should not be

confused with erosion caused by particles

(discussed below).


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Excessive voltage (electrical pitting) is

caused by a high electrical current or

amperage passing through only a fewasperities 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 mayalso 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 occurswhile 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.


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Plastic Deformation

This is the denting, indentations or

depressions in the race or rollers caused byimpact 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 surface-initiated

fatigue (surface cracks) and eventual pit

formation or adhesive wear. Plastic

deformation consists of threesubcategories.

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 seriesof 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.


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Indentation from debris is a form of

plastic deformation but it is caused by a

particle trapped within the dynamicclearances 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.

ErosionErosion could be considered a form ofabrasive 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


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wear may occur. Flow rates have a

significant influence on these wear rates,

which are proportional to at least thesquare 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 highelevations.

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


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journal bearings where the fluid pressure

increases in the load zone of the bearing.

No metal-to-metal contact is needed tocreate 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 ofwhich are described above.

Failure analysis is used to assign a wear

mechanism to a specific failure. If the wear

mechanism can be determined, then some

corrective action can be applied to prevent

the failure from recurring. Often, it can be

useful to use the process of elimination to

determine which wear mechanisms couldnot have produced the observed wear

pattern, thus reducing the number of

possible mechanisms. Unfortunately,

combinations of wear mechanisms exist in

most situations, thus complicating the


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selection of the optimum wear-resistant

system.

Acknowledgment

Several portions of this article may contain

residual wording from an article that was

originally written by Rees Llewellyn of the

National Research Council of Canada for

the Alberta section of the Society of

Tribologists and Lubrication Engineers

(STLE).

basic wear modes in lubricated systems

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