Machinery Lubrication July August 2008

LUBRICATION 101
Basic Wear Modes in
Lubricated Systems
BY ROBERT SCOTT, NORIA CORPORATION
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 wear that 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.
Surface Fatigue
Denting by
soft particles
Denting by
hard particles
Berm
Surface fatigue often begins by
denting due to hard or soft particles.
This creates a stress riser (berm).
Repeat high loading (stress
reversals) on berm or particles
causes surface fatigue and
eventually pits form. This leads to
larger pits, then spalls.
Fatigue
1.1 Subsurface
1.2 Surface Initiated
Wear
2.1 Abrasive
2.2 Adhesive
Corrosion
3.1 Moisture
3.2 Frictional
3.2.1 Fretting Corrosion
3.2.2 False Brinelling
Dent
Pit
Electrical Erosion
4.1 Excessive Voltage
4.2 Current Leakage
Plastic Deformation
5.1 Overload True Brinelling
5.2 Indents from Debris
5.3 Indents from Handling
Fracture
6.1 Forced Fracture
6.2 Fatigue Fracture
6.3 Thermal Fracture
Pitch Arc
Pitch Line
Fatigue Wear
High Risk Contacts:
rolling element
bearings, gear teeth
at pitchline,cams and
rollers.
Controlling Surface Fatigue:
• Increase film thickness
• Reduce surface roughness
• Maximize hardness
• Lower traction coefficient
• High pressure-viscosity
coefficient
• Avoid particle contamination
• Keep oil dry
Pits and dents disrupt
EHD film thickness
Not contained in the ISO classification is Erosion 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 direct metalto-metal
contact for wear to occur. Noncontact modes do
not require the surfaces to come into direct contact for 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 the bearing metal
below the surface. Subsurface microcracks form due to longterm
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 concentrated at a point below
the metal surface.
These microcracks normally propagate to the surface, which
eventually results in a piece of the surface material being
removed or delaminated. They appear as 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. Most
bearings 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
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