||Need for Cultural Change
||Introduction to Systematic
||Perspective on Testing
and Oil/Material Performance
||Key Parameters Controlling
Lubrication and Failure Mechanisms
||Need for Cultural Change
Considering the complexity of tribo-contact systems in service,
one can conclude that future progress in high performance rotating
mechanical systems requires a cultural change in the design
and development process. In the past, substantial advancements
were made with deliberate, but mostly independent, developments
in component designs, materials and lubrication systems. With
continuing demands for increased performance in fuel economy,
power density, cost of ownership, environmental concerns, safety
and development time, major advances in complex mechanical systems
come with increasing developmental risks.
In a highly integrated mechanical drive or engine system,
materials, lubricants and designs must live within numerous
constraints and conflicting requirements. While modeling and
computer-aided design continue to make progress, certain performance-critical
phenomena, that have been missing from the design and development
process, are now a barrier to future success. As we have experienced,
the life and durability of highly stressed bearing and gear
surfaces rely on micro-scale physical and chemical mechanisms.
For these micro-scale mechanisms, predictive design tools
and development approaches are outside traditional engineering
parameters and test methods. The combined actions of component
design, material development and lubricant formulation result
in a melting pot of dynamic interactions that profoundly control
mechanical system performance. Advancing performance, without
leaving critical system attributes behind, is a community
affair. Traditional technical disciplines, business entities
and professional standards organizations are not aligned to
accomplish this type of community action. A new approach is
needed to move forward in the twenty-first century.
||Introduction to Systematic Tribology
||Tribology mechanisms are the lifeblood
of major mechanical systems in aerospace and other industries.
Over a decade of experience in material/lubricant development
and problem solving reveals the enormous risk that companies
have been taking with new ventures into major mechanical systems
that require years to develop. The inability to succeed in the
anticipated tribology performance of a design is a painful experience
that devastates the confidence to reach out into future ventures.
The root cause of the problem lies in the ability to capture
lubrication mechanisms in design and to assure the working of
these mechanisms in service. It is essential that tribology
mechanisms be assured prior to the execution of fabrication
and performance testing of the final system.
Tribology issues must be addressed and validated up front
as part of the design process. Compared to other engineering
and design disciplines, the level of capability for tribology
design and testing is primitive. There is a general lack of
appreciation for the multitude of lubrication and failure
mechanisms that control durability at the contact interface
of major mechanical systems. In aerospace and other industries,
material enhancements for surface fatigue are frequently accompanied
by an offset in abrasive or adhesive wear. High thermal stability
oils frequently come at the expense of wear and scuffing performance.
Unknowingly, when one tribology attribute is sacrificed for
a gain in another, the net result can be disastrous.
It is recognized at the outset that when chemical and material
interactions are in the equation for life and durability,
engineering design for bearings, gears and other mechanical
components is not a rigorous or high precision process. It
is further recognized that, while tribology mechanisms operate
on a micro-scale, we must cope with the requirement of translating
micro-scale processes into engineering scale parameters. The
path to success is through mechanistic testing that captures
an empirical link to design and performance testing. This
process is called "Systematic Tribology". Systematic
Tribology treats the tribo-contact as a system of technologies,
which can be systematically developed to capture tribology
mechanisms that are assured to provide durability in the application
system. The goal of Systematic Tribology is to leave no attribute
||Perspective on Testing and Oil/Material
||The search for an oil qualification
test method and performance limits leads us to conclude that
a rational solution can only come from a broader perspective.
The broader perspective must encompass the fact that the oil
is intimately connected to a complex tribo-system where its
performance is controlled by multiple parameters. These parameters
include temperature and the degree of asperity encounter. Single
tests are not likely to reveal the true picture of lubricating
performance or performance limits. The broader perspective must
also recognize the importance of materials and their surface
attributes that affect chemical response under various thermal
and stress conditions. Materials and oils provide the ingredients
for the tribo-contact system. The dynamics of this melting pot
can take it to the limits of many types of boundaries within
the domain of operation. These boundaries include wear, scuffing
and fatigue mechanisms, which are interactive and competitive
processes in service.
The need for rational material testing is just as urgent
as oil testing. Oil performance cannot be divorced from material
performance. As developments in bearing and gear materials
continue, oil testing must accommodate these changes. Future
changes are anticipated with the introduction of corrosion
resistant materials and hybrid ceramic/metal material pairs.
The broader perspective must also recognize that oil and
material testing is a multilevel activity. While there is
an urgent need for better oil qualification tests, the need
for development, performance prediction (design) and trouble-shooting
tests are equally urgent. The tribology risk in introducing
new materials and oil formulations is a hidden enemy that
strikes too frequently and discourages innovation. The rationale
behind the approach to qualification testing should be the
same as material and oil development tests, tests for design
criteria and tests for performance prediction. The rationale
for oil qualification testing is really the same as higher
level testing. Oil qualification tests should be derived from
higher-level tests, or at least the rationale behind higher-level
From our limited knowledge of failure mechanisms in service
hardware, we can assume that the general categories of wear,
scuffing and fatigue are relevant failure processes for oil
evaluation and performance qualification limits. For high-speed
aircraft propulsion components, we assume the relevant lubrication
mechanisms are hydrodynamic, elastohydrodynamic (EHD) and
boundary (surface film) mechanisms. If this is the case, relevant
tests must encompass the above lubrication and failure mechanisms.
While we may have assumed or perceived notions about how these
mechanisms are at work in service, experience illustrates
how testing methods can be used to invoke these performance
critical mechanisms. The missing link to this is an understanding
of how the oil functions mechanistically in service hardware
or advanced hardware destined for future service.
Without this knowledge, we can still move forward in the
meantime by identifying the key parameters controlling lubrication
and failure mechanisms. Of particular importance are the lubrication
related parameters linked to oil "properties" and
"attributes". Oil "properties" are inherent
and measurable features of the oil. Examples are the physical
properties of viscosity, pressure-viscosity coefficient and
traction coefficient. Oil "attributes" are characteristics
of the oil as revealed in an operating tribo-system. Examples
are boundary lubrication attributes for wear resistance and
scuffing resistance. These attributes are generally associated
with the chemical characteristics of the oil.
Since EHD film generation is directly linked to viscosity
and pressure-viscosity coefficient, these properties become
fundamental to oil performance. Pressure-viscosity coefficient,
which is currently absent in specifications, is essential
for performance. Traction coefficient, which varies with stress
and temperature, is important with respect to heat generation
and contact temperatures. While viscosity, pressure-viscosity
coefficient and traction coefficient are fundamental properties,
they are not sufficient for performance prediction or oil
qualification. In operating contact systems these parameters
set the stage for life and durability. The final control of
life and durability is derived from chemical attributes of
the oil and their response to the contacting materials and
operating environment. Consequently, these chemical attributes
are the missing link to performance prediction and oil qualification.
They cannot be evaluated independently of the tribo-contact
system. They are part of the system which is controlled by
key parameters. It is the identification of these key parameters
and their domain of operation in service that is required
to develop rational test methods that can be linked to service
||Key Parameters Controlling Lubrication and
||For the "primary" load
supporting and high stress contacts in high-speed bearings and
gears, the EHD mechanism is the "miracle mechanism"
for life and durability. The key to unlocking the mysteries
behind oil physical properties and chemical attributes is the
identification of the functional regions of a lubricated contact.
The EHD contact can be divided into three regions. The inlet
region is the convergent section upstream of the high pressure
Hertzian contact region. The inlet region is the functional
region for EHD pressure generation, which has rigorous mathematical
foundations for predicting film thickness within the Hertzian
region. The dynamic motions within the inlet region pumps the
film up. The Hertzian region rides the film, and the exit region
discharges the film. The decoupling of these functional regions
is the secret behind WAM testing machines and testing methods.
Oil and its viscosity and pressure-viscosity properties within
the convergent inlet region control EHD film thickness in
a reliable and predictable fashion. The shear and traction
of the pressurized film within the Hertzian contact region
gives rise to heat generation, which is dissipated within
the contacting bodies. For surface life and durability, the
preservation of the EHD or micro-EHD mechanism is essential.
The primary purpose of oil chemical attributes is to preserve
the EHD mechanisms as much as possible. Loss of "surface
integrity" is the first step toward wear and scuffing
failure mechanisms. While EHD mechanisms are essential for
life and durability, chemical attributes for boundary lubrication
mechanisms are equally essential. Chemical attributes play
a critical role in preserving surface integrity. The processes
of physical adsorption and chemical reaction heal and protect
against local adhesion and disruption of the surface topography.
Tribo-contact systems, which have oil properties for EHD film-forming
capability and chemical attributes for film-forming ability
working together, can achieve remarkable levels durability.
The decoupling of the inlet region EHD lubrication functions
from the Hertzian region boundary and micro-EHD mechanisms
provides a means to identify the key parameters associated
with lubrication mechanisms and failure mechanisms. Failure
mechanisms of wear, scuffing and surface initiated fatigue,
are the result of micro-scale events associated with roughness
features. The level of EHD film thickness relative to surface
roughness height determines the normal stress at asperity
sites. If roughness features do not "run-in" or
have exhibited plastic flow, high normal stresses with repeated
contact cycles can result in surface initiated fatigue or
"frosting". Since EHD film thickness is a function
of viscous properties in the inlet region and the entraining
velocity (Ue), the entraining velocity becomes a key performance
The sliding velocity within the Hertzian region determines
the tangential strain at asperity sites. The tangential stress
at these sites is directly connected to the frictional conditions
created at asperity sites. The chemical or physical processes
that occur at the local interface between roughness features
determine the frictional conditions. These processes can be
highly sensitive to local temperatures. Since sliding velocity
controls the strain at asperity sites, as well as frictional
heating, sliding velocity becomes a key parameter.
The material response to normal and tangential stress and
its reaction with oil chemistry will determine the outcome
on asperity encounters. High normal stresses at asperity sites
can lead to a distressed or frosted surface deterioration.
Tangential stress and strain at asperity sites can lead to
polishing wear, "corrosive" wear or adhesive wear.
Tangential stress and strain under high-speed conditions,
and without sufficient chemical response at the interface,
can lead to scuffing failure. In some contact situations,
wear, scuffing and fatigue may occur simultaneously. The dominant
failure mode will depend upon the key parameters.
With respect to lubrication and failure mechanisms for a
given tribo-contact system, we propose that the key performance
parameters are the entraining velocity, Ue, the sliding velocity,
Us, and temperature. Temperature can be viewed as the bulk
temperature (Tb) of the contacting bodies and contact temperature
(Tc) within the contact itself. The bulk temperature controls
the viscous properties in the inlet region for EHD film thickness.
The contact temperature controls the bulk fluid traction coefficient
in the Hertzian region. It is also a major contributor to
oil and material chemical reactivity within the Hertzian region.
We recognized that contact load or stress is an important
engineering parameter. It certainly is with respect to subsurface
initiated fatigue, where material fatigue is directly related
to shear stresses below the surface. From a tribological perspective
of processes at the surface interface under high-speed lubricated
conditions, load is translated into contact size, heat generation,
and to some degree, asperity stress and strain. While engineers
like to work in terms of load and stress, the critical phenomena
within the contact is really seen to a greater degree as temperature
or tangential stress and strain. In this respect, load capacity
tests are as much temperature tests as they are load tests.
Theories have gone a long way in prediction of EHD film thickness
including micro-EHD mechanisms associated with interaction
of roughness features. These modeling activities support theories
of rolling contact fatigue, where stresses and strains within
the material or at the surfaces are used to predict fatigue
initiation. What is missing from these theories is the chemical
and physical boundary lubricating mechanisms that control
surface topography, friction phenomena and the strength properties
of the surface and near-surface material. While surface analytical
tools can probe the chemical elements of the surface, little
is known about the shear strength of the interfacial material
under stress. The only way forward is to conduct tests for
surface durability under service-like simulated conditions.
These tests are essential for oil and material development
as well as for oil qualification. To make tests relevant to
service performance, the key parameters and their domain of
operation must be understood. The lubrication and failure
mechanisms that these key parameters invoke must then be properly
simulated. The development of Wedeven Associates, Inc. machine
(WAM) technology is devoted to this task.
||WAM High-Speed Load Capacity
||SAE E-34 approved method for jet
||Adhesive Wear Test Method
||For aeropropulsion bearing materials.
||Oil-Off Test Method
||For aeropropulsion lubricants
and bearing materials.
||Debris Tolerance Test Method
||For aeropropulsion lubricants
and bearing materials.
(This method is in development)
||Abrasive Wear Test Method
||For aeropropulsion cage-land bearing
||Surface-Initiated Fatigue Test
||For aeropropulsion lubricants
and bearing materials.
||EHD Film-Forming Capability
||For evaluation of lubricants.