Testing Philosophy

Testing Methods

WAM High Speed Load Capacity Test Method

WAM Load Capacity Testing for Lubricating Capability of Oil Formulations

Cage-Land Abrasive Wear

Adhesive Wear Test Method

Oil-Off Test Protocol

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.

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 behind.

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 tests.

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 performance.

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 parameter.

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.