Gero Burghardt

Cost-efficient long-term estimation of gear oil filterability considering stresses in service

Purpose Wear particles and contaminants are known to reduce lifetime of machine elements such as roller bearings. This can be avoided through fine filtration. However, oil filterability can change in service, and in case of exceeding the maximum pressure at the filter, contaminated oil can flow through a bypass, threatening the function of the lubricated components. The purpose of this study is to provide a cost-efficient method to estimate the long-term oil fine filterability. Design/methodology/approach The estimation of long-term oil fine filterability takes place in two steps. In the first step, oil in its original condition is subjected to different stresses to reproduce changes experienced in service within a relatively short period of time. In the second step, the resulting oil is subjected to filterability tests. Existing test methods were adapted to produce realistic oil changes and deliver a realistic evaluation of filterability. Findings Oils produced using metal-catalyzed thermal-oxidative stresses and humid air are similar to real oil samples from the point of view of nuclear magnetic resonance oil analyses and filterability tests. Test conditions suitable for highly viscous gear oils were derived considering operation at low and high temperatures. Originality/value The test method provides an estimate of deterioration of oil filterability in service through a comparison between the filterability of oil in original condition and the filterability of an artificially produced oil.

Numerical Calculation of Local Adhesive Wear in Machine Elements Under Boundary Lubrication Considering the Surface Roughness

The calculation of wear in boundary lubricated or unlubricated contacts of machine elements requires the knowledge of certain parameters describing the behaviour of the tribological system. An approach for wear calculation in dry sliding contacts is the Archard equation, which is based on the Archard wear coefficient, describing the probability of the formation of a single wear particle. Although a calculation routine for the Archard equation can be formulated numerically, it is still necessary to determine the Archard wear coefficient for each considered tribological system by experiments. The aim of this paper is to transfer the global Archard equation into a numerical wear model, which allows a spatially resolved determination of wear depth for dry and boundary lubricated contacts. Considering the surface roughness, the calculation will be reduced to a single asperity contact. Each asperity can suffer a specific number of load cycles until its cross section is weakened so the asperity detaches from the surface. The numerical calculation of this critical number of load cycles follows the theory of continuum damage mechanics. The required nonlinear material properties can be evaluated by tensile and compression tests. The residual uncertainty of the calculation process is reduced to the specification of the coefficient of friction for the considered tribological system. The presented numerical model is validated with experimental tests on a FE8 test rig.