Volume 4, Issue 9 (October 2007)
The “Practice Relevant Pitting Test”—A New Improved Test Method to Evaluate the Influence of Lubricants on the Pitting Load Capacity of Case Carburized Gears
Pitting is a form of fatigue failure which occurs on the flanks of highly loaded case carburized gears. The lubricants, consisting of base oil and additives, influence the pitting lifetime of the gear flanks. Common calculation methods of the pitting load capacity consider, along with the flank roughness and the tangential velocity, only the nominal viscosity of a lubricant as a lubrication relevant influence parameter. The actual lubricant performance and particularly the influence of the additives can be determined only experimentally. Within a FVA-research project a new “Practice Relevant Pitting Test” was developed to evaluate the influence of lubricants on the pitting load capacity of case carburized gears. The new test method is a mechanical test procedure which is based on the existing FVA-FZG-Pitting Test. It is defined as a single stage test which can be extended to an application test by testing the lubricant with a second load stage. In order to improve the practice relevance and to reduce the scatter of the test results the test uses superfinished test gears with adequate profile modifications. The extremely low flank roughness of the new test gears prevents particularly the appearance of undesired micro-pitting and therefore improves the reproducibility of the reached lifetime within the performed test runs. The new pitting test procedure was verified with different gear transmission lubricants and is seen as an improvement of the existing FVA-FZG-Pitting Test. It provides also a possibility to consider the test results for calculation of the pitting load capacity of industrial and automotive transmission gears with an additional factor ZLp which is to be applied in the calculation method according to DIN 3990. The paper describes the new developed test procedure “Practice Relevant Pitting Test” and discusses the correlation and classification to the existing FVA-FZG-Pitting Test.