Micropitting is a gear flank surface fatigue damage or phenomena, which is strongly influenced by the tribological system between the meshing tooth flanks. It is characterized by numerous small surface cracks called micropits induced by the Hertzian contact between the two mating gear flanks. The micropits are shaped by cracks growing at a shallow angle to the surface, forming a continuous fractured surface, which can be visualized as dull and matte. Main influence factors on the occurrence of micropitting are the used lubricant (base oil, additives, viscosity), the tooth flank geometry (size, profile shift, modifications, roughness, heat treatment) and the operating conditions (speed, load and temperature).
The effects of micropitting on the gear flank load carrying capacity are very manifold. On the one hand, micropitting may lead to a better load distribution on the tooth flank because high loaded areas are worn and therefore the gear lifetime may be massively increased. On the other hand, micropitting may also encourage the development of pitting damages and other modes of gear failures. The propagation of micropitting can terminate but may also continue to progress, depending on the used lubricant and the operating conditions.
In order to quantify the micropitting load carrying capacity of different lubricants the standardized test according to FVA 54/7 which is also known as FVA-FZG-micropitting test is available. This test procedure consists of two parts and is carried out for a specific oil under defined conditions regarding test gearing type C-GF, oil temperature and circumferential speed. First, a step test for predefined load stages with increasing torque is performed. As a result of this part, a classification of the lubricant regarding the micropitting load capacity expressed as a failure load stage (SKS) is available. This SKS-value is defined as the load stage, in which the defined failure criterion (profile form deviation ffm) is exceeded. In a second part, a subsequent endurance test usually is performed which provides information about the progress of micropitting at a higher number of load cycles.
About 10 years ago, first efforts were made to develop a standardized method for assessing the risk of micropitting for a given gear stage based on previous research projects within the FVA (Research Association for Drive Technology). The results of this process were published in 2010 as ISO/TR 15144-1. ISO/TR 15144-1 describes a method for calculating a safety factor against micropitting for a defined gearing under given operating conditions. Main input parameters are the gear geometry and the operating conditions as well as a value characterizing the micropitting load carrying capacity of the used lubricant such as the SKS value. The safety factor acc. to ISO/TR 15144-1 is defined as the quotient of occurring specific lubricant film thickness and permissible specific lubricant film thickness. Both values are calculated with the same set of formulae which were derived from basic calculation principles from EHL-theory and extended by empirically established factors. Using the same formulae for calculating the occurring and the permissible specific lubricant film thickness ensures that some possible inaccuracies do not have a big impact on the calculated safety factor. The industrial practice from the last years shows, that the calculation approach acc. ISO/TR 15144-1 can assess the risk of micropitting in a reliable way. ISO/TR 15144-1 is currently under revision and will likely be published as ISO/TS 6336-22 in 2018.
If the micropitting load carrying capacity is a design-relevant parameter or not is strongly depending on the field of application and the sector's philosophy. Whereas a small amount of micropitting is often tolerated in automotive gearboxes or industrial gearboxes which are subject to regular monitoring, other fields of application such as wind turbine industry usually do not allow any micropitting on the gear flank at all. To take account of these circumstances, the minimum safety factor regarding micropitting should be chosen depending on the quality of the input variables and the tolerated probability of micropitting. A proposal showing values for the minimum safety factor depending on the quality of the input variables and the tolerated probability of micropitting based on test results and experience from industrial application is shown in the following figure.
Overall, the question, whether micropitting is a damage or only a phenomenon, cannot be answered in general because it is strongly depending on the operating conditions and the specific field of application. FZG is presenting more on this topic under the title "Practical use of micropitting test results according to FVA 54/7 for calculation of micropitting load capacity acc. to ISO/TR 15144-1" at the International Conference on Gears, held in Garching from September 13th to 15th, 2017.
Dipl.-Ing. Michael Hein, Teamleiter Flankentragfähigkeit
Co-Autor: Prof. Dr.-Ing. Karsten Stahl, Ordinarius
Co-Autor: Dr.-Ing. Thomas Tobie, Abteilungsleiter Tragfähigkeit Stirnräder und Betriebsfestigkeit
Alle: Technische Universität München, Forschungsstelle für Zahnräder und Getriebebau (FZG)