Influence of the meniscus on wear in grease-lubricated oscillating rolling contacts

Grease-lubricated rolling element bearings that perform oscillating motion are often prone to False Brinelling. Previous investigations on oscillating open contacts, i.e. no presence of a sticking zone, identified the replenishment of the contact by bled oil through capillary forces as a key mechanism for wear prevention. This investigation aims to better understand how the replenished oil is acting around the contact. Optical investigations with greases previously studied in bearing experiments are conducted to observe the meniscus shape around the contact. Comparison with the bearing experiments shows a strong correlation between the onset of severe wear and the inlet length of the grease meniscus. This reveals a new role of the meniscus in boundary or mixed lubrication, besides its prominent function in EHL. The physical background of the measured inlet lengths for varying base oil viscosities and bleeding rates is discussed. Evaluation of the test data with respect to the capillary number and bleeding rate supports these considerations; a critical threshold value is identified. This paper is intended to provide more insight into the lubrication mechanisms under oscillating conditions. Additionally, the approach derived from the measurement data may help to predict the inlet dimensions in a practical manner for bearing calculations.