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On the prediction of the lubrication condition of grease sealing rotary shaft seals


To achieve a reliable sealing function, the sealing lip of a rotary shaft seal must be sufficiently lubricated. Often, a grease-sealing seal is lubricated with a grease optimised for other tribological systems. Grease-lubricated rotary shaft seals frequently show starved lubrication and sealing system failure. However, grease lubricated sealing systems are far less researched than oil lubricated systems. A simple and quick way to predict the lubricity of a grease would allow to pre-select greases, that better lubricate the sealing lip. The presented “tear-in” tests could provide such a method.


The “tear-in” tests were developed to evaluate a possible correlation between the tear-in of a grease dam in a tribological system and the lubricity of the grease. They were carried out with a rheometer as equivalent system. The grease sample was sheared in the gap between a rotating and a stationary plate with a linearly increasing shear rate. The gap and the motion of the grease within this gap were observed optically with a high-speed camera. In addition, the shear stress was evaluated.


The tear-in tests were performed with 23 greases. Their results were compared with functional tests using a rotary shaft sealing system. It was found that most greases with poor lubricity show specific motions in the gap between the parallel plates and form deep tears. This leads to large fluctuations in the measured shear stress. A quantitative evaluation of these fluctuations correlates very well with the lubricity of the greases in the sealing system. Therefore, the tear-in tests can predict the lubricity of a grease and allow a pre-selection of suitable greases to prevent starved lubrication and sealing system failure. This paper presents the conduction of the tear-in tests as well as their evaluation and correlation with the grease performance in the sealing system.

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