Cavitation and film formation in hydrodynamically lubricated parallel sliding contacts

Abstract

Cavitation influences the pressure distribution in hydrodynamically lubricated contacts and therefore also the load carrying capacity. Film formation in parallel sliders is investigated experimentally and numerically by focusing on its relationship with cavitation. Friction torque, contact temperature and film thickness measurements are conducted on textured and non-textured rectangular face seals. The textured seals have bidirectional structures (‘T-shape’ structures) on their axial faces. It is found that the textured seals have a better performance. The friction torque and the contact temperatures of the structured seals are lower at high speeds. Cavitation is observed and film thickness measurements are performed by using the laser induced fluorescence method. Cavitation forms in the divergent zones of the structures for the structured seals. Cavitation is randomly distributed over the seal interfaces of the standard seals and it seems to occur due to macro and micro surface irregularities. The cavitation area ratio increases rapidly with speed in the mixed lubrication regime. In the hydrodynamic regime, the change in cavitation with speed becomes small. Both the film thickness and the cavitation area ratio decrease with increasing sump temperature and pressure. There is a strong correlation between lubricant film thickness and cavitation. Local contact temperatures over the sealing contact are observed via infrared thermography. The structured seals heat up near cavitation zone. The standard seals warm in patches on their axial faces. It is observed that structuring can generate cooling effect.

Hydrodynamic film formation is modelled based on cavitation by a mass conservative Jakobsson-Floberg-Olsson cavitation model solved using the Fischer-Burmeister-Newton algorithm. The numerical results are validated via the experiments. Influences of structuring, macro surface irregularities, waviness, radial taper, and cavitation pressure are investigated. Experimental and numerical results correlate well in the hydrodynamic regime when realistic contact temperatures and realistic cavitation pressures are taken into account. It is shown that surface structuring can generate a considerable hydrodynamic load support. The manufactured surfaces of the seals influence pressure generation and cavitation. Therefore, it is necessary to consider the manufactured surfaces to predict the cavitation and film formation accurately. For the current application, surface structures are found to be more dominant on film formation than waviness and radial taper. A variation of the cavitation pressure showed that increasing cavitation pressure can result in higher film thicknesses and lower cavitation sizes. Comparison of these results with the results from optical experiments enabled a prediction of a feasible cavitation pressure for the system.

Furthermore, the implementation of the numerical model for other applications is described. Surface structuring of a vane pump is analysed. Here, dimple like structures are investigated. The numerical model is further extended and roughness is considered in the application of nominally flat sliding contacts. Thus, first attempts to understand the influence of roughness on film formation and inter-asperity cavitation are described.