Oil Film Thickness Measurements on Surfaces Close to an Aero-Engine Ball Bearing Using Optical Techniques

Abstract

In a civil aero-engine transmission system a number of bearings are used for shaft location and load support. A bespoke experimental test facility in the University of Nottingham’s Gas Turbine and Transmissions Research Centre (G2TRC) was created to investigate oil shedding from a location bearing. An engine representative ball bearing was installed in the rig and under-race lubrication was supplied via under-race feed to three locations under the inner race and cage. The oil was supplied in an engine representative manner but the delivery system was modified to provide circumferentially even flow. An electromagnetic load system was designed and implemented to allow engine representative axial loads between 5 and 35 kN to be applied to the bearing. In this phase of testing the rig was operated at shaft speeds between 1,000 rpm and 7,000 rpm for a range of oil flow rates and low and high load conditions. The rig was designed with good visual access and high speed imaging was used to investigate film formation and movement on surfaces close to the bearing. This paper presents images and qualitative observations of thin film formed on the static surfaces forming the outer-periphery of the bearing compartment as well as the gap between orbiting cage and static outer race. Quantitative film thickness was obtained at two circumferential locations (90° and 270° from top dead centre) and three axial locations, through sophisticated analysis of the high speed images. The effect on film thickness of the varied parameters rotational speed, axial load and oil feed input flow rate are presented in this paper. It was observed that for all axial planes of measurement in both co-current and counter-current regions film thickness decreases with increase in shaft rotational speed. At 5,000 and 7,000 rpm film thicknesses are around 0.75 mm – 1 mm and are similar at 90° and 270°; at 3,000 rpm films tend to be somewhat thicker at around 1.5 mm – 2 mm and are thicker in the counter current region, particularly closer to the bearing. It is suggested that at higher shaft speeds interfacial shear dominates whereas at lower speed the effect of gravity in slowing the film in the counter-current region causes a measureable difference. It was further observed that increasing the input oil flow rate from 5.2 litres per minute to 7.3 litres per minute did not produce significant effect on film thickness. However, the increase of axial bearing load from 10 kN to 30 kN yielded thicker films at the location above the cage. In all cases there was waviness on the film surface at the bearing outer periphery; imaging was not sufficient to see if the film surface close to the bearing is wavy.