Profiling a Ball Bearing Oil Film with Ultrasonic Reflection

Abstract

The balls and raceways in a rolling bearing are separated by a very thin oil layer that is critical to performance. There are several models for predicting the layer thickness and some lab-based methods for studying individual contacts in model apparatuses. However, techniques for in situ measurement of oil films in real bearings are scarce, especially at the level of detail of the film profile. The purpose of this study was to experimentally measure the film thickness profile between the outer ring and balls in a rolling element bearing using an ultrasonic reflection method. A 25-MHz ultrasonic immersion transducer was installed in a water bath above the bearing and the incident pulses were focused onto the moving contacts. The oil film thickness profiles across the ball passage are presented for a 6410 deep-groove ball bearing under loads up to 49 kN and speeds up to 600 rpm. The spatial resolution of an ultrasonic pulse is relatively low, so it is not possible to pick out the detail of the horseshoe constriction at the contact exit. However, there are some indications of its presence and the central film thickness measurement agrees well with theoretical solutions. For a range of loads and speeds, oil films of between 0.1 and 0.7 μm were measured. Data storage limitations mean that the measurements are limited to low-resolution profiles across several balls or a detailed profile over a single ball. The oil films were also measured for successive balls and found to be identical within the accuracy of the method.