Experimental Analysis of the Dynamic Characteristics and Lubricant Film of a Ball Bearing under Combined Static and Dynamic Load

摘要

Being particularly critical components, the serviceability of rolling element bearings is usually essential for the machine’s operation. They have been the subject of research for more than a 100 years. The work of this manuscript investigates two aspects of rolling element bearings. Firstly, the dynamic behaviour of a rolling element bearing is analysed. The study emphasises the influence of the lubricant film on the bearing dynamics. Secondly, the lifetime of the bearing is analysed. The study discusses the influence of external dynamic loads on the lubricant film formation and wear of the raceway surfaces.The bearing dynamics and lifetime are experimentally analysed using an innovative and versatile test rig. The first contribution of the current work consists of the development of this test rig. The test rig is designed and constructed in-house and characterised by three unique features. Firstly, the test rig allows easy adjustment to test different types and sizes of rolling element bearings. The test bearing is mounted in a modular structure consisting of an auxiliary shaft and an intermediate sleeve. Secondly, the test rig allows applying a multi-axial static and dynamic load on the test bearing. The bearing load is controlled in both the radial and axial direction and has a static and dynamic component in both directions. Thirdly, the test rig allows simultaneous measurement of the bearing motion, the bearing dynamics and the behaviour of the lubricant film. Relations between the different measurements can be derived experimentally. The radial, axial and tilt motion are measured free from disturbances of the surrounding structure. The bearing dynamics are identified in a frequency range below the resonances of the surrounding structure’s flexible modes. The behaviour of the lubricant film is analysed based on the electrical resistance through the bearing. In the current study, a single row deep groove ball bearing is used as a test bearing. The bearing motion, the behaviour of the lubricant film and cage slip are analysed for different operational conditions of the bearing. The three properties are derived using techniques which can be applied in most rotating machinery: measurement of the radial bearing displacement and the electrical resistance through the bearing. Clear trends w.r.t. the bearing load, temperature and speed are obtained. Clean analysis of the bearing motion and cage slip further proves precise operation of the test rig. The electrical resistance reveals the formation of the lubricant film during a run-up of the bearing speed. Estimation of the lubricant film thickness based on the electrical resistance is validated using an analytical model. Exploring the possibilities to easily and accurately identify the properties is the second contribution of this work.Accurate knowledge on the bearing dynamic properties is essential during the design of many rotating machinery. They determine the machine’s critical speeds, the forces acting on the different components, etc. This work investigatestwo dynamic properties of the test bearing: the radial stiffness and damping in the direction of the static load. Variation of the bearing temperature reveals the influence of the lubricant’s viscosity on the identified stiffness and damping. Variation of the bearing speed reveals the influence of the film formation on the identified stiffness and damping. In the literature, no experimental measurements showing the relations between the behaviour of the lubricant film and the dynamic properties are reported. Revealing these relations is the third contribution of the current study. The results help to evaluate the significance of modelling the lubricant film when numerically analysing the vibrations of a bearing system.Precise prediction of the lifetime of rolling element bearings is a crucial step towards a reliable design of many rotating machines. For bearings subjected to highly varying loads, a strong reduction of the actual bearing lifetime w.r.t. the classically calculated bearing lifetime is observed. This work investigates the effect of external dynamic loads on the lubricant film formation and surface wear of the test bearing. When introducing a radial or axial excitation on the bearing, the lubricant film thickness follows the imposed variations of the load. An increase of metallic contact or breakdown of the lubricant film is not detected. Under axial excitation, sliding motion between asperities of the contacting surfaces in the bearing does occur. Accelerated lifetime tests under high axial dynamic load reveal polishing of the raceway honing structure. This polishing is clearly observed on SEM images of the inner raceway after a test duration of only 0.5% of the calculated L10 life. Analysis of the lubricant film behaviour under dynamic load is not described in the current literature and is considered to be the fourth contribution of this work. Finally, the study promotes further research to analyse surface wear due to external dynamic loads.