Bearing System In Dynamic Loading Including Solid Contact And Wear

In small reciprocating compressors for household applications the compressor shaft is placed vertically and supports a dynamic load associated to the transversal piston movement. Usually, for design purposes, the bearings are treated separately which precludes a precise description of the shaft orbit. In the present investigation both the main and the secondary journal bearings are modeled coupled and simultaneously, allowing for shaft misalignment within the bearings. A new computational methodology is introduced and tested to determine the pressure distribution in both bearings via a direct procedure. Solid contact and wearing are both allowed to occur causing a significant impact on the frictional power and a minor effect on the shaft orbit. INTRODUCTION Classical references in the literature related to bearing under dynamic loading are Campbell et al. (1967), that presented an extended review of early works on this subject, with theoretical and experimental results, and Jones (1982) that introduced a methodology for dynamic loading including oil holes and grooves as well as starvation effects. Most available works explore formulations and results for just one bearing, with the shaft perfectly aligned with the bearing axis. Effects of shaft misalignment on bearings operating under static loading were investigated by Pinkus and Bupara (1979), and Vijayaraghavan et al. (1990). The present work deals with two bearings working coupled under a dynamic loading, for a small reciprocating hermetic compressor employed for household applications. The formulation includes shaft misalignment during the load cycle. Solid contact is allowed to occur and is modeled using the classic Coulomb law, where the friction force is related to the normal force through a friction coefficient. Recently, Zhou and Rogers (1997) used this same classic relation in a work on the effects of the squeeze film and the solid contact between a tube and its holder in a heat exchanger. Similarly, Sun and Jing Xu (1995) employed Coulomb law in the analysis of a bearing starting characteristics considering the solid friction, without including lubrication. Despite of the high effort to fully understand the radial bearings behavior under every circumstances, only in recent works wear effects have been considered. Del Din and Kassfeldt (1999) presented friction and wear results for radial bearings. Their experiments investigated the influence of oil type, oil contamination, temperature, and shaft rotation on the friction values and bearings wear index. Regarding modeling, a literature survey indicated no previous work related to contact and wear prediction for radial bearing operating under dynamic loading, and in this regard the present paper aims to contribute towards a comprehensive approach for bearing system simulation. CASE STUDY DESCRIPTION To explore the problem geometry to be considered here figure 1 was prepared. According to the figure, the shaft stands vertically with the main bearing (1 bearing) on the top and the secondary bearing (2 bearing) on the bottom. This geometry reproduces the situation encountered in small hermetic compressors where the electric motor is placed on the lower part of the shaft and drives the piston on its upper end. The piston moves horizontally in a reciprocating motion imposing an alternating load on the shaft. In turn, the shaft describes a periodic misalign trajectory within the bearings.