Tribology of Journal Bearings Subjected to Boundary and Mixed Lubrication

The purpose of this research paper is to examine the tribological characteristics and issues related to journal bearings under boundary and mixed lubrication conditions during shaft startup, shutdown, and low speeds. The key issues and problems related to journal bearings, most notably wear, are discussed as well as the subsequent issues created in the field. The mechanical analysis of the loading conditions is documented for both numerical calculations and experimental test methods. Studies prove the negative effect bearing wear has on performance and pressure loads. Solutions include polymer liners, surface hardening, and using lubrication fluids with additives. Ultimately, the solutions help to optimize journal bearing design and improve product reliability. In addition to the on-going pursuit to reduce wear, finding effective environmentally friendly lubricating fluids is a difficult challenge. 2 Summary of Problem A journal bearing is a journal (such as a shaft) which rotates within a supporting sleeve or shell [1]. Hydrodynamic journal bearings use the rotation of the journal to pressurize a lubricant which is supplied to the bearing to eliminate surface-to-surface contact and bear the external load as seen in Figure 1 [4]. Figure 1: Journal Bearing [4] Sliding lubricated surfaces can be broken down into three lubrication regimes corresponding to the Stribeck Curve seen below in Figure 2. The curve represents the minimum value of friction between full fluid separation and direct asperity contact of two surfaces. The friction is plotted as a function of a lubrication parameter ηN/P, where η is the dynamic viscosity, N is the shaft speed and P is the external load [7]. The highest friction condition occurs in the boundary lubrication region, which represents significant or complete asperity contact between the two surfaces. On the other hand, the hydrodynamic lubrication region represents a load fully supported by the lubricating fluid with no asperity contact. Finally, the mixed lubrication region represents partial load support from the lubricating fluid and partial load support from asperity contact.