Lubricating Characteristics of Soybean-Based Oil on Rough Bearings: A Theoretical and Experimental Study

The growing need for environmentally sustainable lubricants has led to increasing interest in vegetable-based oils. Among them, soybean oil demonstrates promising tribological characteristics owing to its high lubricity, biodegradability, and favorable viscosity–temperature relationship. This study investigates the hydrodynamic lubrication performance of rough bearings operating with soybean-based lubricants. A modified Reynolds equation incorporating surface roughness parameters is employed to evaluate pressure distribution, load-carrying capacity, and coefficient of friction under steady-state conditions. Surface roughness is modeled using average roughness parameters based on stochastic theory. Experimental validation is performed using journal bearing setups lubricated with pure and chemically modified soybean oils. The results show that surface roughness tends to decrease load capacity; however, the high polarity and film-forming ability of soybean oil molecules enhance boundary lubrication and reduce frictional losses. Chemical modification (epoxidation and transesterification) further improves thermal stability and viscosity index. Overall, soybean-based lubricants provide an efficient and eco-friendly alternative to conventional mineral oils, particularly for lightly to moderately loaded rough bearing applications.