2D22 Experimental investigation of effective bulk modulus of oil with entrained air bubbles(The 12th International Conference on Motion and Vibration Control)

Abstract

Although hydraulic systems are compact and have high output power, the demand for further downsized systems with increased output power has been on the rise in recent years. The operating pressures of hydraulic systems have recently been increasing, and condition monitoring of the properties of hydraulic fluid has been one of the most important technical issues. Bulk modulus is a property that indicates the compressibility of a hydraulic fluid. Hydraulic fluids typically become aerated during use, and a certain amount of air is always present in a hydraulic fluid. Aeration strongly influences the bulk modulus of hydraulic fluid because air is significantly more compressible than oil. Entrained air in the form of bubbles reduces the effective bulk modulus. Furthermore, air bubbles entrained in hydraulic fluids cause numerous problems in hydraulic systems. Active bubble elimination devices that use swirl flow capable of eliminating air bubbles from hydraulic fluid have been proposed and developed by the authors. We focus on the relationship between the change in bulk modulus and the elimination of air bubbles from hydraulic fluid. Hydraulic fluid from which bubbles have been eliminated is sampled in a vessel and experimentally pressurized by a servo piston. The volume and pressure change of the sampled oil is measured, and bulk modulus change is calculated to evaluate the bubble elimination. We conclude that the change in bulk modulus of the bubble-eliminated oil influences system performance with respect to positioning, power loss, response time, and stability in hydraulic systems.