On the Predicted Effects of Recess Depth and Orifice Diameter on Pneumatic Hammer Instability in Hydrostatic Journal Bearings Using Compressible Fluid and Comparisons With Test Data

Abstract

Abstract Hydrostatic journal bearings (HJBs), employed in highspeed turbomachinery under extreme operating conditions, offer enhanced reliability, durability, load-carrying capacity, and stiffness, even when lubricated with low-viscosity fluids. A significant challenge with these bearings, particularly when deep recesses are involved and compressible fluids are used, is the occurrence of pneumatic hammer instability. This phenomenon poses a substantial risk, potentially leading to catastrophic system failures. The focus of this research is to quantify the influence of orifice diameter and recess depth on pneumatic hammer instability in HJBs. The study employs a predictive model, prioritizing the understanding and mitigation of this instability. Key to this research is the examination of volume and pressure ratios, particularly the recess volume to fluid film volume ratio and the recess pressure to supply pressure ratio. These ratios are fundamentally influenced by the orifice diameter and recess depth, which in turn significantly affect the HJBs’ characteristics and susceptibility to pneumatic hammer instability. The current study reveals that both the load-carrying capacity and flow rate of the bearings increase progressively with the recess pressure ratio. Meanwhile, the bearing stiffness peaks at a specific orifice diameter and recess depth, typically where the recess pressure ratio is around 0.6. Under these conditions, the bearings demonstrate maximum stiffness with minimized risk of instability. This research also proposes a framework for selecting design parameters in HJBs, aiming to maximize stiffness while averting pneumatic hammer instability. The effectiveness of these design strategies is validated through experimental data from two test bearings, each with different recess depths and orifice diameters. This work contributes significantly to the field by offering strategies for optimizing HJBs. It provides a balance between achieving high performance and mitigating the risks associated with pneumatic hammer instability. By careful selection of orifice diameter and recess depth, it is possible to attain maximum bearing performance, effectively circumventing the challenges posed by pneumatic hammer instability.