Compliant Full Film Lubricated Bearings: Concept Design & Development

Full film lubricated bearings are machine components that excel in their ability to carry high loads with very little friction. Their main archetypes, passive hydrodynamic and active hydrostatic bearings, are used in a wide variety of applications, ranging from hydraulic pumps to high precision machine benches. They are most commonly lubricated using oil-based lubricants and have been a staple in improving machine performance for many decades.

The continuous search for higher performing and more sustainable designs has given ample opportunity to revisit these machine components, and to investigate their possible implementation into machines and fields that so far have not been able to make use of their superior performance characteristics. This lack of increased implementation is often due to fundamental limitations of these components. Both the performance of hydrostatic and hydrodynamic bearings is directly coupled to their need for a thin lubricant film to provide stiffness and load capacity. For hydrostatic bearings in particular, this requirement can more accurately be defined as the need for the bearing surfaces to remain parallel to its counter-surface, separated by a thin film of lubricant in the order of 100 micrometer. Not only the performance, but also ecological constraints have been more dominant in recent years. The conventional use of oil-based lubricants has given researchers the motivation to look for cleaner and more sustainable lubricants. The engineering community keeps on moving forward to a higher performing and more sustainable future.

This thesis investigates one fundamental design direction to obtain higher performance and increase the implementation field of full film bearings. By implementing compliant design, or the use of elastic elements, several facets of in particular hydrostatic bearing limitations are investigated. Two fundamental limitations are dominant throughout this dissertation: the design for changing counter surfaces and the design for multiple operating conditions. Several principles are introduced that improve the use of hydrostatic bearings for non-constant counter surfaces. These principles are the use of functionally graded materials to minimize the pre-loading effect the hydrostatic pressure has on the elastic bearing support, distributed whiffletree support-systems to increase bearing deformability, and the introduction of a compliant water-filled universal joint with superior stiffness characteristics compared to the state of the art. The universal joint makes use of the principle of closed-form pressure balancing, which is pressurizing an enclosed body of fluid to obtain low rotation stiffness while maintaining high support stiffness. All these principles are described through design models and are further investigated with the use of finite element models.

The second dominant subject that is investigated in this dissertation is developing hydrostatic bearings that function for multiple operating conditions with the use of compliant elements. This principle, defined in this work as passive shape shifting, gives hydrostatic bearings more flexibility when it comes to designing for more discrete load cases, such as the one that can be found in hydraulic pumps. Besides describing this concept design principle with finite element models, a step towards real life application is also made in a scaled case study. By combining the previously mentioned compliant universal joint and a hydrostatic bearing using the principle of passive shape shifting, a contact-mechanics free alternative for axial piston pumps is proposed. This mechanism is designed for a scaled case study and validated through experimental work and finite element modelling.

This thesis, which is conceptual in nature, combines these different design directions and therefor shows design directions that broaden the field of use for full film lubricated bearings by using elastic elements.