Structural Analysis And Design Of Variable Displacement Linkage Pumps

Abstract

Fluid power systems are ubiquitous, providing high power within a small package. They are also capable of producing extremely high forces and rapid precise response. However, the average efficiency of fluid power systems is just around 21%, while consuming 2% of the total energy in the United States. A large percentage of these energy losses is due to ineffective flow control methods. While variable displacement pumps offer a more efficient method of controlling the speed of an actuator over metering valves, most of them are inefficient at low displacements. The variable displacement linkage pump architecture, developed recently at the University of Minnesota, shows great promise to achieve high efficiency across the full displacement range. Pin joints, which have a linear relationship between energy loss and displacement, are used instead of hydrodynamic planar joints used in conventional pumps. In this thesis, a new generation variable linkage pump prototype, with a displacement of 10cc/rev and maximum pressure of 3500 psi, is presented. The pump was designed to address the issues of previous generation linkage pump prototypes by improving the volumetric efficiency and reducing the size, thereby advancing the pump towards commercialization. A primary focus of this work was supporting design decisions using structural analysis. A specific focus was on minimizing the linkage deflection by designing the links to be in double shear. The finite element analysis results were validated by comparing the rotational deflection of the adjustment mechanism assembly obtained through simulations with that of those from experimental results. Unlike conventional variable displacement pump architectures, linkage pumps have the potential to pump a slurry, as the pumping fluid can be separated from the lubricating fluid. However, conventional reciprocating seals like elastomeric and clearance seals wear quickly and leak when operating with abrasive fluids, whereas packed glands and mechanical seals, which are the popular choice in industrial slurry pumps, result in high leakage and high initial cost respectively. Rolling diaphragm seals offer negligible friction and zero effective leakage, possibly offering a better option compared to other seals. However, commercial diaphragm seals are currently being used for pressures under 60bar and not much research has been done since 1970’s to improve their pressure capability. In this thesis, a preliminary study is presented on analyzing the behavior of rolling diaphragms under various conditions using finite element analysis. The convolution portion of the seal was analyzed by modelling fiber and elastomer individually. Increasing the fiber diameter and number of fibers reduced the deformation of elastomer and stress induced in the fiber. Fiber diameter is found to be a more important parameter than the number of fibers. Analysis showed that a Kevlar reinforced neoprene diaphragm seal would be able to withstand 80 bar before the elastomer fails. The possibility of modifying the design to withstand higher pressures is also discussed in this thesis. In addition to work on the inline triplex variable linkage pump prototype, this thesis also describes structural analysis that supports the design of a cam driven radial configuration of the variable linkage pump. Challenges of this multi-body finite element analysis were identifying proper contact interfaces, generating high quality mesh, and simplifying the geometry to reduce the computational time. Further, it was not practical to model the full detail of commonly found component like bearing due to large number of contacts between rollers and raceway. The contributions of this work include constructing a systematic approach of using FEA to drive the design process, identifying proper contact interfaces, and developing a simplified bearing model. The cam driven linkage pump is used as a case study to demonstrate the application of these analysis tools.