There are several hydraulic applications that require varied flow rates. Traditionally, this was achieved by using a fixed-displacement pump and a metering valve that throttles the flow across an orifice. This is an extremely inefficient control method that converts waste energy into heat. A better method of control is to use a variable displacement hydraulic pump, where the flow per revolution is varied to match demand thus reducing energy loss. A successful demonstration of this approach from Purdue University’s MAHA laboratory is termed “displacement control,” where a 50% reduction in fuel consumption of an excavator has been achieved.
The most common type of variable displacement hydraulic pump is the axial-piston swash-plate architecture. These pumps are relatively simple, light, compact, robust, and efficient at maximum output. However, this architecture suffers from poor efficiency at low displacement, and the performance varies greatly with operating condition. This is largely due to the leakage and viscous friction losses of the fluid film bearing at the piston-shoe/swash-plate interface. These losses remain constant as displacement is reduced at a given pressure and speed. Additionally, the working fluid must also be used as the lubricant, so material choices are limited.
In the MEPS laboratory at the University of Minnesota, a new variable-displacement linkage pump (VDLP) has been developed, which uses an adjustable mechanism to vary the piston trajectory. Bearings in the joints of the VDLP are inherently more efficient than a fluid film interface. The bearing losses scale with displacement, operating speed, and pressure—so the efficiency remains relatively constant independent of operating condition. Additionally, the working fluid can be easily separated from the lubrication fluid making it possible to pump non-lubricating fluids. Furthermore, the new pump maintains the same top dead center position at all displacements, ejecting nearly all of the fluid on each stroke. This nearly eliminates dead volume and compressibility losses. While other variable linkage pumps exist, they require pump shutdown to make adjustments whereas this technology can actively vary the displacement during operation allowing feedback control. This is achieved by varying the position of a ground pivot with a displacement control actuator.
The VDLP design is not without its trade-offs. The non-contact fluid film bearing of the axial-piston pump results in almost no wear during normal operation. The linkage design has mechanical joints, which will require periodic maintenance. However, the bearings can be readily designed for the life of the pump. Another issue is that the current VDLP design is approximately 50% larger than the axial piston pump. However, the technology is young, and work is being done to increase power density and evaluate the potential energy savings in mobile applications.
A low-power single cylinder prototype has been built and tested to validate the efficiency model and the experimental data agrees well with model predictions. A 10 kW three-cylinder prototype, which uses low friction roller bearings in the joints, is currently being fabricated. This second generation prototype has a predicted efficiency greater than 90% for displacements exceeding 15%.
Contributed by Mr. Shawn Wilhelm, PhD Mechanical Engineering (Advisor: Professor James Van de Ven, University of Minnesota)
