Viral S. Mehta

Physical Limitations for the Bandwidth Frequency of a Pressure Controlled, Axial-Piston Pump

The objectives of this paper are to identify the design parameters that have the greatest impact on the bandwidth frequency of a pressure controlled, axial-piston pump. This study is motivated by the fact that a physical limitation for these machines has been observed in practice, as it has been difficult to increase the bandwidth frequency much beyond 25 Hz. Though much research has been done over the past thirty years to understand the dynamical behavior of these machines, the essential design-characteristics that determine the bandwidth frequency of the pump remain elusive. In part, this is due to the fact that the machine is complex and when coupled with a hydraulic control valve that is disturbed by steady and transient fluid-momentum effects this dynamical property becomes difficult to assess. In order to achieve the objectives of this research, this paper presents the most comprehensive pump-and-valve model of a pressure controlled, axial-piston machine available in the literature to date. The pump model includes the effects of the discrete pumping-elements acting on the swash plate, while the valve model includes both steady and transient fluid-momentum forces. To identify the dominate features of the model, nondimensional analysis is employed and the complexity of the model is subsequently reduced by eliminating negligible terms. Furthermore, a closed-form expression for the bandwidth frequency is employed and perturbation analysis is used to identify the dominant set of parameters that impact the bandwidth frequency of the pump. In conclusion, it is shown that, by far, the greatest impact on the bandwidth frequency may be achieved by reducing the swept volume of the control actuator and by increasing the flow capacity of the control valve.

The Shaft Torque of a Tandem Axial-Piston Pump

The objective of this study is to identify the best indexed position of two rotating groups within a tandem axial-piston pump for attenuating the torque ripple amplitude that is exerted on the shaft. By attenuating the torque ripple characteristics of the pump, other vibration aspects of the machine are also expected to be reduced. In particular, the objectives of this paper are aimed at reducing the noise that is generated by the pump. This paper begins by considering the theoretical torque ripple that is created by the discrete pumping elements of a single rotating group within an axial piston machine. From this analysis, an equation is produced that describes a single pulse for the torque ripple as a function of the average torque and the total number of pistons that are used within the rotating group. By superposing another rotating group on top of the first, and by indexing the angular position of one rotating group relative to the other, a second equation is produced for describing the theoretical torque ripple of a tandem pump design. This equation is also a function of the average shaft torque and the total number of pistons that are used within a single rotating group; however, an additional parameter known as the index angle also appears in this result. This index angle is shown to amplify or attenuate the amplitude of the torque ripple depending upon its value. From these results, it is shown that a proper selection of the index angle can reduce the torque ripple amplitude by as much as 75%. DOI: 10.1115/1.2719785

An Energy Audit for a Three-Function Hydraulic Control System: A Consideration of Pump Arrangements

When designing a hydraulic circuit, there are a number of ways to power each hydraulic function in the circuit. For instance, a single hydraulic function may be powered by its own dedicated pump; or, a hydraulic function may share a pump with additional functions in the circuit. The design question is this: “What is the optimal arrangement of pumps for a given circuit that will result in the lowest energy consumption and the smallest machine size?” This research documents an example study in which a duty cycle from the typical wheel loader is used to study the five possible pump-combinations that exist for powering the lift, tilt, and steering functions of the machine. It is shown that the lowest efficiency for the machine is observed when all three functions are powered by a single pump, and that a 15% efficiency increase may be realized over the single-pump design by giving each function its own dedicated pump. In order to achieve this efficiency increase, the original single pump is replaced by the following pump combination: 1) a pump 69% the size of the original pump for the Lift function, 2) a pump 86% the size of the original pump for the Tilt function, and 3) a pump 31% the size of the original pump for the Steering function. This solution increases the overall pump volume on the machine by 86%, nearly doubling the pump volume on the machine.Copyright

Sensitivity Analysis for the Operating Efficiency of an Axial Piston Pump

Axial piston pumps of swash-plate type are extensively used in off-highway machines to convert rotating mechanical power into hydraulic power. Efficiency of such pumps is of considerable importance to hydraulic design engineers. Many researchers have tried to create mathematical models for describing pump efficiency. These models are typically a system of nonlinear algebraic equations dependent upon a total of four variables (pressure, speed, temperature, displacement) and a set of experimentally determined coefficients. Since these models are not of the a-priori type, they are not of much value to a design engineer who is trying to design an efficient pump. Others have tried to use physics based models and numerical programs to accurately predict the influence of component design on efficiency. Such programs are considerably slow to run and of not much use to a design engineer who needs to make quick decisions. Hence the objective of this paper is to understand the sensitivity of various design parameters on the total efficiency of the pump by conducting a dimensionless parameter study of a large set of pump design parameters. Using this method it will be shown that a small group of design parameters have the highest influence on the efficiency of these pumps.Copyright

Numerical Study Optimal Timing of the Axial Piston Pump

In this paper, the theoretical optimal timing of the axial piston pump is first derived to confirm the analysis published by Professor Kevin Edge [1]. It is discovered that the optimal discharge port delay is different from the optimal inlet port delay. The dimensional analysis also shows that higher shaft angular velocity indicates less delay required in both discharge port and inlet port. Numerical studies show that optimal timing can reduce the dynamic pressure ripple greatly, but since it does not affect the kinematic component, it will not eliminate the entire pressure ripple. The optimal timing could also induce an increase in efficiency where the baseline pump design has cross-porting. However, there is certain tradeoff between pressure ripple reduction and efficiency consideration. Actual design consideration to affect independent timing of the portplate is not studied in this work.Copyright

Piston Pump Noise Attenuation Through Modification of Piston Travel Trajectory

Piston pumps are widely used in industrial and mobile applications to transmit power. These pumps emit loud and objectionable noise when operated at high pressure and high speeds. It is generally accepted that large amplitude of flow ripple causes pumps to produce unacceptable noise level. The flow ripple could be thought of composed of two components — a kinematic component resulting out of periodic nature of flow and a dynamic component resulting due to compression and decompression of fluid. There has been considerable research activity to reduce the noise induced by the dynamic component however very little is done to attenuate the noise generated due to the periodic nature of flow. This research investigated one method to reduce noise associated with kinematic component for axial and radial type piston pumps. A theoretical analysis is presented deriving the equations defining the motion of pistons on their regular trajectory as well as their modified trajectory. It is shown here that by altering the trajectory of the piston travel the amplitude of the kinematic flow component could be reduced by up to 85% in some conditions. While effectiveness of the techniques aimed at reducing the dynamic flow component are speed and pressure dependent, the techniques presented here work with same effectiveness throughout the entire spectrum of speed and pressure towards reducing the noise.Copyright

The Theoretical Frequency Response of an Over-Center, Pressure-Controlled, Axial Piston Pump

Axial piston pumps are ubiquitous in the hydraulic systems employed on various off-highway machines such as hydraulic excavators or wheel-loaders. These pumps exhibit a fairly slow response to the commanded input resulting in a slow response of the work tool (implement or transmission) as well, which is undesirable to machine operators and also slows down the machine’s productivity. Hence it is very important to understand the factors affecting the dynamic pump response. Much of the earlier work aims at understanding the steady state behavior of the pump control system using linear transfer function analysis. A closed-form solution for the dynamic frequency response has not been reported in previous research. This work presents the analysis of a variable displacement pump with an adjustable swash-plate for the purposes of identifying parameters that contribute to the response characteristics of the pump. The full model of the pump involves a seventh order model including a large number of non-linear terms. Hence a reduced order model has also been derived for calculating the frequency response of the pump in the closed form and it is shown that the design parameters that impact the frequency response most heavily are the actuator area, the swash-plate moment arm, and the flow gain of the four-way valve. As it turns out, an increased frequency response of the pump may be most readily achieved by increasing the charge pressure which in turn reduces the required actuator area and increases the flow gain of the valve.Copyright