Analysis and Design of Lubricating Interfaces in External Gear Machines for High and Low Viscous Working Fluids

Lubricating interfaces represent a significant design constituent which contribute to the reliability and efficiency of many modern designs of gap compensated external gear machines (EGMs). However, the complex nature of the influence of fluid behavior in these interfaces on the structural and thermal effects of the solid components involved in the lubricating gap makes their design quite challenging. Moreover, the extensive range of applications currently available for EGMs warrant designs which can perform efficiently at extended range of operating conditions as well as with a comprehensive variety of working fluids. In order to improve the understanding of the physics in these lubricating gaps especially in gap compensated units, and thereby achieve virtual prototyping design tools for these conditions, the principal goal of this research is to extend the capabilities of the state of art numerical models for the lubricating interfaces in EGMs. The present research addresses the design and analysis of the lateral lubricating interface between the lateral bushing and the gears in EGMs for critical operating points as well as for reference working fluids with significant differences in their viscosities which challenge the lubricating ability of the thin film interface. A novel mixed film-TEHD (Thermo-Elastohydrodynamic) model for the lateral lubricating gap was developed as a part of this research to capture the effects of such widely varying design parameters. Two different instances of experimental validation of this mixed film lubrication model were carried out for the reference cases of conventional oil based EGMs, namely with measured torque losses and drain leakage measurements. Furthermore, the capabilities of the lateral gap model are utilized in studying the impact of the variations in surface finishes on the performance of a commercially available EGM chosen for this study, by considering lateral plate designs of varying surface roughness. Additional contributions have also been made to the modeling of lateral gaps in EGMs to extend their capabilities, which include consideration of frictional contact forces between the lateral bushing and the housing for the first time. This research demonstrates the significance of considering the effect of friction on the performance of the lubricating gaps in gear pumps by using a reference case of an asymmetrically balanced EGM used for aerospace fuel injection applications. In addition, a mass conserving cavitation algorithm to account for the cavitating conditions in the lubricating interface was also integrated with the mixed lubrication model to improve the stability of the numerical predictions of the pressures in the lubricating gap. Leveraging the design potential of the numerical tool developed in this research, designs to improve the lubrication performance of EGMs are presented in this work, which include surface shaping on the gears as well as achieving an optimal balance configuration in the lateral gap. These unique modifications along with the mixed lubrication model are then applied to a reference EGM case with water as its working fluid where the low viscosity of a fluid further adds to complexity in designing the lateral lubricating interface. A novel water based EGM prototype which can work at high pressures is thus proposed in this work, to implement and validate the proposed design approaches for the reference low viscous fluid. Furthermore, the present work also proposes certain unique variations to the common designs of oil-EGMs that need to be implemented in the prototype water hydraulic EGM to facilitate its practical implementation especially at high pressure operating conditions. The methodologies and models developed in this research along with their proven fidelity using experiments could potentially serve as a design tool to formulate new efficient EGM designs for an extensive range of applications and working fluids

A noise and vibration analysis on positive displacement pumps for fluid power application to reduce flow pulsation and cavitation phenomena

The Fluid Power area handles a profound change to achieve the highest efficiency levels requested by the market to compete with other technologies of different fields, like Hybrid-Electric configuration. The noise emission increases its importance in the fluid power field since the other technologies cannot have the same power density, flexibility, and reliability as the Hydraulic. For this reason, hydraulic components, especially positive displacement pumps and motors, are pushing to conform to the dB limits defined by European regulations for working environments (ref. SNSI, Horizon 2020, and Direttiva Macchine 2006/42/CE), both in mobile and indoor applications. The purpose of this research activity is aimed at the optimization of pump geometries using numerical and experimental techniques

A Numerical Analysis of an Innovative Flow Ripple Reduction Method for External Gear Pumps

In this paper, an innovative solution to minimize noise emission, acting on the flow ripple, in a prototype External Gear Pump (EGP) is presented. Firstly, a new tool capable to completely simulate this pump’s typologies, called EgeMATor, is presented; the hydraulic model, adopted for the simulation, is based on a lumped parameter method using a control volume approach. Starting from the pump drawing, thanks to different subroutines developed in different environments interconnected, it is possible to analyze an EGP. Results have been compared with the outputs of a three-dimensional CFD numerical model built up using a commercial code, already used with success by the authors. In the second section, an innovative solution to reduce the flow ripple is implemented. This technology is called Alternative Capacitive Volumes (ACV) and works by controlling and uniformizing the reverse flow, performing a consistent reduction of flow non-uniformity amplitude. In particular, a high reduction of the flow non-uniformity is notable in the frequency domain on the second fundamental frequency. The technology is easy to accommodate in a pump housing, especially for high-pressure components, and it helps with reducing the fluid-borne noise

Numerical simulation of three-phase flow in an external gear pump using immersed boundary approach

This paper presents a three-phase fully compressible model applied along with an immersed boundary model for predicting cavitation occurring in a two dimensional gear pump in the presence of non-condensable gas (NCG). Combination of these models is capable of overcoming numerical challenges such as modelling the contact between the gears and simulating the effect of NCG in cavitation. The model accounting for the effect of NCG also has broader applicability, since gas dissolved in liquids can come out of the solution when exposed to low pressures; this plays a significant role in the pump performance and cavitation erosion. Here the simulation results are presented for the gear pump at different operating conditions including the contact between gear, gear RPM and % of NCG; their effects on performance and cavitation is demonstrated. The results suggest that modelling the contact between the gears play a role in the cavitation prediction inside the gear pump. An increase in cavitation is observed when the contact is modelled even for the small pressure difference considered between the inlet and outlet. An increase in the RPM of the gears also results in increased cavitation within the pump, whereas an increase in the percentage of NCG content by a small amount can reduce the cavitation to a greater extent. This reduction is due to the expansion of the gas at a lower pressure which recovers the pressure and prevents or delays the phase-change process of the working fluid. The fluctuations in the outflow rate is also found to increase when the gears are in contact and also with increasing gas content

CFD Analyses of Textured Surfaces for Tribological Improvements in Hydraulic Pumps

In any hydraulic machine there are lubricated couplings that could become critical beyond certain operating conditions. This paper presents the simulation results concerning textured surfaces with the aim of improving the performance of lubricated couplings in relative motion. The texturing design requires much care to obtain good improvements, and it is essential to analyze both the geometric features of the dimples and the characteristics of the coupled surfaces, like the sliding velocity and gap height. For this purpose, several CFD simulations have been performed to study the behavior of the fluid bounded in the coupling, considering dimples with different shapes, size, and spatial distribution. The simulations consider the onset of gaseous cavitation to evaluate the influence of this phenomenon on the pressure distribution generated by the textured surface. The analyses have pointed out that it is critical to correctly predict the behavior of the textured surface in the presence of local cavitation, in fact, when cavitation occurs, the characteristic time of the transient in which the phase of the fluid change is very rapid and it is comparable to the time taken by the fluid to move from one dimple to the next

Three-dimensional model of an external gear pump with an experimental evaluation of the flow ripple

A three-dimensional model of an external gear pump and a new application of an algorithm for the measurement of the unsteady flow rate in hydraulic pipes are presented. The experimental delivery flow ripple was compared with the outcomes of a simulation under different operating conditions. A comprehensive computational fluid dynamics model of the pump and of the high-pressure delivery circuit was developed in SimericsMP+. The pump model considers the clearances, which vary according to the shaft angle, between the tip of the tooth and the inner surface of the stator, as well as between the flanks of the teeth that are in contact. The pump delivery circuit is constituted by a straight pipe with a fixed orifice at the end to generate the load. The model of the entire system was preliminarily validated in terms of delivery pressure ripple. Subsequently, the simulated flow ripple was contrasted with the instantaneous flow rate, measured by means of an innovative flow meter. It was found that the proposed flow meter is reliable in assessing the flow oscillations under the various working conditions

Volume 1 – Symposium: Tuesday, March 8

Group A: Digital Hydraulics Group B: Intelligent Control Group C: Valves Group D | G | K: Fundamentals Group E | H | L: Mobile Hydraulics Group F | I: Pumps Group M: Hydraulic Components:Group A: Digital Hydraulics Group B: Intelligent Control Group C: Valves Group D | G | K: Fundamentals Group E | H | L: Mobile Hydraulics Group F | I: Pumps Group M: Hydraulic Component

Investigation of Noise Sources and Propagation in External Gear Pumps

Oil hydraulics is widely accepted as the best technology for transmitting power in many engineering applications due to its advantages in power density, control, layout flexibility, and efficiency. Due to these advantages, hydraulic systems are present in many different applications including construction, agriculture, aerospace, automotive, forestry, medical, and manufacturing, just to identify a few. Many of these applications involve the systems in close proximity to human operators and passengers where noise is one of the main constraints to the acceptance and spread of this technology

Evaluation of Tooth Space Pressure and Incomplete Filling in External Gear Pumps by Means of Three-Dimensional CFD Simulations

The paper presents the computational fluid dynamics simulation of an external gear pump for fluid power applications. The aim of the study is to test the capability of the model to evaluate the pressure in a tooth space for the entire shaft revolution and the minimum inlet pressure for the complete filling. The model takes into account the internal fluid leakages and two different configurations of the thrust plates have been considered. The simulations in different operating conditions have been validated with proper high dynamics transducers measuring the internal pressure in a tooth space for the entire shaft revolution. Steady-state simulations have been also performed in order to detect the fall of the flow rate due to the incomplete filling of the tooth spaces when the inlet pressure is reduced. It has been demonstrated that, despite the need of a compromise for overcoming the limitation of considering fixed positions of the gears’ axes and of the thrust plates, significant results can be obtained, making the CFD approach very suitable for such analyses