<p>Positive displacement gear machines are widely used in a variety of industrial applications ranging from fuel injection applications to fluid handling systems to fluid power machinery. Simulation models for these machines are increasingly being developed with greater applicability and more accuracy to meet the industry needs. In this work, a research study is done on contact forces in positive displacement gear machines towards improving the accuracy of the simulation models, which can help gain insights on the underlying physics that govern the performance of the machines.</p> <p><br></p> <p>First, the importance of considering contact forces in simulating a positive displacement gear machine is addressed. For this purpose, an orbit motor reference unit is chosen. A multi-domain simulation tool to evaluate the performance of this reference unit, considering contact features, is developed. The approach for creating the simulation tool is based on coupling of different models: pre-processor tools are created that can provide information needed by fluid dynamic model; a 2D CFD model is created that can evaluate leakages through the lubricating gaps based on pressures from fluid dynamic model; and a fluid dynamic model that can accept inputs from other models and evaluate the primary flow of the unit using a lumped parameter approach. This approach allows an accurate prediction of performance characteristics of orbit unit and the results are compared with those of experiments in terms of flow rate (maximum deviation up to 2.5%) and torque (maximum deviation up to 10%). Variation of performance of the unit by modification of contact features is presented, thus drawing the importance of contact forces in simulating a positive displacement gear machine.</p> <p><br></p> <p>After presenting the importance of contact forces, emphasis is placed on creating an accurate model of the traction contact force, in terms of traction coefficient. The traction coefficient is evaluated  by solving a mixed thermal EHL system, for the case of lubricated non-conformal contacts, considering possible asperity effects and temperature change. A few required characteristics of the reference lubricant are obtained using experiments, along with asperity friction coefficient for the lubricant-solid combination for two different roughnesses. The solver is further validated, both in magnitude and trend, against experimental results for the variation of roughness and slide-to-roll ratio of the surfaces. The solver is further used to obtain curve-fit relations of the traction coefficient components with reasonable accuracy.</p> <p><br></p> <p>Lastly, the curve-fit relations of the traction coefficient are used to evaluate the meshing torque loss, and thus the hydro-mechanical efficiency for the case of two external gear machine units, having different gear flank roughnesses. The simulated hydro-mechanical efficiencies are further validated using the results from experiments, with a maximum deviation of up to 3%, but less than 0.5% deviation at many operating conditions. The experimentally obtained variation of hydro-mechanical efficiency with respect to gear flank roughness is captured in the simulations at majority of the operating conditions, thus laying emphasis on the importance of accurate contact force models.</p> <p><br></p> <p>The approaches followed in this work, along with the findings and proven accuracy with experiments, can be considered valuable and can be used to create simulation models that can capture the effects of interference/clearance and gear flank roughness on the performance of positive displacement gear units.</p>

<p dir="ltr">In recent years increasingly stringent regulations regarding the pollution emissions and greenhouse gasses (GHG) of off-highway vehicles have emerged. However, recent studies underscores that off-highway vehicles have an average efficiency of 30%. In response, researchers are exploring the possibility of electrifying these vehicles with electric machines (EMs) potentially undertaking one, multiple, or all the vehicular functions previously reliant on internal combustion engines (ICEs).</p><p dir="ltr">Contemporary off-highway vehicle technology revolves around hydraulic systems tailored for diesel engines, tuned to specific torque characteristics and operating at a single speed. While replacing the prime mover with electric machines, the proper hydraulic supply capable of matching the same torque speed characteristics must be found. Furthermore, it must be determined whether an integration capable of reducing the mass, cost, and volume can be implemented, and if energy recuperation is possible. </p><p dir="ltr">In essence, achieving the desired transformation in off-highway vehicle technology necessitates a comprehensive reevaluation of both hydraulic systems and power sources, with electrification emerging as a promising strategy for harmonizing efficiency, emission standards, and performance expectations. </p><p><br></p><p dir="ltr">This work proposes guidelines to systematically design EMs and positive displacement hydraulic gear machines (HMs), along with their integration in an electro-hydraulic unit (EHU). To do so, three different variants of EHU are produced. The first variant features an external gear machine (EGM) integrated in a permanent magnet synchronous electric machine (PMSEM). The second and third variants integrate an internal gear machine (IGM) and a PMSEM, wherein the final variant introduces features endowing its operation at high rotational velocities.</p><p dir="ltr">The EM and HM constituting all variants of EHU are designed using a genetic algorithm-based optimization framework. This optimization framework encapsulates dedicated models for the EM and the HM that allow the calculation of the EHU performance. The first optimization objectives are the minimization of power consumption over the duty cycle of the selected reference machine, the minimization of the pressure and flow ripple, and maximization of the power density of the EHU. The optimization of the second and third variants instead only aims to maximize the total efficiency and power density of the EHU. </p><p dir="ltr">After having determined the parameters of the EHU through the optimization procedure the designs are refined with thorough simulations focusing on the fluid-dynamic features and the design of the axial balance system of the HMs. </p><p dir="ltr">The three variants present an increasing level of HM and EM integration and component reduction. While in the first variant HM and EM have a dedicated housing, and the HM is only positioned in the inert region of the EM, in the latest variants the HM and the EM also share the same casing. The first variant of EHU is air cooled by a radial fan system attached to the EM rotor and openings machined in the casing. The second variant takes advantage of the extreme integration and the differential pressure generated across the HM to liquid cooling the EM. The third variant necessitates the use of an external system to cool the EM. </p><p dir="ltr">To prove the effectiveness of the design process the first two EHU variants are prototyped and tested. The first EHU variant is tested both in a standalone configuration and on the reference machine showing total efficiency values up to 69%, proving its functionality and proving the capability of recuperating energy. The tests conducted on the second variant EHU show a volumetric efficiency that ranges between 81% and above 96% for a pinion rotation velocity of 6000 rpm proving the value of the presented design process. Despite the good quality of the volumetric efficiency values, this EHU variant present morphological limitations that negatively impact its mechanical efficiency. Finally, the third EHU concept is presented not only to remedy the morphological limitations of the second variant but also to address the challenges raised by high rotational velocity operation. </p>