Dissertation under embargo
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A STUDY ON CONTACT FORCES IN HYDRAULIC GEAR MACHINES
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.
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.
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.
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.
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.