In most hydraulic component mathematical models developed by design engineers, the term "viscous friction" is involved. Because of the lack of knowledge on how to calculate the viscous friction or directly measure it, the value of the viscous friction is usually estimated and adjusted to force the model to provide the desired performance. The model validity based on this estimation may be an issue. In addition, viscous friction is function of certain dimensional parameters and operating conditions. Assuming that it a constant is a harsh assumption that affects the model validity. In this paper a study has been made to investigate a proportional valve response where the viscous friction is calculated based on variable working conditions. Simulation runs were made to show the effect of variable viscous friction on the valve stability. Results are presented and discussed. While the study is applicable for many other hydraulic components, it is developed in this paper for a proportional directional valve dynamic response as a typical example.
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