CFD analysis of the squeeze film damping mechanism in the first stage of servovalves

A fundamental component of two-stage servovalves is the flexure tube, since it both constitutes a low friction pivot for the inner flapper and allows the torque motor to be separated from the hydraulic fluid, thus avoiding contamination particles being trapped inside the torque motor. The inertia of the torque motor armature interacting with the flexure tube stiffness gives lightly damped resonances, which may lead to fatigue failure due to excessive bending under vibration, as well as limiting the position control bandwidth of the main spool. This effect is counteracted by the film of liquid interposed between the flapper and the flexure tube, which is "squeezed" during the flapper motion providing damping. However, the underlying physics of the damping mechanism caused by the squeeze film inside the flapper-flexure tube system is not well-understood, and to date the scientific literature has lacked analyses and investigations aimed at providing insights into this phenomenon. Because of this, the aim of this paper is to develop a reliable CFD model which can help to understand where and how the damping forces are generated during the flapper motion because of the squeeze film. The developed model could be used in further investigations, aimed, for example, at studying the effects of fluid properties and geometric parameters upon the damping factor, in order to achieve more effective designs which can enhance the damping factor in the flapper-flexure tube system of new generation servovalves. This work has been carried out as a collaboration between the University of Bath and the Polytechnic University of Bari, and Moog Controls ltd (Tewkesbury, UK), a world leading manufacturer of servovalves.