Abstract: The fatigue failure of the servo-valve's spring tube in vibration environment is used to be hardly quantitatively predictable, in order to provide a solution, by introducing the impulsive inertia force a mathematical model of deflector jet servo-valve in vibration environment is established, thus acquiring the force condition of the spring tube and the cyclic stress of its thin wall's root. Therefore, by the calculation of nominal stress we propose a method for solving the spring tube's fatigue strength in extreme vibration environment. Taking a certain type of deflector jet servo-valve for calculation example, we achieve the result that under 100 g spool axial simple harmonic vibration with hydraulic pressure, the stress amplitude of the spring tube's root is 275 MPa,which is less than the symmetry cycle fatigue strength(10⁸ 361 MPa), while under 20 g (RMS) random vibration without hydraulic pressure, the stress is 120 MPa, which is less than the nonsymmetry cycle fatigue strength(10⁸ 228 MPa), indicating that the fatigue life of the spring tube has exceeded 10⁸ under thus different extreme environment. Furthermore,experiments of the same conditions are carried out, which contains simple harmonic vibration(1.44×10⁷ cycle) and random vibration(5.47×10⁵ cycle). No performances abnormal of servo-valves or fracture of spring tubes do occur, which testified the dependable of the calculating method of spring tube's fatigue strength under vibration environment.

Key words: extreme vibration environment, deflector jet servo-valve, spring tube, random vibration, fatigue strength