Steady-Amplitude, Self-Excited Oscillations of Hydraulic Spool Valves

Abstract

The free oscillations of an inertia-loaded hydraulic spool valve are examined by obtaining first the theoretical conditions for a steady amplitude oscillation of a three-way valve working at low pressures where compressibility of the fluid is unimportant. The predictions are confirmed qualitatively, in experimental work on valves, but quantitatively there is some discrepancy between the experimental and theoretical negative damping forces for short valves. This theory is then extended to the case where compressibility creates a distributed parameter effect in the inlet pipe, and it is shown that jumps in frequency and amplitude occur when the length of the inlet pipe is near to odd multiples of one-eighth of a wavelength. Another resonant system on the output of the valve also occurs when fluid compressibility combines with the inertia of a load on an output actuator, and again the agreement between the theory and experiment is good. It is concluded from the work that a freely oscillating valve could be used as an oscillator, but the control over frequency of oscillation would be poor. Changing load conditions have a marked effect on both frequency and amplitude of oscillation.