Oil Hydraulic Spool Valve Operating Times

Abstract

This paper concerns oil hydraulic spool valves of the on-off type and describes an investigation into the time taken for such valves to open or to close. Operating times depend upon the forces acting on a spool as it slides along the bore and the balance of these axial forces has been both analysed and studied experimentally.Operating times were first estimated from a simplified equation neglecting forces due to fluid flow. Experiments were then made on two solenoid operated valves to measure actual times and to discover the reasons for differences between calculated and measured values.Measurements of the sliding friction force between spool and bore indicated that the spools were never concentric in their bores but were displaced radially at all times.Measurements and analyses were also made of the forces due to fluid flow through the test valves. These suggested that it is important to take account not only of momentum changes (1)† and fluid acceleration effects (2) but also of fluid friction in the valve chamber. The influence of fluid flow on operating times was found to be significant unless flow rates were small.Another equation incorporating flow effects was then developed and shown to give a realistic indication of the behaviour of a valve. It is concluded that a simple form of equation may be adequate for approximate predictions of valve operating times, but that a more satisfactory method is to use the complete equation based on the work reported in this paper.