Hydraulic positioner with direct connection to pump - has control circuit which at rest equalises pressures either side of positioner cylinder

摘要

The hydraulic positioning system contains an hydraulic pump which directly serves the positioner, the pump delivery rate and direction of operation being variable by a separate hydraulic control circuit. It comprises a control unit which receives the pressure from the pump and from the control circuit. This unit is so arranged that, in the no-load stationary condition, it equalises the pressure on both sides of the positioning cylinder at a level of half the control circuit pressure. The unit may comprise a piston valve whose sliding piston is centralised by two springs, the piston collars being so dimensioned, that the surface area of their annular faces equals the cross-sectional area of the piston rod.