Continuously self-compensating hydraulic positioner

A hydraulic positioner comprising a cylinder closed at one end, an axially displaceable gland closing an opposite end of a fluid filled chamber in said cylinder, a piston axially reciprocable in said chamber, a rod axially connected to said piston and slidable through said gland, said rod moveable with said piston between an extended and a compressed condition, valve means in said piston actuatable externally to said cylinder for allowing flow of said fluid across said piston, said valve means being normally closed for locking said piston and said rod against movement in said cylinder, a return spring urging said rod to said extended condition, and internal spring means in said cylinder cooperating with said return spring for jointly urging said gland against said fluid thereby to compensate for changes in volume of said chamber as said rod moves in and out of said chamber and changes in fluid volume caused by leakage or temperature changes;

• stop means between said gland and said piston for limiting the stroke of said piston thereby to divide said chamber into a piston chamber and a reservoir open to each other, and indicator means on said rod external to said cylinder and movable with said gland, the position of said indicator means relative to said cylinder being representative of the amount of fluid in said reservoir.

A hydraulic positioner comprising a cylinder, a piston chamber in said cylinder closed at one end by an axially displaceable gland, a piston axially reciprocable in said piston chamber, a rod axially connected to said piston and slidable through said gland, said rod moveable with said piston between an extended and a compressed condition, hydraulic fluid in said piston chamber, valve means in said piston actuatable for allowing flow of said fluid across said piston, said valve means being normally closed for locking said piston and said rod against movement in said cylinder, wherein said valve means are spring biased to said normally closed condition and said valve means can be driven to an open condition by an external override force applied to said piston rod, a first spring urging said rod to said extended condition, and a second spring cooperating with said first spring for jointly urging said gland against said fluid to compensate for changes in fluid volume in said chamber, said positioner characterized in that said second spring acts on said gland with a force greater than said override force.