Abstract: The piston mechanism comprises a piston (3), a sliding shoe (6) and a coupling member (18) holding together the piston (3) and sliding shoe (6). The coupling member (18) comprises a shaft part (19) and joint heads (20,21) arranged at the end of the shaft part (19). The heads (20,21) have spherical zone-shaped sliding surfaces (9,11,23,24), which cooperate with corresponding sliding surfaces (9,24) in sliding shoe (6) or in the bottom of the piston (3). Due to the fact that the radius of curvature (R1,R2,R3) of the sliding surfaces of the sliding shoe (6) and the piston (3) have the same centre of curvature (10) and therefore the same fulcrum, damage to the sliding surfaces is avoided.

Abstract: A pressure limiting valve has two pipe connections connectable to pipes at different pressures and a main piston axially movable in a sleeve in a housing. One end of the piston is closed and the other, free end is attached to an annular piston having two oppositely facing annular surfaces. One annular surface is exposed to the higher pressure through a fluid circuit portion including a changeover valve and the other annular surface is exposed to the pressure at one of the connections. The interior of the main piston is exposed to the lower pressure through a fluid circuit portion including a servocontrol valve. The resulting structure is a double-acting limiting valve having a very low mass piston and high response sensitivity.