Temperature-compensating hydraulic positioner

A hydraulic positioning element comprising a cylinder member (1) having an axis and two end portions, namely a first end portion and a second end portion and a cavity (8, 7) within said cylinder member (1) axially between said first and said second end portions;

• a first guiding and sealing arrangement at said first end portion and a second guiding and sealing arrangement at said second end portion;
• a piston rod member (2) extending inwards and outwards of said cavity (8, 7) through said first guiding and sealing arrangement;
• a piston unit (3) within said cavity (8, 7) fixed to said piston rod member (2) and dividing said cavity (8, 7) into two working chambers, a first working chamber (8) adjacent said first end portion and a second working chamber (7) adjacent said second end portion;
• a volume compensating rod member extending inwards and outwards of said cavity (8, 7) through said second guiding and sealing arrangement and fixed to said piston rod member (2) for common axial movement therewith;
• a volume of hydraulic fluid within said first and said second working chambers (8, 7);
• a fluid passage extending across the piston unit (3) and interconnecting said first and said second working chambers (8, 7) for fluid exchange therebetween;
• valve means (6) associated with said fluid passage;
• valve means operating means associated with said valve means (6) for opening and closing said fluid passage means;
• and
• fluid expansion compensating means, said fluid expansion compensating means comprising at least one axially movable partition unit (14, 15) being part of at least one of said first and said second sealing and guiding arrangements and sealed with respect to both said cylinder member (1) and a respective one of said piston rod member (2) and said volume compensating rod member, said partition unit (14, 15) being supported in axial direction through an axially deformable compensating body (9) by abutment means (13, 16, 17) fixed with respect to said cylinder member (1), said compensating body having a deformation force-deformation path characteristic which is responsive to temperature within an operational range between room temperture and about 80.degree. C. so that said compensating body is relatively rigid at room temperature and more deformable at about 80.degree. C.