Hydraulic drive apparatus comprising a cylinder

A hydraulic drive apparatus comprising:

• a hydraulic cylinder having a cylinder tube,
• a first closing means adapted to close said cylinder tube at one end,
• second closing means adapted to close said cylinder tube at the other end,
• first and second working ports (30,33) in said first and second closing means, respectively, a piston movably mounted in a working chamber defined by said cylinder tube and said first and second closing means, and defining a first and second pressure chamber, respectively, at opposite sides of said piston, at least one piston rod fixedly mounted to said piston, at least one seal means for said at least one piston rod, said at least one seal means comprising a first seal means (19) adapted to provide a sealing effect between said at least one piston rod and said first closing means, a second seal means (20) spaced axially outwardly from said first seal means and adapted to provide a sealing effect between said piston rod and said first closing means, said sealing effect occurring between said second seal means and said piston rod being controllable by pressure applied to said second seal means, such that a high pressure will cause a firm application of said second means and thus a high sealing effect, while a low pressure will cause a low sealing effect and a low wear condition if the piston rod were to move, and
• a leakage oil connecting means (24) arranged between said first and second seal means in said first and second closing means, respectively.