along the guide rods and guided by them to the upper pressure plate 60 as seen in fig4 a pull rod 64 is attached extending inside the spring in the longitudinal direction of the spring a non elastic but flexible band or wire 66 is attached to the pull rod the band runs around a first pulley wheel 68 which is rotatably arranged to the base plate then around a second pulley wheel 70 rotatably arranged to the base plate the second pulley wheel corresponds to the wheel s 1 of fig1 the end of the band is attached to the end of an arm 72 which arm is rotatably arranged around a shaft 74 attached to the base plate the arm corresponds to the arm 10 of fig1 a second non elastic but flexible band or wire 76 is attached to the same end of the arm as band 66 the second band runs around a third pulley wheel 78 corresponding to the wheel s 2 of fig1 and is attached to the peripheral surface of a wheel 80 which wheel is attached to a shaft 82 which in turn is rotatably attached to the base plate a stop member not shown is arranged to prevent the wheel 80 to rotate anti clockwise more than the initial position shown.