Gas spring of variable spring force

A gas spring comprising a working cylinder (1) having an axis and defining a cavity (8, 9) therein;

• a plunger member (3, 4) extending inwardly and outwardly of said cavity (8, 9) and defining a first working chamber (8) within said cavity (8, 9), said plunger member (3, 4) being axially movable with respect to said working cylinder (1) inwardly and outwardly of said cavity (8, 9);
• a mass of gas within saids first working chamber (8), the pressure of said gas being responsive to the axial position of said plunger member (3, 4) with respect to said working cylinder (1);
• gas exchange means for selectively increasing and decreasing the mass of gas contained within said first working chamber (8), said gas exchange means comprising
• a gas exchange chamber (10) fixed with respect to said working cylinder (1);
• exchange passage means extending between said first working chamber (8) and said gas exchange chamber (10);
• exchange valve means (14) within said exchange passage means;
• and
• exchange operating means separated from said plunger member (3, 4) further characterized in that
• said gas exchange chamber (10) accommodates a pumping piston (12) movable between an innermost position adjacent said first working chamber (8) and an outermost position remote from said first working chamber (8), said pumping piston (12) defines a pumping chamber (10a) within said exchange chamber (10), said pumping chamber (10a) is adjacent said exchange valve means (14), said pumping piston (12) is provided with suction valve means (12a) permitting aspiration of air into said pumping chamber (10a) on outward movement of said pumping piston (12) and closing said pumping chamber (10) on inward movement of said pumping piston (12), said exchange valve means (14) is of the check-valve type and is closed in response to the pressure within said first working chamber (8) exceeding the pressure within said pumping chamber (10a) and is opened in response to the pressure within said pumping chamber (10a) exceeding the pressure within said first working chamber (8);
• said pumping piston (12) is provided with a pump operating member (13a) outside said gas exchange chamber (10) and with an exchange valve actuating pin (16) opening said exchange valve means (14), when said pumping piston (12) is in or adjacent its innermost position;
• and
• said pumping pistion (12) comprises manually operable bleed valve means (17, 18) for connecting said pumping chamber (10a) with atmosphere.