US Patent 7467787 Compression spring rod

A compression spring rod for relatively displacing elements attached to end mounts of the rod assembly comprises a housing having a rod member moveable between extended and retracted positions relative thereto, and a first compression spring in the housing surrounded by a second compression spring for biasing the rod member to one of an extended or retracted position relative to the housing. The two springs are oppositely wound, of different length, of different outside diameter, and of different wire diameter whereby, from a compressed condition, the spring rod exerts an expansion force which increases at a linear rate. Two sets of the springs can be arranged for biasing the rod member to a central position relative to the housing.