Abstract: In a preferred embodiment of the invention an extension section of a frame is held down in its locked position by the same mechanism that folds and unfolds the frame extension. When the frame extension is in its locked down position, it bears against a limit stop that precludes further movement in an unfolding direction. An operating link that is pushed and pulled by a hydraulic cylinder to manipulate the frame extension is controlled in such a way by a guide member that the line of action of the link is directed transversely toward a guide surface of the guide when the frame extension is in its unfolded position. Thus, only a small amount of force is required from the hydraulic cylinder at that time to maintain the link in its locked position, since most of the load from the link is born by the guide surface at this time. In a preferred embodiment, the line of action of the link is approximately perpendicular to the guide surface.

Abstract: In a preferred embodiment of the invention an extension section of a frame is held down in its locked position by the same mechanism that folds and unfolds the frame extension. When the frame extension is in its locked down position, it bears against a limit stop that precludes further movement in an unfolding direction. An operating link that is pushed and pulled by a hydraulic cylinder to manipulate the frame extension is controlled in such a way by a guide member that the line of action of the link is directed transversely toward a guide surface of the guide when the frame extension is in its unfolded position. Thus, only a small amount of force is required from the hydraulic cylinder at that time to maintain the link in its locked position, since most of the load from the link is born by the guide surface at this time. In a preferred embodiment, the line of action of the link is approximately perpendicular to the guide surface.