To protect a joint part from external force and smooth down the motion of rod by setting a collar on rotatable parts of gear and so on, which rotate integrally with a step board, or a edge of rod, which connects to them, to form a wall for damage protection at a collision.

A rack 5 at the edge part of rod is formed as a square section and a collar b is formed on the at least one face of gear 3. The inside of collar b is sledded to the side face of rack 5 between each other. When the force is exerted from the arrow P direction to the step board 1 and a vertical member, which connects to a shaft 7 vertically and integrally, gets loose and rotates slightly, the external force is transmitted to the gear 3. As it becomes necessary to stand up through the collar b at the side face of rack 5, it is possible to support against the external force by the making of collar b. Thus the mesh of rack 5 is kept exactly and the rod moves vertically without looseness.