I am working on a project that uses a box design with a base. The base has two through holes on the the outer edges that will be used to slide up and down with in the box. The box will support two guide rods where the base can slide up and down. My intention was to use springs so that as the box is loaded the base gradually lowers. The travel is about 16'' which is a rather long spring and also takes up considerable space when compressed. Does anyone know some sort of alternative to the spring? My initial thought was some sort of bearing in the base that would only slide when a certain amount of force is applied. So far I am unable to find such a bearing. Let me know of any thoughts or ideas?