Any structure other than chains may be used.Door 612A has an open position, which is generally shown in FIG. 16, where it is not retaining shaft 38, and a closed position (not shown) where it is tightly pressed against and rigidly retains shaft 38 when shaft 38 is positioned in cavity 620A. An aperture 622A is formed in door 612A and a corresponding, vertically aligned aperture (not shown) is form