In vertical testing machines,
the spherical bearing block shall be spring suspended from the upper head of the machine in such a manner that the upper
platen of the machine (lower face of the spherical bearing
block) remains in a central position (spherical surfaces in full
contact) when not loaded.The spherical surfaces shall be well
lubricated, and the center of curvature shall lie on the lower
face of the platen. The diagonal or diameter of the platen shall
be only slightly greater than the diagonal of the 11⁄2-in.
(38.1-mm) square contact blocks to facilitate accurate centering
of the specimens.