Disc clamp employing resilient cone for spreading balls

A clamping mechanism for clamping, on a disc tester, computer hard discs having central openings, said discs each having a predetermined thickness, comprising:

• a cylindrical disc support having a cylindrical opening, side walls, a bottom wall, an upper surface for supporting a hard disc to be clamped, and a central opening in said bottom wall,
• said cylindrical opening of said disc support having a diameter substantially equal to said central opening of said hard disc;
• said disc support being attached to a base of said tester which rotates about an axis, said axis passing through the center of said central opening;
• a cylindrical retainer slidingly inserted into said cylindrical opening of said support and resting upon said bottom wall of said disc support,
• said retainer having more than three through openings in a side wall thereof, said openings being uniformly spaced in a circumferential direction, said openings having axes located higher than the upper surface of one of said hard discs when said disc rests on said upper surface of said support;
• a plurality of rigid balls slidingly located in said respective through openings of said retainer, said balls having diameters exceeding the thickness of said side wall of said retainer so that the edges of said balls can protrude beyond the outer periphery of said retainer and come into contact with said opening of said hard disc, when said hard disc is positioned on said support;
• a pull rod capable of moving in the longitudinal direction, said pull rod having an upper end which passes through said central opening of said bottom wall into said cylindrical retainer;
• a conical element rigidly attached to said upper end of said pull rod, said conical element having a continuous outer conical surface which is resilient and softer than said balls,
• said conical element having a larger diameter end and a smaller diameter end, said larger diameter end located above said balls and said smaller diameter end facing said bottom wall of said support,
• said conical element and said pull rod being moveable between upper and lower positions and being shaped such that when said pull rod and said conical element are in their upper position, the conical surface of said conical element rests upon said balls, and such that when said pull rod and said conical element are pulled down to their lower position, said conical surface will wedge and hold said balls outwardly to positions where they lock said hard disc onto said support.