the coating is thick enough to provide for smooth cushioning but thin enough that it stiffly supports the rotating gantry although polyurethane is preferred other coatings that provide adequate stiffness preventing axial and radial movement of the gantry 24 while preventing metal to metal contact between the race 50 and the bearings 46 are contemplated with reference to fig3 one of the bearings is attached to an external drive motor 52 and becomes a drive bearing 54 the drive bearing 54 is coated with a substance that can be different from the other bearings 46 for improved friction with the bearing race 50 such a substance may be a hardened rubber or the like in order for the conical bearing race 50 and the tapered bearings 46 to be velocity matched an angle of expansion that describes the growth of the diameter of a bearing 46 along the axle is found with reference to fig4 the angle u03c6 is found to cause velocities v o and v i to match on both the race 50 and the bearings 46 first a ratio of race 50 diameter to bearing diameter is found where d o is the outer diameter of the race 50 and d o is the outer diameter of the bearing 46 and r is the ratio of the two measurements it follows that the angular speed of the bearing u03c9 t is found by where u03c9 g is the angular speed of the gantry in rpm from geometry of the system it is known that l c d o d i 2.