The OEM shafts in all EQ8 mount types are 55mm OD and made from turned aluminium. The shaft bearings comprise a pair of opposed tapered roller bearings preloaded by an M54 x 1mm cast iron ringnut. These are mounted to the shafts with clearance. The OEM bearings are of commercial grade, for which the inner race run-out error is permitted to range up to 25 microns. The sum of the bearing/shaft clearance and the inner race run-out error cause unacceptable displacement of the DEC worm wheel relative to the worm gear, before consideration of the actual run-out error of the worm wheel assembly itself.
SAE’s replacement DEC shaft has nearly identical dimensions but is turned then cylindrically-ground from grade 304 stainless steel. It has 3 times the torsional stiffness of the OEM aluminium part, helping raise the maximum imaging payload. The fit to the ID of the shaft bearings is a transition fit, not a clearance fit. The diameter of the most restrictive section of the internal bore through the shaft is increased to aid passing USB Type B connector shells through the mount.
The P5-grade shaft bearings SAE supply reduce the maximum permissible run-out error from 25 to 5 microns and halve the inner race diameter tolerance to 12um. These tighter tolerances greatly reduce variation in Declination worm wheel/worm gear mesh clearance as the Declination shaft is rotated.