I can’t speak directly to the Robinson transmission but in other gearboxes bearing preload is normally controlled by the use of shims. The shims control the amount of compression placed on the bearing inner race so that the balls or tapered rollers have controlled compression between the inner and outer races. In other cases the preload can be applied by the torquing of bolts against a flat plate which compresses the inner race with the outer race being captive. The effect is the same.
