TRUCK SERVICE MANUAL
DOUBLE REDUCTION DRIVE UNIT
Top-mounted double-education drive units in the SUD and STD Series made by North American Rockwell
incorporate hypoid first reduction gears and helical spur gears in the second reduction.
The thru-shafts of the hypoid gear drive units are supported at the forward end by tapered roller bearings in a cage
and at the rear end by a ball bearing. Pinion bearing pre-load is adjusted and maintained by a hardened precision spacer
between the inner and outer tapered bearings which are held in place on the pinion journal by large nuts. Yokes and
flanges are held in place on the thru-shaft by separate thru-shaft nuts.
The inter-axle differential may be either engaged or disengaged by a power actuated shift unit which moves a
sliding collar on the pinion quill assembly.
The shift unit is controlled by a selector switch or lever within the cab of the vehicle and may be engaged or
disengaged under any normal operating conditions. The inter-axle differential when engaged (unlocked) divides the
engine torque between the forward and rear axles, when disengaged (locked) converts the two axles to a through drive
type tandem. The following Schematic Drawing illustrates the action of the inter-axle differential assembly.
The single gray tone shows the differential "locked-
The double gray tone illustrates the differential
up," inoperative, with the tandem functioning as a thru-
operating under normal conditions, distributing equal
amounts of torque to the axles.