Welded center differential

By Sean Kelly

The center differential initiates the torque split between the front and rear differentials in the Getrag W5MG1 5-speed all-wheel-drive transmission. It is comprised of a "front output shaft" which drives the front differential ring gear; center shaft, also acting as the "rear output shaft" which drives the transfer case input spool and effectively the driveshaft and rear differential and rear drive wheels of the vehicle; multi-disc viscous coupling unit (VCU) which drives the "front output shaft", a planetary gear assemble that drives the "rear output shaft" directly. The planetary gear assembly is what we are concerned with. It is comprised of three sets of two pinion gears on a carrier. The two pinion gears are positively engaged to each other.

Of each set, one is driven by the internal gear on the center differential housing and its pinion shaft is coupled with the VCU. The other set is engaged to the center shaft/rear output shaft. Under normal operation, the intermediate shaft of the transmission drives the outer gear on the center differential case. The inner gear of the center differential housing then turns the outer pinion on the planetary gear assembly which directly drives the VCU, and the meshed inner pinion gears rotate the opposite direct on their shaft and drive the center shaft. Hence the center shaft is rotating inside the front output shaft and they are rotating in opposite directions. There is a torque splits between the two and also a slip-limiting/power-sensing effect due to the VCU: when the VCU senses one side (front or rear set of wheels) slipping it distributes torque to the opposite side.

Since the torque split happens at the planetary gear assembly, this is where you would apply a locking device to full couple each set of drive wheels to each other. This can be accomplished by locking the planetary gear assembly so that the inner gear on the center differential housing is locked to the center shaft and the out pinion gear's shaft is coupled to the VCU and front output shaft. We accomplish this by welding the outer and inner pinion gears to each other and for strength we weld the pinion gears to the carrier. This effectively make the planetary gear assembly operate as a solid machine and directly couples the center differential housing to both output shafts.

This is coupled with welding the VCU into a solid piece. If you did not do this, the two parts of the outer center shaft (front output shaft) would turn the plates in the VCU and cause an overheating situation.

After that is done, the center differential (the case and its internals, including the VCU, and planetary assembly) acts as a solid unit and turns the two output shafts at equal speeds with equal torque, locked together via the solid planetary assembly. This allows the use of only one output shaft to be used, which will only drive two wheels; making the car either front wheel drive or rear wheel drive. It also allows the user to lock both differentials into a singular differential speed, so both differentials will always be traveling the same speed. This is good for traction and acceleration but it doesn't no allow turning due to the inability of the outer wheels to turn faster than the inner wheels in a turn. When you lock both differentials, it is effectively making the car a 4x4, like a jeep with a locker. The outside wheel will skip in a turn. That is, until something else accommodates for the speed discrepancy: something snaps! This is not recommended, but use on a drag strip may see gains due to equal torque distribution and the ability to gain more traction.

The alternative use is 2-wheel-drive. This can be optimal for dynamometer tuning if converted to front wheel drive only. Not only will there be reduced power loss due to friction, rates for 2-wheel drive vehicles is typically lower due to the higher cost of equipment made for 4-wheel drive vehicles.