done
10-17-2007, 02:59 PM
The AWD Transfer Case – Theory of Operation
The transfer case in the AWD is based on the Ravigneaux planetary gearset. A typical configuration of the this gearset is shown below. The unique characteristic of the Ravigneaux gearset is the use of two sun gears, yellow and blue in the illustration and the secondary planet gears, white in the illustration. The ring gear is in purple and the planet carrier is in red. Thus, this set has three input/output shafts and a ring gear. This type of gearset is commonly used in automatic transmissions, notably Lexus and Mercedes and some hybrids.
http://www.watsoncard.com/magnum/ravigneaux3.gif
However, in the AWD transfer case, the ring gear is omitted as shown in the next two illustrations.
http://www.watsoncard.com/magnum/ravigneaux.jpg
http://www.watsoncard.com/magnum/ravigneaux2.jpg
For this discussion for the AWD, the red planetary carrier and shaft represents the input shaft from the transmission. The blue sun gear and shaft represents the rear output shaft and the purple sun gear and shaft represents the front output shaft. I will explain the exact configuration of the AWD gearset later.
To begin to understand how this gearset works, first ignore the white planet gears and the purple sun gear and focus on the red carrier, the green planet gears and the blue sun gear.
Now visualize that the red carrier is being turned by the transmission and remember that the blue sun gear is fixed to the rear drive shaft. As the carrier rotates, the green sun gears would simply roll around the blue sun gear and no power would be transferred. In a conventional planetary set, the ring gear would force the planet gears to rotate in the opposite direction of the carrier, thus forcing the blue sun gear to also rotate in the same direction as the carrier.
But we have no ring gear. Now visualize the action of the white planet gears as the green planet gears rotate around the blue sun gear. They must also rotate the white sun gears. If the white sun gears are rotating, then the purple sun gear must also rotate. Since the purple sun gear is driving the front output shaft, that shaft must also rotate, but in the opposite direction of the carrier. But wait, if the front output shaft is rotating, then the rear output shaft, the blue sun gear must also be rotating because the car is moving. Thus, the green sun gears must then be driving the blue planet gear and the rear output shaft also.
Pretty darn clever! What a first glance seems to be a freewheeling gear set actually applies torque to both output shafts. The torque split is the ratio of the size of the two sun gears and thus must always remain constant.
Now, how do we get a front to rear differential action.? First, visualize the front wheels turned to full steering lock with the car moving backwards, it will make the following easier to understand. The turning radius of the front wheels is larger than that of the rear wheels so the front wheels must rotate faster than the rear wheels to maintain the same rolling distance.
If the front output shaft and thus the purple sun gear is rotating faster, the action through the green gears combined with the action of the moving carrier must then force the blue sun gear and the rear output shaft to rotate slower to keep the forces in the gearset in equilibrium. Therefore, we have a front to rear differential action. Again, pretty darn clever.
Reality vs. Illustration
Our actual AWD case is constructed a little differently than the illustration, but the operation is still consistent. The differences are:
The green gears are a single, long gear driving the front output sun gear and the rear intermediate gears.
The carrier is a very substantial structure that supports the main planet gears on both ends.
The front and rear sun gears are reversed in relation to the carrier. Thus the front output shaft is concentric around the transmission input shaft.
The front output shaft actually drives an idler gear which in turn drives the front drive shaft. This also provides the physical offset to clear the engine.
A friction plate exists between the carrier and the front output shaft. As the carrier and the front output shaft rotate in opposite directions, this friction plate keeps the gearset in a constant tension to minimize vibrations as the loads change on the various gears.
The transfer case in the AWD is based on the Ravigneaux planetary gearset. A typical configuration of the this gearset is shown below. The unique characteristic of the Ravigneaux gearset is the use of two sun gears, yellow and blue in the illustration and the secondary planet gears, white in the illustration. The ring gear is in purple and the planet carrier is in red. Thus, this set has three input/output shafts and a ring gear. This type of gearset is commonly used in automatic transmissions, notably Lexus and Mercedes and some hybrids.
http://www.watsoncard.com/magnum/ravigneaux3.gif
However, in the AWD transfer case, the ring gear is omitted as shown in the next two illustrations.
http://www.watsoncard.com/magnum/ravigneaux.jpg
http://www.watsoncard.com/magnum/ravigneaux2.jpg
For this discussion for the AWD, the red planetary carrier and shaft represents the input shaft from the transmission. The blue sun gear and shaft represents the rear output shaft and the purple sun gear and shaft represents the front output shaft. I will explain the exact configuration of the AWD gearset later.
To begin to understand how this gearset works, first ignore the white planet gears and the purple sun gear and focus on the red carrier, the green planet gears and the blue sun gear.
Now visualize that the red carrier is being turned by the transmission and remember that the blue sun gear is fixed to the rear drive shaft. As the carrier rotates, the green sun gears would simply roll around the blue sun gear and no power would be transferred. In a conventional planetary set, the ring gear would force the planet gears to rotate in the opposite direction of the carrier, thus forcing the blue sun gear to also rotate in the same direction as the carrier.
But we have no ring gear. Now visualize the action of the white planet gears as the green planet gears rotate around the blue sun gear. They must also rotate the white sun gears. If the white sun gears are rotating, then the purple sun gear must also rotate. Since the purple sun gear is driving the front output shaft, that shaft must also rotate, but in the opposite direction of the carrier. But wait, if the front output shaft is rotating, then the rear output shaft, the blue sun gear must also be rotating because the car is moving. Thus, the green sun gears must then be driving the blue planet gear and the rear output shaft also.
Pretty darn clever! What a first glance seems to be a freewheeling gear set actually applies torque to both output shafts. The torque split is the ratio of the size of the two sun gears and thus must always remain constant.
Now, how do we get a front to rear differential action.? First, visualize the front wheels turned to full steering lock with the car moving backwards, it will make the following easier to understand. The turning radius of the front wheels is larger than that of the rear wheels so the front wheels must rotate faster than the rear wheels to maintain the same rolling distance.
If the front output shaft and thus the purple sun gear is rotating faster, the action through the green gears combined with the action of the moving carrier must then force the blue sun gear and the rear output shaft to rotate slower to keep the forces in the gearset in equilibrium. Therefore, we have a front to rear differential action. Again, pretty darn clever.
Reality vs. Illustration
Our actual AWD case is constructed a little differently than the illustration, but the operation is still consistent. The differences are:
The green gears are a single, long gear driving the front output sun gear and the rear intermediate gears.
The carrier is a very substantial structure that supports the main planet gears on both ends.
The front and rear sun gears are reversed in relation to the carrier. Thus the front output shaft is concentric around the transmission input shaft.
The front output shaft actually drives an idler gear which in turn drives the front drive shaft. This also provides the physical offset to clear the engine.
A friction plate exists between the carrier and the front output shaft. As the carrier and the front output shaft rotate in opposite directions, this friction plate keeps the gearset in a constant tension to minimize vibrations as the loads change on the various gears.