All-Wheel Drive Disconnect Clutch System: Schaeffler Symposium

27 AWD disconnect clutch system 27 AWD disconnect clutch system AWD disconnect clutch system 27

All-wheel drive disconnect clutch system

Brian Lee

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hicle propulsion or control, therefore signifi- 200 200 Introducti on cant energy is lost due to friction in these drive- 175 175 lines. 150 150 All Wheel Drive vehicles continue to be popular 125 125 around the world for their advantages in safety 100 100 and capability. Therefore, the fuel economy of 75 75 AWD must be improved like their conventional System descripti on Torque in Nm 50 Torque in Nm 50 vehicle cousins. The additional rotating compo- 25 25 0 0 nents offer new opportunities to eliminate loss- The Schaeffler AWD disconnect clutch system is 0 0.05 0.10 0.15 0.20 0 0.05 0.10 0.15 0.20 es. By disconnecting the secondary driven axle incorporated into the input shaft of the front Time in s Time in s and propshaft, the rotating losses in the bear- PTU, thus allowing disconnection of the power Inertial Inertial ings and seals are eliminated. Such a system is flow where the secondary axle driveline branch- Frictional Frictional capable of achieving greater than 5 % fuel econ- es from the primary axle. When used in conjunc- Total Total omy improvement in a typical passenger car. The tion with rear axle disconnects, perhaps simple Figure 3 Synchronizati on torques and ti mes for manual transmission (left ) and AWD (right) system can be packaged in a conventional AWD dog clutches, the secondary driveline rotation powertrain or, where space is a constraint. The can be stopped thus eliminating the parasitic quired for synchronizing driveline velocities. becomes significantly greater than the inertial space and weight savings created by inclusion of power losses. Reconnection of the power flow is The shifting mechanism is packaged radially component. the Schaeffler lightweight differential in a trans- initiated by an electronic control module based around the driveshafts thus eliminating the axle allows introduction of a disconnect clutch on pre-emptive monitoring of multiple driving Removing the velocity parameter from power need for a secondary shift-rail axis. This unique system. conditions. Lock-up of the system can be com- yields the equati on for synchronizati on torque: arrangement produces an extremely efficient pleted within 500 ms. The basic configuration of the system can be package volume capable of high-energy syn- t = I · ω + T seen in Figure 1. AWD vehicles generally consist The disconnect system (Figure 2) is comprised chronizing events. synch t drag of permanently engaged front and rear drive of a high-torque, triple cone synchronizer pack synch While drawing on proven systems and materials, axles connected via 90° ring and pinion gear- energized via an electromotoric actuator. The designing for the dynamics of the AWD requires sets located in the front power transfer unit electric motor is coupled with a force multiply- Figure 3 shows the inﬂ uence of each torque com- a restudy of the synchonization equations. Syn- (PTU) and the rear differential assembly. During ing roller ramp system capable of producing the ponent in a manual transmission versus an AWD chronizer design is governed by the power limits normal driving, i. e. non-slipping road condi- several thousand newtons of shift force re- synchronizati on event, where the green line is total of the chosen materials. Synchronization power tions, the secondary axle, typically the rear, is synchronizati on torque, red is the inerti al compo- can be defined by the following equation, where spinning but contributing no power to the ve- nent and blue is the fricti on component. While it Shift Sleeve Ramp Actuator I is driveline rotational inertia, ω is differential Triple Cone can be clearly seen that torque is higher in the Synchroniser velocity, t is time to synchronize and T is synch drag AWD disconnect system, the power limits of a Assembly the frictional torque to rotate the driveline: manual transmission synchronizer are not exceed- 2 Primary Axle P = I · ω + (T · ω) ed. t drag synch AWD synchronization requires significantly higher torque than a manual transmission shift, and Schaeffler Disconnect The power equation can be easily separated it must be maintained throughout the shift. into two components, one influenced by iner- These requirements dictate a unique design for tial loads and the other by frictional loads. In a the shift actuation system. The traditional shift typical manual transmission shift event, the system design, using a shift rail, shift fork, and power contribution from the inertial loads is on pads becomes unacceptable under these condi- the order of ten times greater than the friction- tions. High loads transmitted through the shift al loads. High differential velocities synchro- fork to the secondary shift rail axis produce ex- nized in very short times produce high inertial cessive bending moments and can cause bind- loads, while well lubricated transmission bear- ing, fatigue and wear problems in the compo- Dog Clutches ings maintain frictional torque at low levels. nents. Looking at the components of the power equa- The Schaeffler disconnect clutch system elimi- 27 tion in the AWD application, we see a reversal of nates the secondary axis of the shift rail and fork Secondary Axle the contribution of each component, particular- and replaces it with a coaxial roller ramp actua- Output Shaft Input Shaft ly at cold temperatures where driveline friction- tor module. This unit consists of an input plate (To PTU) (From Transmission) al drag becomes extremely high. In a cold tem- with ramps, an output plate with ramps and a Figure 1 Typical layout of an AWD powertrain Figure 2 Cross secti on of actuator and synchronizer perature AWD shift, the frictional component caged roller assembly between the two plates.

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The input plate is coupled to the electric actua- Output tor motor via a gear train and is able to rotate Shift Sleeve Plate around the main PTU input shaft axis. The output plate is rotationally fixed to the PTU alumi- num case via tabs on the outer diameter, but is able to move coaxial along the main shaft axis. When the input plate rotates, the rollers ride up the ramps thus forcing the output plate to move axially. The output plate is coupled with the synchronizer shift sleeve, therefore the function of the ramp actuator is to apply force to the synchronizer assembly through the shift sleeve. In this manner, the relatively low torque of the emotor is magnified to create adequate force for synchronizing AWD torques. In a first step the velocity of the parts is synchronized. Once the parts are synchronized the parts are coupled by a spline (Figure 4). In addition, the coaxial positioning of the actuator allows a high system stiffness and thus less likelihood of a shift system failure. Finally, the coaxial package produces a very compact shift Input Plate Output Shaft system, which produces minimal underhood (Driven by packaging complications. Electric Motor)

Figure 4 Actuated AWD disconnect clutch system

Conclusion second axis of a drivetrain a significant reduc- tion of friction losses and thus a much better The AWD disconnect clutch system consists of fuel economy is achieved. In extreme cases the known, reliable parts and is packaged radially second axis is actuated within 500 ms. The sys- around the shaft of the PTU. It is operated by a tem offers fuel savings potential of about 5 %, coaxial, electromotoric actuator. The engange- bringing an AWD vehicle to a similar fuel econ- ment mechanism is especially designed to the omy as the FWD base vehicle with no sacrifice high operation forces. By disconnecting the in driveability.

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