HYDRAULICALLY ACTUATED ELECTRONIC LIMITED SLJP DIFFERENTIAL FOR

FRONT VVHEEL DRIVE VEHICLES

FIELD OF THE INVENTION

[0001] This invention resales to a limited slip differentia! assembly particularly adapted for front wheel drive vehicle applications.

BACKGROUND OF THE INVENTION [0002] Conventional rear-wheel drive motor vehicles provide wheel driving torque through a propeller shaft coupled to left and right axles through a differentia;. Front-wheel drive vehicles couple to front wheel drive axles through a differential •driven by a transaxle. Four-wheel drive and socaited ail-wheel drive vehicles also use differentials to drive front and rear axtes. Differentials allow differences in wheel rotational speed to occur between the left and right side driven axles. The earliest and most basic designs of differentials are known as open differentials in that they provide constant torque between the two axles and do not operate to control the relative rotstsonas speed between the axle shafts. A weil known disadvantage of open differentials occurs when one of the driven wheels engages the road surface with a low coefficient of f notion (μ) with the other having a higher μ. In such case, the low tractive effort force developed at the low μ contact surface prevents significant torque from being developed on either axle. Since the torque between the two axle shafts is relatively constant, little total tractive effort can be developed to pull the vehicle from its position, Similar disadvantages occur in dynamic conditions when operating, especially in low μ or so-called split μ driving conditions. [0003] The above limitations of open differentials are weil known and numerous design approaches have been employed to address such shortcomings.

One approach Is known as a locking differential. These systems are typseal'y mechanically or hydraulicaiiy based or use other strategies to attempt to couple the two axle shafts together to rotate at nearly a constant speed. Thus, sn this operating condition, the two axles are nest mutually torque iirrsited. A mechanically based locking differentia! typically uses a dutch pack or friction material Interface which locks the two axles together when a speed difference between the axles 5$ detected Other systems incorporate fluid couplings between the axles which provide a degree of speed coupling,

[00041 Prodding a limited slip or locking type differentials for front wheel d?ive vehicle applications poses particularly stringent packaging limitations. The presence of engine, transmission,, transaxle axle shafts, suspension and steering components all within the front engme compartment of the vehicle provide little packaging space for additional power train components.

BRIEF SUMMARY OF THE INVENTION [000Si The hydraulicaily actuated electronic limited slip differential in accordance with the present invention is especially adapted for front wheel drive applications and can be directly coupled to the open differentia! of the vehicle Lraπsaxie A central Intermediate shaft passes through the unit from the deferential to one of the front drive shafts through a universal or constant velocity type flexible torque coupling joint. A clutch pack is compressed to couple or decouple the differential carrier to one of the axle shafts through an intermediate shaft which can provide a locking, or modulating condition between the two front drive shafts.

[0006] These and other aspects and advantages of the present invention m\\ become apparent ^y pon reading the following detailed description of the invention in combination with the accompanying drawings.

BRfEF DESCRIPTION OF THE DRAWINGS [0007] Figure 1 is a schematic view of actuation system to? a baking, or modulating differential in accordance with one embodiment of the present invention; [00081 Figure 2 is a sectional view of one embodiment of actuation system of

Figure 1. and

[0009] Figure 3 is a schematic view of an actuation system according to another embodiment.

DETAILED DESCRIPTION OF THE INVENTION [0010] An actuation system for a limited sHp differential assembly is provided in Figures 1 and 2. The actuation system is generally denoted by reference 10. [001 IJ The differential assembly 16 includes basic elements of typical differentia! assemblies, which include a differentia! carrier 24 which is driven by the vehicle's transmission output via a gear or chain drrve (not shown). A pair of planet gears 26 are rotatabie about a common differential shaft mounted to the carrier Planet gears 26 mesh with a pair of side gears 28 which are in turn spiiπecl or otherwise connected with a pair of axle shafts 31.33 through intermediate shafts 30. 32 for the left and right hand wheels 18, 20 of the associated motor vehicle. The above described components of differential assembly 18 are common components of so-called open differentials, front wheel drive vehicles may also use a differential which is a planetary gear set.

Under certain low traction conditions it may be desirable to lock, or modulate the differential assembly 18 providing a constant, or limited slip speed between two axle shafts 31 , 33. Accordingly, the actuation system 10 includes a piston 40 and a clutch pack 42, The piston 40 Is hydrauHcafiy actuated and does not rotate. Im contrast, differential carrier 24 and the intermediate shaft 32 each rotate with regard to the stationary piston 40, Under normal operating conditions, the differential carrier 24 and the intermediate shafl 32 are allowed to rotate at different speeds. Actuator housing 44 is attached to the differential carrier 24 and rotates with the differential carrier 24. The clutch pack 42 Includes two sets of clutch plates AB, 48. The first set of clutch plates 46 engage the actuator housing 44 through a splϊned connection and, therefore, rotates with the differential carrier 24. The second set of clutch plales 48 engage a shaft portion 50 in a spiiπed connection that is attached to and rotates with the intermediate shaft 32. Typically, the first set of clutch plates 46 is interleaved with the second set of clutch plates 48 to maximize the active fπctbna! surface area in the clutch pack 42.

[0013] To provide a constant, or limited slip speed between the axle shafts 31 and 33, ihe actuation system 10 fncfjonaliy locks, or limits the speed difference between the sntermediate shaft 32 to the differential carrier 24 through the clutch pack 42, To lock, or modulate the intermediate shaft 32 to the differential carrier 24, the piston 40 is hydraulicaliy actuated to extend and apply force to thrust bearing 52. The thrust bearing 52 acts against pins 58 that compresses the first and second set of cluich plates 46 and 48, thereby frictionaϋy coupling or limiting the slip speed between the intermediate shaft 32 to the differential carrier 24, To once again operate in a open differential mode, the hydraulic pressure Is relieved and a spring

54 acts to relieve pressure on pins 58 and, consequently, the piston 40 allowing the clutch plates 48 and 48 to rotate independently. in addition, it is readily contemplated that the pin 58 and spring 54 may be eliminated such that the piston 40 acts on the clutch pack 42 directly through the thaist bearing 52. [0014] The hydraulic circuit 58 is used to control the actuation of the piston

40. A hydraulic pump 80 creates a hydraulic system pressure that is provided to a pressure regulator switch 62 through a check valve 84, If the hydraulic system pressure falls below a minimum pressure, the pressure regulator switch 62 activates the motor 5S ⁾ , thereby driving the hydraulic pump 60 to increase the hydraulic system pressure above the minimum pressure. In addition, the flow from the hydraulic pump 80 feeds a pressure accumulator 68. The pressure accumulator 66 maintains a constant system pressure in response to a transient demand for fluid, for example when driving the piston 40. The pressure accumulator 66 is in fluid communication with a primary valve 70 and a secondary valve 68. When activated, the secondary valve 88 provides hydraulic to flow chamber 72 driving piston 40 forward to compress the dutch plates 46, 48 of the clutch pack 42. The primary valve 70 provides a signal ievel pressure feed to valve 88 in order to control the pressure delivered to the piston 40

[001S] An electronic control unit 69 is in electrical communication with a solenoid of the primary valve 70, The amount of current provided to the solenoid by the electronic control unit 69 controls the amount of pressure provided to the secondary valve 68. As such, the secondary valve 68 may be a spool valve such that the pressure from the primary valve 70 creates a force balance between the pressure from the primary valve 70 and a hydraulic feedback loop m communication

with the output or piston side of the secondary valve 88. Accordingly, the force balance causes a balancing of the spool m the spool valve implementation, thereby controlling hydraulic pressure delivered to the piston 40 In addition, a pressure sensor 74 is In electrical communication with the electronic control unit 69. The pressure sensor 74 is in fluid communication with the hydrauhc line 71 between the secondary valve 68 and the chamber 72 of the piston 40, Accordingly the pressure sensor 74 generates an electronic signal based on the pressure driving the piston 40 and forms an electronic feedback bop to the electronic control unit 69. The electronic feedback loop may be used by the electronic control unit 89 to adjust the current provided to the solenoid of the primary valve 70 creating a variable pressure control loop. With the pressure to the piston 40 being variably adjustable, the piston 40 may provide an adjustable actuation pressure to the clutch pack 42 to variably control the friction engagement of the clutch plates 46, 48 and hence the locking or modulation between the differential earner 24 and the intermediate shaft 32. [0016] The system may also be implemented using a single valve design, as shown in Figure 3. Accordingly, the pressure accumulator 66 Is in fluid communication with a valve 80. When activated, the valve 80 provides hydraulic pressure to flow chamber 72 driving piston 40 forward to compress the clutch plates of the clutch pack 42.

[0017] An electronic control unit 69 ss in electrical communication with a solenosd of the valve 80, The amount of current provided to the solenoid by the electronic control unit 69 controls the amount of pressure provided through the valve 80,. thereby controlling hydrauhc pressure delivered to the piston 40 in addφoπ, a pressure sensor 74 Is in electrical communication with the electronic control unit 69.

The pressure sensor 74 Is in fluid communication with the hydraulic line 71 between the valve 80 and the chamber 72 of the piston 40. Accordingly the pressure sensor 74 generates an electronic signal based on the pressure driving the piston 40 and forms an electronic feedback loop to the electronic control unit 69 The electronic feedback loop may be used by the electronic control unit 68 to adjust the current provided to the solenoid . As described above, the piston 40 may provide an adjustable actuation pressure to the dutch pack 42 to variably control the friction engagement of the clutch plates and hence the locking or modulation between the differential carrier 24 and the intermediate shaft 32,

[0018] As a person skilled in the ail will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from the spirit of this invention, as defined in the following claims.