A disengageable four-wheel-drive transmission system for motor vehicles

Abstract: A disengageable four-wheel-drive transmission system for motor vehicles with transverse gearbox (2) includes a gradually-engageable hydraulic clutch (22) controlled automatically by an electronic control unit (43) in dependence on operating parameters including the torque transmitted by the engine of the vehicle at the time in question. The electronic control unit is arranged to cause the clutch to be engaged gradually from a disengaged condition in whicn the drive is transmitted from the gearbox only to the front differential (4) or the rear differential (32) of the transmission system.

Description: The subject of the present invention is a disengageable four-wheel-drive transmission system for a motor vehicle with a transverse gearbox, including a toothed input member meshed with a toothed output member of the gearbox, a front differential which drives the drive shafts of the front wheels of the vehicle, a rear differential which drives the drive shafts of the rear wheels of the vehicle, and means for distributing the torque between the front and rear differentials as specified in the preamble of claim 1. In four-wheel-drive transmission systems of the type defined above, the means for distributing the torque are usually constituted by an epicyclic differential, possibly in association with a FERGUSON-type sliding viscous coupling, a TORSEN-GLEASON distributor, or the like. These known transmission systems do not enable the four-wheel drive to be disengaged selectively when it is not necessary or desired, so as to limit the transmission of torque to the front wheels of the vehicle only or to the rear wheels only. Moreover, the torque is distributed between the front and rear differentials in proportions which are fixed or which can be varied only in proportion to any variations in the road-holding of the front and rear wheels. The object of the present invention is to provide a four-wheel-drive transmission system of the type defined above which can be engaged or disengaged selectively in order to limit the drive either to the front wheels of the vehicle or to the rear wheels and which, in the four-wheel-drive condition, can distribute the transmitted torque to the front and rear wheels in dependence on the prevailing operating parameters of the transmission system and, in particular, on the torque transmitted by the engine of the vehicle. According to the invention, this object is achieved by the transmission system according to claim 1. According to the invention, the electronic control unit is arranged to pilot the operation of the hydraulic clutch in dependence on the instantaneous torque transmitted by the engine of the vehicle. According to a preferred embodiment of the invention, this is achieved by virtue of the fact that the toothed input member of the transmission system and the toothed output member of the gearbox each have helical teeth, and the fact that the input member of the transmission system is supported for rotation by the gearbox housing by means of an axial rolling bearing with a portion which is resiliently deformable in dependence on the axial load transmitted by the helical teeth and which has an associated deformation transducer operatively connected to the electronic control unit and adapted to supply thereto electrical signals indicative of the deformation of the deformable portion of the bearing. The electronic unit pilots the gradual engagement and disengagement of the hydraulic clutch in dependence on the electrical signals supplied by the deformation transducer. The axial bearing conveniently has an outer race which is mounted with clearance relative to the gearbox housing and has an outer annular projection clamped axially relative to the housing and constituting the resiliently deformable portion with which one or more extensometers are associated. The invention will now be described in detail with reference to the appended drawings, provided purely by way of non-limiting example, in which: Figure 1 is a schematic perspective view of a disengageable four-wheel-drive transmission system according to the invention, Figure 2 is a sectional view of part of the transmission system on an enlarged scale, Figure 3 shows a detail of Figures 2, on an enlarged scale, Figure 4 is a partial cross-section of the detail indicated by the arrow V in Figure 3, Figure 5 is a front elevational view taken on the arrow VI of Figure 4, Figure 6 shows the detail indicated by the arrow VII in Figure 4, on an enlarged scale, and Figure 7 is a variant of Figure 4. With reference initially to Figure 1, an internal combustion engine, schematically indicated 1, is intended for mounting in the front part of a motor vehicle with its axis arranged transverse the length of the vehicle, indicated A in Figures 2 and 3. The engine 1 has a gearbox 2 with a main shaft and a countershaft (not shown) which are also arranged transverse the length A of the vehicle. A housing is indicated 3 and houses a generally conventional differential 4, shown in Figures 2 and 3, whose box 5 has two opposed tubular appendages 6, 7 from which the drive shafts 8, 9 of the front wheels of the vehicle project. With reference in greater detail to Figure 2, a helical gear, indicated 10, is carried by the countershaft of the gearbox 2 and meshes with an external helical ring gear 11 carried by a wheel 12 having a tubular hub 13 which is supported for rotation by the housing 3, coaxially with the drive shaft 8, by means of a radial rolling bearing 14 and an axial rolling bearing 15 which will be referred to again below. The wheel 12 is coupled for rotation with a sprocket 16 used for driving a revolution-counter and has an internal ring gear 17 meshed with a hollow toothed member 18 which is permanently coupled at 19 for rotation with the tubular appendage 6 of the box 5 of the front differential 4. The other tubular appendage 7 of the box 5 of the front differential 4 is permanently coupled at 20 for rotation with a tubular element 21 which extends coaxially with the other front drive shaft 9 and constitutes a first entrainment member of a hydraulic clutch, generally indicated 22, which is also housed in the housing 3. The hydraulic clutch 22 also includes a drum 23 which constitutes a second entrainment member and is supported for rotation by the first entrainment member 21. The drum 23 is coupled permanently at 24 for rotation with a hollow member, generally indicated 25, which contains the front differential 4 and is supported for free rotation by the housing 3, coaxially with the tubular appendages 6 and 7. A bevel gear 26 is fixed to the hollow member 4 and is meshed with a bevel pinion 27 at the inner end of a shaft 28 supported for rotation within a hollow appendage 29 of the housing 3, arranged along the length A of the vehicle. The opposite end of the shaft 28 from the bevel pinion 27 is connected by means of a constant velocity universal joint, generally indicated 30 in Figure 1, to a propeller shaft 31 for transmitting the torque to a rear differential 32. The differential 32 transmits the torque to the drive shafts 33, 34 of the rear wheels of the vehicle. With reference again to Figure 2, the hydraulic clutch 22 includes two sets of friction discs 35, 36 coupled alternately for rotation with the first entrainment member 21 and the second entrainment member 23 and axially slidable relative thereto. A thrust disc 37 is associated with the discs 35, 36 and can be moved axially by a piston 38 which is sealingly slidable in a cylinder 39 defining a thrust chamber 40 connected by means of a distributor, schematically indicated 41, to a supply 42 of pressurised hydraulic fluid. The distributor 41 is controlled automatically by means of an electronic control unit 43, in the manner explained below, so as to keep the chamber 40 of the hydraulic clutch 22 discharged or to connect the thrust chamber 40 gradually to the supply 42. In the first case, the hydraulic clutch 22 is disengaged so that the friction discs 35 and 36, and hence the first entrainment member 21 and the second entrainment member 23, are freely rotatable relative to each other. Consequently, in this condition, the rotation of the output gear 10 of the gearbox is transmitted solely to the front differential 4 and not to the bevel gears 26-27, or hence to the rear differential 32, so that the transmission system only drives the front wheels. In the second case, the rotation of the first entrainment member 21 of the hydraulic clutch 22 is also transmitted to the second entrainment member 23, and hence to the rear differential 32 by means of the hollow member 25, the bevel gears 26, 27 and the shafts 28 and 31, in proportion to the coupling force between the friction discs 35, 36. The transmission system thus achieves four-wheel drive with a distribution between the front and rear wheels which is variable up to a maximum of 50% when the hydraulic clutch 22 is in the fully-engaged condition and the coefficient of friction and the weight of the axles are uniform. The engagement and disengagement of the hydraulic clutch 22 is achieved gradually by the electronic control unit 43 in dependence on operating parameters of the transmission system and of the vehicle in which it is fitted, by means of suitable inputs 45 supplied with signals generated by corresponding sensors and indicative of these parameters. These parameters, which may comprise, for example, the speed at which the vehicle is moving, the difference in the rates of rotation of the front and rear axles, the rate of rotation of the engine, environmental data, and travelling conditions (dry, wet or snowy ground) also include, according to the invention, the torque delivered, at the time by the engine, in the driving condition or in the coasting condition. In order to obtain and supply to the electronic control unit 43 an electrical signal indicative of the torque at a particular time, the invention provides the device shown in greater detail in Figures 3 to 6. This consists of a particular conformation of the axial rolling bearing 15 which, as stated, together with the radial rolling bearing 14, rotatably supports the tubular hub 13 of the wheel 12 carrying the external helical ring gear 11 which is meshed with the helical output gear 10 of the gearbox 2. This bearing 15 includes an inner race 46 which is clamped axially between a ring nut 47 screwed onto the tubular hub 13 and a spacer 58 interposed between the bearing 15 and the bearing 14 and an outer race 48 which is mounted with clearance relative to the housing 3 of the gearbox 2. The outer race 48 has an external annular projection 49 which is clamped axially between a stop 50 of the housing 3 and a spacer 51 and is connected to the outer race 48 by means of a resiliently deformable portion 152. Deformation transducers constituted, for example, by extensometers 53 are mounted on the resiliently deformable portion 152 and are connected electrically to the electronic control unit 43. As shown in greater detail in Figure 6, the annular projection 49 has rounded sides 54 which are in contact with the corresponding surfaces of the appendage 50 and the spacer 51 and may have gaps locally. There may be two annular projections 49 rather than one, as in the embodiment of Figures 3 to 6, and they may have an asymmetrical configuration, as indicated 55 in the variant of Figure 7. When the transmission system is in operation, the variations in the torque delivered by the engine of the vehicle and transmitted through the gears 10 and 11 by virtue of the helical teeth of the gears 10 and 11, in driving and coasting conditions, generate axial forces on the tubular hub 13, and hence on the bearing 15, as a result of which the portion 152 connecting the outer race 48 to its annular projection 49 is deformed resiliently. The deformations are detected by the extensometers 53 which provide the electronic control unit 43 with corresponding electrical signals which are processed by the unit 43 and used moment by moment for making corrective adjustments to the degree of engagement-disengagement of the hydraulic clutch 22 and hence to the distribution of the torque between the front and rear axles.

Claims: A disengageable four-wheel-drive transmission system for a motor vehicle with a transverse gearbox (2), including a toothed input member (12) meshed with a toothed output member (10) of the gearbox, a front differential (4) which drives the drive shafts (8, 9) of the front wheels of the vehicle, a rear differential (32) which drives the drive shafts (33, 34) of the rear wheels of the vehicle, and means for distributing the torque between the front and the rear differentials, in which the front differential (4) includes a box (5) and the drive shafts (8, 9) of the wheels project from opposite sides thereof, characterised in that the means for distributing the torque are constituted by a hydraulic clutch (22) which can be engaged and disengaged gradually and which is operatively associated with an electronic control unit (43) arranged to pilot the gradual engagement of the hydraulic clutch (22) automatically in dependence on operating parameters of the transmission system, starting from a disengaged condition in which the drive is transmitted from the gearbox (2) to only the front differential (4) in order to control the distribution of the torque between the front and rear axles, in that one side (6) of the box (5) of the front differential (4) is coupled permanently to the toothed input member (11) of the transmission system, in that the hydraulic clutch (22) includes first and second coaxial entrainment members (21, 23) and friction discs (35, 36) coupled alternately to the two entrainment members, and in that the first entrainment member (21) is coupled permanently for rotation with the other side (7) of the box (5) of the front differential (4) and a toothed output member (26) for transmitting the drive to the rear differential (32) is coupled permanently for rotation with the second entrainment member (23) so that, when the hydraulic clutch (22) is disengaged, the drive is transmitted from the gearbox (2) to the front differential (4) only. A transmission system according to Claim 1, characterised in that the electronic control unit (43) is arranged to pilot the operation of the hydraulic clutch (22) in dependence on the instantaneous torque delivered by the engine (1) of the vehicle. A transmission system according to Claim 2, characterised in that the toothed input member (11) of the transmission system and the toothed output member (10) of the gearbox (2) each have helical teeth, and in that the input member (11) is supported rotatably by the housing (3) of the gearbox (2) by means of an axial rolling bearing (15) with a portion (152) which is resiliently deformable in dependence on the axial load transmitted by the helical teeth (10, 11) and which has an associated deformation transducer (53) operatively connected to the electronic control unit (43) and adapted to supply thereto electrical signals indicative of the deformation of the deformable portion (152) of the axial bearing (15). A transmission system according to Claim 3, characterised in that the axial bearing (15) has an outer race (48) which is mounted with clearance relative to the housing (3) of the gearbox (2) and has at least one external annular projection (49) clamped axially relative to the housing and defining the resiliently deformable portion (152). A transmission system according to Claim 3 or Claim 4, characterised in that the deformation transducer is constituted by extensometers (53). A transmission system according to Claim 4, or Claim 5, characterised in that there is a single annular projection (49). A transmission system according to Claim 4 or Claim 5, characterised in that there are two annular projections (55). A transmission system according to any one of Claims 4 to 7, characterised in that the annular projection has rounded sides (54).

Inventors: GARRONE FELICE, MONTANARO ENRICO, GARRONE, FELICE, MONTANARO, ENRICO
Applicants: FIAT AUTO SPA, FIAT AUTO S.P.A.
Application year: 1990
Main classification: B60K23
IPC codes: B60K17, B60K23