POWER TRANSMITTING SYSTEM FOR A FOUR-WHEEL DRIVE VEHICLE

Abstract: A viscous coupling (35) is provided in transmission of four-wheel drive for transmitting power of an engine to auxiliary driving wheels, and a lock device (45) is provided for locking the viscous coupling so as to directly transmit the power to the auxiliary driving wheels. In operation the driving torque of the transmission is determined and compared with a predetermined driving torque in a control unit (56) and an output signal is produced when the detected driving torque exceeds the predetermined driving torque. An actuator (51) responds to the output signal to operate the lock device (45) to lock the viscous coupling.

Description: The present invention relates to a system for transmitting the power of an engine to four wheels of a four-wheel drive vehicle and more particularly to a system provided with a viscous coupling for transmitting the power. A four-wheel drive vehicle of the type in which a transfer clutch in a form of a dog clutch, hydraulic clutch or electromagnetic clutch is provided for transmitting the output torque of a transmission to auxiliary drive wheels is known. In such a vehicle, a power transmission system can be converted from a two-wheel drive system to a four-wheel drive system by engaging the transfer clutch. However, if a dog clutch is employed as a transfer clutch, the difference of speeds between front wheels and rear wheels cannot be absorbed when the clutch is engaged. Although a hydraulic clutch slips to absorb the speed difference, a complicated hydraulic control system must be installed. Recently, a viscous coupling containing a high viscosity fluid such as a silicon oil has been proposed as a transfer clutch. The viscous coupling operates to transmit the torque in proportion to the difference between input speed and output speed. The viscous coupling may either be used as a device or locking a control differential, or be used as a transfer clutch which operates to transfer the torque to auxiliary wheels to provide a four-wheel drive system when the auxiliary wheels skid. Moreover, the slipping in the viscous coupling prevents a tight corner braking effect during cornering. British Patent Publication No. 2139972 discloses a power transmitting system for a four-wheel drive vehicle provided with a viscous coupling. A four-wheel driving mode or a two-wheel driving mode can be selected by connecting and disconnecting the transfer device. However, in the four-wheel driving mode, power is transmitted to the rear wheels while the viscous coupling slips so that the power transmitting characteristic and drivability of the vehicle decreases and the fuel consumption increases. Additionally, since the temperature of the silicon oil contained in the viscous coupling rises as a result of the continuous slipping, the viscosity of the oil decreases. Thus, clutch torque decreases hence reducing drivability on muddy and snowy rough roads. In order to eliminate such drawbacks, it is preferable to lock the viscous coupling so as to directly connect the front and rear drive systems with each other. EP-A-149302 discloses a transmission for a four-wheel drive vehicle having an engine, main driving wheels and auxiliary driving wheels, comprising: a viscous coupling for transmitting the power from the engine to the auxiliary driving wheels; a lock device provided for locking the viscous coupling to directly transmit the power to the auxiliary driving wheels and an actuator for operating the lock device. However, it is disadvantageous for the driver of the vehicle to have to manually actuate the lock device in order to alter the coupling between the engine and the auxiliary driving wheels. Accordingly the transmission of the present invention is characterised by means for determining the torque applied to the viscous coupling input, and for comparing the determined driving torque with a predetermined driving torque and for producing an output signal when the determined driving torque exceeds the predetermined driving torque; and the actuator responding to the output signal for operating the lock device to lock the viscous coupling. In an aspect of the invention, the lock device is provided to lock a housing of the viscous coupling to a shaft of the viscous coupling, and the actuator is a vacuum operated actuator having a chamber defined by a diaphragm. A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Fig. 1 is a schematic diagram of a four-wheel drive power transmission system of the present invention; and Fig. 2a shows a detailed sectional view of a viscous coupling and a locking device thereof, and Fig. 2b shows a block diagram of a control unit for controlling the locking device. Fig. 1 shows a power transmission system for a four-wheel drive motor vehicle to which the present invention is applied. An engine 1 is laterally mounted on the motor vehicle at a front portion thereof. The power transmission system comprises a clutch 2 housed in a clutch housing 6, transmission 3 and transfer device 5 the latter two housed in a transmission case 7 and an extension case 8. The transmission 3 has an input shaft 10, five pairs of change-speed gears 12 to 16 corresponding to first to fifth (overdrive) speed gears, and synchronizers 17 to 19. The synchronizers 17 to 19 are disposed between the gears 12 and 13, between 14 and 15 and adjacent the gears 16, respectively. A reverse drive gear 20 mounted on the input shaft 10 meshes with a gear 21 formed on one side of a sleeve of the synchronizer 17 through an idler gear (not shown) for reverse drive. A drive gear 22, which is securely mounted on the output shaft 11, meshes with a final reduction gear 23 having a large diameter and connected with a front differential 24 through a shaft, thereby forming final reduction device 4. The front differential 24 is connected to axle shafts 25 so as to transmit the power to front wheels. The transfer device 5 comprises a lateral transfer shaft 26 and a viscous coupling 35 mounted on the shaft 26. The viscous coupling 35 is connected to the final reduction gear 23 through a transfer gear 30. The transfer shaft 26 is connected to a rear drive shaft 28 perpendicular to the transfer shaft 26 through a pair of bevel gears 27, thereby transmitting the power to rear wheels by way of a propeller shaft 29. Referring to Figs. 2a and 2b, the viscous coupling 35 comprises a hollow hub 36 mounted on the transfer shaft 26 through splines and a cylindrical housing 37 rotatably mounted on the hub 36 through a bearing 38. A high viscous fluid is contained in the housing 37 and sealed by a pair of oil rings 39. The hub 36 has a plurality of perforated inner discs 40 and the housing 37 has a plurality of perforated outer discs 41. The inner and outer discs are alternately arranged in the axial direction of the viscous coupling 35. The housing 37 is connected to the transfer gear 30 through a tubular shaft 42 which is rotatably mounted on the shaft 26 and fixed to the housing 37 by bolts 43. When there is a speed difference between the hub 36 and the housing 37, the viscous coupling operates to transmit the torque. The transmitting torque increases with increase of the difference. Thus, the torque is transmitted from a high speed disc side to a lower speed disc side. Mounted on the transfer shaft 26 adjacent the transfer gear 30 is a lock device 45 for locking the hub 36 and the housing 37. The lock device 45 has a similar construction to the synchromesh device. Namely it comprises a hub 46 mounted on the transfer shaft 26 through splines and a selector sleeve 47 slidably mounted on the outer periphery of the hub 46. The sleeve 47 is slidable in the axial direction of the shaft 26 so as to engage with outer teeth 48 of the transfer gear 30. The selector sleeve 47 is further engaged with a fork 50 which is securely mounted on a rail 49 disposed in parallel with the transfer shaft 26 and supported by the transmission case 7. The rail 49 is connected to a diaphragm 52a of a vacuum operated actuator 52 of an automatic change-over device 51 disposed outside of the transmission case 7. A chamber of the actuator 52 is communicated through a passage 61 with a solenoid operated valve 62 which is controlled by output signal of a control unit 56. The valve 62 has a port 62a opened to the atmosphere and a port 62b communicated with an intake passage of the engine to be applied with vacuum pressure, and a solenoid 62c. The control unit 56 has an engine torque calculator 57 where engine torque T e is obtained, for example from a torque graph, in dependence on a throttle opening degree ϑ detected by a throttle position sensor 54 and an engine speed Ne detected by an engine speed sensor 55. The control unit 56 is further provided with a gear ratio providing section 58 which determines a gear ratio i in dependence on an output signal of a select position sensor 53. The select position sensor 53 detects which of the first to fifth and reverse gears is selected. The engine torque T e and the gear ratio i are applied to a driving torque calculator 59 which calculates a driving torque T D in accordance with the following equation. T D = T e × i × IF/R where IF is a final reduction gear ratio and R is a constant for effective radius of tires. The driving torque T D is applied to a viscous coupling lock deciding section 60 wherein the driving torque T D is compared with a predetermined lock torque T S which is obtained from a gripping force of tires. The deciding section 60 produces a lock signal when the driving torque T D is larger than the predetermined torque T S T D > T S so as to operate the solenoid operated valve 62 through a power circuit 62. The operation of the present embodiment is described hereinafter. The power of the engine 1 is transmitted to the transmission 3 through the clutch 2 and the input shaft 10 and further to the front wheels through output shaft 11, drive gear 22, final reduction gear 23, front differential 24 and axle shafts 25. When the vehicle is driven on an ordinary road, the driving torque T D , which is calculated by the calculator 59 dependent on the engine speed Ne, throttle opening degree ϑ and the gear ratio i, is smaller than the predetermined torque T S (T D ≦ T S ). Thus, a lock signal is not applied to the solenoid 62c of solenoid operated valve 62. Accordingly, the vacuum port 62b of valve 62 is closed as shown in Fig. 2a, so that the chamber of actuator 52 is opened to the atmosphere thereby retaining the rail 49 at a position shown in Fig. 2a. Thus, the selector sleeve 47 is not engaged with the teeth 48 of the transfer gear 30. Therefore, the vehicle is driven under a viscous coupling operating mode. During such a driving mode, the torque is transmitted to the rear wheels through final reduction gear 23, transfer gear 30, viscous coupling 35, transfer shaft 26, rear drive shaft 28, and propeller shaft 29. When the speed difference between the front and rear wheels is small, outer discs 41 in the viscous coupling 35 slip with respect to inner discs 40 to transmit a smaller torque to the rear wheels than to the front wheels. If the front wheels skid, the housing 37 operatively connected to the front wheels rotates much faster than the hub 36. Large difference of rotational speeds between the inner discs 40 and outer discs 41 generates a transmitting torque which is transmitted to the lower speed side, namely to the inner discs 40. Thus, the rear torque is increased and the front torque is reduced so that four wheel driving is provided. When the vehicle is driven on a rough road, the driving torque T D increases and exceeds the predetermined torque T S (T D > T S ). Thus, the viscous coupling lock deciding section 60 produces a lock signal to energize the solenoid 62c to open the vacuum port 62b of solenoid operated valve 62. Accordingly, vacuum is applied to the actuator 52 to move the diaphragm 52a and hence slide the rail 49 to the right. The selector sleeve 47 is forced by the fork 50 to slide to the right, thereby engaging the teeth 48 as shown by a chain line in Fig. 2a. Accordingly, the transfer gear 30 is operatively connected to the shaft 26, and the housing 37 is locked to the shaft 26. The torque is transmitted from the final reduction gear 23 to the transfer shaft 26 through the transfer gear 30, selector sleeve 47 and hub 46. Thus, directly connected four wheel driving mode is established. Under the driving mode, the torque is constantly and equally distributed to the front and rear wheels so that preferable characteristics of the four wheel driving are effected at the start of the vehicle and on rough roads. The present embodiment may be applied to motor vehicles having other engine and transmission arrangemenets than the illustrated embodiment or to a vehicle having an automatic transmission. The lock device 45 may be disposed between the hub 36 and the housing 37. Furthermore, the lock device 45 may be driven by a line pressure of the automatic transmission, or by an oil pressure applied by a pump driven by the engine. In accordance with the present embodiment, a viscous coupling provided in a power transmitting system is automatically locked when it is preferable to equally distribute the torque to four wheels of a vehicle. Thus, the durability of the viscous coupling and driveability on rough roads are improved and fuel consumption can be decreased.

Claims: 1. A transmission for a four-wheel drive vehicle having an engine (1), main driving wheels and auxiliary driving wheels, comprising: a viscous coupling (35) for transmitting the power from the engine (1) to the auxiliary driving wheels; a lock device (45) provided for locking the viscous coupling to directly transmit the power to the auxiliary driving wheels and an actuator (51) for operating the lock device (45); characterised by, means (56) for determining the torque applied to the viscous coupling input, and for comparing the determined driving torque with a predetermined driving torque and for producing an output signal when the determined driving torque exceeds the predetermined driving torque; and the actuator (51) responding to the output signal for operating the lock device (45) to lock the viscous coupling (35). 2. A transmission according to claim 1 wherein the viscous coupling (35) comprises a housing (37, 42) mounted to be rotatable around a transfer shaft (26), and the lock device (45) is arranged to lock the housing (34, 42) to the transfer shaft (26). 3. A transmission as claimed in claim 2 wherein the actuator (51) is a vacuum operated actuator having a chamber defined by a diaphragm.

Inventors: UCHIYAMA, SHINTARO
Applicants: FUJI JUKOGYO KABUSHIKI KAISHA
Application year: 1988
Main classification: B60K17
IPC codes: B60K17, B60K23