Continuously variable transmission system having parallel drive paths

Abstract: A transmission system (10) includes at least two continuously variable transmission units (18, 20) of a toroidal or V-belt type, arranged in parallel with each other between a common driving member (16) and a common driven member (28), so as to provide parallel equal drive paths. Both units (16, 20) are coupled with each other so that operating conditions are equal, and connected with a single common control valve which is designed to control shift actuators of both units equally.

Description: BACKGROUND OF THE INVENTION The present invention relates to continuously variable transmissions of various types such as a toroidal type and a V-belt type. JP-A-63-130954 discloses a conventional toroidal type continuously variable transmission (CVT) having input and output cone discs forming a toroidal cavity therebetween, and two friction rollers which are compressed in the toroidal cavity between the input and output discs for torque transmission therebetween and which are inclined so as to continuously vary a transmission ratio of the output speed to the input speed. JP-A-63-125852 discloses another conventional transmission system. This system has two parallel toroidal CVT units each of which has two friction rollers, and two actuators for actuating the two friction rollers, respectively. JP-A-62-83553 and JP-A-63-34363 disclose V-belt type continuously variable transmission systems each having two parallel V-belt CVT units. Each of these conventional examples includes two actuators for actuating the two CVT units, respectively, and two shift control valves which are connected, respectively, with the two actuators. However, these CVT systems having two drive units of the toroidal or V-belt type are still unsatisfactory. For example, an unbalance of torque between the two parallel CVT units readily results in an unbalance of the shift control between both units. Therefore, it is difficult to control the transmission ratio accurately. Especially the calculation of the actual speed ratio becomes more difficult as the rotational speed becomes lower (because of a lowering of the resolution). The transmission system of the above-mentioned JP-A-62-83553 is arranged to absorb the unbalance between the two parallel CVT units with a differential mechanism provided between both units. The addition of the differential mechanism, however, complicates the construction of the transmission system, and increases the size and weight of the system. A toroidal type continuously variable transmission system according to the preamble of claim 1 is known from EP-A 207 184. This known system comprises two parallel toroidal drive units, providing two parallel drive paths and actuating means for varying the positions of the input, output and intermediate members of both drive units. A loading means is provided for applying a pressing force to the first and to the second drive unit. The operating conditions of the drive units are balanced by a equalizing means. DE-C-3200944 discloses a continuously variable transmission having two parallel drive units of the V-belt type. The transmission comprises four pairs of conical discs, two pairs being connected to an input shaft and two pairs being connected to an output shaft, this output shaft being arranged in alignment to the input shaft. The axially movable disc on the input shaft and the axially movable disc of the output shaft are pressurized by a common pressure source. Consequently, the pressing forces are balanced to each other and the influence of the pressing forces to the movement of the discs are minimized. SUMMARY OF THE INVENTION It is the object of the present invention to provide a continuously variable transmission (CVT) system which is simple in construction, and superior in capability of equalizing operating conditions of two or more parallel CVT units. This object is solved according to the invention by a toroidal type continuously variable transmission system according to claim 1. Preferred embodiments of the transmission system according to the invention are subject-matter of the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing a toroidal type continuously variable transmission system according to a first embodiment of the present invention. Fig. 2 is a schematic view showing two parallel drive units of the transmission system of the first embodiment. DETAILED DESCRIPTION OF THE INVENTION Figs. 1 and 2 show an embodiment of the present invention. A continuously variable transmission (CVT) 10 of this embodiment is a toroidal type transmission as shown in Fig. 2. As shown in Fig. 2, the toroidal type continuously variable transmission 10 of this embodiment is connected with a torque converter 12. Power produced by a prime mover such as an engine of a vehicle is inputted through the torque converter 12 to the toroidal transmission 10. The torque converter 12 has a pump impeller (power input member) 12a, a turbine runner (power output member) 12b and a stator (reaction member) 12c, as is well known in the art. The torque converter 12 of this embodiment is a lockup torque converter having a lockup clutch 12d to provide a direct mechanical drive. The torque converter 12 further has an output shaft 14. The toroidal transmission 10 has two or more parallel equal torque paths. In this embodiment, the toroidal transmission 10 has a center transmission shaft 16 which is in line with the torque converter output shaft 14, and first and second drive units 18 and 20 which are arranged around the center transmission shaft 16 in axially spaced relation with each other, so as to provide two parallel equal drive paths. The center transmission shaft 16 is a hollow shaft, and supported in such a manner that the center transmission shaft 16 is axially slidable to a limited extent with respect to a transmission housing 22. The first and second drive units 18 and 20 aresubstantially identical in construction and size with each other. Each drive unit 18 or 20 has an input disc (input member) 18a or 20a, an output disc (output member) 18b or 20b, and an intermediate member coupling the input and output members for torque transmission therebetween. The input and output discs of each unit have confronting toroidal surfaces and form a toroidal cavity therebetween. Each drive unit is a friction drive transmission, and the intermediate member is a friction member which is in contact with both the input and output members. In this embodiment, the intermediate member of each drive unit consists of two friction power rollers 18c and 18d or 20c or 20d. The first drive unit 18 is mounted on a first half of the center transmission shaft 16 between a first end and a middle of the center transmission shaft 16, and the second drive unit 20 is mounted on a second half between a second end and the middle of the center transmission shaft 16. In Fig. 2, the first end of the center shaft 16 is a lefthand end, and the second end is a righthand end. The first output disc 18b of the first unit 18 and the second output disc 20b of the second unit 20 are disposed between the first input disc 18a of the first unit 18 and the second input disc 20a of the second unit 20. Each of the first and second drive units 18 and 20 is arranged like a mirror image of the other. In this embodiment, the input discs 18a and 20a are outside discs, and the output discs 18b and 20b are inside discs. The first and second input discs 18a and 20a are mounted on the center transmission shaft 16 through first and second coupling means 24 and 26, respectively, in such a manner as to prevent relative rotation between the center transmission shaft 16 and each input disc, and to permit relative axial movement between the center transmission shaft 16 and each input disc. In this embodiment, each of the first and second coupling means 24 and 26 comprises ball splines having splines and balls for making the axial movement between the coupled members smooth. An output (driven) terminal member 28 is rotatably mounted on the center transmission shaft 16 through a suitable bearing means. The output terminal member 28 of this embodiment is an output gear. The first and second output discs 18b and 20b are splined to the output gear 28. The torque transmitted from the first input disc 18a to the first output disc 18b and the torque transmitted from the second input disc 20a to the second output disc 20b are both transmitted to the output gear 28. The torque of the output gear 28 is further transmitted through a gear 30a meshing with the output gear 28 and a counter shaft 30 to an output shaft (not shown). The toroidal transmission 10 further includes a first loading means which, in this embodiment, is a loading cam mechanism 34. The loading cam mechanism 34 is mounted on the center transmission shaft 16, at the side of the first input disc 18a. The loading cam mechanism 34 is designed to receive a torque from the prime mover through the torque converter 12, and to produce an axial thrust load in accordance with the input torque. The loading cam mechanism 34 has a loading cam disc (loading cam member) 34a which is rotatably mounted on the center transmission shaft 16 near the first lefthand end, and engaged with the center transmission shaft 16 through a thrust bearing 36. The loading cam mechanism 34 further includes loading rollers compressed between the cam disc 34a and the first input disc 18a. When the input torque is applied to the loading cam disc 34a, the cam disc 34a rotates relative to the first input disc 18a. This relative rotation causes the cam mechanism 34 to compress the loading rollers between the cam disc 34a and the first input disc 18a so as to permit torque transmission therebetween, and to produce an axial force (thrust load) pushing the first input disc 18a toward the first output disc 18b and an opposite axial force pushing an outward flange formed in the first end of the center transmission shaft 16 through the thrust bearing 36 in the opposite axial direction. The toroidal transmission 10 further includes a second loading means which, in this embodiment, is a disc spring (Bellevile spring) 38 disposed between the second input disc 20a and the second righthand end of the center transmission shaft 16. The disc spring 38 applies an axial preload on the second input disc 20a and pushes the second input disc 20a toward the second output disc 20b. The thrust load produced by the loading cam mechanism 34 is applied on the first input disc 18a, and at the same time, transmitted through the thrust bearing 36, the center transmission shaft 16, and the disc spring 38 to the second input disc 20a. The preload produced by the disc spring 38 is applied on the second input disc 20a, and at the same time transmitted through the center transmission shaft 16 and the cam mechanism 34 to the first input disc 18a. The thrust bearing 36, the axially slidable center transmisson shaft 16, the couplings 24 and 26, and a bearing means for supporting the shaft 16 serve as a means for equalizing the thrust loads of the first and second units 18 and 20. The two power rollers 18c and 18d or 20c and 20d of each drive unit 18 or 20 are arranged symmetrically with respect to the center axis C of the center transmission shaft 16. The power rollers of each drive unit are inclined in accordance with an operating condition such as a vehicle operating condition, and continuously vary a transmission ratio which is a ratio between speeds of the output and input discs, as disclosed in JP-A-63-92859. As shown schematically in Fig. 1, the power rollers 18c, 18d, 20c and 20d are, respectively, connected with hydraulic actuators 50, 52, 54 and 56, through rotating shafts 50a, 52a, 54a and 56a, and eccentric shafts 50b, 52b, 54b and 56b. In this embodiment, the first drive unit 18 is connected with a first actuator unit (means) which includes the two actuators 50 and 52, and the second drive unit 20 is connected with a second actuator unit (means) which includes the two actuators 54 and 56. Each power roller 18c, 18d, 20c or 20d is rotatably supported, through the eccentric shaft 50b, 52b, 54b or 56b, on the shaft 50a, 52a, 54a or 56a which is axially moved by the actuator 50, 52, 54 or 56. Each actuator 50, 52, 54 or 56 is capable of inclining the associated power roller 18c, 18d, 20c or 20d by causing the associated shaft 50a, 52a, 54a or 56a to move up and down. Each of the rotating shafts 50a, 52a, 54a and 56a is arranged to rotate when the associated actuator inclines the power roller by moving the rotating shaft up and down. Therefore, the angle of inclination of each power roller 18c, 18d, 20c or 20d is determined by the displacement of the associate actuator 50, 52, 54 or 56, or by the angular displacement of the rotating shaft 50a, 52a, 54a or 56a which is rotated in accordance with the up and down movement. The toroidal transmission sysem further includes a single control valve 60 serving as a controlling means. The control valve 60 produces a control signal which is in the form of a control oil pressure, and delivers the control signal to each of the hydraulic actuators 50, 52, 54 and 56. The control valve 60 has a drive rod 62 which is rotated by a step (stepping) motor 61, a sleeve 63, a spool 64 received in a bore of the sleeve 63, and a spring 65 for pushing the spool 64 in a leftward axial direction away from the motor 61, as viewed in Fig. 1. The drive rod 62 has an externally threaded end portion 62a, which is screwed in an internally threaded hole 63a of the sleeve 63. The sleeve 63 has an axially extending groove formed in the outer circumferential surface, and a pin 66 is received in the groove of the sleeve 63, so that the sleeve 63 is axially slidable without being rotated. A feedback arrangement is connected with the lefthand end of the spool 64, whose opposite righthand end is pushed by the spring 65. The feedback arrangement has a precess cam 67 and a link 67a. The feedback arrangement is arranged to convert a rotational displacement of one of the rotating shafts 50a, 52a, 54a and 56, into an axial displacement, through the precess cam 67 and the link 67a, and to feed back this axial displacement to the control valve 60. The amount of rotation (angular displacement) of each of the rotating shafts 50a, 52a, 54a and 56a is proportional to the amount of inclination of the associated power roller 18c, 18d, 20c or 20d. Therefore, it is possible to feed back the amount of inclination of the power roller by feeding back the amount of rotation of the rotating shaft. Similar precess cam arrangements are disclosed in a U.S. Patent No. 4,434,675 and a copending U.S. Appliation Serial No. 07/352,309. The explanations about the precess cam of these documents are herein incorporated by reference. The spool 64 has a first land 64a, and a second land 64c, which act to distribute a line pressure introduced through an input port 68 formed in a valve housing of the control valve 60, between first and second ports 68a and 68b of the valve housing, in accordance with the movement of the spool 64. Fig. 1 shows the spool 64 in a stable state in which the line pressure is equally distributed between the first and second ports 68a and 68b. As shown in Fig. 1, the input port 68 is located axially between the first and second ports 68a and 68b. Each hydraulic actuator 50, 52, 54 or 56 has a upper fluid pressure chamber A and a lower pressure chamber B, which are separatd by a piston. When a higher fluid pressure is supplied to the upper chamber A, then each hydraulic actuator 50, 52, 54 or 56 causes the corresponding rotating shaft 50a, 52a, 54a or 56a to move downward in Fig. 1. When, on the other hand, the pressure supplied to the lower chamber B is higher than the pressure of the upper chamber A, then each actuator causes the corresponding rotating shaft to move upward as viewed in Fig. 1. The four hydraulic actuators 50, 52, 54 and 56 are connected with the control valve 60 in the following manner. The upper chambers A of the actuator 50 and 54, and the lower chambers B of the actuators 52 and 56 are all connected with the first port 68a of the control valve 60, by a first hydraulic circuit 86. On the other hand, a second hydraulic circuit 87 connects the second port 68b of the control valve 60 to the lower chambers B of the actuators 50 and 54 and the upper chambers A of the actuators 52 and 56. Between the actuators 50 and 52 for the first toroidal drive unit 18, the upper chamber A of the first actuator 50 is connected with the lower chamber B of the second actuator 52, and the lower chamber B of the first actuator 50 is connected with the upper chamber A of the second actuator 52. The actuators 54 and 56 for the second toroidal drive unit 20 are connected with each other in the same diagonal manner. Therefore, the operating direction of the actuators 50 and 54 which are operated together in the same direction is opposite to the operating direction of the actuators 52 and 56 which are operated together in the same direction. The thus-constructed toroidal transmission system of this embodiment is operated in the following manner. In the toroidal transmission 10, the first and second parallel toroidal drive units 18 and 20 transmit torque inputted from the engine through the torque converter 12 to the loading cam mechanism 34, to the output gear 28 while continuously varying the speed ratio between the output and input speeds. The output torque of the transmisison 10 is transmitted through the gear 30a of the counter shaft 30, to drive wheels of the vehicle. During this, the axial thrust load produced by the lading cam mechanism 34 is transmitted through the center transmission shaft 16, and applied equally to the first and second input discs 18a and 20a. Therefore, the pressure compressing the power rollers 18c and 18d of the first drive unit 18 is equal to the pressure compressing the power rollers 20c and 20d of the second drive unit 20. The first and second input discs 18a and 20a are coupled through the center transmission shaft 16 so that they rotate together. Therefore, the phase of the first input disc 18a is equal to the phase of the second input disc 20a, and the torque is transmitted without phase difference. Thus, the first and second toroidal drive units 18a and 20 are equal in the friction condition in which the input, output and intermediate members are in contact with one another, partly because the pressures compressing the power rollers are equal, and partly because the rotational speeds of the first and second input discs are equal. In the control valve 60, the axial position of the spool 64 is determined by the amount of rotation of the step motor 61 which is driven by an electric control signal produced by a control unit (not shown) in accordance with a vehicle operating condition, and by the amount of feedback which is returned from the power roller through the precess cam 67. In accordance with the axial position of the spool 64, the control valve 60 supplies a first control fluid pressure from the first port 68a and a second control fluid pressure from the second port 68b, to the actuators 50, 52, 54 and 56. Each of the first and second control fluid pressures is common to all the actuators 50, 52, 54 and 56. Thus, the toroidal transmission system of this embodiment holds the first and second toroidal drive units 18 and 20 equal in condition of contact among the input, output and intermediate members, and in condition of control fluid pressure supplied to the hydraulic actuators. By so doing, the toroidal transmission system of this embodiment can make the amounts of inclination of the power rollers 18c, 18d, 20c and 20d equal to one another, and make the speed ratios of the first and econd drive units 18 and 20 equal to each other. If at least one of the power rollers 18c, 18d and 20c and 20d becomes unbalanced from some cause, the transmission system of this embodiment continues supplying the unbalanced power roller with the fluid pressure equal to the pressure of the remaining power rollers. Therefore, the transmission system of this embodiment can easily settle down the unbalanced power roller into the original stable inclined state, and prevent the drive unit from being disturbed severely.

Claims: A toroidal type continuously variable transmission system comprising: first and second parallel toroidal drive units (18, 20) which are arranged in parallel to provide two parallel drive paths, each of the first and second drive units comprising an input disc (18a, 20a), an output disc (18b, 20b) and an intermediate member comprising power rollers (18c, 18d, 20c, 20d) for coupling the input and output discs for torque transmission; first actuating means (50, 52) connected with the first drive unit for varying the relative positions of the input disc, output disc and intermediate member of the first drive unit in accordance with a first control signal, and second actuating means (54, 56) connected with the second drive unit for varying the relative positions of the input disc, output disc and intermediate member of the second drive unit in accordance with a second control signal; loading means for applying a first axial force to the first drive unit, and a second axial force to the second drive unit, and equalizing means for equalizing operating conditions of the first and second drive units, the equalizing means comprising a transmission shaft (16) for supporting the input discs (18a, 20a) of the first and second drive units which are mounted on said transmission shaft and for causing the input discs of the first and second drive units rotate together; characterized in that each of the first and second actuating means comprises a hydraulic actuator, and the first and second control signals are first and second control fluid pressures; in that the equalizing means further comprises a common fluid pressure control valve (60) for controlling the first and second fluid pressures so that both pressures remain equal to each other; in that the equalizing means further comprises first and second coupling means (24, 26) for equalizing the first and second axial forces, said first coupling means (24) facilitating axial movement of the input disc (18a) of the first drive unit relative to the transmission shaft (16) and preventing rotation of the input disc of the first drive unit relative to the transmission shaft, and said second coupling means (26) facilitating axial movement of the input disc (20a) of the second drive unit relative to the transmission shaft and preventing rotation of the input disc of the second drive unit relative to the transmission shaft. A toroidal type continuously variable transmission system according to Claim 1 wherein the loading means comprises a first loading means (34) for thrusting the input disc (18a) of the first drive unit axially toward the output disc (18b) of the first drive unit; the transmission shaft (16) is axially slidably supported on a transmission housing (22); the equalizing means further comprises a thrust bearing (36) which is disposed between the transmission shaft and the first loading means. A toroidal type continuously variable transmission system according to Claim 2 wherein each of the first and second coupling means comprises ball splines (24, 26). A toroidal type continuously variable transmission system according to any of Claims 2 ∼3 wherein an output terminal member (28) is rotatably mounted on the transmission shaft (16), and located axially between the input discs (18a, 20a) of the first and second drive units, and the output discs (18b, 20b) of the first and second drive units are splined to the output terminal member (28) so that the output discs rotate together but are movable axially relative to each other. A toroidal type continuously variable transmission system according to Claims 2 ∼ 4 wherein the first loading means comprises a loading cam mechanism (34) which transmits an input torque to the input disc (18a) of the first drive unit, produces the first axial force in accordance with the input torque, and applies the first axial force on the input disc of the first drive unit; the loading cam mechanism comprises a loading cam disc (34a) which is rotatably mounted on the transmission shaft (16) and which is rotatable relative to the input disc (18a) of the first drive unit, and loading rollers which are compressed between the cam disc (34a) and the input disc (18a) of the first drive unit and which apply the first axial force on the input disc of the first drive unit in accordance with relative rotation between the cam disc and the input disc of the first drive unit; and the loading means further comprises a second loading means which comprises a disc spring (38) disposed between the input disc (20a) of the second drive unit and the transmission shaft (16).

Inventors: NAKANO MASAKI, NAKANO, MASAKI
Applicants: NISSAN MOTOR, NISSAN MOTOR CO., LTD.
Application year: 1989
Main classification: F16H15
IPC codes: F16H63, F16H61, F16H15