WO1991007613A1

WO1991007613A1 - Control system for a continuously variable cone pulley-drive belt transmission

Info

Publication number: WO1991007613A1
Application number: PCT/GB1990/001796
Authority: WO
Inventors: Hartmut Roper
Original assignee: Ford Motor Company Limited; Ford Werke Ag; Ford France S.A.; Ford Motor Company; Ford Motor Company Of Canada Limited
Priority date: 1989-11-21
Filing date: 1990-11-21
Publication date: 1991-05-30
Also published as: DE3938539A1; EP0534971A1

Abstract

A continuously variable transmission includes two conical pulleys (3, 4, 5, 6), one pulley located in the input shaft (1), the other pulley located on the output shaft (2). A drive member (7) in frictional contact with the pulleys (3, 4, 5, 6) transmits torque between them. One conical pulley member (3, 5) is axially fixed and the opposing pulley member (4, 6) is axially movable but non-rotatable on its associated shaft (1, 2). The movable pulley members (4, 6) are actuated by hydraulic servos (A1, A2), loaded by pressure supplied by a pump (8) through a control valve (9) for adjusting and maintaining the transmission ratio. Servos (A1, A2) on the primary shaft (1) and secondary shaft (2) each have an associated torque sensor valve (10, 11) different from that of the other servo. One sensor valve (11) is preceded in a hydraulic control circuit by a minimum pressure valve (12) and followed by the other sensor valve (10).

Description

CONTROL SYSTEM FOR A CONTINUOUSLY VARIABLE CONE PULLEY-DRIVEBELT TRANSMISSION

Background of the Invention

1. Field of the Invention

This invention relates to the field of automatic transmissions of the type that produce a stepless variable range of speed ratio by changing the relative diameters of pulleys connected by an endless drivebelt. The invention particularly relates to a system for controlling the operation of such a transmission.

2. Description of the Prior Art

German Patent DE-C2-30 28 490 describes a control system for a continuously variable cone pulley-drivebelt transmission. The transmission includes two conical pulleys, each arranged on a primary shaft or a secondary shaft, connected driveably by an endless belt rotatably supported on the pulleys. One pulley of each conical pulley assembly is axially fixed on its supporting shaft and the other pulley of each conical pulley pair is axially movable and rotatably supported on the corresponding shaft. The axially movable pulleys are connected to cylinders, which cooperate with pistons arranged on the shafts to form hydraulically actuated pressure servo, supplied with fluid from a pump. In order to adjust the transmission ratio and to ensure that contact .pressure is maintained between the belt and pulleys, the pump is supplied with fluid through a transmission ratio control slide valve.

The axially-fixed conical pulley members cooperate with a torque sensor in the form of rolling elements arranged between mutually opposed ball races.

One of the ball races is constructed on an annular member, axially rigidly connected to the primary and/or secondary shaft, and the opposing ball race is constructed on an annular member, axially and rotatably journalled on the primary and/or secondary shaft. One annular member is driveably connected to a driving or driven shaft. The axially movable annular member has an annular piston, which cooperates with an annular cylinder constructed in the fixed cone pulley member, to form a cylinder/piston assembly. A fluid supply opening in the shafts supporting the pulley, a fluid discharge opening controllable by the annular piston, and the cylinder/piston assembly form a valve for delivering a torque-dependent control pressure.

The control system of the '490 patent consists essentially of a fluid pump, a pretensioning valve operating on the basis of the transmission ratio, a transmission ratio control slide valve, and a torque sensor valve.

Figure 2 of the '490 patent shows that the fluid issuing from the fluid discharge opening is guided via a branch channel to a region between the ball races of a torque sensor. This fluid flow path is provided for the purpose of lubrication only.

Another control system for continuously variable conical pulley belt transmission is described in German Patent DE-C1-34 03 704. That system includes an axially fixed conical pulley having, a torque sensor valve that delivers torque dependent pressure, controlled by a basic pressure valve before it is supplied to a transmission ratio control valve.

A control system in which the torque dependent control pressure is initially modulated by a pretensioning valve in accordance with the operating transmission ratio before it is supplied to the transmission ratio control valve, can therefore be inferred from the '490 patent.

The control systems of the '704 and '490 patents 5 have the disadvantage that, because they operate in response to the torque transmitted only on one of the two shafts, they lead to excessively high contact pressures over wide operating ranges of the transmission. These high contact pressures reduce the overall efficiency of 10. the transmission due to the high flow capacity of the pump required to maintain high contact pressures.

SUMMARY OF THE INVENTION

15 An object of the present invention is to improve a control system for a continuously variable transmission such that, by appropriately accounting for torque on both the input shaft and output shaft, the contact pressure for the endless drivebelt can be controlled so accurately

20 that slippage of the drivebelt on the pulleys is prevented. Yet, the control system of this invention operates such that the contact pressure is kept at the minimum pressure required to prevent slippage of the drivebelt on the pulleys. 5 This object is achieved because a torque sensor valve of the control system has a space located between two ball races that forms a sealed cylinder/piston assembly. The active face of the torque sensor valve corresponds to the active face of the cylinder/piston 0 assembly located on a movable annular member of the axially fixed cone pulley. The torque sensor valve delivers a torque-dependent differential pressure and further includes a minimum pressure valve followed by a torque sensor valve delivering torque-dependent absolute 5 pressure. In this ay, a modified torque-dependent - A -

control pressure, which allows control of the contact pressure at a low pressure and protection against slippage, is supplied to a transmission ratio control slide valve. In a continuously variable conical pulley-drivebelt transmission, a drivebelt contact pressure produced by hydraulic pressure servos is adjusted according to the torque transmitted and the transmission ratio. If contact pressure is too low, slippage of the tension member results. Slippage causes excessive wear and possibly seizure; therefore, it must be avoided in all conditions. In conventional control systems for such transmissions, contact pressure is controlled to a sufficiently high magnitude to prevent slippage reliably.

Excessively high contact pressure requires an unnecessarily high pump capacity and impairs efficiency of the transmission.

If the contact pressure is not controlled accurately in accordance with the torque transmitted between the conical pulleys, it causes low operating efficiency of the transmission particularly in the operating ranges in which only partial load or torque capacity of the transmission is produced. The low fuel consumption potential of vehicles equipped with such transmission that should be possible due to optimum control of the engine speed cannot be fully achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail with reference to an embodiment illustrated in the drawings.

Figure 1 shows a schematic diagram of a hydraulic circuit for a continuously variable automatic transmission. Figure 2 shows a graph of the variation of drivebelt clamping pressure with transmission ratio produced by a hydraulic circuit such as that of Figure 1.

Figure 3 is a torque sensor valve arrangement according to the state of the art.

Figure 4 shows a torque sensor valve according to the present invention.

Figure 5 is a schematic diagram of a hydraulic circuit according to the present invention. Figure 6 is a graph showing the variation of drivebelt clamping pressure with transmission ratio produced by a hydraulic circuit according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the hydraulic circuit of Figure 1, hydrostatic pressure P2, which is determined by operation of a ratio-dependent clamping valve 19 and a torque sensor 20 (Type A) arranged in series with the clamping valve, is present in an hydraulic pressure servo A2, located on the secondary or output shaft 23, to accelerate the vehicle from a stationary state.

Fluid is pumped from a sump 24 through a hydraulic pump 26 and a ratio control valve 28, to an hydraulically actuated servo Al located on the primary or input shaft 1. According to empirical experience, pressure within servo Al is about 30 percent higher than required to maintain the selected transmission ratio. The differential pressure produced by clamping valve 19 is dependent on the current operating transmission ratio. This dependency results because the respective positions of the axially movable conical pulley members on the input shaft and output shaft are determined mechanically. The axial position of at least one of the axially displaceable pulleys is transmitted to a pretensioning spring of the clamping valve. The pressure produced by the torque sensor 20 is proportional to torque according to the equation P = f T wherein P is the torque sensor pressure, f is a torque sensor proportionality constant and T is the torque.

The magnitude of the contact pressure produced is therefore the sum of a magnitude dependent on transmission ratio and a magnitude dependent on torque. The graph of Figure 2 shows graphically the relocation between clamping pressure and transmission ratio. Curve a represents the pressure trend required to transmit maximum torque. Curve A shows the actual adjusted contact pressure.

Curve c represents contact pressure required to transmit one-half maximum torque; curve C represents the actual contact pressure required to transmit one-half maximum torque. Because of the difference in curvature between curves A and a, excess clamping pressure of magnitude Δ P is produced in the intermediate transmission ratio range. The excess pressure magnitude, shown in Figure 2, is considerably greater in the partial torque or load range represented by curves c and C. The large excess pressure Δ P cannot be substantially reduced because curve C would extend beneath curve c, if curve C were lowered in the range of low transmission ratios, i. e., close to overdrive. This condition would cause slippage of the drivebelt connecting the conical pulleys.

A known torque sensor valve assembly is described next in comparison with the torque sensor valve according to the present invention, with reference to Figures 3 and 4. - 1 -

Figure 3 shows a torque sensor valve 20 similar to that of the '490 patent. Torque sensor 20 of Figure 3 has been designated "type A" for distinction from the torque sensor valve 22 arrangement according to the present invention, which is designated "type B" and described below.

Pressurized hydraulic fluid from the discharge side of hydraulic pump 26 passes through ratio control valve 28 and clamping valve 18 to torque sensor 20. The fixed cone pulley member 2 is formed integrally with primary shaft 1 and supports torque sensor 20 within a cylinder 11 formed in pulley member 2. The torque sensor includes an axially movable annular member 3, an axially fixed annular member 4, and rolling elements 7 located between mutually opposed ball races 5 and 6. The axially movable annular member 3 is connected by a spline 9 to a driveshaft or driving component such as an element of a clutch 8. Member 4 is prevented from moving axially away from member 3 by a snap ring 29 seated in a recess on fixed pulley 2, and member 4 is joined rotatably by spline 30 to pulley 2. The ball races are annular cam surfaces of variable axial depth, on which surfaces the rolling elements move as member 3 rotates with respect to member 4. The rolling elements transmit torque from member 3 to member 4 by wedging action on the cam surfaces. For example, starting torque applied to clutch 8 rotates member 3 relative to member 4, thereby moving member 3 leftward from the upper position shown in Figure 3 to the lower position of Figure 3 as rolling elements 7 maintain contact with the cam surfaces due to fluid pressure in cylinder behind member 3. In this way starting torque produces axial movement of annular member 3 and its piston part 10 within cylinder 11 on fixed pulley member 2. Pressurized fluid is supplied through channel 13 at rate QI to pressure chamber 12 bounded by piston 10 and cylinder 11. The piston cooperates with the control edge 14 of discharge channel 15, through which fluid 5 issues at rate Q2 reduced only by leakage, to a pressure-free region of the transmission, such as sump 24. A hydrostatic pressure, necessary to hold axially movable annular member 3 of the torque sensor valve against roller element 7 in the position shown in the

10. upper part of Figure 3, is developed in the cylinder.

This hydrostatic pressure opposes the axial force due to torque developed between fixed annular member 4 and movable annular member 3 tending to spread or increase the space between members 3 and 4. At high torque, the

15 control edge tends to reduce size of the opening between chamber 12 and channel 15, thereby increasing the magnitude of pressure in the chamber. However as torque decreases, the control edge throttles fluid through the opening and eventually opens communication between

20 channel 15 and chamber 12, thereby decreasing pressure in the chamber in proportion to torque.

Therefore, a torque-dependent control pressure develops in chamber 12 in direct proportion to the magnitude of torque transmitted through the torque

25 sensor. The torsion path through torque sensor 20 includes clutch 8, spline 9, member 3, rolling elements 7, member 4, spline 30 and input shaft 1.

Figure 4 shows the torque sensor valve 22 assembly (type B) of the present invention marked with

30 identical reference numbers as those used on similar components of the device of Figure 3, but certain components are modified from those of Figure 3 and have the corresponding reference numbers marked with an apostrophe.

35 Fixed annular member 4' is located in movable annular member 3 ' with a dynamic hydraulic seal therebetween. The pressure-loaded annular faces of members 3', 4' are equal in area, and the region between 5 ball races 5 and 6 is loaded by hydraulic pressure supplied through channel 16, which communicates with the torque-dependent control pressure in chamber 12.

The torque sensor valve 22 produces a torque-dependent differential pressure between the

10_. pressure in chamber 12 and the pressure in discharge channel chamber 17, but not a torque-dependent absolute pressure.

Operation of the control system according to the present invention is described next with reference to

15 Figures 5 and 6. Figure 5 shows that the transmission ratio-dependent clamping valve 19 of Figure 1 is replaced by the series arrangement of minimum pressure valve 18 and torque sensor 22 type B.

A torque sensor, either of type A or type B, is 0 located on the primary shaft 1 and another torque sensor, of the type other than the type located on shaft 1, is located on the secondary shaft 23. It is unimportant whether a torque sensor 20 of type A or torque sensor 22 of type B is located on the primary shaft. Different 5 torque sensor valves are located on the two shafts, and the type B torque sensor 22 follows pressure valve 18 and precedes the type A torque sensor 20.

The intake side of pump 26 is supplied from sump 24 and pump discharge is connected to the bore of ratio 0 control valve 28 and to relief valve 32, which limits the magnitude of line pressure in the hydraulic circuit. The spool 34 of valve 28 is connected mechanically by a mechanism, which includes a lever 36 for setting the transmission ratio and a lever 38, connected to axially 5 displaceable pulley 40 for transmitting displacement of pulley 40 to spool 34. When the spool moves rightward, line pressure from the pump is directed to servo 42 on shaft 1, thereby increasing pressure PI. Rightward movement of the spool also causes servo 44 on shaft 23 to 5 communicate through ratio. control valve 28 with minimum pressure valve 18 and torque sensors 22, 20. Similarly, leftward movement of the spool causes servo 42 to communicate with valve 18 and the torque sensors.

Disregarding the effect of the minimum pressure 10_. valve 18, the resultant contact pressure is:

P₂ = f_χ τ_{± +} f₂ τ₂ wherein P is the contact pressure, f is the input shaft torque sensor constant, f is the output shaft torque sensor constant, T₁ is the input shaft torque 15 and T_ is the output shaft torque.

The resultant torque at the output shaft is: τ₂ = i τ_± wherein i is the transmission ratio produced by the pulley-drivebelt combination. 20 From the two preceding equations, the following equations are derived:

^P2 = ^fl ^Tl ^{+ i f}2 ^Tl and

= T, <^fl ^{+ f}2 i)

25 This equation states that contact pressure P is represented as a product of a function linearly dependent on the transmission ratio i and the torque T. The linear transmission function can be given any desired initial value and any desired slope by establishing

30 suitable sensor constants f, and f_.

If f-. and f₂ are selected such that the actual pressure corresponds to the necessary contact pressure with the transmission operating in the lowest and highest ratios, a pressure trend having only very

35 slight excess pressures Δ P at any torque is obtained over the entire transmission ratio range.

Figure 6 shows the pressure ratios attainable with a control system according to the present invention. Curves a, b, c and d represent the necessary contact pressures for the following multiples of maximum torque: 1.0, 0.75, 0.50, and 0.25, respectively.

Curves A, B, C, and D represent the variation of contact pressure or clamping pressure P_? attainable with a control system of the present invention shown in Figure 5. The respective excess pressures Δ P are reduced to much smaller magnitudes than those of Figure 2. With pressure regulation performed according to the last equation above, a torque T. =* 0 would produce a contact pressure of P„ = 0, i. e. , the drivebelt that transmits torque between the conical pulleys is entirely unloaded. Therefore, contact pressure would be entirely ineffective and a drivebelt would slide on the conical pulleys. To prevent this condition, a residual tension or residual contact pressure is maintained by minimum pressure valve 18 in the control system of Figure 5. This valve does not allow contact pressure to fall below a predetermined magnitude, e. g., 3 bar. The control system according to this invention allows pressure regulation of the contact pressure without significant regions of excess pressure. It therefore allows reduction of pump losses and improvement in efficiency of a conical pulley-drivebelt transmission, thereby improving overall efficiency of the transmission. To produce this result, an additional torque sensor valve is included. However, its cost is compensated for by omission of a pretensioning valve 19, which requires a transmission ratio sensor and a mechanical sensor linkage, which are also omitted. The control system according to the present invention is applicable not only to continuously variable transmissions of. the tension drivebelt "Reimers" design but similarly to compression drivebelt transmissions of the "Van Doorne" design. The secondary valve, which determines contact pressure, is replaced by a torque sensor and valve arrangement according to this invention.

While the invention has been described above by reference to specific forms of torque sensors, it should be appreciated that alternative means can be used to sense the torque applied to the first and second shafts, it only being important to ensure that the torques applied to both shafts are combined in setting the pressure at the outlet of the transmission ratio control valve. Indeed, it is not even essential that the torque sensors be hydraulic or mechanical as one may sense and combine the torques electrically and use an electrically controlled valve or throttle to set the pressure at the outlet of the ratio control valve accordingly.

Claims

Cl aims

1. A system for controlling operation of a continuously variable transmission, comprising: an input shaft; an output shaft substantially parallel to the input shaft; a first pulley assembly supported on the input shaft and a second pulley assembly supported on the output shaft, each pulley assembly adapted for driveable engagment with a flexible endless drivebelt, each pulley assembly including pulleys rotatably fixed relative to the corresponding supporting shaft, one pulley of each pulley assembly being displaceable axially on its supporting shaft; a source of pressurized hydraulic fluid; first servo means connected to the fluid pressure source for selectively actuating said displaceable pulley of the first pulley assembly; second servo means connected to the fluid pressure source for selectively actuating said displaceable pulley of the second pulley assembly; ratio control valve means for selectively connecting, in accordance with the transmission ratio produced by the transmission, the fluid pressure source to the first or second servo means and connecting the other of the first and second servo means to an outlet of the ratio control valve means; a low pressure sump; a first torque sensor, including a first cylinder having an inlet communicating with the outlet of the ratio control valve and an outlet; a first member fixed to the input shaft for rotation therewith, restrained in at least one direction against axial displacement, having a first cam surface; a first piston located within the first cylinder, supported for axially displacement along the input shaft and rotation about the input shaft, adapted for driveable connection to a source of torque applied to the input shaft, having a second cam surface facing the first cam surface, urged by pressure in the first cylinder toward the first member, defining a first control edge that opens and closes the first cylinder outlet in accordance with the axial position of the first piston; rolling elements located between the first and second cam surfaces, driveably connecting the first member and the first piston, adapted to change the axial distance between the first member and first piston as the rolling elements move on the cam surfaces in response to differential torque between the first piston and first member; a second torque sensor in series relation with the first torque sensor, including a second cylinder having an inlet communicating with the outlet of the first torque sensor and an outlet communicating to the sump; a second member fixed to the input shaft for rotation therewith, restrained in at least one direction against axial displacement, having a first cam surface; a second piston located within the second cylinder, supported for axially displacement along the output shaft and rotation about the output shaft, adapted for driveable connection to a source of torque applied to the output shaft, having a second cam surface facing the first cam surface, urged by pressure in the second cylinder toward the second member, defining a second control edge that opens and closes the second cylinder outlet in accordance with the axial position of the second piston, space between the first and second cam surfaces being connected to the outlet of the second cylinder; rolling elements located between the first and second cam surfaces, driveably connecting the second member and the second piston, adapted to change the axial distance between the second member and second piston as the rolling elements move on the cam surfaces in response to differential torque between the second piston and second member.

2. The system of claim 1 further comprising a mimimum pressure relief valve means arranged in series relation with the first and second torque sensors, having an inlet communicating with the outley of the ratio control valve and an outlet communicating with the inlet of the first cylinder, for maintaining pressure at a minimum magnitude at the outlet of the minimum pressure relief valve.

3. The system of claim 1 wherein the second member and the second piston define an hydraulically sealed space therebetween, the space bounded at axially opposite ends by faces of the second member and second piston having substantially equal transverse cross sectional areas.

5. The system of claim 1 wherein the first torque sensor is located on the fixed pulley of the input shaft and the second torque sensor is located on the fixed pulley of the output shaft. 6. A system for controlling operation of a continuously variable transmission, comprising: a first shaft; a second shaft substantially parallel to the first shaft; a first pulley assembly supported on the first shaft and a second pulley assembly supported on the second shaft, each pulley assembly adapted_^for driveable engagment with a flexible endless drivebelt, each pulley assembly including pulleys rotatably fixed relative to the corresponding supporting shaft, one pulley of each pulley assembly being displaceable axially on its supporting shaft; a source of pressurized hydraulic fluid; first servo means connected to the fluid pressure source for selectively actuating said displaceable pulley of the first pulley assembly; second servo means connected to the fluid pressure source for selectively actuating said displaceable pulley of the second pulley assembly; ratio control valve means for selectively connecting, in accordance with the transmission ratio produced by the transmission, the fluid pressure source to the first or second servo means and connecting the other of the first and second servo means to an outlet of the ratio control valve means; a low pressure sump; a first torque sensor, including a first cylinder having an inlet communicating with the outlet of the ratio control valve and an outlet; a first member fixed to the input shaft for rotation therewith, restrained in at least one direction against axial displacement, having a first cam surface; a first piston located within the first cylinder, supported for axially displacement along the first shaft and rotation about the first shaft, adapted for driveable connection to a source of torque

4₀ applied to the first shaft, having a second cam surface facing the first cam surface, urged by pressure in the first cylinder toward the first member, defining a first control edge that opens and closes the first cylinder outlet in accordance with the axial position of the first

45 piston; rolling elements located between the first and second cam surfaces, driveably connecting the first member and the first piston, adapted to change the axial distance between the first member and first piston

50 as the rolling elements move on the cam surfaces in response to differential torque between the first piston and first member; a second torque sensor in series relation with the first torque sensor, including

55 a second cylinder having an inlet communicating with the outlet of the first torque sensor and an outlet communicating to the sump; a second member fixed to the second shaft for rotation therewith, restrained in at least one

6₀ direction against axial displacement, having a first cam surface; a second piston located within the second cylinder, supported for axially displacement along the second shaft and rotation about the second shaft, r - adapted for connection to a source of torque applied to the second shaft, having a second cam surface facing the first cam surface, urged by pressure in the second cylinder toward the second member, defining a second control edge that opens and closes the second cylinder outlet in accordance with the axial position of the second piston, space between the first and second cam surfaces of the second torque sensor being connected to the outlet of the second cylinder; rolling elements located between the first and second cam surfaces, driveably connecting the second member and the first piston, adapted to change the axial distance between the second member and second piston as the rolling elements move on the first and second cam surfaces of the second torque sensor in response to differential torque between the second piston and second member.

7. The system of claim 6 further comprising a mimimum pressure relief valve means arranged in series relation with the first and second torque sensors, having an inlet communicating with the outley of the ratio control valve and an outlet communicating with the inlet of the first cylinder, for maintaining pressure at a minimum magnitude at the outlet of the minimum pressure relief valve.

8. The system of claim 6 wherein the second member and the second piston define an hydraulically sealed space therebetween, the space bounded at axially opposite ends by faces of the second member and second piston having substantially equal transverse cross sectional areas.

9. The system of claim 6 wherein the first torque sensor is located on the fixed pulley of the first shaft and the second torque sensor is located on the fixed pulley of the second shaft. 10. The system of claim 6 wherein the first torque sensor is located on the fixed pulley of the second shaft and the second torque sensor is located on the fixed pulley of the first shaft.

11. A system for controlling pressure magnitude in an hydraulic servo actuating components of a continuously variable transmission, comprising: a first shaft; a second shaft substantially parallel to the first shaft; a first pulley assembly supported on the first shaft and a second pulley assembly supported on the second shaft, each pulley assembly adapted for driveable engagment with a flexible endless drivebelt, each pulley assembly including pulleys rotatably fixed relative to the corresponding supporting shaft, one pulley of each pulley assembly being displaceable axially on its supporting shaft; a source of pressurized hydraulic fluid; a low pressure sump; first servo means connected to the fluid pressure source for selectively actuating said displaceable pulley of the first pulley assembly; second servo means connected to the fluid pressure source for selectively actuating said displaceable.pulley of the second pulley assembly; ratio control valve means for selectively connecting the fluid pressure source to the first or second servo means and connecting the other of the first and second servo means to an outlet of the ratio control valve means; a first torque sensor means for producing a pressure signal representing the magnitude of torque applied to the first shaft; second torque sensor -^•--•cci-.i*-**. hydraulically connected to the differential pressure produced by the first torque sensor for producing a torque dependent absolute pressure representing the magnitude of torque applied to the second shaft; passage means for connecting the first and second torque sensors means in series relation between the outlet of the ratio control valve means and the sump.

12. A system for controlling pressure magnitude in an hydraulic servo actuating components of a continuously variable transmission, comprising: a first shaft; a second shaft substantially parallel to the first shaft; a first pulley assembly supported on the first shaft and a second pulley assembly supported on the second shaft, each pulley assembly adapted for driveable engagement with a flexible endless drivebelt, each pulley assembly including pulleys rotatably fixed relative to the corresponding supporting shaft, one pulley of each pulley assembly being displaceable axially on its supporting shaft; a source of pressurized hydraulic fluid; a low pressure sump; first servo means connected to the fluid pressure source for selectively actuating said displaceable pulley of the first pulley assembly; second servo means connected to the fluid pressure source for selectively actuating said displaceable pulley of the second pulley assembly; transmission ratio control valve means for selectively connecting the fluid pressure source to the first or second servo means and connecting the other of the first and second servo means to an outlet of the ratio control valve means; a first torque sensor means for producing a signal representing the magnitude of torque applied to the first shaft; second torque sensor means for producing a signal representing the magnitude of torque applied to the second shaft; means connecting the first and second torque sensor means for producing a pressure signal representing the sum of the torques applied to the first shaft and the second shaft; and means for maintaining the pressure at the outlet of the ratio control valve means at the value of the latter pressure signal.