CA2008770C - Transmissions of the toroidal-race rolling-traction type - Google Patents

Abstract

A roller control system for a continuously-variable-ratio transmission of the toroidal-race rolling-traction type. The hydraulic piston-and-cylinder combination, which operates to apply tangential shift to each roller carriage and so alter the ratio transmitted by that roller, includes a non-interlocking connection between the carriage and at least one of the operating pistons that bear upon it. The invention also includes a low-resistance connection between a cylinder and its hydraulic operating circuit, a means for forming the essential structure of that circuit within the casing of the transmission itself, and forming the cylinder/circuit connection so that if the piston overshoots its permitted stroke, so indicating an "overload"
condition, an obstruction is set up which causes the system pressure to rise and oppose further overshoot.

Description

w r 2008770 r IMPROVEMENTS IN OR RELATING TO TRANSMISSIONS
OF THE TORODIAL-RACE ROLLING-TRACTION TYPE
This invention relates to continuously-variable-ratio transmissions (Which will be referred to as CVT~s) of the toroidal-race, rolling-traction type, and it relates in particular to CVT~s as described in the specification of Canadian patent application No. 2,003,334, which was filed on 20th November, 1989 and which claims priority under the International Convention from UK Patent Application No.
8827140, filed 21st November, 1988.
The present invention relates to a roller control system for a CVT of the toroidal-race, rolling-traction type in which a roller is mounted to spin in a carriage and transmits traction between coaxial rotatable discs presenting input and output races conforming to different parts of the surface of a single imaginary torus and is free to rotate about a diameter and so to change the transmitted ratio, in which an operating mechanism is reciprocable over a predetermined stroke of operating movement and is operable to apply a predetermined control force to the carriage, and in which the operating mechanism and roller assembly seek an equilibrium position and ratio in which the resultant of the control force and of the traction forces experienced by the roller assembly in a plane at right angles to the axis of the discs is zero. Such a roller control system Will now be referred to as a roller control system of the kind described. While the prior art teaches and the invention will be described With relation to toruses of circular cross-section, the invention includes CVT~s in which the torus is generated by rotating any closed figure, of generally curved outline, about a generator line.
According to one aspect of the present invention, in a roller control system of the kind described, the operating mechanism includes a first piston moveable within a first cylinder, this piston and cylinder combination is single-acting and capable of 2~a8~~~
_ 2 _ exerting a force in a first direction, a second piston and cylinder are provided capable of exerting a force in a second and substantially opposite direction) and the second piston and carriage are separate items which abut each other in use in a 05 non-interlocking manner. Preferably the second piston and cylinder are so disposed, relative to the carriage) that depression of the second piston within its cylinder facilitates the assembly of the carriage and roller within the CVT) by allowing easy access of the first piston into its cylinder) and easy introduction of the roller into contact with the discs between: which it is to transmit traction. Once assembly is complete) and the second cylinder is pressurised when the CVT is brought into use, the second piston will be driven out of its cylinder and into working abutment with the carriage.
According to another aspect of the present invention the operating mechanism of a roller control system of the kind described includes a single-acting first piston moveable within a first cylinder which determines the motion of the roller carriage. The cylinder is in communication with a hydraulic operating circuit, and the cross-section of the connexion between the cylinder and that circuit is large in comparison with the cross-section of conventional hydraulic inlet and outlet ports.
An important consequence of this enhanced-area connexion is less resistance to fluid flow into and out of the cylinder when the piston moves suddenly in response to sudden changes in operating conditions - for instance a crash stop or a quick change from normal to icy road surface when the CVT is fitted to a road vehicle - and thus less tendency for the pressure within the cylinder to take up a spurious value because fluid cannot enter or leave the cylinder quickly enough. Preferably the full bore of the back end of the cylinder is in communication with the circuit. The part of the circuit conduit with which the cylinder bore communicates may be a gallery formed within the CVT casing, and the gallery may be ring-like in shape) and coaxial with the main CVT axis. Preferably the gallery conforms to the shape of only an incomplete ring, the break in the ring allowing access for other components to pass through the CVT casing. Where a second piston and cylinder are provided to exert a force in a second and substantially opposite direction upon the roller 05 carriage, the second cylinder may be in communication with a second gallery, with like effect. Where a plurality of rollers -typically three - transmit traction between coaxial rotatable discs in a CVT of the toroidal-race) rolling-traction type, the first and second galleries may be common to the first and second cylinders of the operating mechanisms of all three such rollers.
In a so-called "double-ended" CVT of the toroidal-race, rolling-traction type) as shown for instance in patent specification US-A-4297918, having two separate but symmetrically-arranged sets of torque-transmitting rollers, both sets of rollers may have first and second galleries, the first galleries of the two sets may be connected so that the fluid pressures within them are equal, and the second galleries may be similarly connected.
According to this aspect of the invention- it is also advantageous that the same hydraulic circuit should generate the appropriate contact forces between the rollers and the discs by exerting an end load on the discs, and that the parts of the circuit in communication with the end- load and roller control cylinders should be separated by no substantial resistance, so that substantially equal pressures exist in these two parts of the circuit at all times. The requirement to put the input and output discs under an appropriate end load, that is to say an axially load urging them together and into the appropriate reaction with the rollers between them, is well understood in the art. Many prior patent specifications) for instance US-A-4297918 and US-A-4524641) describe and illustrate the generation of the end load by hydraulic means.
According to another aspect of the present invention, the operating mechanism of a roller control system of the kind described includes pistons moveable within cylinders in communication with a hydraulic operating circuit. The geometry 2008~~°

of the pistons and their cylinders may be such that whenever a piston approaches the end of its permitted stroke (thus indicating an "overload" or other emergency condition of the transmission), it creates an obstruction within the circuit and causes a rise in fluid pressure upstream of the obstruction which acts to oppose further overshoot. Where a sat of rollers - typically three - transmit traction between two discs, and especially where two related such sets of rollers are used as in a "double-ended" CVT, the cylinders of all the rollers may be connected in parallel to the same hydraulic circuit, so that the pressure rise is generated at the pistons associated with only one or two of the rollers, but acts upon the operating mechanisms of the others.
The invention may be summarized, according to one aspect as a CVT of the toroidal race, rolling traction type in which a roller assembly comprises a carriage, bearings mounted on the carriage and a roller defining a roller centre and supported by the bearings, for rotation about a roller axis, in which the roller contacts and transmits traction forces between coaxial rotatable discs, presenting input and output races conforming to different parts of the surface of a single torus and is subjected to traction forces at disc/roller contacts, and a double-acting operating mechanism having a fixed part and a moveable part, the moveable part being reciprocal over a predetermined stroke of operating movement and operable to apply a control force to the carriage, and in Which the operating mechanism and carriage seek an equilibrium position in which the resultant of the control force and of the traction forces experienced by the roller assembly is zero, wherein: the single torus defines a mid-planet the carriage includes a rigid structure relative to which the roller axis and the roller centre are fixedf the operating mechanism includes constraint means defining a single point of constraint which constrains the operating mechanism to the application of the control force along a force application axis passing through the constraint point and the roller centref the constraint point and the disc/roller contacts form a constant triangular relationship _. ~ 2008770 - 4a -the equilibrium position of the roller assembly is determined by only the two disc/roller contacts and the position of the constraint pointy and the roller is free to rotate about the force application axis.
According to another aspect the invention provides a CVT
of the toroidal race rolling traction type in Which a roller assembly comprises a carriage, bearings mounted on the carriage and a roller defining a roller centre supported by the bearings, for rotation about a roller axis, in which the roller contacts and transmits traction forces between coaxial rotatable discs, presenting input and output races conforming to different parts of the surface of a single torus defining a mid-plane and in so doing are subjected to traction forces at disc/roller contacts, and an operating mechanism having a fixed part and a movable part, the movable part being reciprocal over a stroke of operating movement and operable to apply a predetermined control force to a location on the carriage, with the control force acting solely along an axis including both the location on the carriage and the roller centre and in which the operating mechanism and carriage seek an equilibrium position in which the resultant of the control force and of the traction forces experienced by the roller assembly is zero When resolved in the same direction in a plane normal to the axis of the discs, wherein: the carriage includes a rigid structure relative to Which the roller axis and the roller centre are fixedt the operating mechanism includes means capable of effecting translational movement of the roller centre back and forth along a path dictated by the two disc/roller contactst means allowing the roller freedom to rotate about a diameter thereof so as to change the transmitted ratio of the CVT~ the axis and the roller centre are fixed relative to the carriages the roller centre is positioned to impose no loads on the carriage nor on the operating mechanism other than to produce the balance between the control force and the traction forces experienced by the roller assembly, in the plane normal to the axis of the discs and the two disc/roller contacts and the location at which the control force is applied to the roller - 4b -assembly define a triangular relationship establishing a triangle of forces acting in the same plane.
According to yet another aspect the invention provides a CVT of the toroidal race, rolling traction type in Which a roller assembly comprises a carriage, bearings mounted on the carriage and a roller defining a roller centre supported by the bearings, for rotation about a roller axis, in which the roller contacts and transmits traction forces between coaxial rotatable discs, presenting input and output races conforming to different parts of the surface of a single torus defining a mid-plane and in so doing are subjected to traction forces at disc/roller contacts, wherein: the roller assembly is supported at three points only, namely the two contacts of the roller with the discs and a single point of support defined by a supported operating mechanism that applies a control force to the assembly at the point of support, the single point of support permitting the roller centre to freely follow a path dictated by the two disc/roller contacts when the roller assembly is moved by the control force while constraining the application of the control force to a control axis passing through the single point of support and the roller centre the operating mechanism and carriage seek an equilibrium position in which the resultant of the control force and of the traction forces experienced by the roller assembly is zero when resolved in the same direction in a plane normal to the axis of the discs the roller is free to rotate about the control axis.
According to still another aspect the invention provides a CVT of the toroidal race, rolling traction type in which a roller assembly comprises a carriage, bearings mounted on the carriage and a roller defining a roller centre supported by the bearings, for rotation about a roller axis, in which the roller contacts and transmits traction force between coaxial rotatable discs, presenting input and output races conforming to different parts of the surface of a single torus defining a mid-plane and in so doing are subjected to traction forces at disc/roller contacts wherein: a supported operating mechanism has means for applying a control force to a roller assembly - 4c -always along a control axis, passing through a single point of constraint defined by the mechanism and the roller centre, means at the constraint point allowing the roller centre freedom to follow a path dictated by the two disc/roller contacts when the roller assembly is moved by the control force, the roller being located by only three points, namely the constraint point and the two disc/roller contacts the operating mechanism and carriage seek an equilibrium position in which the resultant of the control force and of the traction forces experienced by the roller assembly is zero when resolved in the same direction in a plane normal to the axis of the discsl the roller is free to rotate about the control axis.
The present invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in whichs-Figure 1 shows a roller, its carriage and operating mechanism, partly in elevation and partly in a section taken in a direction generally at right angles to the axis of the discs Figure 2 shows a piston of another operating mechanism Figure 3 shows a hydraulic circuit for use in connection with the operating mechanism of Figure 2, and Figure 4 shows part of an alternative hydraulic circuit.
In Figure 1, a roller 60 transmits traction between the part-toroidal race 85 of an input disc 61 and the corresponding part-toroidal race 87 of an output disc 62, and is mounted in a carriage 67 fixed rigidly at one and to a piston 71, the centre 82 of which is constrained to follow the axis 84 of the cylinder 150 in which it moves. It will be appreciated that while races 85 and 87 are of part-toroidal shape, as just stated, the surfaces indicated by references 85 and 87 in Figure 1 are straight because that part of the Figure is a section taken in the diametral plane of the roller 60.
According to one aspect of the present invention the opposite and of carriage 67 is formed as a spherical face 151 which in use abuts the flat working face 152 of a separate piston 153 moveable within a cylinder 154.

2QQ87'?0 Carriage 67 and piston 153 are thus not only separate, but also without any interlock in use, and are therefore to be contrasted wi th des i gns where there i s , say, a bal 1-i n-socket joi nt between the carriage and the piston. The construction of Figure 1 of the 05 present application has the advantage that roller 60 can be put in place between discs 61 and 62 by the following succession of simple steps 1. With fluid power disconnected, depress piston 153 within cylinder 154;
2. Insert piston 71 into cylinder 150) and introduce roller 60 between discs 61 and 62 until the crown of face 151 is substantially aligned with the axis of piston 153;
3. Release piston 153. Then, when the CVT starts to operate, and input disc 61 rotates and pistons 71 and 153 are exposed to fluid pressure) roller 60 will take up the correct orientation and working face 152 will abut spherical face 151 to exert the restoring force which single-acting piston 71 itself cannot exert.
According to another aspect of the present invention) the inner ends of cylinders 150, 154 are not closed, but the full bores of these cylinders are in communication with part-annular galleries 160, 161 formed within the casing 63 of the CVT.
Galleries 160) 161 are each in communication with a fluid source comprising twin pumps 175 and 176, are associated with control valve means 43) and also communicate with return line s 162 and 163 as customary. By using valve means 43 to vary the fluid pressure within gallery 160, piston 71 imparts tangential shift to carr i age 67 wh i ch wi 11 , as i s we l 1 known i n the art ) have the effect of causing the carriage and its roller 60 to tilt about axis 84, so changing the transmitted ratio. If carriage 67 makes a sudden axial movement in use due to some emergency such as a crash stop or a change in road surface from normal to icy for example, the large cross-section of the access between the galleries 160, 161 and the cylinders facilitates a speedy entry of fluid into cylinder 150 and exit of fluid from cylinder 154) ~a~~~~o or vice versa. Such a rapid entry or exit of fluid could be impossible, and give rise to undesirable back pressures, if the cylinders were blind-ended) and connected to the pressure fluid circuit by way only of the normal small-bore inlet and outlet 05 ports.
In practice there will be not just one roller 60, but typically a set of three such rollers transmitting traction between discs 61 and 62, the rollers being spaced at equal intervals around the common axis of the discs. The same galleries 160) 161 may conveniently communicate with the corresponding pistons 71) 153 of all rollers in the set. Where there are two such sets of three rollers used together in a double-ended CVT having two inwardly-facing input discs and a single double-faced output disc between them, as shown for instance in Figure 1 of patent specification US-A-4297918) galleries 160) 161 may communicate with the pistons of one set and galleries 160a and 161a also formed within casing 63 may communicate with the pistons of the other set. Galleries 160, 160a are connected by a conduit 177 so that the pressures existing within them are equal) galleries 161) 161a are connected to like effect by a conduit 178) and in the example of the invention shown in Figure 1 the control valve means 43 are located on the downstream side of galleries 160a) 161a and therefore communicate with the galleries 160, 161 of the first set of pistons by way of conduits 177, 178.
The galleries 160, 160a) 161 and 161a are preferably each only part-annular in shape, the annuli being coaxial with the discs 61 and 62, so as to leave a sector of the casing 63 that is free of the galleries and that can therefore be formed with apertures allowing access to the casing interior.
Piston 71, moving within cylinder 150 in Figure 2 of the drawings) is modified by an extension 165, the end face 166 of which meets the side wall 167 at a smoothly curved edge 168. The edge of return line 162) where it enters gallery 160) is formed with an angled face at 170. If in use of the CVT an ~f~Q~'~'~0 _ 7 _ overload/abuse condition develops and is reflected in an extreme axial movement (leftwards) as in Figure 2> of piston 71) surfaces 168 and 170 will approach, so imposing an extra resistance on the normal flow of fluid out of gallery 160 into return line 162.
05 Pressure in galleries 160 and 160a will therefore rise) and since the pressure in those galleries operates on all the other pistons 71, all those other pistons will be subjected to an enhanced force which opposes them as they approach the corresponding extremes of their ranges of axial movement. A "hydraulic end stop" effect is therefore set up. Figure 3 diagrammatically shows one possible practical embodiment in which the fluid source again comprises twin pumps 175, 176 connected to the galleries 160) 160a, 161) 161a feeding the operating cylinders 150) 154 of the two sets of cylinders (20, 25> of a double-ended CVT of the toroidal-race) rolling-traction type. The return lines 162, 163 of the hydraulic circuit connect with the cylinders of only one roller (60a)) which acts as the "master" for all the other rollers. Piston 71 of the carriage 67 of this roller carries an extension 165) which exercises an "end stop" action as already described) when piston 71 tends to overshoot within cylinder 150) and piston 153 of that same roller carries a seal 180 which exercises a similar end stop effect) by approaching cylinder end wal 1 181 and obstructi ng outl et port 182 ) when that pi ston tends to overshoot within cylinder 154. It should also be noted that the end load cylinder 183, which generates the end load force urging discs 61, 62 into contact with rollers 60) is connected to a part of the hydraulic circuit which is close to the galleries and to the roller operating mechanisms, with no substantial resistance intervening, so that equal pressures exist in the galleries and cylinder 183 at all times.
Generating the end stop effect by means of the principal piston (71> of the roller operating mechanism may require accurate machining of the edge 168 (Figure 2>, which must conform to part of the surface of a sphere with centre 82. Figure 4 2~~~~~~
_8_ shows part of an alternative design in which the simpler "second piston" 153 of one roller (60b) carries a ring seal 180 and generates the end stop effect in the left-hand side of the hydraulic circuit when that piston tends to overshoot. The 05 adjacent roller 60c is arranged the other way round so that its "second" piston 153 is on the right-hand side of the circuit (corresponding to galleries 161, 161a>, and generates the end stop effect on that side of the circuit by means of a similar ring seal 180 when it overshoots.

Claims (35)

1. A CVT of the toroidal race, rolling traction type in which a roller assembly comprises a carriage, bearings mounted on the carriage and a roller defining a roller centre and supported by the bearings, for rotation about a roller axis, in which the roller contacts and transmits traction forces between coaxial rotatable discs, presenting input and output races conforming to different parts of the surface of a single torus and is subjected to traction forces at disc/roller contacts, and a double-acting operating mechanism having a fixed part and a moveable part, the moveable part being reciprocal over a predetermined stroke of operating movement and operable to apply a control force to the carriage, and in which the operating mechanism and carriage seek an equilibrium position in which the resultant of the control force and of the traction forces experienced by the roller assembly is zero, wherein:
the single torus defines a mid-plane;
the carriage includes a rigid structure relative to which the roller axis and the roller centre are fixed:
the operating mechanism includes constraint means defining a single point of constraint which constrains the operating mechanism to the application of the control force along a force application axis passing through the constraint point and the roller centre;
the constraint point and the disc/roller contacts form a constant triangular relationship;
the equilibrium position of the roller assembly is determined by only the two disc/roller contacts and the position of the constraint point; and the roller is free to rotate about the force application axis.

2. A CVT according to claim 1 wherein the operating mechanism allows the roller centre freedom to move through an arc about the constraint point.

3. A CVT according to claim 2 wherein the operating mechanism allows the roller centre freedom to move through two intersecting arcs, about the constraint point, which lie in different and intersecting planes.

4. A CVT according to claim 3 wherein the constraint point defines a centre of contact between the movable part and the fixed part.

5. A CVT according to claim 1 comprising a casing, the fixed part being mounted on the casing.

6. A CVT according to claim 1 wherein the rigid structure of the carriage extends from the bearings, in which the roller rotates, to the constraint point.

7. A CVT according to claim 3, wherein the constraint point is defined by a ball joint permitting mutual rotation about orthogonal axes.

8. A CVT according to claim 1 wherein the operating mechanism includes a piston moveable within a cylinder, the piston forming part of the movable part and the cylinder forming part of the fixed part.

9. A CVT according to claim 8 wherein the piston and cylinder combination defines the constraint point, is single-acting and is capable of exerting a force in a first direction along the force application axis through the constraint point, and the operating mechanism further includes a second piston and cylinder combination capable of exerting a force in a second and substantially opposite direction along the force application axis through the constraint point.

10. A CVT according to claim 8 wherein the piston defines the constraint point.

11. A CVT according to claim 7 wherein the operating mechanism includes a piston moveable within a cylinder, the piston forming part of the movable part and the cylinder forming part of the fixed part, and the piston is rotatably mounted in said cylinder, the rotation between the piston and the cylinder takes place about the cylinder axis only, and the constraint point is defined by the ball joint permitting rotation about the orthogonal axes.

12. A CVT according to claim 7 wherein the operating mechanism includes a piston movable within a cylinder, the piston forming part of the movable part and the cylinder forming part of the fixed part, the piston having a part spherical outer surface in sliding contact with the cylinder such that the piston forms the ball joint and is rotatably mounted in said cylinder, the rotation between the piston and the cylinder takes place about the cylinder axis and about the orthogonal axes, thereby defining the constraint point.

13. A CVT according to claim 1 in which the movable part of the operating mechanism is located to one side only of the plane which includes the CVT axis and the roller centre.

14. A CVT according to claim 1 wherein the force application axis about which the roller rotates as transmitted ratio of the CVT changes, is inclined to the mid-plane of the torus.

15. A CVT according to claim 1 including a hydraulic piston-and-cylinder combination, by which the control force is applied to the carriage, and a port formed in the cylinder by which the cylinder is in communication with a hydraulic circuit, wherein the cross-section of the port is substantially coincident with and equal to that of the bore of the cylinder itself.

16. A CVT according to claim 15 defining a main CVT axis, and comprising a CVT casing, wherein the hydraulic circuit includes a passage of ring-like shape formed within the CVT casing, the ring-like shape being coaxial With the main CVT axis.

17. A CVT according to claim 9 wherein the second piston and the carriage are separate items which abut each other in use in a non-interlocking manner.

18. A CVT according to claim 1 wherein the operating mechanism includes a hydraulic operating circuit and at least one piston-and-cylinder combination, the piston having a permitted stroke, and the cylinder is in communication with that circuit via an outlet port, and the communication between cylinder and circuit is such that approach of the piston towards an and of its permitted stroke obstructs the outlet port causing fluid pressure to build up in the cylinder thereby opposing further piston overshoot indicating undesirable conditions of the system.

19. A CVT of the toroidal race, rolling traction type in which a roller assembly comprises a carriage, bearings mounted thereon and a roller defining a roller centre and supported by the bearings for rotation about a roller axis, in which the roller contacts and transmits traction forces between coaxial rotatable discs presenting input and output races conforming to different parts of the surface of a single torus and in so doing are subjected to traction forces at disc/roller contacts, and an operating mechanism having a fixed part and a movable part, the movable part being reciprocal over a stroke of operating movement and operable to apply a control force to the carriage, and in which the operating mechanism and carriage seek an equilibrium position in which the resultant of the control force and of the traction forces experienced by the roller assembly is zero, wherein the single torus defines a mid-plane;
the carriage includes a rigid structure relative to which the roller axis and the roller centre are fixed;
the operating mechanism includes constraint means defining a single point of constraint which constraints the operating mechanism to the application of the control force along a force application axis passing through the constraint point and the roller centres;
the constraint point and the disc/roller contacts form a triangular relationship;
the operating mechanism allows the roller centre freedom to move along two intersecting arcs, about the constraint point, which lie in different and intersecting planes;
the equilibrium position of the roller assembly is determined by only the two disc/roller contacts and the position of the constraint point; and the roller is free to rotate about the force application axis.

20. A CVT according to claim 19 wherein the constraint point defines a centre of contact between the movable part and the fixed part.

21. A CVT according to claim 19 comprising a casing, the fixed part being mounted on the casing.

22. A CVT according to claim 19, wherein the constraint point is defined by a ball joint permitting mutual rotation about two orthogonal axes.

23. A CVT according to claim 22 wherein the operating mechanism includes a piston moveable within a cylinder, the piston forming part of the movable part and the cylinder forming part of the fixed part, and the piston is rotatably mounted in said cylinder, the rotation between the piston and the cylinder takes place about the cylinder axis only, and the constraint point is defined by the ball joint permitting rotation about the orthogonal axes.

24. A CVT according to claim 21 wherein the operating mechanism includes a piston movable within a cylinder, the piston forming part of the movable part and the cylinder forming part of the fixed part, the piston having a part spherical outer surface in sliding contact with the cylinder such that the piston forms the ball joint and is rotatably mounted in said cylinder, the rotation between the piston and the cylinder takes place about the cylinder axis and about orthogonal axes thereby defining the constraint point.

25. A CVT according to claim 19 characterised in that the diameter of the roller, about which the roller rotates as transmitted ratio of the CVT changes, is inclined to the mid-plane of the torus.

26. A CVT according to claim 19 wherein a piston-and-cylinder combination, applies the control force to the carriage, and a port formed in the cylinder by which the cylinder is in communication with a hydraulic circuit, wherein the cross-section of the port coincides with substantially that of the bore of the cylinder itself.

27. A CVT according to claim 26 defining a main CVT axis and comprising a CVT casing, wherein the hydraulic circuit includes a passage of ring-like shape formed within the CVT casing, the ring-like shape being coaxial with the main CVT axis.

28. A CVT according to claim 19 wherein the operating mechanism includes a hydraulic operating circuit and at least one piston-and-cylinder combination, the piston having a permitted stroke, and the cylinder is in communication with that circuit via an outlet port, and the communication between cylinder and circuit is such that approach of the piston towards an end of its permitted stroke obstructs the outlet port causing fluid pressure to build up in the cylinder thereby opposing further piston overshoot and indicating undesirable conditions of the system.

29. A CVT according to claim 19, wherein the operating mechanism comprises a piston movable within a cylinder wherein the cylinder forms part of the fixed part of the operating system and the piston forms part of the moving part.

30. A CVT according to claim 29, wherein the piston/cylinder combination is a double acting piston/cylinder combination and the piston is arranged to one side of the mid-plane of the torus.

31. A CVT according to claim 19, wherein the piston/cylinder combination comprises two opposed single acting piston/cylinders, wherein one of said cylinders forms part of the fixed part of the operating system and the pistons form part of the movable part both acting through the constraint point.

32. A CVT of the toroidal race rolling traction type in which a roller assembly comprises a carriage, bearings mounted on the carriage and a roller defining a roller centre supported by the bearings, for rotation about a roller axis, in which the roller contacts and transmits traction forces between coaxial rotatable discs, presenting input and output races conforming to different parts of the surface of a single torus defining a mid-plane and in so doing are subjected to traction forces at disc/roller contacts, and an operating mechanism having a fixed part and a movable part, the movable part being reciprocal over a stroke of operating movement and operable to apply a predetermined control force to a location on the carriage, with the control force acting solely along an axis including both the location on the carriage and the roller centre and in which the operating mechanism and carriage seek an equilibrium position in which the resultant of the control force and of the traction forces experienced by the roller assembly is zero when resolved in the same direction in a plane normal to the axis of the discs, wherein:
the carriage includes a rigid structure relative to which the roller axis and the roller centre are fixed;
the operating mechanism includes means capable of affecting translational movement of the roller centre back and forth along a path dictated by the two disc/roller contacts means allowing the roller freedom to rotate about a diameter thereof so as to change the transmitted ratio of the CVT;
the axis and the roller centre are fixed relative to the carriage;
the roller centre is positioned to impose no loads on the carriage nor on the operating mechanism other than to produce the balance between the control force and the traction forces experienced by the roller assembly, in the plane normal to the axis of the discs; and the two disc/roller contacts and the location at which the control force is applied to the roller assembly define a triangular relationship establishing a triangle of forces acting in the same plane.

33. A CVT according to claim 32 wherein said no loads are achieved by allowing the roller centre to move through intersecting arcs Which lie in different intersecting planes.

34. A CVT of the toroidal race, rolling traction type in which a roller assembly comprises a carriage, bearings mounted on the carriage and a roller defining a roller centre supported by the bearings, for rotation about a roller axis, in which the roller contacts and transmits traction forces between coaxial rotatable discs, presenting input and output races conforming to different parts of the surface of a single torus defining a mid-plane and in so doing are subjected to traction forces at disc/roller contacts, wherein:
the roller assembly is supported at three points only, namely the two contacts of the roller With the discs and a single point of support defined by a supported operating mechanism that applies a control force to the assembly at the point of support, the single point of support permitting the roller centre to freely follow a path dictated by the two disc/roller contacts when the roller assembly is moved by the control force while constraining the application of the control force to a control axis passing through the single point of support and the roller centre;
the operating mechanism and carriage seek an equilibrium position in which the resultant of the control force and of the traction forces experienced by the roller assembly is zero when resolved in the same direction in a plane normal to the axis of the discs;
the roller is free to rotate about the control axis.

35. A CVT of the toroidal race, rolling traction type in which a roller assembly comprises a carriage, bearings mounted on the carriage and a roller defining a roller centre supported by the bearings, for rotation about a roller axis, in which the roller contacts and transmits traction force between coaxial rotatable discs, presenting input and output races conforming to different parts of the surface of a single torus defining a mid-plane and in so doing are subjected to traction forces at disc/roller contacts wherein:
a supported operating mechanism has means for applying a control force to a roller assembly always along a control axis, passing through a single point of constraint defined by the mechanism and the roller centre, means at the constraint point allowing the roller centre freedom to follow a path dictated by the two disc/roller contacts when the roller assembly is moved by the control force, the roller being located by only three points, namely the constraint point and the two disc/roller contacts;
the operating mechanism and carriage seek an equilibrium position in which the resultant of the control force and of the traction forces experienced by the roller assembly is zero when resolved in the same direction in a plane normal to the axis of the discs;
the roller is free to rotate about the control axis.