Abstract: A work vehicle includes an engine; at least one continuously variable power source (CVP) configured to operate according to a commanded torque; an output shaft; a transmission positioned operatively between the output shaft and the engine and the CVP such that the output shaft selectively receives power from one or both of the engine and the at least one CVP; and a prognostics system configured to monitor a respective component associated with at least one of the output shaft or the transmission. The prognostics system includes a prognostics controller having a processor and memory architecture, configured to: receive input data, including the commanded torque; generate a usage value for the respective component for a time period as a function of the commanded torque; aggregate the usage value with previous usage values to generate a prognostics value over a life of the respective component; and store the prognostics value.

Abstract: A system and method for determining surface contact fatigue life may use a finite element method to determine when components, such as a power transmission component, may fail in operation. The method may generate a finite element model based on the material parameters related to a power transmission component, generate a surface pressure time history for a loading event based on one or more loading parameters, determine, based on the surface pressure time history for a loading event, a finite element solution that describes stress in the grain structure, calculate damage in the finite element solution using a damage model, and determine whether a damage threshold is exceeded.

Abstract: Inventive embodiments are directed to components, subassemblies, systems, and/or methods for infinitely variable transmissions (IVT). In one embodiment, a control system is adapted to facilitate a change in operating mode of an IVT. In another embodiment, a control system includes a drive clutch coupled to a source of rotational power; the drive clutch is configured to selectively engage a traction ring and a carrier of the IVT. The control system includes a one-way clutch assembly configured to selectively engage the traction ring and the carrier. In some embodiments, the control system governs the actuation of the one-way clutch to selectively lock and unlock components of the IVT. In some embodiments, the control system implements an IVT mode wherein the carrier selectively couples to a source of rotational power. In other embodiments, the control system implements a CVT mode wherein the traction ring selectively couples to a source of rotational power.

Abstract: In a continuously variable transmission including: a first transmission unit including a first input disc and a first output disc disposed coaxially and a power roller tiltably interposed between these discs; and a second transmission unit disposed coaxially with the first transmission unit, including a second input disc and a second output disc disposed coaxially and a second power roller tiltably interposed between these discs, and disposed such that a back surface of the second input disc is opposed to a back surface of the first input disc, a tubular back surface cylinder wall is provided on each of the back surfaces of the first input disc and the second input disc so as to project therefrom.

Abstract: A traction system for vehicles comprises a first power source of the non-reversible type, a second power source of the reversible type and a transmission which is connected to the power sources and which includes a first differential device. The transmission further comprises a speed variator which is interposed between the first source and the first differential device and which comprises a continuous speed variation device, and a second differential device. The continuous speed variation device is of the toroidal friction wheel type and comprises a lateral motion input disc, a lateral motion output disc and at least two idle oscillating friction roller members. The lateral motion input and output discs have a friction surface of toroidal shape and the idle oscillating roller members have a friction surface shaped in the form of a spherical dome.

Abstract: A method where the spin factor is looked up in a table; the slip factor is measured and the clamping pressure is adjusted to achieve a slip/spin ratio provided in a desired range is described herein. According to another aspect, an active mechanical clamping mechanism using a radially movable contact point is also described.

Abstract: A system and method for controlling the endload force of a variator includes reducing a net force applied to a variator input disc of the variator. The variator includes an endload chamber, a subtraction chamber, and a valve fluidly coupled between the endload chamber and the subtraction chamber. Each of the endload chamber and the subtraction chamber applies an opposing force to the variator input disc. The valve is activateable to reduce the net force applied to the variator input disc. The valve may be activated based on the variator ratio, the position of the variator roller, and/or other parameters of the variator or operation thereof.

Abstract: The present invention relates to control system for a vehicle. The control system includes a manually operable control lever, such as a joystick, an actuator, a sensor and a control unit. The control lever sets a state variable of the vehicle. The actuator applies a force to the control lever. The sensor senses a vehicle parameter and transmits a parameter signal to the control unit. The control unit determines a current operating state of the vehicle. The control unit, depending on the present operating state of the vehicle, controls the actuator and causes it to apply a changed, predetermined force to the control lever, in order to make the operator aware of an unsafe operating state.

Abstract: A variator transmission comprises an input shaft (18), an input disc (10) mounted on the input shaft for rotation therewith and an output disc (12) facing the input disc and arranged to rotate coaxially therewith, the input and output discs defining between them a toroidal cavity. Two rollers (14, 16) are located in the toroidal cavity and first and second roller carriage means are provided upon which the first and second rollers respectively are rotatably mounted and end load means (34, 36) urge the rollers into contact with the input and output discs to transmit drive. The two roller carriage means are mounted on opposite sides of the pivotal axis of a lever (50) and the pivotal axis of the lever (50) is movable in both the radial and non-radial directions with respect to the rotational axis of the input and output discs.

Abstract: Components, subassemblies, systems, and/or methods for continuously variable transmissions (CVT) having a control system adapted to facilitate a change in the ratio of a CVT. In one embodiment, a control system includes a stator plate configured to have a plurality of radially offset slots. Various traction planet assemblies and stator plates can be used to facilitate shifting the ratio of a CVT. In some embodiments, the traction planet assemblies include planet axles configured to cooperate with the stator plate. In one embodiment, the stator plate is configured to rotate and apply a skew condition to each of the planet axles. In some embodiments, a stator driver is operably coupled to the stator plate. Embodiments of a traction sun are adapted to cooperate with other components of the CVT to support operation and/or functionality of the CVT.

Abstract: A full-toroidal continuously variable transmission has a hydraulic actuator (20) for applying pressing force to a pair of disks (5, 15) through a roller (17). The hydraulic actuator (20) includes first and second oil chambers (23, 24). The pressing force of the hydraulic actuator (20) is produced by a pressure difference between the oil chambers (23, 24). A directional control valve (29; 29P; 29Q) connects a hydraulic pressure source (25; 25P; 25Q) to either of the oil chambers, and an oil tank (26) is connected to the other oil chamber. Each corresponding an intermediate section (27a; 27Pa, 27Qa) of a feeding path (27, 27P, 27Q) and intermediate section (28a; 28Pa; 28Qa) of a discharge path (28; 28P; 28Q) are connected by a communicational path (32; 32P; 32Q). A check valve (33; 33P; 33Q) for permitting a flow of operation oil only to the feeding path side (27, 27P, 27Q) is provided in the communication path (32; 32P; 32Q).

Abstract: A continuously variable ratio transmission unit having a housing (116) and input (102, 106) and output (100) discs paired to define first and second toroidal cavities. The discs (100, 102, 106) are mounted to the housing for rotation about a common axis. First (110) and second (112) rollers—or more typically first and second sets of rollers—are respectively arranged in the first and second cavities and serve to transmit drive between the input and output discs. First (130) and second (148, 150, 152) actuators act on the respective rollers. The first actuator is coupled to a first carrier part (126) which can be mated with the housing introducing it to the housing along a direction generally parallel to the variator axis. The second actuator is coupled to a second carrier part (146) which can be mated with the housing by introducing the second carrier part to the housing along a direction non-parallel to the variator axis.

Abstract: A controller determines a target gear ratio from an accelerator opening and a vehicle speed. The controller determines the stroke of a trunnion on the basis of the difference between present inclination and a target inclination, thereby to control stroke by a hydraulic control valve. The stroke is corrected according to input/output disc pushing force.

Abstract: An adjusting device including a CVT planetary roller transmission having two axially-spaced sun wheels that are rotatable about a common axis of rotation and at different speeds of rotation. Planet wheels contact and ride against peripheral surfaces of each of the sun wheels, and an axially displaceable ring wheel lies radially outwardly of the sun wheels and engages a peripheral groove formed in the sun wheels. When the ring wheel is axially displaced relative to the sun wheels the axes of rotation of the planet wheels tilt relative to the sun wheel axis resulting in a difference in the rotational speed of the sun wheels changes. The device can be employed as an adjusting device for adjusting the rotational speed of connected units, and also as a drive line component having a variable transmission ratio and situated in a power train of a motor vehicle.

Abstract: A hydraulic control circuit is disclosed for a continuously variable transmission comprising a continuously variable ratio transmission unit (“variator”) 10, the circuit comprising a supply line 106 and means (which may take the form of a pump 110) for providing a flow of pressurised fluid in the supply line, means (which may comprise a pressure control valve 116) for generating a back pressure in the supply line, and at least one connection for feeding fluid from the supply line to a hydraulic actuator 100 acting on a movable torque transmission 23-37 element (which may comprise a roller 28) of the variator 10. The valve means 152 connected to the supply line allows pressure in the supply line to be selectively modified in response to rate of flow in the supply line.

Abstract: A continuously variable planetary gear set is described having a generally tubular idler, a plurality of balls distributed radially about the idler, each ball having a tiltable axis about which it rotates, a rotatable input disc positioned adjacent to the balls and in contact with each of the balls, a rotatable output disc positioned adjacent to the balls opposite the input disc and in contact with each of the balls such that each of the balls makes three-point contact with the input disc, the output disc and the idler, and a rotatable cage adapted to maintain the axial and radial position of each of the balls, wherein the axes of the balls are oriented by the axial position of the idler.

Abstract: A continuously variable transmission is disclosed for use in rotationally or linearly powered machines and vehicles. The transmission provides a simple manual shifting method for the user. Further, the practical commercialization of traction roller transmissions requires improvements in the reliability, ease of shifting, function and simplicity of the transmission. The present invention includes a continuously variable transmission that may be employed in connection with any type of machine that is in need of a transmission. For example, the transmission may be used in (i) a motorized vehicle such as an automobile, motorcycle, or watercraft, (ii) a non-motorized vehicle such as a bicycle, tricycle, scooter, exercise equipment or (iii) industrial equipment, such as a drill press, power generating equipment, or textile mill.

Abstract: A toroidal-type continuously variable transmission has: an input shaft; an input side disk and an output side disk respectively disposed on the outer periphery of the input shaft, the input side disk including a first cam surface; and a loading cam device interposed between the input side and output side disks and comprising: a loading cam including a second cam surface and rotatable together with the input shaft; a plurality of rollers interposed rollably between the first cam surface and the second cam surface; and a retainer, the retainer including a retainer main body having a circular-shaped fitting hole, and inside-diameter side projecting portions disposed on a retainer main body for maintaining the attitude of the retainer main body, wherein inside diameters of portions of the fitting hole corresponding to the inside-diameter side projecting portions are larger than that of the remaining portions of the fitting hole.

Abstract: In a toroidal CVT includes a power roller interposed between input and output disks under a preload, a trunnion supports the power roller to permit a tilting motion of the power roller about a trunnion axis perpendicular to a power-roller rotation axis and to permit a parallel translation of the power roller relative to the trunnion in the direction of the common rotation axis of the input and output disks. The power roller includes an inner race in contact with the input and output disks, an outer race supported by the trunnion, and a power-roller bearing permitting relative rotation of the inner race to the outer race. The outer race has a low rigidity structure that promotes a deformation of the outer race so that the outer-race deformation follows the inner-race deformation, which may occur due to the preload during operation of the toroidal CVT.

Abstract: The second platenary gear mechanism 18 is a so-called double pinion type planetary gear comprising a sun gear 18a, paired pinions 18b, a carrier 18c which supports the pinions, and a ring gear 18d. The pinions 18b of eeach pair meshes with each other in a state where one of the paired pinions 18b meshes with the sun gear 18a while the other one of the paired pinions 18b meshes with the ring gear 18d. The carrier 18c is coupled to the countershaft 20 and the sun gear 18a is connected to a shaft 11a which is coupled to a gear 11b of a third gear unit 11.

Abstract: A toroidal type continuously variable transmission, in which an input disc and an output disc are arranged on the common axis with being opposed to each other in the rotatable condition, and in which the rolling member is arranged between the input disc and the output disc, so as to transmit a torque between said input disc and said output disc through said rolling member, by applying the clamping force to said input disc and said output disc in said axial direction, characterized by comprising a clamping force controller for controlling the clamping force to be applied to said input disc and said output disc based on a physical quantity relevant to heat stress of said rolling member.

Abstract: A double-cavity half-toroidal type continuously variable transmission comprises an input shaft rotatable by means of a drive source, first and second input disks rotatable integrally with the input shaft, first and second output disks opposed to corresponding input disks, and a hydraulic loading mechanism. The hydraulic loading mechanism includes first and second hydraulic chambers arranged in the axial direction of the input shaft. The first hydraulic chamber is defined by the back surface of the first input disk, an end face of the first disk member, etc. The second hydraulic chamber is defined by the inner end face of the first cylinder, an end face of the second disk member, etc. The first and second hydraulic chambers are simultaneously supplied with pressurized oil by means of a pressure source. Under the pressure of the oil, the first input disk is pressed toward the first output disk, and the second input disk is pressed toward the second output disk through the medium of an interlocking portion.

Abstract: A toroidal type continuously variable transmission; in which a rolling member for mediating the transmission of a torque is clamped between at least a pair of rotary members; and in which a clamping force for clamping the rolling member in the direction not to let the rolling member be released from between the rotary members is applied to the rolling member, comprises: a detector for detecting temperature rise of either, a temperature of a rolling face of the rotary members for clamping the rolling member, or a temperature of the rolling member; and a rolling member controller for reducing the clamping force in case a temperature rise is detected by said detector.

Abstract: A toroidal type continuously variable transmission includes a pair of discs (5, 10) in opposed relation, and a rotary element (17) interposed in a toroidal clearance (S) defined between these discs (5, 10). Torque is transmitted between the pair of discs (5, 10) via the rotary element (17). Sensing the velocity of the rotary element (17) permits the determination of, for example, how much the rotary element (17) slips on the discs (5, 10). This assists the prevention of heat-up of the rotary element (17) and also provides an optimal traction between the rotary element (17) and the discs (5, 10).

Abstract: Enhanced ratio control in a toroidal drive of a T-CVT is provided. A factor of proportionality is computed by which a trunnion axial displacement and a T-CVT ratio rate are related. A filter in the form of a characteristic equation is established. This filter includes a physical quantity indicative of the T-CVT ratio and a physical quantity indicated by an actuator command, as inputs, a quasi-state quantity, as a state quantity, and a transition coefficient for the quasi-state quantity. The transition coefficient includes an observer gain. The quasi-state quantity is computed using the filter. An estimated quantity of a system state quantity of the T-CVT is computed using the quasi-state quantity, the observer gain, and a trunnion angular position. The system state quantity includes at least the first physical quantity. The observer gain is corrected in response to the factor of proportionality to keep the transition coefficient unaltered.

Abstract: The invention relates to a stepless friction drive (1) in which friction wheels (2) are mounted. Said friction drive is provided with a system that allows a compensatory displacement of the corresponding friction wheel (2) when the input and output wheels are elastically deformed. Said system is mounted between the lower end of every support (1) and the hydraulic piston (6). The compensation system comprises an upper bearing shell (3), a lower bearing shell (5) and balls (4) that are maintained in a defined arrangement by means of a cage (9).

Abstract: A toroidal type continuously variable transmission having first and second discs supported around a rotating shaft and receiving power rollers therebetween includes a ball spline having a first spline groove formed in an outer circumferential surface of the rotating shaft, a second spline groove formed in an inner circumferential surface of the first disc, and balls provided between the first spline groove and the second spline groove rollably. An axial position of an end portion of an effective groove portion of the first spline groove is located to correspond to an axial position of an inner end portion of the second spline groove or more closely to the second disc than the axial position thereof when a pressing unit, a preload spring and the first disc are installed around the rotating shaft, pressure oil is not fed to the pressing unit, and the preload spring is not elastically deformed.

Abstract: A loading cam apparatus for a toroidal type continuously variable transmission includes: a first cam surface formed to have concave and convex portions thereon along a circumferential direction thereo, a second cam surface formed to have concave and convex portions thereon along a circumferential direction thereof and to oppose to the first cam surface along an axial direction thereof, a plurality of rolling elements sandwiched between the first cam surface and the second cam surface, and a retainer for holding the plurality of rolling elements in a freely rotatable state. The retainer includes a retainer main body of a circular annular shape, and pockets provided at outer periphery portions of the retainer main body for holding the plurality of rolling elements, respectively. The retainer main body is formed by carbonitriding material of iron system with a carbon concentration in a range from 0.02 to 0.20 wt % both inclusive.

Abstract: The present toroidal-type continuously variable transmission comprises an oil passage formed in an input shaft so as to extend in the axial direction of the input shaft, a plurality of ball spline grooves formed in the outer peripheral surface of the input shaft so as to be spaced from one another at a given interval in the peripheral direction of the input shaft, a plurality of oil grooves not only for connecting together the ball spline grooves but also for guiding lubricating oil along the outer peripheral surface of the input shaft, and two oil holes 1a formed in the input shaft 1 for allowing the oil grooves and oil passage to communicate with each other.

Abstract: A loading cam mechanism of a toroidal type continuously variable transmission is provided with a cam disc that rotates together with an input shaft. A cam surface is formed on the cam disc. The cam disc includes a disc portion having the cam surface formed thereon and a cylindrical portion protruding from the central portion of the cam surface. The cam disc further includes a lathe-turned portion formed by lathe turning, a milled surface formed by means of an end mill, and a ground surface formed by means of a grindstone. Formed on a corner portion between the cylindrical portion and the disc portion are a first corner curved surface worked by means of an arcuate portion on the distal end portion of the end mill and a second corner curved surface worked by means of an arcuate portion on the distal end portion of the grindstone.

Abstract: In a state where an input shaft 15a, an input side disk 2A and a loading cam device 9 are assembled together, thrust to be generated by the loading cam device 9 is measured. In case where this thrust is within a specified range, the other component parts of a toroidal-type continuously variable transmission are combined with the above assembled parts to thereby produce the toroidal-type continuously variable transmission. Accordingly it is possible to reduce the manufacturing cost of a toroidal-type continuously variable transmission which is excellent in efficiency and durability.

Abstract: A drive mechanism for an infinitely-variable-transmission in which a relatively flexible member 32 connects one or other of the discs 14 to the shaft 10, thereby to accommodate a degree of shaft flexing without distortion of the toroidal cavity.

Abstract: The rotation output of a continuously variable transmission (2) which transmits the rotation of an input shaft 1 at an arbitrary speed ratio is input to a sun gear (5A) of a planetary gear set (5) arranged on a final output shaft (6). A ring gear (5C) of the planetary gear set (5) is joined to the final output shaft (6). An output gear (3B) of a fixed speed ratio transmission (3) which transmits the rotation of the input shaft (1) at a fixed speed ratio is arranged on the final output shaft (6) and joined to a planet carrier (5B).

Abstract: A toroidal continuously variable transmission is provided with a power roller 8 which is gripped between input/output disks 5, 6 arranged on the same axis, a cam flange 2 which is disposed on the same axis as the input disk 5 and is connected to an input shaft 1, and cam rollers 3 which are gripped between the cam flange 2 and the rear face of the input disk 5. Bearings 41, 44, 45 are interposed in the contact position on the long radius end face 3b of the cam roller 3 and the inner circumference of the retainer 4. These bearings allow the reduction of frictional resistance on the contact point of the cam roller 3 and the retainer 4 and improve the torque transmission capacity and component lifespan of the toroidal continuously variable transmission.

Abstract: A power roller (18c) comprises a power roller inner wheel (93) in frictional contact with a input/output disks (18a, 18b), a power roller outer wheel (94) provided in a trunnion (17a), a ball bearing (92) interposed between the power roller inner wheel (93) and the power roller outer wheel (94), and an inner wheel supporting shaft (95) which supports the power roller inner wheel (93) such that it is free to rotate, the shaft having a shaft base part (95b) supported in the trunnion (17a), and arranged so that the radial support rigidity is set high in a gyration axis direction and set low in the direction perpendicular to the trunnion gyration axis (19a) and the power roller rotation axis (15d). When one of the disks is pressed and displaced in the direction of the other disk by the press member, the power roller inner wheel (93) displaces with the ball bearing (92) by the displacement of the one of the disks.

Abstract: A toroidal continuous variable transmission is disclosed in which no power is transmitted to toroidal speed changers during the engagement of a direct clutch. The toroidal continuous variable transmission is constructed such that the engagement of the direct clutch causes an input shaft to connect drivingly with an output shaft through a planetary gear system whereby the power may be transmitted from the input shaft to the output shaft. The release of the forward clutch makes reverse means freewheeling so that no load is applied to output disks. As a result, none of the input disks are subjected to the thrust force at the loading cam, regardless of the rotation of the input shaft. Therefore, although the toroidal speed changers may make only the idling in unison with the input shaft, none of the input disks are forcibly urged against the power rollers.

Abstract: A control system for a toroidal continuously variable transmission which can control the transmission ratio without installing a position detection device or an angle detection device on trunnions and secure a sufficient control response to prevent hunting of the transmission ratio. To realize this control system, an axial force calculation device M3 calculates the axial force 2Ft in the trunnion axis direction that the transmission ratio control actuators are required to generate, based on the transmission ratio i calculated by a transmission ratio calculation device M1 from the input disk revolution speed .omega..sub.1 and the output disk revolution speed .omega..sub.2, and on the input torque Ti or output torque To estimated by a torque estimation device M2. An axial force change calculation device M4 calculates the amount of change in the axial force .DELTA.Ft based on the deviation between the target transmission ratio ic and the transmission ratio i.

Abstract: A variator assembly comprises a variator of the toroidal-race rolling-traction type using double-acting pistons to control the rollers and differential end load pressure to load the variator, and control means operative to vary the differential end load pressure in sympathy with changes in the net pressure exerted by the roller-control pistons so as to maintain the traction coefficient at the roller disc interface consistently below the limit at which traction would be lost.

Abstract: A toroidal continuously variable transmission comprises a main shaft having a step section. An input disc is drivably supported on the main shaft and axially movable along the axis of the main shaft. An output disc is rotatably supported on the main shaft and located facing the input disc. A power roller is disposed between and in frictional engagement with the input disc and the output disc so as to make transmission of power from the input disc to the output disc. Belleville springs (each having a center opening) are disposed around the main shaft and located at an opposite side of the input disc with respect to the output disc so as to press the input disc toward the output disc. A loading nut is screwed on the main shaft to restrict an expansion amount of the Belleville springs. The loading nut axially faces the step section of the main shaft. A sleeve is disposed between the loading nut and the step section of the main shaft. The sleeve is separable from the loading nut.

Abstract: The same rotation torque is input to a continuously variable transmission of a vehicle which continuously varies a speed change ratio and a reduction gear unit. A planetary gear mechanism is provided comprising an output shaft which varies a rotation direction according to a relation between the output speed of the continuously variable transmission and the output speed of the reduction gear unit. The continuously variable transmission varies the speed change ratio according to an oil pressure balance between two oil chambers. A direction change-over valve selectively supplies a first oil pressure and a second oil pressure to these oil chambers, and a control valve controls a differential pressure of the first oil pressure and second oil pressure.

Abstract: A power input flange is fixed to the tip of a power input shaft whose diameter increases in steps from a tip thereof along a main body thereof and which is supported within a casing, and a power output disk is supported upon the outer peripheral surface of the power input shaft. A power input disk is supported upon the outer peripheral surface of the power input shaft between the power input flange and the power output disk, and a power roller is provided which is contacted at a variable inclination with both the power input disk and the power output disk. A member which biases the power input disk in the direction of the power output disk is supported upon the power input flange. And the power input flange is supported via a radial bearing upon the outer peripheral surface of a tubular shaped support portion which projects inwards from the casing. By this, the diameter of the support portion is increased as compared with the case in which the power input shaft is directly supported by the casing.

Abstract: A half-toroidal-type continuously variable transmission capable of transmitting great power has two sets of discs, which each include an input side disc and an output side disc, and two sets of three power rollers, one between each of the sets of discs, thus providing a total of six power rollers. Trunnions supporting the power rollers are supported by support posts. Loads applied to the support posts are transmitted to fixing rods and connecting rods and are offset in those rods.

Abstract: A compact structure is provided for an continuously variable transmission, which provides a more direct application of both load and control forces at the pivot trunnion. Load forces are produced from within the pivot trunnion, providing more direct control over load forces and their effect on contact forces between traction rollers and toric discs. Control forces which establish the desired transmission ratio are applied to the pivot trunnion in a manner which substantially eliminates twisting and squirming of the support rollers. High efficiency in operation, and improved assembly is provided with the compact structure disclosed.

Abstract: A loading cam device is constructed of a first cam surface, a second cam surface, a ring-shaped retainer disposed between the first and second cam surfaces and defining pockets at angular intervals, and rolling members maintained in contact with the first and second cam surfaces while rotatably held within the respective pockets. The circumferential width of each pocket is smaller at each side of the retainer but greater along a thicknesswise central axis of the retainer, both compared with the outer diameter of the rolling members. As an alternative, the thickness of the retainer is a little smaller at a location adjacent to each pocket but significantly smaller at a location remote from the pocket, both compared with the thickness the outer diameter.

Abstract: A hydraulic control system for a continuously-variable-ratio-vehicle transmission of the toroidal-race rolling-traction type, in which the mechanism for positioning each roller includes a double-acting ram (11). The two opposed faces of the ram are exposed in normal use to the pressures of two separate lines of fluid (21, 23), each being pressure-controlled by separate valves (32, 34) located downstream of the respective ram face. The outputs of those separate valves combine in a common region (31), and further valve means (40, 41) open to connect that region to each line in response to the occurrence of a predetermined difference in pressure between the region and that line. In such circumstances such connection and the resulting transfer of fluid from the region to the line helps to counteract any tendency to fluid starvation in the line.