WO2016193710A1

WO2016193710A1 - Variator

Info

Publication number: WO2016193710A1
Application number: PCT/GB2016/051593
Authority: WO
Inventors: John Fuller; Brian Dutson
Original assignee: Torotrak (Development) Ltd
Priority date: 2015-05-29
Filing date: 2016-05-31
Publication date: 2016-12-08
Also published as: GB201509327D0

Abstract

A variator comprising an input race and an output race coaxially mounted for rotation about a variator axis and at least one rolling element mounted on a carriage assembly for rotation about a rolling axis, wherein the carriage is able to pivot through a tilt angle which corresponds generally to a variator ratio. The variator further comprises a control member operative to effect a change in variator ratio, and a reaction member arranged to bear loads from the rolling element, wherein the reaction member generates a control signal that is a function of the rolling element loads and wherein the control signal is adapted to change the position of the control member.

Description

VARIATOR

This invention relates to variators. More specifically, it relates to variators that are components of a mechanical transmission system.

In this context, a variator is a transmission component that interconnects two rotatable elements whereby, when rotating, the two elements have rotational speeds related to one another by a ratio (referred to as the "variator ratio") that can vary between a minimum variator ratio and a maximum variator ratio in a substantially stepless manner.

Full toroidal variators

The range of variators known as toroidal variators fall into two principal categories: "half- toroidal" and "full-toroidal" variators, so termed because of the toroidal (or half-toroidal) cavity that exists between input and output working surfaces of the variator. In a full-toroidal variator, each rotatable element is connected to a respective race within the variator, such that each rotatable element is substantially fixed for rotation with its race and such that the races rotate about a common axis (the "variator axis"). Each race has a working surface, arranged such that the working surfaces face one another in a direction parallel to the variator axis. An annular recess of arcuate cross-section is formed within each working surface, coaxial with the variator axis. The recesses are arranged such that they lie on a common hypothetical circle, the plane of which intersects the variator axis and the centre of which is in a plane (the "centre plane") that is parallel to and spaced equally between the working surfaces. By extending the hypothetical circle around the variator axis, a hypothetical torus is described, the working surfaces occupying opposite regions of the boundary of the torus. Therefore, the space between the working surfaces of the races is referred to as the "toroidal cavity". The radius of the centre of the hypothetical circle with respect to the variator axis is termed the "toroidal radius", and the radius of the hypothetical circle is termed the "minor radius of the toroid".

Typically, several rolling elements are provided within the toroidal cavity. Each rolling element typically has a rolling surface that makes contact with (subject to the discussion below) a respective working surface of the two races. Each rolling element is carried in a respective carriage such that it can rotate with respect to the carriage, typically about a rolling axis upon which the rolling surface is centred.

Rotation of one of the races (called the "input race" in this discussion) with respect to the carriages causes each rolling element to rotate, and this, in turn causes a torque to be applied to the other race (called the "output race" in this discussion), in a direction opposite to the rotation of the input race. If the output race is allowed to rotate in response to the applied torque, it will do so in a direction opposite to that of the input race. During such rotation, each rolling element will make contact with the input race and the output race about a respective circular contact locus described on the respective working surfaces. If these two loci are of the same radius (the radius of a locus being generally referred to as the "contact radius"), then output race and the input race will have the same rotational speeds (albeit in opposite directions). However, if the contact radius of on the input race (the "input radius") is not equal to the contact radius on the output race (the "output radius"), then the speed of the output race will be greater than or lesser than the speed of the input race. In general, the magnitude of variator ratio will be equal to the ratio of the input radius to the output radius.

Each carriage is configured such that said carriage can be moved to alter the input and the output radii, this movement being referred to as "tilt". At least when the variator is operating in an equilibrium condition, the input and the output radii are symmetrically disposed about the toroidal radius.

The foregoing description refers to contact between the working surfaces and the rolling elements. However, this is a simplification. Most embodiments of toroidal variators operate using traction drive. That is to say, the working surfaces and rolling elements are at least partially immersed in a traction fluid. This has the property of having a viscosity that increases rapidly when its pressure exceeds a threshold. As the races rotate, traction fluid is drawn into the nips formed between the rolling elements and the working surfaces to create a thin layer of traction fluid between the rolling surfaces and the working surfaces, so there is, literally speaking, no contact between them. In order to achieve a satisfactory traction drive, an end load is applied, which urges the races towards one another along the variator axis. The end-load is optimised to balance the requirement of providing sufficient loading to produce adequate traction at the interfaces between the working surfaces and the rolling surfaces, but low enough not to compromise the efficiency and durability of the variator. In many embodiments, the races may make slight movements along the variator axis in response to the end-load. Often end-load is applied using hydraulic pressure.

The size of the end load has an important effect on variator performance and longevity. Excessive end load reduces efficiency and causes premature wear, leading eventually to variator failure. Inadequate end load can lead to excessive - -slip between the rolling elements and the races. The end load required to prevent excessive slip varies with the torques exerted upon the races. In principle a constant end load can be used, but this must be large enough to sustain traction when the variator is subject to the maximum expected torques, so that under all other conditions the end load is larger than is necessary. Improved efficiency and variator lifetime can be achieved by adjusting the end load in sympathy with the torques handled by the variator.

The traction coefficient of a variator is defined as the (i) tangential contact force divided by (ii) the normal contact force. These quantities are defined as follows:

(i) The tangential contact force (that is, parallel to the interface plane of the rolling element and race, and in the rolling direction of the rolling element) arises from fluid shear at the rolling element-race contact. It is proportional to a force applied to the rolling elements (termed the "reaction force"),. The reaction force may typically be applied to the rolling element via, for instance, a rolling element control actuator.

(ii) The normal contact force (that is, the force normal to the rolling element- race interface plane) is generally determined by the endload that is applied to the variator, although, since the direction of the normal to the interface varies with the rolling element inclination, it is also a function of the cosine of the rolling element's angle to the variator axis. In the variator in which a single rolling element is disposed on a carriage between the races (a "single rolling element variator"), the race input and output torques generated by the rolling elements are of the same sense. Reaction torque may be described as the sum of the variator input and output torques, and is commonly used to describe the torque reacted into a casing of a variator in which the sign of input and output torque are the same.

Proportionality of end load to reaction torque is, with some provisos, a highly efficient mode of end load control in full toroidal variators since, for any given variator ratio, it provides a constant traction coefficient at the rolling element/race interface.

Control of end load based upon reaction torque is, with some provisos, the optimal strategy for a full toroidal variator and conventionally is achieved when hydraulically controlling the reaction torque (reaction forces in the rolling elements) of the variator and applying a common pressure to the endload system.

Within the general arrangement of a full-toroidal variator described above, a great many variations are possible concerning control, mounting and freedom-of-movement of the carriages, number and configuration of races, number and configuration of rolling elements, and so on.

In one modification to the variator described above, each rolling element is replaced by a train of two rolling elements in contact with one another (a "double rolling element variator"). Thus, each rolling element is in contact with one working surface and with the other rolling element. It should be noted that in a variator that incorporates this modification, both races turn in the same direction about the variator axis, so have a positive variator ratio and, since the torques acting on the input and output races are in an opposite sense to one another, the term reaction torque may be conveniently defined as the sum of the absolute values of input torque and output torque. In all following descriptions the term 'reaction torque' is used in the context of a single rolling element full toroidal variator, but it will be appreciated that the invention applies to other variators which feature input and output torques with opposite signs.

The use of the terms "input" and "output" to define the races should not be taken as a functional or structural limitation relating to these components - they are simply labels. The variator may be entirely symmetrical in operation. These will typically be chosen to provide a concise and understandable description in a particular context. For example, in the case of transmission for a vehicle, the input will typically be connected to a prime mover, and the output will typically be connected to a final drive system to indicate the normal direction of power flowing through the variator. However, it will be understood that when the vehicle is in an overrun condition, engine braking will actually cause power to flow from the output to the input of the variator.

In the remainder of this specification, the term "variator" will refer to a full-toroidal variator as described above, unless the context indicates otherwise. However, it will be understood that the invention is applicable to a wide range of rolling contact variators.

There are two main strategies used to control a variator: torque control and ratio control.

Torque control has no direct equivalent in multiple-speed transmissions because it relies upon a feature arising from the nature of a variator. Torque control has been described in many publications, including those parts of WO-A-2010/070341 that relate to Figures 1 to 3, and will be described here only briefly, as required to enable the present invention to be understood.

Torque control relies upon the variator having several design features:

• each carriage has freedom to rotate about a reaction axis that is typically inclined by a small angle to the centre plane; and

• each carriage can move axially along the reaction axis against a force applied to it by an actuator.

Note that the first of these requirements means that while under torque control, tilt angle is not controlled directly by an actuator. Each race applies a force to each rolling element that acts in a direction tangential to the working surface. Therefore, equal and opposite tangential forces must be provided by each actuator to maintain the rolling axis of the corresponding rolling element stationary along the reaction axis. If the force applied by the actuator changes, the forces acting on the carriage become imbalanced, so the rolling element (and its rolling axis) will move. The geometry of the variator is configured (using considerations that will not be discussed here) such that upon movement along the reaction axis, a couple is generated about the reaction axis that causes the carriage to rotate. This changes the tilt angle, in such a way as to reduce the imbalance of forces acting on the carriage. The carriage will therefore move towards a new tilt angle at which the forces return to balance. Due to the geometry of the variator, the carriage assemblies move along their reaction axes and rotate about their tilt axes, such that they accommodate the ratio of the speeds of the input and the output races. Torque control requires the application of force to the rolling elements in the direction of travel along their reaction axes. Such actuation requires work to be done on the rollers, typically through hydraulic actuation. The energy expended reduces efficiency of the variator system, and introduces a requirement for heavy and costly hydraulic apparatus.

Ratio control bears the nearest similarity to the control of a transmission with multiple discrete, spaced ratios. In ratio control, a control system determines the variator ratio required to achieve a desired operating condition and operates actuators to move the carriages in such a way as to directly cause the rolling axes to tilt to the angle required to achieve the target ratio.

At any moment, the ratio of a variator is determined by its instantaneous tilt angle. Therefore, it might seem to be most straightforward to control the ratio by means of an actuator that applies a torque to a roller or its carriage to cause it to rotate about the tilt axis. However, in many cases, this is not an efficient way to control ratio. In order to cause the roller to tilt in this way, sufficient shear forces must be generated at the contacts between the rolling elements and the races to overcome the viscosity of the traction fluid, and then each contact has to be moved across the working surface of the race. Movement of a force through a distance requires work to be done. Therefore, to achieve such movement requires an actuator that can perform such work, and a source to power such an actuator. These components are typically both heavy and expensive.

An alternative approach to ratio control is therefore proposed, which will be referred to as "pitch steer" ratio change.

Consider the motion of a rolling element and a race at their contact regions. The instantaneous movement of each of the rolling element and the race at the contact region can be described by respective velocity vectors. The angle between these two vectors is the steering angle, so termed because generation of such an angle causes the race to steer the rolling element towards a new ratio angle. The steering angle is generated by an input to the rolling element - a pitching input - that causes the rolling element to pivot about a pitch axis defined by a hypothetical line between the rolling contact regions. In a variator with a single roller per carriage, the pitch axis is defined by a line connecting the race-rolling element contacts.

The tilt axis of a rolling element (or carriage when there is only one rolling element per carriage) is typically inclined to the centre plane of the variator by an angle known as the caster angle. This allows the steering angle that arises due to the input applied to the rolling element (or carriage) to be reduced as the roller tilts, until the steering angle becomes zero At this point, the rolling element has reached its new equilibrium tilt angle. In this way the pitch input to the rolling element results in a new tilt angle of the rolling element and hence a new variator ratio to be achieved.

The existence of the caster angle causes the steering angle to reduce as the tilt angle changes until it approaches zero. When this happens, the rolling element tilt angle, and therefore the variator ratio, stabilises at a new, steady value.

During a change in ratio effected by pitch steer, the rolling elements are steered by the races, but do not translate appreciably along the working surfaces. This means that there is no appreciable work done to effect the change of ratio. Pitch steer control can therefore be achieved using low-powered actuators, which can be light in weight and inexpensive. In a variator configured for pitch steer control, the caster angle can be implemented using a carriage (or rolling element) actuation point that is offset from a plane that is perpendicular to the variator axis (being the axis about which the races rotate). This plane may also be the toroidal mid-plane. Typically each rolling element may also be constrained by a pivotal joint at the rolling element centre, said joint preferably being a spherical joint. Alternatively, the caster angle may be implemented as follows: the rolling elements may be mounted on a gimbal such that the rolling element may rotate about its rolling axis but is constrained to precess (that is, pivot) about a caster axis that is defined by a pivoting joint whose axis passes through the centre of the rolling element, and is inclined to the plane that is perpendicular to the variator axis. The gimbal caster pivot axis may pass through the centre of the rolling element.

One advantage of torque control is that it is advantageous when torque, rather than ratio, is the primary variable of interest, for example in the transmission of torque effort to the driving wheels of a vehicle. A torque control device is especially beneficial when used to transmit torque from a flywheel energy storage system to / from an energy sink / source such as the driving wheels of a vehicle. However, ratio control systems can be configured to be mechanically less complex than some torque control systems. Furthermore, because rolling elements are typically moved along their reaction axis in torque controlled variators, work is done on the rolling elements during ratio change which increase power consumption and potentially cost of the actuation system. Another challenge can arise from the damping of torque control systems which are typically second order dynamic systems, as opposed to pitch steer systems in which the rollers approach their final tilt angle asymptotically in which is known as a 'first order' response. The stability margin of tilt steer systems is typically therefore higher than that of torque controlled systems. An aim of this invention is to solve or at least ameliorate some or all of the aforementioned problems. In a first aspect, the present invention provides a variator arrangement comprising:

an input race and an output race each having a working surface, the races being coaxially mounted for rotation about a variator axis and facing each other to define a cavity;

at least one rolling element disposed between the working surfaces, the rolling element having at least one rolling contact with one working surface and a second rolling contact and being mounted on a carriage assembly for rotation about a rolling axis, wherein the carriage is able to pivot through a tilt angle about a tilt axis, the tilt angle corresponding generally to a variator ratio being the ratio of the rotational velocities of the races;

a control member operative to effect a change in variator ratio;

an end-load clamping system (or arrangement) that urges the at least one rolling element into driving engagement with the races further comprising

a reaction member assembly which comprises a reaction member, the reaction member arranged to bear loads from the at least one or one of the rolling elements of the cavity, wherein

the reaction member assembly generates a control signal that is a function of the rolling element loads and wherein

the control signal is adapted to change the position of the control member.

There may also be a single roller on the at least one carriage, in which case the rolling element makes contact with both working races (or the working faces thereof). Whether there is a single, or two, rolling elements per carriage, there may also be a plurality of carriages in the cavity. There may be one cavity, or two cavities for increased efficiency and power capacity.

The reaction member may bear load from a rolling element. It may bear most or all of the load from a rolling element. Preferably it bears most or all of the loads from all rolling elements within the cavity. Alternatively, it may bear a portion of the load (for example where the reaction member has just a single support). Preferably the reaction member has one support which is adapted to sense the loads of the rolling elements, and a second support.

This invention also provides a variator arrangement comprising: an input surface and an output surface, the input and output surfaces being coaxially mounted for rotation about a variator axis, and a toroidal cavity being defined between the working surfaces; a plurality of rolling elements disposed between and being in driving engagement with the input and the output surfaces at respective contact regions, each rolling element being mounted on an associated carriage assembly for rotation about a rolling axis, each rolling element being free to pivot about a tilt axis, the tilt axis passing through the rolling element perpendicular to the rolling axis, and intersecting the rolling axis at a rolling element centre, whereby a change in the variator ratio being the ratio of rotational speeds of the races surfaces occurs with a change in the tilt angle; wherein each carriage assembly is mounted for pivotal movement that results in a change of a pitch angle of the associated rolling element about a pitch axis, the pitch axis passing through the rolling element centre and through the contact regions; the variator further comprising a control member operative to cause or actuate at least one of the carriage assemblies to undertake the said pivotal movement thereby changing the pitch angle of the associated rolling element, so urging the plurality of rolling elements to pivot about their tilt axes and thereby provide a change in variator ratio, further comprising a reaction member assembly that includes a reaction member which bears the loads from the rolling elements wherein the reaction member assembly is configured to generate a control signal that is a function of the rolling element loads and wherein the control signal is adapted to change the position of the control member.

Preferably an actuation point of the carriage or rolling element is offset from the centre plane of the toroidal cavity in a direction parallel to the variator axis. Preferably each carriage assembly is constrained to the said pivotal movement by i) coupling with the control member about an actuation point and ii) coupling about a second reaction point.

In this way, a "pitch steer" variator may possess some or all of the characteristics of the torque controlled variator, thus making it more applicable to applications where torque control is preferable such as a main drive transmission or a flywheel (energy storage) transmission. The invention may have particular application in vehicles such as loading vehicles, fork-lift trucks, wheeled loaders or back-hoe loaders and other handling vehicles, particularly a vehicle that includes an IVT optionally with a geared neutral configuration.

The reaction member preferably is coupled to each rolling element within the cavity. It is preferably coupled at the centre of each rolling element. It is preferably coupled by a spherical joint at each rolling element centre. Use of a single reaction member allows the total torque in the variator cavity to be sensed using a single component. The reaction member preferably may move radially of the variator axis when the load from each rolling element is not equal, the inequality in forces applied to the reaction member causing it to move in a radial direction. Such radial movement causes the rolling elements to move around the toroid surface such that some rolling elements to adjust their tilt angle in a first sense, whilst others adjust their tilt angles in a second sense that is generally opposed to the first sense. This causes the load on rolling elements with greater load to be reduced, and the load on rolling elements with less load to be increased until the rolling element loads are approximately equal. Thus traction conditions on each rolling element may be equalised, power transmission is optimised and no rolling element is under- or over-clamped with respect to the other rolling elements so that durability and traction margins are satisfactory.

The reaction member preferably is mounted for rotation about the variator axis.

It may be preferable to maintain the first order response of the pitch steer system. This may be achieved by preventing, limiting or restricting movement of the rolling elements around the toroid, which may in turn be achieved by preventing, limiting or restricting rotational movement of the reaction member.

The variator arrangement may further comprise a control system that receives a first input indicative of a torque request, a second input indicative of a variator torque, and a comparator that determines an error from the first and second inputs. The control system may further comprise an arrangement that determines an output from the error signal and applies this output to the control member in order to modify the ratio of the variator.

In an embodiment the reaction member may be mounted for rotation about the variator axis against resiliently deformable means. An actuator positon may form the first input to the control system, and the position of the reaction member may form the second input to the control system. The comparator may be a mechanical comparator that receives the first input being the positon of the actuator, the second input being the position of the reaction member, and issues an output that is a function of the difference between the first and second inputs to the control member. Preferably the comparator is a linkage. Preferably the linkage is a tee-bar. Preferably the comparator has a gain of more than 1 , more than 3 or more than 5. A high gain may limit the required movement of the reaction member which is beneficial for stability of the variator.

In another embodiment the reaction member may be mounted such that it bears against pressure generating means which restricts flow from a hydraulic source thus generating a feedback pressure signal that is a function of the reaction torque of the variator. A hydraulic pressure signal generating means that may be a hydraulic torque request signal may form the first input to the control system, and the feedback pressure may form the second input to the control system. The comparator may be a mechanical comparator that receives the first input being the hydraulic pressure request signal, the second input being the feedback pressure signal, and issues an output that is a function of the difference between the first and second inputs to the control member. Preferably the comparator is a hydraulic actuator with first and second faces, for example a piston. Preferably the piston receives the first input (pressure request) on one face and the second input (feedback pressure) on its other face such that an error between request and feedback pressure (corresponding to an error in the requested and feedback torque) causes the actuator to move. Preferably the actuator is operatively coupled to the control member so that as the actuator moves the control member moves and the variator ratio changes. Preferably the comparator has a gain of 1 , that is, the first and second actuator faces are of equal area, so that the arrangement is conveniently housed at low cost. In this arrangement the reaction member does not move appreciably.

In a further embodiment the reaction member may be mounted such that it bears against force or torque sensing means thus generating a feedback signal that is a function of the reaction torque of the variator. A control signal indicative of a required variator torque may form the first input to the control system, and the feedback signal may form the second input to the control system. The comparator may be a software or electronic controller that receives the first and second inputs, and issues an output that is a function of the difference between the first and second inputs to the control member, optionally via one or more means such as an actuator driver and an actuator. Preferably the actuator driver drives the actuator which is operatively coupled to the control member so that as the actuator moves the control member moves and the variator ratio changes. The comparator may comprise a proportional ('Ρ') function in which the signal to the actuator or driver (the proportional effort) is dependent upon the difference between the first and second inputs. Additionally or alternatively the comparator may comprise an integral (T) control function in which the signal to the actuator or driver (the integral effort) is dependent upon the integral of the difference between the first and second inputs. Additionally or alternatively the comparator may comprise a derivative (Ό') control function in which the signal to the actuator or driver (the derivative effort) is dependent upon the derivative of the difference between the first and second inputs. Preferably the comparator possess either a P+l or a P+l+D control functions. In each case, each control function also comprises a gain that may be constant or may be variable, and which may be suitably tuned to provide the required response and stability of the system. In this arrangement the reaction member does not move appreciably.

The rolling elements and/or discs may be formed from steel. The steel surfaces may be hardened. The rolling elements and/or discs may not entirely be formed from steel. In some embodiments, only parts of the rolling elements and/or discs are formed from steel. Only part of the steel surfaces may be hardened. The surface hardness of the working surfaces of the rollers and /or discs is preferably in the range 40-80 Rockwell (Hrc). The variator may be served by a traction fluid which has a viscosity of between 3 and 10cS at 90°C.

The variator end loading mechanism may comprise one or more of a hydraulic piston, a ball- and-ramp cam mechanism or a spring, as is known in the art. Preferably the variator comprises a ball-and-ramp cam mechanism mounted on the variator input, a ball-and-ramp cam mechanism on the variator output, and a clamping spring (preferably a disc spring) arranged in series with both cams on either the variator input or the output that applies a pre-load force to the variator. Preferably this end loading arrangement is used in combination with any of the aforementioned 'pitch steer' torque control embodiments of this invention.

In normal operation, it is preferable that the reaction member is displaced through an arc around the toroidal or race circumference with a length that is less than 5%, or less than 2%, or less than 1 % of their rolling radius.

In certain embodiments, the reaction member may be arranged to be supported at or near both ends rather than solely at a single support. In this way the torque borne by the reaction member may be reacted (for example by the housing), without imparting side loads to the rolling elements. In such arrangements, the force borne at or near one end of the reaction member can be used to sense the loads or torques in the variator, whilst the second support may be either used for such sensing, or simply used to bear loads as described above.

In embodiments the variator may be arranged for uni-directional torque control, or bi- directional torque control. In the hydraulic embodiment this may be achieved by using two torque sensing arrangements such as the ball and flow supply, mounted on either side of the reaction member. Alternatively they may be mounted at or near opposite ends, but on the same lateral side, of the reaction member. The difference in pressures sensed may produce signals from which both sign and magnitude of sensed variator torque may be deduced. In one arrangement, each of the two sensed pressures may be fed to a comparator valve for generation of a differential pressure signal. Such a comparator valve may be a pressure control valve, and optionally may comprise a pilot chamber at each end of the valve. Each sensed pressure may be fed to one of the pilot chambers. The valve may receive a pressure source at its inlet, and an output a differential sense pressure that is indicative of the difference between the two sensed (i.e. pilot) pressures. Such a valve may therefore serve as a differential pressure control valve. Preferably the comparator valve also comprises preload means at one side of the valve such that the valve output pressure is offset from the difference between the sensed pressure. The output pressure of the valve may be termed the differential sense pressure signal. There may also be pressure generating means for generation of a pressure that is indicative of a pressure request signal, this being indicative itself of a requested variator torque magnitude and sign. The pressure request signal may bear on one face of an actuation member that effects movement of the control member, whilst the differential sense pressure signal may bear against a face that acts on said actuation member in an opposing sense to the pressure request signal. In this way, the difference between the pressure request signal and the differential sense (feedback) pressure signal may cause the control member to move in response to an error between requested and sensed variator torque. As a result the variator torque achieved will be similar to that requested.

The invention is illustrated by reference to the non-limiting drawings in which:

Figure 1 shows an end view of a torque control pitch-steer variator with a linkage control mechanism according to the invention;

Figure 2 shows an end view of a torque control pitch steer variator with a hydraulic control system according to the invention;

Figure 3 shows end view of a torque control pitch steer variator with an electronic control system according to the invention;

Figure 4 shows a toroidal variator.

With reference to Figure 4, the races 2 (one only shown) rotate about the variator axis. Rolling elements 1 (in this example only two are shown in the toroidal cavity) are constrained to pivot about a castor axis which is inclined to a plane that is perpendicular to the variator axis. The caster axis is defined by a line connecting the mounting part 20 and the roller centre 30. A spherical joint at each rolling element 1 centre allows for pivotal movement. Movement of the control member 16 in a direction 'C by actuator 5 causes each rolling elements 1 to be pitched about their respective pitch axis, each pitch axis passing through the contact regions between the respective rolling element 1 and the races 2. This causes each rolling element 1 to be steered by the races 2, and to pivot about its respective caster axis. As each rolling element 1 pivots, each undergoes two modes of angular change: (i) tilt, which causes the position of the contact regions to move across the faces of each race 2 radially relative to the variator axis (this therefore causes the variator ratio to change), and (ii) a pitching motion which cancels the initial pitch input to each rolling element 1. The rolling elements 1 thus pivot in response to an input by the actuator 5, and the tilt angle of each rolling element 1 changes until the tendency for the races 2 to steer the rolling elements 1 diminishes towards zero. Each actuator 5 position thus results in a corresponding variator ratio. Movement of the shaft 16 (the control member) in a first direction tends to increase the variator ratio, while moving it in the opposite direction tends to decrease the variator ratio (such bi-directional arrow indicated by letter 'C')-

With reference to Figure 1 , the rolling elements, in this case two rollers 1 , are disposed in the toroidal cavity that is formed between the two toroidal races 2 (one shown only), both races facing one another and being mounted for coaxial rotation about the variator axis. A reaction member 5 is mounted for rotation about the variator axis, and bears the loads generated by the rollers 1. The reaction member 5 is pivotally coupled to each roller 1 at its respective centre by a spherical joint 9. A control member 15 is operatively coupled to roller stems by mounting parts 8, and the stems are coupled to carriages upon which the rollers 1 are rotatably mounted. The control member 15 may move in direction 'C which in turn causes the stem-control member-linkage to adopt a parallelogram shape. In doing so, the rollers 2 experience steering angles in their regions of contact with the races 2 such that they are tilted to a new tilt angle commensurate with a new variator ratio. The new equilibrium tilt angle is achieved by virtue of the castor angle (not shown in Figure 1) formed by the angle made between each stem and the variator cavity centre plane (this being the plane parallel to and mid-way between the races), as is known in the art. The control system will now be described.

The control system comprises a linkage with control arm 6, and an actuator 3. If the reaction member 5 is not displaced then the actuator position effects a ratio change by displacing the upper portion of control arm 6 thereby moving the control member 15 in direction 'C. As the reaction member 5 bears loads from the rollers 1 (known as the reaction torque of the cavity) is displaced against resiliently deformable means (in this example springs, 12) thus moving the lower end of linkage arm 6. If the actuator rod is not displaced then generation of torque causes the control member 15 to move thus changing the ratio of the variator. It is therefore evident that movement of the actuator rod and generation of reaction torque each cause the ratio to change. In fact, the ratio achieved is a function of the actuator position and the torque generated. This offers a strong element of torque control which may be useful in arrangements such as an Infinitely Variable Transmission (IVT) in which, typically, the input of the variator is coupled to an engine and to a first element of an epicyclic gearset, the output of the variator is coupled to a second element of an epicyclic gearset and the third element of the epicyclic gearset is coupled to the wheels of a vehicle. In this situation a condition known as 'geared' neutral' may exist at one particular operating ratio of the variator. Achieving this precise ratio can be challenging for a control system. However, in the present invention requesting a variator ratio close to (but not exactly) that corresponding to geared neutral condition will cause the vehicle move away from rest. If the driver applies the brakes then the variator will generate torque which will cause the reaction member 5 to displace against the resiliently deformable means, this causing the control arm 6 to reduce the ratio requested at the control member 15 such that the variator ratio matches the geared neutral condition, albeit with torque applied. This would be convenient method of ensuring that the geared neutral condition is accommodated, as would happen with a conventional torque controlled variator, whilst applying a level of creep torque that is indicated by the actuator position. Thus this system emulates torque control typically associated with a hydraulically controlled variator, but without the complex apparatus of a hydraulic system. Torque control may also be advantageous for vehicles which include transmissions with a synchronous shift point between regimes of the transmission. A further feature of this system is the endstops 4 that prevent the control member 15 and hence ratio from travelling outside of the permitted range, and a compliant member 7, which allows the endstops 4 to take effect without causing fouling of the linkage mechanism under certain conditions.

The variator may be hydraulically controlled as shown in Figure 2. Reaction member 5 bears against pressure generating means with a force 'F' which is proportional to variator reaction torque Tvariator, in this case a ball bearing 11 , which restricts flow from the fluid supply (typically a flow source) thus generating a feedback pressure proportional to reaction torque behind the ball. This feedback pressure is fed to the chamber which communicates with the left hand end-face of actuator piston 120. A pressure request indicative of a requested reaction torque is generated by pressure control valve 21 which produces a pressure indicative of a torque request, this pressure being fed to the right hand chamber that communicates with the right hand face of actuator piston 120. It should be noted that the actuator piston comprises two hatched areas connected by an unhatched shaft in the diagram of Figure 2, and moves bodily so as to control the pitching input to the rollers 1 Thus any error between request pressure and feedback pressure causes actuator piston 120 to move in direction 'C, the actuator also being the control member in this embodiment. The rollers 1 are therefore pitched, pivoting about the pitch axis passing through their contact regions, and are steered to a new ratio. In this instance, the rollers 1 will continue to experience a change in tilt until the torque indicative of the pressure generated by the pressure control valve is matched by the feedback pressure. Thus this system provides a good approximation to torque control, but advantageously the fluid supply may be a low pressure source that is readily available, such as the lubrication or cooling flow for the roller 1. Such flow may be provided at a pressure which is less than 1 bar, less than 2 bar, but in any case less than 5 bar. The fluid may be provided by a geroter pump or an impellor pump. Preferably the pump is driven by an electric motor. The reaction member may move in a direction (that is, radially of the variator axis) in order to balance or equalise the reaction loads of the rollers 1. It should be noted that the hydraulic arrangement may be configured to provide bi-directional torque control, as described previously. Two ball 1 1 and flow supply arrangements may achieve such function. The arrangement may comprise a comparator pressure control valve for providing a differential sense pressure signal. Such valve may optionally be preloaded with a spring. Such differential sense pressure signal may then be fed to one side of the actuator piston 120, whilst the pressure request signal may be fed to the opposite side of the actuator piston 120, as described earlier. In this way, variator torque achieved should be similar to that requested by the control system.

Figure 3 shows a modification to the embodiment of Figure 1 in that the reaction member 5 bears against a load cell 40, or where bi-directional torque sensing is required, two load cells 40.. In this embodiment the reaction member may slide radially n direction in order to balance the loads of the rollers 1. This feature is especially effective with 2 rollers only in the cavity, and the reaction member may move along a linear guide. Such an arrangement may also be applied in the embodiments of Figures 1 and 2. The load cell issues a signal (or two signals in the case of two load cells 40) which is issued to control processor J, where it may be modified by a gain and/or a filter. Input T issues a request input control signal (which may be a magnitude or a magnitude and sign of load or torque) to summing junction W which determines the difference (the 'error') between the torque request and the feedback reaction torque. The error signal (which may be a magnitude, or a magnitude and a sign of load or torque) is fed to a series of processing stages L which may contain one or any combination of a series of gains, proportional control functions, integral control functions, derivative control functions, combinations of these, or other processing control functions. The final output is issued to the actuator driver M which issues either a series of stepper motor, current, voltage or other power signals to actuator 3, as required. The actuator 3 then alters its position, thereby adjusting the position of the control member 15. This system therefore also emulates the function of torque control, offering advantages of ease of calibration through tuning of parameters in software or electronics. However, this embodiment does contain sensors and electronic or software processors which may add some complexity compared with embodiments shown in Figures 1 and 2.

The elements of torque control provided for variators disclosed herein are particularly useful for main-drive transmissions for vehicles. Creep torque is often provided by torque converters in automatic and power-shift transmissions. The variators as disclosed herein may advantageously provide creep torque by virtue of the elements of torque control provided, but exhibit lower power losses than torque converters. The variators disclosed herein may be used advantageously in main-drive transmissions for vehicles which shuttle backwards and forwards frequently, such as loading or materials handling vehicles. This invention is advantageous for vehicles incorporating an IVT condition such as is made possible by a geared neutral arrangement.

The elements of torque control provided for variators disclosed herein can also provide benefits in auxiliary drive systems, where firing pulses from combustion engines can be attenuated. Such auxiliary drive systems may operate to drive a supercharger which may optionally include a dynamic compressor such as a centrifugal compressor. In such an arrangement, a uni-directional torque control arrangement may be adequate and even preferred.

Claims

1. A variator comprising:

a control member operative to effect a change in variator ratio;

an end-load clamping system that urges the at least one rolling element into driving engagement with the races, further comprising

a reaction member assembly which comprises a reaction member, the reaction member arranged to bear loads from one of the rolling elements of the variator or cavity, wherein

the reaction member assembly generates a control signal that is a function of the said rolling element loads and wherein

the control signal is adapted to change the position of the control member.

2. A variator according to claim 1 , wherein the two rolling contacts define a pitch axis; wherein

At least one the rolling element is mounted for pivotal movement about its pitch axis; the control member is operative to cause said pivotal movement about the pitch axis, and thereby cause the carriage associated with the at least one rolling element to pivot about its tilt axis to change the variator ratio.

3. A variator according to any preceding claim, wherein the loads are reacted predominantly or wholly by the reaction member.

4. A variator according to any one of the preceding claims, wherein the tilt axis is inclined to a plane normal to the variator axis by a castor angle.

5. A variator according to any one of the preceding claims in which there are a plurality of carriages in the cavity.

6. A variator according to claim 5 wherein there is one and only one rolling element per carriage and the second rolling contact is between the rolling element or elements and the other of the input or output working surface.

7. A variator according to any one of the preceding claims wherein the reaction member is pivotally coupled to the centre of each rolling element.

8. A variator according to any one of the preceding claims wherein the reaction member reacts most or all of the loads generated by the rolling elements in the cavity.

9. A variator according to any one of the preceding claims, wherein there are two and only two rolling elements in the cavity.

10. A variator according to any one of the preceding claims further comprising a control system, the control system receiving first and second inputs, the control system comprising a comparator that determines an error signal from the first and second inputs, the control system further comprising an arrangement that determines an output from the error signal and applies this output directly or indirectly to the control member.

1 1. .A variator according to claim 10 wherein the second input is generated by the reaction member assembly.

12. A variator according to claim 1 1 wherein the second input is determined from a variator torque.

13. A variator according to either of claims 10 to 12 wherein the first input is indicative of a torque requirement, a ratio requirement, or a combination of the two.

14. A variator according to any one of claims 10 to 13 wherein the first input is received from actuation means.

15. A variator according to claim 14 wherein the actuation means has an output power of less than 20W.

16. A variator according to either of claims 14 or 15 wherein the actuation means includes at least one of an actuator, a hydraulic cylinder, a linear motor, a motor plus lead screw, or a stepper motor.

17. A variator according to any of claims 14 to 16, wherein the actuation means is operatively coupled to the control member and the reaction member via a control linkage, optionally a control arm linkage.

18. A variator according to claim 17 wherein the linkage serves as the comparator.

19. A variator according to claim 18, wherein the linkage is operatively coupled to the actuation means, the control member and the reaction member.

20. A variator according to either of claims 18 or 19, wherein the reaction member assembly comprises the reaction member and resiliently deformable means (such as a spring), optionally wherein the reaction member bears against resiliently deformable means, optionally wherein the resiliently deformable means bears against a variator housing.

21. A variator according to claim 20, wherein the second control signal is derived from the displacement of the reaction member against the resiliently deformable means.

22. A variator according to any of claims 10 to 21 , wherein the gain between the second control signal or the error signal and the resultant movement of the control member is greater than 1 , preferably greater than 2, and more preferably greater than 4.

23. A variator according to any one of claims 10 to 16 wherein the comparator is a hydraulic comparator, optionally wherein the first signal is a control hydraulic pressure and the second signal is a feedback hydraulic pressure.

24. A variator according to claim 23 wherein the comparator is a piston, optionally wherein the control hydraulic pressure acts on a first face of the piston and the feedback hydraulic pressure acts on a second face of the piston.

25. A variator according to either of claims 23 or 24 wherein the second hydraulic pressure is generated by the reaction member bearing against means for restricting a flow from a fluid supply or a flow source.

26. A variator according to any one of claims 10 to 16 wherein the reaction member bears against a load sensor.

27. A variator according to claims 26 further comprising an electronic controller.

28. A variator according to either of claims 26 or 27 wherein the comparator is a summing junction, and the second signal is received from a load sensor that derives the second signal from a variator torque.

29. A variator according to any one of the preceding claims further comprising limiting means to keep the variator ratio within a working range.

30. A variator according to claim 29 wherein the limiting means comprises at least one end stop for the control member.

31. A variator according to either claim 29 or 30 further comprising a compliant member configured to prevent fouling of any components of the variator when said limiting means are active in keeping the variator ratio within the working range.

32. A variator according to any preceding claim wherein the rolling elements, when in normal operation, are displaced through an arc with a length that is less than 5%, or by less than 2%, or by less than 1 % of their rolling radius around the circumference of the toroid.

33. A variator according to any one of claims 4 to 32 (when dependent on claim 4) wherein the caster angle is less than 20 degrees or less than 15 degrees or less than 10 degrees.

34. A variator according to any one of the preceding claims wherein the reaction member bears against resiliently deformable means, such as a spring, or a rubber bush or member.

35. A variator according to any one of the preceding claims wherein the reaction member is moveable in a radial direction relative to the variator axis for the equalisation of rolling element loads.

36. A drive system for an energy storage flywheel comprising a variator according to any one of the preceding claims.

37. A drive system for an energy storage flywheel drive system according to claim 36 further comprising a high speed flywheel.

38. A vehicle comprising a variator according to any one of claims 1 to 35.

39. A vehicle comprising a drive system according to either of claims 36 or 37.

40. A vehicle according to either of claims 37 or 38 that is a loading or materials handling vehicle.

41. A drive system for an auxiliary drive comprising a variator according to any one of claims 1 to 35.

42. A variator as substantially herein described and with reference to the accompanying figures.