Traction drives employing a variety of arrangements to facilitate drive transfer are well known.

Such arrangements typically involve operative members having rolling surfaces such as discs, rollers or cones, based on the geometry of the circle. Generally, the operative members are captured or otherwise contained and their movement controlled by external means or they are allowed to rotate freely with their disposition being controlled or manipulated by interactions with adjacent members in rolling contact.

Typically, the operative members are constrained within a toroidal assembly. One of the earliest disclosures of such an arrangement is described in US 197,472, dated 1877. In the design described therein, the input and outputs of the transmission system each comprise a respective rotatable plate shaped to correspond to the upper and lower surfaces of a torus. Between and in contact with the two plates is a rotatable disc such that rotation of one rotatable plate is transferred through the rotatable disc to the other rotatable plate. The axis of rotation of the rotatable disc is adjustable. This in turn causes a variation in ratio of revolutions of one plate to the other.

Development of this arrangement has essentially, to the applicant's knowledge, been limited to modifications involving the rotatable disc. A number of proposed arrangements include the use of a plurality of discs to increase contact with the toroidal plate. Cones have been used to provide an alternative arrangement to a plurality of discs.

The present invention arises from the desire to step back from current developments based on discs or their equivalents and consider alternative methods of variable transmission based on toroidal plates.

According to the present invention there is provided a variable transmission system comprising input and output plates each formed with a toroidal surface having inner and outer edges and each being mounted for co-axial rotation; wherein the input and output plates are mounted with their toroidal surfaces opposed and wherein the system further comprises a rotation transmission

element mounted between the input and output plates and in contact therewith. The present invention is characterised in that the rotation transmission element comprises two contacting races of spheres, an input race associated with the input plate and an output race associated with the output plate. The system further comprises adjustment means to enable movement of at least one sphere from a first position in which it contacts the adjacent plate at a position adjacent the outer edge thereof and a second position in which it contacts the adjacent plate at a position adjacent the inner edge thereof.

It will be appreciated that the two toroidal surfaces together form a torus within which the two races are disposed. It will also be appreciated that each sphere has three points of contact, namely with each adjacent sphere in the other race and with the adjacent toroidal surface.

Typically, each of the input and output races comprises six spheres.

In one embodiment, the adjustment means comprises a pressure application assembly applying a substantially constant slight pressure to the input and output plates urging the plates together.

The assembly typically comprises a spring arrangement and is sufficient to ensure that in any position of the races other than when the spheres of the two races are spaced equally (providing a 1: 1 transmission ratio), there will be a tendency for the two races to move in such a way that the spheres in one race mover radially closer to each other and the balls in the other move correspondingly radially away from each other. The adjustment means then further includes a control disc adapted to apply a force upon the races, in turn, to force the balls in one race to move radially apart.

In a second embodiment, the adjustment means comprises a cradle into which one of the spheres in one of the races is captured. The cradle is mounted for rotational movement about an axis perpendicular to the axis of rotation of the toroidal plates, rotation about which axis is prevented.

The above and other aspects of the present invention will now be described in further detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a vertical cross-section of an illustrative embodiment of a transmission system in accordance with the present invention at a first extreme ratio;

Figure 2 is a plan view of the embodiment of Figure 1 with one toroidal plate removed; Figure 3 corresponds to the plan view of Figure 2 at an intermediate ratio; Figure 4 corresponds to the cross-section of Figure 1 at the intermediate ratio of Figure 3; Figure 5 corresponds to the plan view of Figure 2 at a second extreme ratio; Figure 6 corresponds to the cross-section of Figure 1 at the second extreme ratio of Figure 5; Figure 7 is a cross-section of a first practical embodiment of a transmission system in accordance with the present invention at a first extreme ratio; Figure 8 is a plan view of the embodiment of Figure 7 with one toroidal plate removed at the first extreme ratio; Figure 9 is a cross-section of the embodiment of Figure 7 at an intermediate ratio; Figure 10 is a plan view of the embodiment of Figure 7 at an intermediate ratio; Figure 11 is a cross-section of the embodiment of Figure 7 at a second extreme ratio; Figure 12 is a plan view of the embodiment of Figure 7 at the second extreme ratio; Figure 13 is a cross-section through a second practical embodiment of a transmission system in accordance with the present invention at a first extreme ratio; and Figure 14 is a plan view of the embodiment of Figure 13 at a first extreme ratio with one toroidal plate removed.

With reference initially to Figures 1 and 2, the essential principles of a transmission system in accordance with the present invention will be discussed. The transmission system 10 includes an input plate 11 having a toroidal inner surface 12. Input plate 11 is caused to rotate by means of an external prime mover (not shown) being operative coupled to an axial input drive shaft 13.

The system 10 also includes an output plate 14 having a toroidal inner surface 15. Output plate 14 drives an external mechanism (not shown) by means of an output drive shaft 16 extending outwardly from the output plate 14. The input 11 and output 14 plates are arranged such that their toroidal surfaces are opposed, forming a toroidal space 17 therebetween. The spatial orientation of the two plates 11,14 is ensured by use of an axle 20 upon which the input and output drive shafts 13,16 are mounted.

Housed within toroidal space 17 are two races of spheres or balls, an input race 21 associated with the input toroidal plate 11 and an output race 22 associated with the output toroidal plate 14.

In the embodiments shown, each race comprises six balls. However, this number is not essential and other numbers of balls will be equally suitable depending upon the relationship between the radius of the torus forming the toroidal space 17 and the radius of the circular cross-section of the torus.

Each ball of the input and output ball races 21,22 is dimensioned such that it contacts the adjacent input or output plate at a single point and two adjacent balls in the other race each at a single point. Accordingly, each ball of input race 21 contacts the inner toroidal surface 12 of input plate 11 at a single point and is thus in rolling contact therewith. Similarly, each ball of the input race 21 is in rolling contact with two adjacent balls of the output race 22. Thus rotational drive is imparted to the balls 21 of the input race by means of rotation of the input plate. This rotational drive is transferred from the balls of the input race 21 to the balls of the output race 22 and thus via the output toroidal plate 14 to output drive shaft 16. It will be appreciated that the direction of rotation of drive shafts is maintained through transmission of rotational drive through the two races, in contrast to prior art arrangements.

The position of the balls within the toroidal space 17 is influenced by the self aligning and self- centring properties of the balls circumscribed by the torus and upon the reciprocal relationship of the races. It will be seen that as the balls of the input race 21 move radially outwards from the centre of the torus, the balls of the output race move correspondingly inwardly. This is clearly

illustrated by comparison of Figures 1 and 2 which illustrate the arrangement at one extreme position wherein the balls of the input race contact the toroidal surface of the input plate 11 at a point adjacent the inner edge thereof, with Figures 3 and 4 which show an intermediate position and Figures 5 and 6 which show a second extreme configuration for the balls, in which the balls of the input race contact the outer edge of the toroidal surface of the input plate. It will be appreciated that each configuration shown in Figures 1 to 6 corresponds to a different transmission ratio. It will also be appreciated that between the two extreme configurations represented by Figures 1 and 6, there is an infinite number of intermediate configurations.

It will be appreciated that there needs to be some means by which the relationship of the two ball races 21,22 can be controlled or adjusted. A first adjustment arrangement is illustrated in Figures 7 to 10. In the embodiment shown, at the intermediate position giving a ratio of 1: 1 (that is to say, input is equal to output), the balls of the races are induced to move inwardly or outwardly by means of slight pressure imparted by a spring 30 acting upon the output plate 14 urging the input and output toroidal plates closer together along axle 20. The spring 30 could operate equally upon the input plate, both plates could be provided with springs or indeed alternative biasing arrangements can be readily contemplated. Such movement of the balls is controlled by means of a control disc 31 mounted within the toroidal space 17 upon the axle 20.

Under suitable external control (omitted for clarity) control disc 31 is caused to move from a, first position adjacent one of the toroidal plates in which the ball races are forced into a first extreme configuration (ratio) to a second position adjacent the other toroidal plate in which the ball races are forced into a second extreme configuration or ratio. Compare, for example, Figures 7 and 11, through an intermediate configuration shown in Figure 9.

Upon rotation of the input toroidal plate 11, all the balls are caused to spin by virtue of rolling contact which is then transferred to control disc 30, causing it to rotate. Any tendency for the balls to follow the rotation of the toroidal discs rather than spin under rolling contact is resisted by providing that control disc 30 is mounted upon a free-wheeling assembly such that it is only permitted to rotate in a direction opposite to that of the toroidal discs.

An alternative and preferred adjustment arrangement for the ball races is illustrated in Figures 13 and I4. In this embodiment, one of the balls of one of the races is captured in a pivotable assembly. It does not matter which ball is captured and more than one ball may be so captured.

In the arrangement shown, the pivotable assembly comprises a generally C-shaped cradle 40 including opposing cups 41,42 between which a ball 21a of input race 21 is captured and is free to rotate by means of ball races 43,44 provided around the respective peripheries of the cups 41,42. Cradle 40 is supported within a frame 48 which allows rotation of the cradle 40 about an axis perpendicular to that of axle 20. Frame 48 acts to prevent rotation of the cradle about the axis of the transmission system. In order to control rotation of cradle 40 within frame 48, the outer edge of cradle 40 is provided with teeth which engage inner and outer cogs 45,46 mounted conveniently upon the output toroidal plate 14. Inner cog 45 is driven by means of a sliding rack 47 formed on axle 20 such that the cradle is rotatable through an arc 50 about an axis perpendicular to that of axle 20 between a first position as shown in Figure 13 in which the ball of input race 21 contacts the toroidal surface of input plate 11 adjacent the outer edge thereof and a second position (not shown) where the cradle is rotated by about 45° whereby the ball contacts the toroidal surface adjacent its inner edge. This arrangement provides the additional advantage that rotation of the ball races around the toroidal space 17 is prevented. Thus there is no loss of traction which might otherwise occur.

Alternative drive arrangements for cradle 40 will be apparent to those skilled in the art. For example, in alternative arrangements outer cog 46 is driven, inner cog 45 serving merely for additional support to the cradle 40. g, ò The present invention provides a transmission system using toroidal surfaces which, in contrast with systems based on discs to transfer traction between the toroidal plates, is self-centring and self-aligning and provides pure rolling contact. That is to say there is no rubbing contact which might lead to a loss of traction.