Rotary element for transmitting rotary motion

Abstract

A rotary element 1 for a rotary transmission has a helical radial projection 4 disposed about a rotary axis thereof, the helical radial projection having a helical peripheral surface 5 that defines an envelope. The helical peripheral surface has a radial profile which is inclined to a tangent to the envelope. In use with another such rotary element 2 of opposite handedness in a rotary transmission, a point (P) of rolling contact of the helical peripheral surface with a helical peripheral surface of the other such rotary element helically traverses the helical peripheral surfaces of both rotary elements to positively transmit rotary drive between them.

Description

The present invention relates to a rotary element for positively transmitting transmitting rotary motion in a drive arrangement comprising two such rotary elements and relates also to such a rotary drive arrangement. In a crude friction wheel arrangement, the rim of one friction wheel frictionally engages the rim of another and the drive ratio D is determined by the ratio of their diameters. D may accordingly be any value, including an irrational number. Since the rims are circular, there is no sliding friction between them, unless they slip under load; the corollary is that they lack the mechanical engagement between non-tangential surfaces which would ensure positive drive. Nevertheless, continuously variable transmissions (CVTs) such as toroidal CVTs have employed rollers disposed between and frictionally transferring rotary motion between facing toroidal discs whose surfaces are profiled to allow the drive ratio to be varied by varying the contact regions between each roller and the toroidal discs. Gears have the opposite combination of advantage and disadvantage, namely sliding friction between the teeth surfaces, but positive drive between the (non-tangential) engaging surfaces of the interdigitating teeth. This is true of both involute and cycloidal gear profiles and both spur gears and helical gears. The drive ratio is determined by the numbers of teeth and this can impose design compromises if the number of teeth is limited, eg in calendar complications in horology. In order to avoid mutual scraping between the gear teeth it is conventional to truncate the tips of the gear teeth to form a top land, i.e. a flat, tangential peripheral surface, which defines the tip circle and is designed to avoid contact with any surface, particularly the dedendum, of the meshing gear teeth. Thus the peripheral surface of a gear tooth is normally designed to be non-engaging with any other surface. An object of the present invention is to provide a novel rotary element which at least to some degree combines advantages of both friction wheels and gears but which overcomes or alleviates some of their disadvantages. The invention provides a rotary element for a rotary transmission, the rotary element having at least one helical radial projection disposed about a rotary axis thereof, the helical radial projection having a helical peripheral surface which defines an envelope of the rotary element, the helical 2 peripheral surface having a profile in a radial plane which is inclined to a tangent to said envelope, whereby in use with another such rotary element of opposite handedness in a rotary transmission, a region of rolling contact of the helical peripheral surface with a helical peripheral surface of the other such rotary element helically traverses the helical peripheral surfaces to positively transmit rotary drive between the rotary elements. The inclined profile of the helical peripheral engaging surface enables the helical radial projection of one such rotary element to push the helical radial projection of another such rotary element and thus achieve positive drive. This is illustrated in various embodiments in Figures 6 and 8, and in Figures 9A, 9B, 14 and 16, as discussed in more detail below. Preferably said helical peripheral surface has a convex profile in said radial plane. This feature facilitates rolling motion and reduces sliding friction in use. Preferably said profile is elliptical. This feature minimises sliding friction, theoretically to zero in preferred rotary drive arrangements. This is illustrated in Figures 6, 9A and 16, as discussed in more detail below. Preferably the rotary element has a plurality of such helical radial projections forming separate turns or separate partial turns. This feature provides a plurality of contact regions in use, which spreads the load and tends to reduce wear. Preferably the helical radial projections are regularly spaced apart along the rotary axis. Preferably the helical radial projections are partial turns extending from one end face of the rotary element to another and are regularly spaced around the circumference of the rotary element in circumferentially overlapping fashion. This feature enables the length of the helical radial projections to be reduced for a given helix pitch and thereby enables the length (thickness) of the radial element to be reduced, which is an important consideration in some applications, eg watchmaking. In a preferred embodiment the at least one helical radial projection has a transverse cross-section comprising two flank sides on either side of said helical peripheral engaging surface. Unlike the flank sides of a gear tooth, the flank sides of such an embodiment are not required to engage with the flank sides of any other radial element in use, and can optionally be inclined inwardly towards 3 the helical peripheral surface to maximise the strength of the helical radial projection. They can optionally be left unfinished in order to reduce manufacturing costs. Preferably the helical peripheral surface is selectively treated by hardening or polishing or both to reduce rolling friction. For example, a steel radial projection can have its helical peripheral surface hardened by carburising. In low cost applications, the or each rotary element can optionally be formed of plastics material, eg polyamide. In certain embodiments said helical peripheral surface has an envelope which is tapered in the axial direction. In particular, said envelope is optionally frusto-conical. The invention also provides a rotary drive arrangement comprising first and second rotary elements mounted for rotation about respective rotary axes thereof and having helical radial projections disposed about respective rotary axes of the rotary elements, the rotary elements being of opposite handedness, the helical radial projection of each rotary element having a helical peripheral surface which defines an envelope of that rotary element, the helical peripheral surface having a profile in a radial plane which is inclined to a tangent to said envelope, wherein in use a region of rolling contact of the respective helical peripheral surfaces helically traverses the helical peripheral surfaces to positively transmit rotary drive between the rotary elements. Preferably the rotary elements are as defined above. In some embodiments the ratio of the radii from the respective rotary axes of the first and second rotary elements to a point of rolling contact is D such that D > 1.0 or D < 1.0. In some embodiments the ratio of the radii from the respective rotary axes of the first and second rotary elements to a point of rolling contact is D such that D is an irrational number. Preferably the respective helical radial projections have leads inversely proportional to the respective radii from their rotary axes to the centre of the region of rolling contact. This feature ensures that engagement between the helical peripheral projections is maintained throughout multiple revolutions of both rotary elements; in general the faster rotation of the smaller diameter 18 09 24 rotary element is compensated by the correspondingly greater lead of the helix defining its helical peripheral radial projection. Other preferred features are defined in claims 15 and 16. In a preferred embodiment the respective peripheral surfaces of the helical radial projections have elliptical profiles defined by ellipses centred on the respective rotary axes of the rotary elements which ellipses touch at a point of rolling contact, the ellipses having the same major diameter and same minor diameter and the respective rotary axes being spaced apart by the sum of said major and minor diameters. This feature reduces sliding friction between the helical radial projections, theoretically to zero. In an embodiment the relative axial positions of the rotary elements are variable so as to vary the drive ratio. Optionally said helical peripheral engaging surfaces are unlubricated. In an embodiment the rotary elements are bevel in form and their rotary axes are inclined towards each other in a common plane. Preferred embodiments of the invention are described below by way of example only with reference to Figures 1 to 16 of the accompanying drawings, wherein: Figure 1 is a side elevation of a rotary element in accordance with the invention; Figure 2 is a diagrammatic cross-section taken on ll-ll of Figure 1; Figure 3 is a diagrammatic cross-section taken on Ill-Ill of Figure 1; Figure 4 is a diagrammatic cross-section taken on IV-lV of Figure 1; Figure 5 is a side elevation of a rotary drive arrangement in accordance with the invention, showing continuous rolling engagement of the helical peripheral engaging projections of two rotary elements as shown in Figure 1; Figure 6 is an end elevation taken on VI-VI of Figure 5; Figure 7 is a diagrammatic perspective view showing the engagement of the helical radial projections of the rotary drive arrangement of Figures 5 and 6; Figure 8 is a diagrammatic cross-section in the common axial plane of Figure 5 showing the positive driving engagement of one turn of the helical radial projections thereof; Figure 9A is a diagrammatic cross-section in the axial direction showing a variant of the rotary drive arrangement shown in Figure 6 in which the rotary elements have unequal radii; Figure 9B is diagrammatic cross-section in the axial direction of a variant of the rotary drive arrangement shown in Figure 9A; Figure 10 is a diagrammatic side elevation of the variant shown in Figure 9A; Figure 11 is a diagrammatic side elevation of a rotary drive arrangement in accordance with the invention in which each rotary element has two radial projections axially spaced apart by half a turn; Figure 12 is a diagrammatic perspective view of a further rotary element in accordance with the invention, having multiple helical radial projections; Figure 13 is an end elevation of the rotary element of Figure 12; Figure 14 is a diagrammatic end elevation of a rotary drive arrangement in accordance with the invention showing two rotary elements as shown in Figures 12 and 13 positively engaging without interdigitation; Figure 15 is a side elevation of a frusto-conical rotary drive arrangement in accordance with the invention; 18 09 24 Figure 16 is a diagrammatic section on XVI-XVI of Figure 15, showing the geometry of the radial profiles of the helical peripheral surfaces 5 of the rotary elements 1C and 2C of Figure 15. In the following description of Figures 1 to 11, the leading edge of a helical projection will be considered to be the right-hand edge (the trailing edge being the left-hand edge) and the sense, ie handedness, of a helical projection will be considered as viewed from left to right. In the following description generally, what in practice will be a small region of rolling contact will be described as a point of rolling contact because that is consonant with the geometry of the described embodiments when the helical peripheral surfaces 5 are not compressed by a transmitted torque. Referring to Figure 1, the rotary element 1 comprises a shaft 3 mounted for rotation about its axis and a parallel-sided helical radial projection 4 secured to the shaft. The helical radial projection 4 forms one and a half turns and its sense is right-handed. The leading edge 4Aof helical radial projection 4 defines a cylindrical envelope E and the trailing edge 4B defines a similar cylindrical envelope (not shown) of lesser diameter which is coaxial with envelope E. A helical peripheral radial surface 5 formed between leading and trailing edges 4A and 4B is inclined radially inwardly from leading edge 4A towards the trailing edge 4B, as best shown in Figure 4. As shown in Figure 3, the lateral cross-section through the helical radial projection 4 exhibits a convex profile of the helical peripheral radial surface 5. Consequently, as shown in Figure 2, helical peripheral radial surface 5 is convex in the radial plane, and as shown in Figure 4, in the axial plane also. Thus helical peripheral surface 5 is biconvex. As will be explained below, helical peripheral surface 5 is capable of engaging a helical peripheral surface 5 of a similar rotary element at, nominally, a contact point whose locus PT runs along the helical peripheral surface is shown in Figure 1. It should be noted that locus PT is a helix and lies between the leading and trailing edges 4A and 4B of the helical peripheral radial surface 5 over the entirety of the length of locus PT. Only rolling friction occurs between two mutually engaging rotary elements, as will become apparent from the description of Figures 5 and 6 below. Conversely, in a conventional meshing gear arrangement, the locus of the contact point or contact region moves across the leading edge of a flank surface of a gear tooth of one gear as that tooth ends its engagement with a meshing tooth of the other gear. This results in sliding friction. Referring to Figure 2, the profile of helical peripheral surface 5 is inclined in the radial plane, i.e. the plane of Figure 2, relative to its tangent to the envelope of the rotary element 1; for example the centre of the profile of helical peripheral surface 5 which cuts axial plane AX is inclined to tangent T to the envelope E. This inclination enables positive drive between two rotary drive elements, as will become further apparent from the description of Figure 6, and contrasts with the symmetrical arcuate profile FW (shown in phantom in the Figure) of a friction wheel, which cannot positively transmit drive. It also contrasts with the symmetric profile G of a gear tooth, also shown in phantom in Figure 2. Additionally it will be noted that the peripheral surface of a gear tooth is normally a land L as shown, which is specifically designed not to engage any part of the teeth on a meshing gear (not shown) and thus has the opposite function of helical peripheral surface 5 in embodiments of the present invention. In preferred embodiments the contact region (nominally a point in the absence of any distortion of surface 5 under load) lies on the axial plane AX in which the axes of both rotary elements lie. This feature is discussed in more detail below in connection with Figures 9A and 9B. As shown in Figure 6, the preferred profile of peripheral radial surface 5 in the radial plane is convex, eg elliptical. This feature is considered to minimise sliding friction, ideally to zero, but in less preferred embodiments this profile need not be elliptical and in still less preferred embodiments need not be convex. Departures from an elliptical profile geometry shown in Figures 6 and 9A will still achieve positive drive but may result in a degree of sliding friction along the helical radial projections 4. As noted above and as will become apparent from the description of Figure 6, positive drive between the rotary elements is achieved as a result of the inclination of the profile of helical peripheral surface 5 in the radial plane. The engagement between rotary element 1 and a similar rotary element 2 is shown in Figures 5 and 6. Rotary element 2 is left-handed and is thus of opposite sense to rotary element 1. It will be seen that the respective helical radial projections 4 of the rotary elements 1 and 2 engage at a point P which defines equal radii R of the rotary elements. Rotary element 1 rotates anticlockwise to drive rotary element 2 clockwise, as indicated by arrows a. The common tangent TA (Figure 6) to helical peripheral surfaces 5 at P is inclined to the (vertical) normal to the common (horizontal) plane of the axes of the rotary elements 1 and 2, i.e. it is inclined to the plane of the drawing in Figure 5. This inclination of the helical peripheral drive surface 5 in the radial plane ensures positive drive between the the rotary elements 1 and 2 in the directions of rotation a shown in Figures 5 and 6. In this arrangement, rotary element 1 is the driving element and rotary element 2 is the driven element. It will be noted that the driving engagement between the helical radial projections 4 occurs without interdigitation of the helical projections 4, unlike in gear drives. The ellipses E1 and E2 shown in Figure 6 each have a major diameter D and a minor diameter d, the separation S (S = 2R) between their centres being 72 (D + d) such that they touch at a point P lying in their common axial plane (corresponding to plane AX in Figure 2) and on a common tangent TA. Accordingly it will be appreciated that if the rotary element 1 had a cross-section of the complete ellipse E1 and element 2 similarly had a cross-section of the complete ellipse E2 as shown, then rotation of ellipse E1 anticlockwise would rotate ellipse E2 at an increasing drive ratio until the major diameter of ellipse E1 becomes vertical and the major diameter of ellipse E2 becomes horizontal. At this point their common tangent TA would become horizontal and the the ellipse E1 would slide over ellipse E2. i.e. positive drive would be lost. However, in the rotary drive arrangement shown in Figure 6, peripheral engaging surface 5 of rotary element 1 has a profile corresponding to ellipse E1 in the radial plane of Figure 6 and peripheral engaging surface 5 of rotary element 2 has a profile corresponding to ellipse E2 in that radial plane and both peripheral engaging surfaces are helical. Accordingly, as the rotary element 1 rotates anticlockwise as indicated by arrow a, it drives rotary element 2 clockwise and the turns of the respective helical projections 4 of the two rotary elements move in the fashion of the stripes on a barber's pole to maintain their engagement - in other words the situation shown in Figure 6 is maintained but the plane of Figure 6 and associated point of contact P move axially left (in Figure 5). When a pair of engaging turns reaches the left end of the rotary elements 1 and 2 and they 9 disengage, drive is maintained by a succeeding pair of engaging turns and then a further pair of turns at the right end make contact and a continuous cycle is maintained with a constant drive ratio (in this case 1:1). The locus PT (Figure 1) of point of contact P remains between the leading and trailing edges 4A and 4B and there is no sliding friction between the rotary elements. This axial movement of the point of contact P is illustrated in Figure 7, which is an oblique diagrammatic view of the engaging turns of the helical radial projections 4 shown in Figure 6. In the configuration shown in Figure 7, the helical peripheral surfaces 5 of the respective upper and lower rotary elements 1 and 2 engage in a coincident radial plane corresponding to sections PL1 and PL2. This radial plane is the plane of Figure 6. As upper rotary element 1 rotates anticlockwise as indicated by the upper arrow a, its helical radial projection 4 drives the helical radial projection 4 of the lower rotary element 2 clockwise as indicated by the lower arrow a. Consequently, sections PL1 and PL2 and their associated contact point P move rearwardly as viewed in Figure 7 or axially to the left as viewed in Figure 5. Figure 8 shows the location of the contact point P (indicated as a black dot for clarity) in more detail, lying behind the section plane through the upper helical radial projection 4 and on the intersection between the upper and lower helical peripheral engaging surfaces 5. Accordingly, rotation of upper rotary element 1 as indicated by upper arrow a will result in upper peripheral engaging surface 5 pushing lower peripheral engaging surface 5 at point P in the opposite direction of rotation indicated by lower arrow a. Thus positive drive is achieved. It will be appreciated that in practice when an appreciable torque is being transmitted, there will be some compression of the helical peripheral engaging surfaces 5 and what is nominally a point P will in practice be a region of contact. Figure 9A shows a variant of the rotary drive arrangement shown in Figure 6 in which rotary elements 1A and 2A are based on the same ellipses E1 and E2 as shown in Figure 6, with the same spacing between the shafts 3 on which the ellipses are centred, but with a further rotation of the ellipses to a configuration in which their point of contact P is located a position corresponding to unequal radii R1 and R2 of the respective ellipses E1 and E2. Thus upper rotary element 1A is of smaller diameter than lower rotary element 2A. Point of contact P lies in the common plane of the rotary axes of rotary elements 1A and 2A. The helical radial projections 4 of the respective rotary elements 1A and 2A have peripheral engaging surfaces 5 whose profiles correspond to the respective ellipses E1 and E2. The ratio R1 / R2 = D where D is optionally an irrational number. In general, D is determined by the configuration of the ellipses E1 and E2 and can vary between d / D and D / d where D and d are the major and minor diameters of the ellipses (see Figure 6). In certain embodiments, D > 1.0 or D < 1.0. It should be noted that the drive ratio can be varied continuously by varying the relative axial positions of the rotary elements, which (assuming their mutual engagement is maintained) effectively varies the angular configuration of the ellipses E1 and E2 (compare Figures 6 and 9A) and hence the position of the contact point P, which remains in the common axial plane of the rotary elements but moves closer to the axis of one rotary element and away from the other as the ellipses E1 and E2 are moved to an angular configuration different from that shown in Figure 6 to eg that shown in Figure 9A. In a variant shown in Figure 9B, the helical radial projection 4A of the upper rotary element has a peripheral surface 5A whose profile in the radial plane is flat, i.e. a straight line, rather than convex, but aligned with the corresponding portion of ellipse E1. Similarly the helical radial projection 4B of the lower rotary element has a peripheral surface 5B which is flat, i.e. a straight line, rather than convex, but aligned with the corresponding portion of ellipse E2. Ellipse E1 has its centre coincident with the axis of the upper rotary element i.e. upper shaft 3 and ellipse E2 has its centre coincident with the axis of the lower rotary element i.e. lower shaft 3. The location of point of contact P is the same as in the embodiment of Figure 9A, as is its locus along the peripheral surfaces 5A and 5B. Thus convexity is not essential. However the embodiment of Figure 9A is preferred over that of Figure 9B because the load at contact point P is better distributed. Referring to Figure 10, a drive arrangement utilising rotary elements 1A and 2A is shown in more detail. Because each rotation of rotary element 1A will rotate rotary element 2A by R1 / R2 turns, the lead L2 of rotary element 2A is R2 / R1 x L1, the lead of rotary element 1 A, in order to keep the turns in engagement contact point P, as shown. Thus rotation of rotary element 1A anticlockwise (Figure 9) will rotate rotary element 2A clockwise (Figure 9) as indicated by arrows a in Figures 9 and 10. This rotation will move contact point P (Figure 10) axially to the left in Figure 10. 11 It will be noted that the larger diameter rotary element 2A has more than one turn in order to enable contact throughout multiple revolutions of rotary element 2A. This has implications for the axial length of the rotary element for a given helix pitch angle of its helical radial projection 4. One way of reducing the required axial length of a rotary element is to include more than one helical radial projection 4. Such an embodiment is shown in Figure 11, which shows rotary elements 1B and 2B mounted for rotation about their rotary axes on bearings B, the bearings B being mounted on a frame 11 which maintains the spacing between the axes at spacing S (Figure 6). Each rotary element 1B and 2B has two helical radial projections 4A and 4B with helical peripheral surfaces 5 having diameters and elliptical profiles as shown in Figure 6. The rotary elements 1B and 2B are mirror images of each other and their helical radial projections 4 are separated in the axial direction by half a turn. Their helical peripheral surfaces engage at two contact points similarly separated by half a turn. Figures 12 and 13 show a further embodiment of a rotary drive element 10 in which there are multiple helical radial projections 4A, 4B, 4C, 4D, 4E, 4F,.....which are regularly circumferentially distributed distributed and have a common pitch such that there is overlap in the circumferential direction between the termination of each helical radial projection and the start of the immediately adjacent succeeding radial projection, as shown. Each helical radial projection has a peripheral surface having an asymmetric elliptical profile in the radial plane, similar to that shown in eg Figure 6. Accordingly a rotary drive arrangement as shown in Figure 14 comprises an upper rotary element 10 as shown in Figures 12 and 13 and a lower rotary element 20 which is a mirror image of the upper rotary element. Rotation of the upper rotary element 10 in the clockwise direction indicated by upper arrow a drives the lower rotary element 20 anticlockwise as indicated by lower arrow a. The helical peripheral radial surfaces 5 of engage with rolling contact in the same manner as indicated in Figures 5 and 6, and drive is transferred positively without interdigitation of the helical projections. In particular, the helical projection 4C of upper rotary element 10 is shown aligned with a corresponding helical projection 4c of lower rotary element 20, and the helical projection 4D with the corresponding helical projection 4d. 18 09 24 Figure 15 shows a bevel gear arrangement in which an upper, driving rotary element 1C in accordance with the invention in mounted for rotation about an axis X1 and has a helical radial projection 4 located on a frusto-conical support 40. A mirror image lower, driven rotary element 2C in mounted for rotation about an axis X2 which is inclined towards X1 and has a similar helical radial projection 4 which engages the helical radial projection 4 of rotary element 1C at multiple contact points P as shown. Each helical radial projection 4 has a helical peripheral surface 5 of elliptical profile and the geometry of the helical peripheral surfaces 5 at their contact points P is shown in Figure 16 (similar to Figure 6). The ellipses E1 and E2 shown in Figure 16 each have a major diameter D and a minor diameter d, the separation S (S = 2R) between their centres being 1 / 2 (D + d) such that they touch at a point P lying in their common axial plane, as also shown in Figure 15. The directions of rotation of the rotary elements 1C and 2C are indicated by arrows a. Since the spacing between the axes X1 and X2 of the rotary elements 1C and 2C decreases in the axial direction from right to left in Figure 15, the corresponding spacing S between the identical ellipses E1 and E2 will decrease similarly. Accordingly the size of the ellipses E1 and E2 will decrease similarly and hence the profile of the helical peripheral surfaces 5 will vary accordingly. In general, the elliptical profile of each helical peripheral surface at a given axial position will be determined by the spacing S between the axes X1 and X2 at that axial position, which will determine the size of ellipses E1 and E2 and hence the precise profile of surfaces 5 at that axial position. In the more general case that the frusto-conical rotary elements are of different size, i.e. non-unity drive ratio, then the profile at any given spacing S will be determined by a geometry analogous to Figure 9A, with a gradual reduction of the helix pitch of each radial projection 4 from the large diameter ends to the small diameter ends of the the frusto-conical rotary elements. Output shafts 3, secured to respective rotary elements 3D and 4D extend through apertures in the sides of cage CG and may be coupled to eg the road wheels of a vehicle.

Claims

1. A rotary element for a rotary transmission, the rotary element having at least one helical radial projection disposed about a rotary axis thereof, the helical radial projection having a helical peripheral surface which defines an envelope of the rotary element, the helical peripheral surface having a profile in a radial plane which is inclined to a tangent to said envelope, whereby in use with another such rotary element having a helical radial projection of opposite handedness in a rotary transmission, a region of rolling contact of the helical peripheral surface with a helical peripheral surface of the other such rotary element helically traverses the helical peripheral surfaces to positively transmit rotary drive between the rotary elements.

2. A rotary element according to claim 1, wherein said helical peripheral surface has a convex profile in said radial plane.

3. A rotary element according to claim 2 wherein said profile is elliptical.

4. A rotary element according to any preceding claim having a plurality of such helical radial projections forming separate turns or separate partial turns.

5. A rotary element according to claim 4 wherein the helical radial projections are regularly spaced apart along the rotary axis.

6. A rotary element according to claim 5 wherein the helical radial projections are partial turns extending from one end face of the rotary element to another and are regularly spaced around the circumference of the rotary element in circumferentially overlapping fashion.

7. A rotary element according to any preceding claim wherein the at least one helical radial projection has a transverse cross-section comprising two flank sides on either side of said helical peripheral engaging surface.

8. A rotary element according to claim 7 wherein the helical peripheral surface is selectively treated by hardening or polishing or both to reduce rolling friction.

9. A rotary element according to any preceding claim wherein said helical peripheral surface has an envelope which is tapered in the axial direction.18 09 2410. A rotary drive arrangement comprising first and second rotary elements mounted for rotation about respective rotary axes thereof and having helical radial projections disposed about respective rotary axes of the rotary elements, the rotary elements being of opposite handedness, the helical radial projection of each rotary element having a helical peripheral surface which defines an envelope of that rotary element, the helical peripheral surface having a profile in a radial plane which is inclined to a tangent to said envelope, wherein in use a region of rolling contact of the respective helical peripheral surfaces helically traverses the helical peripheral surfaces to positively transmit rotary drive between the rotary elements.

11. A rotary drive arrangement according to claim 10 wherein the rotary elements are as defined in any of claims 2 to 9.

12. A rotary drive arrangement according to claim 10 or claim 11 wherein said region of rolling contact remains in a common plane of said rotary axes.

13. A rotary drive arrangement according to claim 10 or claim 11 or claim 12 wherein the ratio of the radii from the respective rotary axes of the first and second rotary elements to a point of rolling contact is D such that D > 1.0 or D < 1.0.

14. A rotary drive arrangement according to claim 13 wherein the ratio of the radii from the respective rotary axes of the first and second rotary elements to a point of rolling contact is D such that D is an irrational number.

15. A rotary drive arrangement according to any of claims 10 to 13 wherein the respective helical radial projections have leads inversely proportional to the respective radii from their rotary axes to the centre of the region of rolling contact.

16. A rotary drive arrangement according to any of claims 10 to 15 wherein the helical radial projections of the respective rotary elements engage at two or more axially spaced regions of rolling contact during said rotary motion.

17. A rotary drive arrangement according to claim 16 wherein the helical radial projections of the respective rotary elements engage at two or more axially spaced regions of rolling contact throughout said rotary motion.

18. A rotary drive arrangement according to any of claims 10 to 17 wherein the respective peripheral surfaces of the helical radial projections have elliptical profiles defined by ellipses centred on the respective rotary axes of the rotary elements which ellipses touch at a point of rolling contact, the ellipses having the same major diameter and same minor diameter and the respective rotary axes being spaced apart by the sum of said major and minor diameters.

19. A rotary drive arrangement according to claim 18 wherein said helical peripheral surfaces are unlubricated.

20. A rotary drive arrangement according to any of claims 10 to 19 wherein the relative axial positions of the rotary elements are variable so as to vary the drive ratio.

21. A rotary drive arrangement according to any of claims 10 to 19 wherein the drive ratio is constant.CM22. A rotary drive arrangement according to any of claims 10 to 21 wherein the rotary elements are bevel in form and their rotary axes are inclined towards each other in a common plane.CO

Images