Rotary element for transmitting rotary motion

Abstract

A rotary element 1E for a rotary transmission has two coaxial helical radial projections 40A, 40B disposed about a rotary axis thereof, the helical radial projections having leading and trailing edges

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. One such arrangement is disclosed in co-pending UK patent application GB2406339.5, due to be published as GB2627593A. The rotary element disclosed in that application has at least one helical peripheral surface which defines an envelope of the rotary element and has a profile in a radial plane which is inclined to a tangent to the envelope. The at least one helical peripheral surface provides positive unidirectional drive to a corresponding helical peripheral surface of another such rotary element of opposite handedness in a rotary drive arrangement comprising first and second such rotary elements. The present invention provides a rotary element for a rotary transmission, the rotary element having at least first and second helical radial projections disposed coaxially about a rotary axis thereof, the helical radial projections being axially spaced apart and having respective first and second helical peripheral surfaces which define a common envelope of the rotary element, each helical peripheral surface having a profile in a radial plane which is inclined to a tangent to said envelope and the inclinations of the first and second helical peripheral surfaces to the tangent being of opposite sense, whereby in use with another such rotary element in a rotary transmission, the helical peripheral surfaces of the rotary element engage with helical peripheral surfaces of complementary inclination of the other rotary element to positively transmit bidirectional rotary drive between the first and second rotary elements at regions of rolling contact which regions of rolling contact helically traverse the helical peripheral surfaces. The inclined profile of the helical peripheral engaging surfaces enables the helical radial projections of one such rotary element to push corresponding helical radial projections of another such rotary element and thus achieve positive drive. The opposite inclinations of the helical peripheral surfaces enable bidirectional positive drive. This is illustrated in Figures 1 to 3, as discussed in more detail below. Preferably said helical peripheral surfaces have a convex profile in said radial plane. This feature facilitates rolling motion and reduces sliding friction in use. Preferably said profile is elliptical. In a preferred embodiment the first and second helical radial projections each have a transverse cross-section comprising two flank sides on either side of a said helical peripheral 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 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 surfaces are 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. Optionally, said envelope is tapered in the axial direction. In particular, said envelope is optionally frusto-conical, for example. Preferably said inclination is in the range 5° to 45°, more preferably in the range 10° to 30°. Optionally the angles of inclination of the respective helical peripheral surfaces are different from each other. 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 plurality of helical radial projections are regularly spaced apart along the rotary axis. Optionally 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. 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. The invention also provides a rotary drive arrangement comprising mutually engaged first and second rotary elements mounted for rotation about respective rotary axes thereof, each rotary element having at least first and second helical radial projections disposed coaxially about a rotary axis thereof, the first and second helical radial projections being axially spaced apart and having respective first and second helical peripheral surfaces whose profiles in a radial plane are oppositely inclined with respect to a tangent to a common envelope thereof, wherein the helical peripheral surfaces of the first rotary element engage with helical peripheral surfaces of the second rotary element of complementary inclination to positively transmit bidirectional rotary drive between the first and second rotary elements at regions of rolling contact which regions of rolling contact helically traverse the helical peripheral surfaces. Preferably the rotary elements are as defined above. Preferably said regions of rolling contact remain in a common plane of said rotary axes. This feature minimises friction. In certain embodiments the ratio of the radii from the respective rotary axes of the first and second rotary elements to points of rolling contact is D such that D >1.0 or D <1.0. Optionally the ratio of the radii from the respective rotary axes of the first and second rotary elements to points of rolling contact is D such that D is an irrational number. Optionally the helical radial projections of the first and second rotary elements have leads inversely proportional to the respective radii from their rotary axes to the centre of the regions of rolling contact. Optionally the helical radial projections of the respective rotary elements engage at four or more axially spaced regions of rolling contact during rotary drive. Optionally the helical radial projections of the respective rotary elements engage at four or more axially spaced regions of rolling contact throughout said rotary drive. Preferably the respective peripheral surfaces of the first helical radial projections of the first and second rotary elements have elliptical profiles defined by ellipses centred on the respective rotary axes of the first and second rotary elements which ellipses touch at a point of rolling contact of the first helical radial projections, the ellipses having the same major diameter and same minor diameter and the respective rotary axes being spaced apart by half the sum of said major and minor diameters. Preferably the respective peripheral surfaces of the second helical radial projections of the first and second rotary elements have elliptical profiles defined by ellipses centred on the respective rotary axes of the first and second rotary elements which ellipses touch at a point of rolling contact of the second helical radial projections, the ellipses having the same major diameter and same minor diameter and the respective rotary axes being spaced apart by half the sum of said major and minor diameters. These features minimise friction. Optionally said helical peripheral surfaces are unlubricated. Optionally the rotary elements are bevel in form and their rotary axes are inclined towards each other in a common plane. Optionally the rotary elements are bevel in form and the rotary drive arrangement is a differential. In an embodiment the relative axial positions of the rotary elements are variable so as to vary the drive ratio. Preferred embodiments of the invention are described below by way of example only with reference to Figures 1 to 5 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 similar to Figure 2 showing the geometry of the peripheral surfaces 5 of the helical radial projections in Figure 1 and other preferred embodiments, and Figure 5 is a somewhat diagrammatic side elevation of a further rotary drive arrangement in accordance with the invention in which the radii from the respective drive element axes to the points of contact are unequal. 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 1E comprises a shaft 3 mounted for rotation about its axis and two axially spaced-apart oppositely-handed helical radial projections 40A and 40B secured to the shaft. The helical radial projections 40A and 40B are each parallel-sided. The helical radial projections in this embodiment 40Aand 40B each form one and a half turns, helical radial projection 40A being right-handed and helical radial projection 40B being left-handed. Each helical radial projection 40A and 40B has a helical peripheral radial surface 5 formed between its leading and trailing edges 4A and 4B, each helical peripheral radial surface 5 being inclined radially inwardly from leading edge 4A towards the trailing edge 4B, as best shown in Figures 2 and 3. Each helical peripheral surface 5 is convex in the axial plane as shown in Figure 1. In this embodiment, a common envelope E of the helical peripheral surfaces 5 of rotary element 1E corresponds to the envelope of the leading edges 4A but in other embodiments (not shown) in which the radial inclination of the helical peripheral surfaces is lower and / or the convexity of the helical peripheral surfaces 5 is more pronounced it could correspond to a radial extremity of the helical peripheral surfaces intermediate leading and trailing edges 4Aand 4B. 6 In the rotary drive arrangement shown in Figure 1, rotary element 2E is identical to rotary element 1E but reversed in the axial direction whereby each of its helical peripheral surfaces 5 engages a complementary helical peripheral surface 5 of rotary element 1E. Thus the helical peripheral surfaces 5 of the first rotary element 1E engage with helical peripheral surfaces 5 of the second rotary element 2E of complementary inclination in the radial plane. This is best seen in Figures 2 and 3, which show oppositely inclined helical peripheral surfaces 5 engaging at contact points P which lie in the common plane of the axes of shafts 3. Accordingly, referring to Figure 2, anticlockwise rotation (indicated by arrow a) of upper rotary element 1E will cause its helical projection 40Ato drive helical projection 40B and hence lower rotary element 2E clockwise. Similarly, referring to Figure 3, clockwise rotation (indicated by arrow b) of upper rotary element 1E will cause its helical projection 40B to drive helical projection 40A and hence lower rotary element 2E anticlockwise. Thus upper rotary element 1E can be used to drive lower rotary element 2E bidirectionally, or vice versa, with no rotational play, unlike the situation with conventional gearing. However if a degree of lost motion between the upper and lower rotary elements 1E and 2E is required, one or both of helical projections 40A and 40B may be angularly offset, eg as indicated by angularly offset helical projection 40A’ in Figure 3. In that variant, clockwise rotation of upper rotary element 1E as indicated by arrow b will disengage helical projections 40A and 40B of Figure 2 (thus having no effect on lower rotary element 2E) and engage helical projections 40A’ and 40B of Figure 3 only when projection 40A’ reaches the 12 o’clock position. Referring now to Figure 1, the loci PT of the contact points P during rotary drive run along the helical peripheral surfaces 5. It should be noted that loci PT are helices and lie between the leading and trailing edges 4A and 4B of the helical peripheral radial surface 5 over the entirety of the length of loci PT. Rolling rather than sliding friction occurs between two mutually engaging rotary elements 1E and 2E. 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 interdigitation results in appreciable sliding friction. 7 Referring now to Figure 4, the tangent T to the envelope E (closely adjacent contact point P) of upper rotary element 1E and the common tangent TA to the helical peripheral surfaces of the helical projections subtend an angle 0 in the radial plane. 0 is preferably in the range 5° to 45°, more preferably in the range 10° to 30°. If 0 is too small, there is a risk of one or other of the helical radial projections distorting and slipping over the other under load, whereas if 0 is too large, undue friction may occur between the helical peripheral surfaces. In the embodiment of Figures 1 to 3, the inclinations of the tangents (TA in Figure 4) of the helical peripheral surfaces 5 relative to the tangent (T in Figure 3) to the envelope are equal and opposite. However in a variant, the inclinations may be opposite but unequal. As shown in Figure 4, the helical peripheral surfaces of the upper and lower rotary elements 1E and 2E are preferably elliptical, ie defined by identical ellipses E1 and E2 respectively. The spacing between centres c of the rotary axes of the upper and rotary elements is equal to half the sum of the major and minor diameters of ellipses E1 and E2, which corresponds to a contact point P located on the line joining centres C. This is optimal for reducing friction. It will be noted that the inclination of the common tangent TA relative to envelope tangent T can be varied by varying the eccentricity of the ellipses E1 and E2, ie the ratio of their major and minor diameters. However an elliptical profile of helical peripheral surfaces 5 is not essential. Figure 5 shows a variant of the rotary drive arrangement shown in Figure 1 in which radius R1 of upper rotary element 1F is about 50% larger than radius R2 of lower rotary element 2F. To compensate for this, ie to ensure continuous engagement between the helical radial projections 4C and 4D of upper rotary element 1F with the helical radial projections 4c and 4d respectively of lower rotary element 2F, the lead L1 of the upper helical radial projections is about 50% greater than the lead L2 of the lower helical radial projections. In general, R1 / L1 = R2 / L2.The drive ratio R1 / R2 may optionally be an irrational value. Optionally, in view of the virtual absence of sliding friction, the helical peripheral surfaces 5 may be unlubricated.

Claims

1. A rotary element for a rotary transmission, the rotary element having at least first and second helical radial projections disposed coaxially about a rotary axis thereof, the helical radial projections being axially spaced apart and having respective first and second helical peripheral surfaces which define a common envelope of the rotary element, each helical peripheral surface having a profile in a radial plane which is inclined to a tangent to said envelope and the inclinations of the first and second helical peripheral surfaces to the tangent being of opposite sense, whereby in use with another such rotary element in a rotary transmission, the helical peripheral surfaces of the rotary element engage with helical peripheral surfaces of complementary inclination of the other rotary element to positively transmit bidirectional rotary drive between the first and second rotary elements at regions of rolling contact which regions of rolling contact helically traverse the helical peripheral surfaces.

2. A rotary element according to claim 1 wherein said helical peripheral surfaces have 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 wherein the first and second helical radial projections each have a transverse cross-section comprising two flank sides on either side of a said helical peripheral surface.

5. A rotary element according to any preceding claim wherein the helical peripheral surfaces are selectively treated by hardening or polishing or both to reduce rolling friction.

6. A rotary element according to any preceding claim wherein said envelope is tapered in the axial direction.

7. A rotary element according to any preceding claim wherein said inclination is in the range 5° to 45°.

8. A rotary element according to claim 7 wherein said inclination is in the range 10° to 30°.

9. A rotary element according to any preceding claim wherein the angles of inclination of the respective helical peripheral surfaces are different from each other.

10. A rotary drive arrangement comprising mutually engaged first and second rotary elements mounted for rotation about respective rotary axes thereof, each rotary element having at least first and second helical radial projections disposed coaxially about a rotary axis thereof, the first and second helical radial projections being axially spaced apart and having respective first and second helical peripheral surfaces whose profiles in a radial plane are oppositely inclined with respect to a tangent to a common envelope thereof, wherein the helical peripheral surfaces of the first rotary element engage with helical peripheral surfaces of the second rotary element of complementary inclination to positively transmit bidirectional rotary drive between the first and second rotary elements at regions of rolling contact which regions of rolling contact helically traverse the helical peripheral surfaces.

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 regions of rolling contact remain 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 points 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 points of rolling contact is D such that D is an irrational number.

15. A rotary drive arrangement according to claim 13 or claim 14 wherein the helical radial projections of the first and second rotary elements have leads inversely proportional to the respective radii from their rotary axes to the centre of the regions 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 four or more axially spaced regions of rolling contact during said rotary drive.

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

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

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

20. A rotary drive arrangement according to any preceding claim wherein said helical peripheral surfaces are unlubricated.

21. A rotary drive arrangement according to any of claims 10 to 20 wherein the rotary elements are bevel in form and their rotary axes are inclined towards each other in a common plane.

22. A rotary drive arrangement according to any of claims 10 to 21 wherein the rotary elements are bevel in form and the rotary drive arrangement is a differential.

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