WO2013179000A1

WO2013179000A1 - A worm drive, a kit, a device and a fixing method

Info

Publication number: WO2013179000A1
Application number: PCT/GB2013/051364
Authority: WO
Inventors: David Roberts Mcmurtry; Stephen James Trull
Original assignee: Renishaw Plc
Priority date: 2012-05-28
Filing date: 2013-05-24
Publication date: 2013-12-05
Also published as: GB201209393D0

Abstract

This invention concerns a worm drive (101) comprising a worm wheel (102) and a worm (103) mounted such that relative movement can occur between axes (108, 109) of the worm (103) and the worm wheel (102) during transmission of drive. The worm's axis (109) may be free to move relative to the worm wheel's axis (108) to enable the worm (103) to remain in driving contact with the worm wheel (102) when a drive to the worm (103) is reversed. The worm's axis (109) may be free to move relative to the worm wheel's axis (108) in five degrees of freedom, a position of the worm's axis (109) being defined by engagement of the worm (103) with the worm wheel (109). Teeth (120) of the worm wheel (102) and a thread (124) of the worm 103) may be arranged such that, in use and at a moment in time, a tooth (120) discretely engages the thread (124) at two or more regions circumferentially spaced about and located within a half-turn of the thread.

Description

A WORM DRIVE, A KIT, A DEVICE AND A FIXING METHOD

Field of Invention This invention concerns a worm drive and, in particular, but not exclusively, a worm drive for use in positioning a rotary table, such as a rotary table of a coordinate positioning machine, including the non-Cartesian coordinate positioning machines as described in WO2011/107729 and WO2011/107746. Background

It is known to use worm drives for controlling movement of a rotary table. However, when accuracy and precision become important, such as in a coordinate positioning machine, backlash within the worm drive can result in inaccurate movement of the rotary table. To avoid such a problem, worm drives have been developed comprising a worm or wheel having two relatively moveable toothed parts that are biased such that the teeth on each part are forced in opposing directions to grip the teeth on the other of the wheel or the worm, reducing play between the teeth of the worm and the wheel. An example of such a worm drive is described in DE3344133.

Summary of Invention According to a first aspect of the invention there is provided a worm drive comprising a worm wheel and a worm mounted such that relative movement can occur between axes of the worm and the worm wheel during transmission of drive. It is believed that by providing a worm that floats about its axis relative to the axis of the worm wheel, the worm will find a stable position in which a thread of the worm engages two opposed drive faces of the worm wheel. The worm's axis may be free to move relative to the worm wheel's axis to enable the worm to remain in driving contact with the worm wheel when a drive to the worm is reversed. In this way backlash may be reduced.

The worm's axis may be free to move relative to the worm wheel's axis in five degrees of freedom. A position of the worm's axis may be defined in five degrees of freedom by engagement of the worm with the worm wheel. In this way, the worm may naturally find a stable position through engagement with the worm wheel. The worm may be biased into engagement with the worm wheel by suitable biasing devices.

The axis of the worm wheel may be fixed relative to a housing in which the worm drive is mounted. In certain applications, it may be important that the drive provided by the worm drive, which is taken from the worm wheel, is about an axis fixed relative to the housing. In such circumstances, it may be preferable that the worm is mounted such that its axis can move with the worm wheel fixed in place relative to the housing.

The worm wheel and worm may be mounted such that the worm can move to a position in which the worm's axis is not perpendicular to the worm wheel's axis (and therefore, also not parallel to a plane of the worm wheel).

In one embodiment, the worm may be supported by two bearings, at least one of the bearings, and preferably both bearings, capable of translational movement relative to the worm wheel's axis. The translational movement of one of the bearings may be constrained to one direction, which is preferably substantially perpendicular to the worm's axis and at an angle to a plane of the worm wheel, and preferably substantially parallel with the worm wheel's axis. The other bearing may be able to move in directions substantially perpendicular to the worm wheel's axis. Translational movement of the bearings may allow the worm to find a position in which the worm's axis is slightly skew with the plane of the worm wheel, enabling a thread of the worm to engage opposed drive faces of teeth of the worm wheel. The use of bearings may help to reduce wear relative to there being direct contact between ends of the worm and a mounting surface. Translational movement of one of the bearings may be constrained by fixed abutments (fixed relative to the worm wheel's axis) engaged by the bearing and/or worm. One of the fixed abutments may comprise a face, preferably parallel with the axis of the worm wheel, which is engaged by the bearing to constrain movement of the bearing towards the worm wheel.

In order that the worm's axis can find a position that is askew to the plane of the worm wheel, the worm may be mounted such that the worm's axis can rotate about one or more points. The ends of the worm and/or surfaces of housings of bearings may comprise curved surfaces that contact abutments to allow rotation of the worm's axis.

Biasing devices may urge the bearings and/or worm against the fixed abutments and/or worm wheel. In one embodiment, the biasing devices comprise spring loaded plungers having hardened surfaces for engaging the bearings and/or worm. In this way, wear of the biasing devices is reduced.

The worm drive may comprise a device for urging the worm towards the worm wheel. The worm drive may comprise a drive, such as a motor, for rotating the worm, the drive transmitted to the worm by a belt. The belt may engage the worm to exert a force that pulls the axis of the worm towards the worm wheel. The force exerted by the belt may pull one of the bearings against an abutment. This bearing may be capable of translational movement in a direction substantially perpendicular to the force exerted by the belt.

The teeth of the worm wheel and a thread of the worm may be arranged such that, in use and at a moment in time, a tooth discretely engages the thread at two or more regions circumferentially spaced about and located within a half-turn of the thread. A region of the half-turn that is not engaged by the teeth may include a point of the half-turn that is closest to the axis of the worm wheel. This ensures that the worm sits on two discrete points. Each tooth may have a profile in an axial direction comprising first and second sections that can engage the thread separated by a central section that cannot engage the thread. The central section may be a gap/discontinuity in a profile of the tooth that follows a profile of the thread.

According to a second aspect of the invention there is provided a kit for assembling into a worm drive according to the first aspect of the invention. According to a third aspect of the invention there is provided a worm drive comprising a worm wheel and a worm, wherein teeth of the worm wheel and a thread of the worm are arranged such that, in use and at a moment in time, a tooth discretely engages the thread at two or more regions circumferentially spaced about and located within a half-turn of the thread.

According to a fourth aspect of the invention there is provided a device comprising a part that is held in place within a housing by deformations of the housing that engage the part, the deformations formed by forcing elements into recesses in the housing surrounding the part.

The housing may be made of a softer material than the part. For example, the housing may be made of aluminium and the part steel.

In one embodiment, the device is a rotor comprising a shaft mounted for rotation in the housing by a bearing assembly, the bearing assembly comprising an outer sleeve relative to which the shaft rotates, the outer sleeve held in place relative to the housing by deformations of the housing that engage the sleeve, the deformations formed by forcing elements into recesses in the housing surrounding the sleeve.

According to a fifth aspect of the invention there is provided a method of fixing a part in place in a housing comprising forcing elements into recesses in the housing surrounding the part to form deformations in the housing that grip the part. The elements may be forced into the recesses simultaneously. Description of the Drawings

Figure 1 is a perspective view of a worm drive for rotating a table according to one embodiment of the invention; Figure 2 is a cross-sectional view of the worm drive shown in Figure 1 in a housing;

Figure 3 is a cross-sectional view of the worm drive and rotary table in a plane perpendicular to the cross-sectional plane of Figure 2;

Figure 4 is a perspective view of a worm and a wheel of the worm drive; and

Figure 5 is side view of the worm and the wheel.

Description of Embodiments

Referring to the figures, in one embodiment of the invention, a worm drive 101 comprises a worm wheel 102 and a worm 103 mounted such that, in use, relative movement can occur between axes of the worm wheel 102 and worm 103 during transmission of drive. (The axes of the worm wheel 102 and worm 103 are illustrated by lines 108 and 109 respectively). The worm wheel 102 and worm 103 are mounted within a housing 1 12. The worm wheel 102 is connected via shaft 104 to a rotary table 105. The table

105 may be, for example, a rotary table of a coordinate positioning machine. The shaft 104 is mounted for rotation within a bearing assembly 130. The bearing assembly 130 comprises a sleeve 131 held fixed within the housing 112. Bearings 132, in this case ball bearings distributed along the length of the sleeve 131, are retained by the sleeve 131 to provide surfaces against which the shaft 104 can rotate. Shim 138a spaces the fixed sleeve 131 from the rotating worm wheel 102 and is adhered to the sleeve 131. Shim 138b spaces the housing 112 from rotating end cap 140 and is adhered to the housing 112. In this way, both shims 138a and 138b remain stationary with respect to the sleeve 131.

In this embodiment, the sleeve 131 is fixed within the housing 112 by an adhesive. The adhesive may be introduced through ports 133, 134. However, it has been found that when the sleeve 131 is fixed with adhesive alone, the sleeve 131 (and therefore, the rotational axis of the rotary table 105 and worm wheel 102) may become misaligned during manufacture and/or in use. Accordingly, the sleeve 131 is additionally held in place by deformations of the housing 112. These deformations are formed by forcing an oversized ball 141 simultaneously into each one of three recesses 135 in the section 137 of the housing 112 that surrounds the sleeve 131 such that the housing 112 in these areas bulges out to engage and locate the sleeve 112. The three recesses 135 are circumferentially spaced equally around the cylindrical sleeve 131. Section 137 of the housing 1 12 is an annular shaped projection from the main body and comprises cut-outs, in this embodiment, D-shaped cut-outs, 136 along an outer periphery of the annular section and aligned with the recesses 135 in a radial direction. These cut-outs 136 form weakened walls such that when a ball is forced into a recess 135, deformation of the weakened wall will be favoured over deformation of the sleeve 131.

Deformation of the housing 112 will also be favoured over deformation of the sleeve 131 because the housing 112 is made of softer material than the sleeve 131. In this embodiment, the housing 112 comprises aluminium whereas the sleeve 131 comprises steel.

During manufacture, the sleeve 131 is first located by abutment with the housing 112 and held in place through simultaneous insertion of balls into the recesses 135 and then the adhesive is added to ports 133, 134 to provide further means for holding the sleeve 131 in place. The worm 103 is supported by two bearing assemblies 106, 107 both capable of translational movement relative to the worm wheel's axis 109. In this embodiment, spring plunger 113 engages bearing assembly 106 in line with the worm's axis to bias the worm 103 towards abutment 110 and spring plunger 118 engages the bearing assembly 106 in a direction orthogonal to the worm's axis to bias the worm 103 towards the worm wheel 102. The bearing assembly 106 can move against the biasing of spring plungers 113 and 118. Bearing assembly 107 is constrained to translation movement in a direction which is substantially perpendicular to the worm's axis 109 and substantially parallel with the worm wheel's axis 108. Movement of the bearing assembly 107 is constrained by the abutments 110 and 111 fixed to the housing 112.

Bearing assemblies 106, 107 comprise bearing housings 106a, 107a in which the bearings 106b, 107b are retained, the bearing housings 106a, 107a comprising curved surfaces that contact abutments 110, 111 or plungers 113, 118 to allow rotation of the worm's axis about points defined by these curved surfaces. In this way, relative translation movement between the bearings 106, 107/ends of the worm 103 can occur with the worm axis rotating to be askew to the plane of the worm wheel 102. The worm drive comprises a motor 1 14 for rotating the worm 103, the drive from the motor 114 transmitted to the worm 103 by a belt 1 15. The belt 1 15 engages a cog 1 16 attached to the worm 103 to exert a force that urges the axis of the worm 103 towards the worm wheel 102 and towards abutment 11 1. The motor 114 and the cog 116 are arranged such that the belt 115 extends perpendicularly to the worm's axis 109.

The worm 103 is urged towards abutment 110 by a biasing device 113. Now referring to figures 4 and 5, the teeth 120 of the worm wheel 102 comprise first and second sections 121, 122 that can engage the thread 124 of the worm 103 separated by a central section 123 that does not engage the thread 124. In this embodiment, the central section 123 is a gap in a profile of each tooth that follows a profile of the thread 124. In this way, at a moment in time, a tooth 120 of the worm wheel 102 can discretely engage the thread 124 at two or more regions circumferentially spaced about and located within a half-turn of the thread 124. The central sectionl23 may also be used as a reservoir for lubricant, which may be required to reduce wear of the thread 124 and teeth 120.

It is believed that, in use, the worm 103 settles into a stable position in which one or more turns of the thread 124 each engage drive surfaces of the teeth 120 that face in opposed directions. On reversal of drive to the worm 103, the axis of the floating worm 103 can move as dictated by its engagement with the worm wheel to continuously or substantially continuously find stable positions in which drive faces are engaged in both directions. In this way, the worm 103 remains in driving contact with the teeth 120, limiting backlash.

Claims

1. A worm drive comprising a worm wheel and a worm mounted such that relative movement can occur between axes of the worm and the worm wheel during transmission of drive.

2. A worm drive according to claim 1, wherein the worm's axis is free to move relative to the worm wheel's axis to enable the worm to remain in driving contact with the worm wheel when a drive to the worm is reversed.

3. A worm drive according to any one of the preceding claims, wherein the worm's axis is free to move relative to the worm wheel's axis in five degrees of freedom.

4. A worm drive according to claim 3, wherein the worm's axis is free to move relative to the worm wheel's axis such that a position of the worm's axis is defined in five degrees of freedom by engagement of the worm with the worm wheel.

5. A worm drive according to any one of the preceding claims, wherein the axis of the worm wheel is fixed relative to a housing in which the worm drive is mounted.

6. A worm drive according to any one of the preceding claims, wherein the worm wheel and worm are mounted such that the worm can move to a position in which the worm's axis is not perpendicular to the worm wheel's axis.

7. A worm drive according to any one of the preceding claims, wherein the worm is supported by two bearings, at least one of the bearings capable of translational movement relative to the worm wheel' s axis.

8. A worm drive according to claim 7, wherein both bearings are capable of translational movement relative to the worm wheel's axis.

9. A worm drive according to claim 7 or claim 8, wherein the translational movement of one of the bearings is constrained to one direction.

10. A worm drive according to claim 9, wherein the one direction is substantially perpendicular to the worm's axis and at an angle to a plane of the worm wheel.

11. A worm drive according to any one of claims 7 to 10, wherein the or the other bearing is able to move in directions substantially perpendicular to the worm wheel's axis.

12. A worm drive according to any one of claims 7 to 11, wherein translational movement of one of the bearings is constrained by fixed abutments engaged by the bearing and/or worm.

13. A worm drive according to claim 12, wherein one of the fixed abutments comprises a face, which is engaged by the bearing or worm to constrain movement of the bearing towards the worm wheel.

14. A worm drive according to claim 12 or claim 13, wherein at least one biasing device urges the bearings against the fixed abutments

15. A worm drive according to any one of the preceding claims, wherein at least one biasing device urges the worm against the worm wheel.

16. A worm drive according to claim 14 or 15, wherein the at least one biasing device comprises a spring loaded plunger having a hardened surfaces for engaging the bearing and/or worm.

17. A worm drive according to any one of the preceding claims, wherein the worm is mounted such that the worm axis can rotate about one or more points.

18. A worm according to claim 17, wherein surfaces on which the worm is mounted comprise curved surfaces that contact abutments to allow rotation of the worm's axis.

19. A worm drive according to any one of the preceding claims, wherein the worm drive comprises a drive for rotating the worm, the drive transmitted to the worm by a belt, the belt engaging the worm to exert a force that pulls the axis of the worm towards the worm wheel.

20. A worm drive according to claim 19, wherein at least one bearing of the worm is capable of translational movement in a direction substantially perpendicular to the force exerted by the belt.

21. A worm drive according to any one of the preceding claims, wherein teeth of the worm wheel and a thread of the worm are arranged such that, in use and at a moment in time, a tooth discretely engages the thread at two or more regions circumferentially spaced about and located within a half-turn of the thread.

22. A worm drive according to claim 21, wherein a region of the half-turn that is not currently engaged by the tooth includes a point of the half-turn that is closest to the axis of the worm wheel.

23. A worm drive according to claim 21 or claim 22, wherein each tooth has a profile substantially in an axial direction of the worm wheel comprising first and second sections that can engage the thread separated by a central section that cannot engage the thread.

24. A worm drive according to claim 23, wherein the central section is a gap/discontinuity in a profile of the tooth that substantially follows a profile of the thread.

25. A kit for assembling into a worm drive according to any one of claims 1 to 24.

26. A worm drive comprising a worm wheel and a worm, wherein teeth of the worm wheel and a thread of the worm are arranged such that, in use and at a moment in time, a tooth discretely engages the thread at two or more regions circumferentially spaced about and located within a half-turn of the thread.

27. A device comprising a part that is held in place within a housing by deformations of the housing that engage the part, the deformations formed by forcing elements into recesses in the housing surrounding the part.

28. A device according to claim 27, wherein the housing is made of a softer material than the part.

29. A device according to claim 27 or claim 28, wherein the device is a rotor comprising a shaft mounted for rotation in the housing by a bearing assembly, the bearing assembly comprising an outer sleeve relative to which the shaft rotates, the outer sleeve held in place relative to the housing by deformations of the housing that engage the sleeve, the deformations formed by forcing elements into recesses in the housing surrounding the sleeve.

30. A method of fixing a part in place in a housing comprising forcing elements into recesses in the housing surrounding the part to form deformations in the housing that grip the part.

A method according to claim 30, wherein the elements are forced into the recesses simultaneously.