Field of the Invention

The present invention relates to a cardan joint with an adjustable backlash mechanism. Background Cardan joints (also referred to as universal joints) are commonly used in mechanical systems in order to transfer rotary motion from one part to another when said parts are not necessarily parallel.

Generally, a cardan joint comprises a pair of yokes connected by a linkage (also referred to as a spider or cross-shaft). The connection between any given yoke and the linkage may be subject to backlash. Backlash is generally understood to mean the maximum distance or angle through which any part of a mechanical system may be moved in one direction without applying appreciable force or motion to the next part in mechanical sequence. In the context of universal joints, backlash can refer to unnecessary, or unwanted, movement of either the yoke or linkage which does not aid in the transfer of rotational force. US Patent Number 5,062,730 describes a cardan joint having two yokes coupled by a coupling link. The joint as disclosed in this patent is manufactured with a high degree of accuracy in an attempt to mitigate the effects of backlash. However, the degree to which backlash is present is highly dependent on the manufacturing quality, and moreover any backlash introduced by use of the joint cannot be corrected. US Patent Number 6,926,61 1 describes a cardan joint with elastomeric bearings. The bearings are preloaded in compression, and comprise laminated segments with reduced resistance to shear deformation or rubber cords providing reduced torsional stiffness.

However this joint has a large number of components, which renders it complex and may increase the unit price substantially. Furthermore any backlash within the joint cannot be corrected after manufacture or assembly.

Multi-axis positioners are often used in multi-axis machining and other related roles. A multi- axis positioner should accurately move a workpiece of a sample, and many examples do so through the use of cardan joints connected to extensible rods. The movement of the positioner needs to be highly accurate (in some examples to within microns) and therefore any backlash within the cardan joints can negatively affect the overall accuracy of the positioner. Conventional cardan joints cannot be tightened or have their stiffness and/or backlash adjusted.

Accordingly, there is a need for a cardan joint overcoming the problems listed above. Summary Accordingly, in a first aspect, the invention provides a cardan joint comprising:

a first yoke; and

a second yoke;

wherein the first yoke and the second yoke are connected via a linkage;

the cardan joint further comprising:

a backlash adjusting mechanism, the backlash adjusting mechanism configured to reduce or increase an amount of backlash between the linkage and either the first yoke or the second yoke.

Advantageously, the degree of backlash may be preloaded for the joint and moreover can be adjusted without requiring dismantling of the joint and so can be performed after the joint has been incorporated in a larger device (such as a multi-axis positioner). The ability to adjust the backlash can allow the amount of backlash to be maintained at an effectively constant level over the life of the joint, even if there is wear on the components of the joint which would normally lead to increased backlash.

In a second aspect, the invention provides a multi-axis positioner including a cardan joint as set out according to the first aspect. The multi-axis positioner may include several such cardan joints, which may be interconnected, for example by extensible rods.

Conveniently, the rigidity of the joint, which is dependent in part on the degree of backlash, as used in the multi-axis positioner may be adjusted.

As the amount of backlash in the cardan joint(s) in the positioner can be controlled or adjusted, more accurate operation of the positioner may be possible. For example, the amount of backlash in the cardan joint(s) may be adjusted to a predetermined level and account taken of this in the control of the positioner. Alternatively or additionally the backlash may be adjusted to a level where its contribution to the overall error in the position of the positioner is below a desired level. Optional features of the invention will now be set out. These are applicable singly or in any combination with any aspect of the invention.

The first yoke or the second yoke may be configured to rotate about an axis of rotation of the linkage, and the backlash adjusting mechanism may be configured to reduce or increase the amount of backlash in an axis transversal to the axis of rotation.

Advantageously, this can reduce the slackness in the joint (i.e. increase the tightness) by decreasing any gaps between the first or the second yoke and the linkage.

The backlash adjusting mechanism may be configured to reduce or increase an amount of backlash between the linkage and the first yoke, and the joint may further include: a second backlash adjusting mechanism, the second backlash adjusting mechanism configured to reduce or increase a degree of backlash between the linkage and the second yoke.

Conveniently, this can further increase the tightness of the joint and/or provide for control of the backlash in multiple directions.

The or each backlash adjusting mechanism may be provided as one or more slots within the linkage, wherein a width of the or each slot is adjustable.

Advantageously, the slots may be easily machined within the linkage and provide an accurate adjustment mechanism for backlash.

The joint may further include a first shaft, the first shaft being fixed to the first yoke or the second yoke and rotatable relative to the linkage. The first shaft may be disposed within a housing of the linkage, wherein adjusting the width of the slot adjusts a dimension of the housing. The first shaft may be threaded, and may threadedly engage with the linkage to prevent axial displacement of the first shaft relative to the linkage.

The or each backlash adjusting mechanism may be provided as one or more slot(s) within the first yoke and/or the second yoke, wherein a width of the slot(s) are adjustable. The joint may include a second shaft, the second shaft being fixed to the linkage and rotatable relative to the first yoke or the second yoke. The second shaft may be disposed within a housing of the first yoke or the second yoke, and adjusting the width of the slot may adjust a dimension of the housing. The second shaft may be threaded, and may threadedly engage with the first yoke or the second yoke to prevent axial displacement. Advantageously, the threaded engagement can serve to further tighten the joint and improve accuracy.

The or each backlash mechanism may comprise one or more adjustment screws disposed within a threaded bore of either the linkage or the first yoke or the second yoke. The use of adjustment screws may provide a simple way of controlling the backlash.

Conveniently, the adjustment screws can be operated with the joint assembled.

The first or second shaft may be fixed to the yoke(s) or the linkage by nuts or by gluing or welding. By fixing the shafts in this way, backlash within the joint in other directions can also be reduced or eliminated. Preferably the shaft(s) and/or housings are formed of a material with a low-coefficient of friction.

The first yoke may rotate relative to the linkage around a first axis, the second yoke may rotate relative to the linkage around a second axis; and the first axis and the second axis may intersect. The linkage may comprise a cuboidal shaded housing, the housing having four apertures, each on a respective face of the housing; wherein the four shafts are respectively disposed in each of the four apertures, and each shaft is fixed to the housing.

Advantageously, such a joint may have reduced dimensions as the linkage can be made smaller.

The first yoke may rotate relative to the linkage around a first axis, the second yoke may rotate relative to the linkage around a second axis; and the first axis and second axis may not intersect. The linkage may comprise a rectangular shaped housing, the housing have two bores, each on a respective face of the housing; wherein two shafts are respectively disposed within the bores of the linkage, and each shaft is fixed to either the first yoke or second yoke. Advantageously, by spacing the first and second axes of rotation, the degree by which the yokes may rotate can be increased. Brief Description of the Drawings

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 shows an exploded perspective view of a cardan joint according to an embodiment of the present invention;

Figure 2 shows an assembled perspective view of the cardan joint of Figure 1 in a first position;

Figure 3 shows an assembled perspective view of the cardan joint of Figure 1 in a second position; Figure 4 shows an exploded perspective view of a variant cardan joint according to a further embodiment of the present invention; and

Figure 5 shows an assembled perspective view of the cardan joint of Figure 4. Detailed Description and Further Optional Features

Figure 1 shows an exploded view (in perspective) of a cardan joint 100 with improved backlash control according to an embodiment of the present invention. The joint comprises a first coupling unit 103, a second coupling unit 123 and a linkage 1 1 1 that connects the first and second coupling units.

The first coupling unit 103 has a base portion 102 and yoke portion (or yoke) 105. The base portion 102 is suitable for connecting to a rotary shaft or other mechanical linkage via an aperture 104. The yoke 105 includes first and second apertures 106 and 107, through which a shaft 108 passes. The shaft may be fixed in place by, for example, nut 109 and nut 1 10. Alternatively the shaft may be glued or welded to the yoke. As a further alternative, apertures 106 and 107 may be threaded and the shaft may include corresponding threaded portions; in such examples, the shaft may be threadedly engaged with the apertures thereby fixing it in place. Therefore the shaft 108 is not rotatable relative to the yoke 105.

A mid-portion of the shaft 108 is disposed within an aperture 1 13 of a linkage 1 1 1 . The aperture 1 13 extends through a first face 1 12 of the linkage 1 1 1. Connected to the aperture 1 13 is a slot 1 18, the width of which may be controlled by adjustment screws 1 19, the adjustment screws engaging the linkage either side of the slot. Therefore the degree to which there is backlash between the shaft 108 and the linkage 1 1 1 can be controlled by the adjustment screws 1 19 which control a dimension (e.g. a width) of slot 1 18, which in turn adjusts a dimension (e.g. a radius) of aperture 1 13. Linkage 1 1 1 is rotatable around an axis 1 16 which passes through the yoke 105 and its apertures 106 and 107. Backlash may exist in the joint along an axis which is transversal to the axis of rotation 1 16. For example, the gap between the shaft 108 and the aperture 1 13 may vary along the length of the shaft. Therefore the shaft 108 might be rotatable in an axis transversal to the axis of rotation 1 16, which can be described as backlash in the joint. The linkage 1 1 1 is preferably formed of a material with a low-coefficient of friction, as are the shafts 108 and 120.

The second coupling unit 123 is substantially identical to the first, and also comprises a base portion 124 and yoke portion (or yoke) 126. The base portion 124 is suitable for connecting to a rotary shaft or other mechanical linkage via an aperture 125. The yoke 126 includes first and second apertures 128 and 127, through which a second shaft 120 passes. The second shaft may be fixed in place by, for example, nuts 129 and 130. A mid-point of the shaft 120 is disposed within a second aperture 1 15 of the linkage 1 1 1 . The aperture 1 15 being disposed on a different face to aperture 1 13. Aperture 1 15 also includes adjustment slot 121 , which is controlled by adjustment screws 122. Therefore the degree of backlash between aperture 1 15 and the shaft 120 may be adjusted, by adjusting the width slot 121 through adjustment screws 122. The linkage 1 1 1 is rotatable relative to the yoke 126 about an axis 1 17.

Optional features of the first coupling unit, such as the apertures and shaft having threaded portions, are equally applicable to the second coupling unit.

It can be seen that axis of rotation 1 17 and axis of rotation 1 16 are offset relative to one another, i.e. they do not intersect. In a variant example of the joint shown in Figure 1 , the axes of rotation do intersect. The dimensions of the linkage 1 1 1 and yokes 105 and 123 are chosen such that they may rotate by more than ±50°.

Figure 2 shows the joint 100 in an assembled form. As is clearly shown, the first and second shafts 120 and 108 are fixed within respective yokes 126 and 105, by nuts. This fixing can also prevent axial displacement of the shaft. The linkage 1 1 1 is therefore rotatable around axes 1 16 and 1 17 and the degree of backlash between the shafts 120 and 108 and the linkage 1 1 1 is adjustable via screws 1 19 and 122. Figure 3 shows an example where the linkage 1 1 1 has been rotated around axis 1 16 such that linkage 1 1 1 , second coupling unit 123, and axis of rotation 1 17 has been rotated relative to the arrangement shown in Figure 2.

In this example, the adjustment mechanism (i.e. slots 1 18 and 121 ) are installed within the linkage 1 1 1 present between the two yokes.

Figure 4 shows an exploded (in perspective) view of a variant cardan joint 400 with improved backlash control according to a further embodiment of the present invention. In this example, the adjustment mechanisms are mounted within the yokes of the coupling units rather than in the linkage. In detail, the joint 400 comprises a first coupling unit 403 which comprises a base 402 and yoke 405. The base 402 includes an aperture 404 suitable for attachment to a rotating shaft or other mechanical linkage. The yoke 405 includes first and second apertures 406 and 407. In this respect, the coupling unit 403 is identical to the coupling units shown in Figures 1 - 3. However, the apertures of coupling unit 403 each include respective slots 408. A width of the slots may be adjusted by adjustment screws 409. A pair of shafts 410 and 412 are disposed within respective apertures 406 and 407 of the yoke 405. Therefore, by adjustment of the screws 409, the degree of backlash present between the apertures of the yoke and the shafts can be adjusted.

The length of the shafts is such that the shafts extend beyond respective apertures 406 and 407 into corresponding apertures 414 of a linkage 413. The apertures within the linkage may contain a thread, and the shafts may have a corresponding thread such that they threadedly engage to fix the shafts in place relative to the linkage 413. Therefore the shafts may not rotate relative to the linkage 413, but instead rotate relative to the yoke 405.

Similarly, a second coupling unit 424 comprises a base 425 and yoke 423. The second coupling unit 424 is substantially identical to the first coupling unit 403. The base 425 includes an aperture 426 suitable for attachment to a rotating shaft or other mechanical linkage. The yoke 423 includes first and second apertures 422 and 421 . The apertures of coupling unit 424 include respective slots 428. A width of the slots 428 may be adjusted by adjustment screws 430. A pair of shafts 419 and 420 are disposed within respective apertures 422 and 421 of the yoke 423. Therefore, by adjustment of the screws 430, the degree of backlash present between the apertures of the yoke and the shafts can be adjusted. The length of the shafts is such that the shafts extend beyond respective apertures 422 and 421 into corresponding apertures 415 of the linkage 413. The apertures of the linkage may contain a thread, and the shafts may have a corresponding thread such that they threadedly engage to fix the shafts in place relative to the linkage 413. Therefore the shafts may not rotate relative to the linkage 413, but instead rotate relative to the yoke 423.

In contrast to the example shown in Figures 1 - 3, the adjustment mechanism in the example 400 is disposed within respective yokes 405 and 423.

This is clearly shown in Figure 5. It should also be noted that, in the example shown in Figure 5, the first axis of rotation 417 and second axis of rotation 418 intersect one another within the linkage 413.

The shafts shown in the Figures are treated by the application of, for example, a helix tread or similar to thereby increase the surface area and as a result increase the stiffness and static load of the joint. However, untreated shafts may also be used. The slots as discussed above may include an elastic element disposed therein, such that the slots are biased to a rest position.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

List of features 100, 400 Cardan joint

102, 402, 124, 424 Base

103, 403, 123, 425 Coupling unit

104, 404, 125, 426 Aperture within base

105, 405, 126, 423 Yoke

106, 107, 406, 407, 127, Aperture within yoke 128, 421, 422

108, 410, 412, 120, 419, Shaft

420

109, 110, 129, 130 Nut for fixing shaft to yoke 111, 413 Linkage

112 Face of linkage

113, 115, 414, 415 Aperture in linkage 116, 117, 417, 418 Axis of rotation

118, 121, 408, 428 Slot

119, 122, 409, 430 Adjustment screws