Robust kinematic mount and metrological apparatus and method

Abstract

An aspect of the disclosure provides a bearing assembly comprising: a bearing face (12) configured to provide three-point contact with a circularly symmetric load member (34) to rotatably support said load member in a datum position relative to the bearing face (12); an axial support (14), arranged to resiliently support the bearing face (12); an azimuthal positioner (16, 18) comprising at least one stopping element (16) and at least one movable element (18) coupled to the bearing face (12), wherein the stopping element (16) and the movable element (18) are configured to engage with each other to constrain the bearing face (12) at a predetermined axial position and a predetermined azimuthal position.

Description

[0001] Robust Kinematic Mount and Metrological Apparatus and Method

[0002] Field of Invention

[0003] The present invention relates to methods and apparatus , and more particularly to kinematic mountings , such as those which may be used in metrological apparatus .

[0004] Background

[0005] High preci sion apparatus , such as metrological apparatus and other systems , may require high preci sion bearings . Such bearings may be used to rotatably mount components such as a stylus arm or other element in the apparatus . The accuracy and stability of such a bearing i s of great importance . Typically, such bearings must be made from very hard and / or very rigid components to provide such accuracy .

[0006] Such bearings may al so be subj ect to mechanical shock or deforming forces , which can cause the bearings to be damaged and accuracy to be lost .

[0007] Shock resistant j ewel bearings are widely found in mechanical watches and high- speed mechanical dial s such as those found in voltmeters , however the bearing seat is not kinematic, resulting in the bearing re- seating a number of microns away from its original position after an impact .

[0008] Without shock-resi stance features , the critical bearing surfaces may sustain damage which degrades the performance of the bearing .

[0009] Jewel bearings used in watches are not kinematic . Typically they have an annular line of contact between the pivot / shaft and j ewel bearing . As a result , to keep friction low there will be slight clearances between the shaft and the bearing radially leading to non-repeatable radial movement of the shaft . Other instances exi st where a kinematic ball seat has been used as a bearing .

[0010] Summary

[0011] Aspects and examples are set out in the claims and aim to address the above-described technical problems and other problems .

[0012] In an aspect there i s provided a bearing as sembly compri sing : a bearing face configured to provide three-point contact with a circularly symmetric load member to rotatably support said load member in a datum position relative to the bearing face ; an axial support , arranged to resiliently support the bearing face ; an azimuthal positioner compri sing a stopping element and a movable element coupled to the bearing face , wherein the stopping element and the movable element are configured to engage with each other to constrain the bearing face at a predetermined axial position and a predetermined azimuthal position .

[0013] The bearing face typically compri ses a kinematic seat , such as a three-ball kinematic couple . For example , it may compri se three forward facing spherical surfaces - such as part spherical surfaces , for example hemispheres .

[0014] The stopping element may be fixed in position in the bearing assembly . For example it may be fixed in relation to a bore in which the bearing face i s operable to move ( e . g . to advance and retreat along the bore ) . The bore may be provided by a housing in which the axial support and bearing face are mounted, as described below .

[0015] Typically, the azimuthal positioner compri ses three stopping elements and three corresponding movable elements . Each movable element may be arranged to be constrained by (e.g. disposed within) a corresponding one of the stopping elements. Each pair (one stopping element, one movable element) may be azimuthally separated from the adjacent pair by 120° . In addition, each pair may be configured to provide a point or line of contact so that the three pairs together provide pseudo-kinematic positioning of the bearing face. In the present disclosure the term azimuth may refer to rotational angle about the axis of the bearing assembly - e.g. measured in a plane perpendicular to that axis.

[0016] The stopping element may be provided by a housing of the bearing assembly which is to be held in a fixed position, such as by installation in a metrological apparatus, and the movable element may be configured to move relative to the housing - e.g. by retraction and extension of the axial support.

[0017] The, or each, stopping element may comprise a first engaging surface and the, or each, movable element may comprise a second engaging surface, wherein the first engaging surface and the second engaging surface fit together to provide said constraint. The stopping element (s) may be provided by recesses, such as holes, in an internal surface of the bore.

[0018] The fitting together of the stopping element and the movable element may comprise at least two azimuthally opposed contacts between the first engaging surface and the second engaging surface. This may provide a pseudo-kinematic seat of the bearing stage in the sense that in the constrained geometry provided by the bearing stage mounted in the housing, it can provide equivalent function to a full kinematic mount.

[0019] The azimuthally opposed contacts may each comprise a point contact or a line of contact. It will be appreciated in the context of the present disclosure that a so-called point contact corresponds to contact substantially at a single location on the surface of the stopping element and the movable element respectively .

[0020] The opposed contacts may each compri se an azimuthally aligned line contact - e . g . , a line of contact which lies along a substantially straight line on the surface of the stopping element and the movable element respectively . Typically such a line is directed outward from the axi s of the bearing as sembly .

[0021] The two opposed contacts may be azimuthally opposed in the sense that they are spaced apart from each other either side of a notional radial line directed out from the axi s of the bearing assembly . Generally, they are mutually symmetric about thi s notional radial line . Where the two contacts are line contacts the lines of contact may be substantially parallel with such a radial line .

[0022] The stopping element or the movable element may compri se an axial facing convex surface , which may be in the form of a ridge . The other one of the stopping element and the movable element ma have a complementary form, configured to receive that forward facing convex surface . For example it may compri se an axial facing groove . Such a ridge and groove may be shaped and positioned such that , when the bearing face i s at the predetermined axial position, the ridge fits into the groove to provide the constraint of the bearing face .

[0023] The axial support may be configured to resiliently urge the stopping element and the movable element into engagement with each other . Typically the axial support i s provided by a spring, which may be compressed when the stopping element and the movable element are engaged with each other to provide said bias . The bearing assembly may comprise a housing, wherein the stopping element is fixed relative to the housing and the movable element is fixed to relative to the bearing face.

[0024] The housing may have an axial bore, wherein the axial support is configured to provide resilient retreat of the bearing surface into the bore from the predetermined axial position.

[0025] At the predetermined axial position, the bearing face may be recessed in the bore. This may allow a rotatable load member to be seated in a recess in the bore to provide general mechanical restraint and alignment for engagement with the bearing face. This may also permit mechanical shock to be absorbed by engagement between the bore / housing and the sides of the load member, rather than by the bearing itself. This effect may be further assisted by the resilient movability of the bearing face.

[0026] The axial support may be configured to provide resilient return of the bearing face to the predetermined axial position. For example, when it is displaced (e.g. , by force applied to the load) the axial support will urge it back into position after being resiliently deflected thereby protecting the bearing face from damage.

[0027] The housing may be cylindrical. The bore may also be cylindrical and the bearing face may be carried on a bearing stage which fits into the bore.

[0028] Embodiments of the disclosure provide an apparatus, such as a metrological instrument, comprising the bearing assembly and a rotatable load member, configured to engage with the bearing face to support a rotatable load in the datum position. The rotatable load member may comprise a circularly symmetrical element , such as a part- spherical surface ( such as a hemi sphere ) . This spherical surface may be carried at the end of a rod, which extends into a bore of the bearing assembly ( e . g . via an opening at the end of the bore ) to position the rotatable load member on the bearing face . For example , thi s may position the part- spherical surface in engagement with three part-spherical surfaces carried on the bearing face .

[0029] The rod may fit into the bore and at least one of the bore and the rod may comprise a sacri ficial portion . The sacrificial portion may compri se a softer material than the bearing face . The rod may be provided by a shaft or other mounting for carrying stylus of a metrological instrument or other system in which high-reproducibility and accurate positioning are required .

[0030] Any feature of any one of the examples di sclosed herein may be combined with any selected features of any of the other examples described herein . For example , features of methods may be implemented in suitably configured hardware , and the configuration of the speci fic hardware described herein may be employed in methods implemented using other hardware .

[0031] By seating the bearing kinematically, the return position of the bearing after an impact i s much more determini stic . Thi s may reduce errors resulting from the change in position of the bearing and reduces the need for any re-calibration procedures .

[0032] Embodiments may be amenable to miniaturi sation . The use of a pseudo-kinematic seat may be particularly advantageous in this context .

[0033] The load carried by the bearing ( such as a shaft / pivot ) may kinematically engage with the bearing at 3 points of contact . Thi s may make the motion of the shaft signi ficantly more repeatable over time helping to reduce the frequency of recalibration .

[0034] The 3-point contact may reduce preload requirements , reducing the friction of the bearings and allowing for more sensitive rotation of the bearing .

[0035] Brief Description of Drawings

[0036] Embodiments of the di sclosure will now be described in detail with reference to the accompanying drawings , in which :

[0037] Figure 1 shows a bearing as sembly and compri ses

[0038] Fig 1A - a transverse view from the side of the assembly,

[0039] Fig IB - an axial view of the front of the as sembly;

[0040] Fig 1C - is a detailed view of the engagement of a pseudo-kinematic seat in the bearing as sembly .

[0041] Figure 2 shows a system comprising a bearing as sembly such as that shown in Figure 1 and load member to be supported on the bearing assembly;

[0042] Figure 3 shows a metrological apparatus compri sing a system such as that shown in Figure 2 ; and

[0043] Figure 4 shows a further example of a metrological apparatus .

[0044] In the drawings like reference numerals are used to indicate like elements .

[0045] Specific Description

[0046] Figure 1 shows a bearing as sembly 10 which has a bearing face 12 for kinematic mounting held in a housing 28 . Kinematic mounting may be provided by three part-spherical surfaces 9 carried on the bearing face 12 . The bearing face 12 i s carried on a bearing stage 15 which can retract axially backwards into the housing 28 in the event that excess force is applied to the kinematic mounting. It is also supported axially so that it will return, after such retraction, to a predetermined axial position. The bearing assembly comprises an azimuthal positioner configured so that, when the bearing stage is in that predetermined axial position, the azimuthal position of the bearing face 12 (e.g. , its rotational position about the axis) is also predetermined.

[0047] This can provide a robust bearing able to provide highly repeatable positioning of a load carried on that bearing, even after significant mechanical shock.

[0048] The bearing assembly 10 in Figure 1 shows the bearing stage 15 with bearing face 12, and three part-spherical surfaces 9 carried on the bearing face to provide a kinematic seat by way of a uniquely defined three-point contact. The bearing stage 15 is carried by an axial support 14. The assembly 10 also comprises the azimuthal positioner 16, 18, configured to provide return of the bearing face 12 to a predetermined azimuthal position.

[0049] The bearing assembly 10 further comprises a housing 28 having a bore 30. The axial support 14 and the bearing stage 15 may be disposed in the bore 30. The front of the bore 30 comprises an opening 31. The bearing face 14 is carried on the front of the bearing stage 15 facing outward toward the opening 31.

[0050] The bearing stage 15 is slidably disposed in the bore 30 to allow it to advance and retreat along the bore 30. The axial support 14 may be disposed behind the bearing stage 15 in the bore 30 and provides resilient support of the bearing stage e.g. , it may be provided by a spring which is compressed upon retreat of the bearing stage and which biases the bearing stage 15 to advance. The axial support 14 is typically provided by a resiliently deflectable element , such as a coil spring and carries the bearing stage 15 , which is fixed to the front end of the axial support 14 . The back end of the axial support 14 may be fixed to a foot element 15 . The foot element 15 i s fixed to the bore 30 of the housing 28 towards the back of the bore 30 . At the other end of the axial support the bearing face 12 i s carried adj acent the opening 31 and reces sed relative to the front end of the bore 30 .

[0051] The azimuthal positioner 16 , 18 illustrated in Figure 1 comprises three stopping elements 16 and three corresponding movable elements 18 . Each movable element 18 is arranged to engage with a corresponding one of the stopping elements . The movable elements collectively are biased into engagement with the stopping elements 16 because they are fixed in relation to the bearing stage - and so biased forward with it by the axial support .

[0052] Together , the arrangement of stopping element 16 and corresponding movable element 18 provides a pseudo-kinematic seat in the sense that it i s configured to provide , in addition to a constraint on the axial position, a uniquely defined azimuthal position for the bearing stage at that axial position .

[0053] The azimuthal positioner 16 , 18 illustrated in Figure 1 comprises three stopping elements 16 and three corresponding movable elements 18 . Thi s may provide increased robustness and repeatability of position . As illustrated, the three pairs of elements 16 , 18 may be azimuthally spaced apart at 120 ° ( 2n / 3 ) interval s at the radially outer edge of the bearing stage 15 .

[0054] The stopping elements 16 are each fixed relative to the housing 28 and, in thi s example , are each provided by a hole or reces s in a side wall of the bore 30 . The movable elements 18 are fixed to a radially outward edge of the bearing stage 15 and are each positioned for engagement with a corresponding one of the stopping elements 16 in the wall of the bore 28 . The three stopping elements 16 and the three movable elements 18 are angularly distributed about the circumference of the housing as explained above ( e . g . azimuthally separated by 120 ° ) .

[0055] In the example illustrated in Figure 1 , the movable element 18 comprises a protrusion which protrudes radially outward from the the bearing stage 15 . The front of the movable element 18 ( the surface which faces towards the opening 31 of the bore 30 ) i s curved to provide an axially facing radially aligned ridge . For example , when viewed from the side , the front of the movable element may have a curved forward- facing front surface - e . g . a proj ection which has a substantially semi-circular cros s section when viewed from the side as in Fig 1A.

[0056] The stopping element 16 of the example illustrated in Figure 1 comprises a recess in a wall of the bore 30 . This reces s i s of complementary form to the movable element 18 and larger than the movable element 18 to allow the movable element to fit within it . The stopping element 16 has a rearward- facing surface which i s curved to fit with the forward- facing front surface of the movable element 18 .

[0057] The detail shown in Fig 1C of Figure 1 shows the pseudo-kinematic seat which i s provided by engagement between one of the stopping elements 16 and a corresponding one of the movable elements 18 . It can be seen that the curvature of the rearward- facing front surface of the stopping element 16 and the curvature of the forward- facing curved surface of the movable element 18 may be selected so that , when the movable element 18 i s pushed axially forward into engagement with the stopping element 16 , two azimuthally opposed points of contact , XI and X2 , are provided between the stopping element 16 and the movable element 18. These two points of contact are azimuthally separated so as to constrain the movable element at a predetermined azimuthal position .

[0058] The axial position of the stopping element (and the bearing face) is also fixed by the axial position of the forward edge of the stopping element 16. In the configuration illustrated in Figure 1, the bearing face is fully advanced with the movable element engaged against the stopping element. In this position, it can be seen that the bearing face is axially recessed back from the forward end of the bore.

[0059] The stopping element 16 comprises a recess which is longer in axial extent than the movable element 18 and extends along the bore to allow the movable member 18 to be moved backwards away from the front of the bore. This can allow the stage to be moved into the bore so that the bearing face retreats and the axial support 14 can be resiliently deflected (e.g. , by compression of the spring) to allow this retreat.

[0060] It can therefore be seen that the resilient axial support 14 and azimuthal positioner 16, 18 together fix the ordinary axial and azimuthal position of the bearing face in the configuration illustrated in Figure 1. It can also be seen that, in response to a force axially into the bore, the spring can be compressed allowing resilient retreat of the bearing face 12 into the bore.

[0061] The bearing face 12 carries three spherical surfaces 9 which, in the embodiment shown in Figure 1, are provided by hemispheres affixed to a flat front surface of the bearing stage 15. This configuration provides a seat which will support any circularly symmetric element in a datum position - namely a position which is at a fixed location relative to the bearing stage. This circularly symmetric element may, for example , comprise a spherical surface at the tip of a load member such as a rod or axle which i s to be rotatably carried by the bearing as sembly 10 .

[0062] Figure 2 illustrates a bearing as sembly 10 such as that shown in

[0063] Figure 1 with a load member 34 .

[0064] The arrangement shown in Figure 2 compri ses a bearing assembly 10 having a bearing face 12 supported by an axial support 14 in a bore 30 of a housing 28 such as that described above with reference to Figure 1 . In addition, there i s al so provided a rotatable load member 33 compri sing a rod 36 having a sacrificial portion 38 . The rod 36 ends with a circularly symmetric tip 50 configured to engage with the spherical surfaces of the bearing face . In thi s example the tip 50 is shown as being a hemi spherical surface , but any appropriate circularly symmetric element may be used . Spherical and part-spherical elements are preferred .

[0065] The rod 36 i s shaped and si zed to fit in the opening 31 of the bore 30 . When, in use , the load member 36 i s positioned in the bore so the tip 50 engages with the bearing face 14 , the tip 50 i s kinematically supported in a datum position - uniquely defined by the three spherical surfaces of the bearing face .

[0066] The bearing face 12 in turn is held at a predetermined axial position 32 and a predetermined azimuthal position 34 by the interaction between the stopping element 16 and the movable element 18 .

[0067] Figure 3 illustrates a meteorological apparatus 400 compri sing a body 40 and a bearing assembly 10 such as that described above with reference to Figure 1 and Figure 2 . It further compri ses a rotatable load member 33 such as that described above with reference to Figure 2 .

[0068] The rotatable load member 33 i s connected to a load 42 which i s to be supported in the meteorological instrument . Examples of the load 42 include a stylus arm of the meteorological instrument . It can be seen that the rod 36 extends into the opening 31 of the bore 30 of the bearing assembly 10 to allow the tip of the rotatable load member 33 to engage with the bearing face 12 .

[0069] In thi s configuration, the sacri ficial portion 38 of the rotatable load member 33 is able to engage with the internal surface of the bore 30 . In addition, the rotatable load member 33 i s able to rotate about the axis of the bore 30 while the tip of the rod 36 is held in the predetermined axial position by the bearing face 12 . The bore 30 of the bearing as sembly 10 constrains the rotatable load member 33 only to rotate about the axi s of the bore 30 whil st the bearing face 12 fixes the axial position of the rotatable load member 33 .

[0070] In operation, in the event that the load 42 i s subj ect to a mechanical shock, the sacri ficial portion 38 of the rotatable load member 33 and / or the housing 28 surrounding the opening 31 may be damaged or deformed . Thi s damage may absorb some of the mechanical energy as sociated with the shock, which would otherwi se be applied at the point of contact with the bearing face 9 , 12 .

[0071] In addition, the axial support 14 typically i s al so resiliently compressed to allow the bearing face 12 to retreat along the bore back away from the opening . Thi s can protect the bearing face from damage due to the mechanical shock . The axial support 14 also resiliently urges the bearing stage 12 forward so that , after such a shock it will return to its predetermined position by advancing until the front facing surface of the movable element 18 engages with the rearward- facing surface of the stopping element 16 .

[0072] The engagement of the movable element with the stopping element stops the bearing stage with the bearing face 12 in the predetermined axial position 32 . In addition, the relative curvatures of the engaging surfaces of the stopping element 16 and the movable element 18 cause the bearing stage to return to the predetermined azimuthal position 34 .

[0073] In other words , in the predetermined axial position 32 the front edge of the movable element 18 i s bought into engagement with the rearward- facing edge of the stopping element 16 . The complementary shapes of these two surfaces cause the azimuthal positioner 16 , 18 to bring the bearing face into the predetermined azimuthal angle when the bearing face i s at the predetermined axial position 32 .

[0074] Accordingly, in thi s configuration, the kinematic seat of the bearing face ensures that the tip 50 of the rotatable load member 33 is rotatably supported in a single uniquely defined position, to which it will always return even after mechanical shock or deforming force being applied to the bearing .

[0075] Figure 4 illustrates a further example of meteorological apparatus 400 ' compri sing a body 40 ' and two bearing as semblies 10 , 10 ' each being provided by bearing as semblies such as that described above with reference to Figure 1 and Figure 2 . It further compri ses two rotatable load members 33 , 33 ' as described above with reference to Figure 2 , each of which i s arranged in a respective corresponding one of the two bearing as semblies 10 , 10 ' . As with the example in Figure 3 , in Figure 4 a load 42 , such as a stylus arm, i s to be supported in the meteorological instrument. The two bearing assemblies 10, 10' are arranged opposite to each other and are axially aligned, so that their respective bearing faces face towards each other and the two assemblies 10, 10' are disposed on the same, or substantially the same, axis. The two rotatable load members 33, 33' may each be connected to opposite sides of a load 42 and they may also be aligned with each other (on the same axis) but pointing in opposite directions. They may be arranged to provide a balanced mounting of the load 42. The load 42 may be a stylus arm of the metrological apparatus. This apparatus may function as described with reference to Figure 3, but may provide additional stability and improved reproducibility as compared to a single sided mounting .

[0076] In the event that this metrological apparatus is subject to a mechanical shock, that shock may cause the respective bearing faces to be resiliently deflected rather than to cause damage to them. The sacrificial portions of the two bearing assemblies may provide additional protection to further reduce damage to the kinematic function of the bearings.

[0077] It is anticipated that a variety of changes may be made to the constructions described herein. For example the skilled addressee will appreciate that embodiments provide a bearing face that is able to retreat along a predetermined axis and which is biased forward along that axis into a predetermined axial position. The bearing face is also arranged so that, when it is at the predetermined axial position, the azimuthal angle (rotational position about the axis) is constrained by at least one movable element- stopping element pair. Each such pair may provide a two- point contact. The movable element and stopping element pairs described herein provide one implementation of such a pseudo- kinematic seat. For example, three movable element- stopping element pairs may provide three contacts - each of which may be a two-point (or two-line) contact, such as the azimuthally opposed contacts described herein. but other implementations are contemplated. Variations in terms of shape, number, size, materials, and relative arrangement of the structural components described herein are contemplated within this general teaching.

[0078] For example, the movable element has been described as a forwardfacing ridge which is received into a rearward facing concavity provided by the stopping element. This configuration may be reversed in that the forward-facing surface of the movable element may comprise a concavity. The stopping element may comprise a rearward facing convex curved surface arranged to be received into this concavity.

[0079] As a further example, the bearing assembly 10 illustrated in the drawings is shown as having an azimuthal positioner comprising points of contact at three positions about the circumference of the bearing stage (three movable elements and three stopping elements) . However, other numbers of points of contact may be used. For example, some embodiments may comprise just one movable element and just one stopping element. Such embodiments may have a bore 28 and a bearing stage 15 configured to fit together so as to sufficiently constrain the position of the bearing stage to provide the desired repeatability. There may be different ways to achieve this. One example is to machine the bore and stage to fit each other extremely closely, but there are disadvantages to this and providing acceptable repeatability may be challenging. Another approach may be to provide a taper in either the bore or the stage (or both) so that appropriate closeness of fit arises automatically from the form of the two. However it is done, the bore and the stage may be configured to constrain the radial position of the bearing face - e.g. to keep it centrally aligned in the bore 30. In some embodiments having fewer than three such points of contact , the bore 30 of the housing 28 and the bearing stage may be configured to provide constraint of one or more degrees of freedom . In other embodiments , two , three , four or any other practical number of such elements may be used .

[0080] The axial support i s illustrated as being secured by a foot member towards the back end of the bore . However, the foot member i s optional and the axial support may be fixed in place by any appropriate means . For example it may be welded or glued . The foot member need not be present at all . The axial support may be secured in any appropriate way . I f the foot i s present it need not be attached to the walls of the bore and can be attached to the end of the bore .

[0081] The axial support i s illustrated as being a coil spring but any biasing means may be used -any structure can be used provided it i s capable of providing resilient support of the bearing stage to bias the bearing stage forward into the predetermined axial position . Other examples include leaf springs , elastic elements and magnetic and electrical biasing means .

[0082] The embodiments presented herein have presented a housing which may be removable from an apparatus , such as a metrological instrument , in which the bearing as sembly i s to be used . Thi s has the advantage that the bearing as sembly can be easily removed and replaced . However in some embodiments the housing may be provided by a body of the apparatus itself . For example , the bearing stage , bearing face , and axial support may be provided into a bore which is integrated with the apparatus . The bearing stage , bearing face , and axial support may be made and sold separately, for example without the housing to allow installation into a bore which is integrated into an apparatus , such as a metrological instrument for repair of that apparatus . One of the technical problems addressed by the present disclosure, in addition to the provision of a robust bearing with a high degree of reproducibility in positioning the load carried by the bearing, is miniaturisation. Embodiments of the present disclosure are amenable to manufacture in miniature. For example, the bearing assemblies of the present disclosure may have a bore with an internal diameter of less than 5mm, for example, less than 2mm.

[0083] It will be appreciated in the context of the present disclosure that a three-dimensional rigid body has exactly 6 independent degrees of freedom: the translational (X, Y, Z) and the rotational (Rx, Ry, Rz) . A mounting may generally be considered fully kinematic if all degrees of freedom are fully constrained.

[0084] An advantage of a kinematic mount is that it locates one rigid body relative to another with very high repeatability, without over-constraining the body or introducing instability.

[0085] A single kinematic bearing according to the present disclosure locates and orients a rigid body using 3 contact points. Providing two such bearings in opposition to each other (e.g. with the bearing faces facing each other and being mutually aligned on the same axis) allows an article, such as a shaft, rod or axle or any similar rotatable element) to be rotatably mounted between them and constrained to rotate only about a single axis (e.g. an axis aligned with the axes of the two bearing assemblies) . It can thus be used to enable components to pivot in a highly repeatable fashion - e.g. about a single and accurately defined axis of rotation. This may be of particular advantage in metrological instruments, to provide pivotable mounting of a component such as a stylus arm in a contact surface metrology instrument . Other components may al so be kinematically mounted using the bearing as semblies of the present disclosure .

[0086] Any feature of any one of the examples di sclosed herein may be combined with any selected features of any of the other examples described herein . For example , features of methods may be implemented in suitably configured hardware , and the configuration of the speci fic hardware described herein may be employed in methods implemented using other hardware .

[0087] It will be appreciated from the discussion above that the embodiments shown in the Figures are merely exemplary, and include features which may be generali sed, removed or replaced as described herein and as set out in the claims . With reference to the drawings in general , it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein . It will be appreciated however that the functionality need not be divided in thi s way, and should not be taken to imply any particular structure of hardware other than that described and claimed below . The function of one or more of the elements shown in the drawings may be further subdivided, and / or di stributed throughout apparatus of the di sclosure . In some embodiments the function of one or more elements shown in the drawings may be integrated into a single functional unit .

[0088] The above embodiments are to be understood as illustrative examples . Further embodiments are envisaged . It i s to be understood that any feature described in relation to any one embodiment may be used alone , or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments , or any combination of any other of the embodiments . Furthermore , equivalents and modifications not described above may al so be employed without departing from the scope of the invention, which i s defined in the accompanying claims .

[0089] List of Elements bearing assembly 10 bearing face 12 bearing stage 13 axial support 14 foot member 15 azimuthal positioner 16 , 18 stopping element 16 movable element 18 azimuthal angle a axial direction z azimuthally opposed contacts 24 , 26 housing 28 bore 30 opening 31 predetermined axial position 32 predetermined azimuthal angle 34 rotatable load member 33 rod 36 sacri ficial portion 38

Claims

Claims1 . A bearing as sembly compri sing : a bearing face configured to provide three-point contact with a circularly symmetric load member to rotatably support said load member in a datum position relative to the bearing face ; an axial support , arranged to resiliently support the bearing face ; an azimuthal positioner compri sing at least one stopping element and at least one movable element coupled to the bearing face , wherein the stopping element and the movable element are configured to engage with each other to constrain the bearing face at a predetermined axial position and a predetermined azimuthal position .2 . The bearing as sembly of claim 1 wherein the azimuthal positioner compri ses three movable elements and three corresponding stopping elements , wherein each movable element i s configured to engage with a respective corresponding one of the stopping elements to provide said constraint .3 . The bearing as sembly of claim 1 or 2 wherein the , or each, stopping element compri ses a first engaging surface and the , or each, corresponding movable element compri ses a second engaging surface , wherein the first engaging surface and the second engaging surface fit together to provide said constraint .4 . The bearing assembly of claim 1 or 2 wherein the fitting together of the , or each, stopping element and the , or each movable element comprises at least two azimuthally opposed contacts between the first engaging surface and the second engaging surface , for example wherein the azimuthally opposed contacts each compri se a point contact or a line contact .5 . The bearing assembly of claim 4 wherein the opposed contacts each compri se an azimuthally aligned line contact .6 . The bearing as sembly of claim 3 , 4 or 5 wherein one of the stopping element and the movable element compri ses an axial facing ridge and the other one of the stopping element and the movable element comprises an axial facing groove , wherein the ridge and the groove are shaped and positioned such that , when the bearing face i s at the predetermined axial position, the edge fits into the groove to provide the constraint of the bearing face .7 . The bearing as sembly of any preceding claim wherein the axial support resiliently urges the stopping element and the movable element into said engagement .8 . The bearing assembly of any preceding claim comprising a housing, wherein the stopping element i s fixed relative to the housing and the movable element i s fixed to relative to the bearing face .9 . The bearing as sembly of claim 8 wherein the housing has an axial bore , wherein the axial support is configured to provide resilient retreat of the bearing face into the bore from the predetermined axial position .10 . The bearing as sembly of claim 9 wherein, in the predetermined axial position, the bearing face is reces sed in the bore .11 . The bearing assembly of claim 9 or 10 wherein the axial support i s configured to provide resilient return of the bearing face to the predetermined axial position, for example wherein the axial support compri ses a spring .

12. The bearing assembly of any of claims 7 to 10 wherein the housing is cylindrical.

13. An apparatus comprising the bearing assembly of any preceding claim and a rotatable load member, configured to engage with the bearing face to support a rotatable load in the datum position .

14. The apparatus of claim 13 wherein the rotatable load member is carried by a rod, which extends into a bore of the bearing assembly to position the rotatable load member on the bearing face .

15. The apparatus of claim 13 or 14 wherein the rod fits into the bore and at least one of the bore and the rod comprises a sacrificial portion, for example wherein the sacrificial portion comprises a softer material than the bearing face.

16. A metrological instrument comprising: a measurement probe, configured to perform surface contact measurements of a surface of a workpiece; wherein the measurement probe is mounted to the instrument by at least one bearing assembly according to any of claims 1 to12.

17. The metrological instrument of claim 16 wherein the measurement probe comprises a stylus, carried on a stylus arm and the stylus arm is coupled to at least one rotatable element carrying a circularly symmetric load member for engagement with the bearing face of a corresponding one of the at least one bearing assemblies.

18. The metrological instrument of claim 16 or 17, wherein the at least one bearing assembly comprises two bearing assemblies arranged on the same axis and having their respective bearing faces opposing each other.

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