Bearing arrangement

1. A positioning apparatus comprising a moveable member moveable along a bearing guideway in a first dimension, the moveable member comprising at a first end thereof at least first and second bearing members which are preloaded against corresponding first and second bearing surfaces of the bearing guideway, configured such that in a two dimensional coordinate system perpendicular to the first dimension, the resultant reaction force to the preload on the first bearing member and the resultant reaction force to the preload on the second bearing member intersect.

2. A positioning apparatus as claimed in claim 1 , comprising at least one preload effecting member providing a resultant preload force which, in said two dimensional coordinate system, extends non-parallel to the resultant reaction force on the first bearing member and non-parallel to the resultant reaction force on the second bearing member.

3. A positioning apparatus as claimed in claim 2 , in which the preload effecting member comprises a bearing member biased against a third surface of the bearing guideway.

4. A positioning apparatus as claimed in claim 2 , configured such that, the resultant reaction forces on the first and second bearing members, and the line of the resultant preload force are directed so as to substantially converge toward a common point.

5. A positioning apparatus as claimed in claim 1 , in which the moveable member comprises one or more sheets or plates of material configured to provide a load bearing structure comprising first and second planar load bearing members.

6. A positioning apparatus as claimed in claim 5 , configured such that said resultant reaction force on the first bearing member and said resultant reaction force on the second bearing member are respectively substantially aligned with and carried predominantly along the planes of said first and second planar load bearing members.

7. A positioning apparatus as claimed in claim 5 , in which the first and second planar load bearing members are adjoining, and in which the apparatus is configured such that, in said two dimensional coordinate system, the lines of the resultant reaction forces are directed so as to substantially converge toward the junction of first and second planar load bearing members.

8. A positioning apparatus as claimed in claim 2 , configured such that the lines of the resultant reaction forces and the line of the resultant preload force are directed so as to substantially converge toward the junction of the first and second planar load bearing members.

9. A positioning apparatus as claimed in claim 2 in which the preload effecting member is mounted to both the first and second planar load bearing members.

10. A positioning apparatus as claimed in claim 5 , in which for said first and/or second bearing member, its resultant reaction force is substantially parallel to the plane of its respective planar load bearing member, and optionally contained within a volume defined by the main planar surfaces of the sheet or plate of material defining said planar load bearing member.

11. A positioning apparatus as claimed in claim 1 , in which at least in said two dimensional coordinate system, the point at which the resultant reaction forces intersect is located substantially at or within a corner feature of the moveable member.

12. A positioning apparatus as claimed in claim 1 , in which the positioning apparatus comprises a coordinate positioning apparatus, for example a coordinate measuring machine.

13. A positioning apparatus as claimed in claim 1 , comprising at least a second bearing assembly comprising at least third and fourth bearing members which are preloaded respectively against the first and second bearing surfaces of the bearing guideway, configured such that the resultant reaction force to the preload on the third bearing member and the resultant reaction force to the preload on the fourth bearing member intersect at least in said two dimensional coordinate system.

14. A positioning apparatus as claimed in claim 13 , in which in said two dimensional coordinate system, the point at which the resultant reaction forces of the first and second bearing members intersect and the point at which the resultant reaction forces of the third and fourth bearing members intersect are substantially coincident.

15. A positioning apparatus comprising a moveable member moveable along a bearing guideway in a first dimension, the moveable member comprising at a first end thereof at least first and second bearing members which are preloaded against corresponding first and second sides of a concave corner of the bearing guideway.

16. A positioning apparatus comprising a moveable member moveable along a bearing guideway in a first dimension, the moveable member comprising at a first end thereof at least first and second bearing members which are preloaded against corresponding first and second bearing surfaces of the bearing guideway, configured such that the resultant reaction force to the preload on the first bearing member and the resultant reaction force to the preload on the second bearing member converge.

17. A positioning apparatus as claimed in claim 12 , in which the moveable member comprises a horizontally-extending beam which carries a carriage that is moveable along the beam in a second dimension, the carriage carrying a quill which is moveable relative to the carriage in a third dimension.

18. A positioning apparatus as claimed in claim 2 , in which the point at which said resultant reaction forces of the first and second bearing members intersect is located substantially at or within a corner feature of the structure of the moveable member, the corner feature being defined by adjoining first and second planar load bearing members, and wherein the preload effecting member is mounted to the first and/or second planar load bearing members substantially at the corner feature.