WO1995021336A1

WO1995021336A1 - Linear bearing with self adjusting mechanism

Info

Publication number: WO1995021336A1
Application number: PCT/GB1994/000595
Authority: WO
Inventors: Charles William Middleton
Original assignee: Charles William Middleton
Priority date: 1994-02-01
Filing date: 1994-03-23
Publication date: 1995-08-10
Also published as: AU6263094A; GB9401855D0

Abstract

A linear bearing is provided with a self-adjusting mechanism for urging the bearing members (e.g., rollers or sliders) onto a guide surface to compensate for dimensional variation of the guide (e.g., a shaft or rails) on which it runs, and for mechanical wear. In one example a bearing body (1) with a cylindrical bore contains ball races (16) mounted on axles (18) and located in carriers (28). The position of the axles in relation to a ramp (20) on which they rest determines the adjustment of the bearing. Self-adjustment is effected by the use of springs to maintain a tension between the carriers and eliminate play by trapping the axle-roller assemblies between the taper and shaft. A screw-thread system can provide for manual adjustment of the linear bearing. Other embodiments are described.

Description

LINEAR BEARING WITH SELF ADJUSTING MECHANISM.

The invention relates to linear bearings.

A typical form of linear bearing known in the prior art comprises a cylindrical bearing or bush which in operation moves along a round- section precision shaft that passes through its centre. A system of recirculating balls may be interposed between the bearing and shaft to reduce friction and increase load-carrying capability.

Other forms of linear bearing systems employ rollers or balls mounted in a block or carriage plate so as to run in contact with a high precision rail of specialised profile.

Linear bearing systems provide a low-friction sliding action that enables a precision transfer of load to and from any point through which the bearing unit may travel along its shaft. Their use is widespread in applications where smooth running and/or precision linear movement is required, e.g., printing and packaging machinery, sawing and grinding machines, hospital beds and equipment, robotic technology.

Within the context of the present invention, the term linear bearing is to be understood to include linear motion systems for motion in a generally linear direction along the guide surface(s) of an elongate guide (e.g., a shaft or rail(s)).

In order to enable smooth running and linear movement with precision, linear bearings have typically required high-cost, precision manufacturing techniques and materials for the linear bearing and the guide surfaces of the guide (e.g., a shaft or rails). This is true of most or all linear systems that achieve sufficient rigidity as a result of their design and construction to operate smoothly and without sticking whilst subject to loads that are significantly offset.

With a more simple linear bearing of the sliding (bush) type, it has been difficult to maintain accuracy and ease of operation over a period of time due to wear of the linear bearing and/or the guide surfaces. This is particularly true in applications where the linear bearing is subject to turning moments which can cause the bearing to rock or catch on the bearing surfaces. Linear bearings of the re- circulating ball type known as ball bushings are not ideal in such applications since they require a relatively even distribution of weight between the balls under load in order to avoid stiction and achieve a smooth sliding action.

Thus, existing forms of linear guidance that will operate despite severely cocked loading require specialised shafts or rails and are for many applications over-engineered and very expensive. The incentive for the present invention arose as a result of problems experienced in the construction of an apparatus to enable lifting and transfer operations as a component of a device for use by the disabled. This required a linear bearing which could run on a rolled steel shaft, which could include larger variations in diameter and other imperfections and be subject to greater wear than would be the case with an accurately machined and hardened precision shaft. Also the linear bearing needed to be light in operation while being able to cope with torsional forces as a result of the handicapped operator putting weight on an operating handle or bar connected to the linear bearing. A low cost solution was also required.

In summary therefore, a lightweight, low-cost, multi-use linear bearing was required which would be precise in operation and at the same time suitable for low-accuracy and domestic applications.

In accordance with the present invention there is provided a linear bearing for substantially linear motion along an elongate guide having a guide surface, the linear bearing comprising a bearing body, at least one bearing member adjustably mounted in the body for bearing on the guide surface and a mechanism for, in use, urging the bearing member against the guide surface. Thus the bearing member for bearing on a surface of the guide can be adjustably mounted with respect to the body or housing of the bearing for varying the clearance between the body and the surface. By providing the linear bearing with one or more bearing members which are urged against the guide surface, free play between the bearing members and the guide surface can be substantially eliminated, thus ensuring smooth, accurate and light operation. Adjusting the mounting of the bearing member with respect to the housing enables the linear bearing to be adapted to different sizes of guide (e.g., different shaft diameters) and can also be adapted to changes over time as a result of wear and tear. It will be appreciated that the invention is not limited to the specific application which lead to its development, but can also be used in other applications where linear bearings are required, reducing the cost of the bearing and guide surface construction by removing the need for highly machined components and/or providing means for adapting to wear of the device.

Preferably, the linear bearing comprises a self-adjusting mechanism for urging the bearing member against the guide surface. Thus the linear bearing is able to provide self-adjustment of the mounting of the, bearing member with respect to the body or housing. This enables the bearing to adapt itself to inaccuracies and.changes of diameter along a shaft or other guide and also enables automatic adaptation to the wear of the components of the guide and bearing. Thus, for example, the bearing can absorb shaft inaccuracies and/or wear, maintaining substantially zero play on shafts or other guides made, for example, of standard steel tube and bars and shafts of slightly different sizes, as well as precision ground shafts. A simple self-adjusting mechanism can be provided which pre-loads the bearing member(s) against the guide surface(s) , substantially eliminating play between the bearing and the guide surface(s). Preferably, the adjustment mechanism comprises an inclined ramp, the adjusting mechanism urging the bearing member along the ramp whereby the bearing member is caused to be urged against the guide surface.

A simple and effective urging means which can maintain a substantially constant pre-load of the bearing member(s) against the guide surfaces can be in the form of resilient means (e.g. , a spring or springs) for urging the bearing members along the ramp. In order to provide a linear bearing which is particularly resistant to torsional effects, the linear bearing preferably comprises first and second adjustable bearing members at or near opposite ends of the bearing body, said first and second bearing members being moveable along respective first and second ramps with opposed inclines. The resilient means may be in the form of a compression spring for urging the first and second adjustable bearing members apart. Alternatively, they may be in the form of one or more tension springs for urging the first and second adjustable bearing members towards one another. The form and number of resilient elements will depend on the relative slope of the ramps.

A linear bearing according to the invention may include a manually adjustable mechanism for urging the bearing member against the guide surface.

In order to maintain the overall relationship between the bearing and the guide surfaces (for example, if the bearing is to be located symmetrically around a guide shaft) , the bearing preferably comprises first and second sets of adjustable bearing members at or adjacent opposite ends of the bearing body.

To provide a particularly low friction bearing, the bearing members are preferably roller bearings.

However, in order to provide a lower cost solution, the or each bearing member may be a sliding bearing element with at least a bearing surface thereof of low friction material (e.g. polytetrafluoroethylene) . In this case the bearing members are preferably wedge shaped so that one surface thereof is parallel to the surface of a ramp and another surface thereof is parallel to the guide surface. A plurality of wedge-shaped bearing members may be provided distributed around the guide surface. Alternatively the bearing member(s) may be in the shape of a collet.

An example of a linear bearing for a guide in the form of a shaft comprises a bearing body for at least substantially surrounding the shaft and a plurality of bearing members disposed around the shaft for bearing on the shaft.

An example of a linear bearing for a guide having one or more rails comprises a bearing body for at least substantially surrounding the rails and a plurality of bearing members disposed with respect to the bearing body for bearing on the rail(s).

The body of the linear bearing may have a longitudinal slot to permit the linear bearing to pass supports for the guide.

Accordingly, examples of the present invention provide a multi¬ purpose linear bearing assembly that is adjustable in relation to the shaft on which it runs. The bearing consists of a group of bearing members mounted in a housing and in contact with a shaft that passes between them so as to allow rectilinear motion to take place between the bearing and the shaft. Adjustment is effected by the simultaneous positioning of the rollers in relation to a tapered or ramped surface within the housing.

Examples of the invention will be described hereinafter, by way of example only, with reference to the accompanying drawings in which like elements are identified by like reference signs and in which:

Figure 1 is an exploded assembly drawing of a first embodiment of the invention;

Figure 2 is an exploded assembly drawing of a second embodiment of the invention;

Figure 3 is an exploded assembly drawing of a third embodiment of the invention; ^■

Figure 4 is an isometric view of the housing of the third embodiment; Figure 5 is an exploded assembly drawing of a fourth embodiment of the invention;

Figures 6A and 6B are an end view and a cross-sectional view respectively of a fifth embodiment of the invention;

Figures 7A and 7B are an end view and a cross-sectional view respectively of a sixth embodiment of the invention;

Figure 8 is a schematic representation examples of possible dispositions of bearing members for different shaft shapes;

Figure 9 is a side view in cross-section of a seventh embodiment of the invention; Figure 10 is a side view in cross-section of an eighth embodiment of the invention; and

Figure 11 is a side view in cross-section of a ninth embodiment of the invention.

Figure 1 illustrates an exploded assembly drawing of a first example of a linear bearing in accordance with the invention.

The body 10 of the bearing (e.g., of a metal such as steel or aluminium, or a suitable plastics material) consists of a cuboid housing with a round internal bore 12 of round cross-section which has four diagonally opposite slot grooves 14 (at substantially equal angular spacings around the end of the bore 12) at each end of the bore 12 to give clearance for the mounting of eight bearing members in the form of rollers 16. End caps 24 incorporating an oil seal 26 are a press fit to either end of the bearing body 10.

The eight rollers 16 consist of shielded ball races, each provided with an axle 18 that is an interference fit to the centre of the race and converts it to use as a wheel/roller. The rollers 16 may alternatively be constructed in any other suitable manner. The roller axles 18 bridge the recesses 14 at either end of the housing bore 12 and rest on a tapered or inclined surface 20 defining a cone in the internal face of the bore 12. This cone forms an adjustment ramp 20 as will be described hereinafter. Each set of four rollers 16 at either end of the bore 12 are maintained in their correct positions relative to each other and to the housing by a respective hollow cylindrical carrier 28 provided with four slotted recesses 30. The slotted recesses 30 are located at substantially equal annular spacings around each carrier 28 to correspond to the positions of the grooves 14 in the housing bore 12. The carriers 28 fit within the bore 12 of the housing and the end caps 24 with sufficient clearance to enable them to slide freely.

When the carriers 28 are moved longitudinally inside the housing the roller axles are moved in relation to the slope of the ramp or taper 20 on which they rest. In this embodiment the taper or incline of the ramp is such that the internal diameter of the bore 12 at the end of the ramp 20 nearer to the end of the bearing body 10 is greater than the internal diameter of the bore 12 at the end of the ramp 20 nearer the middle of the housing body 10. The ramp at the other end of the bearing body fits the same description, that is it forms a mirror configuration with respect to the ramp 20 illustrated in Figure 1.

As the axles 18 move up the incline of the ramp, the rollers are drawn radially inwards against the shaft (not shown) which in use passes through the hollow centre of the cylindrical carriers 28 and the bore 12 and on which the bearing runs. This has the result of removing clearance that would otherwise exist between the shaft and the bearing. By moving the roller axles 18 down the incline, the rollers are able to move away from the shaft allowing a larger shaft to be fitted.

Four tension springs 32 pull the carriers inwards towards each other, drawing the axles the maximum distance up the incline of the ramp 20 until clearance between the bearing members and the shaft is substantially nil. The strength of the springs can be chosen to suit the dimensions of the bearing and the torsional and other forces to which the bearing is to be subjected, as will be apparent to one skilled in the art.

This self-adjustment mechanism compensates for wear and for variations in shaft dimensions. Figure 2 illustrates a second embodiment of the invention. The general construction of this linear bearing is similar to that of Figure 1 and accordingly only the differences between the embodiments will be described. In the embodiment of Figure 2, the inclines of the adjustment ramps 20- of Figure 1 are reversed to form the ramp 3^ (and a corresponding mirror image ramp at the other end of the bearing body) so that as the carriers move away from one another, clearance between the bearing members and shaft is reduced. Accordingly, in order to urge the carriers 28 away from one another, that is in the direction required to urge the bearing members against the shaft (not shown) which in use passes though the centre of the circular carriers 28 and the bore 37 of the body 3 1 the four tension springs 32 of Figure 1 between the shaft and the bore are replaced by a single compression spring 36 that encircles the shaft. In order to secure the end caps 38, they are arranged as a snap-on fit to a bevelled groove 40 provided at each end of the housing and are therefore external to the bore in which the carriers operate. Suitable oil seals 39 are provided within the end caps 38. Figure 3 illustrates a version of a linear bearing in accordance with the invention with a cylindrical body 44 with an external surface of circular cross section. The internal form and operation of the bearing are generally the same as in the example of Figure 2 except that six rollers 16 are used instead of eight. Also, in this embodiment, rather than being retained in grooves as in the embodiment of Figure 2, the rollers protrude through slots 5 at each end of the cylindrical body and are externally visible components of the assembly. End caps 46 and seals 47 are provided which are adapted to the shape of the body 44. In the embodiments of Figures 1 and 2, the four rollers grouped at each end of the body 10,35 of the bearing can be adjusted simultaneously. In an embodiment such as that illustrated in Figure 3. however, it is necessary to provide adjustability for only one of the rollers at each end to avoid play between the bearing and the shaft. Figure 4 represents a modified version 48 of the body of the bearing of Figure 3. which modified body 48 accepts the axle 18 of two sets of three adjustment rollers 16 but only provides adjustment for one roller 16 of each set. Accordingly a single inclined recess or ramp 50 is provided for the adjustable roller 16 at each end. As the adjustment roller 16 is moved relative to the ramp by the carrier 28, the remaining two rollers simply follow the parallel internal face of the internal wall of the bore of the body 48 and are not therefore subject to adjustment.

Where all rollers are to be adjusted simultaneously, an arrangement of individual flat inclined ramps, such as is illustrated in Figure 4 for the single adjustable roller 16, may be provided for each of individual roller 16, as an alternative to the conical ramp or taper 12/34 of Figures 1 to 3> The adjustment surfaces may be curved or otherwise profiled in order to enhance adjustment performance and/or faced with inserts of a low-friction material such as polytetrafluoroethylene (PTFE) . Figure 5 illustrates a further embodiment of the invention in which, rather than the rollers of Figures 1 to 4, the bearing members are in the form of low friction blocks or segments 52 of a generally wedge-shaped configuration and made of PTFE or a similar material with a very low coefficient of friction. The bearing blocks are located in carriers 4 within slots 56. The bearing blocks are subjected to adjustment by movement within grooves 8 in the bearing body 60. The grooves 58 have suitably inclined radially outward faces so that, for example in the embodiment of Figure 5t movement of the sets of blocks at the ends of the body 60 towards each other causes the blocks to be urged radially inward to take up any play between the blocks and a centrally located shaft (not shown) . Thus it will be seen that the blocks are adjusted in a manner similar to that of the roller bearings of Figure 1 with the exception that the contact with the shaft is a sliding rather than a rolling contact. With the floor, or radially outward wall of each of the grooves 58 inclined at an angle to provide an adjustment ramp, the bearing blocks 52, positioned by a simplified carrier 54 are drawn inwards by the tension springs 32, move up the incline of the ramp and thereby close up against the shaft on which the bearing runs. It will be appreciated that in alternative embodiments similar in configuration to those of Figures 2, 3 and 4, bearing blocks similar to the blocks 52 of Figure 5 can be used in place of the rollers 16, with appropriate detailed modifications to the carriers, slots and grooves as will be apparent to one skilled in the art. Thus, for example, if the ramp incline is reversed and a compression spring substituted for the tension springs, an arrangement similar to that of Figure 2 can be achieved. The use of bearing blocks in this form can lead to a considerable reduction in the width or diameter of the bearing. Additional slots and blocks may be added to enhance performance and load-bearing characteristics.

An alternative embodiment of the invention is illustrated in Figure 6. Figure 6A is an end view of the linear bearing and Figure 6B is a cross-sectional view along the line A - A in Figure 6A. In this embodiment, an internal cone 64 at either end of the bore in a bearing body 62 (e.g. , of a plastics material such as nylon or ABS) contains a bearing member 66 made of PTFE or similar low friction material 66. The external configuration of the bearing member 64 is arranged to match the profile of an end of the bore including the cone shaped portion 64. A central hole 68 accepts the shaft on which the bearing runs.

A number of longitudinal cuts 70 which stop short of the inner extremity 7 of the bearing member 66 divide the bearing member into a plurality of bearing segments 74 to form a collet. As the collets are drawn inwards by tension springs 76, they close against the shaft so maintaining substantially zero clearance.

Figure 7 illustrates a further embodiment of a linear bearing in accordance with the invention. Figure 7A is an end view of the linear bearing and Figure 7B is a cross-sectional view along the line A - A in Figure 7A. A body 80 (e.g., of a plastics material such as nylon or ABS) with a circular external cross-section is provided with a flange 82 at either end to accept inserts 84. Each insert is provided with three dove-tailed grooves 86 with an inclined rear surface forming a ramp. A corresponding wedge shaped bearing member 88 is located in each groove 86. The bearing members are made of PTFE or another low friction material.

The wedge shaped recesses formed by the grooves 86 and occupied by the cooperating wedge shaped bearing members 88 narrow towards the open ends of the bearing body. The wider end 0 of each of the bearing members 88 protrudes internally beyond the grooves 86. Two washers 2 rest against respective internal protrusions 90 of the bearing members 88 at either end of the bearing body and are pushed apart by a compression spring 94.

As the bearing members are pushed outwardly by the washers under the influence of the compression spring, following the angle of the grooves in the insert, they are urged towards the shaft 96 on which the bearing runs. ■

As with previous examples, the angle of inclination of the grooves may be reversed and the compression spring replaced by tension springs. As with the embodiments of Figures 3 and 4, clearance between the bearing and its respective shaft or rail may be controlled by the adjustment of only one bearing member in each group of three, the remaining bearing members being held in a fixed position. The number of bearing blocks may be increased according to the diameter of the bearing and the shaft or guide form required.

In all of the above embodiments, contact between the bearing and the shaft or rail may be increased, and possible rotational movement of the bearing inhibited, by the provision of flat contact surfaces along the length of the shaft. The same effect may be achieved by the use of a shaft that is polygonal in section. Figure 8 illustrates some examples of possible shaft and bearing member options including substantially circular and polygonal shapes as well as bar and cross- form shapes, although it will be appreciated that many other embodiments are possible. In the case of guides in the form of shafts or rails having curved guide surface(s) , contact may also be increased by the use of bearing members with concave contact surfaces (e.g., ball-races with concave outer rings or fitted tyres to suit the radius of round shafts, or bearing blocks with a concave contact surface). A bearing with eight bearing members may be converted to run on both round and square shafts by the selection of modified end-caps and seals. The construction of the bearing may be modified to incorporate larger numbers of bearing members in order to meet specific loading demands or shaft requirements. Shafting for arrangements with multiple bearing members may be round in section or sided or polygonal according to the number of bearing members.

Figure 9 shows yet a further embodiment of the invention wherein roller axles 102 rest on inclined ramps or flats 104 provided in the external surface of a cylinder 106 so as to correspond with clearance slots 108 in a carrier 110. The carriers 110 and spring 112 operate externally so as to adjust the rollers in relation to the internal dimensions of a tube 100 inside which the linear bearing operates. A further possible adaptation interposes the bearing between an external tube and an internal shaft.

Figure ip illustrates a system with a configuration generally similar to Figures 2 and 3. but with a system of manual adjustment where the springs are replaced by a screw thread system. The cylindrical body 120 is provided with an internal screw thread 121 at either end which accepts the corresponding external thread of an adjuster ring 122. An oil seal 123 is interposed between the ring 122 and a carrier 124. As a ring 122 is tightened, the corresponding carrier 124 is moved inwardly urging the bearing members (e.g., rollers 16) against the adjustment ramp 125. so causing the bearing members to be urged against the shaft 126 on which the bearing operates. It will be understood that each end of the bearing may, in this configuration, be adjusted independently of the other end.

Figure 11 illustrates a system similar to that in Figure 10, except that the angle of the adjustment ramps is reversed and an internal thread on the adjustment ring 129 co-operates with a corresponding external thread on the carrier 130. An oil seal is retained by an internal groove in the carrier 130. As a ring 129 is tightened, the corresponding carrier 130 is pulled outwardly so urging the bearing members (e.g, rollers 16) against the shaft 126. Once again, each end of the bearing may be adjusted independently of the other end.

It will be appreciated that a similar principle of fixed adjustment may be applied in place of the resilient adjustment means of the previous embodiments.

The embodiments with a plurality of resilient (e.g., spring) members to provide the resilient force for urging the bearing members against the guide surface(s) can be provided with a split or open portion (not illustrated) of the body of the bearing to enable the linear bearing to pass shaft supports which might be provided along its length. A longitudinal split through one side of the bearing body and a corresponding split through the end-caps and carriers provides clearance for intermediate shaft supports to pass through the bearing and between the tension springs thus enabling a long shaft to support heavy loads without deflection. It will be understood that the spilt in the body, although not illustrated in the drawings, extends from one end of the body to the other in an axial direction substantially parallel to the axis of the shaft or rails so that the bearing may pass supports without colliding with them.

There have been described embodiments of a linear bearing in accordance with the invention. Movement of the linear bearing in accordance with the invention is substantially linear, guided by the contact of the rollers, guide blocks or other guide members against the shaft rails or other guides on which it runs.

Embodiments with self-adjustment provide a major advantage over existing technology in that there is no necessity for the manual adjustment of the bearing or the matching of bearings to shafts in order to achieve correct pre-load or clearance. In many applications the self-adjustment system obviates any necessity for the specialised hardened and precision-ground shafts required by conventional linear guidance systems. An embodiment of the invention has free-running ability that will tolerate a severe overturning moment imposed by cocked loading. Although specific examples of the invention have been illustrated, it will be appreciated that many modifications and/or additions can be made within the scope of the invention. Thus, for example, although embodiments of linear bearings for use with specific shapes of shafts or rails have been described, it will be appreciated that the invention can be used with other configurations of shafts, rails or other guide structures. Thus, for example, the invention could be applied to a rail structure comprising a central support structure and two rails on opposite sides of the support structure. In such a case, or indeed in the examples described above, a longitudinal split in the body of the linear bearing can enable the linear bearing to pass the central support structure of the guide. In the above embodiments with spring loading, the carriages at either end are loaded by means of common springs. However, it will be appreciated that the carriages could be separately loaded by respective springs (e.g. , between a carriage or other bearing mount and a position on the bearing body). Also, it will be appreciated that springs (e.g., leaf springs) of types other than those illustrated could be used for loading the bearing members, either individually or via carriages. Indeed, the springs could be replaced by other suitable loading means, including, if desired, motors, solenoids, etc, as appropriate to a particular application.

In the above embodiment the bearing members are moved along a ramp in the axial direction of the linear bearing so that cooperation with the ramp enables adjustment of the bearing position transversely to the axial direction. Alternatively, however, the bearing members could be arranged merely to move transversely to the axis of the linear bearing, with wedge members being arranged to move axially, for example under the action of springs, so that by cooperation with a ramped surface of the wedge, the bearing members are caused to move towards a shaft or other guide member on which the bearing runs. Also, instead of separate carriages at either end of the linear bearing with ramps having opposed inclines, a single carriage with ramps having the same, that is parallel inclines could be provided. In such a case, loading means between the carriage and the body could be used to urge the bearing members towards the shaft or other guide member on which the bearing runs.

Claims

1. A linear bearing for substantially linear motion along an elongate guide having a guide surface, the linear bearing comprising a bearing body, at least one bearing member adjustably mounted in the body for bearing on the guide surface and a mechanism for, in use, urging the bearing member against the guide surface.

2. A linear bearing according to claim 1 comprising a self-adjusting mechanism for urging the bearing member against the guide surface.

3. A linear bearing according to claim 2, wherein the adjustment mechanism comprises an inclined ramp, the adjusting mechanism urging the bearing member along the ramp whereby the bearing member is caused to be urged against the guide surface.

4. A linear bearing according to claim 3 comprising resilient means for urging the bearing members along the ramp.

5- A linear bearing according to any one of the preceding claims comprising first and second adjustable bearing members at or near opposite ends of the bearing body, said first and second bearing members being moveable along respective first and second ramps with opposed inclines.

6. A linear bearing according to claim 5 comprising a compression spring for urging the first and second adjustable bearing members apart.

7- A linear bearing according to claim comprising one or more tension springs for urging the first and second adjustable bearing members towards one another.

8. A linear bearing according to any one of claims 5. 6 and 7 comprising first and second sets of adjustable bearing members at or adjacent opposite ends of the bearing body.

9. A linear bearing according to claim 1 comprising a manually adjustable mechanism for urging the bearing member against the guide surface.

10. A linear bearing according to any one of the preceding claims wherein the bearing members are roller bearings.

11. A linear bearing according to any one of the claims 1 to 9 wherein the or each bearing member is a sliding bearing element with at least a bearing surface thereof of low friction material.

12. A linear bearing according to claim 11 wherein the low friction material of the bearing member or the bearing surface thereof is polytetrafluoroethylene.

13. A linear bearing according to claim 11 or claim 12 comprising a plurality of wedge shaped bearing members.

14. A linear bearing according to claim 11 or claim 12 comprising one or more bearing members in the shape of a collet.

15- A linear bearing according to any one of the preceding claims for a guide in the form of a shaft comprising a bearing body for at least substantially surrounding the shaft and a plurality of bearing members disposed around the shaft for bearing on the shaft.

16. A linear bearing according to any one of claims 1 to 15 for a guide comprising one or more rails comprising a bearing body for at least substantially surrounding the rails and a plurality of bearing members disposed with respect to the bearing body for bearing on the rail(s) .

17. A linear bearing according to claim 15 or claim 16 wherein the body has a longitudinal slot to permit the linear bearing to pass supports for the guide.