WO1997029303A1

WO1997029303A1 - Nut and screw arrangement

Info

Publication number: WO1997029303A1
Application number: PCT/GB1997/000332
Authority: WO
Inventors: Ivor Gillbe
Original assignee: Ivor Gillbe
Priority date: 1996-02-06
Filing date: 1997-02-05
Publication date: 1997-08-14
Also published as: GB9602321D0; AU1611797A

Abstract

The invention provides a nut and screw arrangement of the type suitable for converting rotary into linear motion or vice-versa. The nut (2) is made up of two or more circumferentially spaced segments (3) which are each radially movable. Each segment (3) is biased radially inwardly, onto the screw (1), by two sliding cam bodies (or other means) (6, 7), which may be common to all of the segments or be provided for each segment separately. The cam bodies (6, 7) bear on cam surfaces (4, 5) of the corresponding nut segments (3) and are preferably in the form of wedges acting on tapered surfaces of the segment to bias it inwardly. The sliding cam bodies (6, 7) are biased in opposite axial directions, for instance away from each other by a biasing means (8) extending between them. With the present invention, each segment (3) is urged radially inwards due to the action of the biasing means (8) to compensate both for backlash and long term wear.

Description

NUT AND SCREW ARRANGEMENT BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a nut and screw arrangement for converting rotary into linear motion or vice versa.

SUMMARY OF THE PRIOR ART

A nut and screw arrangement provides a convenient means of converting rotary into linear motion, which is required for many applications such as machine automation. Conventional methods of achieving this include ball-screws, planetary roller-screws and plain nuts of various materials. For high speeds, ball screws are one solution but the speed is limited by the requirement to re-circulate the balls, which also causes high speed operation. to be relatively noisy. Ball screws also have limited acceleration capability since the balls are prone to skidding under high acceleration, which can lead to premature failure. Planetary roller screws provide higher speeds and accelerations, but need to be manufactured to high precision in order to function correctly and are therefore prohibitively expensive for many applications. High performance plastic bearing materials, loaded with lubricants such as graphite, provide the capability to achieve very high sliding speeds. However, when the load is significant, wear causes a plain nut constructed from these materials to develop end-play (backlash) and hence become unserviceable over a relatively short service life.

There are a number of known methods of eliminating backlash in plain nuts, the most common of which comprises two nuts, mounted back-to-back, separated by a spring as in eg. GB-A-2105816. This causes any end-play caused by wear to be taken-up by relative movement of the two nuts. In another implementation, in US-A-5027671, one end of a nut is split three ways and a spring and ring arrangement causes these elements to be forced against the screw-shaft. The other end of the nut is an integral unit against which the spring bears . A further known system uses a rigid spacer to eliminate end-play in the nut by separating the two nuts, and this spacer is automatically adjusted by means of a fine thread and torsion spring. These configurations, while being effective at eliminating backlash, do not offer an effective solution to bulk material removal from the nut caused by significant long term wear. SUMMARY OF THE INVENTION

At its most general, the present invention proposes that the nut of a nut and screw arrangement has at least two circumferentially spaced segments which are each radially movable and an arranged about a common axis. Preferably, each segment is biased radially inwardly, onto the screw, by two sliding cam bodies, which bear on cam surfaces of the corresponding segment of the nut. The cam bodies are biased in opposite axial directions.

Preferably, the cam bodies are in the form of wedges, which act on tapered surfaces of the segment, biasing the segment inwardly. The opposite biasing of the cam bodies is provided by a spring, or other biasing means, extending between the cam bodies and hence biasing them away from each other, but it is also possible to have separate biasing means biasing the cam bodies towards each other. The use of two cam bodies, acting in opposite directions, has the advantage that two forces may be applied at locations spaced apart in the axial directions of the segment, thereby preventing any twisting apart of the segment away from the screws . Since a plurality of segments are provided around the circumference of the screw it is convenient if the cam bodies extend annularly around the screw, so that they each act on all the segments . The annular cam bodies may then be in the form of a split ring. In such an arrangement, there will normally be a plurality of springs, or other biasing means, around the circumference of the screw, it is then preferable that the biasing means are in housings aligned with the axis of the screw shaft. An alternative to the use of annular cam bodies is to provide separate cam bodies for each nut segment . This may be desirable for ease of manufacture, since such separate cam bodies may be relatively easily manufactured using techniques such as precision die casting, moulding, or extrusion, followed by minimal machining.

Where separate cam bodies are provided for each nut segment it is preferable that they are nevertheless physically linked in order that they move in unison to avoid unwanted jamming and help maintain concentricity. This can be achieved, for example, by using a common biasing means to act on circumferentially adjacent cam bodies . As was mentioned above, the nut has at least two, preferably three circumferential face segments. The use of three segments permits the nut to have a self-centring action on the screw shaft, to ensure the nut and screw remain concentric (or co-axial) as wear occurs. It is possible, however, to have more segments.

With the present invention, each segment is moved radially inwards due to the action of the biasing means, so that new material is provided as wear of the segment occurs at the part in which it is in contact with the screw. In this way, the present invention permits both backlash compensation and compensation for long term wear, together with the ability to carry high loads at high speeds. The action of the cam bodies causes the segments to move inwards so that their radially inner surface is always concentric with the periphery of the nut .

It is also preferable that no part of the thread form on the nut segments is perpendicular to the common axis of the nut segments. Various thread configurations are possible, such as triangular threads or sinusoidal threads . This ensures that radial motion of the nut segment provides additional material to compensate for wear at the surface of the thread.

In practice, frictional heating of the shaft during repeated high speed motion may be a problem. It is therefore preferable that the nut and screw arrangement is lubricated. However, not all lubricants have been found appropriate and it is therefore particularly preferred that lubrication is achieved using an oil bath. The nut and the part of the screw along which it travels are preferably housed in a sealed chamber at least partially filled with a lubricating fluid, such as oil. Housing the nut and screw arrangement in a sealed cavity can also be used, with or without a lubricating fluid, to provide a gas spring effect. This effect can be enhanced by raising the gas pressure in the sealed chamber to a pressure greater than atmospheric pressure. BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 is a sectional view through part of a nut and screw arrangement according to a first embodiment of the present invention;

Fig. 2 is a transverse sectional view through the nut and screw arrangement of Fig. 1, Fig. 1 being along the line Y-Y in Fig . 2 ;

Figs 3a and 3b show part of the nut and screw arrangement of Fig. 1 in more detail;

Fig. 4 shows a perspective view of the nut and screw arrangement of Fig. 1;

Figs. 5a and 5b show an annular ring forming a cam body in a second embodiment of the present invention, Fig. 5a being a view of the ring in the axial direction and Fig. 5b being a sectional view along the line Z-Z in Fig. 5a;

Figs. 6a to 6d show the nut segment used in the second embodiment, Fig. 6a being an end view of the nut segment, Fig. 6b being a sectional view along the nut segment, along the line A-A in Fig. 6a, Fig.6c being a sectional view along the line B-B in Fig 6b, and Fig.6d being a plan view of the nut segment in the second embodiment;

Fig. 7 illustrates a non-return arrangement which may be used in embodiments in the present invention; Fig. 8 shows an alternative non-return arrangement which may be used in embodiments in the present invention;

Figs 9a to 9d show a third embodiment of the present invention, Fig 9a being a sectional view through part of the nut and screw arrangement of the third embodiment, Fig 9b being a sectional view along line C-C of Fig 9a (Fig 9a being along line D-D in Fig 9b) , and Figs 9c and 9d illustrating two arrangements for retaining the springs in the third embodiment; and

Fig 10 shows an embodiment of the present invention incorporated in a linear ram. DETAILED DESCRIPTION Referring first to Fig. 1, a shaft 1 (shown in outline) has a nut 2 mounted thereon. In Fig. 1, the nut 2 is shown schematically, extending along the shaft 1. The nut 2 has a plurality of segments 3 which are radially moveable and has a threaded surface 12 in contact with a threaded surface of the shaft 1. Each segment 3 has tapered surfaces 4, 5, which taper outwardly in opposite directions, each of which surfaces 4, 5, have a respective ring 6, 7, thereon. The rings 6, 7 have wedge-shaped surfaces which bear on the surfaces 4, 5 of the segments 3.

The rings 6, 7 are biased outwardly in the direction of arrows A, and the camming action of the rings 6, 7 against the surfaces 4, 5 forces the segments 3 radially inwards in the direction of arrow B. Thus, the segment 3, and hence the nut 2, is held firmly on the shaft 1. The outward biasing of the rings 6, 7 is, in this embodiment, provided by four springs 8 extending parallel to the axis of the shaft 1, and bearing on the inner surfaces of the rings 6, 7. The spring 8 is supported at the circumference of the nut 2 by a housing 9 which is used to transmit torque to the nut segments. Preferably, four segments 3 are provided around the circumference of the shaft 1, each segment being acted on by the rings 6, 7. The segments 3 and springs 8 are, in the embodiments, repeated around the screw 1 at 90 degree intervals, although three or more segments 3 may be used. The thread form of the shaft 1 is typically a 45 degree triangular thread. An axial load applied to the shaft therefore causes segments 3 to be forced outwards. In most applications, the thrust applied by the shaft also varies cyclically. It is desirable however that the radial force applied by the spring and ring arrangements 6, 7, 8, is less than the radial force generated at the maximum thrust that the nut 2 can produce, since the wear rate of the nut material is a function of pressure and velocity. In order to prevent the segments 3 being forced outwards when the applied thrust exceeds the radial force applied by the spring 8, the annular rings are split at one point 10 on their circumference such that they expand and jam against the bore of the nut housing 11 during periods of peak loading (the "locked" condition) . The correct selection of spring force is such that the segments 3 are free to move (i.e. the annular wedges are "unlocked") during lightly loaded parts of the operating cycle. Therefore, backlash and wear compensation only occurs during the lightly loaded parts of the operating cycle when the annular rings 6, 7 are unlocked. This ensures that in the unloaded state the pre-load applied by the springs 8 is significantly less than the maximum operating load of the nut 2 so that the average product of pressure and velocity is minimised and hence wear is also minimised.

In a practical implementation the annular rings 6, 7 may be constructed such that they have a number of flats, one of each of which engages with a similar flat on each nut segment 3. This is preferable to the implementation shown in Fig. 1, because once a small amount of wear has occurred, an annular ring 6, 7 only has line contact on each side of the segment 3. This is due to the difference in effective diameter of the ring 6, 7 and the opposing face of the segment 3. This may cause an unnecessary stress concentration in the segment 3, which is typically constructed of a plastic material optimised for friction characteristics, rather than mechanical strength. Axial thrust produced by the nut 2 is transmitted via the flat ends of the segments 3 directly to the nut housing 11. The annular rings 6, 7 and their respective springs are free to float when unlocked and there is therefore no significant jamming effect caused by short periods of peak loads.

In typical implementation, based on a 38mm diameter screw, the design can achieve speeds up to lm/s, accelerations of lOm/s with 3KN peak and 1KN continuous loading with virtually silent operation. In the unlubricated state, life is in excess of 5,000 hours, with a total 3.5mm travel of the segments as they wear.

The nut segment 3 may consist of a carrier with one or more moulded thread sections attached. The advantage of this approach is that the carrier can be cast steel for strength and the threaded sections plastic, optimised for friction characteristics rather than strength. Another reason for a separate carrier is that some bearing materials can only be created by sintering, a process which may impose a limit on the surface area of the component which is lower than that required to create a practical nut segment.

As was mentioned earlier, the annular rings 6, 7 may be constructed such that they have a number of flats, each which engages with a corresponding similar flat on each segment 3. Figs. 5a to 6d illustrate a practical implementation of such an arrangement. Figs. 5a and 5b show an annular ring 20 which is similar to angular rings 6, 7 and has four tapered surfaces 21, forming respective flats, each of which engages a similar flat 22 on the nut segment 23 illustrated in Figs. 6a to 6d. The arrangement is thus similar to the engagement of the tapered surface of the rings 6, 7 with the surfaces 4, 5 but instead of both surfaces being annular, the flats 21, 22 are plane. The advantages that the surfaces 21 are then in plane contact with the nut segment 23 , thereby improving the load bearing capabilities of the device. A ring similar to the ring 20 is also provided in contact with the flats 24.

Figs. 5b shows that the ring 20 has four projections 25, which prevent the axis of the ring 20 twisting relative to the longitudinal axis of the nut 23. The projections 25 also engage with cavities 26 on the nuts 23. The projections 25 ensure that the length/diameter ratio of the ring 20 is such that it will not twist in the nut housing 11 (not shown on Fig. 6b) . This ensures that the axis of the ring 20 is always aligned with the axis of the nut housing 11. This is preferable to ensure that the ring 20 does not jam, and also to ensure that the motion of the ring 20 is such that the flats 21 are always concentric with the axis of the nut 23. This latter point is of particular importance in systems where the nut rotates, because if concentricity was lost, vibration of the screw shaft may then result.

The jamming action of the rings can be enhanced by putting a fine saw tooth profile on the tapered surfaces 21 (Fig. 5b) and a corresponding profile on the flats 22, 24 (Fig. 6b) . Alternatively, a similar saw tooth may be provided on the periphery of a split ring and the inside of the nut housing 11. In this manner a ratchet action is achieved, preventing the nut segments 3 moving outwards in a regime where the mechanism is subjected to vibration.

Other variations to the above embodiments are possible. For example, in the embodiment of Fig. 1, the rings 6, 7 are biased axially in opposite directions by the spring 8. Other types of biasing may be used and it is also possible to have the rings 6, 7 biased inwardly by reversal of direction of the camming surfaces. It is also possible to use rings which. are not split, so that they do not lock against the housing when subject to peak loading.

Furthermore, other non-return mechanisms may be used. For example, cam bodies may be used which have friction characteristics and angle of wedge which jam naturally.

Alternatively, a non-return mechanism using a spring clip acting on a shaft may be used, as illustrated in Fig. 7. As shown in Fig. 7, a spring clip 30 of known type is fitted on to a shaft 31 which is attached rigidly to one of the rings 6 and slides in a bore in the second ring 7. The clip 30 is a push-on fixing to the shaft 31, and as the spring 8 extends to take up wear in the rings 6, 7 and nut segment 3, the spring clip 30 moves along the shaft 31. Because of the shape of the spring clip 30 it can only move in one direction along the shaft 31, that direction being the direction of elongation of the spring 8. Thus, a simple non-return mechanism is achieved. This arrangement has the advantage of low cost, but has the disadvantage that stiction is relatively high, which places a high minimum value on the spring rate. This limits the design freedom and it is also necessary carefully to control the diameter of the shaft 31. A further non-return mechanism is shown in Fig. 8.

In this arrangement, a lever action pawl 41 is provided between the spring 42 and one of the rings 7. Spring 42 then acts on the other ring 6 (not shown in Fig. 8) via a similar pawl. The pawl 41 is free to move over the housing 11 in the direction of extension of the spring 42, and thus the rings 6, 7 can move to compensate for wear. However, if the ring 7 is forced in the opposite direction, against the spring 42, the pawl 41 tends to rotate about the point 43 and has a projection 44 which is then jammed against the housing 11. Again, a non-return arrangement is achieved.

Another possible non-return mechanism is similar to Fig. 7, except that the shaft 31 is replaced by a threaded shaft and the spring clip 30 is replaced by a nut. The spring 8 becomes a torsion spring which rotates the nut against the ring.

Other types of cam bodies are possible to cause the segments to move inwards radially. These include a series of rotary cams, which act on the segments and are biased by torsion springs. Also possible are direct application of force by means of springs acting directly on the cam segments, i.e. with their axes at right angle to the axis of the screw shaft and radially dispersed about the axis. For light loads, no non-return mechanism may be required. For heavier loads, the non-return mechanism may consist of a torsion spring acting on a nut or other known means. Another embodiment of the present invention is shown in Figs 9a to 9d.

This embodiment uses cam bodies, 45, which engage with a key-way 46 in the nut housing 47. Two opposing but identical cam bodies 45 support the nut segment 48, via two plain cam faces (wedges) 49 of the segment 48. The cam bodies 45 are urged apart in the directions A, urging the nut segment 48 radially inwards to take up wear at the nut face 50.

Previously it was mentioned that the axial force generated by the nut should preferably be transmitted through the flat ends of the nut segments, rather than via the cam bodies, since the latter may cause a high radial force at the nut face. An improvement in this respect is to construct a cylindrical sleeve housing the nut, which is free to float axially with respect the ends of the nut housing. An example of this is illustrated in figure 9b. The nut segments 48 are constrained from moving axially within the nut housing 47 by two thrust plates 72, 74. These are retained in the housing 47 by means of two large circlips 73. An axial force on the shaft generates a radial force (because of the triangular shape of the thread on the nut segments) which may cause the cam bodies 45 to jam against the nut housing 47, thereby preventing retraction of the nut segments, which is desirable. However, the axial component of the force is transmitted directly via one of the thrust plates, 72, 74. In a practical implementation, the thrust plates will typically be biased in one direction by a large diameter spring washer between one circlip and thrust plate to eliminate backlash in the assembly. The thrust plate may also be coated with a low friction material to facilitate radial movement of the nut segments as wear occurs .

In this embodiment, unlike the embodiments already described, eight separate cam bodies 45 are used, one pair for each of four nut segments 48 around the circumference of the shaft. Each pair of cam bodies is urged apart in the directions A by springs 51 disposed between the adjacent pairs of cam bodies 45.

In one possible arrangement each spring is retained by and acts on a pin, 52, shown in outline in fig 9a and in plan in fig 9c. This pin 52 bridges the gap between two circumferentially adjacent cam bodies. The pin is a sliding fit in the cam bodies to allow each individual cam body 45 to move radially outwards and act like a wedge, jamming against the nut housing 47, and nut segment 48, during periods of high load on the screw, thereby preventing the nut segments, 48, from retracting radially.

An alternative arrangement for retaining the spring 51 is shown in fig 9d. In this arrangement the spring is retained in a closed end spring pocket, 53, formed by two circumferentially adjacent cam bodies, 45. Each end of the spring is retained in this way and acts on the two adjacent cam bodies simultaneously. In both of these retention arrangements each spring

51, acts simultaneously on the four cam bodies, 45, it sits between (two at either end) ; in the first case via pins, 52, and in the second case on the closed ends of the spring pockets . This causes the cam bodies to move approximately together thereby assuring that the concentricity of the nut and shaft is maintained as wear occurs . In this embodiment of the invention the cam bodies,

45, are easier to manufacture than the other described embodiments, since they are not formed as a single ring. Each cam body can be produced from a relatively simple precision die casting or moulding, or by extruding a length of the cam shape and machining the ramp 49. It should be noted that the arrangement shown in fig 9d is not suitable for manufacture by extrusion since the spring pocket terminates some way down its length.

Frictional heating of the shaft 1 during repeated high speed motion may affect the duty cycle capability of a screw arrangement according to the present invention. Many bearing materials are not significantly affected by high shaft operation temperatures, but such temperature may necessitate use of particular lubricants and may affect other components of the arrangement. For instance, it has been found that lubricants comprising organic oils tend to burn off during use and be removed from the shaft by the rubbing action of the nut .

In such circumstances, it is possible to use a hollow screw 1, with forced ventilation through the interior of the screw 1 to reduce the temperature of the screw. More preferably, lubrication is carried out using an oil bath, the nut and screw arrangement being housed in an assembly capable of retaining oil. An example of such an assembly is shown in fig 10, in this case used in a linear ram, although it is equally applicable to other apparatus. The ram comprises a pair of telescoping tubes 55, 57 the tube 57 telescoping into and out of the tube 55 under the influence of a nut and screw arrangement 54, 56 driven by a motor 70. The nut 56 is a nut according to an embodiment of the present invention. The screw 54 is disposed within the tube 55 and is co-axial with it, defining an annular cavity in which the nut 56 travels. The tube 57 is fixed to the nut 56 so that it telescopes as the nut 56 is driven along the shaft 54. Both ends of the tube 55 are sealed; the upper end by a bush and seal arrangement 58 through which the tube 57 passes, and the lower end by a seal 60 through which the screw shaft 54 is connected to the motor 70.

The screw shaft 54 is supported at its lower end by bearings adjacent to the motor 70 and at its upper end by a bearing 59 the periphery of which slides on the inside surface of the tube 57.

The annular space defined between the screw shaft 54 and the inner surface of the tube 55 is partially filled with oil 71. The oil 71 may be of an organic or synthetic composition, the latter being preferred for its typically superior life at high operating temperatures. During operation, as the nut 56 moves along the screw shaft 54 it causes the oil 71 to be dispersed around the inside of the tube 55 coating the nut 56 and screw 54. The oil 71 acts to lubricate the shaft 54 and also to help conduct heat generated by friction from the shaft 54 and nut 56 to the wall of the tube 55, which may be fitted with fins to further aid the dissipation of heat.

If it is desired to further enhance the cooling, the oil may be circulated via a cooling system including e.g. a radiator. However, it is thought that for most applications a sealed unit will provide sufficient cooling and this is preferred for simplicity.

Enclosing the nut and screw arrangement of the present invention in a sealed chamber, for example the tube 55 illustrated in fig 10, can be used to provide a further advantageous effect. Using the fig 10 arrangement as an illustration, it can be seen that the volume of the annular chamber between the screw 54 and the wall of the tube 55 changes as the ram is retracted and extended. With a sealed assembly, this results in a change in gas pressure inside the assembly and hence a force on the ram tending to urge it towards its extended position. In other words, the annular cavity acts as a gas spring. The magnitude of this change in pressure can be altered by varying the relative internal volumes of the tube 57 and the tube 55, and also by the quantity of oil inside the assembly, where an oil bath is used.

If, for example, the assembly is required to support a gravity load, the pressure of gas inside the unit can be increased so that the gravity load is mostly supported by the gas pressure rather than the nut. This has the benefit of reducing both nut wear, frictional heating and armature losses in the motor due to the current flowing in the windings when not rotating.

In some applications the ram may be required to support a gravity-load, which is supported by a swinging arm on which the ram acts . The geometry of the arm and the ram may dictate that the gravity load on the ram is greater when the arm is retracted than when it is extended. Advantageously, careful selection of the relative volumes of the tubes of the ram can result in the change in load due to geometry being approximately equal to the change in force due to the gas pressure. This type of arrangement is, for example, commonly encountered in motion platforms for simulators, for which this invention is particularly suitable due to its inherent smoothness of operation, high acceleration and high speed capability when compared to other types of electrical and linear motion devices.

Claims

CLAIMS :

1. A nut for a nut and screw arrangement comprising at least two discrete nut segments, each having a threaded surface, the threaded surfaces being concave and the nut segments being arranged such that the threaded surfaces have a common axis of curvature, the nut segments being biased radially inwardly towards the common axis.

2. A nut according to claim 1, wherein there are three nut segments.

3. A nut according to claim 1 or claim 2, wherein the walls of the threads of the threaded surfaces of the nut segments are inclined to the common axis at angles which are always less than 90°.

4. A nut according to claim 3 , wherein the threads of the threaded surfaces are triangular.

5. A nut according to any one of the preceding claims, wherein the nut segments are biased radially inwardly by two sliding cam bodies which bear on cam surfaces of the corresponding segment of the nut, said two cam bodies being biased in opposite axial directions.

6. A nut according to claim 5, wherein the cam bodies are in the form of wedges which act on respective tapered surfaces of the segments to bias them radially inwardly.

7. A nut according to claim 5 or claim 6, wherein the two sliding cam bodies are biased axially away from each other by one or more biasing means extending between the two cam bodies .

8. A nut according to any one of claims 5 to 7, wherein the cam bodies extend annularly around the central axis of the nut so that they each act on all of the segments of the nut.

9. A nut according to claim 8, having a plurality of circumferentially spaced biasing means extending between the two cam bodies.

10. A nut according to any one of claims 5 to 7, having two axially opposed cam bodies for each of the two or more nut segments, said opposed cam bodies being biased axially apart by biasing means which extend between said opposed cam bodies and are retained between circumferentially adjacent cam bodies so as to act on said adjacent cam bodies simultaneously.

11. A nut according to any one of claims 5 to 10, having a housing circumferentially surrounding the nut segments, wherein the cam bodies can expand radially outwardly during periods of peak loading to jam against the housing to lock the nut segments in position.

12. A nut according to any one of claims 5 to 11, wherein the radially inner surfaces of the cam bodies have planar portions which engage corresponding planar portions on the radially outer surfaces of the nut segments.

13. A nut according to any one of claims 5 to 12, wherein the cam bodies have longitudinally extending projections or recesses which engage with corresponding longitudinally extending recesses or projections on the nut segments or a housing circumferentially surrounding the cam bodies, to resist twisting of the cam bodies relative to the longitudinal axis of the nut segments.

14. A nut according to any one of the preceding claims, having a non-return mechanism which allows relative movement between the cam bodies and the nut segments in the direction which urges the nut segments radially inwardly, but prevents relative movement in the opposite direction.

15. A nut according to claim 14, wherein the non-return mechanism comprises complimentary saw tooth profiles on mating surfaces of the cam bodies and the nut segments, or mating surfaces of the cam bodies and a housing circumferentially surrounding them.

16. A nut according to claim 14, wherein the cam bodies are biased axially apart by a biasing means extending between them and the non-return mechanism comprises a locking member disposed between one or both ends of the biasing means and the adjacent cam body to prevent movement of said cam body in a direction causing compression of the biasing means.

17. A nut according to any one of claims 5 to 16, wherein each cam segment comprises a carrier and one or more thread sections mounted on the carrier.

18. A nut and screw arrangement comprising a screw and a nut according to any one of claims 1 to 17 mounted on the screw.

19. A nut and screw arrangement according to claim 18, wherein the nut and the part of the screw along which the nut travels are housed in a sealed chamber, a cavity being defined between the screw and an inner wall of the chambe .

20. A nut and screw arrangement according to claim 19, wherein the cavity is at least partially filled with a lubricating fluid.

21. A nut and screw arrangement according to claim 19 or claim 20, wherein the cavity contains a gas at a pressure greater than atmospheric pressure .

22. A nut and screw arrangement according to any one of claims 19 to 21, wherein the volume of the cavity changes as the nut travels along the screw to cause a change in gas pressure in the cavity.