TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to rotative drive screw systems for linearly positioning a cooperating member, e.g. a translation nut.

BACKGROUND ART

[0002] Drive screw systems are typically used to position a cooperating member, which will generically be called drive nut in the present application, in response to rotation of the drive screw. [0003] The linear speed of the drive nut parallel to the rotation axis of the drive screw is directly proportional to the rotation speed of the drive screw. Hence, it is desirable to rotate the drive screw as quickly as possible to achieve linear displacement as quickly as possible.

[0004] However, the variable rotation speed of a drive screw generates vibration in particular when the rotation frequency matches one of the natural frequencies of the drive screw system. Such vibration may be excessive with resultant noise, wear or damage to the system. The natural frequencies of the drive screw system vary in particular with the position of the nut, the diameter of the drive screw and its unsupported length. The smaller the diameter/length ratio, the lower the speed at which the drive screw can be permitted to rotate, if the drive screw is to be prevented from reaching the first natural frequency. Consequently, the rotational speed of longer drive screws must be kept low, and the speed at which the nut moves along the drive screw will therefore be correspondingly slow. Additionally, axial compression loads exerted on the drive screw system may cause buckling. [0005] In order to increase the effective length and rotation speed of a drive screw as well as the axial compression load carrying capacity without buckling, it has been suggested in US 5,531,557 to provide a drive screw system with a plurality of bearing assemblies coaxially disposed relative to each other for supporting the drive screw; The bearing assemblies automatically move back and forth between the proximal and distal ends of the drive screw with the drive nut to automatically maintain a predetermined maximum unsupported length of the drive screw. Each bearing assembly comprises two bearing supports connected to one another at a fixed length. [0006] Thanks to the additional bearing assemblies, the lowest natural frequency of the system is increased, which allows higher rotation speed. However, the additional bearing assemblies cause vibration and noise of their own, in particular when the drive screw changes direction. Moreover, as the bearing assemblies are free to move linearly along the drive screw, they may start to scrawl uncontrollably in one direction or another due the residual vibration of the drive screw.

[0007] In order to avoid such scrawling motion of the additional bearing assemblies, it has been proposed in US 5,720,202 to provide a system of ropes and pulleys to couple the additional bearing assemblies to the drive nut, such that the position of the bearings is always linked to that of the drive nut and the bearings move at half the speed of the drive nut. The system of ropes and pulleys, however, may prove unreliable over time. Moreover, they do not solve the problem of noise generated when the drive screw changes direction.

SUMMARY OF THE INVENTION

[0008] The invention aims at increasing the reliability of a drive screw system. According to a first aspect of the invention, there is provided a drive screw system comprising

- a stationary track,

- a drive screw mounted for rotation relative to the stationary track about a screw axis, - a nut threadingly engaging the drive screw for linear displacement parallel to the screw axis in response to rotation of the drive screw,

- at least one slide bearing for radially supporting and guiding the drive screw, the slide bearing being moveable on the stationary track parallel to the screw axis, wherein the slide bearing is provided with at least a first friction pad moveable relative to the slide bearing in at least a first direction transverse to the screw axis, and spring means biasing the friction pad in said first direction against the stationary track. [0009] Backlash between the slide bearing and the track is compensated by the friction pad, which allows a smooth and safe running of the slide bearing on the track despite large tolerances of the track. The friction pad slides on the track. Thanks to the spring means, a controlled pressure is maintained between the friction pad and the track. This controlled pressure leads to a controlled friction between the friction pad and the track, which contributes to stabilising the position of the slide bearing along the screw axis.

[0010] According to a preferred embodiment, the slide bearing is provided with a first guide face perpendicular to the screw axis for guiding the first friction pad and for preventing movement of the first friction pad relative to the slide bearing parallel to the screw axis. The first friction pad slides on the first guide face so as to move parallel to the first guide face.

[0011] According to a preferred embodiment, the slide bearing is provided with a second friction pad moveable relative to the slide bearing in a second direction opposed the to first direction, and the spring means bias the second friction pad in the second direction against the stationary track. Hence, the slide bearing is maintained without backlash in the first and second direction relative to the track.

[0012] The spring means may include a compression spring between the first and second friction pad for biasing the first and second friction pads in the first and second lateral directions, respectively. This provides a very simple structure and minimises the number of parts. According to a preferred embodiment, the compression spring is charged between two parallel faces of the first and second friction pads oriented such as to bias the first and second friction pads against the first and second guide faces, respectively. In one embodiment, the two parallel faces of the first and second friction pads are perpendicular to a geometric plane including the screw axis and the first and second directions, and have an intersection with said geometric plane which is at an angle relative both to the first and second directions and to the screw axis.

[0013] According to a preferred embodiment, the stationary track includes a channel comprising two opposed lateral faces parallel to the screw axis, the slide bearing being received between said lateral faces, the friction pads resting against the lateral faces. Preferably, the friction pads slide against the lateral faces.

[0014] According to a further preferred embodiment, the first friction pad is moveable relative to the slide bearing in a first perpendicular direction perpendicular to the first direction and to the screw axis, and the spring means bias the first friction pad in the first perpendicular direction. The spring means may include a compression spring acting between the slide bearing and the first friction pad for biasing the first friction pad in the first perpendicular direction.

[0015] Similarly, the second friction pad can be moveable relative to the slide bearing in a second perpendicular direction perpendicular to the second direction and to the screw axis, and the spring means bias the second friction pad in the second perpendicular direction. The spring means may comprise a compression spring acting between the slide bearing and the second friction pad for biasing the second friction pad in the second perpendicular direction. The first and second perpendicular directions can be identical or opposed. [0016] The channel may comprise a further face parallel to the screw axis and perpendicular to the two opposed lateral faces, the friction pads resting against the further face. More specifically, the opposed parallel faces and further face may form a U-shaped channel.

[0017] The drive screw system may further comprise a second slide bearing and a link for maintaining a constant distance between the first and second slide bearings. Preferably, the first and second slide bearings are identical.

[0018] According to a further aspect of the invention there is provided a slide bearing for a drive screw system as disclosed hereinbefore. DESCRIPTION OF THE DRAWINGS

[0019] Other advantages and features of the invention will then become more clearly apparent from the following description of specific embodiments of the invention given as non-restrictive examples only and represented in the accompanying drawings in which:

- figure 1 is a diagrammatic illustration of a drive screw system according to an embodiment of the invention;

- figure 2 is a cross-section of the drive screw system of figure 1;

- figure 3 is an exploded view of a slide bearing of the drive screw system of figure 1;

- figure 4 is an exploded view of a slide bearing according to an alternative embodiment of the invention;

- figure 5 is a side view of the slide bearing of figure 4; and

- figure 6 is a cross-section of a drive screw system including the slide bearing of figure 4.

[0020] Throughout the description and drawings, identical reference numbers will be used to indicate identical or similar elements of the various embodiments.

DETAILED DESCRIPTION OF A PREFERED EMBODIMENT

[0021] With reference to Figure 1, a drive screw system 10 comprises a frame 12 made of an extruded aluminium profile and provided with a central track 14 for receiving a drive screw 16 as well as two lateral tracks 18 on each side of the central track 14. The drive screw 16 is mounted for rotation relative to the stationary frame 12 about a screw axis 20 parallel to the central and lateral tracks 14, 18. One end of the drive screw 16 is powered by a motor (not shown), while the other end 22 is received in a stationary bearing 24.

[0022] A carriage 26 is mounted on top of the frame 12 and supported on roller travellers 28 received in the lateral tracks 18 of the frame 12. One of the roller travellers 28 is connected to a ball nut 30 via a side arm 32. The ball nut 30 is received in the central track 14 and threadingly engages the drive screw 16 so as to move in a fore and aft direction without rotation parallel to the screw axis in response to rotation of the drive screw 16. [0023] The drive screw system 10 further includes a pair of slide bearings 32, each provided with a cylindrical through hole 33, preferably lined with a bushing having the same diameter as the envelop of the drive screw, for receiving the drive screw 16 with sliding contact and for radially supporting the drive screw 16.

[0024] Each slide bearing 32 is guided in the fore and aft direction in the central track 14. In order to compensate for any backlash with the frame each slide bearing 32 is provided with two opposite friction pads 34 moveable relative to the slide bearing 32 in a cross direction towards an upper wall 14.1 of the central track 14 as well as towards an adjacent lateral wall 14.3 of the central track 14. More specifically, each friction pad 34 has an overall triangular prismatic shape with an upper horizontal somewhat triangular base face 34.1 facing the upper wall 14.1 of the central track, a lower triangular base face 34.2 parallel to the upper base face 34.1 and facing a horizontal face 32.2 of the slide bearing 32, a vertical lateral face 34.3 facing the adjacent lateral wall 14.3 of the central track, a vertical inner face 34.4 which is at an angle of between 30° and 60°, and preferably of 45° with the screw axis and a vertical guide face 34.5 perpendicular to the screw axis facing a guide wall 32.5 of the friction pad 32. The guide wall 32.5, which is perpendicular to the screw axis 20, may be provided with a transverse groove 32.51 and the guide face 34.5 may have a corresponding protruding strip 34.51, which is loosely inserted in the groove 32.51 to both guide the friction pad 34 in the transverse direction and provide a limited freedom of movement in the "vertical" direction, i.e. the direction perpendicular to the transverse direction and to the screw axis 20.

[0025] A compression spring 36 is loaded between the inner walls 34.4 of the two friction pads 34. The spring 36 applies on each friction pad 34 a spring force in a direction substantially perpendicular to the inner wall 34.4. Hence, the spring force has a component perpendicular to the to the guide wall 32.5 and a component parallel to the guide wall 32.5, so that each friction pad 34 bears against the corresponding guide wall 32.5 and is resiliently biased laterally towards the adjacent lateral wall 14.3 of the central track.

[0026] A compression spring 38 is loaded between the lower base face 34.2 of each friction pad 34 and the "horizontal" face 32.1 of the slide bearing 32, to resiliently bias the friction pad 34 towards the upper wall 14.1 of the central track 14.

[0027] The slide bearing 32 provided with the friction pads 34 and springs 36, 38 constitutes a subunit, which can be easily preassembled. In the operational position, when the slide bearing 32 is inserted in the central track 14, the friction pads 34 bear against the corresponding lateral walls 14.3 and the upper wall 14.1 of the central track 14, while a lower face 32.6 of the slide bearing 32 bears against a lower wall 14.6 of the track 14.

[0028] The drive screw 16 is preferably provided with at least two such slide bearings 32, which are linked to each other by a pair of rigid rods 40. [0029] To describe the operation of the drive screw system, we shall assume that the nut 30, travellers 28 and carriage 26, which form a subunit, are midway from the ends of the drive screw, and that the two slide bearings 32 are at equal distance from the nut 30, each between the nut 30 and one of the two ends of the drive screw 16. When the motor is powered, the drive screw 16 revolves about the screw axis 20 to move the nut 30, travellers 28 and carriage 26 in one direction towards a first of the two ends of the drive screw 16. At first, the two slide bearings 32 remain stationary and the drive screw 16 revolves with sliding contact in the through holes 33 of the slide bearings 32, which provide a radial support for the drive screw 16. When the nut moves towards said first end of the drive screw 16, it will at some stage reach a first of the two slide bearings 32 and push the first slide bearing 31 along the drive screw 16 towards the first end of the drive screw 16. The second slide bearing 34, linked to the first slide bearing via the rods 40, will be dragged along. When the revolving direction of the drive screw is reversed, the nut 30 moves in the other direction towards the second slide bearing 34 and the second end of the drive screw 30. The two slide bearings 34 will remain stationary as long as the nut 30 has not reached the second slide bearing 34. Once this position has been reached, the second slide bearing 34 will be pushed in front of the nut 30 towards the second end of the drive screw 30 and the first slide bearing 34 will be dragged along.

[0030] The linear movement of the slide bearings 34 ensures that the whole length of the drive screw 16 can be used.

[0031] The tension of the compression springs 36, 38 is chosen so that the slide bearings 34 do not move radially with respect to the track.

[0032] While the above example illustrates a preferred embodiment of the present invention it is noted that various other arrangements of drive screw system 10 may also be considered, which fall within the scope of the appended claims.

[0033] The terms "horizontal" and "vertical" direction have been used under the assumption that the fore and aft direction of the drive screw is horizontal. It is clear, however that the drive screw system can be oriented in any direction, e.g. with a vertical fore and aft direction. [0034] Several pairs of slide bearings can be provided.

[0035] The springs 36, 38 can be of any type, e.g. flat springs. Two separate springs instead of one can be used to urge the two friction pads towards the two lateral walls of the track. Conversely, a single spring can be used for urging the backlash compression pads laterally and upwardly. [0036] Referring now to figures 4 to 6, an alternative embodiment of a slide bearing for a drive screw assembly is illustrated. The slide bearing is provided with a cylindrical through hole for receiving and guiding a screw nut 16. As illustrated in figure 6 the slide bearing is received in the central track 14 of the frame 12 of a drive screw system 10 similar to the drive screw system of figure 1. [0037] The slide bearing 32 is further provided with a friction pad 34 moveable relative to the slide bearing 32 in a direction perpendicular to the direction of the through hole towards an upper wall 14.1 of the central track 14. The friction pad 34 has an overall parallelepiped shape with an upper horizontal base face 34.1 facing the upper wall 14.1 of the central track, a lower base face 34.2 parallel to the upper base face 34.1 and facing a horizontal face 32.2 of the slide bearing 32, two vertical lateral faces 34.3 facing the adjacent lateral walls 14.3 of the central track and two vertical guide faces 34.5 perpendicular to the screw axis and facing guide walls 32.5 of the friction pad 32. The guide walls 32.5, which are perpendicular to the screw axis 20, may be provided with a transverse groove 32.51 and the guide face 34.5 may have a corresponding protruding strip 34.51, which is loosely inserted in the groove 32.51 to both guide the friction pad 34 in the transverse direction and provide a limited freedom of movement in the "vertical" direction, i.e. the direction perpendicular to the transverse direction and to the screw axis 20. The upper base face 34.1 is provided with a groove 34.11 aligned with similar grooves 32.11 provided on the slide bearing and parallel to the fore and aft direction of the screw nut. These grooves 34.11, 32.11 cooperate with a guide rib 14.11 protruding from the upper wall 14.1 of the central track 14 and extending in the fore and aft direction. Remarkably, a minimal play is kept between the sides of the guide rib 14.11 and the sides of the grooves 34.11 and a greater clearance is left between the leading edge of the guide rib 14.11 and the bottom of the grooves 32.11, 34.11.

[0038] A pair of compression springs 38 are loaded between the lower base face 34.2 of each friction pad 34 and the "horizontal" face 32.1 of the slide bearing 32, to resiliently bias the friction pad 34 towards the upper wall 14.1 of the central track 14.

[0039] The slide bearing 32 provided with the friction pad 34 and springs 38 constitutes a subunit, which can be easily preassembled. In the operational position, when the slide bearing 32 is inserted in the central track 14, the friction pad 34 bears against the upper wall 14.1 of the central track 14, while a lower face 32.6 of the slide bearing 32 bears against a lower wall 14.6 of the track 14. The lateral position of the slide bearing 32 and of the friction pad is defined by the guide rib 14.11 and grooves 32.11, 34.11.