Abstract:
A cable or another longilinear transmission component is set into the center of an axial cavity of a screw belonging to a screw and nut movement conversion unit. By means of this arrangement, bending moments exerted on the screw between the point of attachment of the component and the nut have only a small value, which guarantees the screw against bending and sticking and the whole mechanism against excessive friction.

Description:
TECHNOLOGICAL FIELD OF INVENTION 
   The subject of the invention is a transmission including a screw, a nut, and a longilinear component such as a cable attached to the screw, and the movement of which is controlled by means of the screw, by a conversion of movement between rotation of the nut and translation of the screw. 
   BACKGROUND 
   There are many ways of driving motion transmission cables, whether or not wound in a loop but wherein the motor speed should almost always be geared down to increase the force transmitted by the cable. One system encountered includes two pulleys of different diameters on which the cable is wound, possibly making several revolutions around the pulley of smaller diameter in order to increase grip. The small pulley is the driver and communicates a smaller rotation but a more substantial torque to the large pulley. A drawback of this system is that it has a large space requirement, the two pulleys having significant minimum diameters, that slipping is difficult to eliminate completely and that the cable portion on the small pulley is subject to a significant angular speed which increases its fatigue and may therefore cause it to wear quickly. 
   Another system includes gear reductions, but these mechanical means include substantial play which makes them inaccurate and their space requirement is still significant. In other designs, cable movements are controlled by means of a block gearing down the motion between a drive cable and a driven cable, but blocks are complex pieces of equipment and the reliability of the whole is not guaranteed. 
   One final group of motion to cable transmissions, to which the invention is related, includes screw and nut systems one of the elements of which is driven by the motor and the other is attached to the cable. These systems achieve excellent speed reduction and good nominal resistance, but a significant problem emerges in that the components have to be guided and that possible alignment defects have to be taken into account between the screw, the nut and the cable. 
   This latter phenomenon causes significant friction between the screw and the nut and bending moments on the screw, which compromise the yield and lifespan that might have been expected. Mechanisms responsible for guiding the screw and the nut while keeping them in alignment, and which might have overcome these defects, are too cumbersome or increase friction still further. It is for this reason that the proposal has been made (in French patent 2 782 467) to mount the nut on a frame using a cardan joint which allows it to follow the tilting movements of the screw, to which the cable or another connection component is attached, but the cardan joint must surround the nut in such a way that its axes of rotation converge towards the centre of the nut, which gives too voluminous an assembly for many applications. Another cardan joint is moreover provided between the screw and the connection component to which it is attached. 
   A different means is therefore used to improve the alignment between the transmission component such as the cable, the screw to which this component is attached and the nut, or at least to attenuate the effects of alignment defects. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In its most general form, the invention thus relates to a transmission consisting of a longilinear component, a motor, and a screw and nut assembly, where the nut allows the screw to slide, with the nut being driven by the motor and the component being attached to the screw, so as to control the movement of the component by means of the screw via a conversion of movement between a translation of the nut and a rotation of the screw and where a combination of the component and a means of attaching the component with arms is flexible in an angular position relative to the screw; this transmission is characterised in that the screw is bored with an axial cavity and that the attachment means is housed in the cavity. 
   The attachment means is, preferably, at mid-travel position of the nut on the screw. The result of this design is that the bending moment exerted on the screw between the nut and the transmission component attachment point is proportionate to a short lever arm: it is therefore small. 
   The cavity and the longilinear component may emerge from the screw through one end or through both, particularly if the component is wound in a loop. The component may be a fully flexible cable, a rigid rod or again a flexible rush. The attachment means may then be rather different. In the case of a flexible part, one setting in the screw will be sufficient. Otherwise a single or dual articulation, or a ball-and-socket joint, will have to be designed in order to allow a variation in the angle between the rod and the screw. 
   If the transmission component is not rigid under torsion, a means of stopping the pivot rotation of the screw relative to a frame or to another fixed point should normally be provided. This may be a travelling runner pointing in the direction of the screw, or more generally a transverse two-degrees-of-freedom coupling like an Oldham coupling, or any other coupling having the same property. 
   The longilinear component attachment means may be held in the screw by a spacer engaged in the cavity, against which the attachment means is stopped, and a hollowed-out cap mounted on the screw stopping on the spacer, and which the longilinear transmission component passes through. 
   The invention will now be described in more detail by means of  FIGS. 1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 ,  8 ,  9 , and  10  which all show different embodiments of the invention. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  illustrates a schematic view of a screw in accordance with an embodiment of the invention. 
       FIG. 2  illustrates a schematic view of a screw having a blind cavity in accordance with an embodiment of the invention. 
       FIG. 3  illustrates a schematic view of a screw having a blind cavity and rigid rod in accordance with an embodiment of the invention. 
       FIG. 4  illustrates a schematic view of a transmission component in accordance with an embodiment of the invention. 
       FIG. 5  illustrates a schematic view of a transmission component in accordance with an embodiment of the invention. 
       FIG. 6  illustrates a schematic view of a transmission component in accordance with an embodiment of the invention. 
       FIG. 7  illustrates a perspective view of a transmission component in accordance with an embodiment of the invention. 
       FIG. 8  illustrates a perspective view of a transmission component with a leaf spring in accordance with an embodiment of the invention. 
       FIG. 9  illustrates a perspective view of a transmission component with a bellows component in accordance with an embodiment of the invention. 
       FIG. 10  illustrates a perspective view of a transmission component with an Oldham coupling in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an endless screw  1  passed right through by an axial cavity  2  and in the centre of which is found a setting  3  of a transmission cable  4  which also passes right through the cavity  2  and to which is fixed a drive part not shown. The cable  4  may form an endless loop, particularly in this construction, or a segment. The setting  3  is held, on each side of the cavity  2 , by a spacer  5  in the form of a bush which extends between it and one of the apertures of the cavity  2 , where it is held by a cap  6  including a nut portion  7  engaged on the last threads of the screw  1 , a central aperture  8  for the cable  4  to pass through and a stop part  9  which holds the spacer  5  in place in the cavity  2 . 
     FIG. 2  shows a similar construction, but one in which the cavity  10  is blind and the setting  3  is supported against the bottom of this cavity. It is then one end of a cable  11  of finite length that is set. A single spacer  5  and cap  6  system is encountered. 
   The invention may be implemented ( FIG. 3 ) with a rigid rod  12  replacing a flexible cable. The setting  3  is then replaced by an articulation, and in particular by a dual articulation or a ball-and-socket joint  13  in which one end of the rod  12  is held. The other aspects of the constructions in  FIGS. 1 and 2  remain possible, and furthermore a system with a bush acting as a spacer  5  and a cap  6  acting as a stop is encountered again. 
   A complete transmission is shown in  FIG. 4 . A frame  14  houses a hollow shaft motor  15  which drives a hub  16  extending out of the motor  15  to form a receptacle  17  for housing a nut  18  driving a screw in accordance with any one of the previous definitions, like the screw  1  in  FIG. 1 . It is engaged in the nut  18  and, depending on the motion that is imposed on it, may penetrate into the hub  16 , which is passed through in any case by one branch of the cable  4 . The other end of the screw  1  carries a pin  19  supporting rollers  20  rolling in wide-open grooves  21  of a sleeve  22  belonging to the frame  14 . This travelling runner system integral with the screw  1  pits itself against rotations of the screw  1  and therefore allows the nut  18  to slide it along the grooves  21  approximately parallel to the screw  1 , while lending itself to small transverse tilting movements of the screw  1  in the fixed sleeve  22  as a function of the traction exerted on the cable  4 , thanks to the track and aperture widths of the grooves  21 . Finally, the hub  16  is held in the frame  14  by a pair of bearings  23 . 
   The mechanical connection between the screw  1  and the component guiding it, in this case the sleeve  22 , must be approximately equivalent to an Oldham coupling, which has the property of absorbing motion in both transverse directions of both the parts it joins together. In the previous embodiment, the rollers  20  may be displaced to one side without leaving the tracks of the grooves  21 , and in the other direction by lifting themselves from the tracks. It is not necessary for the rollers  20  to be closely guided in the grooves  21  since they are used above all to pit themselves against the rotation of the screw  1 . 
   If the rollers  20  are however arranged in grooves  21  which are too narrowly open to allow sufficient travel of the screw  1 , the necessary flexibility may be restored if the rollers  20  are mounted on a frame  30  equipped with a central slider  31 , oriented in the direction of opening of the grooves  21 , on which an end piece  32  of the screw  1  slides ( FIG. 7 ), or if a leaf support  33 , flexible in the same direction, joins the end piece  32  to a runner  34  carrying rollers  20  ( FIG. 8 ). A bellows could replace the leaf spring  33 ; the assembly would then be flexible in the width direction of the grooves  21 , which would allow them to be replaced and a rigorous guide bush to be used, taking the appearance of sliders  35  parallel to the screw  1  and on which would slide rings  36  connected to a runner  37 , which would also be connected to an end piece  38  of the screw  1  either by a bellows  39  ( FIG. 9 ), or by an Oldham coupling  40  ( FIG. 10 ). 
   It may be seen that the setting  3 , located approximately half-way along the screw  1  and therefore approximately at mid-travel, remains at a short distance from the nut  18  even when the screw  1  is in its extreme position: if vertical bending is imposed on it by the cable  4 , the force which is exerted between it and the nut  18  remains moderate, which substantially reduces the risks of sticking and bending of the screw  1  and of the friction which might be exerted between it and the nut  18  even without permanent damage. Experiments conducted in the laboratory have demonstrated this, and in particular that the friction measured remained close to the theoretical values of the screw and nut system. One effect of the invention is therefore to make this system effective once again. 
     FIG. 5  is a similar design, in which the hollow shaft motor  15  is replaced by a motor  24  external to the frame  14  and which drives one end of the hub  16  using a belt drive  25  tensioned between pulleys or the like. 
     FIG. 6  shows a composite transmission component including, within the screw  1 , a single portion  26  as a single cable, which is joined to an additional portion  27  of the component which is divided at that point and, composed of a plurality of strands of cables  28  joined to the single portion  26  by a spreader  29 . This arrangement may be found on both sides of the screw  1  if necessary. The component may as previously form a loop or not form one. 
   It is apparent that the invention may be implemented in other ways. In this way the rotation stop component becomes unnecessary when the transmission component is rigid under torsion, unable to pivot and connected to the screw by a connection which is rigid under torsion: in these constructions, stopping the rotation of the screw  80  is brought about by the essential components of the mechanism themselves; in the case of  FIG. 3 , the bar  12  is rigid under torsion and therefore holds the screw if it is attached to a body which does not rotate and if a dual articulation replaces the ball-and-socket joint  13 . 
   Numerous applications may be proposed for the invention. The transmission component may rotate a body, translate it, or even cause it to pivot or otherwise move. It is already planned to use the invention to regulate the steering of guide wheels, or to act as active suspension to a wheel suspended from a lever made to rotate by the invention, as a complement to conventional cushioning means. 
   When the transmission component is flexible and has to be tensioned, this may be achieved by a spring, a counterweight or a loop arrangement of the component. 
   It is obvious that the previous description has not exhausted all the forms the invention may take, and in particular all its possible modes of construction; those which have been discussed could be combined in different ways.