Abstract:
The actuator includes a first tubular body, a nut positioned inside the tubular body and having at least a globally helical raceway, balls arranged between the raceway and the tubular body ( 20 ), and driving mechanisms designed to drive the nut in rotation. The driving mechanism has a motor, the rotating nut driving the tubular body in translation relative to the nut. The motor is mounted fixed inside a second body adapted to be driven in translation relative to the first tubular body.

Description:
RELATED U.S. APPLICATIONS  
       [0001]     Not applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       REFERENCE TO MICROFICHE APPENDIX  
       [0003]     Not applicable.  
       FIELD OF THE INVENTION  
       [0004]     The invention relates to the field of the mechanical linear actuators and, in particular, of the mechanical actuators driven by an electric motor (electromechanical actuators).  
       BACKGROUND OF THE INVENTION  
       [0005]     The development of the electromechanical linear actuators is related to the needs in fields such as robotics and home systems. Indeed, in these fields the electromechanical jacks compete with the traditional, hydraulic or pneumatic jacks, because they are more easily controllable, more accurate and do not require an external source of fluid.  
         [0006]     These electromechanical actuators generally include a ball screw on which a nut is mounted. The nut is rotated by an external geared motor. The rotation of the nut drives the screw in translation.  
         [0007]     The drawback of these electromechanical actuators is that they are relatively cumbersome.  
         [0008]     Moreover, since the cost of the ball screws is generally high compared to the other mechanical parts they contain, these actuators remain relatively expensive.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     An object of the invention is to provide a compact actuator structure and the manufacture of which would be simplified compared to the actuator structures of the prior art.  
         [0010]     To this end, the invention provides an actuator including a first tubular body, a nut positioned inside the tubular body and having at least a generally helical ball-race, balls arranged between the ball-race and the tubular body, and driving means for rotating the nut, said driving means comprising a motor, the rotation of the nut driving the tubular body in translation with respect to the nut, characterized in that the motor is mounted fixed inside a second body capable of being displaced in translation with respect to the first tubular body.  
         [0011]     The fact that the actuator comprises an internal nut allows to position the motor inside a second body. In addition, the re-circulation path can be integrated into the nut. This arrangement leads to a compact actuator structure the external appearance of which is similar to that of the pneumatic actuators. In particular, the actuator does not leave visible any external geared motor device. The actuator provided is thus particularly compact, compared to the effort which it is capable of generating.  
         [0012]     In addition, the use of a tubular structure imparts to the actuator a better buckling strength than a traditional actuator having an external nut mounted about an internal screw.  
         [0013]     In an implementation of the invention, the balls are fitted between the race and the first tubular body, with a determined radial prestressing.  
         [0014]     The fact that the balls are fitted with prestressing allows to obtain a linear actuator capable of transmitting significant efforts, compared to its size.  
         [0015]     In an implementation of the invention, the race includes a helical portion extending about the nut according to an angle of less than 360 degrees and a widened portion connecting the adjacent ends of the helical portion, said widened zone constituting a re-circulation zone for the balls.  
         [0016]     This implementation has the advantage of not requiring the formation of an internal re-circulation race in the nut. The balls are automatically “recycled” as soon as they reach the re-circulation zone.  
         [0017]     In addition, the inner surface of the first tubular body can advantageously have helical ball-races the function of which is to guide the balls. These ball-races reduce the risks of sliding of the balls on the inner surface of the first body when the actuator exerts a significant effort. The widened re-circulation zones allow the passing over of the balls from one ball-race to an adjacent race, over a race edge during their re-circulation.  
         [0018]     In a preferred implementation of the invention, the nut includes several aligned elements, of a cylindrical general shape, each having at least a bevel forming a helical cam surface, the bevels forming, two by two, helical ball-races in which balls are positioned. Each element is formed from a cylindrical part with a straight cross-section, one circular edge of which is beveled, in order to form said helical cam surface inclined with respect to the axis of the cylindrical part, the ends of helical surface being joined by a setback surface with a preferably conical general shape.  
         [0019]     Each element of the nut is formed from a cylindrical part with a straight cross-section, i.e. the cylindrical part is limited by two parallel planes orthogonal to its axis of rotation. This is a simple shape. The shape of the elements is therefore easier to be generated than in the prior art.  
         [0020]     According to the technique for carrying out the bevel, the setback surface can also have a general shape that is convex, concave, planar, cylindrical, planar with conical connection or cylindrical connection or the like.  
         [0021]     Advantageously, each helical cam surface forms a setback and two elements are so positioned with respect to each other that their setbacks are in front of each other, said setbacks forming the balls re-circulation zone.  
         [0022]     Advantageously, the prestressing exerted on the balls is generated by tightening the elements with respect to each other.  
         [0023]     To this end, the actuator can include an element adjusting nut for controlling the prestressing exerted onto the balls.  
         [0024]     The effort which can be exerted by the actuator directly depends on the prestressing applied to the balls and adjusted by the adjusting nut.  
         [0025]     Advantageously, the actuator includes elastic means interposed between the adjusting nut and the nut elements through which the adjusting nut exerts prestressing on the elements.  
         [0026]     Preferably, the motor is an electric or hydraulic motor.  
         [0027]     The invention also relates to a nut element aimed at being arranged in an actuator as defined above. The nut element is formed from a cylindrical part with a straight cross-section, one circular edge of which is beveled to form said helical cam surface inclined with respect to the axis of the cylinder, the ends of the helical surface being connected by a setback surface with a conical general shape.  
         [0028]     The invention also relates to a process for obtaining a nut element aimed at being arranged in an actuator according to the invention. The process includes the steps consisting in machining a circular edge of a cylindrical part with a straight cross-section, in order to generate a bevel forming a helical cam surface inclined with respect to the axis of the cylinder, the ends of helical surface being connected by a setback surface with a conical general shape.  
         [0029]     One understands that the process for obtaining the nut element is easy to be implemented with traditional machining means.  
         [0030]     Further characteristics and advantages will clearly appear from the following description, which is purely illustrative and non-restrictive and must be read with reference to the attached figures. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0031]      FIG. 1  shows a longitudinal cross-sectional view of an example of an actuator structure according to an embodiment of the invention in which the driving means include an electric motor.  
         [0032]      FIG. 2  is a diagram representing a prestressed ball,  
         [0033]      FIG. 3  is a perspective view of a cam constituting the nut,  
         [0034]      FIG. 4  is a diagram representing a step of generating a helical cam surface,  
         [0035]      FIG. 5  is a diagram representing the positioning of two cams with respect to each other on the driving shaft of the actuator,  
         [0036]      FIG. 6  schematically shows the positioning of two pairs of cams with respect to each other, in which the ball re-circulation zones are regularly distributed around the driving shaft,  
         [0037]      FIG. 7  shows an example of inner surface of the tubular body having ball-races formed by a wire wound into a spiral,  
         [0038]      FIGS. 8 and 9  schematically show ball-races formed by a first wound wire and a intermediate second wire arranged between the windings of the first wire,  
         [0039]      FIG. 10  schematically shows ball-races formed by plastic distortion of an inner tube arranged in the tubular body,  
         [0040]      FIG. 11  schematically shows a step of welding of the inner tube in the tubular body,  
         [0041]      FIG. 12  shows a longitudinal cross-sectional view of an actuator structure of a telescopic type,  
         [0042]      FIG. 13  shows the actuator of  FIG. 12  in unfolded position,  
         [0043]      FIG. 14  schematically shows the positioning of a ball resting between the nut and a ball-race,  
         [0044]      FIG. 15  is a cross-sectional and perspective view of the balls when they arrive in a re-circulation zone,  
         [0045]      FIG. 16  is a diagram representing the positioning of two cams with respect to each other. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0046]     In  FIG. 1 , the linear actuator includes an inner tube  10  and an outer tube  20  the diameter of which is larger than the diameter of the inner tube  10 . The inner tube  10  extends partly in the outer tube. Both tubes  10  and  20  are locked in rotation with respect to each other and are capable of being actuated to slide with respect to each other in their longitudinal direction.  
         [0047]     To this end, the actuator includes a drive mechanism including a driving shaft  30  extending according to the longitudinal axis of the tubes  10  and  20 . The shaft  30  is rotated by an electric motor  2  fixed at one of its ends and positioned in the inner tube  10 . The motor  2  and the shaft  30  are maintained in the inner tube  10  through a cylindrical support  3  fixed to the inner tube.  
         [0048]     Furthermore, the shaft  30  is guided in the inner tube  10  through two ball bearings  7  and  9  the inner ring of which is fitted on the shaft  30  and the outer ring rests against the inner surface  11  of the inner tube  10 . Both bearings  7  and  9  are maintained at a distance by a spacer  8  in the form of a cylindrical sleeve resting on the inner rings of the bearings  7  and  9  as well as through a spacer  12  pinned in the inner tube  10  and resting on the outer rings of the bearings  7  and  9 . The absorption of the axial forces exerted on the bearings can occur either through the spacer  12  or by any other equivalent means (for example circlips locking the bearing).  
         [0049]     The shaft  30  supports in addition an adjusting nut  4 , a set of Belleville washers  5 , a first clamping washer  6  positioned between the support  3  of the motor and the bearing  7 . The clamping washer  6  rests on the inner cage of the bearing  7 . The shaft  30  also supports a second clamping washer  1  and ball nut  70 , positioned between the bearing  9  and a thrust element  31  at the end of the shaft  30 .  
         [0050]     The nut  70  is formed of a succession of cams  40 ,  50  and  60  with cylindrical general shapes mounted aligned on the shaft  30  and locked in rotation with respect to the shaft by a key. The cams  40 ,  50 ,  60  have helical bevels  41 ,  51  and  52 ,  62 , oriented at 45° with respect to the axis of the shaft  30 . These bevels  41 ,  51 ,  52 ,  62  form, two by two, helical ball-races in which balls  22  are positioned. The balls  22  are into contact, on the one hand, with two surfaces with opposite bevels,  41  and  51 , or  52  and  62  and, on the other hand, with the smooth inner surface  21  of the outer tube  20 . The radial force applied to the balls  22  is controlled by tightening the nut  4 . The adjusting nut  4  applies a compressive force to the Belleville washers  5  according to the longitudinal direction of the shaft  30 . This compressive force is transmitted to the cams  40 ,  50 ,  60  through the clamping washer  6  which transmits and distributes the clamping force on the inner cages of the bearings  7  and  9  and on the clamping washer  1 . The cams  40 ,  50 ,  60  are thus in compressed state between the clamping washer  1 , the balls  22  and the thrust element  31  at the end of the shaft  30 . By tightening the cams  40 ,  50 ,  60 , the adjusting nut  4  advantageously allows to adjust the prestressing exerted on the balls  22 .  
         [0051]     The actuator of  FIG. 1  includes two ball-races formed by three cams  40 ,  50  and  60  aligned on the shaft  30 . Of course, it is possible to form an actuator having only one ball-race or a number of ball-races higher than two. It is enough to change the number of cams mounted on the shaft, each ball-race being formed between two successive cams.  
         [0052]     The force that can be exerted by the actuator of  FIG. 1  directly depends on the prestressing applied to the balls and set by the adjusting nut  4 .  
         [0053]     However, the prestressing force which can be applied to the balls  22  remains limited by the Hertz pressure which the surface of the cams  40 ,  50 ,  60  and the inner surface  21  of the outer tube  20  can be subjected to.  
         [0054]     When the motor  2  of the actuator of  FIG. 1  is operating, it rotates the shaft  30  and, hence, the cams  40 ,  50  and  60  which are keyed on the latter. The balls  22  then roll between their ball-race and the inner surface of the outer tube. The tangential speed of the center of each ball  22  thus has two components: a tangential component, perpendicular to the axis of rotation of the shaft  30  and a longitudinal component parallel to the axis of the shaft  30  due to the pitch of the helix of the ball-race.  
         [0055]     As shown in  FIG. 2 , a ball  22  turns about an axis inclined with respect to the axis of the shaft  30  according to an angle equivalent to that of the helix of the ball-race. Moreover, the point of contact I between the ball  22  and the inner surface of the tube is always positioned on the line perpendicular to the axis of rotation passing through the point  0 . This results into the outer tube  20  being driven in translation at a speed proportional to the speed of rotation of the driving shaft  30  and to the pitch of the helical race.  
         [0056]     The linear actuator of  FIG. 1  can be mounted by carrying out the following steps: 
        mounting the various elements on the shaft  30 : cams  60 ,  50 ,  40 , washer  1 , bearing  9 , spacers  8  and  12 , bearing  7 , washer  6 , Belleville washers  5 , adjusting nut  4 ,     inserting the end of the shaft  30  bearing the cams  40 ,  50 ,  60  into the outer tube  20 , the balls  22  being positioned in the ball-races,     tightening the nut  4  which causes the nearing of the cams  40 ,  50 ,  60  and the prestressing of the balls  22  between surfaces of the bevels and the inner surface  21  of the outer tube  20 .        
 
         [0060]      FIG. 3  shows an example of a cam  40  used in the mounting of  FIG. 1 . The cam  40  has a cylindrical general shape. It includes a central bore  43  aimed at receiving the driving shaft  30 , as well as a key slot  44  formed from the bore  43  and aimed at allowing indexing the cam  40  on the shaft  30 . A helical bevel  41  has been carried out by milling of a circular edge of the cam  40 . This cam is formed from a cylindrical part with a straight cross-section, one circular edge of which is beveled, in order to form said helical cam surface inclined with respect to the axis of the cylindrical part, the ends of helical surface being connected by a setback surface with a conical general shape.  
         [0061]     As shown in  FIG. 4 , the milling operation is carried out using a conical cutter  100  the cutting edges of which form an angle of 45 degrees with respect to its axis of rotation  101 . The cutter is mounted on a rotary machining spindle  102 . A cylindrical rotation part  400  (shown in dotted lines) aimed at forming the cam  40  is mounted on a rotary table. It is so arranged with respect to the cutter  100  that their axes  101  and  401  are parallel and have a given separation e. The part  400  is subjected during the milling operation to a rotational movement with respect to its axis  401  (indicated by arrow R). Simultaneously, the cutter  100  is subjected to a translational movement (indicated by arrow T) along its axis  101 . The translational movement is carried out in a direction in which the cutter  100  moves away from the cylindrical part  400 . The part  400  carries out a rotation of 360 degrees, while the spindle  102  is moved in translation by a distance equal to the pitch of the helical bevel to be generated. This milling operation leads to the generation of the helical bevel  41  oriented at 45 degrees with respect to the axis  401 .  
         [0062]     Traditional heat-treatment and rectifying operations can then be carried out on the helical surface  41  obtained (for example grinding of the helical surface).  
         [0063]     As can be seen in  FIG. 3 , the helical bevel of the cam  40  forms a circumferential surface  41  which widens when it is followed in the opposite direction to the milling and is connected at his ends by a setback with a conical shape  45 . This setback with a conical shape is generated by the shape of the conical cutter when starting its initial radial passage in the part  400 .  
         [0064]     Of course, variants of the above-described embodiment can be contemplated. In particular, the shape of the setback can vary according to the path of the initial passage of the cutter. If the conical cutter penetrates into the part  400  according to a tangential passage start, the setback obtained will have a planar general shape. If the conical cutter penetrates into the part  400  according to an oblique passage start, the setback obtained will have a planar general shape with conical connection.  
         [0065]     It is also possible to use a cylindrical cutter the axis of rotation of which would be inclined with respect to the axis of the cylindrical part and according to the path of the start of the initial passage, in order to obtain a setback with a cylindrical, planar or planar general shape with a cylindrical connection.  
         [0066]     In addition, when the pitch of the race is large with respect to the diameter of the cams, the helical ball-race must be obtained by a different process. For example, a previous step of milling of the cylindrical part using a cylindrical cutter can be carried out, in order to obtain in the first place a helical surface oriented perpendicularly to the axis of the part. Then, a step of milling of the edge of the helical surface using a conical cutter is carried out, to make a helical bevel oriented at 45 degrees with respect to the axis of the part. The helical bevel thus obtained forms a circumferential surface with a constant width which is connected at its ends by a conical setback.  
         [0067]      FIG. 5  shows the positioning of two cams  40  and  50  with respect to each other on the driving shaft  30 . Both cams  40  and  50  have each an identical beveled surface  41 ,  51 . They are positioned side by side on the driving shaft  30 , so that their respective beveled surfaces  41  and  51  are faced to each other, in order to form a helical race for the balls  22 . The cams  40  and  50  are each indexed on the shaft  30  by their key slot  44  or  54 . The key slots  44  and  54  are so positioned with respect to the bore of the cams  40  and  50  that the conical setback surfaces  45  and  55  of the cams  40  and  50  are positioned in front of each other, in an opposite way, when the latter are mounted on the shaft  30 .  
         [0068]     The conical setback surfaces  45  and  55  of both cams  40  and  50  advantageously form a widened zone  81  which accommodates the balls  22  and allows their re-circulation. When the shaft  30  of the actuator is rotated, the balls  22  roll on the ball race formed by the beveled surfaces  41  and  51 . When a ball  22  arrives in the re-circulation zone  81  where the two beveled surfaces  41  and  51  have a maximum width, it is no longer into contact with the inner surface  21  of the outer tube  20 , so that it does no longer roll. The ball  22  remains in the re-circulation zone until it is pushed by the arrival of a next ball and thus automatically re-inserted into the ball-race.  
         [0069]     In  FIG. 1 , the nut  70  formed by the association of the cams  40 , 50 , 60  has the advantage of not requiring the formation of an inner re-circulation race. Thus, in this implementation of the invention, the balls  22  are automatically “recycled” as soon as they reach the re-circulation zone  81  connecting the ends of a ball-race.  
         [0070]      FIG. 6  shows the positioning of the successive cams  40 ,  50  and  60  with respect to each other on the driving shaft  30 . These cams are so arranged that the re-circulation zones of the balls are not aligned. More particularly, the cams are oriented on the driving shaft  30  so that the re-circulation zones are angularly distributed in a regular way about the axis of the shaft  30  (axis of rotation and translation of the actuator). Thus, in  FIG. 6 , the nut including two ball-races formed by the cams  40 ,  50  and  50 ,  60 , respectively, it has two re-circulation zones which are arranged at 180 degrees with respect to each other about the axis of the shaft  30 .  
         [0071]     In the case of a nut including three roll-races which would have three re-circulation zones, the cams would be so oriented that the re-circulation zones are arranged at 120 degrees with respect to each other about the axis of the shaft  30 .  
         [0072]     In a general way, in the case of a nut including N ball-races (formed by N pairs of cams), the cams would be so oriented that the re-circulation zones are arranged at 360/N degrees with respect to each other about the axis of the shaft  30 .  
         [0073]     This feature allows to avoid a rotational movement of the inner tube  10  with respect to the outer tube  20  which can occur when the actuator comprises only one pair of cams (i.e. only one ball-race) or when the re-circulation zones are arranged aligned.  
         [0074]     In a variant of the linear actuator of  FIG. 1 , the inner  10  and outer  20  tubes are made out of a relatively light material: for example, out of a composite or plastic material or out of a light alloy. Ball-races can be formed on the inner surface  21  of the outer tube  20 . These ball-races allow to reduce the Hertz pressure exerted by the balls  22  on the surface of tube  20 . The ball-races are formed by burnishing the inner surface  21  of the tube  20 . The ball-races can advantageously be formed by the balls  22  themselves during the rotation of the shaft  30 . The balls  22  produce a plastic distortion of the surface  21  while forming ball-races.  
         [0075]     In the event the outer tube  20  is made out of a light alloy, after having formed the ball-races, a ceramization treatment for hardening this surface in depth (0.1 to 0.2 mm) is applied to the surface  21  of the tube  20 .  
         [0076]     The constitution of the ball-races allows to apply compressive forces which a smooth cylindrical surface would not withstand. In addition, these races allow to apparently increase the external friction coefficient between the ball and the tube.  
         [0077]     Alternatively, the ball-races allow not to apply too great a prestressing force to the balls. Since the balls are guided by the ball-races, they cannot slide with respect to the outer tubular body  20 .  
         [0078]     These ball-races have a helical pitch substantially equal to the helical pitch of the ball-race formed in the nut  70 .  
         [0079]     In this variant, the actuator includes, in combination, ball-races on interior surface  21  of the outer tube  20  and one nut  70  having re-circulation zones in the form of widened spaces. Thanks to this structure, when a ball arrives in a re-circulation zone, it penetrates radially towards the interior of the nut  70 , so that it is no longer into contact with one of the races formed in the outer tube  20 . Thus, when “recycled”, the ball passes from one ball-race onto an adjacent race, over a race edge, this passing over from one race to another one being possible thanks to the widened space forming the re-circulation zone.  
         [0080]     In still another variant of the actuator of  FIG. 1 , the inner  10  and outer  20  tubes are also made out of a relatively light material. Ball-races are formed on the inner surface of the outer tube  20 . As shown in  FIG. 7 , the ball-races are formed by a high-strength steel wire  91  positioned in a helical way inside the outer tube  20 . In such a variant, the balls  22  roll resting on two successive windings of the wire  91 . This variant allows to obtain a mechanically positive connection between the balls  22  and the races of the tube  20  (there is no longer any friction, but a support). The longitudinal components of the forces of support on the windings of the wire  91  are positive supports. The inner surface  21  of the outer tube  20  includes a helical groove  24  aimed at receiving the steel wire  91 .  
         [0081]     This variant allows to use tubes made out of aluminum, KEVLAR©, carbon fibers or molded plastic, which guarantees the lightness of the final actuator structure obtained.  
         [0082]     In an implementation shown in  FIGS. 8 and 9 , the inner surface  21  of the outer tube  20  is smooth. Ball-races are formed on the inner surface of the outer tube  20 . They are formed by a first high-strength steel wire  91  positioned in a helical way inside the outer tube  20  and on which the balls  22  rest. A second intercalated wire  92  having a diameter smaller than that of the first wire  91  extends between the windings of the first wire. This second wire  92  maintains the separation between the windings of the first wire. It prevents, in particular, the windings of the first wire  91  from separating during the passing through of a ball  22 . In a preferential way, the balls  22  are not into contact with the intermediate wire  92 . This implementation is particularly simple and avoids having to use techniques for machining the outer tube  20 .  
         [0083]      FIG. 10  shows still another variant of the invention in which ball-races are made by plastic distortion in a calibrated inner tube. The inner tube  93  is arranged in the outer tube  20  and welded to the latter.  
         [0084]     The ball-races in the inner tube  93  are made as follows. For example, a burnishing or shaping machine is used, which includes a roller holder provided with three rollers arranged at 120 degrees with respect to each other and oriented according to the helix angle of the race to be obtained. The inner tube  93  is fixed on a chuck the shape of which is close to the inner profile to be achieved. The roller holder is rotated. At the same time, the tube  93  and the chuck are driven in translation. The speed of translation of the tube  93  is set so that the translation distance is equal to the pitch of the helix at each turn of the roller holder. The operation can be carried out in one single pass and the tube  93  is then highly cold hardened, which increases the rigidity and the hardness of the surface. Once shaped, the tube  93  is inserted into the outer tube  20 .  
         [0085]      FIG. 11  shows a step of welding the inner tube  93 , in which the ball-races are formed, in the outer tube  20  of the actuator. In order to make both tubes integral with each other, one proceeds to a series of spot welding operations on the bottom of the race between the two tubes. To this end, one uses, for example, a spot welder including an inner thumb wheel  201  mounted on a shaft  203  and a motorized outer thumb wheel  202 . The inner thumb wheel is inclined with respect to the shaft  203  according to an angle equal to the helix angle of the ball-races. The welding operations are carried out on the bottom of the helical races into contact with the outer tube  20 . The unit thus formed is boxed and the axial distortion of the unit is insignificant. This small distortion guarantees a linearity of the conversion of the rotational movement into a translational movement in the final actuator.  
         [0086]     In the event ball-races are formed on the inner surface of the outer tube  20 , each cam  40 ,  50  or  60  has a bevel oriented according to an angle smaller than or equal to 45 degrees with respect to the axis  401  of the cam, preferably strictly smaller than 45 degrees and preferably of about 35 degrees. This feature allows to decrease the radial force which serves as a support for the reaction of the forces applied to the ball-race. Moreover, this feature facilitates the passing of the balls over the edges of the races during their re-circulation. Indeed, the component of the force which allows a ball to pass over a race edge (formed for example by a wire) passes above the edge of the race.  
         [0087]      FIG. 12  shows a linear actuator of a telescopic type. This actuator is similar to that of  FIG. 1 . It includes an inner tube  10  and an outer tube  20  the diameter of which is larger than the diameter of the inner tube  10 . The inner tube  10  extends partly in the outer tube  20 . It also includes a nut  70  comprised of a succession of cams  40 ,  50  and  60  of cylindrical general shapes.  
         [0088]     The linear actuator shown in  FIG. 12  includes, in addition, a third tube  300  the diameter of which is larger than that of the outer tube  20 . The outer tube extends partly in the third tube  300 . The nut  370  is rigidly connected to the outer tube  20 , so that the outer tube  20  is capable of rotating a nut  370  including cams  340  and  350 .  
         [0089]     The tubes  10  and  300  are locked in rotation with respect to each other and are capable of being driven to slide with respect to each other in their longitudinal direction. The outer tube  20  is mounted floating, i.e. it is locked in rotation neither with respect to the inner tube  10  nor with respect to the third tube  300 .  
         [0090]     When the motor  2  of the actuator of  FIG. 12  is operating, it rotates the nut  70  including the cams  40 ,  50  and  60 . The balls  22  then roll between their ball-race and the inner surface of the intermediate tube  20 . Since the tubes  10  and  300  are locked in rotation with respect to each other, the rotation of the nut  70  causes the inner tube  10  to be displaced in translation with respect to the unit formed of the outer tube  20  and the third tube  300 . This translation is limited by a thrust.  
         [0091]     When the inner  10  and outer  20  tubes are in abutment against each other, the tubes  10  and  20  are then rotated simultaneously. The outer tube  20  then rotates the nut  370  including the cams  340  and  350 . The balls  22  then roll between their ball-race formed by the cams  340  and  350  and the inner surface of the third tube  300 . Since the tubes  10  and  300  are locked in rotation with respect to each other, the rotation of the nut  370  causes the unit comprised of the inner tube  10  and the outer tube  20  to be moved in translation with respect to the third tube  300 .  
         [0092]     This results into the thus produced telescopic actuator unfolding in two steps. In a first step, the inner tube  10  is displaced in translation with respect to the outer tube  20  and to the third tube  300 , then, in a second step, the inner  10  and outer  20  tubes are displaced in translation with respect to the third tube  300 . This unfolding in two steps is due to the fact that the couple necessary to rotate the nut  370  with respect to the third tube is larger than the couple necessary to rotate the nut  70  with respect to the outer tube.  
         [0093]     The unfolding can also occur at random depending on the friction torques occurring in the mechanism.  
         [0094]     Such a telescopic actuator has the advantage of being able to reach larger unfolding lengths than with a simple actuator as shown in  FIG. 1 .  
         [0095]     In the actuator shown in  FIG. 12 , the nut  70  includes two pairs of cams and the nut  370  includes only one pair of cams. Of course, it is possible to manufacture telescopic actuators having a larger number of tubes and a different number of cams.  FIG. 13  shows the actuator of  FIG. 12  in unfolded position.  
         [0096]     The tubes  20  and  300  each have ball-races on their inner surfaces. These races preferably have the same pitch. Thus, the unfolding of the actuator will occur at a constant speed. In addition, it will be possible, by counting the number of revolutions of the motor, to know the exact position of the actuator.  
         [0097]     If the races of the tubes  20  and  300  have different pitches, the unfolding speed of the actuator will vary according to the tube which will be moving at a given moment.  
         [0098]     Generally, in a telescopic actuator including a plurality of tubes capable of being driven in translation with respect to each other, one can choose to establish different race pitches for the various tubes. One thus obtains a telescopic actuator which sequentially unfolds with programmable values of motor/movement reduction coefficient over the total travel distance of the actuator. This feature allows to adapt the evolution of the motor torque provided depending on the profile of the load the actuator has been subjected to during its unfolding, this profile being determined length by length.  
         [0099]     If one wants the tubes to unfold in a given order, it is possible to add means for braking the rotation of the tubes with respect to each other (for example one or several O-ring(s) rubbing against the tubes, so that the latter unfold sequentially.  
         [0100]     The preceding description relates to an example of linear actuator in which the means for driving the nut include an electric motor  2 . It will be understood that it is of course possible to use other types of driving means: hydraulic motor or the like.  
         [0101]     Now it will described more in detail the passing over of a ball from one ball-race to the next one in the case of an actuator including an outer tube the inner surface  20  of which has ball-races.  
         [0102]      FIG. 14  shows a ball  22  with a center  0  maintained between the beveled surfaces  41  and  51  of the cams  40  and  50  and a ball-race formed, for example, by two wires  92  and  94 . The points of contact between the ball  22  and the cam  40 , the cam  50 , the wire  92  and the wire  94 , are designated by B, D, C and A, respectively. The angle between the plane P with a straight cross-section of the actuator passing through 0 and the straight line (OA) are designated by α 1  and the angle between the plane P and the straight line (OB) is designated by α 2 . The forces exerted on the ball by the cams and the ball-race are designated by F A , F B , F C  and F D .  
         [0103]     If α 1 =α 2 , we have F A =F B , so that the ball is in balance and the forces F C  and F D  are zero.  
         [0104]     If α 1 &gt;α 2 , we have F A +F B +F C =0 and the force F D  exerted by the cam  50  is zero.  
         [0105]     If α 1 &lt;α 2 , we have F A +F B +F D =0 and the force F C  exerted by the cam  40  is zero.  
         [0106]     The cams  40  and  50  are rotated so that the ball  22  arrives at a widened re-circulation zone as shown in  FIG. 15 . From that moment on, the ball  22  is no longer into contact with the cam  50 , so that it is no longer balanced, since no force applies at D. The ball  22  is subjected to a force imparting it an acceleration allowing it to separate from the ball-race and to cross the wire  94 , in order to position itself on the adjacent race.  
         [0107]     The passing over of the ball  22  from one race to the next one can occur only if α 1 &lt;α 2 , so that the resultant of the forces on the ball passes over the wire  94 .  
         [0108]     In addition, if one takes into consideration the frictions which are exerted on the ball  22  and which are designated by φ 1  and φ 2 , the friction angles between the ball and the wire  94  and between the ball and the cam  40 , a condition for the passing over of the ball from one race to the next one to occur is: α 1 +φ 1 +φ 2 &lt;α 2 . When assuming by φ 1  and φ 2  in the range of 5 degrees (lubricated contact), and α 1  in the range of 35 to 45 degrees, one deduces therefrom that α 2  must be of more than 45 or 55 degrees.  
         [0109]     In order to facilitate the passing of balls from one race to another and to maintain a good efficiency, α 2  can be chosen between 50 and 60 degrees, preferably to be 55 degrees. When α 2  is in the range of 55 degrees, the cam  40  has a helical beveled surface  41  oriented at 35 degrees with respect to the plane P. A cam having such a helical bevel can be achieved by machining a cylindrical part with a conical cutter having at the top half an angle of 55 degrees.  
         [0110]     Furthermore,  FIG. 16  shows the positioning of two cams  40  and  50  with respect to each other. The plane Q extends transversely to the plane of the diagram and passes through the axis of rotation of the nut  70  including both cams  40  and  50 . The cams  40  and  50  are identical. They are arranged in front of each other, so that the rolling surfaces  41  and  51  face each other. The cams are indexed by their key slots (see  FIG. 5 ), the key slots extending in the plane Q. As shown in  FIG. 16 , the key slots are positioned so as to form an angle θ with respect to the reference mark formed by an end of the helical surface corresponding to the leading plane of the cutter.  
         [0111]     The angle θ can be set in order to minimize the space of evolving of the balls in the re-circulation zone  81 , in order to avoid the presence of several balls at the same time in this zone and to keep the largest possible number of “working” balls. The setting of the angle θ depends namely on the pitch of the ball-race, on the orientation of cam surfaces  41  and  51 , on the diameter of the balls  22 , on the diameter of the wires  92  and  94  used for making the races.  
         [0112]     A way for determining this angle θ consists in determining the volumes in which the center O of a ball moves when the latter is resting against one of the cam surfaces, resting on the other cam surface and resting on the ball-races, respectively. The intersection of these volumes represents the space in which the ball is guided. This space can be modified by varying the angle θ. The space of intersection must both be large enough for a ball to be able to enter into the re-circulation zone and to move on the helical ball-race and sufficiently restricted to prevent several balls from being present simultaneously in the re-circulation zone  81 . The shape of the space obtained depends on angle θ and also on the shape of the setback surfaces of the cams.