Abstract:
A linear electromechanical actuator of the screw type for a parking brake including a tubular nut ( 22 ) coupled by means of an external thread to the casing ( 12 ) of the actuator and operated to perform a screwing movement about and along a longitudinal axis (x). The nut has an internal thread ( 23 ) opposite the external thread, for engaging a threaded rod ( 11 ) which is prevented from rotating with respect to the casing. Operation of the electrical motor ( 13 ) brings about a telescopic movement of the nut ( 22 ) and the rod ( 11 ) with respect to the casing ( 12 ), with a rapid linear translation of the rod (x).

Description:
This is a National Stage entry of International Application PCT/EP2004/007316, with an international filing date of Jul. 5, 2004, which was published under PCT Article 21(2) as WO/2005/005212 A1, and the complete disclosure of which is incorporated into this application by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a linear electromechanical screw actuator for a parking brake of a motor vehicle. 
   Instead of the traditional manually operated lever which imparts a traction movement to one or two Bowden cables connected to the parking brakes of a motor vehicle, it has recently been proposed to effect that traction movement by means of an electromagnetic actuator (see, for example, US2002/0100647 A1). 
   SUMMARY OF THE INVENTION 
   The object of the invention is to provide a linear actuator capable of imparting to the Bowden cables a traction movement for a maximum travel of approximately 40 mm rapidly (less than 1.0-1.5 seconds), in a non-backdrivable manner (that is to say, capable of maintaining the imparted braking force in the absence of an unlocking command) and capable of exerting a high traction force of the order of approximately 3600 N. 
   Another object of the invention is to provide a strong and reliable linear actuator which is easy to mount and which is constituted by a minimum number of components. 
   A further particular object of the invention is to provide an actuator which is such that, when the braking force is applied, the path of forces and reactions which passes through that actuator involves a minimum number of components. 
   Those and other objects and advantages which will be better understood hereinafter are achieved according to the invention by a braking system and by an actuator according to the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A description will now be given of the structural and functional features of some preferred but non-limiting embodiments of a braking system and of an actuator according to the invention; reference is made to the appended drawings in which: 
       FIG. 1  is a diagrammatic plan view of a motor vehicle provided with a braking system having an actuator according to the invention; 
       FIG. 2  is a view in longitudinal section of an actuator according to the invention; 
       FIG. 3  is a view in longitudinal section in accordance with the line III-III in  FIG. 2 ; 
       FIG. 4  is a perspective view of the actuator of  FIGS. 2 and 3 ; and 
       FIG. 5  is a view in longitudinal section of a variant of the actuator according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring first of all to  FIG. 1 , a braking system of a motor vehicle comprises parking brakes B 1  and B 2  associated with the rear wheels W 1  and W 2  of a motor vehicle V. Each parking brake is operated by a respective Bowden cable C 1 , C 2  constrained on a distributor R which, in order to operate the brakes, has to be pulled in the direction of the arrow A by a rod transmission element  11  at the output side of a linear electromechanical actuator generally indicated  10  and illustrated in  FIGS. 2 to 4 . 
   The actuator  10  has a substantially cylindrical rigid casing  12  defining a central longitudinal axis x which coincides with the direction of operation of the rod  11 . The casing  12  is advantageously formed by joining two bodies  12   a,    12   b,  of which the one body  12   a  located on the side where the rod  11  emerges is rigid while the complementary body  12   b,  which does not have to be subjected to appreciable stresses during use, can be thinner. 
   Mounted inside the casing is an electrical motor  13  with an associated reduction unit  14  whose rotary member at the output side comprises a radial flange  15  which is rotatable about the central axis x of the actuator. The flange  15  carries a plurality of axial pins  16 , for example three or four parallel pins arranged in an angularly equidistant manner around the axis x. 
   The head or output portion  12   a  of the casing  12  has an end transverse terminal wall  17  having a central opening  18  through which the rod  11  extends. Secured to the inside of the head portion  12   a  of the casing is a metal bush  19  which is threaded internally with a trapezial thread  20  with which engages the external thread  21  of a tubular nut element  22  having a trapezial internal thread  23  opposite the external thread  21  and coaxial therewith. The nut element  22  co-operates in a threaded manner with the rod  11 , which has an axially internal (or proximal) portion  11   a  having a trapezial thread  24  which is congruent with the internal thread  23  of the nut, and an axially more external (or distal) portion  11   b  having a non-circular cross-section. For example, as shown in  FIG. 4 , the distal portion  11   b  which extends through the opening  18  in the transverse wall  17  of the casing has a prismatic shape or has one or more flat surfaces  11   c,  whose function is described hereinafter, and a terminal portion  11   d  for connection to the distributor R. As will be seen, the rod and the nut co-operate by performing a telescopic movement with respect to the fixed casing. As an alternative to the example illustrated, the thread  20  which is fixedly joined to the casing  12  could be formed in a single piece with the casing instead of being formed by a separate element (the bush  19 ). 
   The nut element  22  has a base portion in the form of a radial flange  25  in which are formed axially oriented through-openings  26  in which the pins  16  carried by the rotary member  15  engage slidingly. 
   In the region of the outlet opening  18  for the rod  11 , the casing has means for preventing the rotation of the rod about the longitudinal axis of the actuator. Those anti-rotation means may be constituted by the opening  18  itself, which may have a non-circular shape corresponding to the cross-section of the distal portion  11   b  of the rod  11 . Alternatively, the anti-rotation means could comprise one or more grub screws  27  ( FIG. 2 ) which, by co-operating with the flat surfaces  11   c  of the rod  11 , prevent the rotation thereof. 
   In the alternative embodiment illustrated in  FIG. 5 , the rotary member  15  at the output side comprises a cylindrical seat having internal axial grooves  15   a.  A grooved peripheral portion  26   a  of the flange  25  of the nut  22  engages slidingly along those grooves. 
   When the electrical motor  13  is activated, the flanged member  15  causes the nut  22  to rotate relative to the threaded bush  19  and to the casing  12 , so that the nut performs a screwing movement about and along the longitudinal axis x. Owing to the anti-rotation engagement between the rod and the casing, and owing to the internal threaded coupling between the nut and the rod, which, as stated, have respective threads  23  and  24  opposite those  20 ,  21  of the external threaded coupling between the nut  22  and the casing, the rotation of the nut brings about a rapid linear translation (without rotation) of the rod  11 , which is rapidly retracted towards the inside of the actuator, thus operating the Bowden cables C 1 , C 2  which control the parking brakes B 1 , B 2 . During the screwing movement about the axis x, the nut  22 , with its flange  25 , approaches (or moves away from, depending on the sense of rotation imparted by the motor) the rotary flange  15 , by sliding on the axial pins  16  (or, in the variant of  FIG. 5 , by sliding along the axial grooves  15   a ). 
   It will be appreciated that, owing to the contribution made by the two threaded couplings, the speed of linear translation of the rod  11  is very high and therefore the parking brakes can be activated rapidly, despite the fact that the threads have small angles of inclination (preferably less than 14° and more preferably of approximately 8°) in order to render the system non-backdrivable. It is desired that, owing to the wear between the mobile portions of the actuator, the rod should be capable of maintaining the retracted position reached and avoiding a situation where, in the absence of an operating command, the rod may be removed from the casing, for example if the vehicle V is parked on a slope. 
   The reaction forces transmitted in return to the actuator during braking may be discharged to the outside (onto a fixed component of the vehicle) by way of a flange  28  formed near the region where the rod emerges from the rigid portion  12   a  of the casing or, alternatively, as illustrated with a broken line in  FIG. 3 , by way of the transverse end wall  17  of the casing, which acts as a check surface for a rigid sheath S which surrounds the rod and which is secured at its opposite end to the vehicle at a point (not illustrated) remote from the actuator. That variant advantageously enables the actuator to be arranged at any point in the vehicle, that is to say, even where there are no particularly rigid anchoring points present. 
   Owing to the proximity of the threaded members to the check portions or surfaces  17  and  28 , the path of the forces acting on the actuator when the latter is activated affects few members (the rod, the nut, the threaded bush and the casing) and those forces are discharged to the outside of the actuator without stressing the reduction unit  14  or the electrical motor  13 . This is advantageous if it is considered that the braking forces (of the order of approximately 3600 N) would inevitably increase the clearance between the transmission members of the reduction unit and the motor, thus reducing the service life of the actuator. The small number of members affected by the braking forces instead defines a very short kinematic chain which guarantees the reliability of the actuator over time.