Abstract:
Friction device includes at least two members, one of which is mobile, or potentially mobile, in terms of relative displacement with respect to the other under the action of an external stimulus. The relative displacement is liable to cause the relative displacement of two friction surfaces subjected to pressing forces, so as to convert the external stimulus into a friction force and to thus control the displacement of the mobile or potentially mobile member. The pressing forces are produced by pre-stressing mechanisms and the members are able to act on the effects of the pre-stressing mechanisms so as to use the resultant effect of the relative displacement of the mobile or potentially mobile member to reduce the pressing forces.

Description:
CROSS-REFERENCE TO RELATED U.S. APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT 
     Not applicable. 
     REFERENCE TO AN APPENDIX SUBMITTED ON COMPACT DISC 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a friction device for controlling force, and a friction damper comprising such a device. 
     2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98. 
     There are already known friction devices for controlling force, namely used in friction damping devices. The general principle consists of using two friction surfaces with relative displacement pressed against each other so as to transform the normal force at said surfaces into a friction force capable of creating a damping force. 
     Generally, these damping devices comprise at least two organs, one of which is movable in displacement with respect to the other one under the action of an external force. The displacement is likely to cause the relative displacement of two friction surfaces subjected to pressing forces, so as to transform said external force into a friction force and to thus dampen the displacement of said movable organ. 
     The friction devices are used in different fields, both, non-restrictively, in that of air, road and railway transport for damping, breaking, clutch or transmission, torque-limiting systems and in that of weapons for damping the return of the firearms. 
     However, controlling the force of a friction device occurs through the normal pressing force, so that it directly depends on the friction coefficient of the materials of the friction surfaces, which leads to uncertainties about the returned force or about its repeatability and stability over the time. 
     From GB 2 60 740 is also known a shock absorber for vehicles aimed at being used to damper springs. The invention comprises friction elements such as discs, movable in displacement with respect to each other under the action of a displacement, associated with springs the action and the thrust of which are controlled to put the friction elements under pressure. This pressure varies according to the direction of said movement, i.e. this pressure is applied in one direction, while it is suppressed in the other direction. To this end, one portion of the friction elements is screwed onto a threaded coaxial element, and the thrust of the springs is axially applied on the side opposite the one including the other portion of the friction elements. Thus, the pivoting in the direction of the screwing conjugated with the thrust causes the friction to increase, while the pivoting in the other direction tends to separate the friction elements and to suppress the contact between them. 
     The operation of such a shock absorber is of the “on-off” type and does not permit control of the force. It is provided for assisting a damper system, i.e. it oscillates according to the normal operation of the damper system and is blocked when the oscillation reaches a determined magnitude in the direction of compression of the spring of said damper system. It does therefore not permit calibration of the damping force in only one direction, since, in order the remove the blocking, a change of direction of the force is necessary. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is aimed at coping with the various above-mentioned drawbacks by providing a friction device for controlling the force permitting that the level of returned force is almost independent from the friction coefficient of the materials the friction surfaces are made of and which therefore permits to control said returned force. 
     The friction device according to the invention is of the type comprising at least two organs, one of which is movable, or potentially movable, in terms of relative displacement with respect to the other one under the action of an external stimulus. The relative displacement is likely to cause the relative displacement of two friction surfaces subjected to pressing forces so as to transform said external stimulus into a friction force and to thus control the displacement of said movable, or potentially movable, organ. The pressing forces are produced by pre-stressing means, and said organs are designed capable of acting on the effects of the pre-stressing means so as to use the resulting effect of the relative displacement of said movable, or potentially movable, organ in order to reduce said pressing forces. 
     Such a friction device permits, starting from a pre-stressing of known intensity, to achieve in an exact way the control of a force, independently from the friction coefficients of the friction surfaces. 
     According to a first embodiment of the friction device according to the invention, the organs are movable, or potentially movable, with respect to each other in a rotation motion and are associated to means for transforming a rotation motion into a rectilinear movement, while an annular element is intercalated between said organs, and said annular element is:
         capable of being blocked in rotation on one of said organs, at least in one direction of rotation, so as to generate a friction torque between said annular element and the other organ,   associated to elastic pre-stressing means capable of increasing said friction torque, and   associated to means capable, under the effect of, on the one hand, the rotation of the movable organ generated by the external force and, on the other hand, the resisting force resulting from the friction, of reducing the effects of said elastic pre-stressing means.       

     According to an additional feature of the first embodiment of the friction device according to the invention, the annular element is formed of a coaxial assembly of two rings having, each, a friction surface and shaped so that the modification of the distance separating them, obtained under the action of the elastic pre-stressing means, acts on the nearing of the friction surfaces of said rings to those of the movable organ. 
     According to another additional feature of the first embodiment of the friction device according to the invention, the friction surface of each of the rings is coaxially truncated, while the friction surface in front of the movable organ is also coaxially truncated and according to the same angle. 
     According to another additional feature of the first embodiment of the friction device according to the invention, it includes a ring blocked in rotation, at least in one direction of rotation, on the immobilized organ. The annular element is made integral in rotation with through balls accommodated in cavities uniformly distributed at the periphery, each being formed by the coincidence of two recesses, one being provided for in said ring and the other one in said annular element. For one half in each of the rings forming same, the recess provided for in said annular element is in the form of ramps so that during a relative rotation motion of said annular element with respect to said ring, said balls rest against said ramps so as to cause the separation of said rings. 
     According to another additional feature of the first embodiment of the friction device according to the invention, the rings each have a truncated external wall so as to confer to the annular element, by nearing said rings to each other, a substantially twin-wheel shape, while the movable organ is in the form of a pulley mounted on said annular element and the inner wall of which, which forms a friction surface aimed at co-operating with the external wall of said annular element, which forms the other friction surface, has a complementary shape and includes to this end a peripheral median ridge, which separates it into two parts that each have of truncated shape, and oriented divergently, and wherein on said pulley is fixed and wound a means capable of being subjected to the external force. 
     According to another additional feature of the first embodiment of the friction device according to the invention, the annular element is connected to one of the organs through a free wheel. 
     According to a second embodiment of the friction device according to the invention, the organs are movable with respect to each other in a rectilinear translation movement, and include means for transforming said rectilinear movement into two rotation movements coaxial to said translation movement and in opposite directions, so as to drive into reversed rotations friction surfaces brought into contact through pressing means. 
     According to an additional feature of the second embodiment of the friction device according to the invention, one of the organs consists of a rod provided with a threaded portion with several left-hand threads, and with a threaded portion with several right-hand threads, onto which are screwed two nuts, one of them with a left-hand thread and the other one with a right-hand thread, each of them being associated to at least one friction surface. 
     According to an additional feature of the second embodiment of the friction device according to the invention, it includes a pile of annular shaped discs inserted onto the rod, each of which being alternately connected in rotation to one of the nuts. 
     According to an additional feature of the second embodiment of the friction device according to the invention, the two nuts and their discs are accommodated in a body that contains the pressing means, which consist of elastic means capable of exerting a pressing force on the pile of discs. 
     According to an additional feature of the friction device according to the invention, irrespective of the contemplated embodiment, the pre-stressing means consist of pressing means of adjustable intensity. 
     According to an additional feature of the friction device according to the invention, the pre-stressing means consist of hydraulic, pneumatic or electric actuators. 
     The advantages and features of the friction device according to the invention will become clear from the following description referring to the attached drawing, which represents several non-restrictive embodiments of same. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  represents a schematic view illustrating the principle of the friction device according to the invention. 
         FIG. 2  represents a perspective schematic view of a first embodiment of a device according to the invention. 
         FIG. 3  represents a schematic median cross-sectional schematic view of the same device. 
         FIG. 4  represents a perspective and exploded schematic view with a cut-out of part of the same device. 
         FIG. 5  represents a partial, perspective and exploded schematic view with a cut-out of a second embodiment of the device according to the invention. 
         FIG. 6  represents a schematic median cross-sectional schematic view of the same device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     When referring to  FIG. 1 , one can see a mounting permitting to illustrate the principle of the friction device according to the invention. 
     This mounting comprises a plate  1 , which carries two pairs  10  and  11  of rollers  12  that are all mounted on axes, respectively  13  and  14 , parallel, perpendicular to a plane of the plate  1 . The alignment of the axes  13  of the rollers  12  of the pair  10  is not parallel to the alignment of the axes  14  of the rollers  12  of the pair  11 , they indeed form a certain angle. 
     The mounting also includes, on the one hand, a rectilinear rod  15  placed on the rollers  12  of the pair  10  and, on the other hand, a wedge  16  intercalated between the rod  15  and the rollers  12  of the pair  11 , the wedge  16  being shaped so that these faces in contact with the rollers  12  form the same angle as the one formed by the aforementioned alignments. 
     Furthermore, the wedge  16  is pushed back, parallel to the rod  15 , against the rollers  12  of the pair  11 , by means of a spring  17  co-operating with a system  18  for adjusting its tension. 
     The spring  17  exerts a force on the wedge  16 , which force is transformed into a normal force between the wedge  16  and the rod  15 . 
     In the case of a relative movement between the rod  15  and the wedge  16  is created a tangential force between the sliding surfaces and, during a relative displacement of the rod  15  with respect to the plate  1 , the system is blocked in the direction of the thrust of the spring  17 , while it is functional in the oppose direction P. 
     It will be understood that the displacement of the rod  15  in the direction of P depends very little, and almost not at all, on the friction coefficient of the materials the rod  15  and the wedge  16  are made of, but that it depends, on the other hand, on the pre-stressing force exerted by the spring  17 , which force, since it is adjustable and also depends on the damper force, permits an adjustment of the returned force. 
     This friction device is not of the on-off type, in a direction of displacement, the opposite direction to P, occurs a blocking, while in the other direction occurs a control of the displacement. 
     When referring now to  FIGS. 2 ,  3  and  4 , one can see a first embodiment  2  of the friction device according to the invention. 
     In particular in  FIG. 3  one can see that this friction device  2  comprises a shaft  20 , on which is mounted a ring  21 , above which is mounted an annular element  3 , above which is mounted a pulley  4 . 
     It should be noted that the ring  21  is mounted on the shaft through a free wheel  22 , which permits only one direction of rotation of the ring  21  on the shaft  20 . 
     When referring also to  FIG. 2 , one can see that the pulley  4  externally includes a groove  40  in which is wound a foil  41 , which is firmly coupled by one end to the pulley  4  at one coupling point  42 . 
     As can be seen more accurately in  FIG. 4 , the annular element  3  consists of a coaxial assembly of two rings  30  having internally a cylindrical wall  31  adapted to the outer wall  23  of the ring  21 , and externally a truncated wall  32 , so that the annular element  3  is twin-wheel shaped. 
     Furthermore, in  FIG. 3  one can observe that the pulley  4  includes an internal wall  43  having a particular shape, i.e. a bi-conical shape. This internal wall  43  indeed includes a peripheral median ridge  44 , which separates into two parts  45  and  46 , each having a truncated shape, and oriented divergently, i.e., its outer edge has a diameter larger than that of the ridge  44 . 
     The slopes of the parts  45  and  46  are of course according to the same angle as the slopes of the walls  32  of the rings  30 , so as to achieve a perfect adaptation of the pulley  4  on the annular element. 
     The nearing to each other of the two rings  30  inside the pulley  4  is performed through a plurality of holding elements  33  distributed at the periphery, each formed of a screw  34  passing through the two rings  30  and a nut  35 , which compress against the latter elastic means  36 , in this case superposed elastic washers, for example of the Belleville type. 
     The two rings  30  are thus compressed, each, through their wall  32  against a part  45  or  46 , and elastically because of the elastic means  36 . 
     Thus, between the annular element  3  and the pulley  4  is created a friction torque the value of which varies according to the nature of the elastic means  36 , according to the number of holding means  33 , and according to the tightening couple of the screws  34  and the nuts  35 . The holding means  33  thus form pre-stressing means permitting to brake a relative displacement in rotation of the pulley  4  on the annular element  3  through a pulling force on the foil  41 . 
     In  FIGS. 3 and 4 , one can see that in the outer wall  23  of the ring  21  are provided for recesses  24 , which are uniformly separated from each other and distributed according to a median line, while the wall  31  of each one of the rings  30  has, on its ridge  37  at the inner side of the annular element  3 , notches  38 , which are uniformly separated from each other and have the same angular distance as the recesses  24 , so that, during the nearing of the two rings  30 , two notches  38  facing each other form a recess that forms, in co-operation with the recess  24 , a cavity  25 , visible in  FIG. 3 , aimed at containing a ball  26 . 
     The balls  26  constitute the only connecting means between the ring  21  and the annular element  3 , the cavities  25  being dimensioned so as to permit only a functional backlash of the balls  26 . 
     Though the recesses  24  have a shape that can be any shape whatsoever, in this case the shape of a cap, the notches  38  have a particular shape, i.e., they are formed of two inclined planes  39 , so that the recesses obtained through the nearing of the rings  30  to each other have a pyramidal shape. 
     During the putting into rotation of the annular element  3  in the direction opposite that permitted by the free wheel  22 , the contact of the balls with the inclined planes  39 , which then form ramps, tends to cause the rings  30  to separate from each other. 
     In use, non-restrictively, the shaft  20  is fixed, the foil  41  is connected to the object in movement to be damped. During a pulling on the foil, the pulley  4  rotates on the annular element  3 , the rotation being limited by the frictions between the internal wall  43  and the walls  32 . 
     If the friction torque is too high, an adhesion of the pulley  4  to the annular element  3  occurs, which results into a relative rotation of the annular element  3  on the ring  21  and thus, because of the co-operation of the balls  26  with the inclined planes  39 , a separation of the rings  30  and a lowering of the friction torque. 
     The friction torque is thus determined by the rivalry between two antagonistic functions exerted, one by the tightening of the rings  30  on the pulley  4  through holding elements  33  and the action of the elastic means  36 , and the other one through the loosening of the rings  30  induced by the relative rotation motion of the annular element  3  on the ring  21  and the action of the balls  26  on the inclined planes  39 . 
     The rotation motion of the pulley  4  on the annular element, i.e., the damper force, depends very little on the friction coefficient of the materials they are made of, it essentially depends on the holding elements  33 , which exert a pressing force in association with the elastic means  36 , which exert a pre-stressing force, while the damper force acts, through the balls  26  and the inclined planes  39 , on the holding elements  33  and the elastic means  39  so as to reduce the pressing force. 
     It should be noted that it is contemplated that the foil  41  be preformed to a diameter smaller than that of the pulley  4 , in order to provide it with a spring function, so that, when releasing the pulling force, it is restored to its initial shape and winds up on the pulley, which is possible because of the interposition of the free wheel  22  between the ring  21  and the shaft  20 . 
     This embodiment of the friction device according to the invention can be used for example as a damper device in a weapon system in order to limit the effects of the return of a barrel of a weapon. The return of the barrel drivel the foil  41  in translation, which causes the pulley  4  to rotate. When the external stress is stopped, the foil  41  recovers its shape and permits the barrel to return to its initial position. 
     Of course, numerous variants are possible, thus, the conical friction surfaces can be replaced by simple flat surfaces or surfaces of multiple type as in the multi-disc brakes, depending on the global geometry of the damper to be designed. 
     The separating system with balls can also be replaced by any other system permitting to carry out the same function, i.e., a helical ramp or a cam. 
     While the system comprised of a foil and a pulley can be replaced by a system comprised of a pulley and a belt with or without notches, a system comprised of a pulley and a cable or a system comprised of a pinion and a rack. 
     When referring now to  FIGS. 5 and 6 , one can see another embodiment of the friction device according to the invention. 
     It comprises a rod  5  aimed at moving in axial translation under the effect of a pushing or pulling force and the external wall of which is provided with, on the one hand, a threaded portion with several left-hand threads  50  and, on the other hand, a threaded portion with several right-hand threads  51 . Thus, the threads  50  and  51  intersect, it should be noted that they do not necessarily have the same pitch. 
     The device also comprises, on the one hand, a nut  6  having a left-hand thread  60  shaped so as to co-operate with the left-hand threads  50  and, on the other hand, a nut  7  having a right-hand thread  71  shaped so as to co-operate with the right-hand threads  51 . 
     One will understand that the relative displacement in translation of the two nuts  6  and  7  in the same direction along the rod  5  causes opposite rotations of the two nuts  6  and  7 . 
     The device also comprises a series of friction discs  62  blocked in rotation on the nut  6 , as well as a series of discs  72  blocked in rotation on the nut  7 , the discs  62  and  72  being alternately piled up in one single pile. 
     To this end, the nut  6  is in the form of a tubular sleeve  61  including internally the left-hand threads  60  and externally longitudinal grooves  63  arranged according to generating lines, while the discs  62  are in the form of rings including, at their internal periphery, channels  64  aimed at co-operating with the grooves  63  so as to achieve a blocking in rotation of the discs  62  on the sleeve  61 . 
     Likewise, the nut  7  is in the form of a tubular sleeve  70  provided internally with right-hand threads  71  and externally with a flange  73  bearing an external skirt  74  in the form of a sleeve having, on its internal wall, longitudinal grooves  75  arranged according to generating lines, while the discs  72  have externally channels  76  aimed at co-operating with the grooves  75  so as to achieve a blocking in rotation of the discs  72  on the skirt  74  and, therefore, through the flange  73 , on the sleeve  70 . 
     One will understand that the relative displacement in translation of the two nuts  6  and  7  in the same direction along the rod  5  causes opposite rotations of the discs  62  and  72  while they are in contact with each other. 
     When referring now in particular to  FIG. 6 , one can see that the unit comprised of the two nuts  6  and  7  their discs, respectively  62  and  72 , is enclosed in a body  8  forming a cage. 
     This body  8  comprises, on the one hand, a tubular sleeve  80 , surrounding externally the two nuts  6  and  7 , and which is closed at each of its ends by an annular part  81  and  82 . 
     The annular part  81  has an opening  83  with a diameter between the outer diameter of the rod  5  and the outer diameter of the sleeve  61 , so as to be able to abut against the latter. 
     The annular part  82  has an internal opening  84  with a diameter larger than the outer diameter of the sleeve  70 , so as to permit an axial displacement of this sleeve  70  in this opening  84 . 
     Finally, the sleeve  80  is shaped so as to be accommodated in an annular space  85  provided for between the flange  73  and the annular part  82 , elastic means  86  capable of separating from each other the flange  73  and the annular part  82  and, hence, of pressing against the ends of the pile of discs  62  and  72 . 
     These elastic means  86  consist in this case of a pile of elastic washers  87  of the Belleville type associated to a thrust  88 , of the needle bearing type, intercalated between the pile and the flange  73  so as to permit the rotation of the nut  7  with respect to the body  8 . 
     According to a non-restrictive mode of operation, an external stress T applied to the rod  5  drives the latter in translation, which generates the rotations in opposite directions of the nuts  6  and  7 , causing the friction discs  62  and  72  to slide on each other. A friction torque appears between each friction surface because of the pressing force exerted by the elastic means  86 . This friction torque is transformed by the two systems comprised of a screw and nut into two axial braking forces onto the rod  5 , in order to create a damper force. 
     In order to make the system reversible, one of the nuts, preferably the nut  7 , should be mounted in series with a free wheel, not shown, so as to permit the return movement of the rod  5 . 
     Furthermore, since the body  8  is designed integral with the object to be damped, or vice-versa, the translation of the rod  5  tends to compress the elastic means, so that the damper force tends to reduce the pre-stressing and hence the friction force, which permits to achieve a regulation of this friction force depending on the damper force. 
     It should be noted that the elastic means  87  can be made in various ways, they can thus consist of pneumatic, hydraulic or electric actuators, for example jacks, which permits an additional adjustment, eventually in real time. 
     Irrespective of the embodiment of the friction device according to the invention, it has been calculated that strongly varying the friction coefficient only varies the returned force in a non-substantial way. 
     The preferred application of the friction device according to the invention consists in a friction damper-device made according to any embodiment. 
     The friction device according to the invention can however find many other applications, such as, non-restrictively, in the field of braking, transmission, servo-control of both braking and clutch.