Abstract:
This electric actuator for driving a home-automation screen is provided with a spring brake ( 105 ) comprising a helical spring ( 130 ), a friction part ( 140 ) having a friction surface ( 141 ) against which the helical spring ( 130 ) bears radially. Said brake further comprises an inlet part ( 110 ) suitable for driving the spring in rotation in a direction reducing the contact force between the spring ( 130 ) and the friction part ( 140 ), and an outlet part ( 120 ) connected to the screen. 
     While the screen is being lowered, the inlet part ( 110; 210 ) drives the spring ( 130; 230 ) in rotation with the contact force being decreased to the extent that the outlet part ( 120; 220 ) is released in rotation, without direct contact between the inlet part and the outlet part. The inlet part ( 110; 210 ) has at least two contact surfaces ( 113   a,    113   d;    213   b,    217   c ) suitable for transmitting drive torque (C M ) for raising the screen ( 2 ), by direct contact, to at least two corresponding contact surfaces ( 123   a,    123   d;    223   b,    227   a ) of the outlet part ( 120; 220 ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an electric actuator for driving a home-automation screen, of any of the following types: roller blind, shade, curtain, gate, projection screen, or garage door. The actuator of the invention is provided with a spring brake. This type of brake is more particularly adapted to tubular motors. 
       STATE OF THE ART 
       [0002]    Use of a helical-spring brake in actuators for home-automation screens is known, in particular from Patent Document FR B 2 610 668. In that document, a helical spring is mounted in a friction part. At least one turn of the spring is stressed radially by a bore in the friction part. Each end of the spring forms a tab extending radially towards the inside of the spring. Each tab can be moved in order to drive the spring in rotation about its axis. The inlet part, the outlet part, and the spring are arranged specifically to obtain the following dynamic behavior: action from the inlet part situated on one side of the first tab causes the spring to move in rotation in a first direction. This movement releases the outlet part, i.e. it tends to reduce the diameter of the outside envelope of the spring. Thus, the friction between the bore in the friction part and the turns of the spring decreases, thereby reducing the radial stress between the spring and the friction part. Conversely, action from the outlet part on the opposite side of the first tab causes the spring to move in rotation in the second direction, i.e. in the opposite direction. This movement blocks the outlet part, i.e. it tends to increase the diameter of the outside envelope of the spring. The friction between the bore in the friction part and the turns of the spring therefore increases. The same applies for the radial stress between the spring and the friction part. In addition, the inlet part can also act on the second tab of the spring in order to drive the spring in rotation in the second direction, while also releasing the outlet part. Furthermore, the outlet part can also act on the second tab of the spring in order to drive the spring in rotation in the first direction. In which case, the outlet part is blocked, or at least is braked by means of the spring rubbing against the friction part. Therefore, the inlet part moving in rotation makes it possible for the spring and for the outlet part to be moved in rotation, while the outlet part moving in rotation blocks the movement begun by the outlet part. 
         [0003]    The main braking of the outlet part is thus obtained by the spring rubbing against the friction part. A second phenomenon contributes to the braking of the outlet part, namely the outlet part rubbing at its guide means. This rubbing is directly related to the torque applied to the brake. When drive torque is exerted on the inlet part, the inlet part applies a force on the outlet part via a tab of the spring. Since that force is asymmetrical about the axis of the outlet part, it induces a radial force that causes the outlet part to be moved until it bears against its guide means. That contact brakes the outlet part. When torque is exerted on the outlet part, said outlet part applies a force on a tab of the spring that tends to hold the spring stationary in rotation. In reaction to that asymmetrical force, a radial force causes the outlet part to be moved until it bears against its guide means. Thus, in conventional spring brake designs, secondary braking torque exists that is added to the main braking torque of the spring against the friction part. That secondary braking torque is then applied both while the screen is being raised and also while it is being lowered. 
         [0004]    In Patent EP-B-0 976 909, a spring brake comprises an inlet part having two teeth, an outlet part also having two teeth, a spring, and a friction part. The drive torque exerted on the inlet part is transmitted to the outlet part via a tooth bearing against one of the tabs of the spring, which tab bears against a tooth of the outlet part. Since the force exerted on the outlet part is asymmetrical, it results in a radial force being applied to said outlet part and thus in secondary braking torque being applied. When torque is applied to the outlet part, a phenomenon occurs that is similar to the phenomenon that occurs in the brake of FR-B-2 610 668. A tooth of the outlet part bears against a tab of the spring that blocks the spring. In reaction to that asymmetrical force, a radial force causes the outlet part to be moved until it bears against its guide means. 
         [0005]    The way in which conventional spring brake designs as described in the preceding examples operate suffers from drawbacks in certain configurations. When the actuator drives a screen in the lowering direction, i.e. when the load torque exerted by the weight of the screen at the outlet part is in the same direction as drive torque from the actuator that is exerted at the inlet part, it is advantageous for secondary braking torque to be added to the main braking torque because that reduces the response time of the brake, thereby making the installation safer. Unfortunately, the existence of secondary braking torque while the screen is being raised, i.e. when the load torque exerted by the weight of the screen at the outlet part is opposed to drive torque from the actuator that is exerted at the inlet part, is particularly disadvantageous because the brake brakes continuously, thereby requiring the motor to be over-dimensioned. The motor must not only raise the load, i.e. exert torque that is greater than the load torque, but must also compensate for the secondary braking torque, since said secondary braking torque is added to the load torque. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention proposes an electric actuator provided with a spring brake that improves the operation of the above-described brakes, while also preserving the advantages of those brakes. In order to optimize dimensioning of the motor, the invention aims to eliminate the secondary braking torque while the load is being raised. To this end, the invention provides an electric actuator for driving a home-automation screen mounted to move between an open position and a closed position, said actuator being provided with a spring brake, said brake comprising:
       a helical spring, each end of which forms a respective tab extending radially or axially relative to a central axis of the spring;   a friction part having a substantially cylindrical friction surface against which at least one turn of the helical spring bears radially;   an inlet part driven by an electric motor of the actuator, and suitable for coming into contact with at least one tab of the spring, in such a manner as to drive the spring in rotation about a central axis of the brake, in a direction making it possible to reduce the contact force between the helical spring and the friction surface; and   an outlet part connected to the screen and suitable for coming into contact with at least one tab of the spring in such a manner as to drive the spring in rotation about the central axis of the brake, in a direction making it possible to increase the contact force between the helical spring and the friction surface.       
 
         [0011]    In this actuator, while the screen is being lowered, the inlet part drives the spring in rotation with the contact force being decreased to the extent that the outlet part is released in rotation, without direct contact between the inlet part and the outlet part. According to the invention, the inlet part has at least two contact surfaces suitable for transmitting drive torque for raising the screen, by direct contact, to at least two corresponding contact surfaces of the outlet part. 
         [0012]    The screen generates load torque at the outlet part, which torque makes it possible to generate secondary braking torque. As a result, this actuator is particularly suitable for screens that move vertically and whose weight makes it possible to generate the preceding load torque. This may be for winding an apron around a tube or for swinging a garage door between a horizontal position and a vertical position. 
         [0013]    The inlet part and the outlet part are in direct contact only while the screen is being raised. Thus, during lowering, these two parts are not in direct contact for transmitting the drive torque. During lowering, the inlet part releases the brake by acting only on one of the tabs of the spring. The drive torque is exerted on that tab. No force is transmitted between the inlet part and the outlet part. The outlet part is retained by the other tab of the spring. As a result, it exerts a force, generated by the load torque, on that tab only, so as to drive the spring in rotation about the central axis of the brake, in a direction making it possible to increase the contact force between the helical spring and the friction surface. 
         [0014]    In the present description “direct contact” between two parts means that one part acts on the other either by direct co-operation of complementary surfaces, or by co-operation between complementary surfaces through another part that is rigid disposed between these surfaces, or else by a combination of the preceding types of co-operation. Direct contact can be obtained by one or more contact surfaces disposed on the outlet part, such a contact surface being a surface against which there comes to bear a complementary contact surface of the inlet part or a complementary surface of an intermediate part urged by the inlet part. In order to implement the invention, it is necessary for the torque to be transmitted via at least two contact surfaces of the outlet part. 
         [0015]    The balancing of the drive torque that makes it possible to reduce the secondary braking torque during raising can be achieved astutely by transmitting the drive torque via a plurality of sets of contact surfaces disposed, about the axis of rotation of the spring, in a manner such that the drive torque is transmitted in substantially balanced manner, making the outlet part relatively unstressed radially. These sets of surfaces can be disposed about the axis of the outlet part in a manner such as to reduce or eliminate the induced radial force. For example, the torque can be transmitted via two contact surfaces of the outlet part that are substantially identical and that are diametrically opposite each other about the axis of the outlet part. This solution is simple to implement. 
         [0016]    Advantageously, operation of the brake is identical, regardless of the direction of the drive torque for raising the screen. This characteristic makes it possible to obtain a versatile actuator that can be installed independently of the configuration of the screen. For example, for a tubular actuator that fits into a winding tube, operation of the actuator is identical regardless of whether the screen is wound in one direction or in the opposite direction. This symmetrical operation of the brake makes it possible to rationalize a product range and to facilitate installation of the actuator because there is no need to distinguish whether the motor should be mounted in a particular manner relative to the screen. 
         [0017]    According to other advantageous but non-essential aspects of the invention:
       in the absence of drive torque, the outlet part exerts a force on the tab of the spring in such a manner as to drive the spring in rotation about the central axis of the brake, in a direction making it possible to increase the contact force between the spring and the friction surface;   at at least one contact surface, the direct contact between the inlet part and the outlet part is achieved by means of a rigid part such as one of the tabs of the spring;   the configuration of the contact surfaces makes it possible to balance the transmission of the raising drive torque, in such a manner as to eliminate or significantly reduce the radial component, relative to the axis of rotation of the spring, of the forces transmitted to the outlet part; and   the two contact surfaces of the outlet part are diametrically opposite each other about the axis of the outlet part.       
 
         [0022]    Provision may be made for the outlet part to be suitable for coming into contact with a part having dynamic behavior different from that of the outlet part, in particular a part secured to or integral with the friction part or the inlet part, when a radial force is exerted on the outlet part, said radial force being generated only while the screen is being lowered. 
         [0023]    The outlet part is advantageously suitable for coming to bear against a centering member for centering the outlet part relative to the inlet part under the effect of the radial component of the resultant of the load torque exerted by the screen, while the screen is being lowered. 
         [0024]    Provision may be made for the outlet part to be guided in rotation relative to the inlet part. The inlet part and the outlet part must be centered relative to each other. The inlet part and the outlet part may be centered by a shaft passing through said parts. The shaft is mounted in tight-fitting manner in the inlet part or in the outlet part and is mounted to slide in the other part, i.e. respectively in the outlet part or in the inlet part. This centering is simple to achieve and is compact. The sub-assembly formed by the inlet part and by the outlet part is then advantageously centered relative to the friction part. This centering may be achieved either by the outlet part, or by the inlet part. Preferably, the sub-assembly is centered by the inlet part, because that makes it possible to reduce the vibration of the brake considerably. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0025]    The invention can be better understood on reading the following description given merely by way of example and with reference to the accompanying drawings, in which: 
           [0026]      FIG. 1  is a diagrammatic view of the architecture of a tubular actuator of the invention that incorporates a spring brake of the invention; 
           [0027]      FIG. 2  is an exploded perspective view of a spring brake belonging to the actuator of  FIG. 1 ; 
           [0028]      FIG. 3  is a diagrammatic cross-section view of operation of the spring brake  2  of  FIG. 2  during raising of a load; 
           [0029]      FIG. 4  is a diagrammatic cross-section view of operation of the spring brake  2  during lowering of a load; 
           [0030]      FIG. 5  is a diagrammatic cross-section view of operation of a prior art spring brake during raising of a load; 
           [0031]      FIG. 6  is an exploded perspective view of a second embodiment of a spring brake that can be part of the actuator of  FIG. 1 ; 
           [0032]      FIG. 7  is an exploded perspective view from a different angle of certain component elements of the spring brake of  FIG. 6 ; 
           [0033]      FIG. 8  is a diagrammatic end view seen looking along arrow F in  FIG. 6 , and partially in cross-section, showing operation of the spring brake of  FIG. 6  during raising of a load that generates torque in the clockwise direction on the outlet part of the brake; 
           [0034]      FIG. 9  is a diagrammatic end view partially in cross-section analogous to  FIG. 8 , showing operation of the spring brake of  FIG. 6  during lowering of a load that generates torque in the clockwise direction on the outlet part of the brake; 
           [0035]      FIG. 10  is a diagrammatic end view partially in cross-section analogous to  FIG. 8 , showing operation of the spring brake of  FIG. 6  during raising of a load that generates torque in the counterclockwise direction on the outlet part of the brake; and 
           [0036]      FIG. 11  is a diagrammatic end view partially in cross-section analogous to  FIG. 8 , showing operation of the spring brake of  FIG. 6  during lowering of a load that generates torque in the counterclockwise direction on the outlet part of the brake. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0037]      FIG. 1  diagrammatically shows a rotary tubular actuator  100  designed to drive in rotation a winding tube  1  on which an apron  2  for closing an opening  0  can be wound to various extents. The tube  1  is driven by the actuator  100  in rotation about an axis of revolution X-X that is disposed horizontally at the top of the opening. For example, the opening O is an opening provided in the walls of a building. The actuator  100 , the tube  1 , and the apron  2  then form a motor-driven roller blind. 
         [0038]    The actuator  100  comprises a stationary cylindrical tube  101  in which a motor-and-gearbox unit  102  is mounted that is made up of an electric motor  103 , a first gearbox stage  104 , a spring brake  105 , a second gearbox stage  106 , and an outlet shaft  107  that projects at one end  101 A of the tube  101 , and that drives a wheel-ring  3  that is constrained to rotate with the tube  1 . 
         [0039]    The winding tube  1  turns about the axis X-X and about the stationary tube  101  by means of two pivot couplings. A bearing-ring  4  mounted on the outside periphery of the tube  101  in the vicinity of its end  101 B opposite from the end  101 A forms the first pivot coupling. The second pivot coupling is installed at the other end of the tube  1  and is not shown. 
         [0040]    The actuator  100  further comprises a fastening part  109  that projects from the end  101 E and that makes it possible to fasten the actuator  100  to a frame  5 . Said fastening part  109  is, in addition, designed to close off the tube  101  and to support a control module  108  for controlling the power supply to the motor  103 . Said control module is powered via a mains power supply cable  6 . 
         [0041]    While the tubular actuator  100  is operating, the motor-and-gearbox unit  102  drives in rotation the shaft  107  which, in turn, drives in rotation the tube  1  via the wheel-ring  3 . For example, when the actuator  100  is installed in a roller blind case, the shaft  103  rotating causes the opening O to be opened and to be closed in alternation. The apron  2  thus moves vertically in the opening O, between an opening high position and a closure low position. 
         [0042]      FIGS. 2 to 4  more particularly show the structure of the spring brake  105  in a first embodiment of the invention. As shown in  FIG. 1 , a rotor of the motor  103  drives an epicyclic gear train of the first gearbox stage  104 . The cylinder  110  of the epicyclic train that carries three planet gears also forms an inlet part of the brake  105 . The brake  105  includes a helical spring  130  having its turns centered on an axis X 130  that coincides with the axis X-X when the brake  105  is in place, as shown in  FIG. 1 . Said spring is mounted in tight-fitting manner inside a bore  141  in a friction part  140 . In other words, the outside envelope  131  of the spring  130 , which envelope is defined by the outside generator lines of its turns, bears against the radial surface of the bore  141 , thereby tending to secure together the spring  130  and the part  140  by friction. 
         [0043]    Each end of the spring  130  forms a tab  132   a,    132   b  extending radially towards the axis X 130  and towards the inside of the spring, from its turns. 
         [0044]    The inlet part  110  is provided with two protuberances or “teeth”  111   a  and  111   b  that fit into the helical spring  130 . Each protuberance  111   a  or  111   b  has a face  113   a  or  113   b  suitable for being in contact respectively with a surface  133   a  of a first tab  132   a  forming the first end of the spring or with a surface  133   b  of the second tab  132   b  forming the second end of the spring. The surface  133   a  is disposed in a manner such that action on said surface causes the spring to be moved in rotation about the axis X 130  in a direction that is opposite from the direction of rotation of the spring if the action is exerted on the surface  133   b.    
         [0045]    Action by one of the teeth  111   a  or  111   b  on a surface  133   a  or  133   b  tends to release the brake, i.e. to move one of the tabs  132   a  or  132   b  in a manner such that the radial stress between the outside envelope  131  of the helical spring  130  and the friction surface of the bore  141  decreases. This action from one of the teeth  111   a  or  111   b  tends to contract the spring  130  radially about the axis X-X, so that its outside envelope moves away from the surface of the bore  141 . The part  110  thus makes it possible to act on the spring  130  to reduce the contact force between the spring and the friction surface of the bore  141 . The spring can then turn about the axis X 130  that coincides with the central axis X 105  of the brake  105 , itself coinciding with the axis X-X when the actuator  100  is in the assembled configuration shown in  FIG. 1 . A direction or a dimension is said to be “axial” when it extends or is measured parallel to the axis X 105 . A direction is said to be radial when it is perpendicular to and intersects the axis X 105 . 
         [0046]    An outlet part  120  of the brake  105  is situated in register with the inlet part  110 . The outlet part is provided with two lugs  121   a,    121   c  also fitting into the helical spring  130 . The lug  121   a  is provided with two recesses or setbacks  122   a,    122   b  disposed on either side of said lug. Each recess  122   a  or  122   b  is designed to receive a respective one of the tabs  132   a,    132   b  of the spring and is defined partially by a surface  124   a,    124   b  suitable for being in contact with a surface  134   a,    134   b  of a tab  132   a,    132   b.  The surfaces  134   a  and  134   b  are opposite from respective ones of the surfaces  133   a  and  133   b.    
         [0047]    Action on one of the surfaces  134   a,    134   b  tends to move the tabs  132   a  and  132   b  apart, thereby causing the turns of the spring  130  to expand radially relative to the axis X 130  and increasing the contact force between the spring  130  and the friction surface of the bore  141 . This results in actuating the brake, i.e. in blocking or in strongly braking the rotation of the spring  130  relative to the part  140 . Thus, the radial stress between the outside envelope  131  of the helical spring and the friction surface  141  increases, thereby holding the part  120  stationary or braking it strongly about the axes X 105  and X 130 . 
         [0048]    In order to enable the brake to operate, it is necessary to have angular clearance between the teeth  111   a  and  111   b  of the inlet part  110  and the tabs  132   a  and  132   b  of the spring. Similarly, angular clearance is also necessary between the lug  121   a  and the tabs  132   a  and  132   b  of the spring. The width of the lug  121   a  is designed for this purpose. In addition, the axial length L 111  or L 121  of the portions  111   a,    111   b,  and  121   a  is slightly greater than the axial length L 130  of the spring. 
         [0049]    The outlet part  120  is also provided with a set of teeth  129  forming the interface with the second gearbox stage  106 . 
         [0050]    The necessary centering of the outlet part  120  relative to the inlet part  110  is achieved by a shaft  118  projecting axially relative to the inlet part, on the same side as the outlet part  120 . Said shaft  118  serves as guide means for guiding the outlet part, by means of a bore  128  provided through the center of said outlet part. 
         [0051]    As appears more particularly from  FIGS. 3 to 4 , the load L constituted by the apron  2  can be considered as being secured to the outlet part  120 , via the elements  1 ,  3 ,  106 , and  107 , as indicated by the vertical dashed line in  FIGS. 3 and 4 . 
         [0052]    The weight of the load L exerts torque C L  on the outlet part  120  that tends to cause it to turn about the axis X 105 , in the clockwise direction in  FIGS. 3 and 4 . 
         [0053]    Reference X 120  designates the central axis of the outlet part  120 , which axis coincides with the axis X 105  when the brake is in the assembled configuration. 
         [0054]    While the load L is being raised, and as shown diagrammatically in  FIG. 3 , rotation of the outlet part  120  in the clockwise direction in  FIG. 3 , which rotation is normally induced by the torque C L , is blocked by the inlet part  110 . The inlet part  110  is driven in rotation in the counterclockwise direction in  FIG. 3  by torque C M  generated by the motor and weighted by the efficiency of the first gearbox stage  104 . The two protuberances  111   a  and  111   b  of the inlet part  110  pivot about the coinciding axes X 105  and X-X until one of the protuberances  111   a  or  111   b  is in contact with a face  123   a  or  123   b  of the lug  121   a  of the outlet part. Whereupon, the other protuberance  111   b  or  111   a  also enters into contact with one of the faces  123   c  or  123   d  of the second lug  121   c  of the outlet part. Therefore, the drive torque C M  is transmitted to the outlet part via two sets of contact surfaces, formed between the faces  113   a  and  113   d  and the faces  123   a  and  123   d  that are diametrically opposite each other about the axis X 105  and about the axis X 120  of the outlet part, thereby causing the radial component of the resultant of the torque C M  exerted on the outlet part  120  to be reduced or eliminated. The drive torque C M  is of opposite direction to the load torque C L . The faces  123   a  and  123   d  constitute the contact surfaces of the outlet part  120 . 
         [0055]    The balance of the forces to which the outlet part  120  is subjected is shown in  FIG. 3 . The load torque C L  is balanced by forces F 1a  and F 1b  resulting respectively from the surface  113   a  of the tooth  111   a  and the surface  123   a  of the lug  121   a  bearing against each other, and from the surface  113   d  of the tooth  111   b  and the surface  123   d  of the lug  121   c  bearing against each other. These two forces F 1a  and F 1b  express in terms of forces the drive torque C M  necessary for overcoming the load torque C L . Since the two forces F 1a  and F 1b  are of substantially the same magnitude and are substantially symmetrical about the central axis X 120  of the outlet part, the radial component of the resultant of the torque C M  of the outlet part  120  is negligible, or even zero. It should be noted that the shaft  118  of the inlet part making it possible to center the outlet part is not in contact with the bore  128  of the outlet part in this configuration, due to the fact that the radial component of the above-mentioned resultant is negligible. 
         [0056]    In order to raise the load, the torque C M  must be greater than the sum of the load torque C L  and of the drag torque of the brake spring due to the residual friction between the outside envelope  131  of the spring and the friction surface of the bore  141 . At start-up, the torque C M  to be exerted must be larger because, in order to release the brake  105 , it is necessary to overcome a static friction force. Thus, the protuberance  111   a  acts on one of the tabs of the spring, which tab is, in this example, the tab  132   a,  received in the recess  122   a,  as soon as the lug  121   a  is driven in rotation. 
         [0057]    While the load L is being lowered, and as shown diagrammatically in  FIG. 4 , the outlet part rotating in the clockwise direction in  FIG. 4  is not stopped by the inlet part but by the spring  130 . Thus, the load torque C L  presses the lug  121   a  against one of the tabs  132   a  or  132   b,  namely the tab  132   a  in this example. The effect of this is to expand the turns of the spring  130  radially and to activate the brake  105 , as explained above. The torque C L  exerted by the lug  121   a  on the surface  134   a  of the tab  132   a  is weighted by the efficiency of the second gearbox stage  106 . The tab  132   a  is engaged in the recess  122   a.  The drive torque C M  is in the same direction as the load torque C L . 
         [0058]    The balance of the forces of the outlet part is shown in  FIG. 4 . The load torque C L  is balanced by two forces F 2a  and F 2b . The first force F 2a  corresponds to the reaction of the face  134   a  of the tab  132   a  of the spring  130  against the bearing face  124   a  of the recess  122   a.  Since said first force F 2a  does not make it possible to compensate for the load torque C L  fully, the outlet part  120  tends to move perpendicularly to the axis X 105 , relative to the preceding bearing configuration, until the outlet part comes into contact with its guide means formed by the shaft  118  that is secured to or integral with the inlet part  110 . The bore  128  for guiding the outlet part thus comes into contact with the shaft  118 , then generating the second radial force F 2b  making it possible to balance the load torque C L . Said second force F 2b  generates friction during the downward movement of the load. This friction brakes the load and is added to the braking torque of the spring. It thus contributes to the reactivity of the brake. The response time of the brake is faster than the response time of a brake for which said friction does not exist. 
         [0059]    It should be noted that, for this embodiment, the inlet part  110  is itself centered relative to the friction part  140  by means of a cylindrical web whose envelope surface (not shown) co-operates with the bore  141  in the friction part. Therefore, the preceding force F 2b  induces an equivalent force (not shown) between the inlet part  110  and the friction part  140 . Said equivalent force participates in the secondary braking torque and contributes to the reactivity of the brake. 
         [0060]    In order to make it possible to lower the load, it is necessary to release the brake. For this purpose, the drive torque C M  drives the protuberances  111   a  and  111   b  of the inlet part  110  in rotation, the protuberance  111   b  being driven by said drive torque until it comes into abutment against the face  133   b  of the tab  132   b  of the spring  130 . By this action, the spring  130  is relaxed and the outlet part  120  can turn, by means of the load torque C L . The parts  110  and  120  are then not in direct contact. 
         [0061]    If the direction of winding of the load is reversed, operation is identical. Operation of the brake is thus symmetrical, which makes it easier for it to be installed because the performance of the brake is the same, regardless of the raising direction of the actuator, i.e. regardless of the direction of the drive torque C M  that serves to raise the screen  2 . 
         [0062]      FIG. 5  shows a conventional prior art spring brake, and more particularly how it behaves during raising. The portions of the brake that are shown in  FIG. 5  and that are analogous to the portions of the brake  105  bear like references minus  100 . For that type of brake, the outlet part is not designed to balance the load torque during raising. The outlet part  20  is provided with one lug  21   a  only. During raising, operation is similar to operation of the brake  105  in the configuration shown in  FIG. 3 . The drive torque C M  drives a protuberance  11   a  in rotation until said protuberance comes into contact with one face  33   a  of a tab  32   a  of the spring  30 . The opposite face  34   a  of the tab is in abutment against a face  23   a  of the lug  21   a  of the outlet part  20  by means of the load torque C L . 
         [0063]    Therefore, the drive torque C M  is transmitted to the outlet part  20  via the tab  32   a  of the spring  30 . 
         [0064]    In the embodiment of the invention that is described above with reference to  FIGS. 1 to 4 , the drive torque is transmitted directly to the outlet part  120  by contact between one face  113   a  of the inlet part  110  and one face  123   a  of the outlet part  120 , the spring tab then being retracted into the recess  122   a  provided for this purpose. This makes it possible to achieve better torque transmission and to stress the parts less. 
         [0065]    In the brake shown in  FIG. 5 , the load torque CL is not sufficiently taken up by a tab  32   a  of the spring to balance said torque, and therefore induces a radial force on the outlet part  20 . That radial force causes the outlet part to move until it is in contact with its guide means that are formed by the bore  41  in the friction part  40 . The outlet part  20  has a cylindrical web whose envelope surface  25  makes it possible to perform the guiding in the bore  41 . Thus, the load torque is balanced firstly by a force F′ 1a  corresponding to the lug  21   a  bearing against the tab  32   a  of the spring  30  and secondly by a force F′ 1b , resulting from the outlet part  20  bearing against the bore  41  in the friction part  40 . Given that, during raising, the outlet part  20  has a relative speed relative to the friction part  40 , said force F′ 1b  generates friction during the load-raising movement. In order to lift the load L, the drive torque C M  must therefore be greater than the sum of the load torque C L , of said friction, and, on start-up, of the torque necessary to release the brake. Therefore, said friction adversely affects the dimensioning of the motor because said motor must be more powerful in order to compensate for the additional friction resulting from the force F′ 1b . 
         [0066]    For lowering the load, operation is analogous to the operation shown in  FIG. 3  for the brake of the invention. Balancing of the forces is, however, more similar to the balancing shown in  FIG. 5 . The load is braked by the braking torque of the spring  30  and by the friction with the guide means formed by the bore  41  in the outlet part. 
         [0067]      FIGS. 4 and 5  show two different guide means for guiding the outlet part  20  or  120 . In  FIG. 4 , the outlet part  120  is guided relative to the inlet part  110 . The inlet part  110  is also centered relative to the friction part  140 . In  FIG. 5 , the outlet part  20  is guided relative to the friction part  40  that is stationary. Tests have shown that the brake  105  behaves better in the  FIG. 4  situation. The centering of the outlet part relative to the inlet part makes it possible to reduce the vibration of the brake. 
         [0068]      FIGS. 6 to 11  show a second embodiment of the brake. The operating principle is close to the first embodiment. The references of these parts are analogous to the references of the first embodiment, plus  100 . 
         [0069]    The outlet of the epicyclic gear train of the first gearbox stage  104  drives in rotation a part  210  forming the inlet of the brake  105 . The inlet part  210  is provided with a polygonal shaft  219  designed to receive and to transmit torque coming from the gearbox stage  104 . The brake  105  includes a helical spring  230  whose turns are centered on an axis X 230  that coincides with the axis 
         [0070]    X-X when the brake  105  is in place as shown in  FIG. 1 . The axes X 230  and X-X coincide with the central axis X 105  of the brake  105  when an actuator  100  incorporating the brake  105  of this second embodiment is in the assembled configuration. 
         [0071]    The spring  230  is mounted in tight-fitting manner inside a bore  241  in a friction part  240 . In other words, the outside envelope  231  of the spring  230 , which envelope is defined by the outside generator lines of its turns, bears against the radial surface of the bore  241 , thereby tending to secure together the spring  230  and the part  240  by friction. 
         [0072]    Each end of the spring  230  forms a tab  232   a,    232   b  extending radially towards the axis X 230  and towards the inside the spring, from its turns. 
         [0073]    The inlet part  210  is provided with a protuberance or “tooth”  211   a  that fits into the helical spring  230 , between the tabs  232   a  and  232   b.  Said tooth  211   a  has two faces  213   a,    213   b  suitable for being in contact respectively with a surface  233   a  of a first tab  232   a  forming the first end of the spring and with a surface  233   b  of the second tab  232   b  forming the second end of the spring. The surface  233   a  is disposed in a manner such that action on said surface causes the spring to be moved in rotation about the axis X 230  in a direction that is opposite from the direction of rotation of the spring if the action is exerted on the surface  233   b.    
         [0074]    Action by the tooth  211   a  on a surface  233   a  or  233   b  tends to release the brake, i.e. to drive the tab  232   a  or  232   b  in rotation about the axes X 230  and X 105 , in a direction such that the radial stress between the outside envelope  231  of the spring  230  and the friction surface of the bore  241  decreases. Action from the tooth  211   a  on one of the faces  233   a  or  233   b  tends to contract the spring  230  radially about the axis X-X, so that its outside envelope moves away from the surface of the bore  241 . The part  210  thus makes it possible to act on the spring  230  to reduce the contact force between the spring and the friction surface of the bore  241 . 
         [0075]    An outlet part  220  of the brake  105  is situated in register with the inlet part  210 . The outlet part is provided with two lugs  221   a,    221   b  also fitting into the helical spring  230 . Each lug is provided with a recess or a setback  222   a,    222   b  designed to receive a respective one of the tabs  232   a,    232   b  of the spring  230 . Each recess  222   a,    222   b  is defined partially by a surface  224   a,    224   b  suitable for being in contact with a surface  234   a,    234   b  of a tab  232   a,    232   b.  The surfaces  234   a  and  234   b  are opposite from respective ones of the surfaces  233   a  and  233   b.    
         [0076]    Action on one of the surfaces  234   a,    234   b  tends to move the tabs  232   a  and  232   b  towards each other, thereby causing the turns of the spring  230  to expand radially relative to the axis X 230  and increasing the contact force between the outside envelope  231  of the spring  230  and the friction surface of the bore  241 . This results in actuating the brake, i.e. in blocking or in strongly braking the rotation of the spring  230  relative to the part  240 . Thus, the radial stress between the outside envelope  231  of the helical spring and the friction surface  241  increases. 
         [0077]    In addition, each lug  221   a,    221   b  of the outlet part  220  is provided with a projecting portion  226   a,    226   b  extending axially towards the inlet part and suitable for being received in respective ones of banana-shaped slots  216   c,    216   d  in the inlet part  210 , once the brake  105  is assembled. Said projecting portions  226   a  and  226   b  are dimensioned and disposed in a manner such that their faces  227   a,    227   b  are in contact with respective ones of inside faces  217   c,    217   d  defining the corresponding slots  216   c,    216   d  when the face  213   b,    213   a  of the tooth  211   a  of the inlet part  210  is in contact with the face  223   b,    223   a  of a lug  221   b,    221   a  of the outlet part  220 . 
         [0078]      FIGS. 8 and 10  show the two possible configurations for the brake  105 . The dimensioning of the slots  216   c,    216   d  is such that, outside the two preceding configurations, the projecting portions  226   a,    226   b  do not come into abutment against any inside surface of the slot. 
         [0079]    In order to enable the brake to operate, it is necessary to have angular clearance between the tooth  211   a  of the inlet part  210  and the tabs  232   a  and  232   b  of the spring. Similarly, angular clearance is also necessary between the lugs  221   a  and  221   b  and the tabs  232   a  and  232   b  of the spring. The width of the tooth  211   a  is designed for this purpose. In addition, the axial length L 211  or L 221  of the portions  211   a,    221   a,  and  221   b  is slightly greater than the axial length L 230  of the spring. 
         [0080]    The necessary centering of the outlet part  220  relative to the inlet part  210  is achieved by a shaft  270 . Said shaft is engaged in a centered bore  218  of the inlet part  210 . A portion of the shaft  270  projects from the same side as the outlet part  220 . 
         [0081]      FIGS. 8 to 11  show how the brake  105  operates.  FIGS. 8 and 9  correspond to the screen being wound on the shaft  1  in the clockwise direction in said figures.  FIG. 8  shows the load being raised, while  FIG. 9  shows the load being lowered.  FIGS. 10 and 11  correspond to the screen being wound on the shaft  1  in the counterclockwise direction in these figures.  FIG. 10  shows the load being raised while  FIG. 11  shows it being lowered. 
         [0082]    Firstly, operation of the brake is explained relative to the first screen-winding configuration, i.e. to winding in the clockwise direction in  FIGS. 8 and 9 . 
         [0083]    By default, the weight of the load L exerts torque C L  on the part  220  that presses one of the lugs  221   a  or  221   b,  namely the lug  221   b  in this example, against one of the tabs  232   a  or  232   b,  namely the tab  232   b  in this example, as shown in  FIG. 9 . The effect of this is to expand the turns of the spring  230  radially and to activate the brake  105 , as explained above. The torque C L  exerted by the lug  221   b  on the surface  234   b  of the tab  232   b  is weighted by the efficiency of the second gearbox stage  106 . This torque is shown by a vector associated with the lug  221   b.  The tab  232   b  is then engaged in the recess  224   b.    
         [0084]    While the load L is being raised, and as shown in  FIG. 8 , the inlet part  210  is driven in rotation by torque C M  generated by the motor and weighted by the efficiency of the first gearbox stage  104 . The protuberance  211   a  of the inlet part then turns until it is in contact with the lug  221   b  of the outlet part, at the interface between the surfaces  213   b  and  223   b.  In order to raise the load, the torque C M  must then be greater than the sum of the torque C L  and of drag torque of the brake spring due to the residual friction between the outside envelope of the spring and the friction surface of the bore  241 . The torque C M  is represented by a vector in dashed lines associated with the inlet part. 
         [0085]    At start-up, the torque C M  to be exerted must be larger because, in order to release the brake  105 , it is necessary to overcome a static friction force. In order to release the brake  105 , the protuberance  211   a  acts on the tab  232   b  received in the recess  222   b  whenever the lug  221   b  is driven in rotation. The drive torque C M  is transmitted from the inlet part  210  to the outlet part  220  by double contact. On one side, the face  213   b  of the protuberance  211   a  bears against the face  223   b  of the lug  221   b.  And, diametrically opposite, the inside face  217   c  of the slot  216   c  bears against the face  227   a  of the projecting portion  226   a.  Thus, the load torque C L  is balanced by efforts F 1a  and F 1b  resulting from the bearing between the portions  211   a  and  221   b,  on one side, and  216   c  and  226   a,  on the other side. Since these two forces are of substantially the same magnitude and are substantially symmetrical about the central axis X 105  of the brake  105  and about the axis X 220  of the outlet part, the radial component of the resultant of the torque C M  on the outlet part is negligible, or indeed zero. The faces  223   b  and  227   a  constitute contact surfaces of the outlet part. 
         [0086]    While the load L is being lowered, as shown diagrammatically in  FIG. 9 , the outlet part  220  is not stopped by the inlet part  210  but rather it is stopped by the spring  230 . Thus, the load torque C L  presses the lug  221   b  against one of the tabs  232   a  or  232   b,  namely the tab  232   b  in this example. The effect of this is to cause the turns of the spring  230  to expand radially and to activate the brake  105 , as explained above. 
         [0087]    The torque C L  exerted by the lug  221   b  on the surface  234   b  of the tab  232   b  is weighted by the efficiency of the second gearbox stage  106 . The tab  232   b  is engaged in the recess  222   b.  The drive torque C M  is in the same direction as the load torque C L . The balance of the forces is then different from the balance during raising. The load torque C L  is balanced by forces F 2a  and F 2b . The first force F 2a  corresponds to the reaction of the spring that blocks the load at the interface between the face  234   b  of the tab  232   b  of the spring  230  and the bearing face  224   b  of the recess  222   b  of the lug  221   b  of the outlet part. Since the first force F 2a  does not make it possible to compensate for the load torque C L , the outlet part  220  tends to pivot relative to the preceding bearing configuration until the outlet part is in contact with its guide means formed by the shaft  270  that is secured to or integral with the inlet part  210 . The bore  228  for guiding the outlet part  220  relative to the shaft  270  thus comes into contact with the shaft  270 , thereby generating the second force F 2b  making it possible to balance the load torque C L . This force is radial relative to the axis X 220 . This force F 2b  generates friction while the load L is moving downwards. This friction brakes the load and is added to the braking torque of the spring. It therefore contributes to the reactivity of the brake. Its response time is faster than the response time of a brake for which such friction does not exist. 
         [0088]    It should be noted that, for this embodiment, the inlet part  210  is itself centered relative to the friction part  240  by means of a cylindrical web whose envelope surface (not shown) co-operates with the bore  241  of the friction part. Therefore, the preceding force F 2b  then induces an equivalent force (not shown) between the inlet part  210  and the friction part  240 . This equivalent force participates in the secondary braking torque contributing to the reactivity of the brake. 
         [0089]    In order to enable the load to be lowered, it is necessary to release the brake. For this purpose, the drive torque C M  drives a protuberance  211   a  on the inlet part in rotation until it comes to bear against the face  233   a  of the tab  232   a  of the spring  230 . By this action, the spring  230  is relaxed and the outlet part  220  can turn, by means of the load torque C L , since the parts  210  and  220  are then not in direct contact. 
         [0090]    Operation of the brake in the second winding configuration is shown in  FIGS. 10 and 11 . 
         [0091]    During raising, and as shown in  FIG. 10 , the load torque C L  is balanced by the forces F 1a  and F 1b  resulting firstly from the contact between the face  213   a  of the tooth  211   a  and the face  223   a  of the lug  221   a,  and secondly from the contact between the inside face  217   d  of the slot  216   d,  and the face  227   b  of the projecting portion  226   b.  Since these forces F 1a  and F 2a  are balanced, the radial component of the resultant of the torque C M  on the outlet part  220  is negligible. The motor must thus deliver drive torque that is greater than the load torque C L  to which only the drag torque of the brake is added, which drag torque results from the friction between the spring  230  and the friction part  240 . There is little or no secondary braking torque generated by the friction between the outlet part  220  and its guide shaft  270 . The faces  223   a  and  227   b  constitute the contact surfaces of the outlet part. 
         [0092]    During lowering, the load torque C L  is balanced by the forces F 2a  and F 2b . The first force F 2a  corresponds to the reaction of the spring  230  blocking the load L at the interface between the face  234   a  of the tab  232   a  of the spring  230  and the bearing face  224   a  of the recess  222   a  in the lug  221   a.  The second force F 2b  corresponds to a localized force at the guide shaft  270  of the outlet part  220 , while the parts  210  and  220  are not in direct contact. This friction generates a radial force braking the load. Thus, the brake reacts rapidly because the secondary braking torque no longer becomes negligible. 
         [0093]    The two embodiments describe a brake spring whose ends are folded over towards the inside of the spring. Naturally, said ends can be folded over towards the outside of said spring. Another variant consists in folding over the ends parallel to the central axis of the spring. The tabs then extend axially on either side of the spring, while extending away from the center of the spring. 
         [0094]    In addition, the spring brake does not specifically have to be received between two gearbox stages. It can be disposed at the outlet of the motor or at the outlet of the gearbox.