Patent Publication Number: US-8995238-B2

Title: Device for displaying time information

Description:
The invention concerns a device for use in a timepiece to display time information with the aid of an indicator member. 
     In some display devices, known as “retrograde” devices, the display is not circular. Instead of effecting a complete revolution, the indicator member follows a path from a departure point A to an arrival point B in front of graduations representing a time related quantity. Once it has followed the path from A to B, the indicator member returns to the point A backwards and instantaneously. This type of display may be used for any type of time related quantity, for example seconds, minutes, hours, dates, days of the week or phases of the moon. 
     A retrograde display device generally comprises a spiral or snail-shaped cam the profile of which features a steep flank. A lever is pressed against the cam by a follower. A return spring presses the lever against the cam. The lever includes a toothed portion or rack designed to mesh with and to drive a pinion driving the indicator member. 
     In normal operation (i.e. in the time related quantity display phase), rotation of the cam in a first direction corresponding to the clockwise direction causes the indicator member to be driven from its departure position A, in which the follower is in a low position at the base of the flank of the cam, to its arrival position B, in which the follower is in a high position at the top of the flank of the cam. The cam continues to rotate when the follower reaches the high position. The follower then goes from its high position to its low position instantaneously, causing the indicator member to return to the departure point A. 
     The presence of this steep flank poses a problem when it is required to correct the time information “in reverse” (i.e. in the counter-clockwise direction). In fact, when the follower reaches the base of the steep flank, this causes jamming or even breaking of the components. It is therefore necessary to prohibit this type of correction. 
     The document EP1918792 describes a timepiece provided with a conventional retrograde display device including a snail-shaped cam and a retrograde display lever in contact with the cam profile via a follower. The display lever drives a hand indicating a time related quantity. A disconnecting device is added to the display device. In the correction phase, notably the reverse correction phase, the disconnecting device enables the follower of the retrograde display lever to be moved away from the path of the cam. Accordingly, in the reverse correction phase, the indicator hand is in a predetermined position and does not indicate a value of the time quantity. The disconnecting device therefore makes it possible to carry out a bidirectional correction of the display of the time related quantity at any time. However, during this adjustment, the correction effected is not visible to the user, which introduces the risk of inducing errors in the adjustment of the timepiece. Moreover, the disconnecting device requires a large number of components to be added to the conventional mechanism. 
     The object of the invention is to provide a device eliminating the aforementioned drawbacks and improving the known prior art devices. In particular, the invention proposes a simple, reliable and robust device enabling reverse correction of a retrograde system. 
     To this end, the invention concerns a display device as claimed in claim  1 . 
     Thus the invention originates in a problem affecting retrograde display devices. However, the applicant does not intend to limit the scope of the application to this particular application example but to extend it to any type of display device including a cam with at least one flank and a control follower designed to ascend that flank. 
     When the follower ascends the cam flank, the flank exerts an action force on the follower. The effect of this action force on the follower is to move it relative to the lever, against the return action of the second spring means, and thereby to prevent damage to the mechanism. In other words, the invention proposes to interpose second spring means between the follower and the lever to enable movement of the follower relative to the lever when ascending the flank. These second spring means are of course separate from the first spring means adapted to urge the lever against the cam via the follower. On movement of the follower relative to the lever, the follower moves toward the lever and is therefore retracted, against the return action of the second spring means. 
     The action force exerted by the flank of the cam on the follower when the flank moves in the second direction of movement of the cam advantageously acts to bring about immobilization of the lever. 
     Accordingly, when the follower ascends the flank, the action force exerted on the follower by the cam flank produces a second effect: this force acts on the lever to bring about immobilization thereof. 
     The action force advantageously acts on the lever to cause pivoting thereof against a stop. 
     Accordingly, the action force of the flank on the follower causes a pivoting effect of the lever. This pivoting is effected against a stop, which acts to cause a stable immobilization of the lever. 
     In one particular embodiment, the lever being adapted to pivot about a pivot point, the action force passes substantially through said pivot point of the lever. In this case, the drive lever including an arm for driving the indicator member, an action angle defined by the action force of the cam on the follower and the line segment connecting the point of contact of the follower and the cam flank and the pivot point of the lever is advantageously located in the half-plane defined by the line segment containing said segment and not containing the arm driving the lever. 
     The line segment containing the point of contact between the follower and the cam and the pivot point of the lever defines two half-planes, one containing the arm driving the lever, the other not containing said arm. Positioning the action angle in the half-plane that does not contain the lever arm has the effect of producing a force tending to cause the lever arm to pivot in the appropriate direction to immobilize it against the stop. 
     In one particular embodiment, said action angle is between 0 and 180°. 
     The stop therefore has a two-fold function:
         on the one hand, it immobilizes the lever at the base of the cam flank at the end of the follower descending the flank during movement of the cam in the first direction;   on the other hand, it immobilizes the lever when the follower ascends the flank during movement of the cam in the second direction.       

     The invention therefore cleverly employs the stop for immobilizing the lever at the end of the descent of the flank also to immobilize the lever during the ascent of the flank. 
     In one particular embodiment, said stop is adapted to immobilize the lever when the follower reaches the base of the flank during movement of the cam in the first direction. 
     The head of the follower is therefore conformed to bring about continuous contact between the follower and the cam flank throughout the movement of the follower along the cam flank during the ascent of the cam flank. 
     The geometry of the follower is advantageously adapted so that there is no contact between the tip of the follower and the flank when the follower moves from the top of the cam flank to the base of the cam flank. 
     In another embodiment, the follower comprises a head flank conformed to bring about continuous contact between the flank of the follower and the top end of the cam flank (i.e. the end located at the level of the cam portion with the largest radius) while the follower is ascending the cam flank. 
     The cam having a snail-shaped profile, the follower advantageously comprises a head provided with a contact tip and a contact flank and is adapted to be in contact:
         with the flank of the cam via said tip of the follower head, and   with at least a portion of the snail-shaped profile of the cam via the flank of the follower head.       

     The follower is advantageously pivotably mounted on the lever. 
     Thanks to this, the follower can pivot relative to the lever without butting. 
     An angle defined by the normal to the cam at the point of contact between the follower and the cam flank and the half-segment originating at the location of said contact and passing through the pivot point of the follower is advantageously greater than the angle defined by the normal at the location of the contact between the follower and the cam flank and the reaction force exerted by the follower on the flank. During movement of the cam in the first direction, which corresponds to normal operation of the display device, the second spring means are slightly deformed because of the action of the cam on the follower. This deformation causes an offset in the display of the time information compared to a display provided by an assembly including a follower rigidly mounted on a lever. This offset is compensated by adapting the cam profile. The modified, adapted cam profile enables a perfectly rigid assembly to be simulated and produces a perfectly accurate display. 
     In one particular embodiment, movement of the cam in the first direction prestresses the second spring means, which prestressing is compensated by a cam profile conformed to ensure accurate display of the time information. 
     In a first embodiment, the second spring means comprise a flexible blade or leaf spring. This minimizes the number of components and reduces the overall size of the mechanism. Furthermore, this makes it possible to prevent balancing defects liable to occur within an assembled lever. 
     The lever, the second spring means and the follower are advantageously made in one piece. The lever is advantageously fastened to a toothed portion adapted to cooperate with the teeth of a pinion for driving the indicator member. 
     The toothed portion may comprise play-compensating teeth. 
     The play-compensating teeth make it possible to minimize display errors. They further make it possible to limit shocks and to attenuate the effects of inertia when the time indicator member returns suddenly to the initial position. 
     The lever may be made from one of the materials from the group comprising Ni, NiP and Si. 
     The invention also concerns a clock movement including a display device as defined above. 
     The invention further concerns a timepiece incorporating the display device defined above or the clock movement defined above. 
     The timepiece may comprise a basic movement and a display device as defined above, the cam of the display device being driven by the basic movement. 
    
    
     
       The invention will be better understood with the aid of the following description of a number of particular embodiments of the display device of the invention, given with reference to the appended drawings, in which: 
         FIG. 1  shows the display device of a first embodiment of the invention for a retrograde display of the hour between 0 h and 24 h (midnight); 
         FIG. 2A  shows the display device from  FIG. 1  at 22 h; 
         FIG. 2B  shows a detail view of a follower and cam assembly from  FIG. 2A ; 
         FIG. 3A  shows the display device from  FIG. 1  just before midnight; 
         FIG. 3B  shows a detail view of the follower and cam assembly from  FIG. 3A ; 
         FIG. 4A  shows the display device from  FIG. 1  at midnight; 
         FIG. 4B  shows a detail view of the follower and cam assembly from  FIG. 4A ; 
         FIG. 5  shows the display device from  FIG. 1  when the follower is ascending a cam flank; 
         FIGS. 6A and 6B  show the display device from  FIG. 1  just after the ascent of the cam flank; 
         FIG. 7A  shows the display device from  FIG. 1  when the follower is ascending a cam flank, showing the reaction force of the follower on the cam; 
         FIG. 7B  shows the display device from  FIG. 1  at the end of the follower ascending the cam flank when the latter reaches the apex of the cam, showing the reaction force of the follower on the cam; 
         FIG. 7C  shows the display device from  FIG. 1  when the follower is ascending the cam flank, showing the action force of the cam on the follower; 
         FIGS. 8A and 8B  show the functional areas of the follower in contact with the cam during conventional operation of the display device from  FIG. 1 ; 
         FIG. 9  shows an adapted cam profile of the device from  FIG. 1  and an initial cam profile; 
         FIG. 10  shows part of the display device of a second embodiment of the invention; 
         FIG. 11  shows part of the display device of a third embodiment of the invention. 
     
    
    
     It will be noted that, for clarity, corresponding elements shown in different figures bear the same references, unless otherwise indicated. 
     A display device in accordance with a first embodiment of the invention is shown in  FIG. 1 , which shows a timepiece  300  of one embodiment of the invention. The timepiece includes a clock movement  200 . The clock movement includes the display device  100 . This display device is designed to be integrated into a clock movement of a timepiece. 
     The  FIG. 1  display device  100  is designed to drive an indicator member  1 , here an indicator hand, indicating a time related quantity (i.e. time information). The hand  1  is seen notably in  FIGS. 2A ,  3 A and  4 A. In the particular example shown in these figures, the time related quantity is the hour represented on a graduated scale of 24 hours covering an angular arc of 180°. In the time display phase, the forward path of the hand  1  goes from a “0 h” position to a “24 h” position. The hand  1  is designed to return instantaneously from the “24 h” position to the “0 h” position at midnight. 
     The invention could of course be applied to any type of time related quantity (seconds, minutes, dates, days of the week, phases of the moon, etc.), to a retrograde display represented on any angular arc, and to a non-retrograde display, as explained in more detail at the end of the description. 
     Referring to  FIG. 1 , the display device includes a cam  2 , a follower  3  and a lever  4 . 
     The cam  2  is snail-shaped with a spiral profile and is mounted on a shaft  20  to rotate about a rotation point  21 . The spiral profile of the cam  2  is referenced  24 . The cam  2  is kinematically coupled to a basic movement of the timepiece by way of a wheel  22  meshing with a drive wheel  8 , for example driving a circular hour indicator hand  80 , as shown in  FIGS. 2A ,  3 A and  4 A. In a plane perpendicular to its rotation axis  21 , the profile  24  of the cam  2  corresponds at least substantially to part of an Archimedes spiral. The cam  2  has a flank  23  connecting the cam portion with the smallest radius and the cam portion with the largest radius. In normal operation (i.e. during the time display phase), the function of the flank  23  is to return the indicator hand  1  to the initial “0 h” position when the “24 h” final position is reached. 
     The lever  4  includes an arm  40 , a toothed portion  41  and a counterweight portion  42 . The lever  4  is pivotably mounted on a shaft  43 , pivoting about a pivot point  44 . The pivot shaft  43  of the lever  4  and the rotation shaft  20  of the cam  2  are fixed relative to each other. 
     The toothed portion  41  is joined to the end of the arm  40  opposite the shaft  43 . It meshes with the teeth of a pinion  10  driving the hand  1 . In other words, the lever  4  carries the toothed portion  41  designed to cooperate with the pinion driving the indicator member. In the particular example described here, the toothed portion and/or the pinion  10  may have play-compensating teeth  45  so as to minimize display errors in the indication provided by the display device. One example of play-compensating teeth usable in the invention is notably described in the document EP2112567.  FIGS. 10 and 11  show embodiments of play-compensating teeth  45 . The teeth are adapted so that they mesh in the “0 h” and “24 h” positions. 
     The counterweight portion  42  is of semicircular general shape and is located on the opposite side of the pivot shaft  43  to the arm  40 . The counterweight  42  is adapted so that the center of mass of the lever  4  and its pivot point  44  at least substantially coincide. 
     The follower  3  includes an arm  30  and a head  31 . 
     The arm  30  extends along a longitudinal axis AX 1  shown in  FIG. 1  in the vicinity of and generally along the arm  40 . It has two ends  33 ,  34 , respectively near the counterweight portion  42  of the lever  4  and near the toothed portion  41 . The end  34  and the end  33  are fastened to the arm  40  and a flexible blade (or leaf spring)  5 , respectively. The arm  30  pivots on the lever  4  at a pivot point  32  situated in the vicinity of the connection between the arm  30  and the lever  4 . The angle defined by the longitudinal axis AX 1  of the arm  30  and the line segment connecting the pivot point  32  of the follower  3  and the pivot point  44  of the lever  4 , shown in  FIG. 1 , is denoted y. 
     The flexible blade  5  is curved and, with the arm  30 , defines an S-shape. The end of the spring  5  opposite that fastened to the arm  30  is fastened to the counterweight portion  42 . The flexible blade acts as a return spring  5  disposed between the follower  5  and the lever  4 , against the action of which the follower  3  is pivoted by the cam  2 , as explained in the description of operation. 
     The head  31  has a triangular profile (as seen perpendicularly to the rotation shafts  20  and  43 ) forming a contact tip  312 . It includes a first flank  310  facing toward the end  33  and a second flank  311  facing toward the end  34 . The flank  310  and the axis A 1  define an angle θ 1  and the flank  311  and the axis AX 1  define an angle θ 2 , as shown in  FIG. 1 . Here the angle θ 1  is an obtuse angle and the angle θ 2  is preferably a right angle or an acute angle. In the particular example shown in  FIG. 1 :
         the angle θ 2  is of the order of 90°;   the angle θ 1  is of the order of 130°.       

     It is emphasized here that the shape of the follower head shown in  FIG. 1  in particular is merely one non-limiting illustrative embodiment. In any event, the follower head has a shape adapted to guarantee correct operation. 
     The follower head  31  is adapted to be in contact with the cam  2  either via the flank  310  or via the tip  312 , as shown in  FIGS. 8A and 8B . The contact flank  310  is designed to come into contact with a cam portion near the cam apex (i.e. the top of the flank  3 , having the maximum cam radius), after ascent or before descent of the cam flank  23  by the follower  3 , as explained later. The contact tip  312  is designed to be in contact with the rest of the cam profile  24 , notably with the flank  23 . 
     In the particular embodiment shown in  FIGS. 8A and 8B , the tangent T 1  to the cam flank  23  passing through the top end of this flank  23  (i.e. the end of the flank located near the cam portion with the largest radius) and the tangent T 2  to the cam profile  24  at the bottom end point of this profile  24  (i.e. the point of the cam profile  24  with the smallest radius at the base of the cam flank  23 ) form a space delimited by an angle δ within which the follower head  31  can move. The angle δ is preferably an obtuse angle. In the particular example shown in  FIGS. 8A and 8B , this angle  6  is at least of the order of 100°. 
     In the particular embodiment shown in  FIG. 1 , the lever  4  and the follower  3  are made in one piece. They may be made by one of the following techniques:
         by photolithography and electroforming of one of the materials from the group comprising Ni (nickel), NiP (nickel phosphorus) and Si (silicon);   by photolithography and deep etching of silicon (Si).       

     A return spring  6  separate from the spring  5  disposed between the lever  4  and the follower  3  acts on the lever  4 , here at the level of the toothed portion  41 , to press the follower  3  against the cam  2 . The spring  6  is adapted to load the lever  4  so that the lever  4  is pressed against the cam  2  by the follower  3  so that the follower  3  is pressed against the cam  2 . 
     The display device also includes a stop  7 , diagrammatically represented in the figures, having a two-fold function:
         it is adapted to immobilize the lever  4  when the follower  3  is at the base of the flank  23  when the cam  2  is driven in a first direction corresponding to the clockwise direction;   it is adapted to immobilize the lever  4  when the follower  3  ascends the flank  23  when the cam  2  is driven in rotation in a second direction corresponding to the counter-clockwise direction and causes the lever  4  to pivot against the stop  7 .       

     The operation of the display device in normal operation, on the one hand, and in the reverse or “counter-clockwise” correction phase, on the other hand, is described next with reference to  FIGS. 2A to 7C . 
     Normal Operation: 
     Normal operation corresponds to the conventional display in the clockwise direction of time information, here the hour on a 24 h graduated scale covering an angular arc of 180°. It is described with reference to  FIGS. 2A-2B ,  3 A- 3 B,  4 A- 4 B. 
     It will be remembered that this is a particular illustrative example and that the invention applies to the display of any other time related quantity (day-night indication, minute, second, date, day of the week, etc.). 
     In normal operation, the cam  2  is driven in rotation in a first direction called the “forward” or “normal operation” direction, here the counter-clockwise direction. The follower head  31 , pressed against the cam  2  by the return action of the spring  6 , drives pivoting of the lever  4  in the clockwise direction, which here in turn drives movement of the hand  1  in the counter-clockwise direction. During rotation of the cam  2  in the counter-clockwise direction, the follower head  31 :
         ascends the spiral cam profile  24  from its low position at the base of the flank  23  to its high position at the top of the flank  23  (i.e. at the apex of the cam  2 );   then goes from the apex of the cam  2  at the top of the cam flank  23  to the base of the cam flank  23 .       

     The shape of the follower head  31  is adapted to prevent contact between the tip  312  of the follower head  31  and the cam flank  23  when the follower head  31  moves from the apex of the cam  2  to the base of the cam flank  23 . 
     The follower head  31  ascending the cam profile  24  causes the hand  1  to move from the “0 h” position to the “24 h” position by rotating through 180° in the counter-clockwise direction. The movement from the apex of the cam  2  at the top of the cam flank  23  to the base of the cam flank  23  causes the hand  1  to return instantaneously to the “0 h” position. 
       FIGS. 2A ,  3 A and  4 A show the display device in normal operation at respective different times, namely 22 h, just before midnight and just after midnight. A circular dial of the basic movement of the timepiece with hour and minute hands  80 ,  81  is also shown. 
       FIG. 2A  shows the display device at 22 h. The lever  4  has pivoted to position the hand  1  in front of the “22 h” indication on the graduated scale.  FIG. 2   b  shows to a larger scale the cam  2  and the follower head  31  in this “22 h” position. Note that the flank  310  of the head  31  is in contact with the upper portion of the cam  2 . 
       FIG. 3A  shows the display device at midnight (24 h) when the follower head  31  reaches the high position on the cam  2  at the top of the flank  23  just before the head  31  of the follower  3  goes from the top of the cam flank  23  to the base of the cam flank  23 .  FIG. 3B  shows to a larger scale the cam  2  and the follower head in the “midnight” position. In this position, the hand  1  indicates “24 h”. The contact tip  312  of the head  31  of the follower  3  bears against the apex of the cam  2 . 
       FIG. 4A  shows the display device just after midnight, the hand  1  indicating “0 h”. This position is reached just after the follower head  31  moves from the top of the cam flank  23  to the base of the cam flank  23 .  FIG. 4B  shows to a larger scale the cam  2  and the follower head  31  in the “0 h” position. As it descends the cam flank  23 , driven by the follower  3 , the lever  4  pivots in the counter-clockwise direction until it comes to bear against and is immobilized by the stop  7 . The pivoting of the lever  4  causes the hand  1  to return instantaneously from the “24 h” position to the “0 h” position by rotating backwards in the clockwise direction. 
     The return spring  5  is sized so as to be as stiff as possible upon rotation of the cam  2  in the forward direction (the counter-clockwise direction in the particular example described). However, this stiffness is limited by stresses within the material and by the permissible torque on the cam  2  during reverse correction of the hour. In fact, the stiffness of the spring  5  must not be too high, because this would require high forces to enable movement of the flank  23 . As a result of this, when, in normal operation, the cam  2  rotates in the forward direction, the return spring  5  is prestressed and is deformed slightly by the action of the cam  2  on the follower  3 . This prestressing and this deformation depend on the action exerted by the spring  6  urging the lever toward the stop  7 . Moreover, this action itself depends on the position of the lever relative to the stop. This deformation induces a temporal offset in the display by the hand  1  relative to the display by a hand connected to a mechanism including a follower mounted rigidly on a lever. This temporal offset is quantifiable and corresponds to a given duration, for example 1 h. It is compensated by adapting the cam profile  24 . In other words, the cam profile  24  is adapted, i.e. modified relative to a so-called “initial” cam profile  25  used with an analogous but totally rigid mechanism (follower, spring and lever).  FIG. 9  shows the initial cam profile  25  and the cam profile  24  of the display device from  FIG. 1 . The two cam profiles  24 ,  25  substantially correspond to an Archimedes spiral. 
     Operation in Reverse Correction Phase: 
     A reverse correction consists in correcting the display in the counter-clockwise direction. Operation in the reverse correction phase is described with reference to  FIGS. 5 to 7C . 
     For reverse correction of the display, the cam  2  is driven in rotation in a second or counter-clockwise direction called the “reverse direction”. 
     Driven by the follower  3  bearing against the cam  2 , the lever  4  drives the hand  1  in rotation in the clockwise direction. 
     As the follower  3  moves along the cam profile  24  from the apex of the cam  2  to the base of the cam flank  23 , the hand  1  moves from the “24 h” position to the “0 h” position. When the hand  1  is in the “0 h” position, the cam follower  31  is positioned at the base of the cam flank  23  and the lever  4  bears against the stop  7 . 
     Once the position at the base of the cam flank  23  has been reached, the follower head  31  ascends the cam flank  23  again, the tip  312  of the head  31  being in contact with the flank  23 .  FIG. 5  shows the display device during the ascent of the flank  23 . During the ascent the cam flank  23 , the latter exerts an action force {right arrow over (F)} on the follower head  31 , as shown in  FIG. 7C . The follower head  31  exerts a reaction force {right arrow over (F′)} on the cam flank  23 . This action force {right arrow over (F)} or the reaction force {right arrow over (F′)} has two effects:
         on the one hand, it acts to cause the follower  3  to pivot about the point  32  relative to the lever  40 , which brings about retraction of the follower  3 , i.e. movement of the follower  3  toward the lever arm  40 , against the return action of the spring  5 , which is compressed;   on the other hand, it acts to drive pivoting of the lever  4  against the stop  7 , which brings about immobilization of the lever  4 .       

     Thus the action force {right arrow over (F)} moves the follower  3  towards the arm  40  of the drive lever  4  and therefore retracts the follower  3  against the return action of the spring disposed between the follower  3  and the lever  4 . Retraction of the follower  3 , here by pivoting about the pivot point  32 , enables ascent of the flank  23  without damaging the mechanism. 
     Immobilizing the lever  4  during the ascent of the flank  23  enables immobilization of the hand  1  and prevents an erroneous indication by the hand  1  during the ascent of the flank  23 . 
     Once the top of the flank  23  is reached, the hand  1  is instantaneously in a display position synchronized with the position of the main hour and minute hands  30  and  40 , as shown in  FIGS. 6A and 6B . 
     In normal operation, the stop  7  immobilizes the lever  4  at the end of the descent of the flank  23  and therefore immobilizes the hand in the “0 h” position just after midnight, following its “retrograde” return from the “24 h” position. Furthermore, during the ascent of the flank  23  in the reverse correction phase, the stop  7  also immobilizes the lever  4  in a stable manner and therefore immobilizes the hand  1  in the “0 h” position. Thanks to this, the hand  1  is perfectly stable during the ascent of the flank  23 . 
       FIGS. 7A to 7C  show the action force vector {right arrow over (F)} of the cam flank  23  on the follower head  31  and the reaction force vector {right arrow over (F′)} of the follower tip  312  on the flank  23  when the follower head  31  ascends the flank  23 . The point C represents the point of contact between the follower head  31  and the flank  23 . The vector {right arrow over (n)} represents the normal to the flank  23  at the contact point C. 
       FIGS. 7A and 7B  show, respectively during and at the end of the ascent of the flank  23  (when the follower tip  312  reaches the top of the cam  2 ):
         the reaction force {right arrow over (F′)},   the vector {right arrow over (n)},   the line segment [C32] connecting the contact point       

     C and the pivot point  32  of the follower  3 ,
         the angle α defined by the normal {right arrow over (n)} and the line segment [C32] joining the contact point C and the pivot point  32 ,   the angle Atan(μ) defined by the normal {right arrow over (n)} and the reaction force {right arrow over (F′)}, corresponding to the arc-tangent of the coefficient of friction μ between the tip  312  of the follower head  31  and the cam flank  23 .       

     The angle α varies as the follower tip  312  ascends the flank  23 . The pivot point  32  is defined so that this angle α is greater than the angle Atan(μ) throughout the ascent of the flank  23  by the follower head  31 . The moment at which the angle α is smallest and tending toward the angle Atan(μ) corresponds to the moment at which the tip  312  reaches the apex of the cam  2  (in other words the top of the flank  23 ), as shown in  FIG. 7B . The pivot point  32  is thus determined so that at the moment the tip  312  reaches the apex of the cam  2  the angle α is greater than the angle Atan(μ) and therefore allows pivoting of the follower  3  relative to the lever  4 . Referring to  FIG. 7B , it will be noted that when the follower tip  312  reaches the apex of the cam  2  the return spring  5  is compressed to the maximum. 
       FIG. 7C  shows:
         the contact point C,   the action force {right arrow over (F)},   the line segment [C44] between the contact point C and the pivot point  44  of the lever  4 ,   the line segment [44-32] between the pivot point  44  of the lever  4  and the pivot point  32  of the follower  3 ,   the oriented angle β defined between the action force {right arrow over (F)} and the line segment [C44], where β=({right arrow over (F)},[C44]), or the line containing this line segment.       

     The pivot point  44  of the lever  4  is defined so that the action force {right arrow over (F)} immobilizes the lever  4  or causes pivoting of the lever  4 , here in the counter-clockwise direction, against the stop  7 , the effect of which is to immobilize the lever  4 . To this end, and as shown in  FIG. 7C , the angle β defined by the vectors {right arrow over (F)} and {right arrow over (C44)} is positive and less than the value π:
 
0&lt;β&lt;π
 
     The action force {right arrow over (F)} and the angle β are located in the half-plane defined by the line segment containing the contact point C and the pivot point  44  of the lever  4  and not containing said lever arm  40 . The action force {right arrow over (F)} therefore acts to cause the lever  4  to pivot against the stop  7  to immobilize it effectively while the follower  3  ascends the flank  23 . The direction of the action force {right arrow over (F)} and the side of the pivot point  44  of the lever  4  on which it is applied therefore causes the lever  4  to pivot against the stop  7 , thereby immobilizing the lever  4 . 
     In other words, the oriented angle β may be defined as being an action angle defined by the action force {right arrow over (F)} of the cam on the follower and the line segment [C44] connecting the follower-cam contact point C and the pivot point  44  of the lever  4 . This action angle β is located in the half-plane defined by the line segment containing this segment [C44] and not containing said lever arm  40 . It is preferably between 0° and 180°. 
     In another embodiment, the action force {right arrow over (F)} of the cam  2  on the follower  3  passes substantially through the pivot point  44  of the lever  4 ; in other words the action angle β has the value 0. In this configuration, the action force {right arrow over (F)} acts to immobilize the lever  4  without causing it to pivot against a stop. 
     In the preceding description, the follower  3 , the lever  4  and the spring  5  are in one piece.  FIG. 10  shows another one-piece embodiment which differs from that just described in terms of the shape of the flexible blade  5 , which with the arm of the follower defines a U-shape. 
     Alternatively, the display device could comprise an assembly including a lever  4  on which is pivoted a follower  3 .  FIG. 11  shows by way of illustration one particular example of such an embodiment. The follower  3  is pivotably mounted on the lever  4  by means of a pivot. The return spring  5  disposed between the lever  4  and the follower  3  takes the form of a flexible blade. 
     Here the latter is fastened to the lever  4  at one of its ends and bears against the arm  30  of the follower  3  at its other end. In operation, during the ascent of the cam flank  23 , the head  31  of the follower  3  and the portion of the arm  30  carrying the head  31  move toward the lever arm  40  against the return action of the spring  5 , thereby bringing about retraction of the follower  3  because of the action of the force {right arrow over (F)} exerted by the cam flank  23 . 
     In the preceding description, the cam flank  23  is substantially straight. Alternatively, it could have a more or less marked curvature notably adapted to guarantee that the action force {right arrow over (F)} always passes through the pivot point  44  of the lever  4 , or substantially through this pivot point  44 , to immobilize the lever  4  (with no pivoting against a stop). 
     The device, various embodiments of which have just been described, may be employed to indicate in a retrograde manner any time information or information derived from the time (indication of the time in a different time zone, calendar indications such as the date, the day of the week, the month, the phase of the moon, etc.). However, the invention is not limited to a retrograde application but concerns any use involving a lever cooperating with a cam (snail-shaped or otherwise) the profile of which includes at least one flank. The invention could for example be applied to a cam-type perpetual date indicator, such as that described in the document EP0191921 for example, or an instantaneous counter chronograph (i.e. a chronograph in which counting of time indications is instantaneous, for example as described in the document CH702137). Moreover, the invention allows bidirectional driving of the display of time information, whether the latter be indicated in a retrograde manner or not. 
     The display device of the invention is integrated into a clock movement itself integrated into a timepiece including a basic movement to which the movement of the invention is kinematically coupled. The invention therefore also concerns the clock movement integrating the display device that has just been described and the timepiece.