Abstract:
The belt tensioner roller device is provided with a fixed support able to hold the device on an engine block, with moving parts that can move with respect to the fixed support, with a rotary roller supported by the moving parts and intended to be in contact with the belt, and with a component able to exert a permanent force between the fixed support and the moving parts. The rotary roller comprises a target element, and the fixed support comprises at least one sensor element mounted facing the target element of the roller with a gap and collaborating with the said target element to detect belt wear as a function of displacement of the rotary roller with respect to the fixed support.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the field of automatic belt tensioner roller devices, particularly those used to ensure adequate belt tension, for example for motor vehicle internal combustion engine timing belts.  
         [0003]     2. Description of the Relevant Art  
         [0004]     A belt tensioner roller device generally comprises a support intended to be fixed to the engine block and a moving part able to move angularly relative to the support. The said moving part is provided with an eccentric and with a rolling bearing on which there is mounted a roller intended to be brought into contact with the belt. A spring exerts a permanent tensile force between the support and the moving part, causing the roller to be brought into contact with the belt, with suitable belt tension. The moving part is mounted on the support with a possibility of adjustment so that the belt tension force can be adjusted. For fuller details, reference may be made to documents FR-A-2 744 506 and FR-A-2 668 567.  
         [0005]     The tensioner roller device thus allows the belt to be kept under the required tension within a certain tolerance range in spite of the dimensional variations in the belt that may be caused by wear or by temperature variations.  
         [0006]     Belt failure, particularly the failure of a motor vehicle combustion engine-timing belt, can cause serious damage to the said engine, and thus adversely affect its dependability.  
         [0007]     To this end, document U.S. Pat. No. 6,666,784 discloses a belt tensioner roller device provided with a fixed support mounted on an engine block, an eccentric that can move angularly relative to the said support, and a damping element positioned at the fixed support. Mounted inside the damping element is a belt wear detection device equipped with at least one magnet fixed to a moving element of the damping element and with a sensor able to detect the linear position of the moving element, the said linear position being associated with the angular position of the eccentric relative to the fixed support.  
         [0008]     The device has the disadvantage of having a relatively high number of intermediate components involved in the drive line between the moving element and the belt, and this can give rise to a certain degree of inaccuracy in the measurements made.  
         [0009]     The Applicant Company has also developed a belt tensioner roller device, filed as French Patent Application No. 01 14 669. The tensioner roller device is provided with a fixed support, with a working eccentric able to move angularly relative to the fixed support, with an encoder element mounted on the eccentric and having a multipolar encoding ring, with a sensor element mounted on the fixed support and able to collaborate with the said encoder element to measure the angular displacement of the working eccentric and with a signal processing module for processing the signals emitted by the sensor element and able to determine information regarding belt wear.  
         [0010]     Such a tensioning roller device, although being particularly precise and effective in detecting angular oscillations and displacements of the eccentric relative to the fixed support, has the disadvantage of having detection elements that are too expensive for simply measuring belt wear thresholds, and which may be incompatible with mass production of tensioning roller devices.  
         [0011]     Furthermore, this device cannot be fitted to a tensioning roller the displacement of which is in a straight line and does not employ an eccentric.  
       SUMMARY OF THE INVENTION  
       [0012]     In an embodiment, these disadvantages may be remedied by a belt tensioner roller device able to detect the tension in a belt in order therefrom to deduce a predetermined wear limit in a way that is particularly simple, precise, reliable and economical.  
         [0013]     To this end, the belt tensioner roller device is provided with a fixed support able to hold the device on an engine block, with moving parts that can move with respect to the fixed support, with a roller supported by the moving parts and intended to be in contact with the belt, and with a component able to exert a permanent force between the fixed support and the moving parts.  
         [0014]     According to embodiment, the rotary roller includes a target element and the fixed support includes at least one sensor element mounted facing the target element of the roller with a gap and collaborating magnetically with the said target element to detect belt wear as a function of displacement of the rotary roller with respect to the fixed support.  
         [0015]     Such a device has the advantage of allowing belt wear to be detected directly from a displacement of the rotating part of the tensioner roller device which is in contact with the belt and maintains the tension therein.  
         [0016]     In other words, the arrangement of a target element directly on the rotary roller allows particularly precise and reliable measurements to be taken, belt wear being associated directly with the belt-induced displacement of the rotary roller relative to the fixed support.  
         [0017]     Furthermore, arranging the sensor element at the fixed support not only makes it easier to fit the device but also, in the event of sensor element failure, makes the intervention needed simpler by not requiring any operation in the region of the moving parts.  
         [0018]     As a preference, the fixed support further includes at least one magnetic element able to create a magnetic field near the sensor element, the said magnetic element collaborating with the target element of the roller to modify the value of the magnetic field as a function of the displacement of the roller relative to the fixed support.  
         [0019]     Combining a sensor element with a magnetic element able to create a magnetic field near the said sensor element in order to detect belt wear makes it possible to obtain particularly precise measurements and to do so economically.  
         [0020]     Advantageously, the gap can vary as a function of the displacement of the roller relative to the fixed support. The gap may be radial or axial.  
         [0021]     In one embodiment, the fixed support includes two sensor elements positioned radially on either side of the target element of the roller, the sensor elements being electrically connected in such a way as to take a differential measurement of the signals supplied by the sensor elements.  
         [0022]     An instrumented tensioner roller device such as this makes it possible to achieve particularly reliable detection of the degree of belt wear while at the same time in particular circumventing the effects of possible dimensional variations due to thermal variations, variations in the properties of the target element or variations in the voltage powering the sensor elements.  
         [0023]     Advantageously, the sensor element detects belt wear as a function of a radial linear displacement of the moving parts with respect to the fixed support.  
         [0024]     With preference, the moving parts include a working eccentric that can move angularly relative to the fixed support and supports the roller, the sensor element detecting belt wear as a function of the position of the roller relative to the fixed support.  
         [0025]     The fixed support may include a plate on which the sensor element is mounted.  
         [0026]     As a preference, the target element includes a ferromagnetic material.  
         [0027]     Advantageously, an end portion of the rotary roller forms the target element. An arrangement such as this makes it possible to obtain an instrumented tensioner roller device that is particularly compact in the axial direction. Furthermore, since a portion of the roller forms the target element, there is no need to provide an additional element which would lead to additional cost in order to measure the belt tension.  
         [0028]     The target element may be attached to or integral with the working eccentric.  
         [0029]     In one embodiment, the target element includes a synthetic material filled with magnetized ferrite powder.  
         [0030]     Advantageously, the device includes a sensor unit, mounted on the fixed support, and in which the magnetic element is embedded and the sensor element is at least partially housed.  
         [0031]     Arranging the magnetic element such that it is embedded within the sensor unit makes it possible to avoid any possible ingress of elements from outside, for example metallic ones, near its external surface and that could vary the field emitted. The belt tension detection is thus particularly precise.  
         [0032]     The sensor element may include a linear or digital magnetosensitive cell. The use of such a sensor element makes it possible to obtain a device of a particularly simple and economical design.  
         [0033]     Furthermore, the use of a magnetosensitive cell by comparison with so-called “incremental” sensors allows for highly reliable measurements. This is because incremental sensors have the disadvantage of not being able to detect the true position of one element relative to another fixed element when an associated electronic memory is no longer electrically powered.  
         [0034]     The invention also relates to a method for monitoring belt wear, including the steps during which a permanent tensile force is exerted between a fixed support and moving parts that can move with respect to the fixed support and that are in contact with the belt, characterized in that belt wear is detected directly as a function of displacement of the roller relative to the fixed support. The displacement is detected through collaboration of sensor and target elements mounted facing one another with a gap between them, the said displacement being linear and a function of the position of the roller relative to the fixed support.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]     The present invention will be better understood upon reading the detailed description of some embodiments taken by way of entirely non-limiting examples and illustrated by the attached drawings, in which:  
         [0036]      FIG. 1  is a view in axial section of a belt tensioner roller device according to a first embodiment;  
         [0037]      FIG. 2  is a detailed view of the device of  FIG. 1 ;  
         [0038]      FIG. 3  is a view in axial section of a tensioner roller device according to a second embodiment;  
         [0039]      FIG. 4  is a schematic view of a processing unit for sensor elements of the device of  FIG. 3 ;  
         [0040]      FIG. 5  is a view in axial section of a tensioner roller device according to a third embodiment;  
         [0041]      FIG. 6  is a view in axial section of a tensioner roller device according to a fourth embodiment;  
         [0042]      FIG. 7  is a view in axial section of a tensioner roller device according to a fifth embodiment. 
     
    
       [0043]     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawing and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]     As can be seen in  FIG. 1 , the tensioner roller device, referenced  1  in its entirety, includes fixed parts provided with an adjusting eccentric  2 , and moving parts mounted pivotably on the fixed parts and provided with a working eccentric  3 . A bearing brass forming a plain bearing  4  is mounted rigidly by clamping in the working eccentric  3 . An actuator including a spring  6  is mounted between the said working eccentric  3  and a fixed support  5  including the adjusting eccentric  2 , a roller or pulley  8  being rotationally mounted on the working eccentric via a rolling bearing  7 , the said roller  8  resting against a belt  9 .  
         [0045]     The adjusting eccentric  2 , of cylindrical overall shape, includes a bore  10  of axis  10   a  offset towards the outside relative to the geometric axis of the said eccentric. The bore  10  allows the passage of a fastener such as a screw  11 , depicted in dotted line, for fastening the said adjusting eccentric  2  to an engine block  12 . The adjusting eccentric  2  can thus pivot angularly relative to the engine block  2  when the screw  11  is slackened, tightening the said screw  111  immobilizing the adjusting eccentric  2  on the said block. The adjusting eccentric  2  includes, at one axial end, a radially outwardly directed radial flange  13 . The flange  13  more or less radially extends a radial bearing surface  14  via which the adjusting eccentric  2  bears against the engine block  12  so as to increase the area over which the said adjusting eccentric  2  bears against the engine block  12 .  
         [0046]     The plain bearing  4 , of axis  4   a , is in the form of an annular bushing including an axial portion  15 , tightly push-fitted into a bore  3   a  of the working eccentric  3 , and able to slide over the exterior cylindrical surface  2   a  of the adjusting eccentric  2 . The axial portion  15  is extended, at one axial end, the opposite end to the flange  13 , by an outwardly directed radial portion  16 . The radial portion  16  bears against a radial transverse surface of the working eccentric  3 .  
         [0047]     The fixed support  5  also includes a base plate  17  equipped with an axial guide portion  17   a  of tubular shape, here exhibiting a short axial dimension and defining a bore that comes into contact with the exterior surface  2   a  of the adjusting eccentric  2 . The axial portion  17   a  is extended outwards, from one axial end, the same end as the flange  13 , by a radial portion  17   b  bearing against the said flange  13 . The radial portion  17   b  is axially extended, from a larger-diameter region at the opposite end to the axial portion  17   a , by an axial portion  17   c  which extends radially outwards from its free end in the form of a radial portion  17   d  situated axially substantially level with the flange  13 . The radial portion  17   d  extends radially outwards, one end being situated radially between the adjusting eccentric  2  and the roller  8 , an opposite end being radially offset outwards relative to the said roller  8 . The base plate  17  is immobilized with respect to the engine block  12 , for example using a pin fixed to the engine block  12  and projecting through a space formed in the thickness of the base plate  17 . Advantageously, the base plate  17  is made by cutting, pressing and folding, from a thin sheet steel blank.  
         [0048]     The support  5  also includes an intermediate plate  18  more or less matching the shape of the base plate  17 . The intermediate plate  18  is fitted axially into and centered on the said base plate  17 , on the same side as the adjusting eccentric  2 . The intermediate plate  18 , advantageously made of steel sheet, includes an axial portion  18   a , of tubular shape, defining a bore that comes into contact with the exterior surface of the axial portion  17   a  of the base plate  17 . The axial portion  18   a  is extended outwards, from one axial end, by a radial portion  18   b  which extends from a larger-diameter region, on the opposite side to the axial portion  18   a , in the form of an axial portion  18   c . The axial portion  18   c  is extended from its free end by a radial portion  18   d  in contact with the radial portion  17   d  of the base plate  17 . The axial portions  18   a ,  18   c  and the radial portions  18   b ,  18   d  of the intermediate plate  18  are formed as one piece.  
         [0049]     The working eccentric  3  includes a radial surface  3   b  positioned axially away from the base plate  17 , a cylindrical exterior surface  3   c  extending over most of its length from the transverse radial surface  3   b , a radial portion  3   d  extending outwards from the end of the cylindrical exterior surface  3   c  and an axial portion  3   e  extending from the free end of the radial portion  3   d  axially away from the transverse radial surface  3   b . The working eccentric  3  is locally extended, in the region of the axial portion  3   e , by an axial extension  3   f  extending axially towards the base plate  17 . The axial extension  3   f  here has axial and radial dimensions markedly greater than those of the axial portion  3   e . The axial extension  3   f  may, for example, extend angularly over between 30 and 40°.  
         [0050]     The radial surface  3   b , the radial portion  3   d , the axial portion  3   e  and the axial extension  3   f  of the eccentric  3  are formed as one piece.  
         [0051]     The radial portion  3   d , the axial portion  3   e  and the radial portion  18   d  of the intermediate plate  18  define a housing in which the spring  6  is positioned. One end of the spring  6  is hooked over the base plate  17 , the other end being secured to the working eccentric  3 . Thus, the working eccentric  3  can rotate about the axis  4   a  relative to the adjusting eccentric  2 , while at the same time being subjected to a return torque exerted by the said spring  6 .  
         [0052]     An axial stop in the form of a ring  19  includes an interior surface  19   a  mounted, for example, by crimping on a cylindrical exterior surface  2   a  of the adjusting eccentric  2 , at one axial end the opposite side to the flange  13 , delimited by two radial surfaces  19   b  and  19   c , and a cylindrical exterior surface  19   d . The radial surface  19   b  forms an axial stop surface for the adjusting eccentric  2  resting against the said surface  19   b , via the axial portion  16  of the bearing  4 . The surfaces  19   a  and  2   a  exhibit complementing shapes and are able to provide an angular connection between the ring  19  and the adjusting eccentric  2 . The ring  19  can thus allow the adjusting eccentric  2  to be angularly adjusted before the screw  11  is tightened.  
         [0053]     The deep-groove rolling bearing  7  of axis  7   a , is inexpensive to manufacture and includes an inner ring  20 , an outer ring  21 , between which a row of rolling elements  22 , here produced in the form of balls, are housed together with a cage  23  that maintains the circumferential spacing of the rolling elements  22  and two seals  24  and  25 .  
         [0054]     The inner ring  20  is of the solid type. The expression “ring of the solid type” is to be understood to mean a ring the shape of which is obtained by machining with the removal of chips (turning, grinding) from tubes, bar stock, forged and/or rolled blanks. The inner ring  20  includes a bore  20   a  of cylindrical shape, in contact with the exterior surface  3   c  of the adjusting eccentric  3  and delimited by two opposed radial lateral surfaces  20   b  and  20   c , the radial surface  20   c  being in contact with the radial portion  3   d , and an exterior cylindrical surface  20   d  from which is formed a circular groove  20   e  exhibiting, in cross section, a concave internal profile able to form a raceway for the rolling elements  22 , the said groove facing outwards.  
         [0055]     The outer ring  21 , also of the solid type, includes an exterior cylindrical surface  21   a  delimited by transverse surfaces  21   b  and  21   c  and a bore  21   d  of cylindrical shape from which there is formed a circular groove  21   e  which, in cross section, exhibits a concave internal profile able to form a raceway for the rolling elements  22 , directed inwards. The bore  21   d  here includes two annular grooves (not referenced) which are mutually symmetric relative to a plane passing through the centre of the rolling elements  22 . Mounted inside the said annular grooves are the seals  24  and  25 , which sit close to portions of the cylindrical exterior surface  20   d  of the inner ring  20  forming narrow passages. The seals  24 ,  25  are positioned radially between the inner and outer rings  20 ,  21 . The seal  24  is mounted axially between the rolling elements  22  and the radial surfaces  20   b  and  21   b  of the rings  20  and  21 , the seal  25  being mounted axially between the rolling elements  21  and the radial surfaces  20   c ,  21   c  of the said rings.  
         [0056]     The roller  8  includes a stepped bore  8   a  push-fitted onto the exterior cylindrical surface  21   a  of the outer ring  21  of the rolling bearing  7 . The rolling bearing  7  thus gives the roller  8  the freedom to rotate relative to the working eccentric  3  and reacts the radial forces.  
         [0057]     The device  1  also includes a sensor unit  26  provided with a support  27  fixed at the level of the base plate  17  of the fixed support  5 , axially on the same side as the roller  8 , by fixing studs  27   a ,  27   b . The fixing stud  27   a  is situated radially between the working eccentric  3  and the roller  8 , near the said eccentric  3 . The fixing stud  27   b  is radially offset outwards relative to the roller  8 . The studs  27   a ,  27   b  are each situated near a radial end of the support  27 . The support  27  also includes an axial extension  27   c  extending axially towards the belt  9  so as to be mounted with a small radial gap with respect to the roller  8 . The fixing studs  27   a ,  27   b  and the extension  27   c  are made as one piece with the support  27 , for example made of synthetic material.  
         [0058]     The sensor unit  26  also includes a sensor element  28  and a magnetic element  29  which are respectively mounted semi-embedded and embedded within the extension  27   c  of the support  27 . The sensor element  28 , for example a magnetosensitive cell, lies flush with a small-diameter cylindrical surface of the extension  27   c  of the support  27  so as to be mounted facing an axial end of the roller  8 , on the outside. The sensor element  28  is thus mounted with a small radial gap to the roller  8 , encouraging the compactness of the device  1 .  
         [0059]     The magnetic element  29  may, for example, include a permanent magnet provided with two magnetic poles of opposed polarities, “north” and “south”, running axially inside the support  27  so that one of the poles faces towards the sensor element  28 , the other pole facing outwards. The magnetic element  29  is axially aligned with the sensor element  28  and radially offset outwards relative to the said sensor element while being mounted a short radial distance therefrom so as to further improve the compactness of the device  1 . The magnetic element  29  is able to create a magnetic field which is detected by the sensor element  28 , it being possible for the said field to be modified according to the position of the roller  8  relative to the sensor unit  26 .  
         [0060]     The sensor unit  26  is also provided with a printed circuit board  30  embedded within the support  27  and positioned near the fixing studs  27   a ,  27   b , it being possible for the said board  30  to include a signal processing circuit for processing the signal emitted by the sensor element  28 . The board  30  is connected (not depicted) to a wired connection  31  able to impart information relating to the signals emitted by the sensor element  28  and processed, to an alarm module (not depicted) belonging to the device  1 . The alarm module may, for example, be produced in the form of a light and/or audible indicator mounted on the dashboard of the motor vehicle so as to alert the driver to the fact that a belt wear threshold has been reached and that the said belt needs to be replaced.  
         [0061]     When mounting the belt tensioner roller device  1  a first step is to set the tension in the belt  9 . To do this, the screw  11  is fitted without tightening it so that the adjusting eccentric  2  can turn relative to the engine block  2  and relative to the fixed support  5 . Using a spanner collaborating with the ring  19 , the adjusting eccentric  2  is pivoted relative to the base plate  17 . The roller  8  is thus brought into contact with the belt  9 . The adjusting eccentric  2  continues to be turned and this, through reaction between the belt  9  and the roller, causes the working eccentric  3  to rotate relative to the support  5  with an increase in the tension in the spring  6 , which exerts a resistive torque between the support  5  and the working eccentric  3 , tending permanently to push the roller  8  against the belt  9 . The device is designed so that the roller  8  exerts on the belt  9  a force that generates sufficient tension in the said belt  9 . The support  5  is then immobilized on the engine block  12  using the screw  11 .  
         [0062]     When fitting the device  1 , an axial end portion  8   b  of the roller  8  is mounted more or less facing the sensor element  28 , the gap between the said roller  8  and the sensor element  28  being small. The sensor element  28  thus detects a first magnetic field created by the magnetic element  29  collaborating magnetically with the roller  8 , advantageously made of ferromagnetic material, the axial end portion  8   b  of the roller  8  being subjected to the field lines of the magnetic element  29 .  
         [0063]     As the belt  9  wears, the working eccentric  3 , under the action of the return spring  6 , will move angularly relative to the adjusting eccentric  2  and thus pivot thereon. The roller  8 , which is mounted on the working eccentric via the rolling bearing  7 , will be displaced relative to the fixed support  5  in a path which will tend to modify the gap between the roller  8  and the sensor element  28 . The radial gap between the sensor element  28  and the axial end of the roller  8  will thus increase, as illustrated in  FIG. 2 . Such a variation in the gap thus modifies the magnetic field lines initially established between the magnetic element  29  and the axial end portion  8   b  of the roller  8 , this modification in the field lines being detected via the sensor element  28 . In other words, the sensor element  28  detects a second magnetic field different from the first.  
         [0064]     The end portion  8   b  of the roller  8  thus forms a target element magnetically collaborating with the magnetic element  29  to allow critical wear of the said belt to be detected; all that is required is predefining of a gap corresponding to the critical belt wear threshold beyond which modification in the magnetic field detected by the sensor element  28  will be exploited by the board  30  of the device as being a signal corresponding to a predetermined state of belt  9  wear and the signal to be forwarded to the alarm module of the device in order to alert the driver.  
         [0065]     The variant embodiment illustrated in  FIG. 3  differs in that the support  27  includes a second axial extension  27   d  extending axially towards the rolling bearing  7  being positioned radially on the opposite side to the first axial extension  27   c  when considering the axial end portion  8   b  of the roller  8 .  
         [0066]     Similarly, a second sensor element  32  and a second magnetic element  33  are mounted respectively semi-embedded and embedded within the axial extension  27   d  of the support  27 . The sensor element  32 , for example a magnetosensitive cell, lies flush with a large-diameter cylindrical surface of the axial extension  27   d  so as to be mounted facing the axial end of the roller  8 , on the inside. The magnetic element  33  is of the same kind as the magnetic element  29  described previously and mounted inside the axial extension  27   d  so that one of the poles of the magnetic element  33  faces towards the sensor element  32 , the other pole facing towards the inside of the device  1 .  
         [0067]     The use of two sensor elements  28 ,  32  positioned radially facing the end portion  8   b , one on the outside and one on the inside, in particular makes it possible to get around the emergence of parasitic phenomena and improve the resolution of the measurement obtained, for example by taking a differential measurement from the signals delivered by each of the sensor elements  28 ,  32 .  
         [0068]      FIG. 4  depicts a block diagram of a processing unit  34  dedicated to the processing of the signals from the sensor elements  28 ,  32  making it possible in particular to compensate for various variations in external parameters, such as the temperature, the magnetic properties of the material of the roller on its periphery, or even the voltage with which the said sensor elements are powered.  
         [0069]     The processing unit  34  includes an analogue/digital converter  35  and a microcontroller  36 . Outputs  37 ,  38  from the sensor elements  28 ,  32  are connected in parallel to the analogue/digital converter  35 , converting the analogue signals supplied by the sensor elements  29 ,  33  into digital signals. The said signals thus converted are forwarded to the microcontroller  36  via one output  39 , the microcontroller  36  being positioned downstream of the analogue/digital converter  35 .  
         [0070]     The microcontroller  36  is able to exploit the voltages V 1  and V 2  delivered respectively by the sensor elements  28 ,  32  and to combine the two measurements taken in order to circumvent parasitic phenomena and improve the resolution of the overall measurement forwarded by an output  40  connected to the microcontroller  36 . It is possible, for example, to subtract the voltages delivered by the sensor elements  28 ,  32  (V 1 −V 2 ) or form a ratio between the said voltages (V 1 /V 2 ) or alternatively, to form a ratio between the difference in voltages and the sum of the said voltages (V 1 −V 2 /V 1 +V 2 ).  
         [0071]     The embodiment illustrated in  FIG. 5  differs in that the rolling bearing  7  includes, on its exterior cylindrical surface  21   a , two annular grooves  41 ,  42  that are mutually symmetric with respect to a plane passing through the centre of the rolling elements  22  and in that a roller  43  is overmoulded onto the exterior cylindrical surface  21   a  of the ring  21  of the rolling bearing  7  and includes a stepped bore  43   a . The bore here includes two frustoconical portions (unreferenced) which are mutually symmetric with respect to a plane passing through the centre of the rolling elements  22  and forming bearing surfaces for the radial lateral surfaces  21   b  and  21   c.    
         [0072]     Here, the roller  43  is made of moulded synthetic material, for example, a glass-fiber-filled nylon-6,6. A target element  44  is mounted embedded within the roller  43 . The target element  44 , for example a ferromagnetic insert, runs more or less axially between the frustoconical portion bearing against the radial surface  21   c  of the outer ring  21  and the axial end of the roller  8 , on the same side as the support block  27 . An axial end of the said insert  44  lies flush with a radial lateral surface of the roller  43 . The sensor elements  28  and  32  are thus positioned radially one on each side of the insert  44  forming the target element. The insert  44  thus collaborates magnetically with the magnetic element  29  to detect the tension in the belt  9  via the sensor elements  28  and  32 .  
         [0073]     Of course, it is equally conceivable to mount the insert  44  semi-embedded within the roller  43  such that the said insert  44  projects from one axial end of the roller  43 . It is also conceivable to provide an insert  44  produced in the form of a ferromagnetic ring attached to the axial surface of the roller  43 , at the same end as the support block  27 .  
         [0074]     The variant embodiment illustrated in  FIG. 6  differs from the preceding embodiment in that the roller  43 , made of moulded synthetic material, in this instance is filled with ferrite powder and magnetized at part of an axial end portion  43   b  of the roller  43  facing the sensor elements  28 ,  32 . The sensor elements  28 ,  32  of the sensor unit  26  are thus positioned radially one on each side of the magnetized part of the axial end portion  43   b.    
         [0075]     In this embodiment, the sensor unit has no magnets associated with the sensors  28 ,  32 . It is the roller which emits a magnetic field the magnitude of which is detected by the sensor elements  28 ,  32  and varies according to the position of the said roller  43  with respect to the fixed support  5  manifested by a modification in the gap between said roller  43  and the sensors  28 ,  32 .  
         [0076]     The embodiment illustrated in  FIG. 7  differs from the preceding embodiments in that the rolling bearing  7  is fixed to a support hub  45 , of axis  46 , and in that the fixed support  5  includes a support plate  47  partially in contact with the engine block  12 . The hub  45  includes an exterior cylindrical surface  45  extending over the major part of its length and in contact with the bore  20   a  of the inner ring  20  of the rolling bearing  7 . The exterior cylindrical surface  45  is radially extended, on the same side as the engine block  12 , by a shoulder  45   b  forming a bearing face for the radial surface  20   c  of the inner ring  20  of the rolling bearing  7 . The hub  45  includes a lateral radial surface  45   c  axially on the same side as the shoulder  45 , from which guide pins  48 ,  49  extend axially towards the engine block  12 . The substantially cylindrical pins  48 ,  49  in this instance are each positioned near the geometric axis  46  of the hub  45 .  
         [0077]     The support plate  47  includes a radial portion  47   a  in contact with the lateral radial surface  45   c  of the hub  45  and in which a slot-shaped opening  50  is formed, this slot being radially bounded by two parallel straight edges. The guide pins  48 ,  49  are positioned to project axially relative to the said opening  50 .  
         [0078]     The radial portion  47   a  is extended, on the same side as the engine block  12 , by axial protrusions  47   b  and  47   c  which are diametrically opposed and of small circumferential size. The axial protrusions  47   b  and  47   c  are extended outwards by first and second radial portions  47   d  and  47   e  respectively, these being axially offset from one another, the axial portion  47   e  forming a sole being in contact with the engine block  12 . The sensor unit  26  is mounted at the axial portion  47   d , the said portion being extended by an axial portion then by the radial portion  47   e  in contact with the engine block  12 .  
         [0079]     An actuator, such as a return spring  51 , is mounted between the hub  45  and the plate  47  of the fixed support  5  and exerts a return force between the hub  45  and the fixed support  5 . The spring  51  extends through a circular opening  52  formed at the axial protrusion  47   c  of the plate  47 . One end of the spring  51  is hooked onto a fixing pin  53  mounted at a free end of the axial lug  47   e , the other end of the said spring  51  being hooked onto the guide pin  49  fixed to the hub  45 , the said pins  49 ,  53  being shaped for this purpose.  
         [0080]     As the belt  9  wears, the hub  45 , the rolling bearing  7  and the roller  8 , which form the moving parts, will move linearly relative to the fixed support  5  in a radial direction towards the spring  51 , the displacement of the said parts being guided by the pins  48 ,  49  sliding in the opening  50 . The radial gap between the sensor element  28  and the axial end of the roller  8  will thus increase, modifying the magnetic field lines initially established between the magnetic element  29  and the axial end portion  8   b  of the roller  8 .  
         [0081]     The device thus makes it possible, during operation, to pick up signals corresponding to the belt tension so as to determine the degree of wear of the said belt in a way that is particularly precise and reliable, making direct use of the displacement of the roller relative to a fixed support of the device.  
         [0082]     Further modifications and alternative embodiments of various aspects of the invention may be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description to the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims. In addition, it is to be understood that features described herein independently may, in certain embodiments, be combined.