Abstract:
A belt tensioner with a retainer can be brought into contact with a belt via a belt support for applying a tensile force. The retainer is joined at a distance from the belt support to a radial plain bearing and it can be swivelled about the axis of rotation of the radial plain bearing, whereby the tensioning and swivel movements of the belt support occur under the action of a coil spring. For a belt tensioner of this type, which with a compact construction runs essentially wear-free, it is suggested that the retainer is subject to the action of a spring, which exerts a force (F 3 ) essentially parallel to the axis of rotation on the retainer, the said force (F 3 ) counteracting the force (F 1 ) exerted by the belt on the retainer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a belt tensioner of the type for automotive vehicles. 
   2. Description of the Related Art 
   Typical belt tensioners are known, for example from DE-A-43 27 141 or DE-A-40 10 928. These belt tensioners are employed in motor vehicles for tightening the V-belt and comprise a retainer with a tension arm, which is fitted with an idler pulley at one end, that is applied to the belt, and which is joined to a radial plain bearing at its other end. The radial plain bearing includes a bearing bush part and a bearing pin part, whereby one of the parts is fixed to the tension arm and the other part is held spatially fixed. A rotary spring in the form of a spiral coil spring is wrapped coaxially around the bearing bush and bearing pin and is held by one end on the retainer and held spatially fixed at the other end. These known belt tensioners are fitted in a state in which the bearing bush and the bearing pin have been rotated against one another under the pretension of the rotary spring, so that the idler pulley already exerts a predetermined force on the belt. Due to the spring force of the pretensioned rotary spring, the belt is held under a predetermined tension, but the idler pulley can deflect or take up the slack if the length of the belt changes. 
   Due to the type of construction, the tension arm bearing the belt idler pulley must, however, be arranged off center in relation to the axial length of the radial plain bearing on the bearing bush or bearing pin. Consequently, the radial plain bearing is, however, stressed by tilting forces which cause increased wear. Attempts have been made in DE-A-43 27 141 to arrange the deflecting point of the swivel spring in the axial direction to the rotating axis so far removed from the radial plain bearing that the resultant of the force introduced into the radial plain bearing meets approximately the center of the axial length of the radial plain bearing. This is however only possible where there is sufficient installation space available. 
   The object of the invention is to develop a belt tensioner such that the susceptibility to wear is reduced with a compact, short construction. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the invention, a belt tensioner is provided with a retainer which can be brought into contact with a belt via a belt support for applying a tensile force and which, at a distance from the belt support, is connected to a radial plain bearing and can be swivelled about its axis of rotation, whereby the tensioning and swivel movements of the belt support occur under the load of a coil spring. The invention is characterized in that the retainer is subject to the action of a spring, which exerts a force essentially parallel to the axis of rotation on the retainer and the force counteracting upon the force exerted by the belt on the retainer. 
   According to the invention the retainer is subject to the action of a spring which acts against the tilting moment which is caused by the force exerted through the belt on the retainer. The application of this spring is possible with a compact construction, so that the installation space required for fitting does not need to be enlarged. 
   Although belt tensioners are already known which exhibit more than one spring, such as for example the belt tensioner according to DE-A-26 08 277, the springs in this case, however, have a different function. With this known belt tensioner, a first compression spring is used which presses friction discs against the retainer and whose spring force defines a threshold value at which the retainer can only then be swivelled. The belt tensioner also has a second compression spring which is formed as a spring strut and presses the idler pulley directly against the belt. 
   The application of springs, which act on the retainer essentially parallel to the rotating axis, is for example known with a belt tensioning arm according to DE-A-195-24-403. Here however, this spring is used as a single spring and in its function replaces the rotary spring of the generic state of the art, i.e. the tensile force is determined through the pretension of the cup spring. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
       FIG. 1  a partial cross-section through a first embodiment of a belt tensioner according to the invention, and 
       FIG. 2  a partial cross-section through a second embodiment of a belt tensioner according to the invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1  illustrates a belt tensioner  1  in partial cross-section as it is used, for example, for tensioning belts in automotive vehicles. The belt tensioner  1  is however also suitable for other applications where belts, chains or other endless transmission elements need to be tensioned. 
   The belt tensioner  1  includes a base section  2  which is provided with a central opening  3  and cup-shaped, high-reaching walls  4  which form part of a housing. The belt tensioner  1  also includes a retainer  5 , which also exhibits a central opening  6  and cup-shaped high-reaching side walls  7 , whereby the walls  4  and  7  are formed such that they form the outer boundary of the belt tensioner  1  and the center lines of the openings  3  and  6  meet in a common center line  8 . 
   The retainer  5  is also provided with a tension arm  9  which is mounted in the axial direction asymmetrically and is offset from the center line  8  on the retainer  5  and protrudes over the wall  7 . An idler pulley  10  is rotationally supported about an indicated axis  11  in the tension arm  9 . The idler pulley  10  forms a belt support for the belt to be tensioned, whereby the axis  11  runs parallel to the center line  8 . 
   One end of a hollow bearing pin  12  is mounted in the opening  3  of the base section  2 , preferably by caulking. A screw, which is not shown, can be passed through the hollow bearing pin  12 , enabling the belt tensioner  1  to be firmly mounted onto an engine part of a motor vehicle or similar. 
   The retainer  5  with a bearing bush  13  sits on this hollow bearing pin  12 , whereby the bearing bush  13  and bearing pin  12  can be displaced relative to one another in the axial direction and the inner surface of the bearing bush  13  forms a radial plain bearing  14  with the outer surface of the bearing pin  12 , so that the retainer  5  can rotate about the bearing pin  12 , whereby the center line  8  forms the axis of rotation. The retainer  5  is held by a locking piece  15  in the form of a disc on the bearing pin  12 , the disc forming a positive locking joint with the free end of the bearing pin  12  opposite the base section  2 . 
   The retainer  5  also includes a friction cone  16  which extends coaxially on the center line  8  and on which a spring bush  17  and a coil bush  18  are arranged in a normal manner, carrying a rotary spring  19  in the form of a spiral-coil spring. The rotary spring  19  is, as is usual with belt tensioners of this type, attached at one end to the base section  2  and to the retainer  5  at its other end, so that tensioning can be provided through the relative rotation of the retainer  5  and the base section  2 . The function and the operation of tensioning with the aid of this type of coil spring is known to the specialist so that further details need not be supplied here. 
   Through the tensioning of the belt it exerts a force F 1  on the retainer  5 . Since the bearing bush  13  is spaced in the axial direction from the idler pulley  10  with regard to the axis of rotation  8 , and since, compared to the belt tensioner  1  of the generic state of the art according to DE 43 27 141, the friction cone  16  and therefore the lower mounting point of the coil spring  19  for introducing a counter force F 2  in the axial direction of the axis of rotation  8  is located closer to the bearing bush  13  than the idler pulley  10 , this would lead to a resultant force, which is applied off center on the plain bearing  14 , and would therefore cause a tilting moment, which would lead to increased wear of the radial plain bearing  14 . 
   To prevent this, the bearing bush  13  of the radial plain bearing  14  has a shorter axial length than the intervening space between the locking disc  15  and the base section  2  in the region around the opening  3 . In the axial intervening space produced by this, a spring  20  is arranged which is formed as a ring-shaped cup or Belleville spring on which the bearing pin  12  is located and acts symmetrically on the axis of rotation  8 . In the region of this Belleville spring  20  preferably the base section  2  is fitted with a base  21  which acts as a thrust pad so that the Belleville spring  20  is evenly compressed. The Belleville spring  20  acts via a supporting ring  22  on the bearing bush  13  such that a force F 3  can be exerted on the bearing bush  13  parallel to the axis of rotation  8 , the force F 3  supporting F 2  as counter force for the introduced force F 1  transferred from the belt, so that essentially a resulting force F R  aligned to the axial center of the radial plain bearing  14  is produced with which a tilting moment is essentially not produced. 
   During assembly, the force F 3  can be set variably via the spring displacement of the Belleville spring  20 , whereby values between about 4000 to 7000 N are preferred. 
   The Belleville spring  20  acts via the supporting ring  22  on a face of the bearing bush  13  and presses it against the locking disc  15  which acts as an abutment. Here, the sides facing one another are formed as axial plain bearings between the locking disc  15  and the bearing bush  13  or between the bearing bush  13  and the supporting ring  22 . These axial plain bearings preferably have steel surfaces which are coated with PTFE. Depending on the type of coating, these plain bearings can also however contribute to friction damping, so that the applied damping is split between the damping due to the coil spring  19  and the damping due to friction. 
   The plain bearings can also be provided by separate discs and/or in another axial position. 
   In an illustration similar to  FIG. 1 ,  FIG. 2  shows a second embodiment of a belt tensioner  30 , whereby components similar to the first embodiment are identified with the same reference symbols and are not explained again. 
   The belt tensioner  30  differs from the belt tensioner  1  only due to the fact that here an additional, increased friction damping is produced by the arrangement of a special damping washer  31 . The damping washer  31  is preferably placed between the bearing bush  13  and the locking disc  15 , whereby, due to the Belleville spring  20 , the bearing bush  13  is pressed via the supporting ring  22  against the damping washer  31  which it presses against the locking disc  15 . This arrangement is particularly advantageous where additional friction damping is required due to very high application-related requirements. Here, friction damping values of up to 60% of the tensile force or the torque can be produced. Wear-free functioning can be ensured due to tuning the damping via the coil spring  19  on one hand and via the Belleville spring  20  on the other, or due to splitting of the damping between both systems, despite an overall damping value of 85% referred to the torque. 
   An advantageous material pairing for the damping washer  31  and the adjacent locking disc  15  is for example a glass-fibre reinforced polyamide, in particular PA 46 with 5% glass-fibre content, for the damping washer  31  and a stainless steel, in particular V2A, for the locking disc  15 . The plain bearings, which are arranged in this case between the bearing bush  13  and the damping washer  31  or between the bearing bush  13  and the supporting ring  22 , can, as with the first embodiment, have bearing surfaces of steel coated with PTFE, which, where required, are arranged on additional washers not illustrated in the drawings. 
   In a modification of the described and drawn embodiments a different sequence of the components is possible. For example, the Belleville spring  20  can be arranged on the side of the bearing bush  13  facing the locking disc  15 . Instead of a Belleville spring a different suitable spring can be used. The selection of materials can be carried out with regard to the service life to be achieved and/or to the required damping values. The invention can furthermore be applied when the tension arm  9  is joined to the bearing pin  12 .