Abstract:
A tensioner comprising a base, a pivot arm comprising cast aluminum, a spring disposed between the base and the pivot arm for urging the pivot arm, a pulley journalled to the pivot arm, a steel shaft overmolded within the pivot arm, the shaft pivotally engaged with the base, the shaft comprising a waist portion for connecting to the overmolded pivot arm, and a pin releasably engaged between the pivot arm and the base.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a tensioner, and more particularly, to a tensioner having a shaft with an overmolded pivot arm. 
       BACKGROUND OF THE INVENTION 
       [0002]    Tensioners are used to apply a preload to belts used in engine accessory drives. A tensioner will typically comprise a pivot arm mounted to a base. A spring between the pivot arm and base urges the pivot arm, and a pulley journalled thereto, into engagement with a belt. This will apply a preload to the belt which assures proper power transmission from the belt to a driven accessory such as an alternator. 
         [0003]    The tensioner pivot arm comprises a pivot shaft and arm. The pivot shaft can be press fit into the pivot arm or the combined shaft and pivot arm are cast as a monolithic piece. 
         [0004]    A press fit pivot shaft can be prone to becoming misaligned. This is due to a bending moment which is applied to the shaft during operation. Misalignment of the shaft within the arm can lead to premature failure of the tensioner. 
         [0005]    For manufacture, overmolding is a process of casting a first material into a predetermined form about a second component. The casting material can either be similar or dissimilar from the receiving component. 
         [0006]    Representative of the art is U.S. Pat. No. 6,895,700 which discloses a soleplate for a clothes pressing iron, composed of: a metal sheet having a lower surface that will be directed toward fabrics to be ironed, a lower surface layer that extends form the lower surface and an interior region that is separated from the lower surface by the lower surface layer, the lower surface layer being mechanically hammer-hardened, or work hardened, to have a hardness greater than the hardness of the sheet prior to being mechanically hardened; and a coating that is hard and resistant to scratching covering the lower surface. 
         [0007]    What is needed is a tensioner having a steel shaft with an overmolded cast aluminum pivot arm. The present invention meets this need. 
       SUMMARY OF THE INVENTION 
       [0008]    The primary aspect of the invention is to provide a tensioner having a steel shaft with an overmolded cast aluminum pivot arm. 
         [0009]    Other aspects of the invention will be pointed out or made obvious by the following description of the invention and the accompanying drawings. 
         [0010]    The invention comprises a tensioner comprising a base, a pivot arm comprising cast aluminum, a spring disposed between the base and the pivot arm for urging the pivot arm, a pulley journalled to the pivot arm, a steel shaft overmolded within the pivot arm, the shaft pivotally engaged with the base, the shaft comprising a waist portion for connecting to the overmolded pivot arm, and a pin releasably engaged between the pivot arm and the base. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings, which are incorporated in and form a part of the specification, illustrate preferred embodiments of the present invention, and together with a description, serve to explain the principles of the invention. 
           [0012]      FIG. 1  is a bottom perspective view of the tensioner pivot arm. 
           [0013]      FIG. 2  is a top perspective view of the tensioner pivot arm. 
           [0014]      FIG. 3  is a top plan view of the tensioner pivot arm. 
           [0015]      FIG. 4  is a side cross-sectional view of the tensioner pivot arm. 
           [0016]      FIG. 5  is a perspective view of the pivot arm shaft. 
           [0017]      FIG. 6  is a cross-sectional view of the pivot arm shaft. 
           [0018]      FIG. 7  is a perspective view of the tensioner. 
           [0019]      FIG. 8  is a cross-sectional view of the tensioner. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]      FIG. 1  is a bottom perspective view of the tensioner pivot arm. The tensioner pivot arm  100  comprises pivot arm shaft  101 . Shaft  101  is overmolded into pivot arm body  102 . 
         [0021]    Portion  103  cooperatively engages a tensioner base  104 . Portion  103  cooperatively engages a pulley  105 . Shaft  101  pivotally engages base  104 . Spring  114  butts against damping stop  117 . Damping stop  117  also reinforces the overmold joint  107  with shaft  101 , see  FIG. 4 . Damping member  116  bears upon damping stop  117 , which damping stop is molded into the pivot arm. Damping member  116  has a frictional engagement with base  104  whereby a pivot arm oscillation is damped. 
         [0022]      FIG. 2  is a top perspective view of the tensioner pivot arm. A fastener engages receiver  105 , thereby fastening a pulley  105  to the pivot arm  102 . 
         [0023]      FIG. 3  is a top plan view of the tensioner pivot arm. A tool receiving portion  106  can be engaged by a tool (not shown) for adjusting the tensioner when installed with a belt (not shown). For example, a suitable tool may comprise a ratchet socket, pliers or wrench. 
         [0024]      FIG. 4  is a side cross-sectional view of the tensioner pivot arm. Shaft  101  is overmolded into pivot arm  102 . Overmold joint portion  107  is molded around portion  108  of shaft  101 . For the instant embodiment the pivot arm material comprises cast aluminum, for example, die cast JIS ADC12 aluminum. Shaft  101  comprises steel, for example, SAE J403 1018 steel. The materials specified are offered as examples and are not intended to limit the scope or function of the invention. Any other metal or material possessing similar physical properties will perform with equal success. The limiting aspect being that the melt temperature of the shaft must be greater than the melt temperature of the overmold cast material. 
         [0025]    In the prior art, a shaft is typically press fit into the pivot arm. The press fit design can cause the damping shoe to tilt unacceptably over time, thereby degrading alignment and performance. This in turn may cause shaft  101  to bend. Further, the hole may elongate and the pivot arm ultimately fails by separating from the shaft. 
         [0026]    The inventive overmolded design corrects the damping shoe tilting problem. Correction of damping shoe tilt ensures stable alignment for a greater duration of operation. Further, by use of overmolding it is possible to maintain better perpendicularity between shaft and arm during manufacturing. During assembly the final alignment of the tensioner assembly will typically fall within 0.5 degree. 
         [0027]    As noted, when the alignment of the tensioner deteriorates, the bending moment on the shaft increases which causes stress on the joint with the pivot arm. If hoop stress resistance in the joint is not adequate then the joint may loosen. During testing of the overmolded joint the cast pivot arm will usually fracture at the limit, but the shaft does not come out of the pivot arm hole. For example, facture may occur at about 12,000 Newtons to 15,000 Newtons for the inventive pivot arm configuration. On the other hand, in a prior art press fit joint the shaft may slip out of the pivot arm hole at approximately 6,000 Newtons. 
         [0028]      FIG. 5  is a perspective view of the pivot arm shaft. Portion  108  comprises a waist portion  109 . Waist portion  109  comprises an arcuate form wherein the diameter of shaft  101  is reduced to approximately 70% of the shaft overall diameter. Waist portion  109  provides means for the overmolded arm to mechanically grip shaft  101 . Mechanical engagement enhances shaft  101  resistance to pull-out from pivot arm  102 . Waist portion  109  may comprise any profile compatible with the desired connection between the shaft and pivot arm, such as arcuate, angular or grooved. An arcuate form is depicted in  FIG. 5 . 
         [0029]      FIG. 6  is a cross-sectional view of the pivot arm shaft. Hole  110  receives a fastener. 
         [0030]      FIG. 7  is a perspective view of the tensioner. Pivot arm  102  is pivotally engaged with base  104 . Lock pin  112  engages portion  118  in base  104 . Lock pin  112  engages pivot arm  102  at portion  119  to hold the pivot arm in a “ready” position during installation. In the ready position spring  114  is preloaded. Once a belt (not shown) is engaged with the pulley  105 , lock pin  112  is removed thereby allowing pivot arm  102  and pulley  105  to pivot into a loaded engagement with the belt. 
         [0031]      FIG. 8  is a cross-sectional view of the tensioner. Dust cover  111  prevents debris from contaminating bearing  115 . Fastener  113  retains bearing  115  on portion  103 . 
         [0032]    Torsion spring  114  is disposed between base  104  and pivot arm  102 . Torsion spring  114  urges pivot arm  102  into engagement with a belt. Damping member  116  damps movement of pivot arm  102  by frictional engagement with base  104 . Fastener  120  engages shaft  101  to pivotally connect pivot arm  102  to base  104 . 
         [0033]    The overmolded design corrects a damping shoe tilting problem for the reasons given and also because the damping stop feature on the cast arm provides the required support for proper seating of the shoe. The damping stop is a part of the overmolded joint  107  with shaft  101 . The overmolded joint can withstand more bending moment than a prior press fit joint. This is advantageous because it is not desirable to package a thicker arm in the available space. For a press fit joint it would be necessary to increase the arm thickness (“T” see  FIG. 4 ) by nearly twice to make an equivalent joint strength compared to the inventive overmolded joint. Increasing the arm thickness T would also have the effect of reducing spring height “H” which would tend to make the spring stiffer. This in turn would adversely affect tension control in a belt drive system. 
         [0034]    Although forms of the invention have been described herein, it will be obvious to those skilled in the art that variations may be made in the construction and relation of parts and method without departing from the spirit and scope of the invention described herein.