Abstract:
A belt tensioner includes a stationary spring case receiving a pivotal arm supporting a pulley and biased in a belt-tensioning direction by a spring element. The arm is provided with a surface supporting an armplate bushing having an inner diameter and juxtaposed with an armplate. The armplate has an inner diameter smaller than an inner diameter of the armplate bushing so as, upon radially riveting the armplate to a stationary shaft during assembly of the tensioner, an inner area of the armplate is resiliently bent. The bushing is provided with a taper, so that the respective juxtaposed surfaces of the bushing and armplate extend complimentary to each other upon deflection of the armplate.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a tensioning device for power transmission belts and the like, in particular a tensioning device mounted on motor vehicle engines and having a pivotal arm, which supports a pulley and is received in a stationary case, and a means for reliably interconnecting these parts without locking up the tensioning device. 
     BACKGROUND OF THE INVENTION 
     It is known to drive various automobile accessory assemblies, for example the water pump, the generator, the fan for cooling the coolant, the power steering pump, and the compressor, by the vehicle engine. This is done by a driving pulley actuated by the engine shaft of the motor vehicle which drives an endless drive belt operating the accessory assemblies through driven pulleys. 
     Different states of operation of the engine are known in which the belt tension increases pulse-like generating a growing torque on the pivot arm which, in response to varying tension of the belt, may deflect in a direction opposite to a belt tensioning direction. In order to prevent the unnecessary sliding friction between the pivotal arm and the stationary case, the prior art teaches numerous resiliently deformable elements which are disposed between the pivotal arm and the stationary house. Usually, such deformable elements are bushings made of friction materials. Increasing the number of bushings leads to complicated kinematics, which may result in structural damages to the tensioner. Typically, the tensioner may have increased undesirable wear of many frictional parts as a result of tensioning forces exerted by new elements. This brings about tension peaks in the belt, which can exceed the permissible belt tension. Ultimately, all of the above described defects can cumulatively cause undesirably large excursions of the pivotal arm. 
     Recognizing this problem, the prior art has designed numerous structures of belt tensioners which allow reduced dynamic loads on the main elements of the tensioner. Two basic premises are usually considered during design and assembly of the belt tensioner. First, the ideal assembly has to be tight enough to have all the clearances removed. Second, the ideal assembly must be sufficiently loose to avoid locking up the tensioner. 
     The structure that has been previously designed is shown in FIGS. 1 and 1A, and as a result of practical observations, is illustrative of the problems inherent in this type of tensioner. The belt tensioner has a disk shaped armplate  5 ′ serving a dual function. First, the armplate  5 ′ holds the assembled tensioner together. Second, the plate removes the clearances between the various components of the tensioner during assembly. The armplate has a wear bushing  4 ′ between it and a tensioner arm  1 ′ as a result of the relative rotational movement that exists between these two components when the tensioner is in operation. Typically, the wear bushing and the armplate have had the same outer and inner diameters. Such dimensions have caused assembly problems due to the dimensional variation of a plurality of components that must be mated together to build a tensioner. 
     The plate, as known in the art, is customarily attached to the tensioner by one of the following methods: 
     According to the first method, the armplate  5 ′ is pressed to a shoulder  16 ′ of a shaft  2 ′ and then radially riveted to secure the plate to the shaft. One of negative consequences of such method is poor removal of the clearances in the assembly because the stackup tolerances in the components are so large that the assembly is either pressed too tight together and, therefore, is locked up or, conversely, it is too loose to perform properly. 
     Pursuant to the other method, the armplate is pressed with a certain force tightly against the armplate bushing  4 ′ and arm  1 ′ to remove all the stackup clearances of the assembly and then is radially riveted to secure it. This approach has not been found entirely satisfactory because the press force is difficult to control and the radial rivet operation tends to push an inner area adjacent to the shaft  2 ′ against the bushing  4 ′ even further and can easily lock the tensioner up. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a belt tensioner of the type described herein which overcomes the above-discussed drawbacks. 
     Another object of the present invention is to provide a belt tensioner having a structure including a limited number of clearances. 
     Still another object of the present invention is to provide a belt tensioner that despite a limited number of clearances, has a tight structure avoiding a possible lockup of the tensioner. 
     The above and other objects according to the invention are achieved by a belt tensioner which is provided with an armplate made of steel and having inner and outer diameters. An outer area of the armplate terminates at a radial distance from spaced apart stops formed on the arm while an inner area is pressed against a stationary shaft. The armplate is also spaced axially apart from a respective surface of a pivotal arm, which carries a pulley, and forms a gap therewith. The gap, in turn, receives a plate bushing pressed against respective surfaces of the plate and the arm and having an inner diameter substantially larger than the inner diameter of the armplate. Such structure of the plate bushing allows the inner area of the armplate to deflect freely during assembly. The plate bushing is formed with a taper to conform a deflected surface of the plate upon riveting and thus maintains a good wear surface. 
     An added benefit to deflecting the armplate is that it retains a small amount of “spring back” and, as the bushing surfaces wear out, it is able to compensate for the wear and keep the tensioner firmly together. 
     Advantageously, the armplate is made of spring steel and is able to create damping which is sometimes desirable. 
     According to another advantageous feature of the present invention, the respective surface of the pivot arm is formed with a taper while the bushing is manufactured with flat surfaces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more readily apparent from the following detailed description of the preferred embodiment of the invention, references being made to the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a belt tensioner according to the prior art. 
     FIG. 1A is a cross-sectional view of the belt tensioner according to the prior art tensioner of FIG.  1 . 
     FIG. 2 is a cross-sectional view in elevation of the belt tensioner according to the invention. 
     FIG. 3 is a cross-sectional view of a part of the belt tensioner according to another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 illustrates an arm tensioning device in accordance with the invention. The arm tensioning device includes a shaft  2  which is mounted on a motor vehicle engine (not show herein) in a known manner and extends along an axis A—A. During assembly of the device, a spring case  3  is fixedly centered on the shaft  2 . The spring case  3  has a generally frustoconical cross-section and is formed with a peripheral annular wall  6  tapering toward an inner mounting end  7  of the shaft  2 . An inner surface  10  of the peripheral wall  6  defines a compartment  9 , which is open axially outwardly to receive a swingable arm  1  carrying a pulley  10   a.    
     The inner surface  10  of the spring case  3  has a portion  10 ′ extending generally parallel to the axis A—A and a portion  11  which tapers axially inwardly. The portion  11  is formed with an annular flange  12  extending axially outwardly and having an inner surface  13  that extends substantially perpendicular to the axis A—A. 
     The peripheral wall  6  further has an annular radially extending surface  19   a  facing an inner surface  21  of the arm  1 . The inner and outer surfaces  21  and  19   a  are spaced axially apart so as to form a clearance  28 ′ therebetween which receives a spring bushing  28  covering the spring case  3  and being in contact with both of these surfaces. The spring bushing  28  is preferably made of plastic. 
     The inner mounting end  7  of the shaft  2  is formed with a part  14  having a reduced diameter and defining a seat  14   a  which receives the inner portion  13  of the annular flange  12  of the peripheral wall  6 . 
     Understandably, the above-described spring case may have different forms and shapes as long as the requirements of easy assembly and reliability of the device are met. 
     The compartment  9  receives the swingable arm  1 , which is formed with an axially extending hollow sleeve  15  having an inner annular surface  19  that is spaced radially outwardly from the shaft  2 . An inner axial end  15 ′ of the hollow  15  sleeve terminates at some distance from the inner portion  13  of the peripheral wall  6 . An outer axial end  20  of the hollow sleeve  15  is spaced axially inwardly from an outer axial end  16  of the shaft  2 . 
     Extending radially outwardly from the outer axial end  20  of the hollow sleeve  15  and formed unitarily therewith is a radial flange  17  having the inner surface  21  that faces the compartment  9  and an outer surface  22 . An outer radial periphery of the flange  17  has a stop  23  and pulley support  24 , which are spaced diametrically apart from each other. The pulley support  24  extends axially outwardly from the flange  17  towards the pulley  10   a.    
     The stop  23  has an inner surface  25  spaced radially outwardly from the outer axial end  16  of the shaft  2  and extending axially outwardly from the outer peripheral surface  22  of the radial flange  17 . The pulley support  24  has an axially extending recess  31 , which receives a shaft  25   a  of the pulley  10   a  swingable with the arm  1  toward a belt (not shown herein). 
     The compartment  9  of the spring case  3  receives a spring element  26  having one end connected to the swingable arm  1  and the other end connected to the spring case  3 . The spring element  26 , which presses against the spring bushing  28 , applies the tensioning force to the arm  1  so as to bias the arm and the pulley  10   a  towards the belt. According to the invention, the shaft  2  does not protrude into the volume of the pulley  10   a,  which has its inner surface  27  spaced axially from the flange  17  of the swingable arm  1 . 
     As shown in FIG. 2, the inner surface  25  of the stop  23  and an inner surface  18  of the support  24  form a radial gap with the shaft  2 . This gap receives a disk-shaped armplate  5  pressed by its inner surface  5 ″ against the outer axial end  16  of the shaft  2 . Thus, the armplate  5  removes, at least partially, undesirable clearances. The armplate  5  further has its outer periphery terminating at a distance from inner surfaces  25  and  18  of the stop  23  and support  24  respectively. Upon assembling the device, the armplate  5  is also spaced axially from the outer surface  22  of the flange  17  of the arm  1 , thereby forming a space  32  with this outer surface  22 . Further, during radially riveting the armplate to the outer axial  16  of the shaft  2  its inner surface  5 ″ is bent inwardly. As a result, a region  5 ′″ of the armplate  5  adjacent to the inner surface  5 ″ deflects inwardly and solidifies the whole structure. 
     The space  32  receives a plate bushing  4  made of friction material and having an outer surface  32   a  and an inner radially extending surface  34 . These outer and inner surfaces of the plate bushing  4  are juxtaposed with an inner surface of the armplate  5  and the outer surface  22  of the flange  17  of the swingable arm  1  respectively, thereby affecting the sliding friction between the rotational and stationary parts of the device. 
     As mentioned above, the invention attacks two problems stemming from dichotomy inherent in any belt tensioner. First, the device should have a very few clearances between the various rotational and stationary components, thereby providing tightness of the assembly. Second, the tensioner must be loose enough to avoid locking up the pivotal arm. 
     In order to prevent a lock up, the plate bushing  4  is formed with an outer diameter substantially equal to, but preferably greater than the outer diameter of the armplate  5 . The plate bushing  4  is sized to reduce a radial clearance formed between the surfaces  25 ,  18  of the stop  23  and support  24  respectively and the outer annular surface of the armplate  5  facing these surfaces. 
     An inner axially extending peripheral surface  35  of the bushing  4  terminates at a substantial distance from the shaft  2 , thus making the inner diameter of the annular plate bushing  4  substantially larger than the inner diameter of the armplate  5 . In structural terms, the plate bushing  4  terminates in the central region of the armplate  5 . 
     The large inner diameter of the plate bushing  4  allows the region  5 ″ of the armplate  5  to deflect freely during assembly. Upon deflection, the armplate  5  is inclined inwardly with respect to the axis A—A. The plate bushing  4  tapers axially inwardly and extends substantially parallel to armplate  5  after it has deflected during assembly. 
     According to anther aspect of the invention shown in FIG. 3, an outer surface  22 ″ of a flange  17 ″ tapers radially inwardly forming an angle β with a horizontal. In this case, the plate bushing  4 ″ can have the uniform thickness and yet extend complimentary to a deflected armplate  5   a  thus providing free deflection of the armplate so as to avoid locking up the tensioner. 
     According to the invention, each of the armplates  5 ,  5   a  is preferably made of spring steel. As a result, the plate retains a small amount of elasticity allowing the armplate to partially spring back to its initial position. As the contact surfaces of the plate bushing  4 ,  4 ″ wear, the armplate is able to compensate for the wear and to keep the tensioner firmly together. Using the increased axial force of the deflected armplate made of spring steel can create damping, which is sometime desirable. 
     While the above disclosure has described various embodiments, it is understood that the invention is not limited thereto, and is susceptible to numerous changes and modifications as known to those skilled in the art. Therefore, this invention is not limited to the details shown and described herein, and includes all such changes and modifications as are encompassed by the scope of the appended claims.