Abstract:
A timing belt antotensioner includes a mechanism actuated by the reversal of timing belt movement direction. In automotive applications reversal of timing belt direction occurs only rarely under inadvertent or accidental circumstances. Such reversals can cause belt slackening sufficient for a toothed belt to jump gear teeth thereby changing engine cam shaft timing. Four related mechanisms are disclosed, each of which will, upon actuation by belt reversal cause the autotensioner pulley axis of rotation to move in a direction that tightens the belt during reverse movement of the belt. Thus, the belt remains tight to the toothed pulleys preventing jump or skip of the belt until the belt returns to proper forward belt movement.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to the field of belt tensioners and belt tensioner systems. More particularly, the present invention relates to improvements in both mechanical type and hydraulic type belt tensioners for use with a camshaft belt drive system in automotive engine applications and the like.  
           [0002]    A timing belt trained about two cooperating pulleys is well-known in the art of tension transmitting assemblies. There are economical advantages to the aforementioned when compared with other types of assemblies, specifically meshing gear assemblies. It is known to use an automatic tensioner in conjunction with a synchronous or timing belt drive system in order to compensate for tension variations in the belt. These variations are commonly attributable to dynamic effects such as cyclic torque variations and thermal effects that introduce changes in the length of a timing belt drive.  
           [0003]    A tensioner is located on the normally slack side of the belt span in a belt drive system. Tensioner design is typically divided into two groups: mechanical tensioners, relying on coulomb friction as means to generate damping; and second, hydraulic tensioners, generally having a piston arrangement with a known leak-through and a one-way valve to create an asymmetrical damping which is proportional to speed. While these types of tensioners are designed to accommodate cyclic torque variations and thermal effects in a belt drive system by controlling belt tension at the slack side of the belt span, such tensioners are not designed to accommodate extreme torque reversal situations (kickback), such as engine backfiring or engine rotation in reverse (e.g., an automobile going backward while in forward gear with the clutch engaged).  
           [0004]    In such extreme torque reversal situations, the slack side of the belt drive system becomes the tight side. The tight belt tension, on the normally slack side, causes the tensioner device to respond to the kickback and rapidly decrease belt tension by moving the pulley and its related pivot-arm away from the belt to slacken the tight side of the belt span. If the pulley movement is extreme, it can over-slacken the belt and result in tooth jump or ratcheting as the slackened belt enters the crank pulley or cam shaft pulleys. Tooth jump or ratcheting is deleterious to the operation of an engine as synchronization of the pulleys is lost.  
           [0005]    Some tensioners have a ratchet and pawl mechanism attached to the tensioner&#39;s pivot arm to eliminate tensioner kickback and avoid tooth jump or ratcheting. U.S. Pat. No. 4,299,584 discloses a ratchet operative with a leaf-spring pawl that allows some compliance at kickback by permitting the leaf-spring to deflect slightly. U.S. Pat. No. 4,634,407 also teaches a ratchet and pawl mechanism where the ratchet operates as a one-way clutch that fixes the position of a pivot-arm such that the tensioner cannot operate to slacken the belt.  
           [0006]    However, a common problem of ratchet/pawl devices is that the tensioner must operate primarily as a fixed idler in one direction as the ratchet mechanism limits the motion of the tensioner pivot-arm. In other words, the tensioner pivot-arm is unable to function in a direction that would allow the belt to be slackened. Under this condition, belt tooth failure and noise is reintroduced into the belt drive system when the belt cannot be at least partially slackened.  
           [0007]    U.S. Pat. No. 5,591,094 teaches an adjustable stop spaced at a distance from the pivot-arm when the pulley is biased in a pressing engagement against a static belt. The spacing is pre-determined to allow pivot-arm movement in a direction to slacken the belt while also preventing belt teeth from becoming disengaged from a toothed pulley (i.e., tooth jump) in an extreme torque reversal situation. The problem with an adjustable stop of this nature is that its distance from the pivot-arm is determined by compensation for the thermal effects of a hot engine. Each component of the belt drive system, however, leaves space for simultaneous tensioner arm vibration. In practice, this distance is large enough to allow tooth jump, especially under conditions such as low temperature and when at least one of the belt and pulleys is covered with a coating of ice.  
         SUMMARY OF THE INVENTION  
         [0008]    The new autotensioners comprise mechanisms actuated by the reversal of movement direction of the timing belts. Such a reversal of belt movement direction, normally a very rare occurrence, usually occurs during a short period of time, after which the belt returns to its normal forward or preferred movement. Each of the four mechanisms disclosed below, upon actuation by reverse belt movement, causes the autotensioner pulley axis of rotation to move in a direction that tightens the belt during reverse movement of the belt. Mechanisms are disclosed below that apply to autotensioners which have a trailing or leading geometry relative to the belt. Applied to autotensioners engaging the slack span of the belt, the mechanisms almost instantly tighten the belt in response to the reversal of belt direction. While disclosed for an automotive application, the invention is useful for any toothed belt applications where skipping or jumping of the belts over toothed gears would be deleterious to the operation of the machines. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The novel features which are believed to be characteristic of the present invention, as to its structure, organization, use and method of operation, together with further objectives and advantages thereof, will be better understood from the following drawings in which a presently preferred embodiment of the invention will now be illustrated by way of example. It is expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. Embodiments of this invention will now be described by way of example in association with the accompanying drawings in which:  
         [0010]    [0010]FIG. 1 is a side elevational view of a timing belt drive system;  
         [0011]    [0011]FIG. 2 is a side elevational view of a first embodiment of the tensioning device of the present invention;  
         [0012]    [0012]FIG. 3 is a side elevational view of a second embodiment of the tensioning device of the present invention;  
         [0013]    [0013]FIG. 4 is a side elevational view of a third embodiment of the tensioning device of the present invention; and  
         [0014]    [0014]FIG. 5 is a side elevation of a fourth embodiment of the tensioning device of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    Reference will now be made to FIGS. 1 through 5.  
         [0016]    [0016]FIG. 1 is a side elevational view of a synchronous timing belt drive  5  shown with a toothed belt comprising spans  16 ,  17 ,  18  and  19 . Teeth  25 , located on the interior periphery of the belt, are spaced at multiple pitch  31 . The belt is entrained and tensioned around toothed pulleys  11 ,  12  and  13 . The pulleys are illustrated as a camshaft drive of an automotive engine design that includes two exterior toothed cam pulleys  11 ,  12  and an exterior toothed crankshaft pulley  13 . A belt-tensioning device  21  is mounted in connection with these pulleys such that it is operative in conjunction with the timing belt drive  5 . As the engine operates over a range of RPM&#39;s, the drive camshaft pulleys  11 ,  12  introduce cyclic torque variations, which cause dynamic belt tension variations in belt spans  16  through  19 . The tensioning device  21  is intended to compensate for torque variations, thermal growth when the engine is running, and stretch and wear of the belt which occurs during the life span of the drive  5 .  
         [0017]    [0017]FIG. 2 illustrates a first embodiment of the present invention. The belt tensioner  21  is mounted on the engine via a pivot shaft  50  having a pivotal eccentric arm  49  to which a predetermined torque is applied, usually via a spring arrangement (not shown here). This torque generates a predetermined belt force which is transmitted to the belt via a pulley  52  attached to eccentric arm  49  by any means as is apparent to one skilled in the art, and generally through a bearing (not shown).  
         [0018]    The tensioning device as shown in FIG. 2 is a trailing type configuration. The center  53  of arm structure  49  is located above line  70  throughout its operational range. Line  70  represents the over-center position of pulley  52  with respect to the pivot shaft  50 . A ratchet wheel  42  is attached to arm structure  49 . A plurality of pawls  40 , located in pockets within the housing structure  41 , and attached to pulley  52 , bias the ratchet wheel  42  to form a one-way clutch and permit the unrestricted rotation of pulley  52  in the counterclockwise rotational direction of the drive  5  as depicted by arrow  61 . In the event of a clockwise rotational direction, depicted by arrow  60  and which generally occurs under kickback and rollback conditions, the pawls  40  engage ratchet wheel  42  locking the pulley  52  and eccentric arm structure  49  together. This generates frictional torque between the belt  18  and pulley  52  in the direction of arrow  60 . The torque upsets the abnormal belt force caused by the belt reversal. Using an existing tensioner device typical of the prior art, pulley  52  is normally pushed in an outward belt direction as the belt force, in conjunction with the arm length  55 , generates an opposing torque which overcomes the spring torque applied to the eccentric arm  49 , slackening the belt, and, in turn, potentially creating tooth jump. When using the tensioning device of the present invention, the belt  18  causes engagement of the ratchet  40 ,  41 ,  42  generating torque and moving the pulley  52  toward the belt, thus increasing the belt tension temporarily on the slack side and preventing tooth jump.  
         [0019]    [0019]FIG. 3 is an enlarged view of the tensioning device of FIG. 1 and illustrates a second embodiment of the present invention. The tensioner  21  functions in the same manner as explained above. The tensioner as shown in FIG. 3 is a leading type configuration. The center  53  of arm structure  49  is located below the line  70  throughout its operational range. A ratchet wheel  44  is pivotally mounted on the cylindrical surface of the eccentric arm  49 . A plurality of pawls  40 , located in pockets within housing structure  41 , attached to pulley  52  bias the ratchet wheel  44  and permit the unrestricted rotation of pulley  52  in the counterclockwise rotational direction, depicted by arrow  61  of drive  5 . In the event of a clockwise rotational direction, depicted by arrow  60  and which generally occurs during kickback and rollback conditions, the pawls  40  engage ratchet wheel  44  enabling rotation of ratchet wheel  44  together with the pulley  52 . The ratchet wheel  44  is meshed with gear  81  through teeth  45  on the inside of the volute. Gear  81  is pivotally mounted on a support structure  80 , and is attached to the pivot structure  50  via a member not shown here for clarity. Thus, pivot structure  80  is fixed. Gear  81  is meshed with teeth  46  which are part of the eccentric arm  49 . This gear train results in the eccentric arm  49  rotating toward the belt and generating an opposing torque. This opposing torque overcomes the belt force generated torque resulting from the clockwise rotational direction of the drive  5  (depicted by arrow  60 ), and increases the belt tension which, in turn, prevents tooth jump.  
         [0020]    [0020]FIG. 4 illustrates a third embodiment of the present invention. The belt tensioner comprises a pulley  52 , an eccentric arm structure  49 , a hydraulic actuator unit  100 , mounted on an engine via a pivot shaft  50  and bolts  93 ,  94 . Pulley  52  is attached to the eccentric arm structure  49  through a bearing fixed to the arm structure  49  via bolt  91 . The arm structure  49 , pivotally trained about pivot shaft  50 , allows the pulley  52  to rotate eccentrically around the center of pivot shaft  50 . Hydraulic actuator  100  exerts a known force at point  110  generating a predetermined torque that is transferred to arm structure  49  in conjunction with arm length  55 . This generates a predetermined belt force that is transmitted to the belt via pulley  52 .  
         [0021]    The tensioner shown in FIG. 4 is a trailing type configuration. The center of pulley  52  is located above line  70  throughout its operational range. Line  70  represents the over center position of the pulley  52  with respect to the pivot shaft  50 . A ratchet wheel  42  is attached to the arm structure  49 . A plurality of pawls  40 , located in pockets within housing structure  41 , attached to pulley  52  bias the ratchet wheel  42  and permit the unrestricted rotation of pulley  52  in the counterclockwise rotational direction of the drive  5  depicted by arrow  61 . In the event of clockwise rotational direction, depicted by arrow  60  and which occurs during kickback and rollback conditions, the pawls  40  engage ratchet wheel  42  locking the pulley  52  and eccentric arm structure  40  together. This generates a frictional torque between the belt  18  and pulley  52  in the direction of arrow  60 . The torque upsets the abnormal belt force caused by the belt reversal, moves the pulley  52  toward the belt increasing the belt tension temporarily, and, in turn, prevents tooth jump.  
         [0022]    In FIG. 5, a belt tensioner  21  is mounted on the engine via a pivot shaft  50  and has a pivotal eccentric arm structure  49  to which a predetermined torque is applied usually via a spring arrangement (not shown here). This generates a predetermined belt force which is transmitted into the belt via a pulley  52  attached to eccentric arm structure  49  through a bearing usually of the type known as ball or roller (not shown here). The tensioner configuration shown is of the leading type, wherein the center  53  of arm structure  49  is below the line  70  throughout its operational range as above. A second pivotal structure is mounted to the base plate  148  of the tensioner, comprised of a second eccentric arm structure  150  and pulley  153 . Attached to the arm structure  150  is pawl  144  which at its tip has a gear mesh  143 . Within pulley  153  is a one-way clutch  152  biasing the arm structure to permit free rotation of the pulley  153  when the belt moves normally in direction  61 . When the engine kicks back or roll back occurs, the belt changes direction to  60 .  
         [0023]    The one-way clutch  152  senses this change of direction and locks pulley  153  and arm structure  150  firmly together. This causes the arm structure  150  to rotate in the direction shown by arrow  145 . Pawl  144  rotates with the arm  150  resulting in the gear mesh  143  engaging mesh  142  which is attached to the first pivotally mounted eccentric arm structure  49 . This gearing results in the eccentric arm structure  49  rotating toward the belt and generating an opposing torque that overcomes the belt force generated torque due to the abnormal direction of the drive  5  depicted by arrow  60 , thus increasing the belt tension and preventing tooth jump. A stop  149  prevents over centering of the second arm structure  150 .  
         [0024]    Throughout this specification, unless the context requires otherwise, the word “comprise,” and variations such as “comprises” or “comprising,” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not to the exclusion of any other integer or step or group of integers or steps. The ratchet and pawl mechanisms forming one-way clutches are to be understood as including equivalents such as spring clutches, sprag clutches and roller ramp clutches.