Abstract:
In a ratchet type chain tensioner, having a toothed rack on its plunger and a pawl cooperable with the rack, a recess is provided between the front end of the rack and the protruding end of the plunger. In the initial stages of operation of the tensioner, the pawl can enter the recess while permitting the plunger to protrude and retract freely within a limited range. Therafter, when the chain controlled by the tensioner becomes elongated, the pawl cooperates with the rack to limit retraction of the plunger.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority on the basis of Japanese patent application 2005-344637, filed Nov. 29, 2005. The disclosure of Japanese application 2005-344637 is hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to a ratchet type tensioner used for applying proper tension to a timing chain in a vehicle engine or the drive chain of a balancer or the like. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a typical timing transmission in a vehicle engine, an endless, flexible, traveling chain transmits rotation from a crankshaft sprocket to one or more camshaft sprockets. A ratchet tensioner is typically used to maintain proper tension in the slack side of the timing chain, that is, the side moving from away the crankshaft sprocket, in order to suppress vibration. The ratchet tensioner has a protruding plunger, and is mounted on the engine body in such a way that a front end of its plunger presses against the back of a pivoted tensioner lever, urging a shoe on the tensioner lever into sliding contact with the chain. A typical timing transmission using such a tensioner is described in U.S. Pat. No. 6,478,703. 
         [0004]    In the case of a balancer, two balance shafts are provided with gears that mesh with each other, and a balancer chain is in mesh with a sprocket mounted on one balance shaft and a crankshaft sprocket. The balancer also includes a ratchet tensioner for maintaining proper tension in the balancer chain, as described in Japanese Laid-open Patent Publication No. 2004-308624. 
         [0005]    An example of a conventional ratchet type tensioner is shown in  FIG. 7 . In the tensioner  21 , a plunger  24  is slidable in a plunger-accommodating hole  23  in a tensioner body  22 , and protrudes forward from the front surface of the tensioner body. A compression spring  26  bears against the bottom of the plunger-accommodating hole in the tensioner body and extends through an opening at the rear of the plunger into a blind hole  25  inside the plunger  24 , its opposite end bearing against the closed end of the blind hole. The spring  26  continuously biases the plunger  24  in the protruding direction. 
         [0006]    A longitudinal rack  27  is formed on the outer circumferential surface of the plunger  24 . The rack  27  comprises a series of rack teeth disposed at a uniform pitch over the entire length of the rack. The length of the rack is determined in accordance with the expected amount of loosening of the timing chain. 
         [0007]    A pawl  29  is pivotably supported on the tensioner body  22  by a shaft  28  at a position adjacent the rack  27 . The pawl  29  is continuously biased clockwise by a spring  30  held in compression between the pawl and the bottom of a spring-receiving hole in the tensioner body. A first pawl tooth  29   a , and a second pawl tooth  29   b  spaced rearward of the first pawl tooth  29   a , are engageable with the rack teeth  27   a . The pawl  29  and the rack teeth  27   a  form a “backstop” mechanism  31  that blocks backward displacement of the plunger  24 . 
         [0008]    The first pawl tooth  29   a , formed on the front side of the outer edge of the pawl  29 , has a triangular shape corresponding to the shape of rack teeth  27   a , and engages with a space between rack teeth substantially without a clearance. The second pawl tooth  29   b  is smaller than the first pawl tooth  29   a . That is, its height, measured along a direction approximately perpendicular to the plunger axis, is lower than the height of pawl tooth  29   a . Tooth  29   b  is formed at a location spaced rearward from tooth  29   b  by at least three times the rack tooth pitch. When the first pawl tooth  29   a  is engaged between two rack teeth  27   a  substantially without any clearance, the second pawl tooth  29   b  is out of contact with the rack teeth  27   a , and does not restrict movement of the plunger  24 . When the chain loosens by an amount permitting the plunger to protrude through a distance corresponding to half the width of one rack tooth  27   a , the plunger  24  begins to move forward and the first pawl tooth  29   a  rides over the top of a rack tooth  27   a . The second pawl tooth  29   b  then comes into contact with another rack tooth  27   a , causing the pawl to rotate clockwise, and causing the first pawl tooth  29   a  to be engaged with another rack tooth  27   a . While the second pawl tooth  29   a  is between two rack teeth, the plunger  24  can move slightly forward or backward in accordance with the looseness or tension in the chain, and the plunger  24  applies proper tension to the chain. 
         [0009]    Both in the case of a timing chain and in the case of a chain for driving auxiliary equipment such as a balancer or the like, there are engines in which it would be desirable to have the backstop mechanism of a ratchet tensioner function only when the chain has become elongated after a lengthy period of operation. However, in a conventional ratchet tensioner, the length of the rack is such that it is opposite the ratchet pawl throughout the entire range of movement of the plunger. In such a tensioner, if the backstop mechanism is effective to prevent jumping of the chain on the sprocket teeth when the chain becomes elongated, it can exhibit insufficient backlash in the initial stages of its operation before the chain has become elongated. The tensioner cannot operate reliably in an engine that requires a tensioner having a backstop mechanism that functions in one region of plunger movement and does not function in another region of plunger movement. 
         [0010]    Accordingly, an object of the invention is to solve the above-mentioned problems and to provide a ratchet tensioner that can be adapted to an engine of the kind that requires a tensioner having a backstop mechnism that functions only when the chain has become elongated, and does not function in the intial stages of operation of the chain. 
       SUMMARY OF THE INVENTION 
       [0011]    In the improved ratchet tensioner according to the invention, as in the case of a conventional ratchet tensioner a plunger is slidable in a longitudinal direction in the plunger-accommodating hole of a tensioner body. The plunger has a front end that protrudes from the plunger-accommodating hole and a rear end within the plunger-accommodating hole. A plunger spring urges the plunger in the protruding direction, and a ratchet mechanism is provided for limiting movement of the plunger in a retracting direction. The ratched mechanism comprises a toothed rack having a set of rack teeth formed on the plunger and arranged in a longitudinal row, a pawl pivotably supported on the tensioner body for engagement with the rack teeth, and a spring biasing the pawl toward the rack teeth. 
         [0012]    However the improved ratchet tensioner according to the invention differs from the conventional ratchet tensioner in that it has a recess formed in the plunger at a location between the toothed rack and the front end of the plunger. The recess is deeper than the tooth gap bottoms of the rack. Thus, the pawl can enter the recess without engaging the bottom wall of the recess, and permits the plunger to move freely in both its protruding and retracting directions through a limited range. 
         [0013]    In a preferred embodiment, the plunger is provided with an additional tooth at a located between the recess and the front end of the plunger. The pawl is engageable with the additional tooth to hold the plunger in a fully retracted position, and prevents the plunger from jumping out of the plunger-accommodating hole prior to installation of the tensioner in an engine. 
         [0014]    A protrusion engageable by the pawl is preferably formed on the tensoner body to limit pivoting movement of the pawl, thereby preventing the pawl from contacting the bottom wall of the recess in the plunger when the pawl enters the recess. 
         [0015]    When the plunger is retracted to a position in which the pawl enters the recess of the plunger forward of the toothed rack, the ratchet mechanism does not function as a backstop mechanism. Thus, the punger is free to move forward and backward within a limited range. Excessive forward movement of the plunger is prevented, and proper tension can be maintained in the chain. However, as the plunger gradually moves forward with elongation of the chain over time, the pawl can engage the rack teeth, and the ratchet mechanism becomes operative to prevent the elongated chain from jumping the sprocket teeth. Thus, the ratchet tensioner of the invention can be adapted to an engine which requires a backstop mechanism having a non-operating range and an operating range, and in which the backstop mechanism becomes effective after the chain has become elongated over time. 
         [0016]    When an additional tooth is provided forward of the recess in the plunger, the pawl can be used to hold the plunger in a fully retracted condition when the tensioner is shipped and transported, and when the tensioner is being installed in an engine either during initial assembly of the engine or in the process of repair of the engine. 
         [0017]    A protrusion is provided to prevent excessive pivoting of the ratchet pawl, and thus prevents contact between the pawl and the bottom of the recess, especially when the bottom of the recess is flat. Furthermore, the protrusion prevents the pawl biasing spring from jumping out of its retaining hole as a result of excessive pivoting movement of the pawl. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a cross-sectional view of a ratchet type tensioner according to the invention; 
           [0019]      FIG. 2  is a cross-sectional view of the tensioner body; 
           [0020]      FIG. 3  is a cross-sectional view of the plunger: 
           [0021]      FIG. 4  is a cross-sectional view showing the tensioner in its initial state; 
           [0022]      FIG. 5  is an explanatory cross-sectional view showing the tensioner in a state in which its ratchet does not perform a backstop function; 
           [0023]      FIG. 6  is an explanatory cross-sectional view showing the tensioner in a state in which its ratchet performs a backstop function; and 
           [0024]      FIG. 7  is a cross-sectional view of a conventional ratchet type tensioner. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0025]    The ratchet type tensioner  1  of the invention differs from the conventional ratchet type tensioner  21  shown in  FIG. 7  in the shape of the rack formed on the outer circumferential surface of its plunger, and in that a protrusion  2   a  is provided in the tensioner body  2  for engagement by the pawl. 
         [0026]    As shown in  FIG. 1 , the tensioner  1  includes a plunger  4 , which is fits slidably into a plunger-accommodating hole  3  ( FIG. 2 ) of tensioner body  2  so that the plunger  4  can be urged in the protruding direction by a compression spring  6 . A ratchet pawl  9  is pivotably supported on a pivot shaft  8  provided in a hole  8   a  ( FIG. 2 ) in the tensioner body  2 , and biased by a spring  10  provided in hole  10   a  ( FIG. 2 ) in the tensioner body. The pawl  9  has a first pawl tooth  9   a  ( FIGS. 4 ,  5  and  6 ), which is engageable with rack teeth  7   c  of the rack  7 , as shown in  FIG. 6 , in order to to block retraction of the plunger  4 . Thus, the pawl  9  and the rack teeth  7   c  serve as a backstop mechanism  11 . A second pawl tooth  9   b  is provided at a location rearward with respect to the pawl tooth  9   a.    
         [0027]    As shown in  FIG. 3 , a tooth  7   a  is provided adjacent the front end of the rack for preventing jumping of the plunger out of the plunger-accommodating hole, and a recess  7   b  is provided immediately rearward of tooth  7   a . The recess in the plunger preferably has a flat bottom. The flat bottom can be formed easily by milling or the like, and provides clearance for the pawl without impairing the strength of the plunger. The rack teeth  7   c , which cooperate with pawl  9  to perform the backstop function, are provided rearward of recess  7   b . The bottom of recess  7   b  (which is a flat vertical surface in  FIG. 3 ) is deeper than the tooth gap bottoms of tooth  7   a  and rack teeth  7   c  so that the first pawl tooth not come into contact with the bottom of recess  7   b.    
         [0028]    As shown in  FIGS. 2 and 4 , a protrusion  2   a  is provided on the tensioner body  2  adjacent pawl  9  for the purpose of limiting the pivoting movement of the pawl under the biasing action of spring  10 , so that, when the ratchet  9  is pivoted by the biasing action of spring  10 , its tooth  9   a  does not come into contact with the bottom of recess  7   b . By preventing excessive pivoting movement of the pawl, the protrusion  2   a  also prevents the pawl biasing spring  10  from jumping out of its retaining hole  10   a  ( FIG. 2 ). 
         [0029]    As shown in  FIG. 4 , when the tensioner is shipped and transported, and when it is being mounted on an engine either during assembly or during engine repair, the first pawl tooth  9   a  of the ratchet  9  engages with tooth  7   a  at the front of the rack  7  so that the plunger  4  is maintained in a retracted condition in the plunger accommodating hole  3  and does not jump out of the hole. Locking pins (not shown) may be removably inserted into holes formed in the pawl and tensioner body to prevent the pawl from pivoting until the ratchet is installed and in proper engagement with a tensioner lever or the like. Suitable locking means for the ratchet are well known and disclosed, for example, in U.S. Pat. No. 6,612,951, the disclosure of which is incorporated by reference. 
         [0030]    When the tensioner  1  is mounted on an engine while in the state shown in  FIG. 4 , the ratchet  9  may be pivoted slightly counterclockwise to disengage the first pawl tooth  9   a  from tooth  7   a . The pawl  9  is then biased clockwise by spring  10  until it abuts protrusion  2   a  on the tensioner body as shown in  FIG. 1 . Then, the first pawl tooth  9   a  opposite to the flat portion of recess  7   b  does not engage any of the rack teeth on the plunger  4 , and the plunger can move freely in the protruding and retracting directions through a range  12  as shown in  FIG. 5 . As mentined above, since the pawl  9  abuts the protrusion  2   a , excessive pivoting movement of the pawl  9  is blocked, so that spring  10  cannot jump out of its hole  10   a  ( FIG. 2 ). 
         [0031]    When the first pawl tooth  9   a  is located within recess  7   b  but out of contact with the flat bottom of the recess, the plunger  4  is biased so as to protrude forward from the tensioner body  2 , and applies proper tension to the slack side of a chain (for example a timing chain, not shown) so that the chain cannot become excessively loose, vibrate, or disengage from its sprockets. Even if a large amount flutter is generated in the chain, the first pawl tooth  9   a  does not engage rack teeth  7   c . Thus, the backstop mechanism  11  does not function while the plunger is operating within range  12 , and excessive tension is not applied to the chain. 
         [0032]    When the chain becomes elongated after a period of operation in which the plunger is free to protrude and retract within range  12 , the plunger  4  gradually protrudes beyond range  12 , and the first pawl tooth  9   a  engages a rack tooth  7   c  as shown in  FIG. 6 . As the plunger moves forward still farther in response to elongation of the chain the first pawl tooth  9   a  passes over a rack tooth  7   c  to engage with a next rack tooth, and so on. Retraction of the plunger is blocked by the backstop mechanism  11  when the plunger is within a range  13  shown in  FIG. 6 . As a result, the plunger  4  applies proper tension to the chain as the chain gradually elongates, and thereby prevents jumping of the chain over the sprocket teeth. 
         [0033]    In the operation of the tensioner described above and shown in  FIGS. 1-6 , since the recess  7   b  is not enaged by the pawl, and rack teeth  7   c  are engageable by the pawl, the plunger can reciprocate freely in a region  12  during the intial stages of the useful life of a transmission chain, but the backstop mechanism becomes active when the chain becomes sufficiently elongated that the plunger extends into region  13 . Thus, excessive tensioning of the chain is prevented, but, as the chain becomes elongated, jumping of the sprocket teeth by the chain can be prevented. The tensioner, therefore, can be adapted to an engine which requires a backstop mechanism that has both a non-operating range, and an operating range in which it becomes effective as the chain becomes elongated. 
         [0034]    The ratchet type tensioner of the invention can be used with a pivoted tensioner lever arranged so that the plunger of the tensioner presses against a back surface of the lever, thereby urging a shoe on the lever into sliding contact with a traveling chain. Alternatively, a member adapted for sliding contact with a travelling chain can be mounted directly on the front end of the plunger.