Abstract:
A hydraulic tensioner comprising a hollow plunger having rack teeth formed on a portion of an outer circumferential surface, slidably received in a bore of the housing, where the plunger has an inner space to form a fluid chamber with the bore. A slider housing portion, having an inclined surface, located in the housing, receives a wedge-shaped slider having a ratchet portion adapted to engage with the rack teeth of the plunger and a second side surface. The wedge-shaped slider being slidable along the inclined surface of the slider housing portion in a direction crossing an axial centerline of the plunger. The hydraulic tensioner further includes a first spring biasing the plunger in a protruding direction, and a second spring biasing the slider such that the slider moves along the inclined slide surface of the slider housing portion. A “wedge-effect” prevents the plunger from moving backwards.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from Japanese application no: 2002-363571, filed Dec. 16, 2002, entitled “Hydraulic Tensioner” and is a continuation-in-part of application number 10/314,692, filed Dec. 9, 2002, entitled “Hydraulic Tensioner” which claims priority from Japanese application number 2001-381280, filed Dec. 14, 2001, entitled “Hydraulic Tensioner.” The aforementioned application(s) are hereby incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention pertains to the field of tensioners for imparting tension to a chain, belt or the like. More particularly, the invention pertains to a tensioner having a ratchet mechanism, including a slider, to prevent a plunger&#39;s retraction when hydraulic pressure is decreased.  
           [0004]    2. Description of Related Art  
           [0005]    A hydraulic tensioner generally includes a housing, a hollow plunger that is fitted slidably into a bore formed in the housing, where the plunger is biased in a protruding direction by a spring, and a fluid chamber defined by the bore of the housing and the plunger. During operation, the force is exerted by a chain or belt on the distal end of the plunger, which balances the elastic force of the spring and hydraulic force in the fluid chamber.  
           [0006]    When a force is exerted by a chain on the distal end of a plunger, an inadequate amount of hydraulic pressure is present in the chamber. This force is exerted at the time of engine start. As a result, the plunger is easily forced to retract into the housing, causing noise or oscillation. In order to prevent such a retraction of a plunger, various kinds of hydraulic tensioners with ratchet mechanisms have been proposed, such as shown in Japanese patent application laying-open publication Nos. 2000-136856 and 2001-304360.  
           [0007]    The hydraulic tensioner shown in Japanese Publication No. 2000-136856 includes a rack supported translatably in a longitudinal hole formed in a housing, a ratchet fitted slidably in a lateral cavity formed in the housing, a rack which is engaged by the ratchet, and a spring fitted in the cavity, which biases the ratchet in an engaging direction with the rack. The ratchet moves in a the direction perpendicular to the movement of the rack at the time of protrusion of the plunger. One problem associated with this tensioner is that the rack is discrete from the plunger, such that the tensioner has an increased number of components. Additionally, in the case of the chain having a relatively long center distance, there exists a demand for increasing a backlash of a ratchet mechanism in some degree, but the above-mentioned tensioner has a drawback in that the backlash of the ratchet mechanism cannot be enlarged more than the backlash between the rack and the ratchet.  
           [0008]    Japanese Publication No. 2001-304360 shows a hydraulic tensioner that includes a rack groove formed on the circumference of a piston. A claw member is adapted to engage the rack groove and slidably fits in a lateral hole. The lateral hole extends in a direction that crosses the axial centerline of the piston in the housing, via a predetermined clearance with an inside wall surface of the lateral hole. The tensioner also includes a spring that fits in the lateral hole that biases the claw member in the engagement direction with the rack. The claw member moves in the direction diagonal to the movement of the rack teeth at the time of protrusion of the piston. However, the amount of movement of the claw member does not fully contribute to the increase of the backlash. In this tensioner, it may be possible to enlarge the backlash of the ratchet mechanism more than the backlash present between the rack groove and the claw member by providing a predetermined clearance between the claw member and the inside wall surface of the lateral hole. However, increasing the backlash may cause chattering due to too great a clearance between the claw member and the lateral hole. Thus increasing the amount of clearance does not increase the backlash of the ratchet mechanism an adequate amount.  
           [0009]    The present invention provides a hydraulic tensioner with a ratchet mechanism, that can reduce the number of components, ensure an adequate backlash, securely prevent a plunger&#39;s retraction, and address the above problems found in the prior art.  
         SUMMARY OF THE INVENTION  
         [0010]    A hydraulic tensioner according to one aspect of the present invention comprises a housing having a central bore that opens at one end thereof, a hollow plunger slidably received in the bore of the housing, the plunger having an inside space that defines a fluid chamber with the bore and rack teeth that are formed at least at a portion of the outer circumference of the plunger. The rack teeth formed on the outer circumference of the plunger eliminate the need to provide a separate rack member and plunger, thereby reducing the number of components of the tensioner. The rearmost end of the rack teeth of the plunger is formed with a stop surface adapted to contact the rear end of the ratchet portion of the slider, such that the slider is sandwiched with the front facing wall at the time of the maximum extension of the plunger preventing the plunger from falling out of the tensioner.  
           [0011]    The rearmost end of the rack teeth of the plunger formed with a stop surface is also adapted to contact a rear end of the ratchet portion of the slider and to sandwich the ratchet portion with the plug member at the time of the maximum extension of the plunger so as to prevent the plunger from falling out of the tensioner. Thereby, prevention of fall-off of the plunger can be realized with a simple structure.  
           [0012]    The tensioner further includes a first biasing member that biases the plunger in a protruding direction, a wedge-shaped slider adapted to permit travel of the plunger in the protruding direction but to prevent travel of the plunger in a backward direction by developing a “wedge-effect” at the time of retraction of the plunger, and a second biasing member that biases the slider in a direction of engagement of the ratchet portion with the rack teeth. The slider has a ratchet portion engageable with the rack teeth of the plunger on a first side surface. The slider is slidable in a direction intersecting an axial centerline of the plunger along an inclined surface formed on a second side surface opposite the first side surface.  
           [0013]    The housing has a supporting block formed with a slider housing portion, the slider housing portion has an inclined slide surface that the inclined surface of the slider slidably contacts, and a second biasing member biases the slider in such a way that the slider moves along the inclined slide surface. The slider housing portion has a stop surface adapted to contact a rear side surface of the slider to block travel of the slider in a backward direction. Thereby, the movement of the slider, and thus the plunger during retraction of the plunger can be securely prevented. The slider housing portion also has a front facing wall oppositely disposed to a front side surface of the slider, and one end of the second biasing member contacts the front facing wall and the other end thereof contacts the front side surface of the slider. At the time of movement of the slider, the slider is guided by the inclined slide surface formed in the slider housing portion of the supporting block under the action of the force of the second biasing member, thus making the movement of the slider smoothly.  
           [0014]    The second biasing member may be a coil spring where the axial centerline is disposed parallel to the inclined side surface of the slider housing portion. In this case, the biasing force of the coil spring can be transmitted to the slider without any loss of contact between the inclined surface of the slider and the slide surface of the supporting block maintained. Thereby, the slider and the second biasing member are integrated and housed inside the supporting block, thus facilitating assembly of the tensioner.  
           [0015]    When the plunger travels in the protruding direction during operation of the tensioner, the slider slides along the inclined surface through engagement between the rack teeth of the plunger and the ratchet portion of the slider, thereby increasing the whole backlash, correspondingly to the amount of sliding movement, allowing an adequate backlash to be secured. The plunger is prevented from retracting by a “wedge-effect,” which is present at the time of backward movement of the plunger by a wedge shaped slider.  
           [0016]    The housing further comprises a first through hole adapted to insert a disengaging pin thereinto between the rear side surface of the slider and the rear stop surface of the slider housing portion in order to disengage engagement of the ratchet portion of the slider with the rack teeth of the plunger. Also, the housing has a second through hole adapted to insert a retaining pin thereinto between the front side surface of the slider and the front facing wall of the slider housing portion in order to retain the plunger in a retracted state. Engagement of the slider with the rack teeth of the plunger can be easily disengaged by inserting the disengaging pin into the first through hole. By this disengagement, a worker can easily push the plunger into the housing before transportation of the tensioner. Then, from this condition, by inserting the retaining pin into the second through hole, the retracted condition of the plunger can be maintained with ease, thereby facilitating assembly of the tensioner into a chain. Additionally, after assembly into the chain, the tensioner is placed in an operatable state by removing the retaining pin.  
           [0017]    The disengaging pin and the retaining pin are preferably the same pin with a tapped distal end. In this case, insertion of the pin into a clearance between the rear end surface of the slider and the stop surface of the slider housing portion, and into a clearance between the front end surface of the slider and the wall surface of the slider housing portion can be conducted with ease. Also, the use of only one pin as a disengaging pin and a retaining pin decreases the number of components.  
           [0018]    A check valve may be provided at a bottom portion of the central bore of the housing to permit fluid flow into the chamber but to block reverse flow of fluid, thereby, securely preventing the retraction of the plunger.  
           [0019]    The housing may also have a recess that adjoins and intersects the bore. The recess has an inclined slide surface that is slidably adapted to contact the inclined surface of the slider. The recess has a pair of flat surfaces oppositely disposed to each other on opposite sides of the slide surface, and opposite side surfaces of the slider is slidably supported by each of the flat surfaces of the recess. In this case, at the time of travel of the slider, the slider is laterally guided in the recess by each of the flat surfaces, thereby preventing lateral swinging movement of the slider. As a result, engagement of the ratchet portion of the slider with the rack teeth of the plunger can be securely maintained.  
           [0020]    At the back of the slide surface of the recess is formed a stop surface adapted to contact the rear end surface of the slider to block travel of the slider in a backward direction, thereby, securely preventing the travel of the slider, and thus, the plunger at the time of retraction of the plunger.  
           [0021]    In front of the slide surface of the recess is provided a plug member opposed to a front end surface of the slider. One end of the second biasing member is engaged with the plug member and the other end of the second biasing member is engaged with the front end surface of the slider.  
           [0022]    The second biasing member biases the slider in such a way that the slider moves along the slide surface of the recess. In this case, at the time of movement of the slider, the slider is guided under the force of the second biasing member by the inclined slide surface of the recess in the housing, thereby allowing for a smooth movement of the slider. Furthermore, in this case, a supporting block does not need to be provided in order to support the slider, thereby simplifying the structure. The second biasing member is a coil spring and an axial centerline of the coil spring is disposed parallel to the slide surface of the recess. Thereby, force of the coil spring can be transmitted to the slider without any loss with contact between the inclined surface of the slider and the slide surface of the recess maintained. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0023]    [0023]FIG. 1 is a longitudinal sectional view of a hydraulic tensioner according to an embodiment of the present invention.  
         [0024]    [0024]FIG. 2 is an enlarged view of a ratchet mechanism of the hydraulic tensioner in FIG. 1.  
         [0025]    [0025]FIG. 3 is a schematic illustrating a ratchet mechanism and a disengaging pin.  
         [0026]    [0026]FIG. 4 is a schematic illustrating a ratchet mechanism and a retaining pin.  
         [0027]    [0027]FIG. 5 is a schematic illustrating operation of the ratchet mechanism at the time of protrusion of the plunger.  
         [0028]    [0028]FIG. 6 is a schematic illustrating operation of the ratchet mechanism at the time of protrusion of the plunger.  
         [0029]    [0029]FIG. 7 is a schematic illustrating operation of the ratchet mechanism at the time of protrusion of the plunger.  
         [0030]    [0030]FIG. 8 is a schematic illustrating operation of the ratchet mechanism at the time of protrusion of the plunger.  
         [0031]    [0031]FIG. 9 is a schematic illustrating operation of the ratchet mechanism at the time of protrusion of the plunger.  
         [0032]    [0032]FIG. 10 is a schematic illustrating operation of the ratchet mechanism at the time of retraction of the plunger.  
         [0033]    [0033]FIG. 11 is an enlarged view of a ratchet mechanism of the hydraulic tensioner according to an alternative embodiment of the present invention.  
         [0034]    [0034]FIG. 12 is a cross sectional view of FIG. 11 taken along line XII-XII.  
         [0035]    [0035]FIG. 13 is a schematic illustrating operation of the ratchet mechanism at the time of the maximum extension of the plunger. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]    As shown in FIG. 1, a hydraulic tensioner  1  includes a housing  2 , a hollow plunger  3  fitted slidably in a bore  2 a formed in the housing  2 , and a spring  4  as a first biasing member that biases the hollow plunger  3  in a protruding direction from the bore  2 .  
         [0037]    In the housing  2 , a fluid chamber  20  is formed by an inner space  3   a  formed in the plunger  3  and an inside wall surface of the bore  2   a . A check valve  5  is provided at the bottom portion of the bore  2   a  in the housing  2  to permit the flow of fluid into the chamber  20  but to block fluid flow in a reverse direction. The check valve  5  is comprised of a ball  50 , a ball seat  51  contacted by the ball  50 , and a ball spring  52  biasing the ball  50  toward the ball seat  51 . Any other suitable structure can be employed to form a check valve. The housing  2  also has a fluid passage  6  to connect the chamber  20  with an outer source of pressurized fluid (not shown).  
         [0038]    Rack teeth  3   b  are formed on a portion of the outer circumference of the plunger  3 . A ratchet mechanism  7  is provided at an open portion of the bore  2   a  to prevent retraction of the plunger  3 . The ratchet mechanism  7 , shown in FIG. 2, is comprised of a supporting block  70  that is disposed in a notch portion  2   b  formed at the open portion of the bore  2   a , a wedge-shaped slider  72  is disposed in a slider housing portion  71  concavely formed in the supporting block  70  where the ratchet portion  72   a  is engagable with the rack teeth  3 b of the plunger  3  on a first side surface, and a spring  73  as a second biasing member biases the slider  72  in such a way that the ratchet portion  72   a  of the slider  72  engages with the rack teeth  3   b.    
         [0039]    The supporting block  70  has a through hole  70   a  to insert a bolt (not shown) thereinto, and is fixed in the notch portion  2   b  of the housing  2  via the bolt. The slider  72  has an inclined surface  72   b  at a second side surface opposite the first side surface containing the ratchet portion  72   a . The slider housing portion  71  has an inclined slide surface  71   a  that the inclined surface  72   b  of the slider  72  slidably contacts. The slide surface  71   a  extends in a direction that intersects the axial centerline L of the plunger  3 . The distance between the slide surface  71   a  and the axial centerline L of the plunger  3  gradually decreases as it goes toward the rearward direction of the plunger  3  (i.e. lefthand direction of FIG. 2). The angle between the slide surface  71   a  and the axial centerline L of the plunger  3  is preferably 45 degrees or less. In this embodiment, the angle is predetermined to be about 30 degrees to ensure an adequate backlash in the ratchet mechanism  7 .  
         [0040]    The slider  72  has a rear end surface  72   c  and a front end surface  72   d  oppositely disposed between the first and second side surfaces. The slider housing portion  71  has a stop surface  71   b  disposed opposite the rear end surface  72   c  of the slider  72  and is adapted to prevent the movement of the slider  72  by contacting the rear end surface  72   c  of the slider  72  at the time of retraction of the plunger  3 . The slider housing portion  71  also has a wall surface  71   c  contraposed to the front end surface  72   d  of the slider  72 . Formed on the front end surface  72   d  of the slider  72 , one end of the spring  73  engages with a concave portion  72   e  and the other end thereof engages with a concave portion  71   d  formed on the wall surface  71   c . In this embodiment, a coil spring is preferably employed as the spring  73 . An axial centerline m of the spring  73  is preferably disposed parallel to the slide surface  71   a  of the slider housing portion  71 , so that contact of the inclined surface  72   b  of the slider  72  with the slide surface  71   a  may be maintained and the force of the spring  73  transmitted to the slider  72  without any loss.  
         [0041]    The ratchet mechanism  7  permits the travel of the plunger  3  in the protruding direction (i.e. righthand direction of FIG. 2), and prevents the retraction of the plunger  3  by developing a “wedge-effect” between the slider housing portion  71  and the outer circumference of the plunger  3  at the time of retraction of the plunger  3 . Due to the integration of the slider  72  and the spring  73  into the supporting block  70 , the assembly of the tensioner is conducted with ease.  
         [0042]    The housing  2  has a first and second through hole  25 ,  26  formed therein to penetrate the outer circumference of the housing  2  in a direction perpendicular to the axial direction (i.e. a direction perpendicular to the page). The first through hole  25  is disposed on the rear end side of the slider housing portion  71  and the second through hole  26  is disposed on the front end side of the slider housing portion  71 . As shown in FIG. 3, the first through hole  25  is a hole adapted to insert a disengaging pin  10  thereinto to disengage the engagement present between the ratchet portion  72   a  of the slider  72  and the rack teeth  3   b  of the plunger  3 . The second through hole  26 , shown in FIG. 4, is a hole adapted to insert a retaining pin  11  thereinto to maintain the plunger  3  in a retracted state. Both the disengaging pin  10  and the retaining pin  11  have tapered distal ends. In addition, a single, common pin may be provided for the disengaging pin  10  and the retaining pin  11 , thus reducing the number of components of the tensioner  1 .  
         [0043]    When the engine starts, engine oil is supplied in the fluid chamber  20  of the housing  2  through the check valve  5  from the oil passage  6 . When the engine oil fills the chamber  20 , hydraulic pressure of the oil causes the plunger  3  to move in a protruding direction. As the plunger  3  starts to move in the protruding direction from the engaging state of the rack teeth  3   b  of the plunger  3  with the ratchet portion  72   a  of the slider  72 , shown in FIG. 5, the slider  72  starts to travel, in the direction marked by the arrow, against the force of the spring  73  through the engagement of the ratchet portion  72   a  with the rack teeth  3   b  along the slide surface  71   a  of the supporting block  70 . In FIGS. 5 and 6- 10 , a dot is marked on a portion of the rack teeth  3   b  of the plunger  3  for the purpose of visualizing the movement of the plunger  3 . As the slider  72  moves, shown in Figure.  6 , the ratchet portion  72   a  of the slider  72  starts to ride on the rack teeth  3   b  of the plunger  3 . When the slider  72  further travels in accordance with the movement of the plunger  3 , as shown in FIG. 7, the ratchet portion  72   a  of the slider  72  rides on the tip of the rack teeth  3   b  of the plunger  3 . At this time, a clearance Bm between the rear end surface  72   c  of the slider  72  and the stop surface  71   b  of the supporting block  70  is the maximum backlash of the ratchet mechanism  7 . During operation of the tensioner, the slider  72  travels within the range of the maximum backlash Bm.  
         [0044]    When the plunger  3  travels further in the protruding direction from the state shown in FIG. 7, the rack teeth  3   b  of the plunger  3  pass over the crest of the teeth of the ratchet portion  72   a  of the slider  72 , as shown in FIG. 8. Then, the slider  72  begins to travel in a reverse direction or in the direction marked by the arrow in FIG. 8, along the slide surface  71   a  of the supporting block  71  by an elastic force of the spring  73 . Thereafter, shown in FIG. 9, the slider  72  stops traveling when the teeth of the ratchet portion  72   a  of the slider  72  engages the rack teeth  3   b  of the plunger  3 . In the case of further movement of the plunger  3  in the protruding direction, the action shown in FIG. 6- 9  is conducted repeatedly. At the time of maximum extension of plunger  3 , a stop surface  3   d  (shown in FIG. 1) is formed at the rearmost end of rack teeth  3   b  of plunger  3  contacts the rear end of the ratchet portion  72   a  of the slider  72  and sandwiches slider  72  with the wall surface  71   c  of the slider housing portion. The plunger  3  is restrained from further movement in a protruding direction and thus plunger  3  is prevented from falling out of the tensioner.  
         [0045]    When tension in the chain increases and a compressive force acts upon the distal end portion  3   c  by the chain, the ball check valve  6  closes and hydraulic pressure in the chamber  20  is imparted upon the plunger  3  as a resistant force, thereby preventing the backward movement of the plunger  3 .  
         [0046]    At the time of backward movement of the plunger  3 , shown in FIG. 10, the slider  72  travels in the backward direction of the plunger  3  along the slide surface  71   a  of the supporting block  70  through engagement between the rack teeth  3   b  of the plunger  3  and the ratchet portion  72   a  of the slider  72 . Thus, the wedge-shaped slider  72  develops a “wedge effect” between the plunger  3  and the slide surface  71   a  of the slider housing portion  71  of the supporting block, thereby preventing the rearward movement of the plunger  3  securely.  
         [0047]    During the operation of the tensioner  1 , since the slider  72  travels along the inclined slide surface  71   a  of the supporting block  70 , the slider  72  travels within the range of the maximum backlash Bm, which is greater than a backlash between the ratchet portion and the rack teeth  3   b . Thereby, the amount of the backlash of the whole ratchet mechanism can be increased and adequate backlash can be secured.  
         [0048]    In the case of inadequate fluid pressure in the chamber  20 , such as at the engine start, a compressive force is applied to the distal end portion  3   c  of the plunger  3  from the chain, similarly, the wedge-shaped slider  72  develops the “wedge-effect” between the plunger  3  and the slide surface  71   a  of the slider housing portion  71  of the supporting block  70  and the rear end surface  72   c  of the slider  72  contacts the stop surface  71   b  of the supporting block  70 , shown in FIG. 10. Thereby, the movement of the slider  72  is prevented and the retraction of the plunger  3  can be securely prevented.  
         [0049]    In the case of the retracted state of the plunger  3  with the plunger  3  disengaged from the slider  72 , such as at the time of transportation or installation of the tensioner, as shown in FIG. 3, a disengaging pin  10  is inserted into the first through hole  25  formed in the housing  2 . Thus, the rear end surface  72   c  of the slider  72  and the stop surface  71   b  of the slider housing portion  71  are separated, and the engagement between the ratchet portion  72   a  of the slider  72  and the rack teeth  3   b  of the plunger  3  is disengaged. From this condition, by pushing the plunger  3  into the housing, the plunger  3  may be placed in a retracted state with ease. With the distal end portion  3   c  of the plunger  3  pressed so as not to protrude the plunger  3 , the disengaging pin  10  is extracted from the first through hole  25  and the retaining pin  11  is inserted into the second through hole  26  formed in the housing, as shown in FIG. 4. Thereby, the ratchet portion  72   a  of the slider  72  and the rack teeth  3   b  of the plunger  3  are re-engaged, thus maintaining the retracted state of the plunger  3 . After the tensioner is fitted to the chain, the tensioner is placed in an operatable condition by extracting the retaining pin  11  from the second through hole  26 .  
         [0050]    According to the present invention, the rack teeth  3   b  are formed around the outer circumference of the plunger  3 , thus eliminating the necessity for providing a rack member aside from the plunger  3 , which reduces the number of components. Also, since the slider  72  travels along the inclined slide surface  71   a  of the slide housing portion  71   a , a sufficient backlash for the whole ratchet mechanism may be acquired. Moreover, since the wedge-shaped slider  72  develops a “wedge-effect” between the slide surface  71   a  and the plunger  3  at the time of retraction of the plunger  3 , the retraction of the plunger  3  may be securely prevented.  
         [0051]    After the tensioner is fitted to the chain, the tensioner is placed in an operatable condition by extracting the retaining pin  11  from the second through hole  26  to cause the plunger  3  and the slider  72  to protrude by the force of the spring  4 .  
         [0052]    In an alternative embodiment of the present invention, the rack teeth  3   b  are formed around the outer circumference of the plunger  3 , thus eliminating the necessity for providing a rack member aside from the plunger  3 , and reducing the number of components. Also, the slider  72  travels along the inclined slide surface  71   a  of the slider housing portion  71 , a sufficient backlash for the whole ratchet mechanism can be acquired. Moreover, since the wedge-shaped slider  72  develops a wedge-effect between the slide surface  71   a  and the plunger  3  at the time of retraction of the plunger  3 , the retraction of the plunger  3  can be securely prevented.  
         [0053]    In the above-mentioned embodiment, a ratchet mechanism is disposed in the slider housing portion of the supporting block that is provided discretely from the housing. The application of the present invention is not limited to this example. A ratchet mechanism may be provided directly in the housing, as shown in FIG. 11 to  13 . In these drawings, like reference numbers indicate identical or functionally similar elements.  
         [0054]    As shown in FIG. 11, at the opening portion of the bore  2   a  of the housing  2  are formed recesses  8 ,  9  that adjoins and intersects the bore  2   a . The recess  8  is positioned on the opening end side of the bore  2   a , and the recess  9  is positioned at the back of the recess  8 . A cross sectional shape of the recess  8 , shown in FIG. 12, is formed of a semicircular portion and flat portions  8   a ,  8   b  that are connected to the semicircular portion and disposed parallel to each other. Similarly, a cross sectional shape of the recess  9  is formed of a semicircular portion and flat portions  9   a ,  9   b  that are connected to the semicircular portion and disposed parallel to each other. In this example, a distance between the flat portions  8   a  and  8   b  is the same as that of the distance between the flat portions  9   a  and  9   b . The flat portion  8   a  is flush with the flat portion  9   a  and the flat portion  8   b  is flush with the flat portion  9   b . The distance between the flat portions  8   a  and  8   b  may be greater than that of the distance between the flat portions  9   a  and  9   b.    
         [0055]    A plug member  11  is press fit into the recess  8  and a slider  12  is provided in the recess  9 . Between the plug member  11  and the slider  12  is provided a spring  13  as a second biasing member. The plug member  11 , slider  12 , and spring  13  form a ratchet mechanism  10  to prevent retraction of the plunger  3 .  
         [0056]    The slider  12  is a generally wedge-shaped member. The slider  12  has a ratchet portion  12   a  formed at a first side surface that is engageable with the rack teeth  3   b  of the outer circumference of the plunger and an inclined surface  12   b  formed at a second side surface opposite the first side surface. The inclined surface  12   b  is in slidable contact with the semicircular portion that acts as an inclined slide surface in the recess  9 . Opposite side surfaces of the slider  12  is in contact with each flat portion  9   a ,  9   b  of the recess  9 , shown in FIG. 12. The slider  12  is thus prevented from swinging in the lateral direction in the recess  9 , thereby maintaining the engagement of the ratchet portion  12   a  of the slider  12  with the rack teeth  3   b  of the plunger  3 .  
         [0057]    The slider  12  has an axial portion  12   c . The plug member  11  has a hole  11   a  formed therein. One end of the spring  13  is engaged with the axial portion  12   c  of the slider  12  and the other end of the spring  13  is fitted into the hole  11   a . The spring  13  biases the slider  12  in such a way that the ratchet portion  12   a  engages with the rack teeth  3   b  of the plunger  3 . An axial centerline m of the spring  13  is preferably parallel to the centerline of the recess  9  and intersects the axial centerline L of the plunger  3  at an acute angle.  
         [0058]    In such a manner, the ratchet mechanism  10  permits an outward movement of the plunger  3  in a protruding direction (or right-hand direction of FIG. 11) and prevents retraction of the plunger  3  by exercising a wedge-effect between the outer circumference of the plunger  3  and the recess  9  at the time of retraction of the plunger  3 .  
         [0059]    The operation of the ratchet mechanism  10  is similar to that of the ratchet mechanism  7  of the above-mentioned embodiment, and it is not explained in detail here.  
         [0060]    At the time of the maximum extension of the plunger  3 , shown in FIG. 13, a stop surface  3   d  formed at the outermost end of the rack teeth  3   b  of the plunger  3  comes into contact with the rear end of the ratchet portion  12   a  of the slider  12  and sandwiches the slider  12  with the plug  11 . Thereby, further protrusion of the plunger  3  is restricted and thus, the plunger can be prevented from falling out of the tensioner.  
         [0061]    In this case as well, similar to the above-mentioned embodiment, the rack teeth  3   b  is formed around the outer circumference of the plunger  3 , thus eliminating the necessity for providing a rack member aside from the plunger  3  and reducing the number of components. Also, the slider  12  travels along the inclined slide surface of the recess  9 , a sufficient backlash for the whole ratchet mechanism is acquired. Moreover, since the wedge-shaped slider  12  develops a wedge-effect between the slide surface of the recess  9  and the plunger  3  at the time of retraction of the plunger  3 , the retraction of the plunger  3  can be securely prevented.  
         [0062]    Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.