Abstract:
In a ratchet-type tensioner having a plunger with rack teeth formed thereon and a piston with ratchet teeth that engage the rack teeth, the ratchet teeth can be disengaged from the rack teeth by insertion of a tool through a tool insertion hole in the tensioner housing and exerting a prying action on the toothed end of the piston to move the piston away from the plunger. Alternatively if an oblique camming surface is formed on the toothed end of the piston, the ratchet teeth can be disengaged from the rack teeth by pushing the tool inward against the oblique camming surface. The piston and the hole in which it slides are shaped to prevent rotation of the piston. The ratchet teeth on the piston and the rack teeth on the plunger can be oblique to allow setback of the plunger when excessive force is exerted on the plunger.

Description:
FIELD OF THE INVENTION 
     This invention relates to a ratchet-type tensioner for applying tension to an endless, flexible, transmission medium such as a timing chain for driving one or more camshafts in an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     A typical ratchet-type tensioner applies tension to a timing chain by exerting force on the chain by means of a plunger which is movable along an advancing and setback direction, in a plunger accommodating hole in the tensioner housing. The plunger is biased in the advancing direction by a spring and external hydraulic pressure provided by oil in a chamber formed by the tensioner housing and the plunger. 
     A typical prior art ratchet-type tensioner is disclosed in Japanese Utility Model No. 2559664, and shown in  FIG. 16 . In the tensioner  500 , a plunger  514  is slidable in a plunger-accommodating hole in a housing  512 , and protrudes from the housing, being biased in the advancing direction by a spring  518  and by oil pressure within a chamber  516  formed by the plunger and the housing. 
     A piston  526  slides in the housing  512  in a direction orthogonal to the direction in which the plunger  514  slides. An oil sub-chamber  520  is formed by the piston  526  and the housing  512 , and an oil passage  544  supplies oil under pressure to the oil sub-chamber  520 , urging the piston away from the plunger  514 . A spring  534  biases the piston  526  toward the plunger, opposing the force exerted by the oil in the sub-chamber  520 . Spring  534  is located within an air chamber  528  on the side of piston  526  opposite from the sub-chamber  520 . An air hole  532  in communication with the air chamber  528  is closable by a rod  524 , to which the piston  526  is attached, when the piston  526  is moved away from the plunger against the biasing force exerted by spring  534  by hydraulic pressure in sub-chamber  520  and the piston  526  moves against a biasing force of the second spring  534 . 
     A rack of teeth  538  is provided on the plunger  514 , and a plurality of ratchet teeth  536 , capable of engaging the rack teeth  538 , is provided at the end of rod  524  opposite form the end that is arranged to close off air hole  532 . Surfaces of teeth  536  and  538  for blocking retraction of the plunger are formed at a right angle to the direction in which the plunger  514  moves. 
     The prior art tensioner requires precise assembly of the cap  530 , which is sealed by press-fitting after the rod  524  has been installed within the air chamber  528 . Thus, the cap  530  cannot be readily removed, preventing ready disengagement of the rack teeth  538  and ratchet teeth  536  for maintenance of the tensioner  500  and timing chain (not shown). 
     Further, because the prior art tensioner  500  comprises a plunger  514  and piston  526 , engagement of the rack teeth  539  and ratchet teeth  536  causes a twist in the rod  524 , which, as the plunger  514  vibrates while the engine is driving, can cause chipping of the teeth. In addition, the twist in the rod  524  can lead to faulty operation of the ratchet mechanism, and to erroneous assembly during manufacture and maintenance of the tensioner. 
     Because the tooth surfaces in the setback direction are perpendicular to the advance/setback direction of the plunger  514 , setback of the tensioner is prevented even when it may be desirable, i.e., when tension of the chain becomes excessive due to temperature change and other causes. Thus, the prior art tensioner  500  experiences excessive tension, wear, noise and eventual seizing. 
     To alleviate the problems of excessive tension, wear, noise, etc., a predetermined backlash is provided in the ratchet mechanism, corresponding to a presumed maximum distance of the movement of the plunger  514  in the setback direction caused by the excessive tension. However, the larger the allowed backlash distance, the larger the “flapping noise” generated on starting the engine. 
     Prior steps to remedy these problems have been the addition of an orifice mechanism and an oil-reserve mechanism or replacement of the spring  518  with a higher-load spring. However, this increases the number of parts, the cost of production, and the size of the tensioner itself. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention aims at solving the problems by providing a ratchet-type tensioner that allows ready release of engagement of the ratchet mechanism with the plunger for maintenance; allows for secure and stable operation of the ratchet mechanism; reduces flapping noise on starting an engine which has sat for a while; and prevents seizure of the plunger by allowing setback when excessive tension occurs. 
     In order to resolve the aforementioned problems, the invention provides, in a first aspect, a ratchet-type tensioner, which includes a housing, a plunger, a high-pressure oil chamber formed by the plunger and the housing, and a ratchet mechanism. The plunger is supported by the housing and moves in opposite advancing and setback directions in a plunger-receiving hole. The plunger has rack teeth provided thereon, and is arranged to advance along the advancing direction to apply tension to a traveling transmission medium engaged with rotating members. The ratchet mechanism is capable of restricting the plunger from setting back due to a reaction force acting in a setback direction from the transmission medium. The ratchet mechanism includes a piston-receiving hole within the housing, and has a front end adjacent the plunger-receiving hole and a rear end remote from the plunger-receiving hole. The piston receiving hole also has an outer circumferential surface. The ratchet mechanism also comprises a piston, which is slidable in the piston-receiving hole and movable in a sliding direction transverse to the advancing and setback directions. The piston has ratchet teeth provided thereon, which are engageable with the rack teeth of the plunger. A piston-biasing spring is provided for biasing the piston in an engaging direction along the sliding direction so that the ratchet teeth engage with the rack teeth. The ratchet mechanism further includes a plug fitted in the rear end of the piston-receiving hole. The piston-biasing spring bears against the plug. 
     The ratchet mechanism also includes a ratchet releasing mechanism for releasing engagement of the ratchet teeth with the rack teeth. The ratchet releasing mechanism includes a tool engaging portion provided at the front end of the piston to abut and engage with an end of a tool and a tool inserting hole provided in the housing. The tool can be inserted through the hole and engaged with the tool engaging portion, thereby pushing the piston away from the plunger so that the ratchet teeth disengage the rack teeth. The ratchet releasing mechanism allows for ease of maintenance of the tensioner, for positioning and replacing parts, and loosening the timing chain when desirable. 
     In one aspect of the invention, the tool engaging portion of the piston includes a stepped abutment surface formed at the front end of the piston, between the ratchet teeth and the outer circumferential surface of the piston. The tool inserting hole is positioned to allow the piston to be pushed away from the plunger by a prying action exerted against the stepped abutment surface by the tool. Therefore, it is possible to release the engagement of the ratchet teeth and the rack teeth easily. 
     In another aspect, the tool engaging portion of the piston includes a tapered abutment surface formed on the piston adjacent the ratchet teeth on the piston. The pin inserting hole is positioned to allow the piston to be pushed away from the plunger by a camming action exerted against the tapered abutment surface by the tool. Therefore, it is possible to release the engagement of the ratchet teeth and the rack teeth readily. 
     Preferably, the ratchet mechanism restricts the plunger from setting back by engaging the ratchet teeth with the rack teeth when the reaction force is a first reaction force generated when the tension in the traveling transmission medium is less than a predetermined excessive tension, but allows the plunger to set back by sliding of the piston in a disengaging direction to disengage the ratchet teeth from the rack teeth when the reaction force is a second, and greater, reaction force generated when the tension in the traveling transmission medium is at least as great as the predetermined excessive tension. The ratchet biasing force is greater than a first component force in the sliding direction, generated from the first reaction force, but smaller than a second component force in the sliding direction, generated from the second reaction force. Accordingly, it is possible to reduce the noise of flapping the transmission medium by restricting movement, thereby blocking setback displacement when the reaction force is at the first level. In addition, no special high load-accommodating plunger-biasing spring is required, the tensioner itself can be downsized, and the number of parts and its manufacturing cost can be reduced. 
     Because the biasing force of the plunger biasing spring is smaller than the component force in the sliding direction of the plunger when the tension in the transmission medium becomes excessive, the second force component acting on the teeth of the piston when the tension is excessive enables the ratchet teeth to disengage the rack teeth on the plunger. The plunger is then set back until the biasing force of the ratchet biasing means becomes greater than the second force component. Consequently it is possible to prevent seizing of the plunger by allowing setback displacement without causing backlash due to excessive tension in the traveling transmission medium. It is also possible to achieve reliable prevention of seizing of the plunger by adjusting the biasing force exerted by the ratchet-biasing means, thereby controlling the timing of tooth disengagement when the transmission medium comes under excessive tension. 
     The rack teeth are preferably concave and convex teeth formed by stop surfaces facing in the setback direction and sliding surfaces facing in the advance direction. The sliding surfaces are inclined relative to the sliding direction and face in a disengaging direction opposite from the engaging direction. The ratchet teeth of the piston are preferably concave and convex teeth formed by stop counterface surfaces inclined toward the advance direction when proceeding in the disengaging direction, and sliding counterface surfaces inclined toward the setback direction when proceeding in the disengaging direction. With this tooth structure, it is possible to prevent the jumping of rack teeth that occurs in the case of rack teeth having stop surfaces that are not inclined, even when retardation of the movement of the piston occurs due to its inertia. 
     When the second reaction force, i.e., a force that sets back the plunger when excessive tension is generated in the transmission medium, acts on the stop surfaces of the ratchet teeth, a component force acts on the stop counterface surfaces of the ratchet teeth, causing the piston to slide in a direction away from the plunger so that its teeth disengage the rack teeth of the plunger. Then, the rack teeth of the plunger slide over the counterface surfaces, clearing the ratchet teeth with which they are engaged, allowing the plunger to set back by a distance corresponding to one tooth. Setback displacement takes place smoothly without restriction on the movement of the plunger in the setback direction. Wear and damage, such as chipping of the rack teeth and ratchet teeth, which would otherwise occur as a result of excessive tension, are prevented. Excessive impact on the ratchet-biasing spring is also avoided. The ratchet mechanism therefore exhibits excellent durability. 
     The inclination angle of the stop surface is preferably smaller than the inclination angle of the sliding surface, the inclination angles being measured relative to the sliding direction of the piston. As a result, it is possible to block the plunger from setting back as a result of the first, lower level, reaction force, and thereby reduce noise caused by the setting back of the plunger. 
     Preferably, the ratchet mechanism comprises a means for preventing rotation of the piston in the circumferential direction of the piston receiving hole when the ratchet teeth and rack teeth are engaged. The rotation preventing means stops the outer circumferential surface of the piston from turning against the inner circumferential surface of the piston-receiving hole regardless whether the rack teeth engage the ratchet teeth, even when the plunger vibrates as the engine is driven. Accordingly, operation of the piston is secure and stable, and engagement of the ratchet teeth with the rack teeth is precise across their entire width because twist in the ratchet mechanism is avoided. 
     In one embodiment, the means for preventing movement is provided by a convex strip on the outer circumferential surface of the piston and a concave groove in the piston-receiving hole in the housing. The groove extends along the sliding direction of the piston and receives the convex strip. Accordingly, it is possible to slide the piston while completely stopping the outer circumferential surface thereof from turning against the inner circumferential surface of the piston-accommodating hole, allowing the piston to operate smoothly. Production of the piston is simplified as compared to an engagement matching mechanism utilizing a convex strip in the ratchet-accommodating hole. 
     Preferably, the length of the piston is greater than the outer diameter thereof. Accordingly, inclination is suppressed, and biasing wear is prevented, even when excessive tension is applied to the piston. 
     In one aspect, the piston-biasing spring is inserted within a spring-accommodating hole formed in the piston along the sliding direction. As compared to a piston in which the ratchet biasing spring is fitted around the outer circumferential surface of the piston, the size of the spring can be reduced and assembly is simplified. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic elevational view of an engine timing transmission incorporating a ratchet-type tensioner in accordance with the invention; 
         FIG. 2A  is a longitudinal cross-section of a ratchet-type tensioner according to the invention; 
         FIG. 2B  is an enlarged view of a part of the ratchet-type tensioner surrounded by a broken line in  FIG. 2A ; 
         FIG. 3  is an enlarged view of rack teeth and ratchet teeth as shown in  FIG. 2B ; 
         FIG. 4  is a cross-section view taken on plane  4 - 4  in  FIG. 2B ; 
         FIG. 5  is an exploded view of a piston, a piston biasing spring and a spring stopping plug; 
         FIG. 6  is a schematic diagram showing the engagement of rack teeth on the plunger with ratchet teeth as the plunger is advancing in starting an engine; 
         FIG. 7  is a schematic diagram showing the engagement of rack teeth on the plunger with ratchet teeth as the plunger is blocked from setting back in starting the engine; 
         FIG. 8  is a schematic diagram showing the engagement of rack teeth on the plunger with ratchet teeth as the plunger begins to set back when the tension of the timing chain becomes excessive; 
         FIG. 9  is a schematic diagram showing the disengagement of rack teeth and ratchet teeth as the plunger sets back when the tension of the chain is excessive; 
         FIG. 10  is a schematic diagram showing the engagement of rack teeth with ratchet teeth after the ratchet teeth disengage the rack teeth with which they were previously engaged, as the plunger sets back when the tension in the chain is excessive; 
         FIG. 11A  is a longitudinal cross section of a plunger of the invention including the ratchet releasing mechanism showing a releasing of the ratchet; 
         FIG. 11B  is an enlarged view of a part of the plunger surrounded by a broken line in  FIG. 11A ; 
         FIG. 12A  is a longitudinal cross-section of a ratchet-type tensioner according to the invention; 
         FIG. 12B  is an enlarged view of a part of the ratchet-type tensioner surrounded by a broken line in  FIG. 12A ; 
         FIG. 13  is a cross-section view taken on plane  13 - 13  in  FIG. 12B ; 
         FIG. 14  is an exploded view of a piston, a piston biasing spring and a spring stopping plug; 
         FIG. 15A  is a longitudinal cross section of a plunger of the invention including the ratchet releasing mechanism showing a releasing of the ratchet; 
         FIG. 15B  is an enlarged view of a part of the plunger surrounded by a broken line in  FIG. 15A ; 
         FIG. 16  is a cross-section view of a prior art ratchet-type tensioner. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIG. 1 , the ratchet-type tensioner  100  of the invention is attached to an engine (not shown) on a slack side of a timing chain C which is engaged with a sprocket S 1  rotated by a crankshaft and a pair of driven sprockets S 2  fixed to camshafts. 
     The ratchet-type tensioner  100  has a housing  110  and a plunger  120  that slidably projects out of a front surface of the housing  110 . The plunger  120  applies tension to the slack side of the timing chain C, i.e., the side that travels from the crankshaft sprocket S 1  toward one of the camshaft sprockets S 2 . The plunger applies tension through a movable lever L on which the chain slides, and which is pivoted on the engine block, and presses the plunger  120  at a location remote from the lever&#39;s pivot axis. 
     The basic configuration of the housing  110  of the tensioner  100  may be one that supplies oil under pressure from an oil pump (not shown) to the oil supply passage  111  formed within the housing  110 , or one that has a concave oil reservoir formed on a back part of the housing  110  to contain the oil supplied from the oil pump before introducing it into the oil supply passage  111 . 
     A stationary guide G for guiding travel of the timing chain C is mounted to the engine block on the tension side of the timing chain C, i.e., the side of the chain that travels from the other of the camshaft sprockets  52  toward the crankshaft sprocket S 1 . Arrows indicate the direction of travel of the chain C and the direction of rotation of the sprockets. 
     As shown in  FIG. 2 , the ratchet-type tensioner  100  has a housing  110  having an oil supply path  111  for introducing oil supplied under pressure from the engine, a plunger-accommodating hole  112  formed in the housing  110  to accommodate the plunger  120 , which is hollow, having an interior space  121  open at the end opposite from the protruding end. The plunger  120  is reciprocable in the advance/setback direction and protrudes out of the plunger-accommodating hole  112  to apply tension to the chain as shown in  FIG. 1 . A high-pressure oil chamber R is formed between the plunger-accommodating hole  112  of the housing  110  and a hollow interior space  121  of the plunger  120 . A plunger-biasing spring  130  is disposed within the high-pressure oil chamber R and biases the plunger  120  in the advancing direction. A check valve  140  in the plunger-accommodating hole  112  blocks the oil from flowing backward from the high-pressure oil chamber R to the oil supply path  111 . 
     A piston-receiving hole  113  is formed in the housing  110 , and a piston  150  is inserted into the piston-receiving hole  113  and slides in a direction transverse the advance/setback direction of the plunger  120 . A piston-biasing spring  160  is provided for biasing the piston  150  toward the plunger  120  so that ratchet teeth  151  provided on the plunger-side edge of the piston  150  engage with rack teeth  122  engraved on a side of the plunger  120 . A spring stopping plug  170  is fitted in the rear end area of the piston-receiving hole  113  and the piston-biasing spring  160  bears against the plug  170 . 
     The check valve  140  may be any known type of check valve. The check valve may be in any conventional location, including a location within the setback direction end of the plunger-accommodating hole  112 , inside the high pressure oil chamber R in the interior of the plunger  120 , or in the oil path outside of the housing  110 . The check valve allows oil to be introduced through the oil supply path  111  to the high-pressure oil chamber R, and blocks the oil from flowing in the reverse direction from the high-pressure oil chamber R to the oil supply path  111 . 
     In the embodiment shown in  FIG. 2A , the check valve unit  140  has a ball seat  141  having an oil passage  141   a  communicating with the oil supply path  111  in the housing  110 , a check ball  142  seated on a valve seat  141   b  of the ball seat  141 , a ball-biasing spring  143  for biasing the check ball  142  against the ball seat  141 , and a bell-shaped retainer  144  for supporting the ball-biasing spring  143  and restricting movement of the check ball  142 . 
     The ratchet-type tensioner  100  of the invention is provided with a means for preventing rotation of the piston in the circumferential direction of the piston receiving hole  113  when the ratchet teeth  151  and rack teeth  122  are engaged. In the embodiment shown in  FIG. 4 , the means for preventing rotation of the piston  150  is provided by a convex strip  152  on the outer circumferential surface of the piston  150  and a concave groove  113   a  in the piston-receiving hole  113  in the housing  110 . The groove  113   a  extends along the sliding direction of the piston and receives the convex strip  152 . This allows the piston to protrude out of the piston-receiving hole, while stopping the outer circumferential surface of the piston  150  from turning against the inner circumferential surface of the ratchet-accommodating hole  113 . It also allows the ratchet teeth  151  to engage with the rack teeth  122  precisely across the entire width, even when the plunger  120  vibrates when the engine is driven. 
     In another embodiment, as shown in  FIG. 13 , the means for preventing rotation of the piston is provided by a piston having an oval cross=section and a ratchet-accommodating hole  213  formed in the housing also having an oval cross-section, substantially in the same shape as the piston. This prevents the outer circumferential surface of the piston from turning with respect to the inner circumferential surface of the piston-accommodating hole  213  that extends in a direction orthogonal to the direction in which the plunger  220  slides. This arrangement permits precise engagement of the ratchet teeth  251  with the rack teeth  222  across the entire width of the teeth, even when the plunger  220  is caused to vibrate by the operation of the engine. This arrangement requires no precise mechanical work, and reduces the burden of production. 
     The engaging direction is the direction of movement of the piston  150  when the piston approaches the plunger  120 , and the disengaging direction is the direction opposite the engaging direction, i.e., the direction in which the piston  150  recedes from the plunger  120 . 
     Any means for preventing rotation of the piston  150  in the circumferential direction of the piston receiving hole  113  can be used, provided the means is constructed so as to stop the outer circumferential surface of the piston  150  from turning against the inner circumferential surface of the piston-receiving hole  113 , but allows the piston  150  to slide easily in the sliding direction and to protrude from the piston-receiving hole  113  to allow engagement of the rack teeth  122  with the ratchet teeth  151 . Means for preventing rotation of the piston  150  in the circumferential direction of the piston receiving hole  113  include a convex pin protruding from a point in the outer circumferential surface of the piston  150 , in combination with a matching concave groove extending along the inner circumferential surface of the piston-receiving hole  113 . Additional means for preventing rotation of the piston  150  in the circumferential direction of the piston receiving hole  113  include a convex pin protruding from a point in the inner circumferential surface of the piston-receiving hole  113 , in combination with a matching concave groove extending along the outer circumferential surface of the piston  150 . Still additional means for preventing rotation of the piston are provided by a concave groove formed on the outer circumferential surface of the piston and a convex strip on the inner circumferential surface of the piston-receiving hole  113 . The groove extends along the sliding direction of the piston  150  and receives the convex strip on the piston-receiving hole  113 . 
     By the term “inner circumferential surface”, it is meant the inner edge of the item thereafter named, e.g., the plunger receiving hole. However, the term is not limited to edges which are circular in shape. Semi-circular, oblong, oval, and irregularly shaped holes, without limitation, are also included in the definition of this term. 
     By the term “outer circumferential surface”, it is meant the outer edge of the item thereafter described, e.g., the piston. However, the term is not limited to edges which are circular in shape. Semi-circular, oblong, oval, and irregularly shaped holes, without limitation, are also included in the definition of this term. 
     As shown in  FIG. 5 , the length W of the piston  150  is greater than an outer diameter D thereof. This arrangement permits the ratchet mechanism comprising the plunger  120  and the piston  150  to operate more smoothly by suppressing inclination and preventing wear of the piston-receiving hole  113  caused by bias of the piston  150 , that would otherwise be caused when an overload is applied to the piston  150 . 
     In one embodiment, as shown in  FIGS. 3 and 5 , to disperse the engagement load evenly, the plunger  150  is provided with three ratchet teeth  122  for engagement with the rack teeth  122  engraved on the side surface of the plunger  120 . The three ratchet teeth  151  are disposed at intervals equal to the distance between the rack teeth  122 . In addition, the pitch and tooth height of the rack teeth  122  are equal to the pitch and height of the ratchet teeth  151 . 
     In a modified embodiment, as shown in  FIG. 14 , the plunger is provided with two ratchet teeth  251  to engage with the rack teeth  222  ( FIG. 15 ) engraved on the side surface of the plunger  220 , to disperse the engagement load evenly. The two ratchet teeth  251  are disposed at intervals equal to the distance between the rack teeth  222 . In addition, the pitch and tooth height of the rack teeth  222  are equal to the pitch and height of the ratchet teeth  251 . The plunger may be provided with any number of teeth, preferably in the range from two to ten teeth. 
     The term “plunger-side” refers to a position closer to the plunger  120  in the sliding direction, and the term “non-plunger-side” refers to positions farther from the plunger  120  in the sliding direction, i.e., the side opposite from the plunger-side. 
     As shown in  FIG. 5 , the plunger-biasing spring  160  is inserted into a spring-accommodating hole  153  which is a hollow space inside the plunger  150 , the axis of which extends along the direction in which the piston slides. 
     The spring  160  is held in place by a spring retaining plug  170 , which is in the form of a washer having a plural resilient tongues  171  projecting from its periphery. The tongues  171  fit into the non-plunger-side end of the ratchet receiving hole  113  and allow the washer to be rigidly and readily assembled without falling out of the piston-receiving hole. The plug reduces flapping noise by stably securing the ratchet mechanism while the engine is operating, and prevents seizing of the plunger. The biasing force Fs exerted by the spring  160  ( FIGS. 6-10 ) depends on the position of the spring retaining plug  170 . 
     As shown in  FIG. 3 , in the tensioner  100 , the rack teeth  122  are concave/convex in form, having stop surfaces  122   a , which are inclined toward the advance direction when proceeding in the disengagement direction of the piston  150 , and sliding surfaces  122   b , which are inclined toward the setback direction, proceeding in the disengagement direction of the piston  150 . 
     The ratchet teeth  151  are similarly concave/convex in form, having stop counterface surfaces  151   a , which are inclined toward the advance direction when proceeding in the direction of disengagement of the piston  150 , and sliding counterface surfaces  151   b , which are inclined toward the setback direction when proceeding in the direction of disengagement the piston  150 . 
     The inclination angle θ of stop surface  122   a  formed on the plunger  120  is smaller than the inclination angle α of the sliding surface  122   b . For instance, the inclination angle θ is preferably 30° or less, and the inclination angle α is preferably greater than the inclination angle θ, but less than 90°. The inclination angles θ and α are angles measured from a line extending parallel to the sliding direction and lines tangent to the tooth surfaces, all said lines being in a common plane to which the advance/setback direction of the plunger and the sliding direction of the piston are mutually parallel. 
     The inclination angle θ is determined by experiment and simulation so that disengagement of the rack teeth  122  and the ratchet teeth  151 , and setback of the plunger  120 , are blocked when the first reaction force F 1  acts on the plunger  120 , but so that disengagement of the rack teeth  122  and the ratchet teeth  151 , and setback of the plunger  120 , are allowed when the second reaction force F 2  acts on the plunger  120 . In contrast to a conventional rack, in which the stop surfaces are not inclined, the rack teeth of the plunger in the invention can have a range of inclination angles θ. 
     The inclination angle α is similarly determined by experiment and simulation so that disengagement of the rack teeth  122  and the ratchet teeth  151 , and advancing movement of the plunger  120  can take place when an advancing force F 1  ( FIG. 6 ) acts on the plunger  120 . 
     In all examples, a first reaction force F 1  is a reaction force that is exerted on the plunger when the timing chain tension is smaller than an excessive tension, and the second reaction force F 2  is a force having a magnitude greater than the first reaction force F 1 . 
     As shown in  FIG. 6 , during starting and normal operation of the engine when the tension in the timing chain is not excessive, the plunger  120  is moved in the advancing direction by an advancing force F 1  due to the plunger-biasing spring  130  and the pressure of the oil within the high-pressure oil chamber R. The biasing force Fs exerted by the plunger-biasing spring  160  is set so that it is smaller than the force component f 1  acting on the piston  150  in the disengagement direction. Component f 1  is the component of the advancing force F 1  acting in the disengagement direction as a result of the action of the rack teeth  122  on the ratchet teeth  151 . 
     When the component force f 1  surpasses the resultant of the biasing force Fs and the sliding direction component of the frictional force acting between the rack teeth  122  and the ratchet teeth  151 , the plunger  120  advances, following the lever L ( FIG. 1 ) while pushing the plunger  150  in the disengagement direction.  FIG. 6  shows, in two-dot broken lines, the positions of the front end of the plunger  120  and the piston  150  before the plunger  120  advances. 
     When the plunger  120  advances to apply tension to the timing chain C during operation of the engine as shown in  FIG. 6 , the relationship of the magnitudes of the component force f 1  in the sliding direction generated by the advancing force F 1  acting on the plunger  120 , and the biasing force Fs of the piston  150  is:
 
 f 1= F 1×sin α×cos α f 1&gt; Fs  
 
     As shown in  FIG. 6 , broken lines at the protruding side of the plunger indicate the starting position of the plunger before the plunger protrudes in the advancing direction, upon starting of the engine. 
     Referring to  FIG. 7 , F 1  is the first reaction force exerted by the timing chain through the lever in the setback direction upon starting the engine when the hydraulic pressure in the high-pressure oil chamber  131  is low, or while the engine is operating normally without excessive tension in the timing chain. The corresponding first force component f 1  acts in the disengagement direction on the piston by the action of the rack teeth  122  on the ratchet teeth  151 . The ratchet biasing force Fs is set so that it is greater than the magnitude of component f 1 . Consequently, the engagement of the ratchet teeth  151  with the rack teeth  122  restricts movement of the plunger  120  and blocks setback displacement of the plunger  120  limiting the setback displacement of the plunger to its backlash. 
     When setback of the plunger  120  is blocked on starting the engine, for example as shown in  FIG. 7 , the relationship of the magnitudes of the first component force f 1  in the sliding direction generated by the first reaction force F 1  exerted on the plunger  120  by the timing chain on starting the engine, and the biasing force Fs of the piston  150 , is:
 
 f 1= F 1×cos θ×sin θ×μ f 1 &lt;Fs;  
 
where μ is the coefficient of friction between the rack teeth  122  and the ratchet teeth  151 .
 
     When the magnitude of the reaction force is at a high level, the reaction force is a second reaction force F 2 , as shown in  FIG. 8 , and a second component force f 2  acts on the piston  150  in the disengagement direction, through engagement of the rack teeth with the ratchet teeth. The biasing force Fs is set so that it is smaller than the second component force f 2 . 
     The reaction force can reach the magnitude F 2  when excessive tension is generated in the timing chain C. This excessive tension can occur, for example, after the plunger  120  has advanced excessively due to elongation of the timing chain, or as a result of other causes such as fluctuations in the tension of the timing chain C due to thermal expansion of the engine, the timing chain C, or both, due to temperature changes in the engine. 
     When the plunger  120  advances excessively due to a temperature change in the engine, as shown in  FIGS. 8 and 9 , the timing chain can be under excessive tension, and setback of the plunger  120  is allowed. Here, the relationship of magnitudes of the second component force f 2  generated by the second reaction force F 2  exerted on the plunger  120  by the timing chain, and the biasing force Fs, is:
 
 f 2= F 2×cos θ×sin θ×μ f 2&gt; Fs;  
 
where μ is the coefficient of friction between the rack teeth  122  and the ratchet teeth  151 .
 
     As shown in  FIG. 8 , when the reaction force F 2  is exerted on the plunger when the tension in the timing chain C becomes excessive after the engine is started, the component force fh is exerted by the stop surface  122   a  on the stop counterface surface  151   a . The second component force f 2  becomes greater than the resultant of the biasing force Fs and the frictional force. As shown in  FIG. 9 , the piston  150  slides in the disengagement direction, and the ratchet teeth  151  disengage the rack teeth  122 . Then, the plunger  120  sets back by a distance corresponding to one tooth or several teeth of the rack teeth  122  until the reaction force F returns to the first reaction force F 1 , and the first component force f 1  acts on the piston  150  as shown in  FIG. 10 . Thus, when the tension of the timing chain C becomes excessive, the tensioner  100  does not restrict movement of the plunger  120  in the setback direction, and allows setback displacement beyond the setback permitted by backlash. 
     The biasing force exerted by the plunger-biasing spring  130  in the advancing direction ( FIG. 2A ) can be greater than the biasing force Fs exerted by the piston-biasing spring  160 . By adjusting the biasing force Fs within the range described above. It is possible to adjust the condition under which disengagement of the ratchet teeth from the rack teeth is caused by excessive tension in the chain after starting of the engine. 
     When the second reaction force F 2  is exerted on the plunger by the timing chain when the tension in the timing chain C is excessive, this second reaction force acts on the stop counterface surfaces  151   a  through the stop surfaces  122   a  to produce a second component force f 2  in the disengagement direction along the direction of sliding movement of the piston. The second component force overcomes the ratchet-biasing force and friction, and causes the ratchet teeth  151  to disengage the rack teeth  122 . Then, as shown in  FIGS. 9 and 10 , the rack teeth return by a distance corresponding to one or more teeth, sliding on slide counterface surfaces, and engaging stop counterface surfaces. 
     As shown in  FIGS. 8-10 , broken lines at the protruding side of the plunger indicate the starting position of the plunger when the tension of the chain becomes excessive ( FIG. 8 ), and as the plunger sets back ( FIGS. 9 and 10 ). 
     The ratchet releasing mechanism allows for release of engagement of the ratchet teeth  151  and the rack teeth  122  by a tool T, as shown in FIG.  11 . The ratchet releasing mechanism includes a tool engaging portion  154  provided at the front end of the piston  150  to abut and engage with the end T 1  of a tool T. The tool T is inserted through a tool inserting hole  114  provided in the housing  110  toward the tool engaging portion  154 . 
     In one embodiment, the tool engaging portion  154  comprises a stepped abutment surface  154   a  formed between the ratchet teeth  151  and the ratchet-side convex strip  152  which is a part of the outer circumferential surface of the piston at the front end of the piston  150 . The pin inserting hole  114  is arranged that that a tool T inserted through the pin inserting hole  114  can engage the tool engaging portion  154  and push back the piston  150  in a direction away from the plunger by a prying action exerted against the stepped abutment surface  154   a  by the tool T. Thus, it becomes possible to push and set back the plunger  120  and easily release the engagement of the ratchet teeth  151  to the rack teeth  122 , to considerably reduce the burden of maintenance on the tensioner. 
     In another embodiment, as shown in  FIGS. 14 and 15 , the ratchet releasing mechanism comprises a tool engaging portion  254  which comprises a tapered abutment surface  254   a  formed on the piston by partly cutting away the front end of the piston  250  toward the rear-end at an oblique angle relative to a plane to which the axis of the piston is perpendicular. The tapered abutment surface  254   a  is formed such that when the tool T is inserted into the tool inserting hole  214 , the tool T pushes the piston  250  away from the plunger  220  by a camming action exerted against the tapered abutment surface  254   a  by the tool T. Thus, it becomes possible to push and set back the plunger  220  and easily release the engagement of the ratchet teeth  251  to the rack teeth  222 , to considerably reduce the burden of maintenance on the tensioner.