Abstract:
To provide a chain tensioner and a relief valve unit that enable a reduction in the number of parts to be machined and the number of assembling steps, reduce the amount of oil flowing out to the outside, and prevent damage on a check valve, without having to increase the size of the chain tensioner. The chain tensioner includes a check valve unit that suppresses reverse flow of oil flowing into an oil pressure chamber, and a relief valve unit that releases the oil when the pressure inside the oil pressure chamber reaches or exceeds a predetermined high pressure. The check valve unit is disposed as a valve element of the relief valve unit, and the relief valve unit and the check valve unit are formed integral with each other.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a chain tensioner that includes a tensioner body that has a cylindrical plunger bore with an open end, a cylindrical plunger that is slidably inserted into the plunger bore, biasing means, accommodated so as to freely expand and contract inside an oil pressure chamber formed between the plunger bore and the plunger, for biasing the plunger in a protruding direction of the plunge, this chain tensioner maintaining appropriate tension of a chain. 
         [0003]    2. Description of the Related Art 
         [0004]    It has been common practice to use a chain tensioner for maintaining appropriate tension of a chain. For example, a chain guide mechanism has been known, which slidably guides, by means of a guide shoe, a transmission chain such as an endless roller chain passing over respective sprockets of a crankshaft and a cam shaft inside an engine room, and which uses a chain tensioner to press a pivoting chain guide having the guide shoe to maintain appropriate tension. 
         [0005]    For example, as shown in  FIG. 8 , an endless timing chain CH passes over a drive sprocket S 1  attached to a crankshaft and a pair of driven sprockets S 2  and S 3  attached to cam shafts in an engine room. A chain guide mechanism is formed, wherein a pivoting chain guide G 1  and a fixed chain guide G 2  guide this timing chain CR. 
         [0006]    The fixed chain guide G 2  is fixed in position in the engine room with two mounting shafts B 1  and B 2 , while the pivoting chain guide G 1  is attached such as to be pivotable around the mounting shaft B 0  in the plane in which the timing chain CH runs in the engine room. 
         [0007]    The chain tensioner  500  presses the pivoting chain guide G 1  and thereby maintains the tension of the timing chain CH at an appropriate level as well as reduces its vibration. 
         [0008]    The known chain tensioner  500  used in such a chain guide mechanism includes, for example, as shown schematically in  FIG. 9 , a tensioner body  510  that has a cylindrical plunger bore  511  with an open end, a cylindrical plunger  520  that is inserted into the plunger bore  511  to freely slide against a cylindrical surface portion  513  of the plunger bore  511 , and biasing means that biases the plunger  520  in a protruding direction of the plunger from the plunger bore  511 . 
         [0009]    The biasing means are formed by a coil spring  540  accommodated inside a cylindrical recess  521  in the cylindrical plunger  520  and compressed between the plunger and the bottom  512  of the plunger bore  511 . 
         [0010]    Oil is supplied from an oil supply hole  514  formed in the plunger bore  511 , so that an oil pressure chamber  501  formed between the plunger bore  511  and the plunger  520  is filled with the oil, and that the plunger  520  is biased in the protruding direction of the plunger by the oil. A check valve  550  (schematically shown as a check ball) stops the oil from flowing out from the oil supply hole  514 . 
         [0011]    As the plunger  520  reciprocates, the oil flows through a small gap between the plunger  520  and the plunger bore  511 , and the flow resistance provides the damping effect of slowing down the reciprocal movement of the plunger  520 . 
         [0012]    In such a chain tensioner, in the event of an excessive tension developed in the timing chain during use, the pressure in the oil pressure chamber will be too high, which could generate noise or vibration, or cause damage on the timing chain. 
         [0013]    To prevent such a situation, chain tensioners provided with a relief valve for relieving pressure when the pressure inside the oil pressure chamber reaches or exceeds a predetermined limit have been known (see Japanese Patent Application Laid--open Nos. 2002-327810, 2011-226534, 2002-130401, 2002-235818, and 2006-125430, etc). 
       SUMMARY OF THE INVENTION 
       [0014]    The chain tensioner known from Japanese Patent Application Laid-open Nos. 2002-327810, 2011-226534 and the like has its relief valve provided to the tensioner body via an additional oil passage and not disposed inside the plunger bore. Therefore, the chain tensioner itself is increased in size, and so are the number of parts to be machined and the number of assembling steps. 
         [0015]    The chain tensioner known from Japanese Patent Application Laid-open Nos. 2002-130401, 2002-235818, 2006-125430 and the like has its relief valve disposed inside the plunger bore and so a size increase of the chain tensioner itself is avoided. On the other hand, the internal structure of the plunger is more complex and therefore the number of assembling steps is increased. 
         [0016]    Another problem is that, since the oil released by the relief valve flows out to the outside, more oil is consumed, and accordingly the oil pump needs higher power. 
         [0017]    Moreover, in known chain tensioners including those of Japanese Patent Application Laid-open Nos. 2002-327810, 2011-226534, 2002-130401, 2002-235818, and 2006-125430, etc, the valve is configured such that the check ball moves inside the check valve. Therefore, in an operating condition wherein the pressure inside the oil pressure chamber rises suddenly, the check baLl sits on the check valve seat with a large impact immediately before the relief valve relieves the pressure, because of which there was a possibility that the check valve function could be deteriorated due to damage on the check ball or check valve seat. 
         [0018]    An object of the present invention is to solve these problems and provide a chain tensioner and a relief valve unit that enable a reduction in the number of parts to be machined  and the number of assembling steps, reduce the amount of oil flowing out to the outside, and prevent damage on the check valve, without having to increase the size of the chain tensioner. 
         [0019]    The chain tensioner of the present invention includes: a tensioner body that has a cylindrical plunger bore with an open end; a cylindrical plunger that is slidably inserted into the plunger bore; biasing means, accommodated so as to freely expand and contract inside an oil pressure chamber formed between the plunger bore and the plunger, for biasing the plunger in a protruding direction of the plunger; a check valve unit that suppresses reverse flow of oil flowing into the oil pressure chamber; and a relief valve unit that releases the oil when pressure inside the oil pressure chamber reaches or exceeds a predetermined high pressure, the check valve unit being disposed as a valve element of the relief valve unit, and the relief valve unit and the check valve unit being formed integral with each other. The problems described above are solved by this feature. 
         [0020]    The relief valve unit according to the present invention is a relief valve unit incorporated in a chain tensioner and includes: a relief sleeve that slidably holds therein a check valve unit; a relief valve seat that opens and closes as the check valve unit slides; and a unit pressing mechanism for pressing the check valve unit toward the relief valve seat. The problems described above are solved by this feature. 
         [0021]    In the chain tensioner according to claim  1 , the check valve unit is disposed as a valve element of the relief valve unit, and the relief valve unit and the check valve unit are formed integral with each other. Since the relief valve is disposed inside the plunger bore, the size of the chain tensioner itself is not increased, and neither is the number of parts to be machined. 
         [0022]    Since the check valve unit is disposed as a valve element of the relief valve unit, and the relief valve unit and the check valve unit are formed integral with each other, they can be assembled in one piece into the tensioner after assembling the relief valve unit in advance. Thus, the number of assembling steps can be reduced. 
         [0023]    Also, the tensioner is configured such that the pressure is relieved by the relief valve unit toward the side from which the oil is supplied, so that the amount of oil flowing out to the outside can be reduced, and, since the pressure difference between the oil pressure chamber and the pressure relief side is smaller, the speed at which the relief valve unit returns to its closed state can be increased. 
         [0024]    Moreover, even when the relief valve unit is activated before the oil supply is started, such as when starting the engine, the oil is circulated rather than flowing out to the outside because of the structure in which the pressure is relieved toward the side from which the oil is supplied, and therefore, rattling at the start of the engine can be reduced. 
         [0025]    According to the configuration set forth in claim  2 , the relief valve unit includes a relief sleeve that slidably holds therein the check valve unit, a relief valve seat that opens and closes as the check valve unit slides, and a unit pressing mechanism that presses the check valve unit toward the relief valve seat. Therefore, in an operating condition wherein the pressure inside the oil pressure chamber rises suddenly, the check valve seat moves at the moment when the check ball sits on the check valve seat and the impact is absorbed by the unit pressing mechanism. Damage on the check ball or the check valve seat is thus prevented, and deterioration of the check valve function can be prevented. 
         [0026]    According to the configuration set forth in claim  3 , the check valve unit includes a check ball, a retainer holding therein the check ball, and a check valve seat that opens and closes as the check ball sits thereon and separates therefrom, and the retainer is configured to sit on the relief valve seat of the relief valve unit. This way, the check valve unit can be incorporated into the relief valve unit as a valve element only by giving the retainer a prescribed outer shape, so that the number of parts to be machined or the number of assembling steps is not increased. 
         [0027]    Also, the number of components can be reduced as compared to known counterparts that have a check valve unit separately from a relief valve unit. 
         [0028]    According to the configuration set forth in claim  4 , the relief sleeve includes a release section that is on a rear side of the relief valve seat and causes the oil, which is released when the pressure reaches or exceeds a predetermined high pressure, to flow toward an outer circumferential surface of the relief sleeve. Thus the oil released when the pressure reaches or exceeds a predetermined high pressure smoothly circulates from the outer circumferential surface of the relief sleeve toward the side where the oil is supplied, and accordingly the amount of oil flowing out to the outside can be reduced. 
         [0029]    According to the configuration set forth in claim  5 , the relief valve unit is disposed on a bottom part of the plunger bore, with the relief valve seat being oriented toward the plunger. When applied to chain tensioners of the type that supplies oil from the bottom side of the plunger bore, the number of parts to be machined and the number of assembling steps are reduced, and so is the amount of oil flowing out to the outside, as well as damage on the check valve can be prevented, without having to increase the size of the chain. tensioner. 
         [0030]    According to the configuration set forth in claim  6 , the plunger includes an oil reservoir chamber inside, which communicates with the oil supply hole, with the relief valve unit disposed inside the oil reservoir chamber, and with an oil passage formed between the outer circumference of the relief sleeve and the inner circumference of the plunger for causing released oil to circulate back to the oil reservoir chamber. When applied to chain tensioners of the type that has an oil reservoir chamber inside the plunger, the number of parts to be machined and the number of assembling steps are reduced, and so is the amount of oil flowing out to the outside, as well as damage on the check valve can be prevented, without having to increase the size of the chain tensioner. 
         [0031]    Moreover, the oil can circulate back to the oil reservoir chamber when the check valve opens, so that oil loss from the oil reservoir chamber can be reduced. 
         [0032]    According to the configuration set forth in claim  7 , the relief valve unit is formed to extend over the entirety of the oil reservoir chamber, with an oil reservoir space being formed inside the relief sleeve. When applied to chain tensioners of the type in which an oil reservoir chamber is formed by inserting a sleeve inside the plunger, the number of parts to be machined and the number of assembling steps are reduced, and so is the amount of oil flowing out to the outside, as well as damage on the check valve can be prevented, without having to increase the size of the chain tensioner. 
         [0033]    Also, since the relief sleeve of the relief valve unit serves also as the sleeve for forming the oil reservoir chamber, the number of components and the number of assembling steps can further be reduced. 
         [0034]    According to the configuration set forth in claim  8 , the relief sleeve includes a sleeve connection hole in a rear side face of a sliding range of the check valve unit, this sleeve connection hole connecting the outer circumference of the relief sleeve with the oil reservoir space on the inner side of the relief sleeve. Since the oil can smoothly circulate back to the oil reservoir chamber through the sleeve connection hole, oil loss from the oil reservoir chamber can further be reduced. 
         [0035]    According to the configuration set forth in claim  9 , the relief valve unit is disposed slidable inside the plunger, with the relief valve seat of the relief valve unit being oriented toward a direction in which the plunger protrudes. When applied to chain tensioners of the type that supplies oil from the bottom side of the plunger bore and that has an oil reservoir chamber inside the plunger, the number of parts to be machined and the number of assembling steps are reduced, and so is the amount of oil flowing out to the outside, as well as damage on the check valve can be prevented, without having to increase the size of the chain tensioner. 
         [0036]    Also, since the relief sleeve of the relief valve unit serves also as the sleeve for forming the oil reservoir chamber, the number of components and the number of assembling steps can further be reduced. 
         [0037]    The relief valve unit according to claim  10  includes a relief sleeve that slidably holds therein the check valve unit, a relief valve seat that opens and closes as the check valve unit slides, and a unit pressing mechanism that presses the check valve unit toward the relief valve seat. Since the relief valve unit and the check valve unit are formed integral with each other, they can be assembled and united before incorporated into the chain tensioner, and thus the number of assembling steps can be reduced. 
         [0038]    According to the configuration set forth in claim  11 , the relief sleeve includes a release section that is on a rear side of the relief valve seat and that causes the oil, which is released when the pressure reaches or exceeds a predetermined high pressure, to flow toward an outer circumferential surface of the relief sleeve. When incorporated in the chain tensioner, in use, the oil released when the pressure reaches or exceeds a predetermined high pressure smoothly circulates from the outer circumferential surface of the relief sleeve toward the side where the oil is supplied, and accordingly the amount of oil flowing out to the outside can be reduced. 
         [0039]    According to the configuration set forth in claim  12 , the check valve unit includes a check ball, a retainer holding therein the check ball, and a check valve seat that opens and closes as the check ball sits thereon and separates therefrom, and the retainer is configured to sit on the relief valve seat of the relief valve unit. This way, the check valve unit can be incorporated into the relief valve unit as a valve element only by giving the retainer a prescribed outer shape, so that the number of parts to be machined or the number of assembling steps is not increased. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]      FIG. 1A  is a front cross-sectional view and  FIG. 1B  is a side cross-sectional view of a chain. tensioner according to a first embodiment of the present invention; 
           [0041]      FIG. 2A  is a plan view,  FIG. 2B  is a front view,  FIG. 2C  is a perspective view, and  FIG. 2D  is a cross-sectional view of a relief valve unit according to the first embodiment of the present invention; 
           [0042]      FIG. 3A  is a front cross-sectional view and  FIG. 3B  is a side cross-sectional view of a chain tensioner according to a second embodiment of the present invention; 
           [0043]      FIG. 4A  is a plan view,  FIG. 4B  is a front view,  FIG. 4C  is a bottom view, and  FIG. 4D  is a cross-sectional view of a relief valve unit according to the second embodiment, of the present invention; 
           [0044]      FIG. 5  is a side cross-sectional view of a variation example of the chain tensioner according to the second embodiment of the present invention; 
           [0045]      FIG. 6A  is a front cross-sectional view and  FIG. 6B  is a side cross-sectional view of a chain tensioner according to a third embodiment of the present invention; 
           [0046]      FIG. 7  is a side cross-sectional view of a variation example of the chain tensioner according to the third embodiment of the present invention; 
           [0047]      FIG. 8  is an explanatory diagram of the chain tensioner used in a chain guide mechanism of an engine; and 
           [0048]      FIG. 9  is a schematic explanatory diagram of a conventional chain tensioner. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       [0049]    A chain tensioner  100  and a relief valve unit  160  according to a first embodiment of the present invention will be described with reference to the drawings. 
         [0050]    The chain tensioner  100  according to a first embodiment of the present invention includes, as shown in  FIG. 1A  and  FIG. 1B , a tensioner body  110  that has a cylindrical plunger bore  111  with an open end, a cylindrical plunger  120  that is slidably inserted into the plunger bore  111 , and a coil spring  140  that is biasing means, accommodated to freely expand and contract inside an oil pressure chamber  101  formed between the plunger bore  111  and the plunger  120 , for biasing the plunger  120  in a protruding direction of the plunger  120 . 
         [0051]    A relief valve unit  160  is provided at the bottom  112  of the plunger bore  111  of the tensioner body  110 , and an oil supply hole  114  is provided near the bottom  112 . 
         [0052]    The chain tensioner  100  according to this embodiment is a type known as a ratchet tensioner, which includes a ratchet portion on an outer circumferential surface of the plunger  120 . This ratchet portion engages with a pawl pivotably attached to the tensioner body  110  in a cut-off portion of the cylindrical surface  113 , thereby to restrict the protruding and retracting movement of the plunger  120 . 
         [0053]    This ratchet mechanism may have any known structure and operate as known, and the tensioner need not necessarily be a ratchet type, and so it will not be described in detail. 
         [0054]    The relief valve, unit  160  includes, as shown in  FIG. 2A  to  FIG. 2D , a relief sleeve  161  that slidably holds therein. a check valve unit  150 , a relief valve seat  162  that opens and closes as the check valve unit  150  slides, a unit pressing spring  163  that is a unit pressing mechanism for pressing the check valve unit  150  toward the relief valve seat  162 , and a unit pressing spring holder  164  that supports and secures one end of the unit pressing spring  163 . 
         [0055]    The relief sleeve  161  includes, on the rear side of the relief valve seat  162 , release holes  165  that are openings for causing the oil, which is released when the pressure reaches or exceeds a predetermined high pressure, to flow toward the outer circumferential surface of the relief sleeve  161 . The relief sleeve  161  has a smaller diameter than the relief valve seat  162  from the release holes  165  downward. 
         [0056]    Bottom grooves  167  are provided at the bottom of the unit pressing spring holder  164  for allowing smooth oil passage. 
         [0057]    The check valve unit  150  includes a check ball  151 , a retainer  152  holding the check ball  151  inside, a check valve seat  153  that opens and closes as the check ball  151  sits thereon and separates therefrom, and a ball pressing spring  154  that lightly presses the check ball  151  toward the check valve seat  153  inside the retainer  152 . 
         [0058]    The retainer  152  has a cylindrical outer shape and is configured to sit on the relief valve seat  162  of the relief valve unit  160 . 
         [0059]    How the chain tensioner  100  and relief valve unit  160  according to the first embodiment of the present invention configured as described above operate will be explained. 
         [0060]    When the plunger  120  protrudes, the oil supplied from the oil supply hole  114  flows in from the outer circumference of the relief sleeve  161  via the bottom grooves  167  below the unit pressing spring holder  164 , reaches the check valve unit  150 , pushes the check ball  151  up from the check valve seat  153 , and enters the oil pressure chamber  101 . 
         [0061]    When the plunger  120  is pressed in to retract, the pressure inside the oil pressure chamber  101  rises, whereby the check ball  151  is pressed against the check valve seat  153 , and the oil is stopped from flowing out of the check valve unit  150 . The pressure then acts to press the check valve unit  150  itself downward against the pressing force of the unit pressing spring  163 . 
         [0062]    When the pressure inside the oil pressure chamber  101  reaches or exceeds a predetermined high pressure, the unit pressing spring  163  is compressed and the entire check valve unit  150  retracts, which causes the retainer  152  to separate from the relief valve seat  162  of the relief valve unit  160 , thus relieving the pressure inside the oil pressure chamber  101 . 
         [0063]    In this embodiment, as shown, the relief valve seat  162  is formed in a concave shape, and closed by the retainer  152  that fits therein to a predetermined depth. That is, with this structure, the pressure inside the oil pressure chamber  101  is relieved after the retainer  152  has retracted until it is disengaged, which means that pressure leakage when the valve is closed is minimized. 
         [0064]    When the pressure inside the oil pressure chamber  101  suddenly rises to or exceeds a predetermined high pressure, the contact pressure between the check ball  151  and the check valve seat  153  will increase rapidly. 
         [0065]    If the pressure suddenly rises to or exceeds a predetermined high pressure in a state where the check ball  151  is not in contact with the check valve seat  153 , the check ball  151  will collide against the check valve seat  153 . 
         [0066]    Since the check valve seat  153  retracts at this time, the impact caused by a sudden increase in the contact pressure or collision between the check ball  151  and the check valve seat  153  can be mitigated, so that deterioration of the check valve function due to damage on the check ball or check valve seat can be prevented. 
         [0067]    Also, since the relief valve unit  160  is in one piece with the check valve unit  150  as shown in  FIG. 2A  to  FIG. 2D , and can be prepared in advance as a single component as shown in  FIG. 2A  to  FIG. 2D , the number of parts of the tensioner body  110  or the plunger  120  that need to be machined, and the number of process steps of assembling the chain tensioner  100  can be greatly reduced. 
         [0068]    As the retainer  152  separates from the relief valve seat  162  to relieve the pressure inside the oil pressure chamber  101 , the oil flows out of the oil pressure chamber  101  through the release holes  165  to the outer circumferential surface of the relief sleeve  161 , and thus the oil circulates toward the oil supply hole  114  without flowing out to the outside. 
         [0069]    When the pressure inside the oil pressure chamber  101  reduces to or below a predetermined level, the retainer  152  is again moved by the unit pressing spring  163  and sits on the relief valve seat  162 , to keep it shut and maintain the pressure inside. 
         [0070]    Since the pressure is relieved by the relief valve unit  160  toward the side from which the oil is supplied from the oil supply hole  114  in this structure, the pressure difference between the oil pressure chamber and the pressure relief side is smaller as compared to conventional structures wherein the pressure is relieved out to the outside with a relief valve, and therefore the speed at which the relief valve unit  160  returns to its closed state is increased. 
         [0071]    In this embodiment, the unit pressing spring holder  164  is provided to the relief valve unit  160  so that they can be prepared as a single component in advance. Alternatively, the unit pressing spring holder  164  may be omitted, and the unit pressing spring  163  may be directly held by the bottom  112  of the plunger bore  111 , and the relief valve seat  162  may be fixedly fitted in the plunger bore  111 . 
         [0072]    Also, while the oil supply hole  114  is provided in the cylindrical surface  113  in  FIG. 1B , it may be provided in the bottom  112 . 
       Embodiment 2 
       [0073]    A chain tensioner  200  and a relief valve unit  260  according to a second embodiment of the present invention will be described with reference to the drawings. 
         [0074]    The plunger  220  of the chain tensioner  200  has an oil reservoir chamber  223  inside, as shown in  FIG. 3A  and  FIG. 3B , which is communicated with the oil supply hole  214  via a supply passage  222  and a plunger hole  221 . The relief valve unit  260  is disposed inside the oil reservoir chamber  223 , and an oil passage  224  is formed between the outer circumference of the relief sleeve  261  and the inner circumference of the plunger  220  for causing released oil to circulate back to the oil reservoir chamber  223 . 
         [0075]    The relief valve unit  260  is formed to extend over the entirety of the oil reservoir chamber  223  as shown in  FIG. 4A  to  FIG. 4D  and configured similarly to the relief valve unit  160  according to the first embodiment (with reference numerals starting with number 2 as the hundred&#39;s digit). 
         [0076]    This relief valve unit  260  is disposed in the oil reservoir chamber  223  inside the plunger  220 , and an oil reservoir space is formed inside the relief sleeve  261 . 
         [0077]    The coil spring  240  that is biasing means for biasing the plunger  220  in a protruding direction of the plunger is formed to apply pressure, with its one end held on the bottom  212  of the plunger bore  211  of the tensioner body  210 . The oil pressure chamber  201  is formed at the bottom  212 . 
         [0078]    The chain tensioner  200  according to this embodiment is also a type known as a ratchet tensioner. The ratchet mechanism may have any known structure and operate as known, and the tensioner need not necessarily be a ratchet type, and so it will not be described in detail. 
         [0079]    How the chain tensioner  200  and relief valve unit  260  according to the second embodiment of the present invention configured as described above operate will be explained. 
         [0080]    Oil is supplied from the oil supply hole  214  to the oil reservoir chamber  223  via the supply passage  222 , plunger hole  221 , oil passage  224 , and bottom grooves  267  of the unit pressing spring holder  264 . 
         [0081]    When the plunger  220  protrudes, the oil inside the oil reservoir chamber  223  presses the check ball  251  down from the check valve seat  253  and flows into the oil pressure chamber  201 . 
         [0082]    When the plunger  220  is pressed in to retract, the pressure inside the oil pressure chamber  201  rises, whereby the check ball  251  is pressed against the check valve seat  253 , and the oil is stopped from flowing out of the check valve unit  250 . The pressure then acts to press the check valve unit  250  itself upward against the pressing force of the unit pressing spring  263 . 
         [0083]    When the pressure inside the oil pressure chamber  201  reaches or exceeds a predetermined high pressure, the unit pressing spring  263  is compressed and the entire check valve unit  250  retracts, which causes the retainer  252  to separate from the relief valve seat  262  of the relief valve unit  260 , thus relieving the pressure inside the oil pressure chamber  201 . 
         [0084]    In this embodiment, similarly to the first embodiment, the relief valve seat  262  is formed in a concave shape, and closed by the retainer  252  that fits therein to a predetermined depth. That is, with this structure, the pressure inside the oil pressure chamber  201  is relieved after the retainer  252  has retracted until it is disengaged, which means that pressure leakage when the valve is closed is minimized. 
         [0085]    As the retainer  252  separates from the relief valve seat  262  to relieve the pressure inside the oil pressure chamber  201 , the oil flows out of the oil pressure chamber  201  through the release holes  265  into the oil passage  224  on the outer circumferential surface of the relief sleeve  261 , and thus the oil circulates back to the oil reservoir chamber  223  through the bottom grooves  267  of the unit pressing spring holder  264  without flowing out to the outside. 
         [0086]    When the pressure inside the oil pressure chamber  201  reduces to or below a predetermined level, the retainer  252  is again moved by the unit pressing spring  263  and sits on the relief valve seat  262 , to keep it shut and maintain the pressure inside. 
         [0087]    In this embodiment, the unit pressing spring holder  264  is provided to the relief valve unit  260  so that they can be prepared as a single component in advance. Alternatively, the unit pressing spring holder  264  may be omitted, and the unit pressing spring  263  may be directly held by the distal end of the plunger  220 , and the relief valve seat  262  may be fixedly fitted in the plunger  220 . 
         [0088]    Alternatively, as in the chain tensioner  200   b  shown in  FIG. 5 , the relief valve unit  160  similar to that of the first embodiment may be used to increase the capacity of the oil reservoir chamber  223   b.    
       Embodiment 3 
       [0089]    A chain tensioner  300  according to a third embodiment of the present invention will be described with reference to the drawings. 
         [0090]    The chain tensioner  300  according to the third embodiment of the present invention includes, as shown in  FIG. 6A  and  FIG. 6B , similarly to the first embodiment, a tensioner body  310  having a cylindrical plunger bore  311  with an open end, a cylindrical plunger  320  slidably inserted in the plunger bore  311 , and a coil spring  340  that is biasing means, accommodated so as to freely expand and contract inside an oil pressure chamber  301  formed between the plunger bore  311  and the plunger  320 , for biasing the plunger  320  in the protruding direction of the plunger. 
         [0091]    The same relief valve unit  260  as that of the second embodiment shown in  FIG. 4A  to  FIG. 4D  is disposed inside the plunger bore  311  of the tensioner body  310  such as to be slidable into the plunger  320  from the bottom  312 , with the relief valve seat  262  of the relief valve unit being oriented toward the direction a which the plunger  320  protrudes. An oil supply hole  314  is provided near the bottom  312 . 
         [0092]    Thus the oil pressure chamber  301  is formed inside the plunger  320  on the side toward which the plunger protrudes, while the oil reservoir chamber  323  is formed at the bottom  312  of the plunger bore  311  by the oil reservoir space formed inside the relief sleeve  261  of the relief valve unit  260 . 
         [0093]    The chain tensioner  300  according to this embodiment is also a type known as a ratchet tensioner. The ratchet mechanism may have any known structure and operate as known, and the tensioner need not necessarily be a ratchet type, and so it will not be described in detail. 
         [0094]    How the chain tensioner  300  according to the third embodiment of the present invention configured as described above operates will be explained. 
         [0095]    When the plunger  320  protrudes, the oil supplied from the oil supply hole  314  flows in from the outer circumference of the relief sleeve  261  via the bottom grooves  267  below the unit pressing spring holder  264 , reaches the check valve unit  250 , pushes the check ball  251  up from the check valve seat  253 , and enters the oil pressure chamber  301 . 
         [0096]    When the plunger  320  is pressed in to retract, the pressure inside the oil pressure chamber  201  rises, whereby the check ball  251  is pressed against the check valve seat  253 , and the oil is stopped from flowing out of the check valve unit  250 . The pressure then acts to press the check valve unit  250  itself downward against the pressing force of the unit pressing spring  263 . 
         [0097]    When the pressure inside the oil pressure chamber  301  reaches or exceeds a predetermined high pressure, the unit pressing spring  263  is compressed and the entire check valve unit  250  retracts, which causes the retainer  252  to separate from the relief valve seat  262  of the relief valve unit  260 , thus relieving the pressure inside the oil pressure chamber  301 . 
         [0098]    As the retainer  252  separates from the relief valve seat  262  to relieve the pressure inside the oil pressure chamber  301 , the oil flows out of the oil pressure chamber  301  through the release holes  265  to the outer circumferential surface of the relief sleeve  261 , and thus the oil circulates back to the oil reservoir chamber  323  through the oil supply hole  314  and the bottom grooves  267  of the unit pressing spring holder  264  without flowing out to the outside. 
         [0099]    When the pressure inside the oil pressure chamber  301  reduces to or below a predetermined level, the retainer  252  is again moved by the unit pressing spring  263  and sits on the relief valve seat  262 , to keep it shut and maintain the pressure inside. 
         [0100]    While the oil supply hole  314  is provided in the cylindrical surface  313  in this embodiment, it may be provided in the bottom  312 . 
         [0101]    Alternatively, as in the chain tensioner  300   b  shown in  FIG. 7 , a relief valve unit  260   b  that has a sleeve connection hole  266  in the rear side face of the relief sleeve  261  of the sliding range of the check valve unit  250  may be used, with this sleeve connection hole connecting the outer circumference of the relief sleeve with the oil reservoir space on the inners side of the relief sleeve, in order to increase the amount of oil circulating back to the oil reservoir chamber  323 . 
         [0102]    While specific examples of the chain tensioner according to the present invention have been described in the embodiments and variation examples above, the chain tensioner according to the present invention is not limited to these examples, and the shapes, positions, sizes, and positional relationships with each other of various constituent parts may be modified in various manners, or variously combined as appropriate. 
         [0103]    In various embodiments and variation examples described above, illustrations in which the plunger protrudes upwards were used and the plunger was described to protrude in this direction, but the plunger may be arranged to protrude in any direction in accordance with the mode in which it is used. 
         [0104]    In the second and third embodiments where there is an oil reservoir chamber, it is preferable to dispose the chain tensioner such that the oil reservoir chamber is located above the check valve. The relief valve will then be located above the oil pressure chamber and the chain tensioner can have enhanced robustness with respect to air leak possibilities. 
         [0105]    The chain tensioner of the present invention may not necessarily be applied to a chain guide mechanism that uses a guide shoe for slidably guiding a transmission chain such as an endless roller chain passing over respective sprockets of a crankshaft and a cam shaft in an engine room, but may also be adopted in applications where the chain is slidably guided directly by the distal end of the plunger. 
         [0106]    The chain tensioner may not necessarily be applied to a chain transmission mechanism but also used for similar transmission mechanisms that use belts, ropes and the like, and can be applied in a variety of industrial fields.