Abstract:
A hydraulic tensioner applies appropriate tension to an endless power transmitting belt. The hydraulic tensioner includes a plunger body, an oil supply path, a check valve, a relief valve, a pressure maintaining valve interposed in a relief path and a pressure accommodating hole. The hydraulic tensioner is configured such that a path that is branched from the oil supply path for operating the pressure maintaining valve is formed independently of a supply path to the check valve.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application Nos. 2011-077424 and 2011-077426, filed in Japan on Mar. 31, 2011, respectively. The entirety of each of the above-identified applications is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a hydraulic tensioner that applies appropriate tension to an endless power transmitting member, such as an endless chain or an endless belt for the power transmission of a valve motion mechanism of an internal combustion engine. 
     The present invention also relates to leak oil emitting device for a hydraulic tensioner that applies tension to an endless power transmitting member configured from a chain, a belt or the like used in a valve motion of an internal combustion engine. 
     2. Description of Background Art 
     An internal combustion engine incorporated in a vehicle such as a motorcycle includes a hydraulic tensioner for pressing an endless power transmitting member, which drives a camshaft of a valve motion mechanism in order to prevent flapping of the endless power transmitting member upon operation of the internal combustion engine. 
     A hydraulic tensioner is available, wherein a pressure maintaining valve for preventing a pressure drop in a high pressure oil chamber in the hydraulic tensioner in a state in which the oil supplying pressure drops upon stopping of operation of an internal combustion engine is provided in an oil supplying path (refer to, for example, Japanese Patent Laid-Open No. 2009-180359). 
     An internal combustion engine incorporated in a motorcycle or the like has an endless power transmitting member for driving a cam of a valve motion. In order to prevent flapping of the endless power transmitting member upon operation, the internal combustion engine includes a hydraulic tensioner for pressing the endless power transmitting member. As a conventional example, an example is available wherein the internal pressure of the hydraulic tensioner, which becomes a high pressure, is kept at an appropriate pressure by a gap appearing between a plunger made of a flexible material and a hollow sleeve fitted in the inside of the plunger, and leak oil is emitted from an emitting flow path provided in a housing (refer to, for example, Japanese Patent No. 3594420). However, since the oil entering the gap between an outer face of the plunger and a chamber wall face (plunger accommodating hole) cannot flow out quickly to the outside, movement of the plunger upon forward motion is suppressed, and the follow-up property for a movement of a tensioner slipper is deteriorated. However, if it is tried to carry out accuracy management of the gap over the overall contact area between the plunger outer face and the plunger accommodating hole, then since the accuracy management region is wide, the cost increases. 
     SUMMARY OF THE INVENTION 
     With the hydraulic tensioner disclosed in Japanese Patent Laid-Open No. 2009-180359, wherein a pressure maintaining valve is provided in an oil supplying path, by blocking the oil supplying path and an oil discharging path, also upon stopping of operation of the internal combustion engine, the oil pressure in the high pressure oil chamber can maintain a high level similarly as upon operation of the internal combustion engine. Also, upon re-starting of the internal combustion engine, the hydraulic tensioner can have an appropriate tensioning characteristic to the endless power transmitting member similarly as in the operation state of the internal combustion engine. 
     However, in order to obtain such a characteristic as just described, it is necessary to enhance the pressure resisting property and the sealing property of parts in the hydraulic tensioner, and there is the possibility that an increase of the cost may be provided. 
     The present invention relates to improvements in a hydraulic tensioner, which overcome such a defect as described above, and it is an object of the present invention to provide a hydraulic tensioner which can maintain an endless power transmitting belt in an appropriate tension state in whichever operation or stopping state an internal combustion engine is. 
     In order to achieve the object described above, according to an embodiment of the present invention, a hydraulic tensioner, includes a plunger, said plunger being acted upon by reactive force from an endless power transmitting belt; a plunger body cooperating with said plunger to form a high pressure oil chamber for storing pressure oil therein; a biasing device accommodated in said plunger, said biasing device being configured to push out said plunger toward said endless power transmitting belt; an oil supply path configured to supply oil into said high pressure oil chamber; a check valve configured to permit supply of the oil from said oil supply path into said high pressure oil chamber; a relief valve configured to communicate a relief path with an oil pressure higher than a predetermined oil pressure in said high pressure oil chamber; and a pressure maintaining valve interposed in said relief path for being opened and closed by the oil pressure of said oil supply path, wherein a path that is branched from said oil supply path for operating said pressure maintaining valve is formed independently of an oil supply path to said check valve. 
     According to an embodiment of the present invention, the hydraulic tensioner is configured such that the relief path and the oil supply path are in communication with each other by an oil supply pressure to the pressure maintaining valve. 
     According to a further embodiment of the present invention, the hydraulic tensioner is configured such that the pressure maintaining valve is provided in a returning path in communication with the relief path and the oil supply path, and in the returning path, the valve body of the pressure maintaining valve is formed in a cylindrical shape and a top portion side circumferential face of the cylindrical valve body is formed with a reduced diameter. 
     According to a further embodiment of the present invention, the hydraulic tensioner is configured such that a drain oil path in communication with the outside is provided in the plunger body, and the relief path and the drain oil path are in communication with each other by an oil supply pressure to the pressure maintaining valve. 
     According to a further embodiment of the present invention, the hydraulic tensioner is configured such that the pressure maintaining valve is formed in a cylindrical shape, and a cylindrical portion top face upon which operating oil acts and an emission hole for emitting the operating oil to the drain oil path side are provided on a cylindrical portion side face such that the operating oil is emitted through the inside of the pressure maintaining valve. 
     According to a further aspect of the present invention, the hydraulic tensioner is configured such that the oil supply path is configured through the inside of a valve body which configures the relief valve, and oil is supplied from an end portion of the relief valve. 
     According to a further embodiment of the present invention, the hydraulic tensioner is configured such that a holding plate for holding an end portion of a coil spring, which biases the relief valve, is disposed at a location of the oil supply path to which the oil is supplied from the end portion of the relief valve, and an oil introduction hole is provided in the holding plate. 
     According to a further embodiment of the present invention, the hydraulic tensioner is configured such that the pressure maintaining valve is provided in a converging relief path to which the relief paths converge after being formed radially from a hydraulic cylinder. 
     The present invention also enhances the emission property of oil through a gap between an inner circumferential face of a plunger body and an outer circumferential face of a plunger to reduce the sliding resistance of the plunger by oil resistance thereby to smoothen the movement of the plunger. 
     The present invention solves the problem described above, and according to an embodiment of the present invention, the hydraulic tensioner further comprises a gap portion, through which oil in said high pressure oil chamber can flow out to the outside, is provided between an inner circumferential face of said plunger body and an outer circumferential face of said plunger, wherein a downstream side gap of said gap portion through which the oil flows out has an area greater than that of an upstream side gap of said gap portion. 
     According to an embodiment of the present invention, the hydraulic tensioner is configured such that inner and outer diameters and working tolerances are set such that a gap dimension where the dimension of the downstream side gap is formed smallest is greater than a gap dimension where the dimension of the upstream side gap is formed greatest. 
     According to an embodiment of the present invention, the hydraulic tensioner is configured such that an oil emitting groove is formed on a downstream side sliding face of the plunger body such that an upstream end thereof extends to the upstream side gap. 
     According to an embodiment of the present invention, the hydraulic tensioner is configured such that the plunger has a plunger stepped portion formed from a plunger large diameter portion on the upstream side and a plunger small diameter portion on the downstream side; the plunger body has a plunger accommodating hole stepped portion formed from a plunger accommodating hole large diameter portion corresponding to the plunger large diameter portion on the upstream side of the plunger and a plunger accommodating hole small diameter portion corresponding to the plunger small diameter portion of the plunger; the plunger stepped portion of the plunger engages with the plunger accommodating hole stepped portion of the plunger body to form a coming off preventing structure for the plunger; and the oil emitting groove is formed on the plunger accommodating hole small diameter portion such that an upstream end thereof extends to the plunger accommodating hole large diameter portion. 
     According to a further embodiment of the present invention, the hydraulic tensioner is configured such that the oil emitting groove is formed by machining on the plunger accommodating hole small diameter portion from the downstream end side, and the tip of the working tool reaches the plunger accommodating hole large diameter portion. 
     According to an embodiment of the present invention, the hydraulic tensioner is configured such that the oil emitting groove is disposed at a position clear of the direction of movement of an endless timing chain. 
     According to a further embodiment of the present invention, the hydraulic tensioner is configured such that, in a state in which the hydraulic tensioner is attached to a vehicle, the hydraulic tensioner is disposed such that an oil emission direction of the oil emitting groove is directed downwardly with respect to a horizontal direction. 
     According to a further embodiment of the present invention, the hydraulic tensioner is configured such that the oil emitting groove is provided in a pair at opposing positions with respect to the center line of a plunger accommodating hole of the plunger body. 
     In the hydraulic tensioner according to the present invention, the oil released from the high pressure oil chamber by the relief valve is filled only between the downstream of the check valve and the pressure maintaining valve in the relief path, and the pressure is maintained. Consequently, the pressure on the relief valve side becomes equal to the pressure in the oil supply path. Therefore, oil of a high pressure is not filled, and accordingly, a high sealing property is not required, and reduction of the cost can be anticipated. 
     With the hydraulic tensioner according to the present invention, in a state in which the pressure maintaining valve comes under oil pressure, since the relief path and the oil supply path are in communication with each other and oil is circulated, a state in which oil is always filled can be maintained, and consequently, the possibility of insufficient supply can be reduced. 
     With the hydraulic tensioner according to the present invention, the returning path can be configured in a simplified shape, wherein only part of the shape of the valve body of the pressure maintaining valve is modified. 
     With the hydraulic tensioner according to the present invention, since the amount of oil is not returned to the supply side, the oil pressure on the supply side is not raised and the necessity to enhance the sealing property is eliminated. 
     With the hydraulic tensioner according to the present invention, since the drain oil structure can be achieved utilizing the inside of the pressure maintaining valve, the path can be simplified. 
     With the hydraulic tensioner according to the present invention, the oil supply path can be configured utilizing the relief valve, and consequently, the path can be simplified. 
     With the hydraulic tensioner according to the present invention, the oil introduction hole is formed in the holding plate, and oil supply can be carried out smoothly. 
     With the hydraulic tensioner according to the present invention, since the pressure maintaining valve is disposed in the converging relief path, simplification of the path by the single pressure maintaining valve can be anticipated. 
     In an embodiment of the present invention, the area of the upstream side gap set for oil pressure adjustment is formed small while the area of the downstream side gap is formed greater than that of the upstream side gap. Consequently, the oil emission performance on the downstream side, which does not act for oil pressure adjustment, is enhanced thereby to reduce the sliding resistance of the plunger by oil resistance. Consequently, the movement of the plunger can be smoothened. 
     In an embodiment of the present invention, the inner and outer diameters and working tolerances are set such that the downstream side gap is greater with certainty than the upstream side gap. Consequently, the dimension management of the gaps can be made sure and the oil emitting property of the downstream side can be enhanced. 
     In an embodiment of the present invention, since the oil emitting groove is formed on the sliding face downstream side, the emitting performance of oil on the downstream side can be improved. 
     In an embodiment of the present invention, the oil emitting groove is formed on the plunger accommodating hole small diameter portion with which the plunger stepped portion of the plunger large diameter portion is engaged such that it extends to the plunger accommodating hole large diameter portion. Accordingly, the oil emission property of the plunger accommodating hole small diameter portion can be improved. 
     In an embodiment of the present invention, the oil emitting groove is formed by machining on the plunger accommodating hole small diameter portion from the downstream end side such that the tip thereof reaches the plunger accommodating hole large diameter portion. Accordingly, the oil emitting groove can be formed readily. 
     In an embodiment of the present invention, the oil emitting groove is disposed clear of the direction of movement of the endless timing chain. Consequently, when the plunger body vibrates upon movement of the endless timing chain, the plunger small diameter portion can be prevented from being damaged by an edge of the oil emitting groove. 
     In an embodiment of the present invention, the hydraulic tensioner is attached in the downward direction with respect to the horizontal direction to the vehicle. Consequently, in the state in which the hydraulic tensioner is attached to the vehicle, also emission is caused by the self weight of oil. Therefore, the oil can be emitted well. 
     In an embodiment of the present invention, the oil emitting grooves are disposed not at neighboring positions but are spaced away from each other at the opposing positions. Consequently, oil can be emitted well while suppressing a drop in strength of the plunger body. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  shows a hydraulic tensioner lift of an embodiment of the present invention and shows an example wherein the hydraulic tensioner is applied to a timing chain, which configures a power transmitting mechanism of a valve motion mechanism in a four-stroke cycle DOHC type internal combustion engine incorporated in a motorcycle; 
         FIG. 2  is a view as viewed in the direction indicated by an arrow mark II of  FIG. 1  showing a shape of a cap of the hydraulic tensioner; 
         FIG. 3  is a rear elevational view of a tensioner body of the hydraulic tensioner as viewed in the direction indicated by the arrow mark II of  FIG. 1   
         FIG. 4  is a vertical sectional view of the hydraulic tensioner taken along line IV-IV of  FIG. 2 ; 
         FIG. 5  is a vertical sectional view of the hydraulic tensioner in a state in which a plunger projects forwardly from the tensioner body and a tension attaching portion of a cylinder head in  FIG. 4 ; 
         FIG. 6  is a vertical sectional view of the hydraulic tensioner taken along line VI-VI of  FIG. 2 ; 
         FIG. 7  is a fragmentary longitudinal sectional view of a check valve and a relief valve of the hydraulic tensioner; 
         FIG. 8  is a fragmentary longitudinal sectional view in a state in which the check valve and the relief valve of the hydraulic tensioner are assembled to a valve holder and a purge valve is assembled to the tensioner body while a pressure maintaining valve is disassembled; 
         FIG. 9  is a vertical sectional view of a hydraulic tensioner of another embodiment of the present invention and is a vertical sectional view taken along a plane similar to that of  FIG. 4 ; 
         FIG. 10  is a vertical sectional view of the hydraulic tensioner of the embodiment shown in  FIG. 9  and is a vertical sectional view taken along a plane similar to that of  FIG. 5 ; 
         FIG. 11  is a right side elevational view of essential part illustrating a state in which an internal combustion engine including a hydraulic tensioner is attached to a vehicle body frame of a motorcycle; 
         FIG. 12  is a rear elevational view of the tensioner, particularly a view as viewed in the direction indicated by an arrow mark II of  FIG. 11 ; 
         FIG. 13  is a view of a tensioner body as viewed from the rear; 
         FIG. 14  is a vertical sectional view taken along line IV-IV of  FIG. 12  and is a longitudinal sectional view of the tensioner; 
         FIG. 15  is a fragmentary perspective view of a plunger, a valve holder, a check valve, a relief valve and so forth inserted in a plunger accommodating hole; 
         FIG. 16  is a view illustrating a disassembled state of a pressure maintaining valve and a purge valve; 
         FIG. 17  is a sectional view taken along line VII-VII of  FIG. 12 ; 
         FIG. 18  is a view illustrating a state in which the pressure in an oil reserving chamber rises to project the plunger forwardly; 
         FIG. 19  is a partial enlarged view of a gap portion (between the plunger accommodating hole and the plunger) of  FIG. 14 ; 
         FIG. 20  is a front elevational view of the tensioner body  20 ; and 
         FIG. 21  is a sectional view taken along line XI-XI of  FIG. 20 ; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described with reference to the accompanying drawings wherein the same or similar elements will be identified by the same reference numeral. 
     In the following, a hydraulic tensioner  0  according to an embodiment of the present invention shown in  FIGS. 1 to 8  is described. 
       FIG. 1  is a right side elevational view of an essential part illustrating a state in which an internal combustion engine  3  which includes the hydraulic tensioner  0  is attached to a vehicle body frame  1  of a motorcycle (which may otherwise be a road surface traveling vehicle such as an automobile). 
     In the present embodiment, forward and backward, upward and downward, and leftward and rightward directions signify the forward and backward, upward and downward, leftward and rightward directions of the vehicle body. In  FIG. 1 , the forward and backward directions correspond to the rightward and leftward directions on the plane of the figure; the upward and downward directions correspond to the upward and downward directions on the plane of the figure; and the leftward and rightward directions correspond to the interior side direction and this side direction with respect to the plane of the figure, respectively. 
     As shown in  FIG. 1 , the four-stroke cycle DOHC type internal combustion engine  3 , which includes the hydraulic tensioner  0  is incorporated in a motorcycle and has a particular structure that the internal combustion engine  3  is attached to a hanger  2  provided at a front portion of a vehicle body frame  1  of the motorcycle and a rear portion of the vehicle body frame  1  by two bosses  8  projecting from the internal combustion engine  3 . 
     In the internal combustion engine  3 , a cylinder block  5 , a cylinder head  6  and a head cover  7  are successively placed in order in the upward direction on a crankcase  4  and are coupled integrally by a coupling device such as bolts (not shown). 
     Further, an endless timing chain  13  extends between and around a driving sprocket wheel  10 , which is integrated with a crankshaft  9  supported for rotation, and a driven sprocket wheel  12 , which is integrated with a pair of camshafts  11  supported for rotation on the cylinder head  6  in the cylinder head  6  and the head cover  7 , at a position sandwiched between the crankcase  4  and the cylinder block  5  of the internal combustion engine  3 . Consequently, a rotating torque of the crankshaft  9 , which is driven to rotate in the clockwise direction in the figure by upward and downward movement of pistons (not shown) fitted for upward and downward sliding movement in cylinder bores (not shown) of the cylinder block  5  is transmitted to the paired camshafts  11  through the driving sprocket wheel  10 , endless timing chain  13  and driven sprocket wheel  12  so that intake and exhaust valves not shown are driven for opening and closing movement. 
     In the four-stroke cycle internal combustion engine  3 , a plurality of cylinder bores (not shown) are arrayed in the vehicle widthwise direction. In this internal combustion engine  3 , fuel in a combustion chamber burns in each of the cylinder bores every time the crankshaft  9  rotates twice, and the pistons are intermittently pushed toward the crankshaft  9  by pressure of the combustion gas. Further, since the traveling resistance varies in response to concaves and convexes of the road surface on which the motorcycle travels, the tension state of the endless timing chain  13  varies and the endless timing chain  13  is liable to flap in the forward and backward directions. 
     In order to prevent this, a chain guide  14  is disposed in contact with the endless timing chain  13  on the tension side on the front positioned rightwardly in  FIG. 1  while a tensioner slipper  15  is disposed in contact with the endless timing chain  13  on the relax side on the rear positioned leftwardly in  FIG. 1 . Further, the hydraulic tensioner  0  is assembled to the cylinder head  6  rearwardly of and adjacent to the tensioner slipper  15 . The hydraulic tensioner  0  has such a structure and a characteristic as hereinafter described in detail, and flapping of the endless timing chain  13  on the relax side can be suppressed effectively by the superior characteristic of the hydraulic tensioner  0 . 
     The shell of the hydraulic tensioner  0  shown in  FIG. 1  is configured from a tensioner body  20  and a cap  21 . Bolts (not shown), which are fitted in a pair of left and right bolt fitting holes  21   a  provided in the cap  21  shown in  FIG. 2 , which is a view as viewed in the direction as indicated by an arrow mark II in  FIG. 1 , extend through bolt fitting holes  20   b  of the tensioner body  20  shown in  FIG. 3  and are screwed in a lifter attaching portion  6   a  of a rear portion of the cylinder head  6  to mount the hydraulic tensioner  0  integrally on the cylinder head  6 . As shown in  FIGS. 4 to 6 , a plunger accommodating hole  20   a  is formed in the tensioner body  20 , and a plunger  23  is accommodated for sliding movement in the plunger accommodating hole  20   a . Consequently, the tensioner body  20  plays a role as a plunger body. 
     Further, as shown in  FIG. 3 , a packing fitting groove  20   d  is formed on a rear end face  20   c  of the tensioner body  20  in such a manner as to surround the plunger accommodating hole  20   a , and an annular packing  20   e  is fitted in the packing fitting groove  20   d . As shown in  FIGS. 4 to 6 , an oil reserving chamber  28  is configured from a rear end face  20   c  formed on a front end face  21   b  of the cap  21  and the rear end face  20   c  of the tensioner body  20 . 
     Further, a base end portion  22   a  of a valve holder  22 , formed in such a manner as shown in  FIG. 7 , is fitted in a rear portion of the plunger accommodating hole  20   a  of the tensioner body  20  as shown in  FIGS. 4 to 6 , and the plunger  23  is fitted for forward and backward sliding movement in the plunger accommodating hole  20   a  of the tensioner body  20 . Further, in a high pressure oil chamber  31  in the plunger accommodating hole  20   a  and the plunger  23 , a coil spring  24  as a biasing device is interposed between a stepped front end face  22   b  of the base end portion  22   a  of the valve holder  22  and an inner face  23   b  of a tip portion  23   a  of the plunger  23  such that the plunger  23  is biased so as to project forwardly by spring restoring force of the coil spring  24 . It is to be noted that an abutting portion  23   c  is mounted integrally at the tip portion  23   a  of the plunger  23 . 
     Furthermore, a valve guide  25   a  of a check valve  25  is fitted integrally at a front portion of a valve accommodating hole  22   c  (refer to  FIG. 7 ) formed at the base end portion  22   a  of the valve holder  22 . A check valve coil spring  25   b  and a spherical valve body  25   c  are fitted in order in the forward direction from the rear on the valve guide  25   a.    
     Besides, a valve body  26   a  of a relief valve  26  is fitted for sliding movement in the valve accommodating hole  22   c  of the valve holder  22 , and in a valve chamber  26   b  of the valve body  26   a  of the relief valve  26 , a relief valve coil spring  26   d  is interposed between a relief valve seat  27  disposed adjacent the cap  21  in the oil reserving chamber  28  and a tip portion  26   c  of the valve body  26   a  of the relief valve  26 . The oil reserving chamber  28  is connected to the high pressure oil chamber  31  through an opening  27   a  of the relief valve seat  27 , relief valve  26  and check valve  25 . 
     A pressure maintaining valve  29  will now be described. 
     As shown in  FIG. 3  in which the tensioner body  20  is viewed from the rear of the vehicle body toward the front of the vehicle body, a pressure maintaining valve accommodating hole  20   f  is formed in parallel (refer to  FIGS. 4 and 5 ) to the plunger accommodating hole  20   a  and positioned obliquely rightwardly downwards with respect to the plunger accommodating hole  20   a . The pressure maintaining valve accommodating hole  20   f  is open at a rear end thereof to the oil reserving chamber  28  surrounded by the annular packing  20   e.    
     Further, before the cap  21  is mounted on the tensioner body  20 , a spring receiver  29   a , a closing coil spring  29   b  and a valve body  29   c  of the pressure maintaining valve  29  are successively fitted through an opening of the pressure maintaining valve accommodating hole  20   f  of the tensioner body  20 . A rear cylindrical circumferential face  29   d  of the valve body  29   c  is formed with a reduced diameter. The valve body  29   c  is biased rearwardly as shown in  FIG. 4  by spring restoring force of the closing coil spring  29   b  so that a rear end face  29   e  of the rear cylindrical circumferential face  29   d  of the valve body  29   c  is abutted by the front end face  21   b  of the cap  21  to close up a communication port  20   g  of the tensioner body  20  which is in communication with a relief valve port  22   e  of the valve holder  22 . 
     It is to be noted that the relief valve port  22   e  formed in the valve holder  22  is configured from a circumferential groove  22   f  formed on an outer circumferential face of the base end portion  22   a , and a plurality of communication holes  22   g  formed on the bottom of the circumferential groove toward the center of the valve accommodating hole  22   c  in a spaced relationship from each other by an equal distance in the circumferential direction. If oil is relieved from the relief valve  26 , then the oil flows from a relief path formed from the relief valve port  22   e  and the communication port  20   g  to a returning path formed from the outer circumferential face of the pressure maintaining valve  29  and the pressure maintaining valve accommodating hole  20   f  into the oil reserving chamber  28 . 
     A purge valve  30  will now be described. 
     As shown in  FIG. 3  in which the tensioner body  20  is viewed from the rear of the vehicle body toward the front, a purge valve accommodating hole  20   h  having a circular transverse section is formed in parallel to the plunger accommodating hole  20   a  at a position obliquely leftwardly upwards with respect to the plunger accommodating hole  20   a . The purge valve accommodating hole  20   h  is in communication at a rear portion thereof with the high pressure oil chamber  31  through a purge path  30   a  as shown in  FIGS. 4 and 5 . 
     Further, as shown in  FIG. 8 , a valve body  30   b , a coil spring  30   e  and a spring receiving tubular body  30   f  are fitted successively in the purge valve accommodating hole  20   h  from a front open end of the purge valve accommodating hole  20   h . The spring receiving tubular body  30   f  is screwed integrally in the purge valve accommodating hole  20   h.    
     Further, an annular groove  30   c  of a substantially rectangular (or square) cross section is formed on a circumferential face of the valve body  30   b , and an annular valve body  30   d  having a substantially rectangular (or square) cross section is fitted in the annular groove  30   c . If a pressure variation in the high pressure oil chamber  31  is transmitted to the valve body  30   b  through the purge path  30   a , then the valve body  30   b  is moved back and forth in the forward and backward directions by the elastic restoring force of the coil spring  30   e . Thereupon, air contained in the oil is separated from the oil and discharged into the atmospheric air. 
     Finally, an oil supplying system will be described. 
     As shown in  FIG. 6 , the oil reserving chamber  28  configured from the rear end face  20   c  of the tensioner body  20 , concave face  21   c  of the cap  21  and rear end face  20   c  of the tensioner body  20  is connected at a lower portion thereof to a cylinder head oil path  33  formed in the lifter attaching portion  6   a  of the cylinder head  6  through a tensioner oil path  32  extending in a downwardly inclined relationship from a rear portion to a front portion through the tensioner body  20  as shown in  FIGS. 3 and 6 . As shown in  FIG. 1 , the cylinder head oil path  33  is connected to an oil filter  35  through an oil path  34  of the crankcase  4  through an oil path not shown in the cylinder block  5 . The oil filter  35  is connected to a discharge port of an oil pump  37  through an oil path  36 , and oil reserved on the bottom of the crankcase  4  is sucked into the oil pump  37  through a strainer  38  by the oil pump  37  which is placed into an operative state in an interlocking relationship with operation of the internal combustion engine  3 . Oil discharged from the oil pump  37  is supplied to the oil reserving chamber  28  through the oil path  36 , oil filter  35 , oil path  34 , cylinder head oil path  33  and tensioner oil path  32 . 
     In the present embodiment, an oil supply path  40  signifies a path up to a valve hole  26   e  configured from the tensioner oil path  32 , the oil reserving chamber  28  and the valve chamber  26   b  of the relief valve  26 . 
     In the embodiment shown in  FIGS. 1 to 8 , if the internal combustion engine  3  starts its operation, then the oil pump  37  is placed into an operative state, and oil is fed from the discharge port of the oil pump  37  to the cylinder head oil path  33  of the lifter attaching portion  6   a  of the cylinder head  6  as described hereinabove. Then, the oil is fed into the oil reserving chamber  28  through the tensioner oil path  32  of the tensioner body  20  as shown in  FIG. 6  and flows into the valve chamber  26   b  of the relief valve  26  from the opening  27   a  of the relief valve seat  27 . 
     If the pressure in the oil reserving chamber  28  rises and exceeds a cracking pressure, then the check valve  25  is opened and the oil flows into the high pressure oil chamber  31  through the valve hole  26   e  of the relief valve  26 , whereupon the plunger  23  is projected forwardly until the abutting portion  23   c  of the plunger  23  is abutted with the tensioner slipper  15 . Then, the plunger  23  forwardly pushes the tensioner slipper  15  strongly with the oil pressure in the high pressure oil chamber  31  to place the endless timing chain  13  on the relax side into a tensioned state thereby to suppress flapping of the endless timing chain  13 . 
     On the other hand, in a state in which the internal combustion engine  3  stops and the oil pressure in the oil reserving chamber  28  is low, the pressure maintaining valve  29  is closed by the spring restoring force of the closing coil spring  29   b  of the pressure maintaining valve  29  as shown in  FIG. 4 . However, if the oil pressure in the oil reserving chamber  28  rises in an interlocking relationship with starting of the internal combustion engine  3 , then it overcomes the spring restoring force of the closing coil spring  29   b  thereby to push the valve body  29   c  of the pressure maintaining valve  29  forwardly so that the rear cylindrical circumferential face  29   d  of the reduced diameter of the pressure maintaining valve  29  approaches the communication port  20   g  of the tensioner body  20 . Then, when the communication port  20   g  is opened as shown in  FIG. 5 , the oil flows into the relief valve port  22   e  of the valve holder  22 . 
     If, in such a state as just described, the tensioner slipper  15  is tilted rearwardly and the plunger  23  is pushed back strongly by variations of the rotating torque of the driving sprocket wheel  10  by an intermittent pressure rise of the pressure in the fuel chambers in the internal combustion engine  3  or the like, then the oil pressure in the high pressure oil chamber  31  rises, whereupon the valve body  26   a  of the relief valve  26  moves rearwardly. Consequently, an abutting face  26   f  of the relief valve  26  is spaced away from a valve seat  25   d  of the check valve  25 , whereupon the oil in a valve chamber  25   e  of the check valve  25  flows from between the valve seat  25   d  and the abutting face  26   f  of the tip portion  26   c  of the relief valve  26  back into the oil reserving chamber  28  through the relief valve port  22   e  of the valve holder  22  and the communication port  20   g  and the pressure maintaining valve accommodating hole  20   f  of the tensioner body  20 . Consequently, an extraordinary increase in tension of the endless timing chain  13  is inhibited and flapping of the tensioner slipper  15  is suppressed. 
     Further, in the hydraulic tensioner disclosed in Japanese Patent Laid-Open No. 2009-180359 described hereinabove, since the pressure maintaining valve and the check valve are connected in series, oil fed from the discharge port of the oil pump cannot flow into the high pressure oil chamber if it does not pass through the check valve after it passes through the pressure maintaining valve. However, in the present embodiment, since the pressure maintaining valve  29  and the check valve  25  are connected in parallel to each other, even in a state in which the pressure maintaining valve  29  is closed, if the check valve  25  is opened, then the oil can flow into the high pressure oil chamber  31 . Therefore, in immediate response to starting of the internal combustion engine  3 , the oil is supplied into the high pressure oil chamber  31 . Consequently, the starting responsibility of the hydraulic tensioner  0  is improved, and the internal combustion engine  3  can start its operation smoothly. 
     Further, in the hydraulic tensioner disclosed in Japanese Patent Laid-Open No. 2009-180359 1 described hereinabove, the check valve is not opened until after oil pressure from the oil pump has been fed by a volume equal to the product of the area of the transverse section of the valve hole of the pressure maintaining valve and the stroke necessary for opening of the pressure maintaining valve. Therefore, the hydraulic tensioner cannot exhibit its function. However, in the present embodiment, if the check valve  25  is opened, then the hydraulic tensioner  0  can immediate exhibit its function independently of the opening of the pressure maintaining valve  29 . Therefore, the hydraulic tensioner of the present embodiment can exhibit its superior function sufficiently particularly in a hybrid vehicle wherein operation of the internal combustion engine  3  stops when the vehicle stops. 
     Furthermore, in the hydraulic tensioner disclosed in Japanese Patent Laid-Open No. 2009-180359 described hereinabove, the pressure maintaining valve is disposed on the oil pump side with respect to the check valve and the relief valve, and the pressure in the high pressure oil chamber acts upon the pressure maintaining valve. Therefore, the pressure maintaining valve requires a sealing property equivalent to that of the check valve, and consequently, there is the possibility that the cost may increase. However, in the present embodiment, since the pressure maintaining valve  29  is disposed in parallel to the check valve  25  and the relief valve  26 , it does not require a high sealing property and is advantageous in terms of the cost. 
     A second embodiment of the present invention shown in  FIGS. 9 and 10  will be described. 
     A pressure maintaining valve  39  is used in place of the pressure maintaining valve  29  shown in  FIGS. 1 to 8 . In this pressure maintaining valve  39 , a pressure maintaining valve port  39   f  is formed on an outer circumferential portion  39   e  of a valve head portion  39   d  of a valve body  39   c , and a communication hole  39   g  is formed in a spring receiver  39   a . This communication hole  39   g  is in communication with an oil draining path  6   b  of the cylinder head  6  through a drain oil path  20   i  of the rear end face  20   c , and the spring receiver  39   a , a closing coil spring  39   b  and the valve body  39   c  are successively fitted through an opening of the pressure maintaining valve accommodating hole  20   f . Where the oil pressure in the oil reserving chamber  28  is lower than a predetermined pressure, the valve head portion  39   d  is abutted with the front end face  21   b  of the cap  21  under the spring restoring force of the closing coil spring  39   b  of the pressure maintaining valve  39  as shown in  FIG. 9 . Consequently, the communication between the pressure maintaining valve port  39   f  provided in the valve head portion  39   d  and the communication port  20   g  of the valve holder  22  is blocked. 
     However, if the rotational frequency of the oil pump  37  increases until the check valve  25  is opened by the increasing oil pressure in the oil reserving chamber  28 , then the oil is supplied into the high pressure oil chamber  31  and the plunger  23  is projected to push the tensioner slipper  15  sufficiently so that suitable tension can be applied to the endless timing chain  13 . In this state, the increasing oil pressure in the oil reserving chamber  28  is transmitted to the pressure maintaining valve accommodating hole  20   f , and by the oil pressure, the pressure maintaining valve port  39   f  of the pressure maintaining valve  39  is in communication with the communication port  20   g  and the relief valve port  22   e . Therefore, when the oil pressure in the high pressure oil chamber  31  exceeds the relief pressure of the relief valve  26 , the relief valve  26  is opened, and the oil in the high pressure oil chamber  31  is fed to the oil draining path  6   b  through the drain oil path  20   i  passing the relief path configured from the relief valve port  22   e , communication port  20   g , pressure maintaining valve port  39   f  and communication hole  39   g  from the relief valve  26 . Meanwhile, the oil in the high pressure oil chamber  31  whose pressure exceeds the relief pressure of the relief valve  26  is discharged into the space in the internal combustion engine from the oil draining path  6   b  irrespective of the oil in the oil reserving chamber  28 . As a result, extraordinary tension to the endless timing chain  13  can be prevented. 
     Also in the present embodiment, similarly as in the embodiment of  FIGS. 1 to 8 , the circuit which establishes communication from the oil reserving chamber  28  to the pressure maintaining valve accommodating hole  20   f  of the pressure maintaining valve  39  is connected in parallel to the circuit which establishes communication from the oil reserving chamber  28  to the high pressure oil chamber  31  through the valve chamber  26   b  of the relief valve  26 , valve chamber  25   e  of the check valve  25  and relief valve port  22   e . Therefore, this point provides effects similar to those in the first embodiment. 
     In the first and second embodiments described above, in the state in which the oil pump  37  stops, oil released from the high pressure oil chamber  31  by the relief valve  26  is filled only between the downstream of the check valve  25  and the relief valve port  22   e  and communication port  20   g  which form the relief path communicating with the pressure maintaining valve  29  and the pressure maintaining valve  39 , and the pressure is maintained. Consequently, the oil pressure in the relief valve  26  is equal to the oil pressure in the oil supply path  40 . Therefore, oil of a high pressure is not filled, and consequently, the relief valve  26  does not require a high sealing property. This makes it possible to achieve reduction of the cost. 
     Further, in the state in which the pressure maintaining valves  29  and  39  are acted upon by oil pressure from the oil reserving chamber  28 , the relief valve port  22   e  and the communicating port  20   g  which form the relief path and the oil supply path  40  are in communication with each other and oil is circulated. Consequently, insufficient supply of oil to the hydraulic tensioner  0  is suppressed. 
     Furthermore, only by partly modifying the shape of the valve bodies  29   c  and  39   c  of the pressure maintaining valves  29  and  39 , the returning path can be configured in a simplified structure. 
     Furthermore, since oil is not returned to the oil reserving chamber  28  side, an increase of the oil pressure in the oil supply path  40  is suppressed, and the necessity to raise the sealing property of the check valve  25 , relief valve  26  and pressure maintaining valves  29  and  39  is eliminated and reduction of the cost of the hydraulic tensioner  0  can be anticipated. 
     Besides, since the valve bodies  29   c  and  39   c  of the pressure maintaining valves  29  and  39  have a simple cylindrical shape, also the pressure maintaining valve accommodating hole  20   f  between the relief valve port  22   e  and communication port  20   g  and the drain oil path  20   i  of the tensioner body  20  may be formed in a simple shape. Besides, since the pressure maintaining valve  39  is structured such that high pressure relief oil is discharged into the drain oil path  20   i  through the inside of the valve body  39   c  of the pressure maintaining valve  39 , the drain oil path  20   i  is simplified and reduction of the cost can be anticipated. 
     Further, since the oil supply path  40  can be configured utilizing the valve holder  22 , the oil supply path  40  is simplified and reduction of the cost can be anticipated. 
     Furthermore, supply oil can be supplied smoothly into the hydraulic tensioner  0  through an opening  7   a  formed in the relief valve seat  27  serving as a holding plate. 
     Furthermore, in the relief valve port  22   e  formed in the valve holder  22 , a converging discharge path is configured from a plurality of communicating holes directed in radial directions from the valve accommodating hole  22   c  and a circumferential groove formed on an outer circumferential face of the base end portion  22   a  in a communicating relationship with the communicating holes. Therefore, a simple oil path can be configured from the single pressure maintaining valve  29  or  39  by disposing the pressure maintaining valve  29  or  39  such that it is connected to the converging discharge path. 
       FIG. 11  is a right side elevational view of essential part illustrating a state in which an internal combustion engine  3  which includes a hydraulic tensioner  0  is attached to a vehicle body frame  1  of a motorcycle. In  FIG. 11 , the rightward direction is the forward direction of the vehicle. Referring to  FIG. 11 , the four-stroke cycle DOHC internal combustion engine  3  which includes the hydraulic tensioner  0  is incorporated in the motorcycle. The internal combustion engine  3  is attached to a hanger  2  provided at a front portion of the vehicle body frame  1  of the motorcycle and a rear portion of the vehicle body frame  1  through two bosses  8  projecting from the internal combustion engine  3 . 
     In the internal combustion engine  3 , a cylinder block  5 , a cylinder head  6  and a head cover  7  are successively placed in order in the upward direction on a crankcase  4  and are coupled integrally by a coupling device such as bolts. A crankshaft  9  is supported for rotation at a position sandwiched between the crankcase  4  and the cylinder block  5  of the internal combustion engine  3 . A pair of camshafts  11  is supported for rotation on the cylinder head  6  in the cylinder head  6  and the head cover  7 . An endless timing chain  13  extends between and around a driving sprocket wheel  10 , which is integrated with the crankshaft  9 , and a driven sprocket wheel  12 , which is integrated with the paired camshafts  11 . The crankshaft  9  is driven to rotate by upward and downward movement of pistons fitted for sliding movement in cylinder bores of the cylinder block  5 , and rotating torque of the crankshaft  9  is transmitted to the paired camshafts  11  through the driving sprocket wheel  10 , endless timing chain  13  and driven sprocket wheel  12  so that intake and exhaust valves are driven for opening and closing movement. 
     In the four-stroke cycle internal combustion engine  3 , fuel in a combustion chamber burns every time the crankshaft  9  rotates twice, and the pistons are intermittently pushed toward the crankshaft  9  by pressure of the combustion gas. Further, since the traveling resistance varies in response to concaves and convexes of the road surface on which the motorcycle travels, the tension state of the endless timing chain  13  varies and the endless timing chain  13  is liable to flap in the forward and backward directions. In order to prevent this, a chain guide  14  is disposed in contact with the endless timing chain  13  on the tension side positioned rightwardly (forwardly of the vehicle) in  FIG. 11  while a tensioner slipper  15  is disposed in contact with the endless timing chain  13  on the relax side positioned leftwardly (rearwardly of the vehicle) in  FIG. 11 . Further, the hydraulic tensioner  0  is assembled to the cylinder head  6  rearwardly of and adjacent to the tensioner slipper  15 . By a characteristic of the hydraulic tensioner  0 , flapping of the endless timing chain  13  on the relax side can be suppressed effectively. 
       FIG. 12  is a rear elevational view of the tensioner  0 , that is, a view as viewed in the direction indicated by an arrow mark II of  FIG. 11 . The shell of the hydraulic tensioner  0  is configured from a tensioner body  20  and a cap  21  ( FIGS. 12 and 14 ). The cap  21  has a pair of left and right bolt fitting holes  21   a  provided therein. 
       FIG. 13  is a view of the tensioner body  20  as viewed from the rear with the cap  21  removed. Bolts not shown which are fitted in the paired left and right bolt fitting holes  21   a  ( FIG. 12 ) provided in the cap  21  shown extend through bolt fitting holes  20   b  ( FIG. 13 ) of the tensioner body  20  and are screwed in a tensioner attaching portion  6   a  ( FIG. 11 ) of a rear portion of the cylinder head  6  to mount the hydraulic tensioner  0  integrally on the cylinder head  6 . In  FIG. 13 , a plunger accommodating hole  20   a  ( FIG. 13 ) in which a plunger  23  and a valve holder  22  ( FIG. 14 ) are fitted extends through the center of the tensioner body  20 . 
       FIG. 14  is a sectional view taken along line Iv-Iv of  FIG. 12  and is a longitudinal sectional view of the tensioner  0 . Arrow marks Fr and Re indicate directions as forward and backward directions of the tensioner itself in order to indicate the positions of the member of the tensioner  0 , and Fr indicates the forward direction and Re indicates the rearward direction. The shell of the tensioner  0  is configured from the tensioner body  20  and the cap  21 . In a state in which the cap is removed, a plunger  23 , a coil spring  24  and a valve holder  22  are successively mounted from the rear end side of the tensioner body  20  in the plunger accommodating hole  20   a  provided at a central portion of the tensioner body  20 . The plunger  23  is slidable forwardly and backwardly in the plunger accommodating hole  20   a . The plunger  23  is biased so as to project forwardly by spring restoring force of the coil spring  24 . It is to be noted that an abutting portion  23   c  is integrally mounted at a tip portion  23   a  of the plunger  23 . 
     A check valve  25 , a relief valve  26  and a relief valve seat  27  are successively mounted in a valve accommodating hole  22   c  ( FIG. 15 ) formed at a base end portion  22   a  ( FIG. 15 ) of the valve holder  22 . A pressure maintaining valve  29  is mounted from a rear end of the tensioner body  20  in a pressure maintaining valve accommodating hole  20   f  provided at a lower portion of the tensioner body  20 . A purge valve  30  is mounted from a front portion of a purge valve accommodating hole  20   h  ( FIG. 13 ) provided at an upper portion of the tensioner body  20 . An oil reserving chamber  28  is defined by and between an inner face of the cap  21  and a rear end of the valve holder  22 . The circumference of the oil reserving chamber  28  is surrounded by an annular packing  20   e  ( FIG. 13 ) fitted in a packing fitting groove  20   d  ( FIG. 13 ). A high pressure oil chamber  31  is defined by and between an inner face of the plunger  23  and an outer face of a coil spring holding body  22   d  of the valve holder  22 . 
       FIG. 15  is a fragmentary perspective view of the plunger  23 , coil spring  24 , valve holder  22 , check valve  25 , relief valve  26 , relief valve seat  27  and cap  21  inserted in the plunger accommodating hole  20   a . The coil spring  24  is interposed between an inner face  23   b  of the tip portion  23   a  of the plunger  23  and a stepped portion front end face  22   b  of the valve holder  22 . A valve guide  25   a  of the check valve  25  is fitted at a front portion of the valve accommodating hole  22   c  formed in the base end portion  22   a  of the valve holder  22 . A check valve coil spring  25   b  and a spherical valve body  25   c  are successively fitted in the forward direction from a rear portion with the valve guide  25   a.    
     A valve body  26   a  of the relief valve  26  is fitted for sliding movement in a rear portion of the valve accommodating hole  22   c , and a relief valve coil spring  26   d  is interposed between the relief valve seat  27  disposed in the oil reserving chamber  28  and a tip portion  26   c  of the valve body  26   a  of the relief valve  26  in a valve chamber  26   b  of the valve body  26   a  of the relief valve  26 . The oil reserving chamber  28  is connected to the high pressure oil chamber  31  through an opening  27   a  of the relief valve seat  27 , the relief valve  26 , the check valve  25  and a through-hole  22   f  of the coil spring holding body  22   d  of the valve holder  22 . 
     Referring to  FIG. 13 , the pressure maintaining valve accommodating hole  20   f  is formed in parallel to the plunger accommodating hole  20   a  and positioned obliquely rightwardly downwards with respect to the plunger accommodating hole  20   a , and the pressure maintaining valve  29  is accommodated in the pressure maintaining valve accommodating hole  20   f . The pressure maintaining valve accommodating hole  20   f  is open at a rear end thereof to the oil reserving chamber  28  ( FIG. 14 ) surrounded by the annular packing  20   e.    
       FIG. 16  illustrates a disassembled state of the pressure maintaining valve  29 . In a state in which the cap is removed, a spring receiver  29   a , a closing coil spring  29   b  and a valve body  29   c  of the pressure maintaining valve  29  are successively fitted from the opening of the pressure maintaining valve accommodating hole  20   f  of the tensioner body  20 . A rear cylindrical circumferential face  29   d  of the valve body  29   c  is formed with a reduced diameter. The valve body  29   c  is biased rearwardly as illustrated in  FIG. 14  by spring restoring force of the closing coil spring  29   b  until a rear end face  29   e  of the rear cylindrical circumferential face  29   d  of the valve body  29   c  is abutted with a front end face  21   b  ( FIG. 14 ) of the cap  21 . Consequently, a communication port  20   g  ( FIGS. 4 and 6 ) of the tensioner body  20  which is in communication with the relief valve port  22   e  ( FIGS. 4 and 5 ) of the valve holder  22  is closed up. It is to be noted that the relief valve port  22   e  formed in the valve holder  22  is configured from a circumferential groove formed on an outer circumferential face of the base end portion  22   a  and a plurality of communicating holes formed toward the center of the valve accommodating hole  22   c  in an equally spaced relationship from each other in the circumferential direction on the bottom of the circumferential groove. 
     Referring to  FIG. 13 , the purge valve accommodating hole  20   h  having a circular transverse section is formed in parallel to the plunger accommodating hole  20   a  obliquely leftwardly upwards with respect to the plunger accommodating hole  20   a , and the purge valve  30  is accommodated in the purge valve accommodating hole  20   h . The purge valve accommodating hole  20   h  is communicated at a rear portion thereof with the high pressure oil chamber  31  through a purge path  30   a  as shown in  FIG. 14 . 
       FIG. 16  illustrates a disassembled state of the purge valve  30 . A valve body  30   b , a coil spring  30   e  and a spring receiving tubular body  30   f  are fitted successively in the purge valve accommodating hole  20   h  from a front open end of the purge valve accommodating hole  20   h . The spring receiving tubular body  30   f  is screwed integrally in the purge valve accommodating hole  20   h . An annular groove  30   c  of a substantially rectangular cross section is formed on a circumferential face of the valve body  30   b , and an annular valve body  30   d  having a substantially rectangular cross section is fitted in the annular groove  30   c . If a pressure variation in the high pressure oil chamber  31  is transmitted to the valve body  30   b  through the purge path  30   a , then the valve body  30   b  is moved back and forth in the forward and backward directions by the elastic restoring force of the coil spring  30   e . Thereupon, air contained in the oil is separated from the oil and discharged to the outside from a front opening  30   g  ( FIG. 14 ) of the purge valve  30  and an air emitting path  39  ( FIG. 14 ) of the tensioner attaching portion  6   a  through a gap between the annular groove  30   c  and the annular valve body  30   d.    
       FIG. 17  is a sectional view taken along line VII-VII of  FIG. 12 . A tensioner oil path  32  (refer also to  FIG. 13 ) is provided at a lower portion of the oil reserving chamber  28  configured from a rear end face  20   c  of the tensioner body  20  and a concave face  21   c  of the cap  21  and extends forwardly in a downwardly inclined relationship from the tensioner body  20  through the tensioner body  20 . This path  32  is connected to a cylinder head oil path  33  formed in the tensioner attaching portion  6   a  of the cylinder head  6 . As shown in  FIG. 11 , the cylinder head oil path  33  is connected to an oil filter  35  through an oil path not shown in the cylinder block  5  and through an oil path  34  of the crankcase  4 . The oil filter  35  is connected to a discharge port of an oil pump  37  through an oil path  36 , and oil reserved on the bottom portion of the crankcase  4  is sucked into the oil pump  37  through a strainer  38  by the oil pump  37  which is placed into an operative state in an interlocking relationship with operation of the internal combustion engine  3 . The oil discharged from the oil pump  37  is supplied into the oil reserving chamber  28  of the hydraulic tensioner  0  through the oil path  36 , the oil filter  35 , the oil path  34 , the cylinder head oil path  33  and the tensioner oil path  32 . Further, the oil is supplied from the oil reserving chamber  28  into the high pressure oil chamber  31  of the tensioner through the valve chamber  26   b  and a valve hole  26   e  of the relief valve  26 , a valve chamber  25   e  and an opening  25   f  of the check valve  25  and the through-hole  22   f  of the valve holder  22  to drive the plunger  23 . 
       FIG. 18  illustrates a state in which the plunger  23  is projected forwardly when the pressure in the oil reserving chamber  28  rises and exceeds a cracking pressure to open the check valve  25  and the oil flows into the high pressure oil chamber  31  from the valve hole  26   e  of the relief valve  26  through the valve chamber  25   e  and opening  25   f  of the check valve  25  and the through-hole  22   f  of the valve holder  22 . Then, the abutting portion  23   c  of the plunger  23  is abutted with the tensioner slipper  15  to push forwardly the tensioner slipper  15  ( FIG. 11 ) strongly with the oil pressure in the high pressure oil chamber  31  to place the endless timing chain  13  on the relax side into a tensioned state thereby to suppress flapping of the endless timing chain  13 . 
     In a state in which the internal combustion engine  3  stops and the oil pressure in the oil reserving chamber  28  is low, the pressure maintaining valve  29  is closed by the spring restoring force of the closing coil spring  29   b  of the pressure maintaining valve  29  as shown in  FIG. 14 . However, if the oil pressure in the oil reserving chamber  28  rises in an interlocking relationship with starting of the internal combustion engine  3 , then it overcomes the spring restoring force of the closing coil spring  29   b  thereby to push the valve body  29   c  of the pressure maintaining valve  29  forwardly so that the rear cylindrical circumferential face  29   d  of the reduced diameter of the pressure maintaining valve  29  approaches the communication port  20   g  of the tensioner body  20 . Consequently, since the communication port  20   g  is opened as shown in  FIG. 18 , the oil flows into the relief valve port  22   e  of the valve holder  22 . 
     If, in such a state as just described, the tensioner slipper  15  is tilted rearwardly and the plunger  23  is pushed back strongly by variations of the rotating torque of the driving sprocket wheel  10  by an intermittent pressure rise of the pressure in a fuel chamber in the internal combustion engine  3  or the like, then the oil pressure in the high pressure oil chamber  31  rises, whereupon the valve body  26   a  of the relief valve  26  moves rearwardly. Consequently, the abutting face  26   f  ( FIG. 15 ) of the tip portion  26   c  of the relief valve  26  is spaced away from the valve seat  25   d  ( FIG. 15 ) of the check valve  25 , whereupon the oil in a valve chamber  25   e  of the check valve  25  flows from between the valve seat  25   d  and the abutting face  26   f  of the tip portion  26   c  of the relief valve  26  back into the oil reserving chamber  28  through the relief valve port  22   e  of the valve holder  22  and the communication port  20   g  and the pressure maintaining valve accommodating hole  20   f  of the tensioner body  20 . Consequently, an extraordinary increase in tension of the endless timing chain  13  is inhibited and flapping of the tensioner slipper  15  is suppressed. 
     Referring to  FIG. 16 , the plunger accommodating hole  20   a  of the tensioner body  20  is configured from a large diameter portion  41  at a rear portion and a small diameter portion  42  at a front portion, and a stepped portion  43  is formed on the boundary between them. Referring to  FIG. 15 , also the plunger  23  includes a large diameter portion  44  at a rear portion and a small diameter portion  45  at a front portion, and a stepped portion  46  is formed on the boundary between them. The plunger  23  is inserted into the tensioner body  20  from the rear. When the plunger is pushed by the pressure in the high pressure oil chamber  31  and projected forwardly until the stepped portion  46  of the plunger  23  is abutted with the stepped portion  43  of the plunger accommodating hole  20   a , then the plunger  23  stops its forward movement. In other words, both stepped portions act as stoppers for preventing coming off of the plunger. 
     First Leak Oil Emission Device: 
       FIG. 19  is a partial enlarged view of a gap portion  40  of  FIG. 14 . Although the gap between the tensioner body  20  and the plunger  23  is a gap in which the plunger  23  can slidably move, it is small to such a degree that no play occurs. In response to sliding movement of the plunger  23 , oil in the high pressure oil chamber  31  enters the gap and enters a portion  49  of a large gap sandwiched between the stepped portions  43  and  46 . When the plunger  23  moves forwardly, the oil in the portion  49  of the large gap is compressed. If the leak amount from a small diameter portion gap  48  at the front portion is small, then the forward movement of the plunger  23  is blocked. Accordingly, the area of the small diameter portion gap  48  on the downstream side is made greater than the area of a large diameter portion gap  47  so that flowing out of the oil is not blocked. In the present embodiment, with regard to both of the plunger accommodating hole  20   a  and the plunger  23 , the upstream side is formed as a large diameter portion and the downstream side is formed as a small diameter portion in the flow of the flowing out oil. 
     Since the gap is very small, upon working of the inner diameter of the plunger accommodating hole  20   a  and the outer diameter of the plunger  23 , a method for determination of the dimension of the small diameter portion gap  48  is described below assuming that reference dimensions (inner diameters and outer diameters) and working tolerances are set appropriately so that the large diameter portion gap  47  may have a required dimension. In particular, the inner diameters and the outer diameters (reference dimensions) and the working tolerances of the required portions are set such that the gap dimension B when the dimension of the small diameter portion gap  48  is formed smallest may be greater than the gap dimension A when the dimension of the large diameter portion gap  47  is formed greatest. In particular, where A=“allowable maximum value of the inner diameter of the plunger accommodating hole large diameter portion  41 ”—“allowable minimum value of the outer diameter of the plunger accommodating hole large diameter portion  44 ” and B=“allowable minimum value of the inner diameter of the plunger accommodating hole small diameter portion  42 ”—“allowable maximum value of the outer diameter of the plunger accommodating hole small diameter portion  45 ,” the reference dimensions (inner diameters and outer diameters) and the working tolerances (allowable maximum values and allowable minimum values) with which the relationship of A&lt;B may be satisfied are set. 
     While the leak oil emission device tries to manage the accuracy in area or dimension of the entire gap between the plunger accommodating hole  20   a  and the plunger  23 , the following description is directed to leak oil emission device which forms an oil emitting groove  51  on a small diameter portion of the plunger accommodating hole  20   a.    
     Second Leak Oil Emission Device: 
       FIG. 20  is a front elevational view of the tensioner body  20 .  FIG. 21  is a sectional view taken along line XI-XI of  FIG. 20 . A working tool  50  is applied to the small diameter portion  42  of the plunger accommodating hole  20   a  from the front face of the tensioner body  20  to form the oil emitting groove  51 , and the groove  51  is formed until the tip of the working tool  50  reaches the large diameter portion  41  of the plunger accommodating hole  20   a . This can enhance the emitting performance of oil. 
     The leak oil emission device described in detail above has the following characteristics and effects. 
     1. Of the gaps between the plunger accommodating hole  20   a  and the plunger  23 , the area of the small diameter portion gap  48  on the downstream side from which oils flows out is greater than the area of the large diameter portion gap  47  on the upstream side. By making the gap area on the upstream side set for oil pressure adjustment small and making the gap area on the downstream side greater than that on the downstream side, the oil emission performance on the downstream side which does not act for oil pressure adjustment is enhanced to reduce the sliding resistance of the plunger  23  by oil resistance. Consequently, the movement of the plunger  23  can be smoothened. 
     2. The reference dimensions and the working tolerances of the plunger accommodating hole  20   a  and the plunger  23  are set such that the gap dimension where the dimension of the small diameter portion gap  48  on the downstream side is formed smallest is greater than the gap dimension in the case where the dimension of the large diameter portion gap  47  on the upstream side is formed greatest to carry out working. Consequently, the dimension management of the gaps can be made sure and the oil emitting property of the downstream side can be enhanced. 
     3. The oil emitting groove  51  is formed on the sliding face of the plunger accommodating hole small diameter portion  42  of the tensioner body  20 , and the upstream end thereof reaches the plunger accommodating hole large diameter portion  41 . Consequently, the emitting performance of oil on the downstream side can be improved. 
     4. The plunger  23  has the stepped portion  46  formed from the plunger large diameter portion  44  on the upstream side and the plunger small diameter portion  45  on the downstream side, and the plunger accommodating hole  20   a  of the tensioner body  20  has the plunger accommodating hole stepped portion  43  formed from the plunger accommodating hole large diameter portion  41  corresponding to the plunger large diameter portion  44  on the upstream side of the plunger  23  and the plunger accommodating hole small diameter portion  42  corresponding to the plunger small diameter portion  45  on the downstream side of the plunger. This plunger stepped portion  46  is engaged with the plunger accommodating hole stepped portion  43  to form a coming off preventing structure for the plunger  23  ( FIG. 19 ). The oil emitting groove  51  is formed on the plunger accommodating hole small diameter portion  42  such that the upstream end thereof extends to the plunger accommodating hole large diameter portion  41 . Accordingly, the oil emission property when the plunger  23  moves forwardly can be improved. 
     5. The oil emitting groove  51  is formed by machining on the plunger accommodating hole small diameter portion  42  from the downstream end side and the working tool  50  is worked such that the tip thereof extends to the plunger accommodating hole large diameter portion  41  ( FIG. 21 ). Accordingly, the oil emitting groove  51  can be formed readily. 
     6. The oil emitting groove  51  is disposed at a position clear of a plane (C-C plane of  FIG. 20 ) including the direction of movement of the endless timing chain  13  and the axial line of the plunger  23 . This is because, since the plunger  23  vibrates in the direction of movement of the endless timing chain  13 , if the oil emitting groove  51  is provided in the plane including the direction of movement of the endless timing chain  13  and the plunger  23 , then there is the possibility that an edge of the oil emitting groove  51  may damage the plunger  23 , it is intended to avoid this. 
     7. In a state in which the hydraulic tensioner  0  is attached to the vehicle, the hydraulic tensioner  0  is attached such that the oil emitting direction of the oil emitting groove  51  is directed downwardly from the horizontal direction ( FIG. 11 ). Consequently, since also emission by the self weight of oil becomes possible, the oil can be emitted well. 
     8. The oil emitting groove  51  is provided in a pair at opposing positions with respect to the center line of the plunger accommodating hole  20   a  of the tensioner body  20  ( FIG. 20 ). By disposing such oil emission grooves not at neighboring positions but in a spaced relationship in opposing disposition, oil can be emitted well while suppressing a drop in strength of the tensioner body  20 . 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.