Abstract:
To provide an adequate biasing load to an endless transmission belt by biasing forces of a plurality of springs with different biasing forces and oil pressure biasing force to permit an easy adjustment. In a structure which includes an almost cylindrical plunger, a tensioner body into which the plunger is fitted, high pressure oil chambers are formed by the tensioner body and the plunger and supplied with oil pressure. Two tensioner springs, a rigid spring and a soft spring are supported by the tensioner body and are arranged in series for biasing the plunger. The plunger in its fully stretched state is supported by the serially arranged rigid and soft tensioner springs. In a condition when the plunger is pushed back from this fully stretched state by a prescribed amount or further, the plunger is supported by the rigid tensioner spring.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present non-provisional application claims priority under 35 USC 119 to Japanese Patent Application No. 2004-270953 filed on Sep. 17, 2004 the entire contents thereof is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a hydraulic tensioner lifter which applies tension to an endless transmission belt such as a chain or belt used in the transmission mechanism of an internal combustion engine. 
   2. Description of Background Art 
   Conventionally, in an internal combustion engine, a hydraulic tensioner lifter has been used in order to apply tension to an endless transmission belt used in the transmission mechanism, such as a chain. This hydraulic tensioner lifter uses a method which pushes a plunger by means of tensioner spring and oil pressure, where as the plunger stretches, a stretched lifter gives the chain a prescribed tension to suppress oscillation of the chain and ensure stable chain drive. See, for example, JP-A No. 287092/2003 Pages 6-7 and FIG. 2. 
   The hydraulic tensioner lifter as described in JP-A No. 287092/2003 includes a tensioner lifter body, a hole made in the lifter body, a plunger which is slidably fitted in the hole for forming an oil chamber in the hole of the lifter body and a tensioner spring which biases the plunger in a way to push it out of the hole, where the tensioner spring is located between the bottom of the hole and a plunger adjacent to the oil chamber side end face of an orifice member and as the plunger is pushed out by this tensioner spring and oil pressure fed into the oil chamber, it applies tension to the chain of the internal combustion engine. 
   In the above hydraulic tensioner lifter, while the internal combustion engine is not working, the hydraulic pump also stops working and the oil in the oil circuit of the hydraulic tensioner lifter falls down by the pull of gravity, causing air to enter the oil circuit of the hydraulic tensioner lifter. Therefore, when the engine is started, it takes some time to remove the air from the oil circuit of the hydraulic tensioner lifter and fill the oil circuit with oil. 
   Before the oil circuit of the hydraulic tensioner lifter is filled with oil, the endless transmission belt turns with the operation of the internal combustion engine and thus a pushing force is irregularly applied to the plunger of the hydraulic tensioner lifter. Consequently, the plunger is deeply pushed inward by a large pushing force and the plunger base end touches the tensioner body. Also, at the time of start, noise may be generated during transition of the chain from a loose state to a tense state. 
   If the spring constant of the tensioner spring is increased in order to avoid this, the spring force of the tensioner spring would rapidly grow with an increase in the plunging amount of the plunger, which would cause an excessive force to be applied to the endless transmission belt and thus resulting in an excessive tension on the endless transmission belt. 
   SUMMARY AND OBJECTS OF THE INVENTION 
   According to an embodiment of the present invention, a hydraulic tensioner lifter includes a tensioner body having a housing hole, a plunger slidably fitted into the housing hole, a high pressure oil chamber which is surrounded by the tensioner body and the plunger which is fed with oil pressure, and a tensioner spring which biases the plunger in the high pressure oil chamber for movement. The tensioner spring consists of a plurality of springs arranged in series. 
   An embodiment of the present invention includes a plunger that when it is in a position pushed inwardly by a prescribed amount from its most projecting position, the length of a spring disposed on the plunger side among the plurality of springs is fixed. 
   An embodiment of the present invention includes a spring that is constant of a spring disposed on the tensioner body side that is the largest among the plurality of springs. 
   An embodiment of the present invention includes a base of the tensioner body that faces the high pressure oil chamber, an oil inflow hole which opens into the high chamber that is made in the base, a check valve for preventing a backward flow from the high pressure oil chamber that is provided in the oil inflow hole and a relief valve that is provided downstream of the high pressure oil chamber. A first oil channel and a second oil channel are arranged in parallel from the oil inflow hole to the relief valve. The first oil passage is fitted with an orifice. In the second oil channel, the path to the relief valve is blocked off when the plunger is pushed inwardly by a prescribed amount from the most projecting position. 
   An embodiment of the present invention includes a valve disc of the relief valve that is conical. 
   An embodiment of the present invention includes a tip of the conical valve disc of the relief valve that projects from the relief valve body of the relief valve toward the first oil channel and the second oil channel. 
   According to an embodiment of the present invention, when the plunger is extremely projecting, the total spring length of the series of springs is longer and the spring constant of the series of springs is smaller than any of the spring constants of the individual springs. Therefore, the tensioner lifter softly bears the pushing force from the endless transmission belt in response to a change in the tension of the endless transmission belt in a plunger projecting condition, so that the endless transmission belt is stably held in place. Thus, the generation of a large tension on the endless transmission belt is prevented, thereby improving the durability of the endless transmission belt. 
   According to an embodiment of the present invention, while the spring constant of the series of springs is small as mentioned above with the plunger almost in its most protruding position, the length of a spring disposed on the plunger side is fixed with the plunger in a position pushed inward by a prescribed amount from its most protruding position; and when the plunger is pushed inward further, the spring on the plunger side does not function as a spring and the spring constant with the plunger in that position is larger than the spring constant with the plunger in its most protruding position. Therefore, when the plunger is more deeply pushed inwardly and the base end of the plunger comes close to the tensioner body, a large spring force is generated and contact of the plunger base end with the tensioner body is avoided, thereby preventing the generation of noise. 
   According to an embodiment of the present invention, because, when the plunger is pushed inwardly further from its position when pushed inwardly by a prescribed amount from its most projecting position, the spring constant is much larger than the spring constant with the plunger in its most projecting position. Thus, even when the plunger is pushed inwardly with a further pushing force, contact of the plunger base end with the tensioner body is surely avoided and the possibility of noise generation is completely eliminated. 
   According to an embodiment of the present invention, since a check valve that opens into a high pressure oil chamber is provided to prevent a backward flow from the high pressure oil chamber, in such a situation that after the plunger projects due to an instant slack of the endless transmission belt, the endless transmission belt becomes tense again and the plunger is about to be pushed inwardly. Thus, oil pressure which is fed to the high pressure oil chamber as a result of an increase in the capacity of the high pressure oil chamber due to the plunger&#39;s projection is prevented from flowing backward from the high pressure oil chamber to the oil path on the hydraulic pump side by the check valve when the plunger is pushed inward further. 
   According to an embodiment of the present invention, a relief valve is provided downstream of the high pressure oil chamber and a first oil channel and a second oil channel are arranged in parallel from the oil inflow hole to the relief valve. The first oil channel is fitted with an orifice. In the second oil channel, the path to the relief valve is blocked off when the plunger is pushed inwardly by a prescribed amount from the most projecting position. Therefore, when the plunger is between the most projecting position and the position pushed inward by the prescribed amount, the oil pressure in the high pressure oil chamber flows from the first oil channel to the relief valve and also flows from the second oil channel to the relief valve. As a consequence, the plunger is pushed inwardly with a relatively small resistance. 
   However, when the plunger is pushed more deeply than by the prescribed amount, the second oil channel is blocked off and thus the oil pressure in the high pressure oil chamber flows only through the first oil channel into the relief valve and due to the orifice fitted in this first oil channel the flow resistance of oil pressure is large and consequently the plunger is pushed inwardly with a large resistance. Hence, when the plunger is pushed deep into the tensioner body and the plunger base end comes closer to the base end of the tensioner body an increase in the resistance against the plunger pushing force prevents collision of the plunger base end against the base end of the tensioner body and also suppresses the generation of noise. 
   According to an embodiment of the present invention, since the valve disc of the relief valve is conical, the conical valve disc stably opens while oil pressure passes through the valve, which prevent chattering. 
   According to an embodiment of the present invention, since the tip of the conical valve disc of the relief valve or the relief valve body projects towards the first oil channel and the second oil channel, the position of the valve disc in the relief valve can be visually checked easily and properly. 
   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  is a sectional view of the key part of a DOHC engine in which a hydraulic tensioner lifter according to the present invention is used as a tensioner for its timing chain; 
       FIG. 2  is a sectional view of the hydraulic tensioner lifter according to the present invention, taken along the line II-II of  FIG. 5 ; 
       FIG. 3  is an exploded view of the hydraulic tensioner lifter according to the present invention; 
       FIG. 4  is a sectional view of the hydraulic tensioner lifter according to the present invention, taken along the line IV-IV of  FIG. 5 ; 
       FIG. 5  is a sectional view of the hydraulic tensioner lifter according to the present invention, as viewed from the direction of arrow V; 
       FIG. 6(   a ) to  6 ( c ) are sectional views showing three different positions of the plunger of the hydraulic tensioner lifter according to the present invention, wherein  FIG. 6(   a ) shows a condition that the plunger is pushed the furthest inwardly,  FIG. 6(   b ) is a condition wherein the plunger is in the middle projecting position, and  FIG. 6(   c ) is a condition wherein the plunger projects the furthest; 
       FIG. 7  is a sectional side view of a hydraulic tensioner lifter according to another embodiment of the present invention, wherein the plunger is pushed the most inwardly; 
       FIG. 8  is a view showing that the plunger in the embodiment shown in  FIG. 7  that is less projecting; 
       FIG. 9  is a view showing that the plunger in the embodiment shown in  FIG. 7  that is moderately projecting; 
       FIG. 10  is a view showing that the plunger in the embodiment shown in  FIG. 7  that is projects the most; and 
       FIG. 11  is a view of a hydraulic tensioner lifter according to a further embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Next a description will be given of a hydraulic tensioner lifter  0  as an embodiment of the present invention, which is shown in  FIGS. 1 to 6 . This hydraulic tensioner lifter  0  is applied to a transmission mechanism  10  of a valve train of a DOHC engine  1 . This hydraulic tensioner lifter  0  is mounted on a small vehicle with the centerline X (see  FIG. 1 ) of the cylinder of the hydraulic tensioner lifter  0  inclined toward the front of the body of a small vehicle such as a motorcycle (not shown). 
   The above internal combustion engine  1  is a single-cylinder engine or an in-line multi-cylinder engine in which a plurality of cylinders are arranged in parallel in the vehicle width direction at regular intervals where in a cylinder block  2  of the engine  1 , a crankshaft  5  is supported in a manner that it can rotate clockwise as viewed in  FIG. 1 , and a cylinder head  3  and a head cover  4  are laid over the top face of the cylinder block  2  in sequence. The cylinder block  2 , cylinder head  3  and head cover  4  are joined integrally by bolts or the like (not shown). 
   The above cylinder is almost perpendicular to the joint face of the cylinder block  2  and cylinder head  3  and a piston slidably fitted into this cylinder is connected to the crankshaft  5  through a connecting rod (not shown), so that as the piston goes up and down, the crankshaft  5  rotates clockwise as viewed in  FIG. 1 . 
   A pair of intake and exhaust camshafts  6  are rotatably supported in the position of the joint face of the cylinder head  3  and head cover  4  in parallel to the crankshaft  5  and a transmission mechanism  10  lies in a transmission chamber  7  hermetically sealed by the cylinder block  2 , cylinder head  3  and head cover  4 , surrounding the crankshaft  5  and camshafts  6 . 
   In the transmission chamber  7 , a drive sprocket  11  is integrally fitted to the crankshaft  5  and driven sprockets whose pitch diameter is twice that of the drive sprocket  11  are integrally fitted to the pair of camshafts  6  and an endless timing chain  13  is put on the drive sprocket  11  and driven sprockets  12  so that in conjunction with clockwise rotation of the crankshaft  5 , the camshafts  6  rotate at a speed which is half the speed of rotation of the crankshaft  5 . 
   In the transmission chamber  7 , a chain guide  14  is provided on and in touch with the tense side  13   a  of the timing chain  13  (right side in  FIG. 1 ) and the lower end of a tensioner slipper  15  is swingably pivoted on the loose side  13   b  of the timing chain  13  along the outer face of the timing chain  13  and the hydraulic tensioner lifter  0  is located in a rear wall mounting seat  3   a  of the cylinder head  3  in a way for a front end contact member  57  of the plunger  50  of the hydraulic tensioner  0  to touch the upper part of the tensioner slipper  15 . Thus, a required level of tension is applied to the loose side  13   b  of the timing chain  13  by pushing the upper part of the tensioner slipper  15  with a required pushing pressure, as set forth below. 
   Next, details of the structure of the hydraulic tensioner lifter  0  will be described referring to  FIGS. 2 to 5 . 
   The hydraulic tensioner lifter  0  has, in the rear wall mounting seat  3   a  of the cylinder head  3 , the following components, a tensioner body  20  detachably fitted with a bolt (not shown) passing through a flange  21  as shown in  FIG. 4  and a check valve body  31  of a check valve  30  fitted into a circular check valve body housing hole  22  of the tensioner body  20  with a plunger  50  slidably fitted in a plunger housing hole  26  of the tensioner body  20 . A relief valve  60  is provided on the front end contact member  57  of the plunger  50  with an air purge valve  80  housed in an air purge valve housing hole  23  of the tensioner body  20 , wherein the check valve body housing hole  22  and the air purge valve housing hole  23  are parallel to each other. 
   In the tensioner body  20 , there is a tensioner body oil feed path  24 , one end of which is in communication with an engine side oil feed path  17  as shown in  FIG. 1  and the other end of which opens into a circular oil feed groove  34  of an inflow oil path  37  of the check valve body  31 , where there is a connecting oil feed path  25  formed perpendicularly to the check valve body housing hole  22  and air purge valve housing hole  23 , the centerline of the tensioner body  20  coincides with the centerline of the plunger housing hole  26  and there is a circular plunger housing hole  26  in a position nearer to the front end than the check valve body housing hole  22  (the base end side is left and the front end side is right in  FIGS. 2 ,  3  and  4  in this embodiment). 
   The check valve body  31  consists of a base end large-diameter part  32  fittable into the check valve body housing hole  22  of the tensioner body  20  and a front end small-diameter part which is on the same axis as the base end large-diameter part  32  and has a diameter smaller than the base end large-diameter part  32 . The circular oil feed groove  34  is formed around the base end large-diameter part  32  and two circular grooves  35  are formed with the circular oil feed groove  34  between them with ring seals  36  being fitted into the circular grooves  35 . The base end large-diameter part  32  is oil-tightly fitted into the outer end of the check valve body housing hole  22  of the tensioner body  20  through the seals  36 . The check valve body  31  has an inflow oil path  37 , oriented in the radial direction of the check valve body  31 , which opens into the circular oil feed groove  34 . 
   An oil feed path  38  is in communication with the inner end of the inflow oil path  37  and oriented toward the front end of the check valve body  31  along the center axis of the check valve body  31  with a valve seat  39  being positioned at the front end of the path  38 . A ball valve housing hole  40  is provided that has a diameter larger than the oil feed path  38 . An outflow oil path  41  lies nearer to the ball valve housing hole  40  than the valve seat  39  that is radially formed and opens to the outer periphery of the front end small-diameter part  33 . 
   In a throttle valve body  42  pressed into the ball valve housing hole  40  of the check valve body  31 , a spring housing hole  43 , an oil path  44  with a smaller diameter than the hole  43 , a throttle  45  and a conical surface  46  are arranged in the order as identified from the base end of the check valve body  31  to its front end. A valve spring  47  is inserted in the spring housing hole  43 . The oil feed path  38 , valve seat  39 , ball valve housing hole  40 , outflow oil path  41 , valve spring  47  and ball valve  48  make up the check valve  30 . When there is no oil pressure in the oil feed path  38 , the ball valve  48 , freely fitted in the ball valve housing hole  40 , is pressed against the valve seat  39  by the spring force of the valve spring  47  to close the check valve  30 . 
   The plunger  50  consists of a cylindrical member  51 , a front end contact member  57  fitted to the front end of a front end small-diameter inner peripheral surface  55  in a front end small-diameter part  54  of the cylindrical member  51 , and a relief valve  60  fitted into a relief valve body housing hole  58  of the front end contact member  57 . The base end large-diameter part  52  of the cylindrical member  51  is slidably fitted into the check valve body housing hole  22  of the tensioner body  20  with the front end small-diameter part  54  of the cylindrical member  51  being slidably fitted into the plunger housing hole  26  of the tensioner body  20 . An inner step end face  56   a  of a step end face  56  with which a floating sleeve  71  (stated later) can engage is formed at the boundary between the base end large-diameter inner periphery  53  of the base end large-diameter part  52  and the front end small-diameter inner peripheral surface  55  of the front end small-diameter part  54 . A guide groove  54   a  is formed that is oriented toward the generating line direction on the outer peripheral surface of the front end small-diameter part  54 . Since the tip of a screw  28  is inserted into the guide groove  54   a  through the outer wall  27  of the plunger housing  26  of the tensioner body  20 , the plunger  50  does not rotate and can slide axially inside the check valve body housing hole  22  and plunger housing hole  26  of the tensioner body  20 . 
   In the front end contact member  57 , there are an oil reservoir recess  62  which opens into the front end of the relief valve body housing hole  58  (at the rightmost end in  FIGS. 2 and 3 ). A discharge oil path  63  is provided that is in communication through the oil reservoir recess  62  with ambient air. A relief valve body  61  is fitted into the relief valve body housing hole  58  with a valve holding sleeve  64  being slidably fitted onto the inner peripheral surface of the relief valve body  61 . The relief valve disc  66  of the relief valve  60  is fitted to the small-diameter part  65  of the valve holding sleeve  64  with the valve spring  68  being inserted between the end face  65   a  of the small-diameter part  65  of the valve holding sleeve  64  and the front end face  58   a  of the relief valve body housing hole  58 . Thus, by the spring force of this valve spring  68 , the front end conical surface  67  of the relief valve disc  66  is tightly pressed against the valve seat  69  of the relief valve  60  to close the relief valve  60 . 
   With the front end conical surface  67  of the relief valve disc  66  being in contact with the valve seat  69  of the relief valve  60 , when the base end face  59  (leftmost end in  FIGS. 2 and 3 ) of the base end large-diameter part  52  of the plunger  50  is in contact with the stepped end face  49  of the check valve body  31  of the check valve  30  (see  FIG. 2 ), a valve chest  70  exists between the front end face of the throttle valve body  42  and the base end face of the relief valve body  61 . In addition, some clearance is provided between the front end conical surface  67  of the relief valve disc  66  and the conical surface opening  46  of the throttle valve body  42 . 
   A rigid tensioner spring  72  with a large spring constant k 1  and a soft tensioner spring  73  with a small spring constant k 2  are arranged in series, making up a combination tensioner spring The rigid tensioner spring  72  and the floating sleeve  71  are fitted to the front end small-diameter part  33  of the check valve  30  from the front end side of the base end large-diameter part  32  and are located nearer to the front end than the floating sleeve  71 . The soft tensioner spring  73  is fitted to the front end small-diameter part  33  and the front end small-diameter inner peripheral surface  55  of the plunger  50 . 
   A circular oil feed groove  83  is provided on the outer peripheral surface of the air purge valve body base end  81  of the air purge valve  80  with an inflow oil path  84  which opens into the circular oil feed groove  83  along the radial direction of the air purge valve body base end  81 . An oil feed path  85  is in communication with the inner end of the inflow oil path  84  and is oriented toward the front end of the air purge valve body base end  81  along the center axis of the air purge valve body base end  81  with a valve seat  86  being provided at its front end. 
   At the base end of the air purge valve body front end  82  of the air purge valve  80  there is a ball valve housing hole  87  having a diameter larger than the oil feed path  85 . At the front end of the ball valve housing hole  87  there is a stepped oil path  88  having a diameter smaller than the ball valve housing hole  87 . A valve spring  89  is inserted into the large-diameter portion  88   a  of the stepped oil path  88  and a ball valve  90  is housed in the ball valve housing hole  87 . 
   With the valve spring  89  and ball valve  90  housed inside the air purge valve body front end  82 , after the air purge valve body front end  82  is inserted into the air purge valve housing hole  23  of the tensioner body  20 , the air purge valve body base end  81  is inserted into the air purge valve housing hole  23 . Thereafter, a tool with a hexagonal columnar head (not shown) is inserted into and engaged with a hexagonal hole  91  at the base end of the air purge valve body base end  81  with the male thread  92  of the air purge valve body base end  81  being screwed into the female thread  29  of the tensioner body  20  by turning the tool in one direction so that the air purge valve  80  is built into the air purge valve housing hole  23  of the tensioner body  20 . 
   Since the air purging structure of the air purge valve  80  is the same as that of the air purging structure as described in JP-A No. 287092/2003, a detailed description of it is omitted here. 
   Next, an explanation will be given of a spring reactive force against a pushing force with no oil pressure fed to the hydraulic tensioner lifter  0 . 
   As illustrated in  FIG. 2 , for a spring system consisting of the rigid tensioner spring  72  with a large spring constant k 1  and the soft tensioner spring  73  with a smaller spring constant k 2  which are arranged in series along the direction in which a load is applied, the combination spring constant is calculated as k 1 ·k 2 /(k 1 +k 2 ) and this combination spring constant is smaller than the spring constant k 1  of the rigid tensioner spring  72  and also smaller than the spring constant k 2  of the soft tensioner spring  73 . 
   When the rigid tensioner spring  72  and soft tensioner spring  73  that are arranged in series constitute a combination spring, as shown in  FIG. 6   c , the plunger  50  projects largely from the tensioner body  20  and the front end face  71   a  of the floating sleeve  71  is off the inner step end face  56   a  of the plunger  50  with the outer step end face  56   b  of the front end small-diameter part  54  and base end large-diameter part  52  being in contact with the front end side stepped end face  49   b  as the boundary step between the plunger housing hole  26  and the check valve body housing hole  22 . When the plunger  50  projects the most from the tensioner body  20 , the rigid tensioner spring  72  and the soft tensioner spring  73  become serially connected and the spring constant in this condition is the abovementioned small combination spring constant k 1 ·k 2 /(k 1 +k 2 ) and the plunger  50  is slightly pushed into the check valve body housing hole  22  of the tensioner body  20 . 
   As the plunger  50  is progressively pushed into the check valve body housing hole  22  of the tensioner body  20 , the spring reactive force increases in proportion to the amount of pushing. When the plunger  50  is deeply pushed into the check valve body housing hole  22  of the tensioner body  20  and as illustrated in  FIG. 6(   b ), the front end face  71   a  of the floating sleeve  71  comes into contact with the inner step end face  56   a  of the plunger  50 , the pushing force applied to the plunger  50  is transmitted, without the intermediation of the soft tensioner spring  73 , from the inner step end face  56   a  of the plunger  50  through the floating sleeve  71  and the rigid tensioner spring  72  to the base end large-diameter part  32  of the plunger housing hole  26 . Therefore, in this case, the spring constant is equal to the spring constant k 1  of the rigid tensioner spring  72  only and the rate of increase in the reactive force against the force pushing the plunger  50  becomes higher. 
   As illustrated in  FIG. 6(   b ), when the front end face  71   a  of the floating sleeve  71  is beginning to touch the inner step end face  56   a  of the plunger  50 , if the force pushing the plunger  50  increases and the plunger  50  is pushed into the check valve body housing hole  22  in a way to come closer to the base end large-diameter part  32  of the tensioner body  20 , the reactive force against the force pushing the plunger  50  by the spring force of the rigid tensioner spring  72  only becomes larger. 
   As illustrated in  FIG. 6(   a ), when the base end face  59  of the plunger  50  comes into contact with the base end side stepped end face  49   a  of the check valve body  31 , the plunger  50  cannot be further pushed into the check valve body housing hole  22 . 
   When the internal combustion engine  1  stops working and no oil pressure is fed to the hydraulic tensioner lifter  0 , if a new timing chain  13  is put on the drive sprocket  11  and driven sprockets  12 , the plunger  50  is moved inwardly by a smaller distance (ΔX) than when the inner step end face  56   a  of the plunger  50  is beginning to touch the front end face  71   a  of the floating sleeve  71  as illustrated in  FIG. 6(   b ). The tensioner body  20 , plunger  50 , rigid tensioner spring  72  and soft tensioner spring  73  are made so that the amounts of projection of the plunger  50 , X 1 , X 2  and X 3  as shown in  FIGS. 6(   a ),  6 ( b ) and  6 ( c ), have the relation of X 2 −X 1 &lt;X 3 −X 2 . Here, each of distances X, X 2 , and X 3  represent the distances in an axial direction from inner end  57   a  of the front end contact member  57  of plunger  50  to face  20   a  of the tensioner body  20 . Face  20   a  of the tensioner body  20  abuts against a rear wall mounting seat  3   a  of the cylinder head  3 . 
   Because the embodiment as shown in  FIGS. 1 to 6  is constructed as mentioned above, when a new timing chain  13  is put on the drive sprocket  11  and driven sprockets  12 , the internal combustion engine  1  stops working and no oil pressure from a hydraulic pump is fed to the hydraulic tensioner lifter  0 , due to the tensile reactive force of the timing chain  13 . Thus, the plunger  50  is pushed into the check valve body housing hole  22  of the tensioner body  20   b  more deeply by ΔX than in the condition as illustrated in  FIGS. 6(   a ) to  6 ( c ). At this moment, the spring force of the soft tensioner spring  73  does not work and the plunger  50  is pushed or biased outwardly only by the spring force of the rigid tensioner spring  72 . 
   In the initial operational stage where the internal combustion engine  1  begins working and the timing chain  13  begins turning between the drive sprocket  11  and the driven sprockets  12 , oil pressure from the hydraulic pump does not reach the hydraulic tensioner lifter  0  yet and only the spring force of the rigid tensioner spring  72  with a large spring constant k 1  bears the pushing force to the hydraulic tensioner lifter  72  as the timing chain  13  turns. 
   If the torque transmitted to the crankshaft  5  of the internal combustion engine  1  changes irregularly due to an intermittent combustion in the engine  1  and the tension of the timing chain  13  changes and the loose side  13   b  of the timing chain  13  seriously slackens for a moment, the plunger  50  projects from the tensioner body  20  further than in the condition as illustrated in  FIG. 6(   b ) and the inner step end face  56   a  of the plunger  50  is off the front end face  71   a  of the floating sleeve  71 . In this case, the plunger  50  is pushed outwardly by the spring force with combination spring constant k 1 ·k 2 /(k 1 +k 2 ) which is smaller than the spring constant k 1  of the rigid tensioner spring  72 , so that the hydraulic tensioner lifter  0  can adequately absorb a small tension change. 
   Furthermore, even when the timing chain  13  is used over a long time and its length becomes larger than the original length, the hydraulic tensioner lifter  0  works in the same way as mentioned above. 
   As the internal combustion engine  1  begins working and a given time elapses, oil pressure from the hydraulic pump (not shown) is sent through the engine side oil feed path  17  of the engine  1 , the tensioner body oil feed path  24  of the tensioner body  20 , the circular oil feed groove  34  and the inflow oil path  37  to the oil feed path  38 . The supplied oil pressure in the oil feed path  38  opens the check valve  30 . Thus, oil pressure is supplied into the ball valve housing hole  40  with some of the supplied oil pressure in the ball valve housing hole  40  being supplied to the valve chest  70  through the spring housing hole  43 , oil path  44 , throttle  45  and conical surface opening  46  (first oil channel). Thereafter, the oil pressure is supplied from the valve chest  70  to a small-diameter oil chamber  75  and at the same time the remaining supplied oil pressure in the ball valve housing hole  40  is supplied to the oil feed path  85  through the inflow oil path  41 , large-diameter oil chamber  74 , communication oil path  25 , and circular oil feed groove  83 . 
   More specifically, at the early stage of hydraulic pump operation, air remains in the oil pressure circuit to the hydraulic tensioner lifter  0  and in the oil pressure circuit in the hydraulic tensioner lifter  0  and the oil pressure flowing in the oil pressure circuits contains much air. The air in the supplied oil pressure in  85  is released to the atmosphere (inside the transmission chamber) through the large-diameter part  88   a  and small-diameter part  88   b  of the stepped oil path  88  and the air exhaust hole  23   a  of the air purge valve housing hole  23 . In addition, the air contained in the oil pressure in the oil pressure circuit to the hydraulic tensioner lifter  0  and in the oil pressure circuit of the hydraulic tensioner lifter  0  is discharged into the transmission chamber  7 . 
   As the pressure of the oil pressure in the oil feed path  85  is increased, due to the oil pressure the ball valve  90  touches the base end edge  88   c  of the large-diameter part  88   a  of the stepped oil path  88  and the air exhaust from the air purge valve  80  stops. Also, during a low speed operation just after the start of the internal combustion engine  1 , the plunger  50  is slightly pushed more towards the base end large-diameter part  32  in the check valve body housing hole  22  of the tensioner body  20  than in the condition as shown in  FIG. 6(   b ). Thus, the front end face  71   a  of the floating sleeve  71  touches the inner step end face  56   a  of the plunger  50  and the large-diameter oil chamber  74  and the small-diameter oil chamber  75  are disconnected. Consequently, oil pressure never goes around through the outflow oil path  41 , small-diameter oil chamber  75  and large-diameter oil chamber  74  (second oil channel) into the valve chest  70 . 
   When the pressure of the oil pressure which is supplied into the ball valve housing hole  40  and led through the spring housing hole  43 , oil path  44 , throttle  45  and conical surface opening  46  into the valve chest  70  exceeds the relief pressure of the relief valve  60 , the front end conical surface  67  of the relief valve disc  66  of the relief valve  60  gets off the valve seat  69  and the relief valve  60  opens. If the force pushing the plunger  50  is almost constant, a large quantity of oil pressure supplied to the hydraulic tensioner lifter  0  is sent from the relief valve  60  through the relief valve body housing hole  58  and oil reservoir recess  62  to the discharge oil path  63  and discharged into the transmission chamber  7 . 
   If no considerable tension reactive force is generated in the timing chain  13  and the amount of the projection of the plunger  50  is larger than X 2  as shown in  FIG. 6(   b ), the rigid tensioner spring  72  and soft tensioner spring  73  function as a serial combination spring. Thus, the combination spring constant k 1 ·k 2 /(k 1 +k 2 ) is smaller than the spring constant k 1  of the rigid tensioner spring  72  and also smaller than the spring constant k 2  of the soft tensioner spring  73 , so that the pushing force from the timing chain  13  can be borne flexibly in response to a change in the tension of the timing chain  13 . 
   In addition, when the force pushing the plunger  50  decreases and the front end face  71   a  of the floating sleeve  71  is released from the inner step end face  56   a  of the plunger  50  as shown in FIG. ( 6 ), the large-diameter oil chamber  74  and the small-diameter oil chamber  75  are connected. In contrast to the situation when the front end face  71   a  of the floating sleeve  71  is in contact with the inner step end face  56   a  of the plunger  50 , the oil pressure going through the check valve  30  into the ball valve housing hole  40  is supplied not only through the spring housing hole  43 , oil path  44 , throttle  45  and conical surface opening  46  to the valve chest  70  but also through the outflow oil path  41 , large-diameter oil chamber  74  and small-diameter oil chamber  75  to the valve chest  70 , so that even if the force pushing the plunger  50  suddenly decreases, the plunger  50  immediately projects from the tensioner body  20  in response to this situation. 
   When the force pushing the plunger  50  increases, the plunger  50  is pushed towards the base end large-diameter part  32  in the check valve body housing hole  22  of the tensioner body  20  and the pushing force applied to the plunger  50  can be borne by the spring force increase of the rigid tensioner spring  72  corresponding to the pushing amount. In addition, the pressure rise in the valve chest  70  and small-diameter oil chamber  75  attributable to the flow resistance of the oil pressure passing through the throttle  45 . On the other hand, when the force pushing the plunger  50  decreases, the plunger  50  projects by the spring force of the rigid tensioner spring  72  and the supply of oil pressure to the valve chest  70  by the closing of the relief valve  60  produces a drop in the oil pressure in the valve chest  70 . As a consequence, the tension of the timing chain  13  can be maintained almost constant. 
   Furthermore, if the force pushing the plunger  50  increases unusually, this large pushing force can be borne because the front end face  71   a  of the floating sleeve  71  and the inner step end face  56   a  come into contact with each other. Consequently the large-diameter oil chamber  74  and the small-diameter oil chamber  75  are disconnected, and oil pressure from the ball valve housing hole  40  flows only through the spring housing hole  43 , throttle  45  and conical surface opening  46  into the valve chest  70  with a large flow resistance and also because of the large spring force of the rigid tensioner spring  72  that has a large spring constant. Therefore, the plunger  50  is pushed very deeply and as shown in  FIG. 6(   a ), it is possible to prevent the base end face  59  of the base end large-diameter part  52  of the plunger  50  from colliding with the base end stepped end face  49   a  of the check valve body  31  of the tensioner body  20  to thereby prevent noise which might be generated upon contact of the plunger  50 . 
   Since the front end conical surface  67  is formed on the relief valve disc  66  of the relief valve  60 , the change in the pressure of oil flowing between the front end conical surface  67  and the valve seat  69  is continuous and consequently chattering hardly occurs in the relief valve  70 . 
   In the embodiment as shown in  FIGS. 1 to 6(   c ), when the front end face  71   a  of the floating sleeve  71  comes into contact with the inner step end face  56   a  of the cylindrical member  51 , the large-diameter oil chamber  74  and the small-diameter oil chamber  75  are disconnected by the floating sleeve  71  and the plunger  50  and the floating sleeve  71  move together in the check valve body housing hole  22  of the tensioner body  20  and the cylindrical member  51  and the floating sleeve  71  are designed so that the soft tensioner spring  73  does not bear the pushing force applied to the front end contact member  57  of the plunger  50 . However, it is also possible that the base end large-diameter inner peripheral surface  53  of the cylindrical member  51  consists of a large-diameter part  53   a , and a small-diameter part  53   b  which can touch the outer peripheral surface  71   c  of the floating sleeve  70 . 
   In the embodiment as shown in  FIGS. 7 to 10 , the plunger is slightly more inward than in its most projecting state ( FIG. 10 ), and as shown in  FIG. 9 , the outer peripheral surface front end edge  71   d  of the floating sleeve  71  is close to the small-diameter part  53   b  of the base end large-diameter inner peripheral surface  53  of the cylindrical member  51 . As the plunger  50  is further pushed inwardly and moves from its position of  FIG. 9  to the position of  FIG. 8 , the large-diameter oil chamber  74  and the small-diameter oil chamber  75  are disconnected. Thus, the oil pressure in the ball valve housing hole  40  flows from the ball valve housing hole  40  into the valve chest  70  only through the spring housing hole  43 , oil path  44 , throttle  45  and conical surface opening  46  because the large-diameter oil chamber  74  and small-diameter oil chamber  75 , constituting the second oil channel, are disconnected, where the flow resistance of the oil pressure from the ball valve housing hole  40  to the valve chest  70  is high and the resistance against the force pushing the plunger  50  is larger than when the plunger  50  moves from the position of  FIG. 10  to the position of  FIG. 9 . 
   However, while the floating sleeve  71  is moving from the position of  FIG. 9  to the position of  FIG. 8 , the soft tensioner spring  73  shrinks as a spring bearing the pushing force of the plunger  50 ; therefore, the spring constant of the spring which works on the plunger  50  is the small spring constant of the combination spring consisting of the rigid tensioner spring  72  and the soft tensioner spring  73 . Thus, when the floating sleeve  71  moves from the position of  FIG. 9  to the position of  FIG. 8 , the spring force to resist the pushing force of the plunger  50  is smaller than when the floating sleeve  71  moves from the position of  FIG. 8  to the position of  FIG. 7 . 
   When the floating sleeve  71  moves from the position of  FIG. 8  to the position of  FIG. 7 , the large-diameter oil chamber  74  and the small-diameter oil chamber  75  are disconnected and the spring constant of the spring which works on the plunger  50  is the spring constant k 1  of the rigid tensioner spring  72  only and larger than the spring constant of the serial combination spring, k 1 ·k 2 /(k 1 +k 2 ). Thus, the resistance against the force pushing the plunger  50  is large. 
   What has been described above is summarized as follows. In the condition as shown in  FIGS. 10 to 9 , the spring constant is k 1 ·k 2 /(k 1 +k 2 ), namely small and also the large-diameter oil chamber  74  and the small-diameter oil chamber  75  are connected and the flow resistance of the oil pressure is small, so that the resistance against the force pushing the plunger  50  is the smallest. In the condition as shown in  FIGS. 9 to 8 , the spring constant still remains small at k 1 ·k 2 /(k 1 +k 2 ). However, since the large-diameter oil chamber  74  and the small-diameter oil chamber  75  are disconnected by the floating sleeve  71 , the flow resistance of the oil pressure is large so the resistance against the force pushing the plunger  50  is moderate. In the condition as shown in  FIGS. 8 to 7 , the spring constant is large at k 1 ; since the large-diameter oil chamber  74  and the small-diameter oil chamber  75  are disconnected by the floating sleeve  71 , the flow resistance of the oil pressure is large so the resistance against the force pushing the plunger  50  is the largest. 
   While the resistance against the force pushing the plunger  50  changes in two steps in the embodiment as shown in  FIGS. 1 to 6(   c ), the resistance against the force pushing the plunger  50  changes in three steps in the embodiment as shown in  FIGS. 7 to 10 . 
   Another possible embodiment of the present invention is as shown in  FIG. 11 . This embodiment includes an air purge valve  100  as a purging mechanism for air and oil that is not integral with the tensioner lifter  0  but is separate from it. In addition, the air purge valve  100  is mounted in the already assembled tensioner body  20 . Thus, the basic structure and functionality of the air purge valve  100  are virtually the same as in the above embodiments and not described here. 
   The air purge valve  100  is disposed and mounted perpendicularly to the longitudinal direction of the tensioner body  20  and this is achieved by screwing the thread of the air purge valve  100  into a screw hole b made in the side of the tensioner body  20  and fixing the valve integrally. The air purge valve  100  comprises a base  101  which is directly screwed in the side of the tensioner body  20  with a valve spring holder  105 , screwed in the base  101 , which houses a ball valve  103  pressed into the valve seat  102  of the base  101  with a spring  104  through contact and joint of its joint surface with the base  101  in a manner to allow the valve to come into contact or out of contact freely. An extension passage  106  is screwed in the valve spring holder  105 . 
   The structural members  101  to  106  are serially connected with each other and extend perpendicularly to the tensioner body  20 . The air purge valve  100  may be mounted on the tensioner body  20  after the structural members  101  to  106  are joined in advance or the individual structural members  101  to  106  may be mounted on the tensioner body  20  one by one. 
   In this embodiment, since the air purge valve  100  is a separate unit, the structure of the tensioner lifter  0  is simplified, which makes its manufacture easy. In addition, because the air purge valve  100  can be removed as a separate unit from the tensioner body  20  for repair or adjustment purposes, repair or adjustment work can be easier, which improves working efficiency and offers an advantage in terms of cost. 
   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.