Abstract:
A hydraulic tensioner includes a build-in relief valve having a relief hole formed in a housing of the tensioner, a ring-shaped valve element disposed in the housing at a side opposite to the front end of a plunger slidably mounted on the housing, and a spring urging the valve element in a direction to close the relief hole. The valve element and the spring of the relief valve are disposed coaxially with the plunger and a check valve provided between the housing and an oil chamber defined between the housing and the plunger. With this arrangement, the housing is made compact in size and hence enables downsizing of the tensioner. The relief valve disposed interiorly of the housing is highly reliable in operation.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a hydraulic tensioner with built-in relief valve (hereinafter referred to as “relief valve built-in hydraulic tensioner) used to apply an appropriate tension to a timing chain of an automobile engine.  
           [0003]    2. Related Art  
           [0004]    Hydraulic tensioners used to apply an appropriate tension to a timing chain of an automobile engine include a hydraulic tensioner having a built-in relief valve that, when there occurs an excessive tension in the timing chain, relieves the excessive tension.  
           [0005]    [0005]FIG. 5 is a front sectional view illustrating one example of the conventional relief valve built-in hydraulic tensioner. As shown in this figure, the conventional relief valve built-in hydraulic tensioner  1  includes a housing  2 , a plunger  3  slidably fitted in a blind hole  2 A formed in the housing  2 , a spring  4  that urges the plunger  3  in a direction projecting outward from the blind hole  2 A, an oil chamber  5  formed between the housing  2  and the plunger  3 , a check valve  6  provided on the bottom of the blind hole  2 A in the housing  2 , and a relief valve  7  provided in the housing  2  at position located radial outward of the check valve  6 .  
           [0006]    The check valve  6  includes a ball seat  6 B having an oil path  6 A and press-fitted in a hole  2 B formed in the bottom of the blind hole  2 A in the housing  2 , a check ball  6 C, a retainer  6 D, and a spring  6 E. The hole  2 B formed in the housing  2  is connected with an oil path (not shown) formed for supplying an oil into the oil chamber  5  from a hydraulic pressure generator or source such as an oil pump (not shown). The check valve  6  has a function that permits an oil to flow into the chamber  5  and in reverse blocks an oil to flow out from the oil chamber  5 .  
           [0007]    The relief valve  7  includes a valve element insertion hole  7 A formed in the housing  2 , a connecting hole  7 B that communicates the oil chamber  5  with the valve element insertion hole  7 A, a valve element  7 C slidably received in the valve element insertion hole  7 A, a spring  7 D urging the valve element  7 C, a relief hole  7 E that communicates the valve element insertion hole  7 A in the housing  2  with the outside air, and a plug  7 G press-fitted in the the valve element insertion hole  7 A and having a central hole  7 F communicating the valve element insertion hole  7 A with the outside air. With the relief valve  7  thus constructed, when the oil pressure inside the oil chamber  5  exceeds a set pressure, the valve element  7 C moves backward against the force of the spring  7 D to such an extent that the connecting hole  7 B communicates with the relief hole  7 E to thereby relieve the oil pressure inside the oil chamber  5 .  
           [0008]    When a looseness occurs in the chain during the engine being in driving, the plunger moves forward by the fore of the spring  4 , and at the same time, the check ball  6 C moves out off the ball seat  6 B whereupon the oil is supplied into the oil chamber  5  by way of the oil path (not shown) formed in the housing  2  and the oil path  6 A formed in the ball seat  6 B. Thus, the tension in the chain is maintained.  
           [0009]    When an excessive tension occurs in the chain, the plunger  3  receives a pressing load in the reverse urging direction from a tensioner lever not shown. At this moment, the check valve  6  functions to block the flow-out of an oil in the oil chamber  5 , and the oil in the oil chamber  5  being an incompressible fluid deters the plunger  3  from returning. However, receiving an oil pressure generated in the oil chamber  5 , the valve element  7 C of the relief valve  7  slightly slide backward against the force of the spring  7 D. Thus, the excessive tension occurred in the chain is absorbed. When the pressing load on the plunger  3  increases further and the valve element  7 C of the relief valve  7  retreats further, the connecting hole  7 B communicates with the relief hole  7 E, which permits the oil in the oil chamber  5  to discharge, thereby reducing the oil pressure in the oil chamber  5 . Thus, the excessive tension in the chain is relieved. As the oil pressure in the oil chamber  5  decreases accompanied with the discharge of the oil, the valve element  7 C urged by the spring  7 D closes the relief hole  7 E, thus maintaining the set value of the oil pressure in the oil chamber  5 . Further, as the oil pressure in the oil chamber  5  decreases excessively, the check valve  6  functions to flow an oil into the oil chamber  5 , whereby the oil pressure in the oil chamber  5  returns to a normal value.  
           [0010]    [0010]FIG. 6 is a front sectional view illustrating another example of the conventional relief valve built-in hydraulic tensioner. As shown in this figure, the conventional relief valve built-in hydraulic tensioner  11  includes a housing  12 , a plunger  13  slidably fitted in a blind hole  12 A formed in the housing  12 , a spring  14  that urges the plunger  13  in a direction to project from the blind hole  12 A, an oil chamber  15  formed between the housing  12  and the plunger  13 , a check valve  16  provided on the bottom of the blind hole  12 A in the housing  12 , and a relief valve  17  provided on a front end of the plunger  13 .  
           [0011]    The check valve  16  includes a ball seat  16 B having an oil path  16 A and press-fitted in a hole  12 B formed in the bottom of the blind hole  12 A formed in the housing  12 , a check ball  16 C, a retainer  16 D, and a spring  16 E. The hole  12 B formed in the housing  12  is connected with an oil path (not shown) which is used to supply an oil into the oil chamber  15  from a hydraulic pressure generator or source such as an oil pump (not shown). The check valve  16  has a function that permits an oil to flow into the chamber  15  and in reverse blocks an oil to flow out from the oil chamber  15 .  
           [0012]    The relief valve  17  includes a valve element insertion hole  17 A formed in the front end of the plunger  13 , a connecting hole  17 B that communicates the oil chamber  15  with the valve element insertion hole  17 A, a valve element  17 C slidably received in the valve element insertion hole  17 A, a spring  17 D that urges the valve element  17 C, a relief hole  17 E that communicates the valve element insertion hole  17 A formed in the plunger  13  with the outside air, and a plug  17 G press-fitted in the valve element insertion hole  17 A and having a central hole  17 F communicating the valve element insertion hole  17 A with the outside air. When the oil pressure inside the oil chamber  15  exceeds a set pressure, the valve element  17 C moves backward against the force of the spring  17 D, and the connecting hole  17 B communicates with the relief hole  17 E; thus the relief valve  17  displays a function to reduce the oil pressure inside the oil chamber  15 .  
           [0013]    The relief valve built-in hydraulic tensioner  11  maintains the tension of the chain in such a manner that, when a looseness occurs in the chain during the engine being in driving, the plunger  13  moves forward by the force of the spring  14 , and at the same time, the check ball  16 C moves out off the ball seat  16 B to thereby permit an oil to be supplied into the oil chamber  15  by way of the oil path (not illustrated) formed in the housing  12  and the oil path  16 A formed in the ball seat  16 B.  
           [0014]    When an excessive tension occurs in the chain, the plunger  13  receives a pressing load in the reverse urging direction from a tensioner lever not illustrated. At this moment, the check valve  16  functions to block the flow-out of an oil in the oil chamber  15 , and the oil in the oil chamber  15  being an incompressible fluid deters the plunger  13  from returning. However, receiving an oil pressure generated in the oil chamber  15 , the valve element  17 C of the relief valve  17  slightly slide backward against the energizing force of the spring  17 D. Thus, the relief valve built-in hydraulic tensioner  11  absorbs the excessive tension that occurred in the chain. When the pressing load to the plunger  13  increases further and the valve element  17 C of the relief valve  17  retreats further, the connection hole  17 B communicates with the relief hole  17 E, which permits the oil in the oil chamber  15  to discharge, thereby reducing the oil pressure in the oil chamber  15  to relieves the excessive tension of the chain. As the oil pressure in the oil chamber  15  decreases accompanied with the discharge of the oil, the valve element  17 C urged by the spring  17 D moves to close the relief hole  17 E, thus maintaining the set value of the oil pressure in the oil chamber  15 . Further, when the oil pressure in the oil chamber  15  decreases excessively, the check valve  16  functions to flow an oil into the oil chamber  15 , whereby the oil pressure in the oil chamber  15  returns to a normal value.  
           [0015]    However, in the conventional relief valve built-in hydraulic tensioner  1  as shown in FIG. 5, since the relief valve  7  is provided on the radial outside of the check valve  6  in the housing  2 , the housing  2  has to be made larger, and the tensioner  1  as a whole cannot be made compact.  
           [0016]    Further, in the conventional relief valve built-in hydraulic tensioner  11  as shown in FIG. 6, since the relief valve  17  is incorporated in the plunger  13 , the inertial mass of the valve element  17 C accompanied with the advancing and retracting motions of the plunger  13  exerts a bad influence on the motion of the relief valve  17 , thus making the operation of the relief valve  17  unstable.  
           [0017]    Accordingly, in view of the foregoing problems, an object of the present invention is to provide a relief valve built-in hydraulic tensioner having structural features which make the tensioner compact in size, facilitate the boring of the housing, and stabilize the operation of the relief valve.  
         SUMMARY OF THE INVENTION  
         [0018]    To achieve the foregoing object, the present invention provides a relief valve built-in hydraulic tensioner which comprises: a housing; a plunger slidably fitted in the housing and urged by a spring in a direction to project outward from the housing; an oil chamber formed between the housing and the plunger; a check valve provided between the housing and the oil chamber and operative to permit an oil to flow into the oil chamber from the outside of the housing and block the oil to flow out from the oil chamber; and a relief valve communicating with the oil chamber. The relief valve is comprised of a relief hole formed in the housing, a ring-shaped valve element disposed in the housing at a side opposite to a front end of the plunger, and a relief valve spring urging the ring-shaped valve element in a direction to normally close the relief hole. The central axis of the ring-shaped valve element and the central axis of the relief valve spring urging the ring-shaped valve element are substantially coincident with the respective central axes of the check valve and the plunger.  
           [0019]    Since the central axis of the ring-shaped valve element and the central axis of the relief valve spring are substantially coincident on one line with the respective central axes of the check valve and the plunger, the housing can be made compact to thereby enable downsizing of the tensioner as a whoale, and the boring of the housing can easily be achieved.  
           [0020]    Further, since the relief valve is installed inside the housing of the tensioner, the ring-shaped valve element can slide smoothly and reliably. The relief valve is, therefore, highly reliable in operation.  
           [0021]    In one preferred form of the present invention, the housing has a hole defining a part of the oil chamber, and a cylindrical plug disposed concentrically in the hole with an annular space defined between an inner surface of the hole and an outer surface of the cylindrical plug, the plug being attached to the housing at one end of the hole. The plunger is slidably inserted into the hole from the other end of the hole. The ring-shaped valve element is slidably received between the inner surface of the hole and the outer surface of the plug, and said relief valve spring is disposed in said the annular space.  
           [0022]    The hole formed in the housing may be a through-hole having a uniform diameter throughout the length thereof, and the plug may be a flanged plug having an annular flange press-fitted in the through-hole. The housing further has a stop ring attached to the inner surface of the through-hole for restricting movement of the ring-shaped valve element in the direction to close the relief hole.  
           [0023]    As an alternative, the hole formed in the housing may be a through-hole having a stepped portion, and the plug may be a flanged plug having an annular flange press-fitted in the through-hole. The the ring-shaped valve element is normally held in abutment with the stepped portion of the stepped through-hole by the force of the relief valve spring.  
           [0024]    The hole formed in the housing may be a blind hole closed at the one end thereof and having a small-diameter portion adjacent the closed one end. The plug may be a flange-less plug press-fitted in the small-diameter portion of the blind hole. The housing further has a stop ring attached to the inner surface of the blind hole for restricting movement of the ring-shaped valve element in the direction to close the relief hole.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 is a front sectional view of a relief valve built-in hydraulic tensioner according to a first embvodiment of the present invention;  
         [0026]    [0026]FIG. 2 is a front sectional view illustrating an operational state of the relief valve of the relief valve built-in hydraulic tensioner shown in FIG. 1;  
         [0027]    [0027]FIG. 3 is a front sectional view of a relief valve built-in hydraulic tensioner according to a second embodiment of the present invention;  
         [0028]    [0028]FIG. 4 is a front sectional view of a relief valve built-in hydraulic tensioner according to a third embodiment of the present invention;  
         [0029]    [0029]FIG. 5 is a front sectional view illustrating one example of the conventional relief valve built-in hydraulic tensioner; and  
         [0030]    [0030]FIG. 6 is a front sectional view illustrating another example of the conventional relief valve built-in hydraulic tensioner.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    Certain preferred embodiment of the present invention will be described with reference to the accompanying drawings wherein like reference characters designate like or corresponding parts throughout the several views.  
         [0032]    [0032]FIG. 1 shows a relief valve built-in hydraulic tensioner according to a first embodiment of the present invention. As shown in this figure, the relief valve built-in hydraulic tensioner  21  includes a housing  22 , a plunger  23  slidably fitted in a through-hole  22 A formed in the housing  22 , a spring  24  that urges the plunger in a direction to project from the housing  22 , an oil chamber  25  formed between the housing  22  and the plunger  23 , a check valve  27  mounted on a flanged plug  26  press-fitted at an annular flange  26 B thereof in the through-hole  22 A from an end opposite to the plunger  23 , and a relief valve  28  disposed in the housing  22  and located radially outward of the check valve  27 . The through-hole  22 A has a uniform diameter throughout the length thereof.  
         [0033]    The check valve  27  includes a ball seat  27 B having an oil path  27 A and press-fitted in the blind hole  26 A of the flanged plug  26  press fitted in the housing  22 , a check ball  27 C, a retainer  27 D, and a spring  27 E. The blind hole  26 A of the plug  26  is connected with an oil path (not shown) for supplying an oil from a hydraulic pressure generator or source such as an oil pump (not shown) into the oil chamber  25 . The check valve  27  has a function that permits an oil to flow into the chamber  25  and in reverse blocks an oil to flow out from the oil chamber  25 .  
         [0034]    The relief valve  28  includes a ring-shaped valve element  28 A slidably installed between an inner surface of the through-hole  22 A formed in the housing  22  and an outer surface of the plug  36 , and a relief valve spring  28 B that urges the ring-shaped valve element  28 A in one direction (rightward direction in FIG. 1) to close a relief hole  28 C formed in the housing  22 . The spring  28 B is disposed in an annular space S defined between the inner surface of the through-hole  22   a  and the outer surface of the plug  26 . Further, a stop ring  29  is attached to the inner surface of the through-hole  22 A and disposed radially outward of the check valve  27 . The stop ring  29  restricts the movement of the ring-shaped valve element  28 A in rightward direction. In operation of the relief valve  28 , when the oil pressure inside the oil chamber  25  exceeds a set pressure, the valve element  28 A moves backward (leftward direction in FIG. 1) against the force of the spring  28 B to allow the oil chamber  25  to communicate with the relief hole  28 C, thereby relieving the oil from the oil chamber  25  to lower the oil pressure inside the oil chamber  25 .  
         [0035]    The relief valve  28  is arranged such that the central axis of the ring-formed valve element  28 A and the central axis of the relief valve spring  28 B are substantially coincident with the central axes of the check valve  27  and the plunger  23 . In other words, the ring-shaped valve element  28 A, the relief valve spring  28 B, the check valve  27  and the plunger  23  are coaxial with one another.  
         [0036]    With the relief valve built-in hydraulic tensioner  21  thus constructed, when a looseness occurs in the chain during the engine being in driving, the plunger  23  advances or moves forward by the force of the spring  24 , and at the same time, the check ball  27 C is caused to separate from the ball seat  27 B to thereby allow an oil to be supplied into the oil chamber  25  by way of the oil path (not shown) formed in the flanged plug  26  in communication with the blind hole  26 A and the oil path  27 A formed in the ball seat  27 B.  
         [0037]    When an excessive tension occurs in the chain, the plunger  23  is subjected to a load or pressure applied from a tensioner lever (not shown) in a direction to move the plunger  23  backward against the force of the spring  24 . At this moment, the check valve  27  functions to block the flow-out of an oil in the oil chamber  25 , and the oil in the oil chamber  25  being an incompressible fluid deters the plunger  23  from moving backward. However, receiving an oil pressure generated in the oil chamber  25 , the ring-shaped valve element  28 A of the relief valve  28  slightly slides backward against the force of the relief valve spring  28 B to thereby absorbs the excessive tension in the chain. When the load or pressure on the plunger  23  increases further and the ring-shaped valve element  28 A of the relief valve  28  retreats further, the oil chamber  25  communicates with the relief hole  28 C, whereupon the oil in the oil chamber  25  is allowed to discharge, as indicated by the arrow shown in FIG. 2. Thus, the oil pressure in the oil chamber  25  decreases to thereby relieve the excessive tension of the. With the discharge of the oil, the oil pressure in the oil chamber  25  decreases, so that the ring-shaped valve element  28 A moves forward by the force of the spring  28 B to close the relief hole  28 C, thereby maintaining the set value of the oil pressure in the oil chamber  25 . Further, when the oil pressure in the oil chamber  25  decreases excessively, the check valve  27  operates to supply an oil into the oil chamber  25 , whereby the oil pressure in the oil chamber  25  returns to a normal value.  
         [0038]    In the relief valve built-in hydraulic tensioner  21  of the foregoing embodiment, since the central axis of the ring-shaped valve element  28 A and the central axis of the relief valve spring  28 B are substantially coincident with the central axes of the check valve  27  and the plunger  23 , the housing  22  can be made compact in size, and the boring or drilling of the through-hole  22 A formed in the housing  22  can easily be achieved. The tensioner  21  having such compact housing  22  is made compact as a whole.  
         [0039]    Further, since the relief valve  28  is installed interiorly of the housing  22  of the tensioner  21 , sliding movement of the ring-shaped valve element  28 A is highly smooth and reliable. Thus, the relief valve  28  can operate stably.  
         [0040]    [0040]FIG. 3 illustrates a relief valve built-in hydraulic tensioner  21 ′ according to a second embodiment 2 of the presenrt invention. The tensioner  21 ′ differs from the tensioner of the first embodiment shown in FIGS. 1 and 2 only in a point described below. In the relief valve built-in hydraulic tensioner  21  of the first embodiment 1 described above with reference to FIGS. 1 and 2, the stop ring  29  attached to the inner surface of the through-hole  22 A is used to restrict the movement of the ring-shaped valve element  28 A in the rightward direction. However, in the relief valve built-in hydraulic tensioner  21 ′ of the second embodiment, the through hole  22 A formed in the housing  22  is a stepped through-hole, having a step  22 B engaged with an end of the ring-shaped valve element  28 A to restrict the rightward movement of the valve element  28 A.  
         [0041]    The relief valve built-in hydraulic tensioner  21 ′ of the second embodiment 2 has the same function and effect as the relief valve built-in hydraulic tensioner  21  of the first embodiment 1. Accordingly, the same parts are designated by the same reference characters and a further description thereof can, therefore, be omitted.  
         [0042]    [0042]FIG. 4 illustrates a relief valve built-in hydraulic tensioner  21 ″ of according to a third embodiment of the present invention. The tensioner  21 ″ differs from the tensioner  21  of the first embodiment shown in FIG. 1 in that the flanged plug  26  press-fitted in the through-hole  22 A in the housing  22  of the first embodiment is replaced with a combination of a stepped blind hole  22 C formed in the housing  22  and having a small-diameter portion  22 D adjacent to the bottom threof, and a flange-less cylindrical plug  26 ′ press-fitted in the small-diameter portion  22 D of the stepped blind hole  22 C. The relief valve built-in hydraulic tensioner  21 ″ of the third embodiment has the same function and effect as the relief valve built-in hydraulic tensioner  21  of the first embodiment shown in FIG. 1. Accordingly, the same parts are designated by the same reference characters, and no further description thereof is needed.  
         [0043]    Although the embodiments shown in FIGS.  1 - 2 , FIG. 3 and FIG. 4, respectively, do not have a ratchet mechanism constructed to prevent backward movement of the plunger with a backlash, the invention can also be applied to a relief valve built-in hydraulic tensioner equipped with such latchet mechanism.  
         [0044]    As described above, since the central axis of the ring-shaped valve element of the relief valve and the central axis of the relief valve spring that urges the ring-shaped valve element are substantially coincident with the central axes of the check valve and the plunger, the housing can be made compact in size and thus downsizes the tensioner as a whole, and the boring or drilling of the housing can easily be achieved.  
         [0045]    Further, since the relief valve is installed inside the housing of the tensioner, sliding movement of the ring-shaped valve element can be performed smoothly and reliably. Thus, the relief valve is highly reliable in operation.  
         [0046]    Obviously, various minor changes and modifications of the present invention are possible in the light of the above teaching. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described.