Abstract:
In a ratchet tensioner, a linearly moving ratchet for engagement with rack teeth on a plunger is operated a piston that is spring-biased in a direction to engage the ratchet teeth with the rack, and movable by oil pressure in a direction to disengage the ratchet teeth from the rack. The piston is located within a hole in the housing and, with that hole, forms a second high pressure oil chamber that can receive oil through an oil supply path independent of the oil supply path that carries oil to a first high pressure oil chamber formed by the plunger and the tensioner housing.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under Title 35, United States Code, §119 (a)-(d) on the basis of Japanese Patent Application No. 2009-121762, filed on May 20, 2009. The disclosure of Japanese Patent Application No. 2009-121762 is herein incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to a ratchet tensioner for applying a tensioning force to a timing chain for driving an engine camshaft and, optionally, other engine components. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is known to apply tension to a timing chain by means of a tensioner having a plunger slidable in the plunger-accommodating hole of a tensioner housing. The plunger and housing cooperate to form a high pressure oil chamber, and the plunger is urged in a direction to exert a tensioning force on the timing chain by a plunger-biasing spring and also by hydraulic pressure applied from an external pressure source to the oil in the oil chamber. 
         [0004]      FIG. 7  illustrates a prior art tensioner  500  having a piston  526  incorporated into a tensioner housing  512  for movement in a direction orthogonal to the direction in which a plunger  514  slides. The piston  526  and the housing form a second oil chamber  520  which receives oil from an oil passage  544 . An air chamber  528  is formed in the housing  512  on the side of piston  526  opposite from the second oil chamber  520 . Within this air chamber, a second spring  534  biases the piston  526  toward the second oil chamber  520 . An air communication hole  532  connects air chamber  528  to the external atmosphere. The air communication hole  532  can be closed by an end of piston rod  524  when the piston  526  moves against the biasing force exerted by spring  534  as a result of pressure applied to the piston  526  by the oil in the second oil chamber  520 . A toothed rack  538  is engageable by teeth  536  formed at the end of the piston rod  524  to block retracting movement of the plunger  514 . The tensioner of  FIG. 7  is described in Japanese Utility Model No. 2559664. 
         [0005]    In the above-described prior art tensioner  500 , the main high pressure oil chamber  516  in which hydraulic pressure is applied to the plunger  514 , and the second oil chamber  520  for applying hydraulic pressure to the piston  526 , are connected by oil passage  548  and an oil passage  544 , which are branches of an oil path connected to an oil reservoir  550 . If the engine in which the tensioner is installed is inoperative for a long period of time, air enters the second chamber  520 . As a result, the hydraulic pressure in the second oil chamber  520  may not be sufficient to cause disengagement of teeth  536  on rod  524  from the rack  538  on plunger  514 . It is possible that the engagement of the teeth  526  from the rack  538  will not be released completely and quickly. Accordingly, in the tensioner  500 , smooth forward and backward movement of the plunger  514  may not be achieved promptly after starting of the engine. Teeth  536  on the piston rod  524  or the teeth of rack  538  may be broken, with the result that the biasing force of the plunger  514  may not adequately compensate for elongation of the transmission chain. Moreover, abnormal noises caused by the flapping of the timing chain cannot be eliminated by completely blocking backlash of the plunger  514  as a result of forces received from the timing chain. 
         [0006]    The prior art tensioner  500  also requires a tough plunger-biasing spring  518  to sustain backlash of the plunger  514 , and, as a result, the size of the tensioner is increased. 
       SUMMARY OF THE INVENTION 
       [0007]    The ratchet tensioner according to the invention comprises a housing having a plunger-accommodating hole with an end opening and a bottom. A first oil supply passage is provided for introduction of oil under pressure into the plunger-accommodating hole. A plunger for maintaining tension in a traveling transmission chain is slidable in the plunger-accommodating hole, and protrudes from the end opening of the plunger-accommodating hole. The plunger is hollow and has an elongated toothed rack formed on its external surface. The toothed rack extends along the protruding direction of the plunger and has a series of teeth arranged in succession along the protruding direction. A plunger-biasing spring for biasing the plunger in its protruding direction is disposed in a first high-pressure oil chamber formed by the plunger-accommodating hole and the hollow portion of the plunger. a check valve unit is assembled at the bottom of the plunger-accommodating hole to block reverse flow of oil from the first high-pressure oil chamber to the first oil supply passage. A cylindrical piston-accommodating hole is also provided in the housing. The piston-accommodating hole has an axis extending in a direction transverse to the protruding direction of the plunger. A piston, slidable in the piston-accommodating hole along its axis, has at least one ratchet tooth engageable with the toothed rack on the plunger. A piston-biasing spring biases the piston toward the toothed rack so that at least one ratchet tooth can engage the toothed rack on the plunger. The least one ratchet tooth and the teeth of the toothed rack are shaped to permit displacement of the plunger in its protruding direction and to block retracting displacement of the plunger in the direction opposite to the protruding direction. The tensioner further comprises a second high-pressure oil chamber formed by the piston and the piston-accommodating hole, and located between the piston and the plunger, for receiving oil under pressure and thereby displacing the piston in a direction to release engagement of the at least one ratchet tooth from the rack teeth on the plunger against the biasing force of the plunger-biasing spring. A second oil supply passage is provided for introducing oil under pressure into the second high-pressure oil chamber. The second oil supply passage is isolated from the first oil supply passage sufficiently that the oil pressure in the second oil supply passage can be different from the pressure in the first oil supply passage. 
         [0008]    Because the biasing force of the plunger-biasing spring acts on the piston, the tensioner permits forward displacement of the plunger but blocks backward displacement, thereby suppressing backlash and flapping noises generated by the timing chain causing backlash. 
         [0009]    After the engine is started, oil pressure in the second high pressure oil chamber releases the engagement of the ratchet teeth with the rack teeth on the plunger, permitting smooth forward and backward movement of the plunger. It is possible to adjust the timing of disengagement of the ratchet by adjusting the biasing force of the piston-biasing spring. 
         [0010]    After starting of the engine following an extended inoperative interval, when hydraulic pressure within the second high-pressure oil chamber increases, and the force imparted by the hydraulic pressure to the piston exceeds the force exerted by the piston-biasing spring, the ratchet tooth or teeth are disengaged from the rack teeth without skipping, and forward and backward movement of the plunger can take place. 
         [0011]    In some embodiments of the invention, the second oil supply passage formed in the housing is arranged to supply oil to the second high pressure oil chamber directly. In this case, the second oil supply passage is independent of the first oil supply passage and the pressure of oil in the second high pressure oil chamber is independent of the pressure of the oil in the first high pressure oil chamber. 
         [0012]    Where the second oil supply passage is independent of the first oil supply passage, i.e., neither passage is a branch of the other it is possible for the second high-pressure oil chamber to avoid being directly influenced by pulsation in pressure of the oil introduced into the first oil supplying passage, even when the capacity of the second high-pressure oil chamber is smaller than that of the first high-pressure oil chamber. As a result, stable generation of hydraulic pressure within the second high pressure oil chamber takes place, and it becomes possible to achieve quick forward and backward movement of the plunger smoothly. 
         [0013]    Another advantage of independent oil supply passages is that the second oil supply passage can be disposed at any convenient position in the tensioner housing, oil can be introduced into the second high-pressure oil chamber through a short path, and fabrication of the second oil supply passage can be simplified. 
         [0014]    The second oil supply passage can be provided with an orifice for absorbing pulsation in the pressure of the oil introduced into the second high pressure oil chamber through the second oil supply passage of the external pressure oil. The orifice suppresses the amount of oil introduced into the second high-pressure oil chamber through the second oil supply passage when it the supply of oil is excessive, and also suppresses leakage of oil from the high-pressure oil chamber when the oil supply is insufficient. Accordingly, with the orifice in place, the effect of pulsations on the ratchet mechanism can be avoided, and a stable supply of oil to the second oil chamber can be ensured. 
         [0015]    In other embodiments of the invention, the second oil supply passage directly connects the second high pressure oil chamber to the first high-pressure oil chamber. Even so, because the check valve, at least when closed, isolates the oil supply paths from each other, the pressures in the oil supply paths can be different. These embodiments have the advantage that the ratchet piston is actuated in synchronism with pulsations in the high-pressure oil chamber. Accordingly, the ratchet tensioner instantly releases the ratchet tooth or teeth from the rack teeth after starting of the engine, and permits smooth forward and backward movements of the plunger. 
         [0016]    In certain preferred embodiments, a projection extends from the piston to the exterior of the housing, the projection being manually accessible outside the housing for forcibly releasing engagement of the at least one ratchet tooth from the toothed rack to permit retracing movement of the plunger. Manual release of the ratchet mechanism simplified tensioner installation and engine maintenance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic elevational view of an engine timing transmission incorporating a ratchet tensioner according to the invention; 
           [0018]      FIG. 2  is a section view of the ratchet tensioner shown in  FIG. 1 ; 
           [0019]      FIG. 3  is a side view of the ratchet tensioner shown in  FIG. 1 ; 
           [0020]      FIG. 4  is a section view showing a state of engagement of a ratchet and a plunger; 
           [0021]      FIG. 5  is a section view showing a state in which the engagement of the ratchet and the plunger is released; 
           [0022]      FIG. 6  is a section view of the ratchet tensioner according to a second embodiment of the invention; and 
           [0023]      FIG. 7  is a section view of a prior art ratchet tensioner. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    The advantages of the invention can be realized in various embodiments, provided that they comprise a housing having a plunger-accommodating hole, a first oil supply passage for introduction of oil under pressure into the plunger-accommodating hole, a hollow plunger slidable in the plunger-accommodating hole, and protruding from an end opening thereof, an elongated toothed rack formed on the external surface of the plunger and extending along the protruding direction of the plunger with a series of teeth arranged in succession along the protruding direction, a plunger-biasing spring biasing the plunger in its protruding direction and disposed in a first high-pressure oil chamber formed by the plunger-accommodating hole and the hollow portion of the plunger, a check valve unit at the bottom of the plunger-accommodating hole to block reverse flow of oil, a cylindrical piston-accommodating hole having an axis extending in a direction transverse to the protruding direction of the plunger, a piston slidable in the piston-accommodating hole along its axis, a piston-biasing spring biasing the piston toward the toothed rack, at least one ratchet tooth on the piston and engageable with the toothed rack on the plunger, the teeth being shaped to permit displacement of the plunger in its protruding direction and to block retracting displacement of the plunger in the direction opposite to the protruding direction, a second high-pressure oil chamber formed by the piston and the piston-accommodating hole, and located between the piston and the plunger, for receiving oil under pressure and thereby displacing the piston in a direction to release engagement of the at least one ratchet tooth from the rack teeth against the biasing force of the plunger-biasing spring, and a second oil supply passage for introducing oil under pressure into the second high-pressure oil chamber, and the oil supply passages are isolated from each other sufficiently that the oil pressures in the oil supply passages can be different. 
         [0025]    The ratchet tensioner blocks backlash of the plunger due to force receiving from the timing chain on starting the engine, when the engine has been inoperative for an extended interval, and generates hydraulic pressure stably within the second high-pressure oil chamber to achieve quick forward and backward movement of the plunger smoothly. 
         [0026]    Oil supplied from an oil pump can be introduced directly into the first oil supply passage in the housing or can be introduced through an oil reservoir formed on a back of the housing for temporarily storing oil supplied from the oil pump before introducing it into the first oil supply passage. 
         [0027]    The check valve unit used in the ratchet tensioner of the invention can be any of various check valve types as long as it is assembled at the bottom of the plunger-accommodating hole and blocks reverse flow of the oil from the high-pressure oil chamber to the first oil supply passage. For example, the check valve unit may have a ball seat that communicates with the first oil supply passage and supplies oil under pressure to the high-pressure oil chamber, a check ball that faces a seating surface of the ball seat, a ball-biasing spring for pressing and biasing the check ball against the ball seat, and a bell-like retainer for restricting displacement of the check ball. 
         [0028]    As shown in  FIG. 1 , the ratchet tensioner  100  according to a first embodiment of the invention is attached to an engine adjacent the slack side of a timing chain C that is in meshing engagement with a driving sprocket S 1  on an engine crankshaft and one or more driven sprockets S 2  on the engine camshafts. A plunger  120  of the tensioner protrudes from the tensioner housing  110 , and applies tension to the slack side of the timing chain C through a movable lever L pivotably supported on the engine block by pressing against the back of the lever L at a location spaced from the lever&#39;s pivot axis. 
         [0029]    A stationary guide G for guiding the travel of the timing chain C is fixed on the engine block adjacent the tension side of the timing chain C. 
         [0030]    Rotation of the crankshaft sprocket S 1  is transferred by the timing chain to the camshaft sprockets S 2 . The directions of rotation of the sprockets, and the direction of movement of the timing chain, are indicated by arrows in  FIG. 1 . that 
         [0031]    As shown in  FIG. 2 , the housing  110  of tensioner  100  has a first oil supply passage  111  for introducing oil supplied under pressure from the engine block (not shown) to which the tensioner housing is attached. Hollow plunger  120  is slidable in a plunger-accommodating hole  112  in the housing, and protrudes outward from the housing toward the chain as shown in  FIG. 1 . A plunger-biasing spring  130  is disposed in a high-pressure oil chamber R formed by the plunger-accommodating hole  112  and the hollow interior  121  of the plunger. The spring  130  biases the plunger in the protruding direction. 
         [0032]    A check valve unit  140 , assembled at the bottom of the plunger-accommodating hole  112   a , blocks reverse flow of oil from the high-pressure oil chamber R to the first oil supply passage  111 . A piston  150  having a set of ratchet teeth  151  fits in a cylindrical hole  113  formed in the housing  110 , and is slidable in a direction transverse to the direction in which the plunger  120  moves. A plunger-biasing spring  160  urges the piston  150  in a direction such that its ratchet teeth  151  can engage with a toothed rack  122  formed on the plunger and extending longitudinally on a side of the plunger. The rack teeth are arranged as a series of teeth in succession along said protruding direction of the plunger. 
         [0033]    Although the axis of the piston-accommodating hole  113  is slightly oblique with respect to the direction of protrusion of the plunger, the hole  113  can be configured in any direction transverse to the protruding direction of the plunger as long the teeth  151  can engage and disengage the rack teeth as a result of movement of the piston in the piston-accommodating hole. A retaining plug  170  retains the piston  150  and the piston-biasing spring  160  within the piston-accommodating hole  113 . 
         [0034]    Any of various known check valve units can be used as long as it is located at the bottom of the plunger-accommodating hole  112  and is capable of blocking reverse flow of oil from the high-pressure oil chamber R to the first oil supply passage  111 . In the embodiment shown, the check valve unit  140  is composed of a ball seat  141  having an oil passage  141   a  connected to the first oil supply passage  111 , a check ball  142  seated on a valve seat  141   b  formed at an end of ball seat  141 , a ball-biasing spring  143  for pressing and biasing the check ball  142  against the ball seat  141 , and a bell-shaped retainer  144  for supporting the ball biasing spring  143  and restricting displacement of the check ball  142 . 
         [0035]    A second high-pressure oil chamber P is formed by the piston  150  and the piston-accommodating hole  113 . Oil pressure in the chamber P exerts a force of the piston opposing the biasing force exerted by spring  160 . When the force on the piston resulting from hydraulic pressure within oil chamber P exceeds the biasing force exerted by spring  160 , after starting of the engine, when the engine has been inoperative for a long interval, the ratchet teeth  151  are pulled away from the rack teeth by displacement of the piston  150 , thereby releasing the ratchet without skipping the rack teeth  122  on the plunger, and forward and backward movement of the plunger  120  can then take place without restriction by the ratchet mechanism. 
         [0036]    Oil is supplied to the second high pressure oil chamber P through a second oil supply passage  114 . Passage  114  in this embodiment is independent of, and not a branch of, the first oil supply passage  111 . Passage  114  is formed in housing  110  and configured to introduce oil under pressure into chamber P directly from an oil port in the engine block without significant influence by pulsations of pressure in the oil introduced into the first high pressure oil chamber R through the first oil supply passage  111 . The independent supply of oil to the second oil chamber P avoids excessive response by the piston  150  to variations in pressure in chamber R that could occur in the case of direct communication between chambers P and R, especially if the capacity of chamber P is much smaller than the capacity of the chamber R. Thus, it is possible to achieve quick forward and backward movement of the plunger  120  by generating hydraulic pressure within the second high-pressure oil chamber P in a stable manner, so that forward and backward movement of the plunger  120  is permitted after starting of the engine. 
         [0037]    Because the second oil supply passage  114  can be disposed at any of various positions in the housing irrespective of the location of the first oil supply passage  111 , oil can be introduced into the second high-pressure oil chamber P through a short passage, and the short passage can be formed easily. 
         [0038]    The second oil supply passage  114  described can be provided with a restricted orifice  114   a  as shown in  FIG. 3 . The orifice absorbs pulsation in the oil supplied under pressure from the engine block that result from operation of the engine oil pump. The orifice  114   a  also suppresses the amount of the oil introduced under pressure into the second high-pressure oil chamber P through the second oil supply passage  114  when it the oil supply is excessive, and suppresses leakage of oil from chamber P when the oil supply is insufficient. Thus, the ratchet tensioner  100  ensures a stable supply of oil under pressure to the second high pressure oil chamber P. 
         [0039]    A ratchet-releasing projection  152  is provided on the rear part of the piston  150  as shown in  FIGS. 2 and 4 . This projection extends through a hole  171  in the retaining plug  170 , and can be grasped manually from the outside of the housing  110 . When pulled, projection  152  releases the engagement of teeth  151  with rack teeth  122  on the plunger  120 , so that the plunger can move forward or backward as necessary when the tensioner is being attached to the engine block and when and various maintenance operations are carried out. 
         [0040]    In the operation of the tensioner  100  described above, at first, while the engine is stopped, and on starting the engine, no oil is supplied from the engine block through the second oil supply passage  114 , and the force exerted on piston  150  by hydraulic pressure within the second high-pressure oil chamber P is smaller than the biasing force exerted by piston biasing spring  160 . The ratchet teeth  151  are held engaged with the rack teeth  122  on the plunger by the biasing force of the piston biasing spring  160  as shown in  FIG. 4 , and block backlash of the plunger  120  which would otherwise occur due to forces exerted by the timing chain. 
         [0041]    After the engine is started, oil is supplied under pressure from the engine block through the second oil supply passage  114  and the oil pressure in chamber P causes the piston  150  to move, as shown in  FIG. 5 , in a direction X against the biasing force exerted by spring  160 , withdrawing the ratchet teeth  151  from the rack teeth  122  on the plunger without skip. Hydraulic pressure is generated in a stable manner within the second high-pressure oil chamber P allowing forward and backward movements of the plunger  120  along direction Y to take place smoothly. 
         [0042]    When the orifice  114   a  is in place to absorb pulsation in the oil supplied directly to the second high-pressure oil chamber P, the tensioner suppresses flapping noises produced by the timing chain on starting of the engine, especially when the engine has been inoperative for a long interval. The flapping noises are suppressed by permitting forward displacement while blocking backward displacement of the plunger  120 . After engine start-up, the ratchet teeth  151  are disengaged from the rack teeth  122  so that the plunger can move both forward and backward. 
         [0043]    The time at which the ratchet teeth are disengaged from the rack teeth after engine start-up can be adjusted by adjusting the biasing force of the piston-biasing spring  160 . Moreover, as mentioned previously installation and maintenance of the tensioner are simplified by the provision of the piston releasing projection  152 . 
         [0044]      FIG. 6  shows another version of the ratchet tensioner of the invention, in which the first and second high pressure oil chambers are connected. Parts of the tensioner of  FIG. 6  that are identical to parts of the tensioner of  FIGS. 1-5  are designated by reference numbers that exceed the reference numbers of the corresponding parts by 100. 
         [0045]    The ratchet tensioner  200  shown in  FIG. 6  is also attached to an engine block, and associated with a timing chain, in the same manner as illustrated in  FIG. 1 . 
         [0046]    The ratchet tensioner  200  differs from ratchet tensioner  100  primarily in the position and configuration of the second oil supply passage  214  for introducing oil under pressure into the second high-pressure oil chamber P. 
         [0047]    The second oil supply passage  214  is formed in the housing  210  and is independent of the first oil supply passage  211  in that it is connected directly to first high pressure oil chamber R, whereas passage  211  is connected to chamber R through the check valve unit. The second oil supply passage  214  is connected to the first high-pressure oil chamber R near the bottom of the plunger-accommodating hole  212 , and delivers oil from chamber R into the second high-pressure oil chamber P. 
         [0048]    Even if the hydraulic pressure in the second high-pressure oil chamber P receives pulsations from the high-pressure oil chamber R, the piston  250  acts in synchronism with these pulsations so that the engagement of ratchet teeth  251  with the rack teeth  222  is released instantly, and smooth forward and backward movement of the plunger  220  can commence after starting of the engine. 
         [0049]    As in the case of the first embodiment, the ratchet tensioner  200  suppresses flapping noise of the timing chain that can take place at the time of engine start-up especially when the engine is started after being inoperative for a long interval. The flapping noise is suppressed by permitting forward displacement while blocking backward displacement of the plunger  220  on engine start-up. 
         [0050]    Since the tensioner  200  releases the engagement of the ratchet teeth  251  from the rack teeth  222  on the plunger by actuating piston  250  in synchronism with the pulsations in the high-pressure oil chamber R, it permits smooth forward and backward movement of the plunger  220  after engine start-up. 
         [0051]    As in the first embodiment disengagement timing can be adjusted by adjusting the biasing force exerted by the piston-biasing spring  260 , and installation and maintenance of the tensioner can be simplified by provision of a piston releasing projection  252  for manual release of the ratchet.