Abstract:
In a hydraulic tensioner, a tensioner housing is provided with an oil outflow channel communicating with a high pressure oil chamber formed by a plunger and the tensioner housing. A control valve in the oil outflow channel can be adjusted manually or automatically to optimize the damping performance of the tensioner in different engines and under different operating conditions, thereby obviating individualized machining of tensioner components to achieve optimum damping.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority on the basis of Japanese Patent Application 2010-095730, filed on Apr. 19, 2010. The disclosure of Japanese Patent Application 2010-095730 is incorporated by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a chain tensioner, comprising a tensioner housing in which a cylindrical plunger-accommodating hole is formed, and a cylindrical plunger, slidable in the plunger-accommodating hole and protruding therefrom through an opening in a wall of the housing. In a chain tensioner, the plunger and the plunger-accommodating hole cooperatively form a high pressure oil chamber, and the plunger is biased in the protruding direction in order to maintain adequate tension in a transmission chain such as the timing chain of an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     In a typical application of a chain tensioner, a transmission chain is engaged with, and driven by, an engine crankshaft sprocket, and is in driving relationship with one or more sprockets provided on engine valve-operating camshafts. A part of the chain, usually the part that moves from the crankshaft sprocket toward a camshaft sprocket, is in sliding relationship with shoe of a movable tensioner lever, which is biased by a tensioner in a direction to maintain tension in the chain. 
     As shown in  FIG. 8 , in a conventional engine timing drive, a timing chain C is engaged with a driving sprocket S 1 , mounted on a crankshaft, and with a pair of driven sprockets, S 2  and S 3 , mounted on valve-operating camshafts. The timing drive includes a stationary chain guide G 2 , mounted on bolts B 1  and B 2 , and a movable chain guide G 1 , pivoted on a bolt B, and pressed against the chain by the plunger of a tensioner  500  in order to maintain tension in the chain. 
     An example of a known chain tensioner is described in United States Patent Application publication No. 2002/0052259, published May 2, 2002. As shown in  FIG. 7 , the known chain tensioner  500  comprises a tensioner housing  510  having a cylindrical plunger-accommodating hole  511  in which a plunger slides. The plunger has a cylindrical exterior surface through part of its length, which fits closely the cylindrical interior surface of the plunger-accommodating hole. The plunger protrudes through an opening at one end of the plunger-accommodating hole. A coil spring  540  extends into a hole  521  in the plunger  520 , and is compressed between the bottom of the plunger-accommodating hole  511  and the end of hole  521  in the plunger. The coil spring serves as a biasing means, continuously urging the plunger  520  in its protruding direction. 
     Oil is supplied under pressure to the plunger-accommodating hole  511  through an oil inflow channel  512  provided in the housing, and fills a high pressure oil chamber  513  formed by the plunger-accommodating hole  511  and the plunger  520 . As illustrated in  FIG. 5 , slow leakage of oil from the high pressure oil chamber  513  through a small gap between the exterior of the cylindrical part of the plunger and the cylindrical internal wall of the plunger-accommodating hole  511  causes damping of the reciprocating movement of the plunger  520 . The damping property of the tensioner depends on factors such as oil viscosity and the dimensions of the gap between the plunger and the cylindrical wall of the plunger-accommodating hole. 
     As shown in  FIG. 7 , the plunger  520  is also provided with a relief valve  522 , which allows oil to flow out the high-pressure oil chamber  513  when the pressure within the chamber becomes excessive, as disclosed in United States Patent Application publication No. 2002/0052259. 
     The plunger damping effect is important for the suppression of fluctuations in chain tension and sinusoidal vibrations of the chain. However, the effectiveness of the damping action is highly dependent, not only on the engine, but on changes in the operating conditions of a given engine. Accordingly, adjustments must be made for optimum damping. However, the gap between the plunger  520  and the plunger-accommodating hole  511  is determined when the tensioner is designed, with the primary objective of ensuring that the plunger reciprocates smoothly and uniformly. Hence it is difficult to make different adjustments for optimum damping of tensioners individually. 
     Although it is possible to control damping by providing a V-shaped groove  523  on the side the plunger  520  as shown in  FIG. 6 , and to control damping property by varying the width and depth of the groove, it is necessary to determine the appropriate groove dimensions for each engine in which the tensioner is to be used, and, once the damping property has been set, it is impractical to change it to accommodate different operating conditions. 
     Accordingly, the damping properties of a tensioner have been a compromise, arrived at by determining a setting that provides adequate, but not optimum, damping under a variety of operating conditions. In addition, it has not been practical heretofore to correct the damping properties of a tensioner when they depart from designed damping properties as a result of machining errors. 
     Another problem with the known chain tensioner shown in  FIG. 7  is that the relief valve  522 , which is provided as a countermeasure against excessive pressure in the oil chamber  513 , cannot be used to adjust the tensioner&#39;s damping properties. 
     SUMMARY OF THE INVENTION 
     The invention addresses the above-described problems by providing a chain tensioner that suppresses fluctuation in tension and sinusoidal vibrations of a chain by optimizing a tensioner&#39;s damping properties without the need for machining tensioners individually to accommodate different engines and different operating conditions. 
     The chain tensioner, according to the invention comprises a tensioner housing and a plunger. A cylindrical plunger-accommodating hole is formed in the tensioner housing. The plunger is a cylindrical plunger, slidable in the plunger-accommodating hole, and protruding therefrom through the opening in the wall of the tensioner housing. The plunger and the plunger-accommodating hole cooperatively form a high pressure oil chamber. Biasing means, preferably a coil spring, urge the plunger in the protruding direction. The tensioner has an oil inflow channel through which oil can be supplied to the high pressure oil chamber, and an oil outflow channel through which oil can be discharged from the high pressure oil chamber. The tensioner includes a control valve for controlling the amount of oil discharged through the oil outflow channel. 
     An advantage of the oil discharge control valve is that, by changing the aperture of the valve either manually or automatically, it becomes possible to optimize the plunger damping property, to suppress fluctuations of tension and sinusoidal vibrations of the chain, and to accommodate different engines and different operating conditions, without custom machining of tensioner components individually. 
     Another aspect of the invention is the inclusion of a check valve for blocking flow of oil from the high pressure oil chamber through the oil inflow channel. By blocking reverse flow of oil, it becomes possible to prevent the damping property from being affected by the reverse flow through the oil inflow channel and to adjust the damping property of the tensioner more accurately by adjustment of the aperture of the control valve. 
     Still another aspect of the invention is the inclusion of an actuator for adjusting the control valve to control the amount of oil discharged through the oil outflow channel. When the aperture of the control valve is adjusted by an actuator, it becomes possible to adjust the damping properties of the tensioner while the tensioner installed on an engine, and to achieve more accurate adjustment of the damping property. 
     When the tensioner is mounted on an engine and arranged to control tension in a timing chain, a controller may be utilized to control the actuator during operation of the engine. Because the actuator is operable by the controller during operation of the engine it is becomes possible to achieve dynamic adjustment of the damping property of the tensioner to take into account the operating conditions of the engine. 
     The actuator can be operated by means of a piezoelectric element, and in this case, more accurate adjustment of damping can be achieved and operational failures due to vibration, engine heat, and similar causes are less likely to occur. Moreover, the piezoelectric element can be small in size, so that the advantages of the controlled discharge of oil from the high pressure chamber can be realized without significantly increasing the size of the actuator. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective sectional view of a chain tensioner according to a first embodiment of the invention; 
         FIG. 2  is a schematic sectional view of the chain tensioner of the first embodiment of the invention; 
         FIG. 3  is a perspective section view of a chain tensioner according to a second embodiment of the invention; 
         FIG. 4  is a schematic sectional view of the chain tensioner of the second embodiment of the invention; 
         FIG. 5  is a schematic diagram explaining the operation of a prior art chain tensioner; 
         FIG. 6  is a perspective view of the plunger of the prior art chain tensioner; 
         FIG. 7  is a section view of a prior art chain tensioner; and 
         FIG. 8  is a schematic elevational view of a timing chain transmission of an engine incorporating a tensioner. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in  FIGS. 1 and 2 , a chain tensioner  100  comprises a housing  110  having a cylindrical plunger-accommodating hole  111  having an end opening in a wall of the housing. A plunger  120 , at least part of the outer peripheral surface of which is cylindrical, is slidable in the plunger-accommodating hole  111  and protrudes therefrom through the end opening. A plunger-biasing coil spring  140 , partly within a spring-receiving hole  121  inside the plunger, is in compression between and end of hole  121  adjacent the protruding end of the plunger, and the bottom of the plunger-accommodating hole  111 . 
     The housing  110  is provided with an oil inflow channel  112  for supplying oil under high pressure to a high pressure oil chamber  113  formed by the plunger  120  and the plunger-accommodating hole  111 . A check valve  150  prevents the oil from flowing from the plunger-accommodating hole through the oil inflow channel  112 . 
     The housing is also provided with an oil outflow channel  114  for discharging oil from the high pressure oil chamber  113 . The oil outflow channel  114  is provided with a control valve  160 , which is capable of adjusting an amount of oil discharged through channel  114 . 
     The control valve  160  includes a spool  161  movable in an axial direction within a cylindrical spool chamber  165 . The chamber  165  has an inlet port  166  opening in the axial direction of the chamber and connecting to a part of channel  114  leading from the high pressure oil chamber  113  and an output port  163  opening in the radial direction and connecting to apart of channel  114  leading from the spool chamber to the exterior of the tensioner housing. A spool  161 , which is reciprocable within the spool chamber  165 , adjusts the aperture of the output port  163 . A biasing spring  164  urges the spool  161  in one direction, in this case, the direction in which the aperture of the output port  163  increases, and an actuator  162  is provided for moving the spool  161  against the biasing force exerted by spring  164 . 
     Damping of the reciprocating movement of the plunger  120 , which is dependent on the rate of flow of oil from chamber  113 , is determined not only by the gap between the plunger-accommodating hole  111  and the plunger  120 , but also by the aperture of the valve  160  in the oil outflow channel  114 . Accordingly, even if the gap between the plunger-accommodating hole  111  and the plunger  120  is optimally designed for smooth sliding of the plunger in the plunger-accommodating hole, it becomes possible to make significant adjustments in the damping properties of the tensioner by adjusting the control valve  160  and thereby changing the rate of flow of oil through the outflow channel  114 . 
     As shown in  FIG. 2 , a controller  170  can be provided for controlling the operation of the actuator  162 . With the controller, it is possible to adjust the damping property of the tensioner dynamically, so that the damping property corresponds to actual operating conditions. The optimum damping property can be determined by calculations based on input from a sensor  171 , which can sense parameters such as the revolution rate of the engine and oil temperature. 
     The sensor  171  may also be arranged to detect fluctuations of tension and vibration of the chain, and thereby provide feedback control enabling the controller  170  to optimize the fluctuations of tension and vibration of the chain. 
     In a second embodiment of the invention, as shown in  FIGS. 3 and 4 , instead of spool valve, the control valve  160  is in the form of an axially movable needle valve, having an element  167  with a tapered end  168  that cooperates with a port  166  of outflow channel  114  by movement in the axial direction to adjust the valve aperture. 
     The actuator  162  may take any of various forms and arrangements as long as it is capable of effecting reciprocating movement of the needle valve element  167 . For example, it is possible to operate the actuator a piezoelectric element, in which case advantage can be taken of the small size of the piezoelectric element to reduce the size and weight of the tensioner. For purposes of illustration, the size of the control valves in  FIGS. 1-4  is exaggerated. In practice the control valves can be very small in size. 
     In a case in which dynamic adjustment of the damping property of the tensioner is not important, the needle valve or the second embodiment, or the spool valve of the first embodiment, can be adjusted manually by means of a screw or similar mechanism instead of by means of an the actuator. 
     In either of the first and second embodiments described above, and in modifications thereof, a ratchet mechanism or a relief valve, or both, as shown in  FIG. 7 , can be provided 
     In any embodiment, the chain tensioner suppresses fluctuations in tension and sinusoidal vibrations of the transmission chain by enabling optimization of the damping properties of the tensioner without the need to machine tensioner parts individually and differently to accommodate different engines and different operating conditions. 
     The tensioner housing and the plunger of the chain tensioner according to the invention may be composed of any of a variety of materials as long as the material has sufficient strength. It is preferable to use ferrous materials such as steel and cast iron because of their strength, workability and low cost.