Abstract:
The hollow plunger of a hydraulic tensioner, slidably receives a sleeve that divides the high pressure oil chamber into two parts. The first part is formed by the plunger and a plunger-accommodating hole of the tensioner housing. The second part is formed by the plunger and the sleeve. Two springs, one being in the first part, and the other being in the second part, urge the plunger in the protruding direction. When the second part is filled with oil during ordinary engine operation, a negative pressure in the second part exerts a force opposing protruding movement of the plunger. Therefore, the springs can be made strong enough to prevent excessive chain vibration and noise during engine start-up when the parts of the high pressure oil chamber are not filled with oil, without causing the tensioner to exert excessive force on the chain during ordinary engine operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority on the basis of Japanese patent application 2008-035574, filed Feb. 18, 2008. The disclosure of Japanese application 2008-035574 is hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a hydraulic tensioner for maintaining proper tension in an endless, flexible, traveling transmission medium such as a timing belt or a timing chain in a vehicle engine. 
       BACKGROUND OF THE INVENTION 
       [0003]    Hydraulic tensioners incorporating check valves have been widely used to maintain proper tension, and to suppress vibration, in a timing belts or timing chain used to transmit rotation between a crankshaft and one or more camshafts in a vehicle engine. 
         [0004]    As shown in  FIG. 7 , a conventional hydraulic tensioner  500  is typically mounted on an engine adjacent the slack side of a timing chain C, which is driven by a crankshaft sprocket S 1  and in driving relationship with camshaft sprockets S 2 . A plunger  520  protrudes slidably from a housing  510  and applies tension to the slack side of the timing chain C by pressing against the back of a pivoted lever L 1  on which the chain slides. A fixed guide L 2  is provided on the tension side of the timing chain C. The sprockets and chain move in the directions indicated by arrows. 
         [0005]    As shown in  FIG. 8 , in the hydraulic tensioner  500 , the plunger  520 , which is hollow, fits slidably in a plunger-accommodating hole  511  formed in the housing  510 . A high pressure oil chamber R is formed by the plunger  520  and a plunger-accommodating hole  511 . The plunger is urged in the protruding direction by a plunger-biasing coil spring  530 . 
         [0006]    A check valve unit  540  is press-fit into the bottom portion of the plunger-accommodating hole  511 . The check valve unit allows oil to flow from a source (not shown) of oil under pressure into the high pressure oil chamber R, but blocks reverse flow of oil. 
         [0007]    The check valve unit  540  comprises a ball  541 , a ball guide  542 , which envelops the ball  541 , a retainer  543 , fixed to one end of the guide  542 , and a ball seat  544 , fixed to the opposite end of the guide  542 . The ball can move toward and away from the seat through a distance limited by the retainer. When the ball guide is moved away from the seat, oil can flow through the check valve unit  540  into the high pressure oil chamber R. When the ball is in engagement with the seat, it blocks reverse flow of oil. 
         [0008]    The tensioner also has a rack  522  on the plunger  520  and a pawl  560 , pivoted to the housing  510  and biased into engagement with the rack  522  by a pawl-biasing spring  561 . The rack and pawl serve as a ratchet mechanism, which limits retracting movement of the plunger to a distance corresponding to the backlash of the ratchet mechanism. 
         [0009]    In operation of the tensioner, oil in the high pressure oil chamber R leaks through a slight clearance between the outer circumferential surface of the plunger  520  and the inner circumferential surface of the plunger-accommodating hole  511 , and is discharged to the outside of the housing  510 . Because of the viscosity of the oil, there is a resistance to flow through the clearance between the plunger and the plunger-accommodating hole. The resistance to flow enables the tensioner to exert a damping action, absorbing impact forces exerted on the plunger  520  and reducing vibration of the plunger  520 . The force exerted on the plunger by the pressure of the oil supplied to the high pressure oil chamber R pressure oil is added to pressing force of the plunger-biasing spring  530 . An example of a hydraulic tensioner having the above-described features is found in United States Patent Application Publication US2005/0227799. 
         [0010]    In a conventional hydraulic tensioner, oil is supplied to the high pressure oil chamber by a pump driven by an engine. When the engine is stopped, the supply of oil to the high pressure oil chamber is also stopped. Some of the oil left in the chamber leaks through the clearance between the plunger and the inner circumferential surface of the plunger-accommodating hole and is discharged and replaced by air. When the engine is re-started after having been stopped for a long time, a considerable amount of time is required for replenishment of the oil in the high pressure oil chamber of the tensioner, and the damping action of the tensioner is therefore delayed. Thus the conventional hydraulic tensioner has different properties at the time of engine start-up and during ordinary engine operation. 
         [0011]    More particularly, at the time of engine start-up, when oil in the high pressure oil chamber R depleted, the contribution of the viscous flow of oil through the leakage path to resistance to pressing-in of the plunger is substantially zero. The load on the plunger is resisted primarily by the plunger-biasing spring  530  both when the plunger is moving in the protruding direction and when the plunger is moving in the retracting direction. On engine-start-up, the pressing force and the damping action of the tensioner are at a lower level than during ordinary operation of the engine. 
         [0012]    On the other hand, during ordinary operation of an engine, as the plunger  520  is pressed inward, the check valve  540  blocks flow of oil out of the high pressure oil chamber R as shown in  FIG. 9 . Thus, the pressure within the high pressure oil chamber R is increased as a result of resistance to flow of oil through the small clearance between the plunger  520  and the inner circumferential surface of the plunger-accommodating hole  511 . The pressing force exerted by the plunger  520  is the result of the force exerted by the pressure of the oil added to the force exerted by the plunger-biasing spring  530 . When the plunger  520  moves in the protruding direction, the check valve  540  is opened so that oil is supplied under pressure into the high pressure oil chamber R as shown in  FIG. 10 . Here, the pressing force exerted by the plunger  520  is the result of the force exerted by the pressure of the oil supply added to the force exerted by the plunger-biasing spring  530 . 
         [0013]    The above-described conditions are summarized in  FIG. 11 . On engine start-up, the pressing force T 01 , exerted by the plunger as it is pressed inward, and the pressing force T 02 , exerted by the plunger as it protrudes, are both equal to the force Ts, exerted by the plunger-biasing spring. That is T 01 =T 02 =Ts 
         [0014]    During ordinary operation of the engine, the pressing force T 11  exerted during pressing-in of the plunger is the sum of the spring force Ts and the pressing force Tp 1  exerted by the oil compressed in the high pressure oil chamber. That is, T 11 =Ts+Tp 1 . 
         [0015]    During ordinary operation of the engine, the pressing force T 12 , exerted during protruding movement of the plunger is the sum of the spring force Ts and the force Tp 2  exerted by the pressure of the oil supply. That is, T 12 =Ts+Tp 2 . 
         [0016]    When the forces T 11  and T 12 , exerted during ordinary engine operation, are adjusted to optimum levels, the forces T 01  and T 02 , exerted during engine start-up are too low. Consequently, the tension-applying force is inadequate to prevent generation of vibration and noise. 
         [0017]    Furthermore, if the pressing force Ts of the plunger-biasing spring is increased in order to increase forces T 01  and T 02  and thereby avoid generation of vibration and noise, the pressing forces T 11  and T 12  of the plunger become excessive. In particularly, when the pressing force T 12 , exerted during protrusion of the plunger, becomes excessive, excessive tension is applied to the timing chain, resulting in skidding contact sounds, increased wear, and an increased risk of breakage of the timing chain and the timing transmission sprockets. 
         [0018]    Accordingly, objects of the invention is to solve the above-described problems and to provide a hydraulic tensioner in which vibration and noise, occurring on engine start-up before the tensioner becomes filled with oil, are reduced, and in which an appropriate pressing force is maintained during ordinary operation of the engine, when the tensioner is filled with oil. 
       SUMMARY OF THE INVENTION 
       [0019]    The hydraulic tensioner according to the invention comprises a housing having a plunger-accommodating hole, and a plunger slidably protruding from the plunger-accommodating hole through the opening, and, with the hole, defining a high pressure oil chamber. A plunger-biasing spring, disposed within the high pressure oil chamber, biases the plunger in the protruding direction. A check valve unit incorporated into the bottom of the plunger-accommodating hole allows oil to flow into the high pressure oil chamber and blocks flow of oil out of the high pressure oil chamber. An inner sleeve-accommodating hole is formed in the plunger. The inner sleeve-accommodating hole has a sleeve opening facing the bottom of the high pressure oil chamber. An inner sleeve extends slidably into the inner sleeve-accommodating hole and divides the high pressure oil chamber into two parts. An inner sleeve-biasing spring is engaged with the end of the inner sleeve remote from the end of the inner sleeve-accommodating hole, and urges the sleeve into abutment with the check valve unit. 
         [0020]    During ordinary operation of the engine, the part of the high pressure oil chamber within the inner sleeve-accommodating hole reaches a negative pressure during protrusion of the plunger. The negative pressure opposes the force exerted on the plunger by the inner sleeve biasing spring. Thus, the pressing force exerted by the plunger does not become excessive during ordinary operation of the engine, and an appropriate pressing force can be maintained. 
         [0021]    Preferably, the shape of the front end of the inner sleeve forms a clearance for the flow of oil through the check valve unit into the high pressure oil chamber. As a result the rate of flow of oil into the hydraulic tensioner can be the same as the rate of flow of oil into a conventional hydraulic tensioner having no inner sleeve, and the same damping performance can be exhibited during ordinary engine operation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a cross-sectional view of a hydraulic tensioner according to the invention. 
           [0023]      FIG. 2  is a cross-sectional view illustrating the pressing-in of the plunger on starting the tensioner; 
           [0024]      FIG. 3  is a cross-sectional view illustrating the protrusion of the plunger on starting the tensioner; 
           [0025]      FIG. 4  is a cross-sectional view illustrating the usual operation of the tensioner; 
           [0026]      FIG. 5  is a cross-sectional view illustrating protrusion of the plunger during the usual operation of the tensioner; 
           [0027]      FIG. 6  is a table illustrating various conditions affecting the pressing force exerted by the plunger of the tensioner; 
           [0028]      FIG. 7  is a schematic elevational view of the timing system of a conventional vehicle engine; 
           [0029]      FIG. 8  is a cross-sectional view of a conventional hydraulic tensioner; 
           [0030]      FIG. 9  is a cross-sectional view illustrating the pressing-in of the plunger on starting the conventional tensioner; 
           [0031]      FIG. 10  is a cross-sectional view illustrating the protrusion of the plunger on starting the conventional tensioner; and 
           [0032]      FIG. 11  is a table illustrating various conditions affecting the pressing force exerted by the plunger of the conventional tensioner. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0033]    Briefly, in the hydraulic tensioner according to the invention, an inner sleeve divides the high pressure oil chamber into two parts. During protrusion of the plunger, a negative pressure within a part of the high pressure oil chamber opposes the spring force exerted on the plunger and prevents the pressing force exerted by the plunger from becoming excessive during ordinary operation of the engine. 
         [0034]    The tensioner can utilize any of various types of check valves, and the inner sleeve can be composed of any of various materials, including, for example, a metal such as iron, or a resin or the like. 
         [0035]    As shown in  FIG. 1 , in a hydraulic tensioner  100 , a hollow, cylindrical plunger  120  is slidable in a plunger-accommodating hole  111  formed in a housing  110 , and a hollow inner sleeve  150  fits slidably in an inner sleeve accommodating hole  121  formed in the plunger. The hollow plunger has a blind hole having an opening facing toward the bottom of the plunger-accommodating hole, and the hollow sleeve has a blind hole having an opening facing in the opposite direction. 
         [0036]    The inner sleeve  150  separates the high pressure oil chamber into two parts R 1  and R 2 . Part R 1  is defined by the plunger-accommodating hole  111 , the plunger  120 , and the exterior of the sleeve  150 . A restricted oil leakage path is provided between a cylindrical external part of the plunger and the inner wall of the plunger-accommodating hole. Part R 2  is defined by the interior of the plunger  120  and the interior of the inner sleeve  150 . A restricted oil leakage path is provided between a cylindrical part of the exterior of the sleeve and the cylindrical interior wall of the plunger. A coiled plunger-biasing spring  130 , accommodated in the first part R 1 , surrounds the sleeve  150  and presses against the part of the plunger surround the opening of the hole formed in the plunger, biasing the plunger  120  in the protruding direction. In the second part R 2 , which is formed by the inner sleeve  150  and the inner sleeve-accommodating hole  121 , a coiled inner sleeve biasing spring  155  urges the inner sleeve  150  out of the plunger and toward the bottom of the plunger-accommodating hole. At the same time, spring  155  augments the force exerted on the plunger by spring  130 . Consequently, both springs  130  and  155  contribute additively to the total force urging the plunger  120  in the protruding direction. 
         [0037]    A check valve unit  140 , is incorporated into a bottom portion of the plunger-accommodating hole  111  for allowing oil to flow under pressure from a source (not shown) into part R 1  of the high pressure oil chamber, while blocking reverse flow of oil. 
         [0038]    The check valve unit  140  comprises a check ball  141 , a ball seat  144 , a ball guide  142 , which envelops the ball while allowing the ball to move freely toward and away from the ball seat, and a retainer  143 , fixed to the ball guide  142 . The retainer holds check ball in the ball guide  142 , while allowing the ball to move toward and away from the seat  144  through a limited distance. 
         [0039]    A rack  122  on the plunger  120  is engaged by a pawl  160 , which is pivoted on the housing  110  and biased by a spring  161  into engagement the rack  122  to allow the plunger to move in the protruding direction while limiting retraction of the plunger to an amount corresponding to the backlash of the ratchet mechanism. The ratchet mechanism is, of course, optional. 
         [0040]    The protruding end  151  of the inner sleeve  150  abuts the ball guide  142 , limiting movement of the inner sleeve relative to the housing  110 . The protruding end is tapered and in the shape of a truncated cone, providing a clearance for the flow of oil through the check valve into chamber R 1 . When the plunger  120  is pressed inward at the time of engine start-up, as shown in  FIG. 2 , both chambers R 1  and R 2  are in an oil-depleted condition. Resistance to flow of oil does not materially contribute to the pressing force exerted by the plunger, and therefore, the pressing force T 01  exerted by the plunger  120  is the sum of the force Ts 1  exerted by the plunger-biasing spring  130  and the force Ts 2  exerted by the inner sleeve biasing spring  155 . Thus, T 01 =Ts 1 +Ts 2 . 
         [0041]    When the plunger  120  moves in the protruding direction at the time of engine start-up, as shown in  FIG. 3 , both chambers R 1  and R 2  are still in an oil-depleted condition. Here also, resistance to flow of oil does not contribute materially to the pressing force exerted by the plunger. The pressing force T 02  exerted by the plunger  120  is also the sum of the force Ts 1  exerted by the plunger-biasing spring  130  and the force Ts 2  exerted by the inner sleeve biasing spring  155 . Thus, T 02 =Ts 1 +Ts 2 . 
         [0042]    During ordinary engine operation, when the parts R 1  and R 2  of the high pressure oil chamber are completely filled with oil, when the plunger  120  is pressed inward, the pressing force Ts 1  of the plunger-biasing spring  130  and the pressing force Ts 2  of the inner sleeve biasing spring  155  act additively as components of the total pressing force T 11  exerted by the plunger. Additionally, the oil pressure in chamber R 2  is increased because of resistance to flow of the viscous oil through the small clearance between an outer circumferential surface of the inner sleeve  150  and an inner circumferential surface of the inner sleeve accommodating hole  121 . Reverse flow of the oil is blocked by the check valve unit  140 . Thus, the pressure in part R 1  of the high pressure oil chamber is also increased as a result of resistance to flow of oil through the small clearance between the outer circumferential surface of the plunger  120  and the inner circumferential surface of the plunger-accommodating hole  111 . Consequently, the pressing force Tp 11  due to pressure in part R 1  of the oil chamber and the pressing force Tp 21  due to pressure in part R 2  of the oil chamber R 2  act additively. The total pressing force T 11  exerted by the plunger is given by T 11 =Ts 1 +Ts 2 +Tp 11 +Tp 21 . 
         [0043]    When the plunger  120  is moved in the protruding direction during ordinary operation of the engine, the pressing force Ts 1  of the plunger-biasing spring  130  and the pressing force Ts 2  of the inner sleeve biasing spring  155  act additively, as components of the total force T 12  exerted by the plunger  120 . Additionally, since the check valve  140  opens as the plunger moves in the protruding direction, oil flows into part R 1  of the high pressure oil chamber, and the pressure therein corresponds to the oil supply pressure, resulting in a pressing force component Tp 12 , which is added the spring force components Ts 1  and Ts 2 . 
         [0044]    On the other hand, because of the viscosity of the oil, there is a resistance to flow of oil into part R 2  of the high pressure oil chamber through the small clearance between the outer circumferential surface of the inner sleeve  150  and the inner circumferential surface of the hole  121  in the plunger as the plunger moves in the protruding direction and the volume of part R 2  expands. The result is a negative pressure within part R 2  which resists protruding movement of the plunger. The negative pressure results in a force Tp 22 , acting in a direction urging the plunger into the plunger-accommodating hole  111 . This force Tp 22  is subtracted from the sum of the spring forces and the force due to the oil supply pressure, with the result that the force T 12  exerted by the plunger is given by T 12 =Ts 1 +Ts 2 +Tp 12 −Tp 22   
         [0045]    The forces exerted by the plunger  120  under the four conditions described above are summarized in the table in  FIG. 6 . 
         [0046]    During engine start-up, when parts R 1  and R 2  of the high pressure oil chamber are not filled with oil, the force exerted by the plunger is given by T 01 =T 02 =Ts 1 +Ts 2 . 
         [0047]    During ordinary engine operation, with parts R 1  and R 2  filled with oil, when the plunger is pressed inward, the force T 11  exerted by the plunger is given by T 11 =Ts 1 +Ts 2 +Tp 11 +Tp 21 . 
         [0048]    During ordinary engine operation, with parts R 1  and R 2  filled with oil, when the plunger moves in the protruding direction, the force T 12  exerted by the plunger is given by T 12 =Ts 1 +Ts 2 +Tp 12 −Tp 22   
         [0049]    Even if the spring forces Ts 1  and Ts 2  are set to a large value in order to avoid vibration and noise to a inadequate chain tension on engine start-up, the pressing force T 12 , exerted by the plunger during ordinary engine operation, can be maintained at an appropriate level due to the fact that the second part R 2  of the high pressure oil chamber is under a negative pressure as the plunger moves in the protruding direction. Therefore, the tensioner does not apply excessive tension to the chain, sliding contact sounds and wear are avoided, and the risk of breakage of the chain and sprockets is reduced. 
         [0050]    Furthermore, when the pressing force Ts 1  of the plunger-biasing spring  130 , the pressing force Ts 2  of the inner sleeve biasing spring  150  and the resistance to flow of oil through the clearance between inner sleeve  150  and the hole  121  are appropriately selected, the pressing forces T 11  and T 12  exerted by the plunger  120  during ordinary engine operation can be set to optimum values. 
         [0051]    Therefore, the tensioner according to the invention reduces noise and vibration during engine start-up, and also prevents the tensioner from exerting excessive pressing force on the chain during ordinary engine operation.