Abstract:
A hydraulic tensioner having a piston slidably fitted in a bore in a housing forming a fluid chamber. The piston is biased in a protruding direction by a spring and fluid. A check valve permits flow of fluid into the fluid chamber and prevents flow of fluid in the reverse direction. A pressure relief valve is provided in the fluid chamber. The pressure relief valve works with a vent disc having two separate vent paths. The pressure relief valve and vent disc provide controlled venting of fluid from the fluid chamber during both conventional operation and during periods of excessive pressure and thus, prevent collapse of the tensioner.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to hydraulic tensioners, and particularly to a hydraulic tensioner having an extending piston which is useful for constantly imparting and maintaining tension to wrapped power transmission devices such as chains, belts and the like. The invention is more particularly directed to a hydraulic tensioner having a spring and fluid actuated piston in which a self-contained pressure relief valve and vent disc is used to vent high pressure fluid from the tensioner hydraulic chamber in a controlled fashion. 
     Tensioning devices, such as hydraulic tensioners, are used as a control device for a power transmission chain, or any similar power transmission devices, as the chain travels between a plurality of sprockets. Generally, it is important to impart and maintain a certain degree of tension to the chain to prevent noises, slippage, or the unmeshing of teeth in cases of a toothed chain. Prevention of such slippage is especially important in the case of a chain driven camshaft in an internal combustion engine because slippage may alter the camshaft timing by several degrees, possibly rendering the engine inoperative or causing damage. In the harsh environment in which an internal combustion engine operates, chain tension can vary between excessively high or low levels as a result of the wide variations in temperature and differences between the coefficients of linear expansion among the various parts of the engine, including the chain and the tensioner. It is also necessary to provide some measures to remove excessive tensioning forces on the tight side of the chain and to insure the necessary tension forces on the slack side of the chain. Camshaft and crankshaft induced torsional vibration cause chain tension to vary considerably. This tension variation results in chain elongation. Moreover, wear of the chain components during prolonged use can cause elongation of the chain that results in a decrease in the tension of the chain. 
     One example of a device used to control tension in a wrapped power transmission device is described in Biedermann, U.S. Pat. No. 4,713,043. Biedermann, discloses a hydraulic ball-type check valve tensioner having a plunger, also referred to as a piston, slidably fitted into a chamber and biased by a spring in a protruding direction. The plunger extends against a lever arm that imparts tension to a chain according to the degree of slackening of the chain. A clearance, which is formed between the ball and seat of a check valve, permits the free flow of fluid therethrough into the chamber. Therefore, the hydraulic pressure from an external source, such as an oil pump or the like, flows into the chamber through passages formed in the housing, advancing the plunger easily by the combined efforts of the hydraulic pressure and the spring force. 
     On the other hand, when the plunger tends to move in a reverse direction, the ball is tightly contacted with the ball seat to restrict outflow of fluid from the chamber. Only a small clearance between the plunger and the housing wall permits some fluid to escape thereby allowing the piston to retract. In such a fashion, the tensioner achieves a so-called no-return function, i.e., movements are easy in one direction but difficult in the reverse direction. 
     However, this no-return function may present difficulties in accommodating tension spikes or surges in the chain, belt or similar wrapped power transmission devices. When a timing device operates at its resonant frequency, the chain load increases significantly. The small clearance between the plunger and the housing wall is not sufficient to quickly release the hydraulic fluid in the chamber to accommodate the sudden overload of the chain. 
     One example of an attempt to alleviate this problem in a hydraulic tensioner is described in Suzuki, U.S. Pat. No. 4,881,927. Suzuki discloses a hydraulic tensioner having a piston slidably fitted into a chamber and biased by a spring in a protruding direction. This tensioner includes a relief valve having a sleeve slidably fitted in an auxiliary chamber in communication with the first chamber, with a spring biasing the sleeve into a depressed position to block a discharge port. Oil in the first chamber flows into the auxiliary chamber to force the sleeve against the biasing spring action to unblock the discharge port. 
     Unfortunately, this relief valve may be slow to open and close due to high mass and subject to variable friction between the sleeve and auxiliary chamber wall. This may vary the pressure at which the relief valve operates. As well, because the flow area is proportional to the pressure in the chamber, extreme pressure spikes may cause too much fluid to flow out of the chamber resulting in too little pressure in the chamber to maintain proper chain tension after the external cause of the pressure spike recovers. Too little pressure in the chamber may result in tensioner collapse and loss of proper chain tension. 
     Another example of an attempt to provide a hydraulic tensioner with a relief valve is described in Mittermeier, U.S. Pat. No. 4,507,103. Mittermeier discloses a hydraulic ball-type check valve tensioner having a piston slidably fitted into a chamber and biased by a spring in a protruding direction. This tensioner includes a relief valve in a bore at the protruding end of the piston. This relief valve is a spring-biased ball type valve with the spring set against a treaded throttle plug capping the bore. Oil in the first chamber forces open that ball, upon reaching a set high pressure, and flows into the bore, past the throttle plug threads to the atmosphere. Unfortunately, this relief valve may be slow to release large displacements of oil because of the restricted path past the threads and resultant back-pressure build against the ball. 
     Accordingly, it is an object of the present invention to provide a hydraulic tensioner for chain, belt or similar wrapped power transmission devices which maintains a substantially constant tensioning force. 
     It is another object of the invention to provide a hydraulic tensioner with a pressure relief mechanism to allow the piston to return when excessive loads are seen by the chain. 
     It is another object of the present invention to provide a hydraulic tensioner with a pressure relief valve that has an even spring rate, low mass and a high natural frequency. 
     It is another object of the present invention to provide a hydraulic tensioner with a pressure relief valve with a low response time and a controlled flow through. 
     SUMMARY OF THE INVENTION 
     The above objects are achieved by providing a hydraulic tensioner with a low mass self-contained pressure relief valve and a dual path vent disc which are used together in a conventional hydraulic tensioner. The dual path vent disc has a pair of tortuous vent paths in parallel operation; one vent path controls conventional venting of fluid from the fluid chamber and the other vent path controls venting of fluid from the compact pressure relief valve. 
     According to one aspect of the present invention, there is provided a tensioner for a wrapped power transmission device, e.g. a chain linking at least two rotating members such as a pair of sprockets. A hollow, cylindrical piston, internally provided with a fluid chamber, slidably fits with a bore formed within a tensioner housing. A piston spring is provided to bias the piston in a protruding direction from the housing and toward the chain. The present invention is directed to a hydraulic tensioner having pressure relief and fluid control mechanisms. The fluid is typically oil, which may include some air. 
     The hydraulic tensioner of the present invention includes a compact, self-contained pressure relief valve preferably operating in combination with a vent disc. The relief valve and vent disc are located in the fluid chamber adjacent the upper end of the piston. 
     During operation of the hydraulic tensioner the pressure relief valve and vent disc provide a mechanism for conventional venting of fluid and air from the fluid chamber and additional venting capacity during periods of extreme high fluid pressure. Furthermore, the present invention includes a mechanism to prevent collapse of the tensioner during periods of extreme high pressure fluid venting by controlling release of fluid by the relief valve. 
     In one embodiment, the pressure relief valve includes a generally cylindrical valve body adapted to reside in the upper end of the piston. The pressure relief body has an inverted cup shape. The pressure relief valve includes a ring-shaped valve seat. The valve seat is essentially a flattened ring held in place by a circlip. The circlip is located and held in the valve body in an interior groove formed in the inside lower skirt portion of the valve body. The valve seat has a central opening to permit fluid to pass through the interior of the valve body. 
     A ball-shaped valve member is located inside the valve body and biases the ball against the opening of the valve seat with a stack of belleville washers. During a period of high pressure, and the pressure in the fluid chamber exceeds a predetermined maximum, the ball is forced away from the valve seat. Fluid passing by the ball is allowed to exit the pressure relief valve through an oil exit passage formed in the top of the valve body. 
     Use of belleville springs as spring members allows for a more compact design than a helical spring and also is much lighter, making the natural frequency of the system higher. In addition, it is possible to design and obtain a belleville washer to provide a nearly constant load over a large portion of its travel. This allows a much looser manufacturing tolerance while still tightly controlling the activation or pop-off pressure of the valve. 
     In another embodiment of the present invention the valve member is a conical steel disk provided in place of the ball and held against the valve seat by a similar stack of belleville washers. The disk is lighter than the ball, further lowering the mass and increasing the natural frequency of the system. This design has the advantage of being more compact than the previous one. 
     The present invention preferably includes a vent disc with a pair of tortuous oil paths molded therein. The vent disc is located in the fluid chamber at the exit end of the relief valve. The vent disc is in fluid communication with both the fluid chamber and the pressure relief valve. One oil path in the vent disc allows the tensioner to vent fluid conventionally from the fluid chamber during normal operation of the tensioner. The other path in the vent disc controls the flow of fluid exiting the oil exit passage of the pressure relief valve. Fluid from both paths is released into the exhaust vent of the hydraulic tensioner. 
     In operation, as the pressure in the fluid chamber increases, air in the chamber is forced through a groove formed in the sidewall or skirt of the vent disc and through a tortuous vent path in the top surface of the vent disc. The tortuous vent path directs the air and some fluid to the exhaust vent formed at the upper end of the tensioner piston. At a predetermined maximum pressure level, the fluid forces the pressure relief valve member from the valve seat. Fluid flows past the ball or disc and exits by way of the valve fluid passage or oil exit passage. From the oil exit passage fluid flows into and through another tortuous passage formed in the bottom surface of the vent disc and is directed into the exhaust vent of the piston. 
    
    
     The advantages and features of the chain tensioner apparatus of the present invention will be better understood by reference to the embodiments which are hereafter presented and depicted by way of example in the following description taken in conjunction with the accompanying drawings in which like reference numbers are used to indicate like elements. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a first embodiment of the pressure relief valve and vent disc of the present invention depicted in a generalized hydraulic tensioner. 
     FIG. 2 is a cross-sectional view of a second embodiment of the pressure relief valve and vent disc of the present invention. 
     FIG. 3 is a flow chart depicting the dual fluid flow paths in parallel relationship. 
     FIG. 4 is a cross-sectional view of a third embodiment of the present invention. 
     FIG. 5 is a perspective view of the vent disc of the present invention showing the top surface. 
     FIG. 6 is a perspective view of the vent disc shown in FIG. 5 depicting the bottom surface and the groove formed in the side skirt of the vent disc. 
     FIG. 7 is an exploded view of the pressure relief valve and vent disc depicted in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, FIG. 1 illustrates a generalized hydraulic tensioner  10  incorporating the pressure relief valve  20  and dual path vent disc  30  of the present invention. It should be understood that any hydraulic tensioner of the general type shown may benefit from the present invention. The hydraulic tensioner  10  includes a tensioner housing or tensioner body  12  with a bore  14 . A hollow piston  16  is slidably fitted into the bore  14  of the tensioner housing  12  forming a fluid chamber  18  therebetween. A spring  19  positioned in the fluid chamber  18  between the housing  12  and the piston  16  is provided to bias the piston in a protruding direction from the bore. A check valve  40  is provided between the housing inlet passageway  42  and the fluid chamber  18  to permit fluid to flow into the fluid chamber and prevent flow in the reverse direction. 
     The pressure relief valve  20  and dual path vent disc  30  are located in the fluid chamber  18  adjacent the upper end  17  of the piston  16 . The upper end  17  of the piston is the end nearest the protruding portion of the piston which extends outwardly from the tensioner bore  14  and includes the exhaust vent port  15 . The pressure relief valve  20  includes a generally hollow cylindrical valve body  22 . Located within the valve body  22  are a valve member  24  and a stack of belleville springs  26  biasing the valve member  24  in the closed position against the valve seat  28 . In this first embodiment, the valve member  24  is a conical metal disc. The valve seat  28  is in the form of a flat ring with a central opening  29 . When pressure in the fluid chamber  18  exceeds a specified maximum, typically on the order of 200 to 300 pounds per square inch, the valve member  24  becomes unseated. Oil then flows from the fluid chamber through the relief valve  20  through the lower opening  23  in the valve body and past the valve seat  28 . An oil passage  25  in the top of the valve body allows the oil to exit the relief valve body and directs the oil to the vent disc  30 . 
     The vent disc  30  is located atop the relief valve  20  adjacent to and in contact with the upper end  17  of the piston  16 . The outside diameter of the vent disc  30  is sized to provide a sealed fit with the internal diameter of the inside of the piston to prevent significant or uncontrolled loss of fluid between the inner piston wall and the outside of the vent disc. The vent disc is held in place by the fit with the inside of the piston. Also, pressure from the spring  19  acts to keep the relief valve in contact with the bottom of the vent disc and the vent disc pressed against the upper inside bore of the piston. 
     The pressure relief valve body  20  nests in the bottom of the vent disc  30  and is held in place by a circular sidewall portion  32  depending from the vent disc  30 . Oil exiting the oil exit passage  25  of the valve body is directed to the first spiral groove  34 , which is referred to as a tortuous path, formed in the bottom surface of the vent disc. The oil follows the groove  34  which spirals inwardly and exits a central vent disc bore  38 . The oil from the central vent disc bore  38  is free to exit the tensioner via the piston exhaust vent port  15 . The tortuous vent path  34  provides a predetermined amount of resistance to exiting oil traveling therethrough and thus prevents rapid or uncontrolled pressure loss and collapse of the tensioner. 
     In addition to the vent path  34  controlling fluid release from the pressure relief valve  20 , the vent disc  30  has a second groove or vent path  36  to control conventional venting from the fluid chamber  18 . Fluid and air in the fluid chamber  18  may exit the tensioner  10  bypassing the pressure relief valve  20  by traveling through the groove  39  formed in the sidewall portion  32  of the vent disc  30 . The groove  39  connects the fluid chamber  18  and the second tortuous path  36  which is located in the top surface of the vent disc. Oil flows through the groove  39  and the second vent path  36  and exits the vent disc  30  by way of the vent disc bore  38 . 
     As shown in FIG. 2, the pressure relief valve  120  of another embodiment of the present invention includes a generally cylindrical hollow valve body  122 . Inside the valve body are a plurality of stacked belleville washers  126  operating to bias a ball  124  against a valve seat  128 . This embodiment operates in a manner similar to the embodiment shown in FIG.  1 . However, the ball-shaped valve member  126  has greater mass, and thus, has a slower response time and lower natural frequency than a thin metal disc or even a hollow metal ball. The exact configuration will depend on the dynamic response designed for the intended service. The valve member may be constructed from any suitable metal, or ceramic or engineered plastics, such as polyamide. 
     FIG. 3 symbolically illustrates the dual path function of the present invention. Fluid from a pressurized oil supply  50  enters the fluid chamber through the check valve  40 . Venting of oil during a pressure relieving condition is accomplished when fluid opens and passes through the pressure relief valve and exits through the first tortuous vent path  134  and exhaust vent  115 . Conventional venting of trapped air and some fluid is accomplished by the second route through the second tortuous path  136  and exhaust vent  115 . 
     As shown in FIG. 4, in a preferred embodiment the valve seat  228  is located in a groove  262  formed in the inside of the valve body  222 . The valve seat  228  is held in place by a flat circlip  260  located in a separate lower groove  264  in the valve body  222 . A passageway or a plurality of passageways (shown in FIG. 7) formed through the top of the valve body  222  allows oil to exit the pressure relief valve. 
     The vent disc (also shown in FIGS. 5 and 6 in top and bottom perspective views) is located atop the relief valve body  222  adjacent to and in contact with the top of the valve body  222 . A downward extending sidewall or skirt portion  232  of the vent disc  230  receives the top portion of the valve body  222 . A groove  239  formed in the sidewall or skirt portion  232  of the vent disc allows fluid from the fluid chamber to bypass the relief valve. Fluid travels through the sidewall groove  239  in the vent disc and is directed to a second tortuous vent path  236  in the shape of a inwardly spiraling groove in the top surface of the vent disc. A central bore  238  leads fluid from the second vent path  236  and leads to the exhaust vent formed in the upper end of the piston. The fluid exits the tensioner through the exhaust vent. 
     FIGS. 5 and 6 illustrate the vent disc in more detail. The second vent path  236  is shown clearly in FIG. 5 on top of the vent disc  230 . The second vent path  236  leads oil from the sidewall groove  239  at the edge of the vent disc to the central bore  238  of the vent disc. FIG. 6 shows the first vent path  234  formed on the underside of the vent disc  230 . The first vent path  234  leads oil from the pressure relief valve to the central bore  238  of the vent disc. Oil in the central bore  238  is vented from the tensioner through the exhaust vent. 
     FIG. 7 is an exploded illustration of the pressure relief valve and vent disc shown in FIG.  4 . The pressure relief valve includes a generally cylindrical body  222 . A stack of belleville washers  226  bias a spherical valve member  224  against a disc-shaped valve seat  228 . A circlip  260  holds the valve seat in the valve body. Exit passages  225   a ,  225   b ,  225   c  formed at the top of the body allow fluid to exit the relief valve. A vent disc  230  fits over the top of the valve body  222 . A first vent path (not shown) formed in the underside of the vent disc  230  controls the release of fluid from the exit passages  225   a ,  225   b ,  225   c  and delivers the fluid to the central bore  238 . 
     A groove  239  formed in the sidewall is in communication with the second tortuous vent path  236  formed on the top surface of the vent disc. A bore  238  is formed in the center of the vent disc  230  in communication with both vent paths permitting venting of fluid from the tensioner. 
     Those skilled in the art to which the invention pertains may make modifications and other embodiments employing the principles of this invention without departing from its spirit or essential characteristics, particularly upon considering the foregoing teachings. The described embodiments are to be considered in all respects only as illustrative and not restrictive and the scope of the invention, is therefore, indicated by the appended claims rather than by the foregoing description. Consequently, while the invention has been described with reference to particular embodiments, modifications of structure, sequence, materials and the like would be apparent to those skilled in the art, yet still fall within the scope of the invention.