Abstract:
A hydraulic tensioner having a piston slidably received in a housing bore and biased in a protruding direction by a spring. A pressure relief valve is positioned in the nose of the piston. The pressure relief valve includes a valve member and seat member. The valve member biased by a spring disposed between the seat member and a retaining plate on the valve member.

Description:
This application claims the benefit of U.S. Provisional Application Ser. No. 60/082,459, filed Apr. 20, 1998. Reference is made to U.S. Pat. No. 5,577,970, entitled “Hydraulic Tensioner With A Pressure Relief Valve,” and U.S. Pat. No. 5,707,309, entitled “Hydraulic Tensioner With Modular Inlet Check Valve With Pressure Relief,” both of which are owned by the assignee of the present application and are incorporated herein by reference, which relate to the subject matter of the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     A tensioning device, such as a hydraulic tensioner, is used as a control device for a power transmission chain, or similar power transmission device, as the chain travels between a plurality of sprockets. In these devices, the chain transmits power from a driving sprocket to a driven sprocket. One portion of the chain span between the sprockets is tight or under tension while the other portion is slack. Generally, it is important to also impart and maintain a certain degree of tension in the slack portion of the chain to prevent noise, slippage, or the unmeshing of teeth in the case of a toothed chain. 
     Prevention of such slippage is particularly important in the case of a chain driven camshaft in an internal combustion engine because jumping of teeth will throw off the camshaft timing, possibly causing damage or rendering the engine inoperative. However, in the harsh environment of an internal combustion engine, various factors can cause fluctuations in the chain tension. 
     Wide variations in temperature and thermal expansion coefficients among the various parts of the engine can cause the chain tension to vary between excessively high or low levels. During prolonged use, wear to the components of the power transmission system can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations may cause considerable variations in chain tensions. Reverse torque on an engine, occurring for example in stopping or in failed attempts at starting, can also cause fluctuations in chain tension. For these reasons, a mechanism is desired to remove excessive tensioning forces on the tight side of the chain and to ensure the necessary tension on the slack side of the chain. 
     Hydraulic tensioners are a common method of maintaining proper chain tension. In general, these mechanisms employ a lever arm that pushes against the chain on the slack side of the power transmission system. This lever arm must push toward the chain, tightening the chain, when the chain is slack, and must retract away from the chain when the chain tightens. 
     To accomplish this result, a hydraulic tensioner typically comprises a hollow piston or plunger, which is biased in the direction of the chain by a tensioner spring. The plunger is housed within a cylindrical bore in the tensioner body or housing, which has an interior space which is open at one end. A fluid chamber is formed in the interior space of the bore between the bore and the interior of the hollow piston. The fluid chamber receives fluid through a fluidic connection with a reservoir or exterior source of hydraulic fluid. 
     Typically, two types of valves are employed to regulate the flow of fluid into and out of the pressure chamber: inlet check valves and pressure relief valves. The inlet check valve typically includes a ball-check valve that opens to permit fluid flow through the valve and into the fluid chamber when the pressure in the chamber has decreased as a result of outward movement of the plunger. When the pressure in the pressure chamber is sufficiently high, the inlet check valve closes, preventing fluid from exiting the pressure chamber, which in turn prevents the piston from retracting, achieving a so-called “no-return” function. 
     The pressure relief valve performs its function when the pressure in the chamber exceeds a predetermined maximum limit. The pressure relief valve opens to permit fluid to exit the chamber and allow the tensioner to retract in response to large increases in chain tension (and the associated fluid pressure). The pressure relief valve typically includes a spring biased check valve. 
     In operation, the inward force of the chain on the piston is balanced by the outward force of the tensioner spring and the reaction force from the hydraulic fluid. As the tension in the chain increases, the chain exerts an increased force on the plunger in the direction of plunger retraction. As the plunger is forced in the retraction direction, the fluid pressure in the pressure chamber increases, but the inlet check valve prevents the fluid from exiting the pressure chamber. If the pressure exceeds a predetermined maximum level, the pressure relief valve opens, allowing the fluid to exit the pressure chamber. 
     Examples of pressure relief valves are shown in the above-mentioned U.S. Pat. No. 5,577,970 and U.S. Pat. No. 5,07,309. In U.S. Pat. No. 5,577,970, the pressure relief valve is in the form of a reed valve that opens to permit fluid to exit the high pressure chamber. In U.S. Pat. No. 5,707,309, the pressure relief valve is integral with the inlet check valve. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a tensioner for providing a tensioning force to a power transmission chain that connects at least two rotating members such as a pair of sprockets. A hollow piston slidably fits within a bore in a piston housing, forming a fluid chamber. A spring is positioned within the fluid chamber to bias the piston outward from the bore. An inlet check valve permits flow from a source of pressurized fluid, or a reservoir, into the fluid chamber and prevents flow out of the chamber in the reverse direction. 
     A pressure relief valve is located in the nose of the piston. This valve permits fluid to exit the high pressure fluid chamber when the pressure in the pressure chamber reaches (or exceeds) a certain specified limit. The valve member includes a stem and rounded portion that closes against a seat member. A valve spring positioned between the seat and a retainer member biases the valve member into a closed position against the seat member. At a predetermined maximum pressure value, the resisting force of the valve spring is exceeded and the spring compresses to permit the valve member to move from the seat member. The seat member is either fixed or biased against the nose of the piston by a piston spring. 
     For a better understanding of these and other aspects and objects of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, which are not to scale, 
     FIG. 1 is a side view of a tensioner of the prior art with a inlet check valve and pressure relief valve with the tensioner maintaining the tension in a power transmission chain. 
     FIG. 2 is a side cross-sectional view of one embodiment of the tensioner of the present invention with the pressure relief valve in the nose of the piston. 
     FIG. 3 is a side cross-sectional view of another embodiment of the pressure relief valve of the present invention. 
     FIG. 4 is a top view of an alternate embodiment of the plug member. 
     FIG. 5 is a side cross-sectional view another embodiment of the tensioner and pressure relief valve of the present invention. 
     FIG. 6 is a side cross-sectional view of the relief valve seat and plug member. 
     FIG. 7 is a top view of the valve and plug member of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, the present invention is directed to providing a pressure relief valve for a hydraulic tensioner. The pressure relief valve is located in the nose or upper portion of the piston of the tensioner. 
     FIG. 1 illustrates a hydraulic tensioner with an inlet check valve and a pressure relief valve of the prior art. A power transmission chain system  10  comprises a chain  12  operating between two sprockets  14 ,  16 . A tensioner arm  18 , or chain guide, mounted on a pivot  20  presses against the chain to maintain tension. A hydraulic tensioner  100  has a piston  110 , which applies force to the lever arm  18 . The inlet check valve and pressure relief valve of this tensioner are described in U.S. Pat. No. 5,707,309. The tensioner  100  in FIG. 1 is not to scale in order to illustrate the inlet check valve and pressure relief valve 
     FIG. 2 is a side view of the cross-section of a tensioner with one of the embodiments of the pressure relief valve of the present invention. The tensioner  100  includes a housing  102  having a cylindrical bore  104  and an inner circumferential surface  106  of this bore. The housing has an aperture  108  at the closed end  110  of the bore. This aperture connects to a reservoir or an external supply of pressurized fluid (not shown). 
     A cylindrical hollow piston  130  is slidably assembled within the bore  104 . This piston comprises a cylindrical body  132  with a closed upper end or nose  134 . In some embodiments, a seal ring  136  is housed in a groove  138  on the outer circumferential surface  139  of the piston. This seal ring  136  forms a partial seal or flow restriction between the outer circumferential surface  139  of the piston and the inner circumferential surface  106  of the bore  104 . 
     Assembled concentrically within the piston  130  is the piston spring  170 . The piston spring  170  biases the piston  130  toward the outside of the housing  102  so that the piston tip  131  at the upper end of the piston nose  134  pushes against the lever arm  18  and associated chain as shown is FIG.  1 . 
     Assembled concentrically or radially within the piston spring  170  is the inlet check valve  200 . The check valve has a generally cylindrical housing  202 , with a closed end and an open end. The closed end includes fluid openings  212  for the passage of fluid. In its preferred embodiment, the check valve comprises a conventional ball check valve with a valve housing  202  and a ball  214  with a valve spring  216  that moves the ball  214  away from the housing  202 . The ball check valve permits flow through the valve into a fluid chamber  180  formed inside the piston  130 , when the ball  214  is moved from the valve seat  218  by a pressure differential across the valve. The ball check valve prevents fluid flow through the check valve in the reverse direction as the ball seats against the valve seat  218 . 
     A high pressure fluid chamber  180  is formed within the interior of the hollow piston and the bore. The high pressure fluid chamber  180  is annular in shape. Its outer circumferential surface is formed by the inner circumferential surface  137  of the piston and the inner circumferential surface  106  of the piston housing bore  104 . The seal ring  136  minimizes fluid that escapes the high pressure fluid chamber through the space between the piston  130  and the piston housing  102 . 
     As shown more clearly in FIG. 2, a pressure relief valve  300  is located in the nose  134  of the piston  130 , which permits fluid to exit the high pressure fluid chamber, but only to provide pressure relief when the pressure of the fluid reaches a certain specified maximum level. The valve  300  includes a valve member  302  that is formed of a valve stem  304  and a rounded or tapered end  305  that is held against a valve seat member  306 . A spring retainer washer or retainer member  310  is assembled within the valve stem  304  such that its axial position with respect to the valve member  102  is fixed. A pressure relief spring  312  is mounted on the spring retainer washer  310  and surrounds or is concentric with the valve stem  304 . 
     The annular pressure relief disc member or valve seat  306  is positioned concentrically within the piston body and radially outward from the valve stem  304 . One end of the pressure relief spring  312  is biased against an internal base  316  of the valve seat while the opposite end of the spring  312  is biased against the retainer washer  310 . In this way, the pressure relief spring  312 , braced against the fixed spring retainer washer  310 , biases the valve member  302  in the direction of the tapered end  318  of the valve seat  306 . When pressure in the pressure fluid chamber exceeds a predetermined maximum level, the tapered end  305  of the valve member  302  moves away from the tapered end  318  of the valve seat to release fluid and relieve pressure in the high pressure fluid chamber. 
     Biased toward the valve seat, the pressure relief valve member  302  seats against the tapered end  318  of the valve seat  306 . The pressure relief valve seat  306  is a formed member that fits within the nose  134  of the piston body. A stepped outer surface  320  abuts an internal step  322  in the piston body. A gasket  324  can be placed therebetween to limit, minimize or eliminate flow of fluid between the seat and the piston body at that area. Alternatively, a limited clearance between the surface  320  and the step  322  of the piston body can permit limited fluid flow and also serve as a vent mechanism for air flow. 
     The pressure relief valve seat member  306  may be held against the piston body by the piston spring  310  or may be press fit and fixedly mounted in the upper end of the nose of the piston. While a presently preferred structure has been shown, the pressure relief valve seat may be any component fixed with respect to the piston body and capable of forming a seal with the pressure relief valve member. 
     Flow through the pressure relief valve may be regulated by a number of adjustable mechanisms or fluid flow passages. The spring constant of the pressure relief spring  312  provides one such adjustment mechanism. Similarly, the surface area of the tapered contact portion  318  between the cupped portion of the valve member  305  and the seat  306  is another such adjustment mechanism. Additionally, the clearance between the stem  304  of the valve member and the inner surface  326  of the center aperture in the seat  306  also provides an adjustment mechanism. The distance that the valve retainer washer or member  310  can travel in the axial direction until it abuts the bottom surface  328  of the seat  306  also limits the opening of the pressure relief valve member. 
     A small fluid reservoir  330  is also created at the top of the valve member within the nose of the piston. This small reservoir acts to prevent ingestion of air into the tensioner during back-flow conditions as the valve is closing against the seat. 
     An alternate construction of the pressure relief valve is shown in FIG.  3 . Only the upper portion of the piston  400  is shown in this Figure. In this embodiment, the axially moveable valve member is in the form of a spring biased ball  402 . The pressure relief valve seat member  406  may be press fit or held in the piston by the piston spring  408 . The ball  402  is biased by a spring  409  against a tapered surface  410  adjacent an opening  412  in the base  414  of the seat member. At a predetermined maximum pressure, the ball  402  moves off of the tapered surface  410  of the seat to permit pressure relief by permitting fluid to exit through the opening  412  in the valve. 
     Preferably, the valve member is a ball, but it may have various geometric configurations. For example, the valve member may be a tapered disc or a tapered plug (not shown). The exact configuration will, of course, depend on the dynamic response desired. For example, a solid ball will have a greater mass and thus a slower response time and a lower natural frequency compared to a small light-weight disc, or even a hollow metal ball. The valve member may also be constructed of ceramic or engineered plastics, such as polymide. 
     The pressure relief valve spring  409  is biased against a plug member  416 , shown as being formed of powder metal material in FIG.  3 . The plug member  416  is inserted into the valve seat  406  after insertion of the ball  402  during assembly of the valve. 
     An alternate construction of the plug member  516  is shown in FIG.  4 . The plug member  516  includes slots  518  to permit flow through the center  420  of the seat member  406 . Allowing flow through the center of the seat  406  permits a threaded attachment of the seat  406  to the piston  400  along sides  422  and  424 . 
     In operation, as the piston moves away from the bore, and the pressure in the high pressure fluid chamber becomes low enough, the inlet check valve member  200  will unseat. Fluid will then flow into the high pressure fluid chamber  180 . Thus, the inlet check valve dictates the pressure required to allow fluid to flow into the high pressure fluid chamber  180  from the external source of pressurized fluid. 
     The pressure relief function is performed as the pressure relief valve member unseats and moves away from the pressure relief valve seat. Under typical conditions, this valve prevents fluid from exiting the high pressure fluid chamber  180 . Hydraulic pressure, applied by the fluid against the outside of the pressure relief valve member, and applied by the fluid against the valve member, urges the pressure relief valve open, as the valve member moves away from the pressure relief seat member. This force is resisted by the pressure relief spring. If the pressure from inside the high pressure fluid chamber becomes too great, and the pressure relief valve member unseats, fluid will then flow from the high pressure fluid chamber, through the aperture in the base end of the valve seat, through the space between the outside diameter of the valve member and the inside diameter of the seat, and to atmosphere. 
     FIG. 5 illustrates another embodiment in which the tensioner housing  500  holds a sleeve  502 . The piston  504  slides within the sleeve  502 . The piston spring  506  biases the piston in a direction away from the housing  500 . An inlet check valve  510 , formed in the base of the sleeve, includes a ball  512  and valve seat  514 . 
     The pressure relief valve  520  is held against a seal  522  by piston spring  506 . The seal  522  includes a tortuous vent path for escape of air along its upper surface. The pressure relief valve  520  includes a plug member  524  formed of powder metal that is press fit into the valve seat  526 . A ball  528  in the base of the valve is held against an opening  530  in the seat by a valve spring  532 . 
     FIGS. 6 and 7 illustrate the pressured relief valve  520  in more detail. The plug member  524  is press fit in the seat  526 . The upper portion of the plug member  524  has recessed sides  540 ,  542 , as shown in FIG. 7, to permit the passage of fluid through the plug member and seat. 
     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.