Abstract:
A hydraulic tensioner having a hydraulically actuated rack member. A pair of check valves permit fluid to flow from an external source of pressurized fluid into a fluid chamber. The two check valves form a substantially fluid tight chamber in order to provide sufficient pressure to prevent the piston from retracting upon deenergization of the source of pressurized fluid.

Description:
This application claims benefit of provisional application U.S. Ser. No. 60/101,261, filed Sep. 21, 1998. Reference is made to co-pending U.S. application Ser. No. 09/232,388, filed Jan. 15, 1999 now issued as U.S. Pat. No. 5,989,139, entitled “Hydraulic Tensioner With External Rack” the subject matter of which relates to the present invention and is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a hydraulic chain tensioner having an external rack or similar component to provide a no-return function in the tensioner. More particularly, the hydraulic tensioner of the present invention has a hydraulically controlled rack that limits or restricts backlash to provide a no-return function for the piston. 
     Hydraulic tensioners are typically used as a control device for a chain drive in an automobile engine timing system. The tension in the chain can vary greatly due to the wide variation in the temperature and the linear expansion among the various parts of the engine. Moreover, wear to the chain components during prolonged use can produce a decrease in the tension of the chain. A hydraulic tensioner is used to take up the slack in the chain or belt that connects the camshafts to the crankshaft of the engine timing system. The tensioner piston must be able to extend outward as the chain stretches from higher engine speed and withdraw back inward when the chain loads have decreased with lower engine speeds. The piston travel from idle to maximum engine speed for most overhead cam engines ranges from 1 to 4 mm. 
     A typical hydraulic tensioner is comprised of a housing having a bore, a piston biased in a protruding direction from the bore by a spring, and a fluid chamber defined by the hollow piston and bore. A check valve is also included in the hydraulic tensioner to permit fluid flow from a source of pressurized fluid into a reservoir or oil supply passage into the fluid chamber, while preventing back flow in the reverse direction. The force of the chain against the piston in an inward direction is balanced by the resistance force of the fluid and the force of the spring in an outward direction. 
     A typical hydraulic tensioner usually has a no-return function, where the piston moves easily in one direction, but with more difficultly in the reverse direction. When the engine is started, the oil supply pressure to the tensioner is delayed by several seconds. During this time, the tensioner may not have enough oil to fill the fluid chamber. As a result, the piston could be pushed to the bottom of the tensioner bore from the chain motion. A proper load would not be maintained on the chain and noise could be generated. In addition, the lower piston position might even allow the chain to jump a tooth on either the crank or cam sprockets. 
     One example of a tensioner having a no-return function is shown in Winklhofer et al., U.S. Pat. No. 3,802,286. The piston of the Winklhofer et al. tensioner has a spiral rack on the inside wall of the bore to limit back travel and prevent the piston from retracting. Another example of a tensioner having a no-return function, Yoshida, U.S. Pat. No. 3,812,733, has a ratchet system with grooves on the outside of a piston and a detent with a spring to prevent the piston from advancing and retracting. Similarly, in U.S. Pat. No. 4,713,043, Biedermann includes grooves on the outside of the piston with a spring-loaded catch. 
     The rack or no-return system must also permit some backlash or limited backward piston movement. In U.S. Pat. No. 4,792,322, Goppelt addresses the problem of insufficient backlash by including an internal ring and groove system. An additional ring and groove are also used to hold the piston in place during shipping. This system is expensive because the grooves must be on the inside of the tensioner bore as well as on the outside of the piston. 
     Suzuki, U.S. Pat. No. 4,822,320 also provides an anti-backlash rack with grooves broached into the outside of the piston. A ratchet is pivotally connected to a housing to allow positive backlash. Suzuki also provides this ratchet system in U.S. Pat. No. 4,874,352, where the ratchet is supported by a spring, and in U.S. Pat. No. 5,006,095, where the number of teeth on the ratchet is n times that of the teeth on the rack. In addition, Shimaya, U.S. Pat. No. 5,073,150, incorporates the ratchet mechanism of Suzuki with a different tensioner. 
     Another example of a ratchet mechanism is disclosed in Deppe et al., U.S. Pat. No. 5,304,099. The ratchet mechanism of Deppe et al. includes grooves on the outside of a piston and a ratchet plunger biased by a spring. The ratchet is disengaged during normal operations and engaged during shut down to maintain the tensioner in an operative position. 
     An example of a mechanism that limits the travel of a shaft device is disclosed in Ojima, U.S. Pat. No. 5,004,448. A coil portion contacts a tension rod. The coil acts as a friction brake by causing an enlargement to prevent advancement of the rod or a shrinkage of the diameter of the coil portion to release the rod from the tensioner. 
     Mott, U.S. Pat. No. 5,259,820, provides an internal ratchet system positioned within the mounting cavity and constructed from a cylinder having two helical openings. The piston engages with the helical openings when the piston experiences sufficient force to be pushed inward. As a result, this tensioner provides tension to the chain when the fluid pressure to the tensioner is low. 
     Similarly, in the present invention, a hydraulically actuated rack is provided to provide tension during low pressure conditions. The anti-backlash feature or no-return feature prevents the piston from retracting when the oil pressure in the tensioner decreases. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a hydraulic chain tensioner having a hydraulically actuated rack. The tensioner includes a housing with a central bore. A hollow piston is slidably received within the bore and creates a fluid chamber with the bore. The hollow piston, or plunger, is biased in a protruding direction from the housing by a spring. 
     A passage is provided in the housing to connect the fluid chamber with a source of pressurized fluid. A check valve is provided between the chamber and the source of pressurized fluid to permit fluid flow into the chamber, while blocking flow in the reverse direction. The check valve may be a ball and spring check valve, a spring valve, or a variable orifice check valve, as presently known in the tensioner art. 
     The tensioner also includes an external rack with an anti-backlash or backlash limitation feature. When oil is supplied to the tensioner through the main check valve, a vent check valve also opens. When the main check valve closes, the vent check valve also closes, which provide an oil-tight system that includes the high pressure chamber and the vent passage. The hydraulic pressure is maintained in the oil-tight system and retraction of the piston is prevented. 
     When oil is again supplied to the piston, the vent check valve opens and the oil that was trapped in the chamber is allowed to escape or vent to atmosphere. A tortuous path vent may also be included in the vent opening. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a conventional hydraulic tensioner of the prior art with an external rack and ratchet system. 
     FIG. 2 is a sectional view of the tensioner of present invention illustrating the hydraulic circuit to maintain oil pressure in the chamber. 
     FIG. 3 is a sectional view of another embodiment of the tensioner of present invention with a tortuous path vent in the high pressure chamber. 
     FIG. 4 is a sectional view of another embodiment of the tensioner of present invention with a tortuous path vent in the vent path opening. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a hydraulic tensioner  10  of the prior art. The tensioner includes a housing  20  having a fluid chamber  22  filled with fluid through a passageway  24  from a source of pressurized fluid (not shown). The fluid source may be an oil pump or a reservoir. The chamber, which is typically cylindrical, receives a hollow piston  40 , also typically cylindrical, having an interior space  42  and an upper end  44  with an aperture  90 . The upper end  44  contacts a lever or arm (not shown) to provide tension along a chain strand. A spring  60  contacts the inside  48  of the upper end of the piston to bias the piston in a protruding or outward direction from the bore or housing. 
     During start-up, fluid enters through passageway  24  and fills the chamber  22  while pushing air to t he upper end of the chamber, pas t the vent disc  91 . As the chamber fills with fluid, the piston moves outward from the chamber due to the force of the spring and the pressure of the fluid from the external source. 
     A check valve  100  is provided between the chamber  22  and the passageway  24  to permit fluid flow into the chamber but to block fluid flow in the reverse direction. The check valve includes a ball  102 , and a spring biasing the ball toward the seat  106 . 
     The tensioner includes a conventional rack and ratchet assembly to provide a mechanical no-return function. The piston is provided with an external rack  28  that is secured to the piston at the upper end  110  of the rack. As the piston moves outward, the rack also moves outward. The rack is provided with a flange  112  at its upper end that contacts a shoulder  50  on the upper end of the piston. The housing also includes a lateral cavity  30  in which a ratchet  120  and spring  122  are received. The spring  122  biases the ratchet into meshing arrangement to provide the mechanical no-return function. 
     During start-up of the hydraulic chain tensioner, after the tensioner has been installed, the piston is pushed outward by the force of the piston spring on the piston. As a result of the pressure differential formed across the check valve  100 , fluid enters through passageway  24  and flows through the check valve and into the chamber  22  while pushing air to the upper end of the chamber  22 . The chamber  22  continues to fill with fluid until the force inward on the piston  40  by the chain (not shown) is balanced by the force of the spring  60  and the resistance force of the fluid in the chamber  22 . 
     The tensioner of the present invention, in one embodiment, is illustrated in FIG.  2 . The piston  200  is received in a bore  202  in the housing  204 , which is sealed by piston seal  206 . A high pressure fluid chamber  208  is formed between the base of the piston  210  and the hollow space inside of the housing. As previously described, the tensioner includes a piston spring  212  and main check valve  214 , which includes a ball  216 , housing  218 , and spring  220  to bias the ball away from the housing  218  and against a seat  222 . 
     Oil supply pressure from source  224  flows through passageway  226  to the main check valve  214  and also through a rack or secondary passageway  228 . The fluid in second passageway  228 , acts on normally closed vent check valve  230 , which includes a ball  232  and spring  234  to bias the ball in the closed position. The vent includes an opening  236  to permit venting of air to atmosphere. The ball  232  and spring  234  are biased against a spacer  238  located in the housing. 
     During operation, upon the creation of a pressure differential across the main check valve  214 , the check valve opens and permits fluid to flow into the chamber  208 . The secondary or vent check valve  230  also opens and permits fluid to flow from the chamber via the third passageway  240  between the chamber  208  and the check valve  230 . 
     When the oil pressure drops, such as upon shut-down of the engine, the main check valve  214  and the vent check valve  230  remain closed and keep the fluid within the chamber in an substantially oil-tight system. In this manner, the oil pressure remains sufficiently high in the chamber so that the piston does not retract into the bore, and the tensioner achieves a no-return function. Upon start-up of the engine, before oil pressure reaches normal operating pressure, the tensioner piston is hydraulically maintained in a no-return state and provides proper load to the chain. 
     A preferred embodiment is shown in FIG. 3, which includes many of the same components of the tensioner of FIG.  2 . The similar components have been given the same numerical indications in FIG. 3 as in FIG.  2 . In this embodiment, however, the piston  300  includes a base portion  302 . The base portion  302  provides a series of passageways  304  between the base portion and the third passageway  240 . A vent disc  306  is provided in the upper end of the chamber  208  against the base portion of the piston. 
     In this embodiment, the vent disc  306  acts to prevent the escape of oil from the chamber  208  into passageway  304 . 
     Another embodiment of the tensioner of the present invention is shown in FIG. 4, which includes many of the components of the embodiment of FIG.  3 . In this embodiment, no vent disc  306  is used in the chamber. Instead, a vent disc  402  is included in the vent opening  236 . This embodiment operates in the same manner as the embodiment of FIG.  3 . However, a passage  404  permits fluid to leave the chamber  208  and enter the fourth passageway  304 , which assists in the pressure balance to maintain the piston in an extended position. 
     In operation of all three of the described embodiments, backlash or backdrive of the piston is limited or prevented by the maintenance of pressure in the chamber. 
     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.