Hydraulic tensioner with a hydraulically controlled rack

Abstract: A hydraulic tensioner having a hydraulically actuated rack member. A pair of check valves (214,230) permit fluid to flow from an external source (224) of pressurized fluid into a fluid chamber (208). The two check valves form a substantially fluid tight chamber in order to provide sufficient pressure to prevent the piston (200) from retracting upon deenergization of the source of pressurized fluid.

Description: 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. Patent 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. Patent 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. Patent 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. Patent 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. Patent 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. Patent No. 4,874,352, where the ratchet is supported by a spring, and in U.S. Patent 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. Patent 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. Patent 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 Ojinia, U.S. Patent 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. Patent 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. In GB-A-2122685 on which the preamble of claim 1 is based there is described a hydraulic chain tensioner in which a hollow piston is urged outwardly by a spring with respect to a housing with which the piston encloses a pressure chamber, and the piston includes a passage controlled by a ball and spring check valve arranged to open, in the event of pressure peaks occurring in the pressure chamber, so as to reduce the pressure peaks. There is no provision for venting the pressure chamber when oil is supplied into the chamber via a main check valve. In accordance with the present invention there is provided a hydraulic tensioner comprising: a housing having a bore; a piston slidably received within the bore, said piston forming a fluid chamber with said bore; a piston spring biasing said piston in an outward direction from said bore; a main check valve provided between the chamber and a source of pressurized fluid to permit fluid flow from a first passageway past said main check valve and into the fluid chamber upon energization of said source of pressurized fluid while blocking flow in the reverse direction; a vent check valve for permitting fluid flow past said vent check valve from said fluid chamber; and said main check valve and said vent check valve providing a substantially fluid tight outlet from said chamber to prevent fluid from exiting said chamber after being permitted to enter said chamber, said substantially fluid tight outlet providing sufficient fluid pressure in said chamber to prevent retraction of said piston into said chamber upon deenergization of said source of pressurized fluid; characterised in that the vent check valve is provided in a second passageway communicating with said source of pressurized fluid, and is connected to the fluid chamber by a third passageway, whereby the vent check valve is opened upon energization of the source of pressurized fluid and flow of fluid into the fluid chamber through the first passageway, and the vent check valve is closed under the fluid pressure in the third passageway upon deenergization of the source of pressurized fluid. In a preferred construction the tensioner has a hollow piston slidably received within the bore so that the fluid chamber is within the piston, and the third passageway extends into the interior of the piston. 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 When oil is supplied to the tensioner through the main check valve, the vent check valve also opens. When the main check valve closes, the vent check valve also closes, which provides an oil-tight system that includes the high pressure chamber and the vent passage. The hydraulic pressure in 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 air 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 not fully in accordance with the 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 the upper end of the chamber, past 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 secondary 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 secondary passageway 240 between the chamber 208 and the check valve 230. When the oil pressure drops, such as upon shutdown of the engine, the main check valve 214 and the vent check valve 230 remain closed and keep the fluid within the chamber in a 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 show 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 and 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 secondary passageway 240. A vent disc 306 is provided in the upper end of the chamber 208 against the base portion. 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 secondary passage 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.

Claims: A hydraulic tensioner comprising: a housing (204) having a bore (202); a piston (200) slidably received within the bore, said piston forming a fluid chamber (208) with said bore; a piston spring (212) biasing said piston in an outward direction from said bore; a main check valve (214) provided between the chamber (208) and a source of pressurized fluid (224) to permit fluid flow from a first passageway (226) past said main check valve (214) and into the fluid chamber upon energization of said source of pressurized fluid while blocking flow in the reverse direction; a vent check valve (230) for permitting fluid flow past said vent check valve from said fluid chamber (208); and said main check valve (214) and said vent check valve (230) providing a substantially fluid tight outlet from said chamber (208) to prevent fluid from exiting said chamber after being permitted to enter said chamber, said substantially fluid tight outlet providing sufficient fluid pressure in said chamber (208) to prevent retraction of said piston into said chamber upon deenergization of said source of pressurized fluid; characterised in that the vent check valve (230) is provided in a second passageway (228) communicating with said source of pressurized fluid (224), and is connected to the fluid chamber (208) by a third passageway (240), whereby the vent check valve (230) is opened upon energization of the source of pressurized fluid (224) and flow of fluid into the fluid chamber (208) through the first passageway (226), and the vent check valve (230) is closed under the fluid pressure in the third passageway (240) upon deenergization of the source of pressurized fluid. The hydraulic tensioner of claim 1 wherein the third passageway (240) extends into the interior of said piston (200). The hydraulic tensioner of any one of claims 1 to 2 wherein said fluid chamber (208) is located within the interior of said piston (300). The hydraulic tensioner of claim 1 wherein the third passageway (240) forms a part of said fluid chamber (208). The hydraulic tensioner of any one of the preceding claims wherein said second passageway (228) includes a vent opening (236) to atmosphere. The hydraulic tensioner of claim 5 wherein said vent opening (236) includes a vent disc (402).

Inventors: SIMPSON ROGER T, SIMPSON, ROGER T.
Applicants: BORG WARNER INC, BORGWARNER, INC.
Application year: 1999
Main classification: F16H7
IPC codes: F16H7, F02B67