Hydraulic tensioner with pawl-style external rack

Details Hydraulic tensioner with pawl-style external rack Hydraulic tensioner with pawl-style external rack

1999-08-23

2001-06-12

Bucci, David A. (Department: 3682)

Endless belt power transmission systems or components

Means for adjusting belt tension or for shifting belt,...

Tension adjuster or shifter driven by electrical or fluid motor

C474S138000

Reexamination Certificate

active

06244981

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a hydraulic chain tensioner having an external rack. More particularly, the hydraulic tensioner of the present invention has a pawl-style external rack on the outside of the piston to limit piston travel and limit backlash.
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 difficulty 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 Deppeet 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, an external rack is provided along the outside of the piston to provide tension during low pressure. The piston is still permitted to move back several millimeters more than the average piston when the engine is running. The pawl slides back and forth in a series of slots or grooves formed along the exterior surface of the piston, while a spring band biases the pawl against the piston.
SUMMARY OF THE INVENTION
The present invention is directed to a hydraulic chain tensioner having an external 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 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 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 a rack and ratchet assembly that has several features. First, the assembly provides a mechanical no-return function, or anti backlash feature. An external rack is formed with pawl style wedges that fit within a series of corresponding wedge-shaped grooves in a rack formed on the outside of the piston. The pawl wedges slide back and forth within the corresponding grooves formed on the piston. A spring-steel band around outside of the pawl biases the pawl wedges toward the grooves formed on the outside or exterior surface of the piston.
The piston retention feature of the rack and ratchet system limits the outward travel of the piston. After the wedges on the pawl rack pass the last rack member, or groove, on the piston, the wedges are biased toward the piston and catch in final stepped groove of the piston. As a result, no further outward movement of the piston is permitted.
In another embodiment of the present invention, the hydraulic tensioner has a pair of pawls. The two pawls are located opposite each other in grooves in the tensioner bore and held in place by a set of circlips. One of the two pawls is located higher than the other in the tensioner body. As the piston extends it engages first one pawl and then the other. By off setting the openings in the tensioner body in which the pawls are inserted, the pitch of the grooves or steps on the piston and the pitch on the pawls may be made larger for ease of manufacturing.
Another feature of the present invention is the flexible tabs that serve as shipping retention tabs to hold the piston in the innermost position for shipping and automatically release when the tensioner is installed in an engine or when the engine is first started.


REFERENCES:
patent: 3802286 (1974-04-01), Winklhofer et al.
patent: 3812733 (1974-05-01), Yoshida
patent: 4713043 (1987-12-01), Biederman
patent: 4792322 (1988-1

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