Hydraulic tensioner with pawl-style external rack

A hydraulic tensioner having a pawl-style rack member. The rack member is located in a groove in a bore in the tensioner housing. The tensioner piston has grooves along its exterior surface that correspond to the wedges or grooves in the pawl rack member and prevent the piston from being pushed inward. The pawl rack member may be a pair of pawls located in the tensioner bore. A flexible tab on the upper portion of the rack member contacts a groove on the upper portion of the piston to retain the piston in place for shipping.

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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 illustrates the hydraulic tensioner 10 having a housing 20, a
 retaining pin 5, and a pawl rack member 3 fitted in place in a groove in
 the bore 15. As shown in FIGS. 2 and 5, a bore 15 within the tensioner
 housing 20 forms a fluid chamber with the interior of hollow piston 2. The
 fluid chamber 22 is filled with fluid through a passageway 24 from a
 pressurized fluid source 25. The fluid source may be an oil pump or a
 reservoir.
 The chamber, preferably cylindrical, receives a hollow piston 2, also
 preferably cylindrical. The outside of the piston 2 has several grooves
 11, or steps, integrally formed on the piston. The grooves 11 form a
 piston rack that contacts corresponding wedges or grooves 12 on the pawl
 rack member 3 as the piston moves outward from housing. The pawl rack
 member 3 includes at least one flexible tab 13, which contacts an upper
 groove 14 of the piston to hold the piston in place for shipping. A vent
 disc 8 is placed in the upper end of the inside of the piston, and a
 piston spring 4 contacts the vent disc 8 and piston to bias the piston 2
 in a protruding or outward direction.
 During start-up of the hydraulic chain tensioner, after the tensioner has
 been installed and clip 5 has been removed, 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 21, fluid enters
 through passageways 23 and 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 2 by the chain (not shown) is balanced by the force of the spring 4
 and the resistance force of the fluid in the chamber 22.
 The check valve 21 is provided between the chamber 22 and the source of
 fluid pressure to permit fluid flow into the chamber 22, while blocking
 fluid flow in the reverse direction. The check valve 21 includes a ball 7,
 ball retainer 6, and spring 9. The spring 9 biases the ball 7 against the
 seat formed by the passageway 24.
 FIG. 3 shows the top of the tensioner 10, and FIG. 4 shows a sectional view
 of the tensioner 10 along line 4--4 of FIG. 3. FIG. 5 illustrates the
 separate parts of the tensioner 10.
 The pawl rack member 3 of the present invention is shown in detail in FIGS.
 6, 7, and 8. As shown in FIG. 6, one feature of the rack, is the piston
 retention feature by which the flexible tab 13, which is normally biased
 radially outward from the piston, is hooked within the uppermost groove 14
 of the piston to hold the piston in its innermost position. The piston is
 securely held in this position by insertion of clip 5 through the aperture
 28 in the piston and corresponding aperture 29 in the housing. This
 feature is used during shipping of the piston. The clip 5 is removed upon
 installation in the engine.
 FIGS. 7 and 8 illustrate the details of the pawl rack member. The base 31
 of the pawl member 3 is held in a groove in the housing by a spring steel
 piece 33. The spring steel member can be secured to the housing by rivets
 35, screws, or other attaching mechanisms. Wedges or grooves 12 are formed
 along the surface of the pawl member in order to contact the corresponding
 grooves 11 in the piston.
 As shown in FIG. 6, a stepped groove 37 is formed so that the rack grooves
 catch in the stepped groove and stop the piston 2 from leaving the
 tensioner housing. This provides an upper limit on piston travel.
 An alternative embodiment of the pawl rack member is shown in FIG. 9. In
 that embodiment, two pawls 50 and 52 are shown on opposite sides of the
 rack member.
 FIGS. 10-12 show a preferred embodiment of the dual pawl rack tensioner
 110. As shown in FIG. 10, the tensioner 110 includes the tensioner housing
 170 which receives a bore 120. A hollow piston 102 is slidably received
 within the bore 120. In this embodiment, the spring band member is in the
 form of a pair of circlips 160 which are located on the outside of the
 bore 120 and retain pawl 152 by engaging slots formed on the outside of
 the pawl 152. In addition, the circlips 160 normally bias the pawl 152
 toward the piston 102.
 As shown in FIG. 11, which is a sectional view of the tensioner shown in
 FIG. 10 along line A--A, fluid is supplied to a fluid chamber formed
 between the bore 120 and the hollow piston 102 through the passageway 123
 in the housing 170 and the passageway 124 from a pressurized fluid source
 125. The piston 120 is biased outwardly from the housing and bore by a
 piston spring 104. Left pawl 150 and right pawl 152 are located opposite
 one another in slots formed near the top of the bore 120. A pair of
 circlips 160 are located on the outside of the bore 120 and retain the
 left and right pawls 150,152 by engaging slots formed on the outside of
 the pawls 150,152. The left pawl 150 is located a half pitch lower on the
 bore with respect to the right pawl 152, both pawls having teeth formed on
 an inside surface which engage grooves or steps 111, (shown in FIG. 12),
 on the outside of the piston 102.
 FIG. 12 shows the hydraulic tensioner of FIGS. 10 and 11 in an exploded
 perspective view. The tensioner includes a housing 170 having a bore 120
 with a pair of pawls 150,152 held in bore slots by circlips 160. The
 piston 102 has grooves or steps 111 for engagement with the pawls 150,152,
 the piston being slidably received within the bore and biased outwardly by
 a piston spring 104. The pawls 150,152 are located in slots or grooves on
 opposite sides of the bore 120 and are vertically offset with regard to
 one another by a half pitch. In another words, one of the pawls is located
 a half pitch closer to the open end of the bore near the tip or distal end
 of the piston with regard to the other.
 The advantage of off setting the pawls is that both the pitch of the
 grooves or teeth on the outside of the piston and the inside of the pawls
 can be made larger and the outward motion of the piston can be controlled
 in smaller increments. For example, in a single pawl design with a 1 mm
 pitch, the pitch defined as the distance between two adjacent grooves or
 teeth, the piston must move 1 mm to engage the next tooth of the pawl. In
 a dual pawl design where the pawls are offset by a half pitch, the piston
 needs to move only 0.75 mm to engage the next tooth of a pawl where a
 pitch of 1.5 mm is provided both on the piston and the inside surface of
 each pawl. A benefit of a larger pitch is ease of manufacturing. In
 addition, the control of piston movement in smaller increments provides
 improved control of piston backlash.
 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.