Hydraulic tensioner with a pawl-style external rack

Abstract: A hydraulic tensioner having a pawl-style rack member (3). The rack member is located in a groove in a bore (15) in the tensioner housing (20). The tensioner piston (2) has grooves (11) along its exterior surface that correspond to the wedges or grooves (12) in the pawl rack member and prevent the piston from being pushed inward. The pawl rack member may be a pair of pawls (150,152) located in the tensioner bore. A flexible tab (13) on the upper portion of the rack member contacts a groove (14) on the upper portion of the piston to retain the piston in place for shipping. A spring band member (33) or circlips (160) surround the pawl rack member to bias it toward the piston.

Description: The present invention relates to a hydraulic chain tensioner as described in the preamble of claims 1 and 12. 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. 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, which comprises the features of the preamble of claims 1 and 12, 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 Ojima, 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, 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 millimetres 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. The present invention is directed to a hydraulic chain tensioner having an extended 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 pressurised fluid. A check valve is provided between the chamber and the source of pressurised fluid to permit 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. According to one aspect of the present invention there is provided a hydraulic tensioner comprising: a housing having a bore; a hollow piston slidably received within the bore, said piston forming a fluid chamber with said bore and having a plurality of grooves formed along the outside of the piston; a pawl rack member positioned radially outward from said piston in a groove in said housing, said pawl rack member having a plurality of stepped wedges constructed and arranged to contact said grooves along the outside of said piston to limit the piston travel in the axial direction, said wedges being normally biased towards said piston by spring means to contact said piston grooves; a piston spring biasing the piston in an outward direction from said bore; and a check valve provided between the chamber and a source of pressurised fluid to permit fluid flow into the chamber while blocking flow in the reverse direction; characterised in that said spring means comprises a generally circular spring band member surrounding said pawl rack member, disposed around a portion of the housing and operably connected to said pawl rack member. The tensioner 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. According to another aspect of the present invention there is provided a hydraulic tensioner comprising: a housing having a bore; a hollow piston slidably received within the bore, said piston forming a fluid chamber with said bore and having a plurality of grooves formed along the outside of the piston; a piston spring biasing the piston in an outward direction from said bore; and a check valve provided between the chamber and a source of pressurised fluid to permit fluid flow into the chamber while blocking flow in the reverse direction ; characterised by a pair of pawls mounted in slots formed in said bore, each of said pawls having a plurality of stepped wedges constructed and arranged to contact said plurality of grooves along the outside of said piston; and at least one circlip surrounding said bore and engaging an outside surface of said pair of pawls, said wedges being normally biased toward said piston by the at least one circlip so as to contact said piston groove, thereby to limit piston travel in the axial direction. The two pawls are preferably 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 preferably 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. In an advantageous development of the present invention, flexible tabs are provided 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. In order that the present invention may be well understood, there will now be described some embodiments thereof, given by way of example, reference being made to the accompanying drawings, in which: FIG. 1 is a front view of the hydraulic tensioner of the present invention; FIG. 2 is a sectional view of the tensioner of FIG 1 taken along the line 2-2; FIG. 3 is a top view of the tensioner of FIG 1; FIG. 4 is a sectional view of the hydraulic tension of FIG. 3 taken along the line 4-4; FIG. 5 is an exploded perspective view of the hydraulic tensioner of FIG. 1; FIG. 6 is a portion of the sectional view of FIG. 2 illustrating the details of the pawl rack member and flexible tab; FIG. 7 is a perspective view of a portion of the pawl member; FIG. 8 is a perspective view of a portion of the pawl member illustrating the side and rack teeth; FIG. 9 is a perspective view of an alternative embodiment illustrating two pawls; FIG. 10 is a side view of another embodiment of the hydraulic tensioner of the present invention; FIG. 11 is a sectional view of the tensioner in FIG. 10 along line A-A; and FIG. 12 is an exploded perspective view of the tensioner of FIGS. 10 and 11. 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 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.

Claims: A hydraulic tensioner (10) comprising: a housing (20) having a bore (15); a hollow piston (2) slidably received within the bore (15), said piston (2) forming a fluid chamber (22) with said bore (15) and having a plurality of grooves (11) formed along the outside of the piston (2); a pawl rack member (3) positioned radially outward from said piston (2) in a groove in said housing (30), said pawl rack member (3) having a plurality of stepped wedges (12) constructed and arranged to contact said grooves (11) along the outside of said piston (2) to limit the piston travel in the axial direction, said wedges (12) being biased towards said piston by spring means to contact said piston grooves (11); a piston spring (4) biasing the piston (2) in an outward direction from said bore (15); and a check valve (21) provided between the chamber (22) and a source of pressurised fluid (25) to permit fluid flow into the chamber (22) while blocking flow in the reverse direction; characterised in that said spring means comprises a generally circular spring band member (33) substantially surrounding said pawl rack member (3), disposed around a portion of the housing (20) and operably connected to said pawl rack member (3). A hydraulic tensioner according to claim 1, wherein said spring band member is a circular spring steel clip (33) which engages in a groove in said housing (20). A hydraulic tensioner according to claim 1 or claim 2, wherein said piston includes a stepped groove (37) for contacting said wedges (12) to provide a position of maximum outward axial travel of said piston (2). A hydraulic tensioner according to any of the preceding claims, wherein said piston grooves (11) comprise a plurality of wedges. A hydraulic tensioner according to any of the preceding claims, wherein axial movement of said pawl rack member (3) is limited by said housing groove. A hydraulic tensioner according to any of the preceding claims, wherein said rack pawl member (3) includes at least one flexible retention tab (13) for holding said piston (2) at an innermost axial position. A hydraulic tensioner according to claim 6, wherein said flexible retention tab (13) is normally biased in a direction radially outward from said piston (2). A hydraulic tensioner according to claim 6 or claim 7, wherein said flexible retention tab (13) cooperates with a retention groove (14) at the uppermost portion of said piston (2) to retain said piston (2) in said innermost position. A hydraulic tensioner according to any of claims 6 to 8, wherein said flexible tab (13) extends radially inward from said pawl rack member (3). A hydraulic tensioner according to any of claims 6 to 8, wherein said flexible tab (13) extends axially outward from said pawl rack member (3). A hydraulic tensioner according to any of the preceding claims, wherein said pawl rack member (3) includes a plurality of pawl members mounted within said groove. A hydraulic tensioner comprising: a housing (170) having a bore (120); a hollow piston (102) slidably received within the bore (120), said piston (102) forming a fluid chamber with said bore (120) and having a plurality of grooves (111) formed along the outside of the piston (102); a piston spring (104) biasing the piston (102) in an outward direction from said bore (120); and a check valve (121) provided between the chamber and a source of pressurised fluid (125) to permit fluid flow into the chamber while blocking flow in the reverse direction; characterised by a pair of pawls (150,152) mounted in slots formed in said bore, each of said pawls (150,152) having a plurality of stepped wedges constructed and arranged to contact said plurality of grooves (111) along the outside of said piston (102); and at least one circlip surrounding said bore and engaging an outside surface of said pair of pawls, said wedges being normally biased toward said piston (102) by the at least one circlip (160) so as to contact said piston groove (111), thereby to limit piston travel in the axial direction. A hydraulic tensioner according to claim 12, wherein a plurality of circlips (160) surround said bore and engage the outside surface of said pair of pawls (150, 152). A hydraulic tensioner according to any of the claims 12 or claim 13, wherein said pair of pawls (150,152) are located opposite one another by a half pitch. A hydraulic tensioner according to any of claims 12 to claim 14; wherein said pair of pawls (150,152) are located opposite one another in said bore(120). A hydraulic tensioner according to any of claims 12 to 15, wherein said plurality of grooves (111) of said piston (102) and said plurality of stepped wedges of said pair of pawls (150, 152) have a pitch of 1.5 mm.

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