Hydraulic tensioner with force limiting tensioner spring

Abstract: A hydraulic tensioner (10) having a pre-loaded spring (50) at the upper end of the piston (20) to reduce the peak operating loads in a timing chain system. The spring member (50) is located between the primary piston (20) and an upper secondary piston (62).

Description: The present invention is directed to a hydraulic tensioner having a pre-loaded spring, or force limiting spring, contacting the upper end of the piston to reduce the peak operating loads in a timing chain system. Hydraulic tensioners are typically used as a control device for a chain drive system in an automobile timing system. Generally, it is important to impart and maintain a certain degree of tension to the chain to prevent noises or slippage. Prevention of slippage is especially important in the case of a chain driven camshaft in an internal combustion engine because slippage may alter the camshaft timing by several degrees, possibly causing damage. The tension in the chain may vary greatly due to the wide variation in the temperature and the linear thermal expansion among the various parts of the engine. Camshaft and crankshaft induced torsional vibrations cause chain tension to vary considerably. This tension variation results in chain elongation. 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. A typical hydraulic tensioner is comprised of a housing having a bore, a fluid chamber defined by the bore, and a hollow piston biased in a protruding direction from the bore by a spring. A check valve is also included in the hydraulic tensioner to permit fluid flow from a source of pressurized fluid 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 force of the spring in an outward direction. When the piston tends to move in the reverse direction, the check valve closes to restrict outflow of fluid from the chamber. Only a small clearance between the piston and the housing wall permits some fluid to escape, thereby allowing the piston to retract. In such a fashion, the tensioner achieves a so-called no-return function. However, a potential problem with hydraulic tensioners is that the no-return function may present difficulties in accommodating tension spikes or surges in the chain. When a timing device operates at its resonant frequency, the chain load increases significantly. The small clearance between the piston and the housing wall is not sufficient to quickly release the hydraulic fluid in the chamber to accommodate the sudden overload of the chain. One example of a tensioner that addresses this problem, is described in Suzuki, US-A-4 881 927, upon which the precharacterising clause of claim 1 is based. Suzuki discloses a hydraulic ball-type check valve tensioner having a piston slidably fitted into a chamber and biased by a spring in a protruding direction. This tensioner includes a relief valve having a sleeve slidably fitted in an auxiliary chamber in communication with the first chamber, with a spring biasing the sleeve into a depressed position to block a discharge part. Oil in the first chamber flows into the auxiliary chamber to force the sleeve against the biasing spring action to unblock the discharge port. Unfortunately, this relief valve may be slow to open and close due to high mass and subject to variable friction between the sleeve and auxiliary chamber wall. This may vary the pressure at which the relief valve operates. Another example of a hydraulic tensioner that addresses the problem of overload is described in Mittermeier, US-A-4,507,103. Mittermeier discloses a hydraulic tensioner having a piston slidably fitted into a chamber and biased by a spring in a protruding direction. This tensioner includes a relief valve in a bore at the protruding end of the piston. This relief valve is a spring-biased ball type valve with the spring set against a threaded throttle plug capping the bore. Oil in the first chamber forces the ball open, upon reaching a set high pressure, and flows into the bore past the throttle plug threads to the atmosphere. Unfortunately, this relief valve may be slow to release large displacements of oil because of the restricted path past the threads and resultant back-pressure build up against the ball. Accordingly, it is an object of the present invention to provide a tensioner, which can maintain a substantially constant tensioning force. A pre-loaded spring device eliminates the need for an expensive pressure relief valve. A pre-loaded spring device also eliminates expensive wear face materials used in timing systems and especially in extraordinary strong timing chains used in systems with high resonance loads. The present invention is additionally directed to a hydraulic tensioner having a cushioning device. This device further reduces the peak operating loads in a timing chain system. That is, the pre-loaded spring provides a mechanical force limiting system to permit inward movement of an auxiliary portion of the piston under peak force conditions and the cushioning device limits the travel of the force limiting piston. According to the present invention there is provided a hydraulic tensioner comprising: a housing having a bore; a primary piston slidably received within said bore, said primary piston having an opening at a lower end, said lower end opening defining a fluid chamber with said bore; a first spring member located within said bore, said first spring member biasing said primary piston in an upward protruding direction from said bore; a check valve provided between the fluid chamber and a source of pressurized fluid to permit fluid flow from said fluid source into the chamber while blocking flow in the reverse direction; and a passage in the housing to connect the fluid chamber with the source of pressurized fluid; characterized in that a secondary piston is guided for axial movement with respect to the upper end of the primary piston, a restriction member limiting said axial movement of said secondary piston in an upward direction; a second spring member is located between said primary piston and said secondary piston, said second spring member biasing said secondary piston upward in an axial direction and away from said primary piston; and a cushioning member is located between said secondary piston and said primary piston. A hydraulic tensioner in accordance with the invention has the advantage that as the chain slackens and tightens, the piston protrudes or resists inward movement in response to the vibration of the chain. The outward force of the spring and fluid on the piston balances the inward force of the chain on the piston. The check valve blocks outflow of fluid when a load is applied to the piston by the chain in a direction opposite to the biasing direction, thereby increasing the fluid pressure in the chamber. At a predetermined reaction load, the nose piece moves against the pre-loaded spring to disrupt the chain system resonance. In one embodiment, the secondary piston, or force limiting piston, slides within the upper portion of the primary piston. A pair of springs are interposed between the primary piston and the upper end of the force limiting piston. One of these springs is a force limiting spring, while the second of these springs is the cushioning member which limits the travel of the force limiting piston. At a predetermined reaction load, the force limiting spring compresses, which absorbs the reaction loads that exceed the predetermined maximum reaction load. As the limit of the force limit spring is approached, the resistance of the larger cushioning member dominates or operates to limit the travel of the force limit piston. In a second embodiment of the present invention, the cushioning member is an elastomeric disc located between the second spring member and the primary piston, which compresses when the second spring member has retracted a predetermined amount. Preferably a tensioner nose piece is held to the main piston by a retention clip. The pre-loading spring is interposed between the piston and the nose piece. Screw threads may also be used in lieu of a retention clip. Alternatively, the end of the piston can be fixed or staked to the nose piece. In order that the 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 cross-sectional view of an example of a hydraulic tensioner not forming part of the present invention; FIG. 2 is a cross-sectional view of the hydraulic tensioner with a spring assembly according to the present invention; FIG. 3 is a cross-sectional view of another embodiment of the tensioner of the present invention in which a pair of springs are interposed between a primary piston and a second, force limiting piston; FIG. 4 is a schematic graphical representation of the expected effect of the force limiting spring and the cushion spring on the relationship between force and displacement in the embodiment of Figure 3; and FIG. 5 is another example of a hydraulic tensioner not forming part of the present invention in which a retention clip holds the nose piece to the primary piston. FIG. 1 illustrates an example of hydraulic tensioner 10 incorporating a pre-loaded spring system. The tensioner 10 includes a housing 25 having a bore 23 in its center, which is filled with fluid through a passageway 24 from a pressurized fluid source (not shown). The fluid source may be an oil pump or a reservoir. The housing 25 receives a piston 20, having an upper end 30 with a cylindrical aperture 32, and a lower, hollow end that forms the fluid chamber 22 with the bore 23. A nose piece 60, having a cylindrical bottom 61 and wider elongated top 62, fits into the aperture 32 of the piston. The top 62 of the nose piece contacts a tensioner arm or chain to provide tension along a chain strand. The nose piece 60 is connected to the piston 20 with a retaining pin 70 that fits into a slot 71. The slot extends through the piston 20 and through the bottom 61 of the nose piece in order to allow connection of the piston to the nose piece by the pin 70. The slot 71 is larger than the retaining pin 70 so that the piston 20 can move axially with respect to the top 62 of the nose piece. A first spring 40 contacts the piston 20 to bias the piston 20 in a protruding or outward direction. A second spring 50 is placed between the upper end 30 of the piston and the bottom of the flat portion 60 of the nose piece. The second spring 50 surrounds the cylindrical bottom 61 of the nose piece and biases the nose piece 60 in a protruding or outward direction. As shown in FIG. 2, a disc 135 may be provided between the second spring 50 and the upper end 30 of the piston. The spring 50 may be compression coil springs or stacked Belleville washers, or the like, which provide a known relationship between force and displacement. Disc 135 may be elastomeric acting as a cushion. Upward movement of the nose piece with respect to the piston 20 is restricted by the pin 70 in the slot 71. A check valve 100 is provided between the chamber 22 and the passageway 24 as the source of fluid pressure to permit fluid flow into the chamber 22, while blocking fluid flow in the reverse direction. The check valve 100 includes a ball 102 and spring 104 within the check valve that biases the ball toward a ball seat 106. The check valve 100 may be a variable orifice check valve as shown and described in US-A-5,259,820 and US-A-5,277,664. During start-up, fluid enters through passageway 24 and opens the check valve 100 such that fluid flows into chamber 22. As fluid fills the chamber 22, the piston 20 moves outward by the force of the pressurized fluid and the spring 40. The piston 20 continues to move outward until the inward force of the chain on the piston 20 balances the outward force of the spring 40 and the pressurized fluid in the chamber 22. At this point, the check valve 100 closes and prevents further fluid flow. When a chain slackens due to wear or load fluctuations, the piston 20 moves outward again and the check valve 100 reopens to allow more fluid into the chamber 22. The piston 20 moves independently in conjunction with the force of the fluid and the spring 40. The second spring 50, which is preloaded in compression between the piston 20 and nose piece 60, acts as a high force relief system. When the tension from the chain increases and forces the nose piece 60 downward against the piston 20, the fluid pressure in the fluid chamber 22 correspondingly increases. This increased pressure against the check valve 100 prevents fluid flow back through the valve 100. As the pressure in the fluid chamber 22 increases beyond a predetermined maximum value, the force against the second spring 50 will overcome the second spring 50 bias and force the second spring 50 to further compress. The nose piece 60 will move inward with respect to the piston 20 as the second spring 50 compresses. As the nose piece 60 moves inward, excess force beyond the maximum allowed will be accommodated by the system. FIG. 3 illustrates another embodiment of the tensioner of the present invention. In this embodiment, a force limit piston 160 slides within the upper end of the primary piston 120. The primary piston includes a flat internal member 122 at its upper portion, which acts to retain a tortuous path vent 164. The primary piston 120 has an upwardly extending portion 166 that extends beyond the flat internal member 122. The outer edge of the force limit piston 160 slides within the inner edge of the upwardly extending portion 166. A retention clip 168 retains the force limit piston within the primary piston upwardly extending portion and provides an upper limit on travel of the force retention piston. The retention clip is preferably in the form of an expansible clip member concentrically mounted between the secondary piston and the primary piston. The primary or main piston 120 is biased in an outward direction by primary or main spring 140. The main piston 120 forms fluid chamber 123 between the lower portion of the piston and the bore of the housing 125. Fluid is permitted to enter the fluid chamber 123 through check valve 202, which is sealed against the housing by check valve seal 200. Air may vent from the fluid in chamber 123 by passing through tortuous path vent 164. However, fluid does not fill the inside of the secondary piston 160. The secondary or force limit piston 160 is biased outward from the lower or primary piston by a spring 150. Force limit spring 150 provides a resistance force against the inward movement of secondary piston 160. A second, cushion spring 151, with a higher stiffness rate than the force limit spring 150, is nested concentrically inside spring 150. A gap between spring 151 and piston 160 is provided so that spring 151 is not acted upon until the limit of the preload spring 150 has been exceeded. The two springs 150, 151 are shown as concentric coil springs. However, other combinations of resistance members, such as Belleville springs, are possible. The cushion spring 151 is intended to provide a resistance to movement after the compression resistance limit of the force limit spring has been exceeded. However, in practice, the two springs may overlap in providing compressive resistance to inward movement of the force limit spring. The embodiment of FIG. 3 operates in generally the same manner as the embodiments of FIGS. 1 and 2. The expected relationship between force and piston displacement is shown in the graph of FIG. 4. In the graph, the first portion 300 of the curve shows the displacement of the tensioner being resisted by the increasing force of the main piston spring and the hydraulic fluid within the main piston. It is understood that, after the spring has caused the piston to move outward and the chamber fills with hydraulic fluid, the combination of resistance force from the hydraulic fluid and compression spring 140 will resist displacement of the tensioner arm in a relationship that is generally linear and shown in the graph portion 300. At some point, the main spring 140 and hydraulic fluid in chamber 123 will have met the pre-load force of the second spring, which is shown as point 301 on the graph of Figure 4. In the second portion of the curve 302, the force limit spring 150 operates to provide a force limiting mechanism, or mechanism to permit further inward movement of the tensioner arm in response to increased load. In the second portion 302 of the curve, a load or force peak is absorbed by the force limit spring inside the force limit piston. This spring permits displacement of the force limit piston with respect to the main or primary piston to absorb the additional force requirements. At a point 303 on the curve, the force limiting piston has moved into contact with the cushion spring. In portion 304, the cushion spring 151 provides additional resistance, which provides some additional cushion of the force limit piston. In this embodiment, the force limit piston is providing a force relief function. In other words, once the pre-determined resistance force of the hydraulic fluid in the main chamber and the resistance force of the main spring have been reached and exceeded, the force limit piston provides a further displacement, or force relief to limit the force on the chain system. The cushion spring may be replaced by an elastomeric bumper or cushion. In this embodiment, the elastomeric bumper will be compressed when the force limiting spring and piston contract. FIG. 5 illustrates another example of a hydraulic tensioner not forming part of the present invention. In this example, the tensioner nose piece 260 is held to the main piston by a retention clip 270. A preloaded spring or set of springs 250 is placed between the nose piece 260 and an upper portion 230 of the main piston 220. The retention clip 270 holds the nose piece to the main piston and provides the force to preload the spring. Instead of a retention clip, the nose piece may be held to the piston by screw threads. Alternatively, the piston may be staked to capture the nose piece. This example operates in a similar manner to the earlier embodiments.

Claims: A hydraulic tensioner (10) comprising: a housing (25) having a bore (23); a primary piston (20) slidably received within said bore (23), said primary piston (20) having an opening at a lower end, said lower end opening defining a fluid chamber (22) with said bore (23); a first spring member (40) located within said bore (23), said first spring member (40) biasing said primary piston (20) in an upward protruding direction from said bore (23); a check valve (100) provided between the fluid chamber (22) and a source of pressurized fluid to permit fluid flow from said fluid source into the chamber (22) while blocking flow in the reverse direction; and a passage (24) in the housing (25) to connect the fluid chamber (22) with the source of pressurized fluid; characterized in that a secondary piston (60) is guided for axial movement with respect to the upper end of the primary piston (20), a restriction member (70) limiting said axial movement of said secondary piston (60) in an upward direction; a second spring member (50) is located between said primary piston (20) and said secondary piston (60), said second spring member (50) biasing said secondary piston (60) upward in an axial direction and away from said primary piston (20); and a cushioning member (135,151) is located between said secondary piston (60) and said primary piston (20). A hydraulic tensioner as claimed in claim 1, wherein said second spring member (150) is compressed a predetermined distance when interposed between said secondary piston (160) and said primary piston (120), said secondary spring member (150) being further compressed when the force against said secondary piston (160) exceeds a predetermined amount. A hydraulic tensioner as claimed in claim 2, wherein said cushioning member (151) is compressed during at least a portion of said further compression of said second spring member (150). A hydraulic tensioner as claimed in any of the preceding claims, wherein said restriction member is an expansible clip (270) concentrically mounted between said secondary piston (260) and said primary piston (220). A hydraulic tensioner as claimed in any of the preceding claims, wherein said secondary spring member (250) is concentrically mounted about an upper portion of the primary piston (220). A hydraulic tensioner as claimed in any of the preceding claims, wherein said secondary piston (60) includes a nose piece forming a cylindrical base portion (61) of said secondary piston (60), said secondary spring (50) surrounding said cylindrical base portion. A hydraulic tensioner as claimed in any of the preceding claims, wherein said cushioning member is a third spring member (151) located between said secondary piston (160) and said primary piston (120). A hydraulic tensioner as claimed in claim 7, wherein said third spring member is an elastomeric pack. A hydraulic tensioner as claimed in claim 7, wherein said second spring member (150) and said third spring member (151) are coil springs, said third spring member (151) being nested within said second spring member (150). A hydraulic tensioner as claimed in any of claims 1 to 8, wherein said second spring member (50;150) is comprised of a plurality of Belleville washers. A hydraulic tensioner as claimed in any of claims 1 to 8, wherein said second spring member (50;150) is a coil spring. A hydraulic tensioner as claimed in any of claims 1 to 6, wherein the cushioning member is an elastomeric disc. A hydraulic tensioner as claimed in claim 12, wherein said elastomeric disc (135) is located between said second spring member (50) and upper end of said primary piston (20). A hydraulic tensioner as claimed in claim 12 or claim 13, wherein said elastomeric disc (135) surrounds a cylindrical base portion (61) of said secondary piston (60).

Inventors: MARKLEY GEORGE L, SIMPSON ROGER, WIGSTEN MARK M, MARKLEY, GEORGE L., SIMPSON, ROGER, WIGSTEN, MARK M.
Applicants: BORG WARNER INC, BORGWARNER INC.
Application year: 1998
Main classification: F16H7
IPC codes: F16H7