Chain tensioner

A chain tensioner includes a sleeve fixedly disposed in a cylinder, with a first end of the sleeve inserted in a plunger and a second end of the sleeve protruding out of the plunger. A cylindrical space is defined between an outer periphery of the sleeve and an inner periphery of the cylinder. An oil supply passage opens to the cylindrical space. A communication passage is defined in the sleeve and provides communication between the cylindrical space and a reservoir chamber.

TECHNICAL FIELD

The present invention relates to chain tensioners used to maintain tensions of mainly chains for driving cams of automotive engines, and chains for driving oil pumps.

BACKGROUND ART

Chain transmission systems used for engines of, e.g., automobiles are used, for example, to transmit the rotation of crankshaft to camshafts; to transmit the rotation of a crankshaft to engine accessories such as an oil pump, a water pump and a supercharger; to transmit the rotation of a crankshaft to a balancer shaft; and to couple together intake cams and exhaust cams of a twin-cam engine. Chain tensioners are used to keep the tension in the chain of such a chain transmission system within a proper range.

The inventor of the present application proposed a chain tensioner used for the above-described purposes in JP 2010-286090 A. The chain tensioner disclosed in JP 2010-286090 A includes a tubular cylinder having an open end and a closed end; a tubular cylinder axially slidably supported within the inner periphery of the cylinder; a return spring biasing the plunger in the direction in which the plunger protrudes out of the cylinder; a sleeve fixed in position within the cylinder with one end thereof inserted in the plunger and the other end protruding out of the plunger, a check valve disposed at the end of the sleeve inserted in the plunger; a pressure chamber defined within the plunger such that its volume changes with the axial movement of the plunger; and a leakage gap defined between the outer periphery of the sleeve and the inner periphery of the plunger. The cylinder has an oil supply passage for introducing hydraulic oil from outside the cylinder into the cylinder. The end of the oil supply passage located at the inside of the cylinder opens to a space inside of the sleeve.

The chain tensioner of JP 2010-286090 A is configured such that when the tension in the chain increases while the engine is running, the plunger moves in the direction in which the plunger is pushed into the cylinder (this direction is hereinafter referred to as the “pushed-in direction) by the tension in the chain, thereby absorbing the tension in the chain. At that time, due to the viscous resistance of the hydraulic oil flowing out from the pressure chamber through the leakage gap, a damping force is produced, allowing the plunger to move slowly. On the other hand, when the tension in the chain decreases while the engine is running, the plunger moves in the direction in which the plunger protrudes out of the cylinder (this direction is hereinafter referred to as the “protruding direction) due to the biasing force of the return spring, thereby taking up the slack in the chain. At that time, the check valve opens, allowing hydraulic oil to flow through the oil supply passage into the pressure chamber, so that the plunger moves rapidly.

Since the chain tensioner of JP 2010-286090 A is structured such that hydraulic oil in the plunger flows through the leakage gap between the inner periphery of the plunger and the outer periphery of the sleeve, and flows out to the end of plunger inserted in the cylinder, if the chain tensioner is used with the protruding direction of the plunger being an obliquely downward direction, air is less likely to enter the pressure chamber in the plunger, and also, any air that may enter the pressure chamber can be effectively discharged through the leakage gap.

SUMMARY OF THE INVENTION

Object of the Invention

The inventor of the present application prepared, and assessed the performance of, a chain tensioner as disclosed in JP 2010-286090 A, and discovered that this chain tensioner is poor in followability to the slack in the chain. Then, upon investigation, the inventor discovered that the followability is poor because: between the outer periphery of the sleeve and the inner periphery of the cylinder, a cylindrical space is present of which the volume changes with the axial movement of the plunger; when the plunger moves in the protruding direction, and as a result, the volume of the cylindrical space increases, the pressure in the cylindrical space decreases because no hydraulic oil or air is supplied into the cylindrical space; and the decreased pressure in the cylindrical space interferes with the movement of the plunger in the protruding direction.

Also, with the recent trend of using smaller oil pumps in automobiles, the need to reduce the oil consumption by a chain tensioner is increasing.

An object of the present invention is to provide a chain tension which shows excellent followability to the slack of the chain and consumes less oil.

Means for Achieving the Object

In order to achieve this object, the present invention provides a chain tensioner comprising:

a tubular cylinder having an open end and a closed end;

a tubular plunger axially slidably supported by an inner periphery of the cylinder, the plunger having an open end inserted in the cylinder, and a closed end protruding out of the cylinder;

a return spring biasing the plunger in the direction in which the plunger protrudes out of the cylinder;

a sleeve fixedly disposed in the cylinder, with one end of the sleeve inserted in the plunger and the other end of the sleeve protruding out of the plunger;

a check valve disposed at the end of the sleeve inserted in the plunger, the check valve being configured to allow only a flow of hydraulic oil from inside the sleeve toward the outside of the sleeve;

a pressure chamber defined in the plunger such that the volume of the pressure chamber changes with axial movement of the plunger;

a leakage gap defined between an outer periphery of the sleeve and an inner periphery of the plunger such that, when the volume of the pressure chamber decreases, hydraulic oil leaks from the pressure chamber through the leakage gap;

a cylindrical space formed between the outer periphery of the sleeve and the inner periphery of the cylinder, the cylindrical space being defined by the plunger such that the volume of the cylindrical space changes with the axial movement of the plunger;

an oil supply passage formed in the cylinder so as to introduce hydraulic oil from outside the cylinder to inside the cylinder, the oil supply passage opening to the cylindrical space; and

a communication passage defined in the sleeve and providing communication between the cylindrical space and the interior of the sleeve.

In this arrangement, when the plunger moves in the protruding direction, and the volume of the cylindrical space increases, hydraulic oil directly flows into the cylindrical space from the oil supply passage, so that the pressure in the cylindrical space is less likely to drop. This allows smooth movement of the plunger in the protruding direction, and thus provides improved followability of the plunger to the slack in the chain. When the plunger is moved in the pushed-in direction, thereby reducing the volume of the pressure chamber, hydraulic oil leaks from the pressure chamber, through the leakage gap, and into the cylindrical space. From the cylindrical space, hydraulic oil flows through the communication passage and returns to the interior of the sleeve. This reduces the amount of hydraulic oil that is discharged from the chain tensioner to the outside thereof, by an amount equal to the amount of hydraulic oil returned to the interior of the sleeve, which in turn reduces the consumption of oil by the chain tensioner.

Preferably, a seating surface to be fixed to a vertically extending engine wall surface is formed on the outer periphery of the cylinder, and the communication passage is disposed such that, with the seating surface fixed to the engine wall surface, the communication passage is located at the upper half portion of the entire circumference of the sleeve.

In this arrangement, any air present in the sleeve can be smoothly discharged through the communication passage. Further, immediately after the engine is started, and while hydraulic oil has not yet been supplied to the chain tensioner, hydraulic oil retained in the sleeve flows into the pressure chamber, thus producing a damping force. In this regard, because the communication passage is disposed at the upper half of the entire circumference of the sleeve, it is possible to retain a large amount of hydraulic oil in the sleeve.

The end of the sleeve protruding out of the plunger may be press-fitted into a sleeve fitting recess formed in the closed end of the cylinder such that, due to the press-fitting, the sleeve is fixed in position.

In this arrangement, because the sleeve is fixed in position by press-fitting, even when the tension in the chain changes sharply, it is possible to prevent axial movement of the sleeve, thereby producing a stable damping force.

The sleeve may be pressed by one end of the return spring toward the closed end of the cylinder such that, due to the pressing force of the return spring, the sleeve is fixed in position.

In this arrangement, the chain tensioner can be manufactured at a lower cost than when the sleeve is fixed in position, e.g., by press-fitting.

Preferably, a reservoir chamber is defined in the interior of the sleeve, the reservoir chamber having a diameter larger than the diameter of the valve hole of the check valve.

In this arrangement, any air that may be mixed into hydraulic oil supplied from an oil pump is less likely to enter the pressure chamber, so that the chain tensioner can produce a stable damping force. Further, immediately after the engine is started, and while hydraulic oil has not yet been supplied to the chain tensioner, hydraulic oil retained in the sleeve flows into the pressure chamber, thus producing a damping force. In this regard, because the reservoir chamber has a diameter larger than the diameter of the valve hole of the check valve, it is possible to retain a large amount of hydraulic oil in the sleeve.

Preferably, the sleeve includes a large outer diameter portion defining the leakage gap between the large outer diameter portion and the inner periphery of the plunger, and a small outer diameter portion continuously connected, via a step, to the end of the large outer diameter portion closest to the end of the sleeve protruding out of the plunger, with the large outer diameter portion entirely received in the plunger.

In this arrangement, regardless of the axial position of the plunger, the length of the large outer diameter portion is defined as the axial length of the leakage gap between the outer periphery of the sleeve and the inner periphery of the plunger. This means that even when the plunger moves in the axial direction, the axial length of the leakage gap remains unchanged. The chain tensioner therefore provides a uniform damping force irrespective of the axial position of the plunger.

Preferably, a seal member is disposed between the sliding surfaces of the plunger and the cylinder to prevent leakage of hydraulic oil from the cylindrical space to the outside of the cylinder.

In this arrangement, because the seal member between the sliding surfaces of the plunger and the cylinder prevents leakage of hydraulic oil from the cylindrical space through the gap between sliding surfaces of the plunger and the cylinder to the outside of the cylinder, it is possible to effectively reduce the consumption of oil by the chain tensioner.

The communication passage may be a through hole extending radially through the sleeve at a location between the leakage gap and the end of the sleeve protruding out of the plunger.

A helical groove may be formed in the outer periphery of the sleeve to provide communication between the pressure chamber and the cylindrical space.

This arrangement allows accurate control of the magnitude of the damping force by changing the depth and/or lead angle of the helical groove.

The chain tensioner may further comprise:

a plurality of circumferential grooves disposed on the outer periphery of the plunger so as to be axially adjacent to each other;

a ring receiving groove formed in the inner periphery of the cylinder; and

a register ring received in the ring receiving groove, and configured to be elastically squeezed around any of the circumferential groove;

wherein each of the circumferential grooves includes:a tapered surface configured such that, when a load is applied to the plunger in the direction to protrude the plunger out of the cylinder, the tapered surface allows movement of the plunger while radially expanding the register ring; anda stopper surface configured such that, when a load is applied to the plunger in a direction to push the plunger into the cylinder, the stopper surface engages the register ring, thereby restricting the movement of the plunger.

Advantages of the Invention

The chain tensioner according to the present invention is structured such that, when the plunger moves in the protruding direction, and as a result, the volume of the cylindrical space increases, hydraulic oil flows from the oil supply passage directly into the cylindrical space. Thus, the pressure in the cylindrical space is less likely to drop, so that the plunger moves smoothly in the protruding direction. This chain tensioner therefore shows excellent followability to the slack in the chain. When the plunger moves in the pushed-in direction, and as a result, the volume of the pressure chamber decreases, hydraulic oil leaks from the pressure chamber, through the leakage gap, and into the cylindrical space. Then, from the cylindrical space, hydraulic oil flows through the communication passage and returns to the interior of the sleeve. This reduces the amount of hydraulic oil that is discharged from the chain tensioner to the outside thereof, by an amount equal to the amount of hydraulic oil returned to the interior of the sleeve, which in turn reduces the consumption of oil by the chain tensioner.

EMBODIMENTS

FIG. 1shows a chain transmission system including the chain tensioner1according to the first embodiment of the present invention. This chain transmission system includes a sprocket3fixed to a crankshaft2of an engine, sprockets5fixed to camshafts4, and a chain6coupling the sprockets3and5together to transmit the rotation of the crankshaft to the camshafts4, thereby opening and closing valves (not shown) of combustion chambers by the rotation of the camshafts4.

While the engine is running, the crankshaft2always rotates in the same direction (clockwise direction in the example ofFIG. 1), and while the engine is rotating in the clockwise direction, the section of the chain6moving toward the sprocket3(right-hand side of the chain inFIG. 1) becomes the tight side, while the section of the chain6moving away from the sprocket3(left-hand side inFIG. 1) becomes the slack side. A chain guide8is in contact with the slack side of the chain6while being pivotally supported about a pivot shaft7. The chain tensioner1presses the chain6via the chain guide8.

As shown inFIG. 2, the chain tensioner1includes a tubular cylinder9having an open end and a closed end, and a plunger10axially slidably supported by the inner periphery of the cylinder9. The plunger10has a protruding end protruding out of the cylinder9and pressing the chain guide8.

The cylinder9is a one-piece member made of a metal (such as an aluminum alloy). The cylinder9has a plurality of mounting pieces11integrally formed on the outer periphery of the cylinder9, and is fixed to an engine wall surface13(shown inFIG. 3) by inserting bolts12through the mounting pieces11and tightening the bolts12. A flat seating surface14is formed on the outer periphery of the cylinder9so as to extend parallel to the center axis of the cylinder9. The seating surface14is fixedly attached to the engine wall surface13, which extends in the vertical direction. The cylinder9is mounted to the engine wall surface13such that the protruding direction, i.e., the direction in which the plunger10protrudes out of the cylinder9, is an obliquely downward direction. The engine wall surface13is a side surface of an engine block15.

The plunger10is a tubular member having an open end inserted in the cylinder9with the protruding end, i.e., the end of the plunger9protruding out of the cylinder9, closed. The plunger10is made of an iron-based material (e.g., a steel material such as “SCM” or “SCr”). The outer periphery of the plunger10is a cylindrical surface, and the inner periphery of the cylinder9is also a cylindrical surface. The gap between the outer periphery of the plunger10and the inner periphery of the cylinder9is minute. Specifically, the difference in radius between the outer periphery of the plunger10and the inner periphery of the cylinder9is adjusted within the range of 0.015-0.080 mm.

A sleeve16is disposed in the cylinder9with one end of the sleeve16inserted in the plunger10and the other end protruding out of the plunger10. The other end of the sleeve16, i.e., its end protruding out of the plunger10, is press-fitted into a sleeve fitting recess17formed in the closed end of the cylinder9, and due to this press fitting, the sleeve16is fixed in position. As with the plunger10, the sleeve16is also made of an iron-based material (such as SCr or SCM material).

The inner periphery of the plunger10is axially slidably fitted to the outer periphery of the sleeve16, and due to this fitting, a pressure chamber18is defined in the plunger10. The volume of the pressure chamber18increases when the plunger10moves in the protruding direction, and decreases when the plunger10moves in the pushed-in direction. Between the outer periphery of the sleeve16and the inner periphery of the plunger10, a leakage gap19is defined through which hydraulic oil in the pressure chamber18leaks when the volume of the pressure chamber18decreases. The leakage gap19is a minute cylindrical gap having a radial width within the range of 0.015-0.080 mm.

The sleeve16has a large outer diameter portion20which defines the leakage gap19between the portion20and the inner periphery of the plunger10; and a small outer diameter portion21continuously connected, via a step, to the end of the large outer diameter portion20closest to the end of the sleeve16protruding out of the plunger10. The large outer diameter portion20of the sleeve16is entirely received in the plunger10, so that the axial length of the leakage gap19never changes when the plunger10moves in the axial direction according to fluctuations in tension in the chain6.

A check valve22is disposed at the end of the sleeve16inserted in the plunger10, and allows only a flow of hydraulic oil from inside the sleeve16to outside the sleeve16. The check valve22comprises a valve seat23at the end of the sleeve16inserted in the plunger10; a valve hole24extending through the valve seat23in the axial direction of the sleeve16; a spherical valve element25for opening and closing the valve hole24from the side of the pressure chamber18; and a retainer26restricting the moving range of the valve element25. The sleeve16defines therein a reservoir chamber27larger in diameter than the valve hole24of the check valve22and located upstream of the check valve22. The check valve22restricts a flow of hydraulic oil from the pressure chamber18toward the reservoir chamber27, and allows only a flow of hydraulic oil from the reservoir chamber27toward the pressure chamber18.

A return spring28is mounted in the pressure chamber18. The return spring28has one end thereof supported by the sleeve16through the check valve22, and the other end thereof axially presses the plunger10, thereby biasing the plunger10in the direction in which the plunger10protrudes out of the cylinder9.

Between the outer periphery of the sleeve16and the inner periphery of the cylinder9, a cylindrical space29is defined by the plunger10such that the volume of the cylindrical space29changes with axial movement of the plunger10. In the example shown, the cylindrical space29is defined radially between the outer periphery of the small outer diameter portion21of the sleeve16and the inner periphery of the cylinder9such that the volume of the cylindrical space29changes with a change in the length of the portion of the plunger10that is inserted into this space.

The sleeve16has a communication passage30through which the cylindrical space29communicates with the reservoir chamber27. The communication passage30is a through hole extending radially through the sleeve16at a location between the leakage gap19and the end of the sleeve16protruding out of the plunger10. The communication passage30is disposed such that, with the seating surface14of the cylinder9fixed to the engine wall surface13, the communication passage30is located at the upper half of the entire circumference of the sleeve16. Specifically, the communication passage30is disposed at the radially upper portion of the sleeve16within the range equivalent to half the circumference of the sleeve16. As used herein, the term the “radially upper portion” refers to the portion of the outer periphery of the sleeve16located, with the chain tensioner1mounted to the engine, above the center axis of the sleeve16. In the example shown, the communication passage30is provided so as to be disposed, with the chain tensioner1mounted to the engine, at the apex of the outer periphery of the sleeve16.

As shown inFIG. 3, the cylinder9has an oil supply passage31for introducing hydraulic oil from outside the cylinder9into the cylinder9. The oil supply passage31is a through hole extending radially through the cylinder9. The oil supply passage31has an oil inlet which is open at the seating surface14on the outer periphery of the cylinder9so that the oil inlet is connectable to an oil hole32formed in the engine block15. The oil outlet of the oil supply passage31is open at a cylindrical surface on the inner periphery of the cylinder9(which defines the outer perimeter of the cylindrical space29). The oil supply passage31introduces hydraulic oil supplied from an oil pump (not shown) through the oil hole32into the cylindrical space29.

An exemplary operation of the chain tensioner1is now described.

When the tension in the chain6increases while the engine is running, the plunger10is moved by the tension in the chain6in the direction in which the plunger10is pushed into the cylinder9, thus reducing the tension in the chain6. As the plunger10is moved in this direction, the volume of the pressure chamber18decreases according to the amount of movement of the plunger10, so that hydraulic oil flows out of the pressure chamber18by an amount corresponding to the reduced volume of the pressure chamber18, and this hydraulic oil then flows through the leakage gap19and through the cylindrical space29. While flowing through the leakage gap19, the hydraulic oil produces a damping force due to its viscous resistance, so that the plunger10moves slowly. As shown inFIG. 4, the hydraulic oil that has flowed through the leakage gap19flows through the cylindrical space29, and then partially flows through the communication passage30and returns to the reservoir chamber27. Thus, the amount of hydraulic oil that flows through the gap between the sliding surfaces of the plunger10and the cylinder9and out of the cylinder9decreases by an amount equal to the amount of hydraulic oil returned to the reservoir chamber27.

On the other hand, when the tension in the chain6decreases while the engine is running, the plunger10moves in the protruding direction due to the biasing force of the return spring28, thus removing the looseness of the chain6. As the plunger10moves in this direction, the volume of the pressure chamber18increases according to the amount of movement of the plunger18. This opens the check valve22, and hydraulic oil flows from the reservoir chamber27into the pressure chamber18, so that the plunger10moves quickly. While the plunger10is moving in this direction, hydraulic oil flows through the oil supply passage31, through the cylindrical space29, and then through the communication passage30, and flows into the reservoir chamber27.

As the plunger10moves in the protruding direction, the volume of the cylindrical space29increases according to the movement of the plunger10. If, in this state, hydraulic oil or air is not supplied into the cylindrical space29, the pressure in the cylindrical space29will decrease, and the decrease in pressure in the cylindrical space29could interfere with the movement of the plunger10in the protruding direction. Also, with the recent trend of using smaller oil pumps in automobiles, the need to reduce the oil consumption by a chain tensioner such as the chain tensioner1is increasing.

In this regard, because this tensioner1is configured such that when the plunger10moves in the protruding direction and the volume of the cylindrical space29increases, hydraulic oil flows into the cylindrical space29directly from the oil supply passage31, the pressure in the cylindrical space29is less likely to decrease. As a result, the plunger10can smoothly move in the protruding direction, and thus shows excellent followability to the slack in the chain6.

Moreover, because this chain tensioner1is configured such that when the plunger10moves in the pushed-in direction and the volume of the pressure chamber18decreases, hydraulic oil leaks from the pressure chamber18, through the leakage gap19, and into the cylindrical space29, and from the cylindrical space29, hydraulic oil partially flows through the communication passage30and returns to the interior of the sleeve16, it is possible to reduce the amount of hydraulic oil discharged from the chain tensioner to the outside, by an amount equal to the amount of hydraulic oil returned to the interior of the sleeve16. Thus, this chain tensioner1consumes less oil.

Further, because this chain tensioner1is positioned such that the communication passage30is disposed at the upper half of the entire circumference of the sleeve16, any air that may be present inside the sleeve16can be smoothly expelled through the communication passage30. Further, immediately after the engine is started, and while hydraulic oil has not yet been supplied to the chain tensioner1, hydraulic oil retained in the sleeve16flows into the pressure chamber18, thus producing a damping force. In this regard, because the communication passage30is disposed at the upper half of the entire circumference of the sleeve16, it is possible to retain a large amount of hydraulic oil in the sleeve16.

Moreover, because the sleeve16of this chain tensioner1is fixed in position by press-fitting the end of the sleeve16protruding out of the plunger10, into the sleeve fitting recess17formed in the closed end of the cylinder9, even when the tension in the chain6changes sharply, the sleeve16is prevented from axial movement under the pressure of hydraulic oil in the reservoir chamber27. Thus, this chain tensioner is capable of producing a stable damping force.

Moreover, because this chain tensioner1has in the sleeve16a reservoir chamber27having a diameter larger than the diameter of the valve hole24of the check valve22, a large amount of hydraulic oil can be retained in the sleeve16. Thus, immediately after the engine is started and while hydraulic oil has not yet been supplied to the chain tensioner1, hydraulic oil retained in the sleeve6flows into the pressure chamber18, providing a damping force.

As described above, the sleeve16of this chain tensioner1includes a large outer diameter portion20that defines a leakage gap19between the portion20and the inner periphery of the plunger10; and a small outer diameter portion21continuously connected, via a step, to the end of the large outer diameter portion20closest to the end of the sleeve16protruding out of the plunger10, such that the large outer diameter portion20is entirely received in the plunger10. Thus, regardless of the axial position of the plunger10, the length of the large outer diameter portion20is defined as the axial length of the leakage gap19between the outer periphery of the sleeve16and the inner periphery of the plunger10. This means that even when the plunger10moves in the axial direction, the axial length of the leakage gap19remains unchanged. The chain tensioner therefore provides a uniform damping force irrespective of the axial position of the plunger10.

FIG. 5shows the chain tensioner according to the second embodiment of the present invention. Elements corresponding to those of the first embodiment are denoted by identical reference symbols, and their description is omitted.

A seal member33is disposed between the sliding surfaces of the plunger10and the cylinder9to prevent leakage of hydraulic oil from the cylindrical space29to the outside. Here, the seal member33is an annular rubber member (such as an O-ring). The seal member33is received in a ring groove34formed in the inner periphery of the cylinder9, and is in contact with the outer cylindrical surface of the plunger10so as to be slidable in the axial direction.

In the chain tensioner of the second embodiment, because the seal member33between the sliding surfaces of the plunger10and the cylinder9prevents hydraulic oil from the cylindrical space29, through the gap between the sliding surfaces of the plunger10and the cylinder9, to the outside of the cylinder9, it is possible to effectively reduce the amount of oil consumed by the chain tensioner.

FIG. 6shows the chain tensioner according to the third embodiment. Elements corresponding to those of the first embodiment are denoted by identical reference symbols, and their description is omitted.

The end of the sleeve16protruding out of the plunger10abuts a flat surface formed on the closed end of the cylinder9so as to be perpendicular to the axis of the cylinder9. The sleeve16is pressed by one end of the return spring28toward the closed end of the cylinder9, and fixed in position by the pressing force from the return spring28. An outwardly extending flange16ais integrally formed at the end of the sleeve16protruding out of the plunger10(i.e., at the end of the small outer diameter portion21farthest from the large outer diameter portion20).

The chain tensioner of the third embodiment can be manufactured at a lower cost than the chain tensioner of the first embodiment, in which the sleeve16is fixed in position by press-fitting.

FIG. 7shows the chain tensioner according to the fourth embodiment of the present invention. Elements corresponding to those of the first embodiment are denoted by identical reference symbols and their description is omitted.

A circumferentially extending ring receiving groove35is formed in the inner periphery of the cylinder9at its portion close to the open end of the cylinder9. A register ring36having one circumferential portion thereof cut apart is axially movably received in the ring receiving groove35. The outer periphery of the plunger10has a plurality of axially adjacent circumferential grooves37such that the register ring36can be fitted in any of the circumferential grooves37while being elastically squeezed around the circumferential groove37. Each circumferential groove37includes a tapered surface38rising at the end of the circumferential groove37close to the end of the plunger10that is inserted in the cylinder9, and a stopper surface39rising at the end of the circumferential groove37close to the end of the plunger10protruding out of the cylinder9. The tapered surface38rises less steeply, and when a load is applied to the plunger10in the direction to protrude the plunger10out of the cylinder9, the tapered surface38allows movement of the plunger10while radially expanding the register ring36. On the other hand, the stopper surface39rises steeply, and when a load is applied to the plunger10in the direction to push the plunger10into the cylinder9, the stopper surface39engages the register ring36, thereby restricting the movement of the plunger10.

The ring receiving groove35includes a tapered inner peripheral surface40rising at the end of the ring receiving groove36close to the closed end of the cylinder9, and an end surface41rising at a right angle relative to the axial direction, at the end of the ring receiving groove35close to the open end of the cylinder9. The tapered inner peripheral surface40rises less steeply, and when the plunger10moves in the pushed-in direction, the tapered inner peripheral surface40receives the register ring36so as to restrict radial expansion of the register ring36. The end surface41receives the register ring36when the plunger10moves in the protruding direction so as to prevent any further movement of the register ring36, while allowing radial expansion of the register ring36in this state.

By using the chain tensioner of the fourth embodiment, even if, when the engine is stopped, the camshafts4(seeFIG. 1) stops at positions where the tension in the chain6is high, the engagement of the register ring36and the circumferential groove37prevent movement of the plunger10in the pushed-in direction. Thus, when the engine is restarted, the chain6is less likely to become slack, which allows smooth start of the engine.

FIG. 8shows the chain tensioner according to the fourth embodiment of the present invention. Elements corresponding to those of the first embodiment are denoted by identical reference numerals and their description is omitted.

A helical groove42is formed in the outer periphery of the sleeve16at its portion in sliding contact with the inner periphery of the plunger10. Via the helical groove42, the pressure chamber18and the cylindrical space29communicate with each other. As shown inFIG. 9, the helical groove42is a groove extending obliquely relative to the circumferential direction with a predetermined lead angle.

By using the chain tensioner of the fifth embodiment, it is possible to adjust the damping force by changing the depth and/or lead angle of the helical groove42.

While the chain tensioner of each of the embodiments is incorporated into a chain transmission system that transmits the rotation of a crankshaft2to camshafts4, the chain tensioner according to the present invention may be incorporated into a chain transmission system that transmits the rotation of a crankshaft2to engine accessories such as an oil pump, a water pump and/or a supercharger; a chain transmission system that transmits the rotation of a crankshaft2to a balancer shaft; or a chain transmission system coupling together intake cams and exhaust cams of a twin-cam engine.

It should be understood that the embodiments disclosed herein are all mere examples and not intended to restrict the invention. The scope of the present invention is defined by the claims and not by the description above, and the present invention covers all modifications that are within the scope of the claims, both literally and equivalently.

DESCRIPTION OF THE REFERENCE SYMBOLS