Patent Description:
Chain transmission systems used in the engines of, e.g., automobiles include, e.g., a system that transmits the rotation of a crankshaft to a camshaft or camshafts; a system that transmits the rotation of a crankshaft to engine accessories such as an oil pump, a water pump and a supercharger; a system that transmits the rotation of a crankshaft to a balancer shaft; and a system that couples the intake and exhaust cams of a twin-cam engine to each other. Chain tensioners are used to keep the tension in the chains of such chain transmission systems within an appropriate range.

For example, the below-identified Patent Document <NUM> discloses a chain tensioner used for such a purpose. The chain tensioner of Patent Document <NUM> includes a tubular cylinder of which one axial end is a closed end and the other axial end is an open end; a plunger axially slidably inserted in the cylinder; a return spring biasing the plunger in the direction in which the plunger protrudes out of the cylinder; a pressure chamber surrounded by the cylinder and the plunger; a leakage gap defined between the outer periphery of the plunger and the inner periphery of the cylinder; an oil supply path through which oil supplied from outside the cylinder is introduced into the pressure chamber; and a check valve disposed at the end of the oil supply path close to the pressure chamber.

In the chain tensioner of Patent Document <NUM>, when the tension in the chain increases during operation of the engine, due to this tension in the chain, the plunger moves in the direction in which the plunger is pushed into the cylinder (this direction is hereinafter referred to as the "push-in direction"), thereby absorbing the tension in the chain. At this time, since a damper force is generated by the viscous resistance of the oil flowing out of the pressure chamber through the leakage gap, the plunger moves slowly.

On the other hand, when the tension in the chain decreases during operation of the engine, due to the biasing force of the return spring, 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"), thereby absorbing any slack in the chain. At this time, since the check valve opens, and oil flows into the pressure chamber through the oil supply path, the plunger moves quickly.

The chain tensioner of Patent Document <NUM> further includes a relief valve for adjusting the pressure in the pressure chamber. Specifically, the cylinder includes a pressure release path extending from its inner to outer periphery, and a relief valve that opens when the pressure in the pressure chamber exceeds a predetermined pressure is mounted to the pressure release path. This relief valve prevents the damper force due to the leakage gap from becoming excessive in the high rotation range of the engine, and, as a result, it is possible to obtain an optimum damper force both in the low and high rotation ranges of the engine.

In order to reduce fuel consumption, increasingly smaller oil pumps are used in today's automobiles, and as a result. there is a growing need to reduce the amount of oil consumed by chain tensioners.

To answer this need, the applicant of the present application proposed, in the below-identified Patent Document <NUM>, a chain tensioner that consumes a smaller amount of oil.

The chain tensioner of Patent Document <NUM> includes a tubular cylinder of which one axial end is a closed end and the other axial end is an open end; a tubular plunger axially slidably inserted in the cylinder, and having an open insertion end inserted in the cylinder, and a closed protruding 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 partition wall (valve seat) that axially divides, the space surrounded by the cylinder and the plunger, into a pressure chamber and a reservoir chamber; a valve hole axially extending through the partition wall; a check valve that allows only movement of oil from the side of the valve hole close to the reservoir chamber to the side of the valve hole close to the pressure chamber; a leakage gap through which oil leaks from the pressure chamber when the plunger axially moves such that the volume of the pressure chamber decreases; and an oil supply path through which oil supplied from outside the cylinder is introduced into the reservoir chamber.

The oil supply path is connected to the end of the leakage gap on the outlet side thereof such that oil that has leaked from the pressure chamber through the leakage gap returns to the oil supply path.

In the chain tensioner of Patent Document <NUM>, when the plunger moves in the push-in direction, and the volume of the pressure chamber decreases, oil leaks through the leak gap from the pressure chamber. At this time, since a portion of the oil that has leaked from the pressure chamber returns to the oil supply path, the amount of oil discharged from the chain tensioner to the outside is small. Therefore, it is possible to reduce the amount of oil consumed by the chain tensioner.

A chain tensioner according to the preamble of independent claim <NUM> is disclosed in <CIT>.

The applicant of the present application discussed, among themselves, adding to the chain tensioner of Patent Document <NUM> a relief valve for adjusting the pressure in the pressure chamber, as with the chain tensioner of Patent Document <NUM>.

Specifically, in the chain tensioner of Patent Document <NUM>, the applicant considered forming, in the cylinder, a pressure release path extending from the inner to outer periphery of the cylinder; and mounting, to the pressure release path, a relief valve that opens when the pressure in the pressure chamber exceeds a predetermined pressure.

However, the chain tensioner of Patent Document <NUM> has a problem in that, if a pressure release path is formed which extends from the inner to outer periphery of the cylinder, and a relief valve is mounted to the pressure release path, the amount of oil consumed by the chain tensioner increases.

Specifically, if a relief valve is mounted to the pressure release path extending from the inner to outer periphery of the cylinder, when the relief valve opens, oil in the pressure chamber is discharged through the pressure release path to the outside of the cylinder. Therefore, in the high rotation range of the engine, the amount of oil discharged to the outside of the cylinder when the relief valve opens increases, and the amount of oil consumed by the chain tensioner increases.

It is an object of the present invention to provide a chain tensioner in which the optimum damper force can be obtained both in the low and high rotation ranges, and the oil consumption is small.

In order to achieve the above object, the present invention, which is defined in independent claim <NUM>, provides a chain tensioner comprising: a tubular cylinder of which one axial end is a closed end and the other axial end is an open end; a tubular plunger axially slidably inserted in the cylinder, and having an open insertion end inserted in the cylinder, and a closed protruding end protruding out of the cylinder; a partition wall that axially divides a space surrounded by the cylinder and the plunger into (i) a pressure chamber of which a volume changes as the plunge axially moves, and (ii) a reservoir chamber of which a volume remains unchanged even when the plunger axially moves; an oil supply path through which oil supplied from outside the cylinder is introduced into the reservoir chamber; a first through hole axially extending through the partition wall; a check valve that allows only movement of oil from a side of the first through hole close to the reservoir chamber to a side of the first through hole close to the pressure chamber; a leakage gap that allows leakage of oil from the pressure chamber when the plunger axially moves such that the volume of the pressure chamber decreases; and a return spring biasing the plunger in a direction in which the plunger protrudes out of the cylinder, wherein the oil supply path is connected to an outlet end of the leakage gap such that oil that has leaked from the pressure chamber through the leakage gap returns to the oil supply path, wherein the chain tensioner further comprises: a second through hole axially extending through the partition wall in parallel with the first through hole; and a relief valve disposed at the second through hole, and configured to release oil from the pressure chamber into the reservoir chamber when pressure in the pressure chamber exceeds a predetermined pressure.

With this arrangement, since a relief valve is provided which releases oil from the pressure chamber if the pressure in the pressure chamber exceeds a predetermined pressure, it is possible to prevent the damper force due to the leakage gap from becoming excessive in the high rotation range of the engine. As a result, it is possible to obtain an optimum damper force both in the low and high rotation ranges of the engine.

Also, since the outlet end of the leakage gap is connected to the oil supply path, oil that has leaked from the pressure chamber through the leakage gap when the plunger moves in the axial direction in which the volume of the pressure chamber decreases, returns to the oil supply path. Also, since the relief valve is disposed at the second through hole of the partition wall, which separates the pressure chamber from the reservoir chamber, when the relief valve opens, the oil flowing from the pressure chamber through the relief valve returns to the reservoir chamber. Therefore, it is possible to reduce the amount of oil consumed by the chain tensioner.

According to the present invention, the first through hole is formed at a position displaced from a center of the plunger to one of two opposite sides of the center of the plunger, and the second through hole is formed at a position displaced from the center of the plunger to the other of the two opposite sides of the center of the plunger.

Since the first through hole and the second through hole are displaced from the center of the plunger to the opposite sides of the center of the plunger, respectively, it is possible to ensure both the flow path area of the first through hole, and the flow path area of the second through hole. Therefore, by ensuring the flow rate when the check valve opens, the chain tensioner can absorb slack in the chain quickly. Also, by ensuring the flow rate of oil when the relief valve opens, it is possible to effectively hinder the tension in the chain from becoming excessive in the high rotation range of the engine.

It is preferable that the second through hole is a stepped hole directly formed in the partition wall, and having a larger diameter on a side of the pressure chamber, and a smaller diameter on a side of the reservoir chamber, and that the relief valve is retained in the second through hole by a second annular member inserted and fixed in position, in an end portion of the second through hole on the side of the pressure chamber.

With this arrangement, it is possible to reliably prevent falling off of the relief valve due to, e.g., vibrations of the plunger, or the pressure which the relief valve receives from the oil in the pressure chamber. Specifically, the relief valve is inserted, from the side of the pressure chamber, into the second through hole, which is a stepped hole having a large diameter on the side of the pressure chamber, and a small diameter on the side of the reservoir chamber. Since the direction of the pressure applied to the relief valve from the oil in the pressure chamber is the same as the direction in which the relief valve is inserted into the second through hole (stepped hole), it is possible to reliably prevent falling off of the relief valve from the second through hole due to. e.g., vibrations of the plunger, or the pressure which the relief valve receives from the oil in the pressure chamber.

In this case, it is preferable that the first through hole is a stepped hole directly formed in the partition wall, and having a larger diameter on the side of the pressure chamber, and a smaller diameter on the side of the reservoir chamber, and that the check valve is retained in the first through hole by a first annular member inserted and fixed in position, in an end portion of the first through hole on the side of the pressure chamber.

With this arrangement, since, when mounting the check valve and the relief valve to the partition wall, the direction in which the check valve is inserted into the first through hole, and the direction in which the relief valve is inserted into the second through hole are the same direction, it is not necessary to change the direction of the partition wall when mounting the check valve and the relief valve to the partition wall. Therefore, it is possible to assemble the chain tensioner easily/efficiently.

The following arrangement may be used: the second through hole includes: a second small-diameter portion having a constant inner diameter, and extending from an end surface of the partition wall on the side of the reservoir chamber toward the pressure chamber; a second step radially expanding from the second small-diameter portion toward the pressure chamber; and a second large-diameter portion extending from the second step toward the pressure chamber, and having an inner diameter larger than the inner diameter of the second small-diameter portion, the relief valve includes: a second valve element movable between (i) a closed position where the second valve element is in contact with the second annular member and (ii) an open position where the second valve element has been moved from the closed position toward the reservoir chamber; and a second valve spring biasing the second valve element from the side of the reservoir chamber toward the pressure chamber, and an end of the second valve spring close to the reservoir chamber is supported by the second step.

With this arrangement, since the second annular member functions as a valve seat of the relief valve (member on which the second valve element is seated), and the second step of the second through hole functions as a valve retainer of the relief valve (member retaining the second valve element and the second valve spring), it is possible to minimize the number of components constituting the relief valve.

The following arrangement may be used: the second through hole includes: a second small-diameter portion having a constant inner diameter, and extending from an end surface of the partition wall on the side of the reservoir chamber toward the pressure chamber; a second step radially expanding from the second small-diameter portion toward the pressure chamber; and a second large-diameter portion extending from the second step toward the pressure chamber, and having an inner diameter larger than the inner diameter of the second small-diameter portion, the relief valve includes: an annular second valve seat; a second valve element movable between (i) a closed position where the second valve element is in contact with the annular second valve seat and (ii) an open position where the second valve element has been moved from the closed position toward the reservoir chamber; a second valve spring biasing the second valve element from the side of the reservoir chamber toward the pressure chamber; and a valve sleeve retaining, as a single assembly, the second valve seat, the second valve element, and the second valve spring, and the valve sleeve is inserted in the second large-diameter portion, while retaining, as a single assembly, the second valve seat, the second valve element, and the second valve spring, and the valve sleeve is prevented from being pulled out of the second large-diameter portion by the second annular member.

With this arrangement, since the relief valve includes a valve sleeve retaining, as a single assembly, the second valve seat, the second valve element, and the second valve spring, it is possible to first assemble, as a single assembly, the components constituting the relief valve before mounting the relief valve into the second through hole, and then to mount the single assembly into the second through hole.

If the check valve includes: a first valve element configured to open and close the first through hole from a side of the pressure chamber; and a first seat surface on which the first valve element is configured to be seated to close the first through hole, and the relief valve includes: a second valve element configured to open and close the second through hole from a side of the reservoir chamber; and a second seat surface on which the second valve element is configured to be seated to close the second through hole, an integral member formed with the first seat surface and the second seat surface is preferably used as the partition wall.

With this arrangement, since the first seat surface of the check valve and the second seat surface of the relief valve are formed on a common integral member, it is possible to reduce the manufacturing cost of the check valve and the relief valve.

The partition wall may be formed of a resin or a sintered alloy.

It is preferable that the first valve element has a spherical shape, and the second valve element has a spherical shape.

With this arrangement, the opening and closing actions of both of the check valve and the relief valve become stable and reliable.

The following arrangement may be used: the first seat surface is formed on a peripheral edge of an end opening of the first through hole on the side of the pressure chamber, the check valve further includes: a first valve retainer retaining the first valve element so as to be movable between (i) a closed position where the first valve element is seated on the first seat surface, and (ii) an open position where the first valve element is separated from the first seat surface by moving from the closed position toward the pressure chamber; and a first valve spring biasing the first valve element from the open position toward the closed position, the first valve retainer comprises: a first flange portion supported by an axial end surface of the partition wall on the side of the pressure chamber; a first tubular portion which axially extends from the first flange portion toward the pressure chamber, and in which the first valve element is axially movably received; and a first end plate disposed at an axial end of the first tubular portion on the side of the pressure chamber, and supporting the first valve spring, the second seat surface is formed on a peripheral edge of an end opening of the second through hole on the side of the reservoir chamber, the relief valve further includes: a second valve retainer retaining the second valve element so as to be movable between (i) a closed position where the second valve element is seated on the second seat surface, and (ii) an open position where the second valve element is separated from the second seat surface by moving from the closed position toward the reservoir chamber; and a second valve spring biasing the second valve element from the open position toward the closed position, and the second valve retainer comprises: a second flange portion supported by an axial end surface of the partition wall on the side of the reservoir chamber; a second tubular portion which axially extends from the second flange portion toward the reservoir chamber, and in which the second valve element is axially movably received; and a second end plate disposed at an axial end of the second tubular portion on the side of the reservoir chamber, and supporting the second valve spring.

With this arrangement, since the first valve element of the check valve is configured to open and close the opening of the first through hole at its end on the side of the pressure chamber, it is possible to easily ensure the flow path area of the first through hole when the check valve opens. Also, since the second valve element of the relief valve is configured to open and close the opening of the second through hole at its end on the side of the reservoir chamber, it is possible to easily ensure the flow path area of the second through hole when the relief valve opens. Therefore, it is possible to increase both of the flow rate of oil when the check valve opens, and the flow rate of oil when the relief valve opens.

The following arrangement may be used: the first seat surface is formed on an inner periphery of the first through hole, the first valve element is received in the first through hole so as to be axially movable between (i) a closed position where the first valve element is seated on the first seat surface, and (ii) an open position where the first valve element is separated from the first seat surface by moving from the closed position toward the pressure chamber, a first valve spring is disposed in the first through hole, and biases the first valve element from the open position toward the closed position, a first spring supporting member is disposed in an end opening of the first through hole on the side of the pressure chamber, to support the first valve spring, the second seat surface is formed on an inner periphery of the second through hole, the second valve element is received in the second through hole so as to be axially movable between (i) a closed position where the second valve element is seated on the second seat surface, and (ii) a second opened valve position where the second valve element is separated from the second seat surface by moving from the closed position toward the reservoir chamber, a second valve spring is disposed in the second through hole, and biases the second valve element from the open position toward the closed position, and a second spring supporting member is disposed in an end opening of the second through hole on the side of the reservoir chamber, to support the second valve spring.

With this arrangement, since the first valve element and the first valve spring, which constitute the check valve, are received inside of the first through hole of the partition wall, and the second valve element and the second valve spring, which constitute the relief valve, are received inside of the second through hole of the partition wall, the axial dimensions of the check valve and the relief valve are short. Therefore, the size of the chain tensioner is small.

The following arrangement may be used: the partition wall is fixed to the plunger so as to axially move in unison with the plunger, the leakage gap is a first gap portion which is, of an annular gap defined between an outer periphery of the plunger and an inner periphery of the cylinder, an end portion close to the insertion end of the plunger, the chain tensioner further comprises: an enlarged gap portion which is a portion of the annular gap on a side of the first gap portion remote from the insertion end of the plunger, the enlarged gap portion having a gap dimension larger than a gap dimension of the first gap portion; and a second gap portion which is a portion of the annular gap on a side of the enlarged gap portion remote from the insertion end of the plunger, the second gap portion having a gap dimension smaller than the gap dimension of the enlarged gap portion, and the oil supply path comprises: a cylinder-side oil path portion formed in the cylinder such that oil supplied from outside the cylinder is introduced into the enlarged gap portion through the cylinder-side oil path portion; and a plunger-side oil path portion extending through the tubular plunger from the outer periphery of the plunger to an inner periphery of the plunger such that the enlarged gap portion and the reservoir chamber communicate with each other via the plunger-side oil path portion.

With this arrangement, even when the axial position of the plunger relative to the cylinder changes, the cylinder-side oil path portion and the plunger-side oil path portion can be kept in communication with each other via the enlarged gap portion. Also, the oil that has leaked from the pressure chamber through the first gap portion (leakage gap) between the outer periphery of the plunger and the inner periphery of the cylinder can be returned to the oil supply path (enlarged gap portion).

It is preferable that the plunger has: a cylindrical first large-diameter radially outer surface on an end portion of the outer periphery of the plunger close to the insertion end of the plunger; a small-diameter radially outer surface on a portion of the outer periphery of the plunger on a side of the first large-diameter radially outer surface remote from the insertion end of the plunger, the small-diameter radially outer surface having an outer diameter dimension smaller than an outer diameter dimension of the first large-diameter radially outer surface; and a second large-diameter radially outer surface on a portion of the outer periphery of the plunger on a side of the small-diameter radially outer surface remote from the insertion end of the plunger, the second large-diameter radially outer surface having an outer diameter dimension equal to the outer diameter dimension of the first large-diameter radially outer surface, the first gap portion is defined between the first large-diameter radially outer surface and the inner periphery of the cylinder, the enlarged gap portion is defined between the small-diameter radially outer surface and the inner periphery of the cylinder, and the second gap portion is defined between the second large-diameter radially outer surface and the inner periphery of the cylinder.

With this arrangement, even when the plunger axially moves relative to the cylinder, the axial length of the leakage gap does not change, i.e., kept constant, thus making it possible to obtain a stable damper force.

It is preferable that the cylinder-side oil path portion has an oil outlet open to the inner periphery of the cylinder, and the cylinder has, in the inner periphery of the cylinder, a circumferential groove arranged such that the oil outlet is entirely located within a range of the circumferential groove.

With this arrangement, since the cylinder has, in its inner periphery, a circumferential groove such that the entire oil outlet of the cylinder-side oil path portion is located within the range of the circumferential groove, even if, when forming the cylinder-side oil path portion in the cylinder, burrs occur on the oil outlet of the cylinder-side oil path portion, which is open to the inner periphery of the cylinder, such burrs are prevented from interfering with the first and second large-diameter radially outer surfaces on the outer periphery of the plunger. Therefore, the plunger behaves stably.

Since the chain tensioner of the present invention is provided with a relief valve which releases oil from the pressure chamber when the pressure in the pressure chamber exceeds a predetermined pressure, it is possible to prevent the damper force due to the leakage gap from becoming excessive in the high rotation range of the engine, and to obtain an optimum damper force both in the low and high rotation ranges of the engine. Also, since the outlet end of the leakage gap is connected to the oil supply path, when the plunger axially moves such that the volume of the pressure chamber decreases, the oil that has leaked from the pressure chamber through the leakage gap returns to the oil supply path. Also, since the relief valve is disposed at the second through hole of the partition wall, which separates the pressure chamber from the reservoir chamber, the oil flowing from the pressure chamber through the relief valve when the relief valve opens returns to the reservoir chamber. Therefore, the amount of oil consumed by the chain tensioner is small.

<FIG> illustrates a chain transmission system in which a chain tensioner <NUM> according to the first embodiment of the present invention is used. This chain transmission system includes a sprocket <NUM> fixed to the crankshaft <NUM> of an engine; sprockets <NUM> fixed to respective camshafts <NUM>; and a chain <NUM> coupling the sprocket <NUM> to the sprockets <NUM>. The rotation of the crankshaft <NUM> is transmitted to the camshafts <NUM> by the chain <NUM> so as to rotate the camshafts <NUM>, thereby opening and closing the valves (not shown) of the combustion chambers of the engine.

While the engine is running, the crankshaft <NUM> always rotates in the same direction (clockwise direction in <FIG>), and the side of the chain <NUM> moving toward the sprocket <NUM> (right side in <FIG>) becomes a tight side, whereas, the side of the chain <NUM> moving away from the sprocket <NUM> (left side in <FIG>) becomes a slack side. A chain guide <NUM> supported for pivotal motion about a pivot shaft <NUM> is in contact with the slack side of the chain <NUM>. The chain tensioner <NUM> presses the chain <NUM> via the chain guide <NUM>.

As illustrated in <FIG>, the chain tensioner <NUM> includes a tubular cylinder <NUM> of which one axial end is a closed end and the other axial end is an open end; and a plunger <NUM> axially slidably inserted in the cylinder <NUM>. The plunger <NUM> has a protruding end protruding out of the cylinder <NUM>, and pressing the chain guide <NUM>.

The cylinder <NUM> is integrally formed of a metal (such as an aluminum alloy). A plurality of attachment pieces <NUM> are integrally formed on the outer periphery of the cylinder <NUM>. By inserting bolts <NUM> (see <FIG>) through the respective attachment pieces <NUM>, and tightening the bolts <NUM>, the cylinder <NUM> is fixed to an engine wall surface <NUM> (see <FIG>).

The plunger <NUM> is a tubular member having an open insertion end inserted in the cylinder <NUM>, and a closed protruding end protruding out of the cylinder <NUM>. The plunger <NUM> is made of an iron-based material (e.g., steel such as SCM (chrome molybdenum steel) or SCr (chrome steel)).

A partition wall14 is disposed on the inner periphery of the portion of the plunger <NUM> inserted in the cylinder <NUM>. The partition wall <NUM> axially divides the space surrounded by the cylinder <NUM> and the plunger <NUM> into (i) a pressure chamber <NUM> of which the volume changes as the plunge <NUM> axially moves, and (ii) a reservoir chamber <NUM> of which the volume does not change even when the plunger <NUM> axially moves. When the plunger <NUM> axially moves in the direction in which the plunger protrudes out of the cylinder <NUM>, the volume of the pressure chamber <NUM> increases. When the plunger <NUM> axially moves in the direction in which the plunger is pushed into the cylinder <NUM>, the volume of the pressure chamber <NUM> decreases.

The partition wall <NUM> is fixed to the plunger <NUM> so as to axially move in unison with the plunger <NUM>. The pressure chamber <NUM> is, of the space surrounded by the cylinder <NUM> and the plunger <NUM>, the area between the partition wall <NUM> and the closed end of the cylinder <NUM>. The reservoir chamber <NUM> is, of the space surrounded by the cylinder <NUM> and the plunger <NUM>, the area between the partition wall <NUM> and the protruding end of the plunger <NUM>.

The partition wall <NUM> is formed with a first through hole <NUM> axially extending through the partition wall <NUM>, and a second through hole <NUM> axially extending through the partition wall <NUM> in parallel with the first through hole <NUM>. The first through hole <NUM> has a round cross section, and axially extends with its center displaced from the center of the plunger <NUM> to one side (lower side in the example shown). The second through hole <NUM> has a round cross section, and axially extends with its center displaced from the center of the plunger <NUM> to the other side thereof (upper side in the example shown). The partition wall <NUM> can be formed of a resin or a sintered alloy.

A check valve <NUM> is disposed at the end of the first through hole <NUM> close to the pressure chamber <NUM> so as to restrict the movement of oil from the side of the first through hole <NUM> close to the pressure chamber <NUM> toward the side of the first through hole <NUM> close to the reservoir chamber <NUM>, while allowing only the movement of oil from the side of the first through hole <NUM> close to the reservoir chamber <NUM> toward the side of the first through hole <NUM> close to the pressure chamber <NUM>. A relief valve <NUM> is disposed at the end of the second through hole <NUM> close to the reservoir chamber <NUM> so as to release oil from the pressure chamber <NUM> into the reservoir chamber <NUM> when the pressure in the pressure chamber <NUM> exceeds a predetermined pressure.

A return spring <NUM> is mounted in the pressure chamber <NUM>. The return spring <NUM> is a compression coil spring formed by a helically winding a wire material made of metal. One end of the return spring <NUM> is supported by the closed end of the cylinder <NUM>, and the other end axially presses the plunger <NUM> via the partition wall <NUM>, thereby biasing the plunger <NUM> in the direction in which the plunger <NUM> protrudes out of the cylinder <NUM>.

The plunger <NUM> has, on its outer periphery, and, from the side close to the insertion end of the plunger <NUM> (left side in <FIG>) toward the side remote from the insertion end (right side in <FIG>), a first large-diameter radially outer surface <NUM>, a small-diameter radially outer surface <NUM>, and a second large-diameter radially outer surface <NUM>. The first large-diameter radially outer surface <NUM> is a cylindrical radially outer surface formed on a portion of the outer periphery of the plunger <NUM> close to the insertion end of the plunger <NUM>. The small-diameter radially outer surface <NUM> is a radially outer surface (i) which is formed on the portion of the outer periphery of the plunger <NUM> adjacent to the side of the first large-diameter radially outer surface <NUM> remote from the insertion end of the plunger <NUM>, and (ii) which has an outer diameter dimension smaller than that of the first large-diameter radially outer surface <NUM>. The second large-diameter radially outer surface <NUM> is a cylindrical radially outer surface (i) which is formed on the portion of the outer periphery of the plunger <NUM> adjacent to the side of the small-diameter radially outer surface <NUM> remote from the insertion end of the plunger <NUM>, and (ii) which has the same outer diameter dimension as the first large-diameter radially outer surface <NUM>. In other words, the first large-diameter radially outer surface <NUM> and the second large-diameter radially outer surface <NUM> are cylindrical surfaces machined in a single step.

An annular gap is present between the outer periphery of the plunger <NUM> and the inner periphery of the cylinder <NUM>, and the annular gap includes a first gap portion <NUM> defined between the first large-diameter radially outer surface <NUM> of the plunger <NUM> and the inner periphery of the cylinder <NUM>. The first gap portion <NUM> is an end portion of the annular gap close to the insertion end of the plunger <NUM>. The first gap portion <NUM> constitutes a leakage gap through which oil leaks from the pressure chamber <NUM> when the plunger <NUM> axially moves such that the volume of the pressure chamber <NUM> decreases. The first gap portion <NUM> (leakage gap) is a gap whose cross-sectional shape perpendicular to the center axis is constant, i.e., does not change, in the axial direction, for example, a cylindrical gap. In the embodiment, the first gap portion <NUM> is a cylindrical minute gap having a radial width within the range of <NUM> to <NUM>.

The annular gap between the outer periphery of the plunger <NUM> and the inner periphery of the cylinder <NUM> further includes an enlarged gap portion <NUM> defined between the small-diameter radially outer surface <NUM> of the plunger <NUM> and the inner periphery of the cylinder <NUM>. The enlarged gap portion <NUM> is adjacent to the side of the first gap portion <NUM> remote from the insertion end of the plunger <NUM>, and has a gap dimension larger than that of the first gap portion <NUM>. The enlarged gap portion <NUM> has an axial length larger than the moving stroke of the plunger <NUM> when the chain <NUM> is stretched over time, for example, has an axial length of <NUM> or more.

The annular gap between the outer periphery of the plunger <NUM> and the inner periphery of the cylinder <NUM> further includes a second gap portion <NUM> defined between the second large-diameter radially outer surface <NUM> of the plunger <NUM> and the inner periphery of the cylinder <NUM>. The second gap portion <NUM> is adjacent to the side of the enlarged gap portion <NUM> remote from the insertion end of the plunger <NUM>, and has a gap dimension smaller than that of the enlarged gap portion <NUM>. The second gap portion <NUM> is a cylindrical minute gap having a radial width within the range of <NUM> to <NUM>.

As illustrated in <FIG> and <FIG>, the cylinder <NUM> and the plunger <NUM> are provided with an oil supply path <NUM> through which oil supplied from outside the cylinder <NUM> is introduced into the reservoir chamber <NUM>. The oil supply path <NUM> is constituted by a cylinder-side oil path portion <NUM> formed in the cylinder <NUM> through which oil supplied from outside the cylinder <NUM> is introduced into the enlarged gap portion <NUM>; and a plunger-side oil path portion <NUM> formed in the plunger <NUM> through which the enlarged gap portion <NUM> and the reservoir chamber <NUM> communicate with each other.

As illustrated in <FIG>, the cylinder-side oil path portion <NUM> is a hole extending through the cylinder <NUM> from its outer periphery to its inner periphery. The cylinder-side oil path portion <NUM> has an oil inlet <NUM> open to the outer periphery of the cylinder <NUM>, and an oil outlet <NUM> open to the inner periphery of the cylinder <NUM>. The oil inlet <NUM> is open to a seat surface <NUM> on the outer periphery of the cylinder <NUM> so as to be connected to an oil hole <NUM> open to the engine wall surface <NUM>. The seat surface <NUM> is an abutment surface configured to abut against the engine wall surface <NUM>. The oil hole <NUM> is an oil supply hole through which oil sent out from an oil pump (not shown) is supplied to the chain tensioner <NUM>.

As illustrated in <FIG>, the cylinder <NUM> has a circumferential groove <NUM> in its inner periphery. The circumferential groove <NUM> is disposed at the position of the oil outlet <NUM> of the cylinder-side oil path portion <NUM> such that the entire oil outlet <NUM> is located within the range of the circumferential groove <NUM>. The axial width of the circumferential groove <NUM> (i.e., the axial width of a portion of the cylinder <NUM> recessed from the cylindrical surface of the inner periphery of the cylinder <NUM>) is <NUM> times or more and <NUM> times or less larger than the axial width dimension of the opening of the oil outlet <NUM>.

The plunger-side oil path portion <NUM> is a hole extending through the tubular plunger <NUM> from its outer to inner periphery. The end of the plunger-side oil path portion <NUM> at the outer periphery of the plunger <NUM> is open within the range of the small-diameter radially outer surface <NUM> of the plunger <NUM>.

The enlarged gap portion <NUM> of the annular gap constitutes the oil supply path <NUM>, and is connected to the outlet end of the first gap portion <NUM> (leakage gap). Thus, the oil that has leaked from the pressure chamber <NUM> through the first gap portion <NUM> (leakage gap) flows/returns to the oil supply path <NUM> (enlarged gap portion <NUM>).

As illustrated in <FIG>, the check valve <NUM> includes a first valve element <NUM> configured to open and close the first through hole <NUM> from the side of the pressure chamber <NUM>; a first seat surface <NUM> on which the first valve element <NUM> is configured to be seated to close the first through hole <NUM>; a first valve retainer <NUM> retaining the first valve element <NUM> so as to be movable between a closed position where the first valve element <NUM> is seated on the first seat surface <NUM>, and an open position where the first valve element <NUM> is separated from the first seat surface <NUM> by moving from the closed position toward the pressure chamber <NUM>; and a first valve spring <NUM> biasing the first valve element <NUM> from the open position toward the closed position.

Similarly, the relief valve <NUM> includes a second valve element <NUM> configured to open and close the second through hole <NUM> from the side of the reservoir chamber <NUM>; a second seat surface <NUM> on which the second valve element <NUM> is configured to be seated to close the second through hole <NUM>; a second valve retainer <NUM> retaining the second valve element <NUM> so as to be movable between a closed position where the second valve element <NUM> is seated on the second seat surface <NUM>, and an open position where the second valve element <NUM> is separated from the second seat surface <NUM> by moving from the closed position toward the reservoir chamber <NUM>; and a second valve spring <NUM> biasing the second valve element <NUM> from the open position toward the closed position.

The second valve spring <NUM> is a compression coil spring formed by winding a wire material having a cross-sectional area larger than that of the wire material forming the first valve spring <NUM>. The second valve spring <NUM> presses the second valve element <NUM> with a force larger than the force with which the first valve spring <NUM> presses the first valve element <NUM>.

The first valve element <NUM> has a spherical shape. The first seat surface <NUM> is formed on the peripheral edge of the end opening of the first through hole <NUM> on the side of the pressure chamber <NUM>. The first seat surface <NUM> is a tapered inner peripheral surface inclined in the radially expanding direction toward the pressure chamber <NUM>. The first seat surface <NUM> may be an inner peripheral surface having a circular arc-shaped cross section.

As with the first valve element <NUM>, the second valve element <NUM> also has a spherical shape. The second seat surface <NUM> is formed on the peripheral edge of the end opening of the second through hole <NUM> on the side of the reservoir chamber <NUM>. The second seat surface <NUM> is a tapered inner peripheral surface inclined in the radially expanding direction toward the reservoir chamber <NUM>. The second seat surface <NUM> may be an inner peripheral surface having a circular arc-shaped cross section.

The partition wall <NUM> is an integral plate-shaped member formed with the first seat surface <NUM> and the second seat surface <NUM>. The outer periphery of the partition wall <NUM> is a cylindrical surface. The partition wall <NUM> is fitted to the inner periphery of the plunger <NUM>, and axially positioned by a shoulder <NUM> formed on the inner periphery of the plunger <NUM> such that the diameter of the portion of the plunger <NUM> on the side of the protruding end of the plunger <NUM> (right side in <FIG>) is smaller than the diameter of the portion of the plunger <NUM> on the side of the insertion end of the plunger <NUM> (left side in Fig. <NUM>).

The first valve retainer <NUM> is constituted by a first flange portion <NUM> supported by the axial end surface of the partition wall <NUM> on the side of the pressure chamber <NUM>; a first tubular portion <NUM> which axially extends from the first flange portion <NUM> into the pressure chamber <NUM>, and in which the first valve element <NUM> is axially movably received; and a first end plate <NUM> disposed at the axial end of the first tubular portion <NUM> on the side of the pressure chamber <NUM>, and supporting the first valve spring <NUM>. The first flange portion <NUM> is fixed in position by being axially sandwiched between the return spring <NUM> and the partition wall <NUM>. The first valve retainer <NUM> is formed with openings <NUM> for oil passage through which oil can pass from inside to outside of the first tubular portion <NUM> (in the example shown, the openings <NUM> are slits circumferentially dividing the first tubular portion <NUM> and the first flange portion <NUM>).

The second valve retainer <NUM> is constituted by a second flange portion <NUM> supported by the axial end surface of the partition wall <NUM> on the side of the reservoir chamber <NUM>; a second tubular portion <NUM> which axially extends from the second flange portion <NUM> into the reservoir chamber <NUM>, and in which the second valve element <NUM> is axially movably received; and a second end plate <NUM> disposed at the axial end of the second tubular portion <NUM> on the side of the reservoir chamber <NUM>, and supporting the second valve spring <NUM>. The second flange portion <NUM> is fixed in position by being axially sandwiched between the partition wall <NUM> and the shoulder <NUM>. The second valve retainer <NUM> is formed with openings <NUM> for oil passage through which oil can pass from inside to outside of the second tubular portion <NUM> (in the example shown, the openings <NUM> are slits circumferentially dividing the second tubular portion <NUM> and the second flange portion <NUM>).

Next, an exemplary operation of this chain tensioner <NUM> is described.

When the tension in the chain <NUM> increases during operation of the engine, due to this tension in the chain <NUM>, the plunger <NUM> moves in the direction in which the plunger <NUM> is pushed into the cylinder <NUM>, thereby absorbing the tension in the chain <NUM>. At this time, since the pressure in the pressure chamber <NUM> exceeds the pressure in the reservoir chamber <NUM>, the check valve <NUM> is closed. Also, since the volume of the pressure chamber <NUM> decreases due to the above movement of the plunger <NUM>, an amount of oil corresponding to the decreased volume of the pressure chamber leaks from the pressure chamber <NUM> into the enlarged gap portion <NUM> through the first gap portion <NUM> (leakage gap). At this time, a damper force is generated due to the viscous resistance of the oil flowing through the first gap portion <NUM> (leakage gap), thereby preventing flapping of the chain <NUM>. The oil that has leaked from the pressure chamber <NUM> through the first gap portion <NUM> (leakage gap) returns to the enlarged gap portion <NUM> (oil supply path <NUM>), and then partially (flows into the plunger-side oil path portion <NUM>, partially returns to the cylinder-side oil path portion <NUM>, and partially lubricates the sliding surfaces of the plunger <NUM> and the cylinder <NUM> by passing through the second gap portion <NUM>.

On the other hand, when the tension in the chain <NUM> decreases during operation of the engine, due to the biasing force of the return spring <NUM>, the plunger <NUM> moves in the protruding direction, thereby absorbing any slack in the chain <NUM>. At this time, since the volume of the pressure chamber <NUM> increases due to the above movement of the plunger <NUM>, the pressure in the pressure chamber <NUM> falls below the pressure in the reservoir chamber <NUM>, and the check valve <NUM> opens. Therefore, oil flows from the reservoir chamber <NUM> into the pressure chamber <NUM> through the first through hole <NUM>, so that the plunger <NUM> moves quickly. At this time, oil is introduced into the reservoir chamber <NUM> from the oil hole <NUM> (see <FIG>), which is outside of the cylinder <NUM>, through the oil supply path <NUM> (i.e., through its cylinder-side oil path portion <NUM>, enlarged gap portion <NUM>, and plunger-side oil path portion <NUM>).

While the engine is operating in the high rotation range, since a high-frequency excitation force continuously acts on the plunger <NUM> from the chain <NUM>, an excessive damper force tends to be generated by the oil flowing through the first gap portion <NUM> (leakage gap). At this time, due to a pressure rise in the pressure chamber <NUM>, the relief valve <NUM> opens, thereby allowing oil to flow into the reservoir chamber <NUM> from the pressure chamber <NUM> through the second through hole <NUM>. This reduces a rise in the pressure in the pressure chamber <NUM>, thus preventing an excessive damper force from being generated. At this time, the oil that has flowed out of the pressure chamber <NUM> through the second through hole <NUM> returns to the reservoir chamber <NUM>.

When the engine is stopped and then restarted, since the surface of oil in the oil hole <NUM> (see <FIG>) is generally at a low level, it takes time until oil starts to be supplied to the chain tensioner <NUM> from the oil hole <NUM>. During this period, i.e., from the restart of the engine until oil starts to be supplied from the oil hole <NUM>, oil remaining beforehand in the reservoir chamber <NUM> flows into the pressure chamber <NUM> through the first through hole <NUM>, thereby keeping the pressure chamber <NUM> filled with oil. Therefore, it is possible to generate a damper force right after restarting the engine, and thus to reduce flapping of the chain <NUM>.

Since this chain tensioner <NUM> includes a relief valve <NUM> that releases oil from the pressure chamber <NUM> when the pressure in the pressure chamber <NUM> exceeds a predetermined pressure, it is possible to prevent the damper force due to the first gap portion <NUM> (leakage gap) from becoming excessive in the high rotation range of the engine, and thus to obtain an optimum damper force both in the low and high rotation ranges of the engine.

Also, in this chain tensioner <NUM>, since the outlet end of the first gap portion <NUM> (leakage gap) is connected to the oil supply path <NUM> (enlarged gap portion <NUM>), when the plunger <NUM> axially moves such that the volume of the pressure chamber <NUM> decreases, the oil that has leaked from the pressure chamber <NUM> through the first gap portion <NUM> (leakage gap) returns to the oil supply path <NUM>. Also, since the relief valve <NUM> is disposed at the second through hole <NUM> of the partition wall <NUM>, which separates the pressure chamber <NUM> from the reservoir chamber <NUM>, when the relief valve <NUM> opens, the oil flowing out of the pressure chamber <NUM> through the relief valve <NUM> returns to the reservoir chamber <NUM>. Therefore, this chain tensioner <NUM> consumes a smaller amount of oil.

Moreover, in this chain tensioner <NUM>, since the first through hole <NUM> and the second through hole <NUM> are displaced from the center of the plunger <NUM> to the opposite sides of the center of the plunger <NUM>, respectively, it is possible to ensure both the flow path area of the first through hole <NUM>, and the flow path area of the second through hole <NUM>. Therefore, by ensuring the flow rate of oil when the check valve <NUM> opens, the chain tensioner can absorb slack in the chain <NUM> quickly. Also, by ensuring the flow rate of oil when the relief valve <NUM> opens, it is possible to effectively reduce an excessive rise in tension in the chain <NUM> in the high rotation range of the engine.

Also, in this chain tensioner <NUM>, since the first seat surface <NUM> of the check valve <NUM> and the second seat surface <NUM> of the relief valve <NUM> are formed on a common integral member (partition wall <NUM>), the manufacturing cost of the check valve <NUM> and the relief valve <NUM> is low.

Also, in this chain tensioner <NUM>, since the first valve element <NUM> has a spherical shape, and the second valve element <NUM> also has a spherical shape, when the check valve <NUM> and the relief valve <NUM> open and close, both of these valves behave stably and highly reliably.

Also, since this chain tensioner <NUM> is configured such that the first valve element <NUM> of the check valve <NUM> opens and closes the end opening of the first through hole <NUM> on the side of the pressure chamber <NUM>, it is possible to easily ensure the flow path area of the first through hole <NUM> when the check valve <NUM> opens. Also, since this chain tensioner <NUM> is configured such that the second valve element <NUM> of the relief valve <NUM> opens and closes the end opening of the second through hole <NUM> on the side of the reservoir chamber <NUM>, it is possible to easily ensure the flow path area of the second through hole <NUM> when the relief valve <NUM> opens. Therefore, it is possible to increase both of the flow rate of oil when the check valve <NUM> opens, and the flow rate of oil when the relief valve <NUM> opens.

Also, in this chain tensioner <NUM>, even when the axial position of the plunger <NUM> relative to the cylinder <NUM> changes, the cylinder-side oil path portion <NUM> and the plunger-side oil path portion <NUM> can be kept in communication with each other via the enlarged gap portion <NUM>. Also, the oil leaking from the pressure chamber <NUM> through the first gap portion <NUM> (leakage gap) between the outer periphery of the plunger <NUM> and the inner periphery of the cylinder <NUM> can be returned to the oil supply path <NUM> (enlarged gap portion <NUM>).

Also, in this chain tensioner <NUM>, since the plunger <NUM> has, on its outer periphery, and, from the side close to the insertion end of the plunger <NUM> toward the side remote from this insertion end, a first large-diameter radially outer surface <NUM>, a small-diameter radially outer surface <NUM>, and a second large-diameter radially outer surface <NUM>, such that the first gap portion <NUM> between the first large-diameter radially outer surface <NUM> and the inner periphery of the cylinder <NUM> acts as the leakage gap, even when the plunger <NUM> axially moves relative to the cylinder <NUM>, the axial length of the leakage gap remains unchanged, i.e., constant, thus making it possible to obtain a stable damper force.

Also, in this chain tensioner <NUM>, since the cylinder <NUM> has, in its inner periphery, a circumferential groove <NUM> such that the entire oil outlet <NUM> of the cylinder-side oil path portion <NUM> is located within the range of the circumferential groove <NUM>, even if, when forming the cylinder-side oil path portion <NUM> in the cylinder <NUM>, burrs occur on the oil outlet <NUM> of the cylinder-side oil path portion <NUM>, which is open to the inner periphery of the cylinder <NUM>, such burrs are prevented from interfering with the first and second large-diameter radially outer surfaces <NUM> and <NUM> on the outer periphery of the plunger <NUM>. This ensures stable behavior of the plunger <NUM>.

A seal member may be disposed between the outer periphery of the plunger <NUM> and the inner periphery of the cylinder <NUM> so as to seal the second gap portion <NUM>. For example, an annular seal member may be mounted in a seal groove (not shown) formed in the second large-diameter radially outer surface <NUM> on the outer periphery of the plunger <NUM>, so as to come into sliding contact with the inner periphery of the cylinder <NUM>. Such a sealing arrangement very effectively reduces the amount of oil consumed by the chain tensioner <NUM>.

<FIG> illustrates a chain tensioner <NUM> according to the second embodiment of the present invention. The elements of the second embodiment corresponding to those of the first embodiment are denoted by the same reference numerals, and their description is omitted.

As illustrated in <FIG>, the check valve <NUM> of this embodiment includes a first valve element <NUM> configured to open and close the first through hole <NUM> from the side of the pressure chamber <NUM>; and a first seat surface <NUM> on which the first valve element <NUM> is configured to be seated to close the first through hole <NUM>.

The first seat surface <NUM> is formed on the inner periphery of the first through hole <NUM>. The first seat surface <NUM> is a tapered inner peripheral surface inclined in the radially expanding direction toward the pressure chamber <NUM>. Also formed on the inner periphery of the first through hole <NUM> is a cylindrical inner peripheral surface <NUM> continuously extending from the first seat surface <NUM> toward the pressure chamber <NUM>. The inner peripheral surface <NUM> has an axial length larger than the diameter of the first valve element <NUM>. The first valve element <NUM> is received in the first through hole <NUM> so as to be axially movable between (i) a closed position where the first valve element <NUM> is seated on the first seat surface <NUM>, and (ii) an open position where the first valve element <NUM> is separated from the first seat surface <NUM> by moving from the closed position toward the pressure chamber <NUM>.

A first valve spring <NUM> is disposed in the first through hole <NUM> to bias the first valve element <NUM> from the open position toward the closed position. A first spring supporting member <NUM> is fixed to the end opening of the first through hole <NUM> on the side of the pressure chamber <NUM> to support the first valve spring <NUM>. In the example shown, the first spring supporting member <NUM> is an annular member made of metal and fitted to the inner periphery of the first through hole <NUM> at its end.

Similarly, the relief valve <NUM> includes a second valve element <NUM> configured to open and close the second through hole <NUM> from the side of the reservoir chamber <NUM>; and a second seat surface <NUM> on which the second valve element <NUM> is configured to be seated to close the second through hole <NUM>.

The second seat surface <NUM> is formed on the inner periphery of the second through hole <NUM>. The second seat surface <NUM> is a tapered inner peripheral surface inclined in the radially expanding direction toward the reservoir chamber <NUM>. Also formed on the inner periphery of the second through hole <NUM> is a cylindrical inner peripheral surface <NUM> which continuously extends from the second seat surface <NUM> toward the reservoir chamber <NUM>. The inner peripheral surface <NUM> has an axial length larger than the diameter of the second valve element <NUM>. The second valve element <NUM> is received in the second through hole <NUM> so as to be axially movable between (i) a closed position where the second valve element <NUM> is seated on the second seat surface <NUM>, and (ii) an open position where the second valve element <NUM> is separated from the second seat surface <NUM> by moving from the closed position toward the reservoir chamber <NUM>.

A second valve spring <NUM> is disposed in the second through hole <NUM>, and biases the second valve element <NUM> from the open position toward the closed position. A second spring supporting member <NUM> is fixed to the end opening of the second through hole <NUM> on the side of the reservoir chamber <NUM>, to support the second valve spring <NUM>. In the example shown, the second spring supporting member <NUM> is an annular member made of metal and fitted to the inner periphery of the second through hole <NUM> at its end.

The partition wall <NUM> is an integral plate-shaped member formed with the first seat surface <NUM> and the second seat surface <NUM>. The outer periphery of the partition wall <NUM> is a cylindrical surface. The partition wall <NUM> is fitted to the inner periphery of the plunger <NUM>, and axially positioned by a shoulder <NUM> formed on the inner periphery of the plunger <NUM>.

In this chain tensioner <NUM>, since the first valve element <NUM> and the first valve spring <NUM>, which constitute the check valve <NUM>, are received in the first through hole <NUM> of the partition wall <NUM>, whereas the second valve element <NUM> and the second valve spring <NUM>, which constitute the relief valve <NUM>, are received in the second through hole <NUM> of the partition wall <NUM>, the axial dimensions of the check valve <NUM> and the relief valve <NUM> are short. Therefore, the size of this chain tensioner <NUM> is small. Otherwise, this chain tensioner <NUM> is the same in its function and effects as the chain tensioner of the first embodiment.

<FIG> illustrates a check valve <NUM> and a relief valve <NUM> of a chain tensioner <NUM> according to the third embodiment of the present invention, and the vicinity of these valves. The elements of the third embodiment corresponding to those of the first and second embodiments are denoted by the same reference numerals, and their description is omitted.

The check valve <NUM> is inserted in the first through hole <NUM>, which is, in this embodiment, a stepped hole directly formed in the partition wall <NUM>. The first through hole <NUM> is a stepped hole having a larger diameter on the side of the pressure chamber <NUM>, and a smaller diameter on the side of the reservoir chamber <NUM>. The check valve <NUM> is retained in the first through hole <NUM> by a first annular member <NUM> inserted and fixed in position in the end portion of the first through hole <NUM> on the side of the pressure chamber <NUM>.

The first through hole <NUM> includes a first small-diameter portion <NUM> having a constant inner diameter, and extending from the end surface of the partition wall <NUM> on the side of the reservoir chamber <NUM> toward the pressure chamber <NUM>; a first step <NUM> radially expanding from the first small-diameter portion <NUM> toward the pressure chamber <NUM>; and a first large-diameter portion <NUM> extending from the first step <NUM> toward the pressure chamber <NUM>, and having an inner diameter larger than the inner diameter of the first small-diameter portion <NUM>. The end portion of the first large-diameter portion <NUM> on the side of the pressure chamber <NUM> is formed with a shoulder <NUM> axially supporting the end of the first annular member <NUM> on the side of the reservoir chamber <NUM>.

The check valve <NUM> includes a first valve element <NUM> movable between (i) a closed position where the first valve element <NUM> is in contact with the first step <NUM> and (ii) an open position where the first valve element <NUM> has moved from the closed position toward the pressure chamber <NUM>; and a first valve spring <NUM> biasing the first valve element <NUM> from the side of the pressure chamber <NUM> toward the reservoir chamber <NUM>.

The first valve spring <NUM> is a coil spring disposed in, and coaxial with, the first large-diameter portion <NUM>. The end of the first valve spring <NUM> close to the pressure chamber <NUM> is supported by the first annular member <NUM>, while the end of the first valve spring <NUM> close to the reservoir chamber <NUM> presses the first valve element <NUM>. The first annular member <NUM> is fixed to the partition wall14 by being inserted, with interference, into the end of the first through hole <NUM> on the side of the pressure chamber <NUM>. The first annular member <NUM> is formed of a sintered alloy or a resin.

The members constituting the check valve <NUM>, which are the first annular member <NUM>, the first valve spring <NUM>, and the first valve element <NUM>, are arranged in this order from the side of the pressure chamber <NUM> toward the reservoir chamber <NUM>. The first step <NUM> in the first through hole <NUM> constitutes a valve seat on which the first valve element <NUM> of the check valve <NUM> is configured to be seated.

The relief valve <NUM> is inserted in a second through hole <NUM> comprising a stepped hole directly formed in the partition wall <NUM>. The second through hole <NUM> is a stepped hole having a larger diameter on the side of the pressure chamber <NUM>, and a smaller diameter on the side of the reservoir chamber <NUM>. The relief valve <NUM> is retained in the second through hole <NUM> by a second annular member <NUM> inserted and fixed in position, in the end portion of the second through hole <NUM> on the side of the pressure chamber <NUM>.

The second through hole <NUM> includes a second small-diameter portion <NUM> having a constant inner diameter, and extending from the end surface of the partition wall <NUM> on the side of the reservoir chamber <NUM> toward the pressure chamber <NUM>; a second step <NUM> radially expanding from the second small-diameter portion <NUM> toward the pressure chamber <NUM>; and a second large-diameter portion <NUM> extending from the second step <NUM> toward the pressure chamber <NUM>, and having an inner diameter larger than the inner diameter of the second small-diameter portion <NUM>. The end portion of the second large-diameter portion <NUM> on the side of the pressure chamber <NUM> is formed with a shoulder <NUM> axially supporting the end of the second annular portion <NUM> on the side of the reservoir chamber <NUM>. The second small-diameter portion <NUM> has an inner diameter smaller than the inner diameter of the first small-diameter portion <NUM>.

The relief valve <NUM> includes a second valve element <NUM> movable between (i) a closed position where the second valve element <NUM> is in contact with the second annular member <NUM> and (ii) an open position where the second valve element <NUM> has moved from the closed position toward the reservoir chamber <NUM>; and a second valve spring <NUM> biasing the second valve element <NUM> from the side of the reservoir chamber <NUM> toward the pressure chamber <NUM>.

The second valve spring <NUM> is a coil spring disposed in, and coaxial with, the second large-diameter portion <NUM>. The end of the second valve spring <NUM> close to the reservoir chamber <NUM> is supported by the second step <NUM>, while the end of the second valve spring <NUM> close to the pressure chamber <NUM> presses the second valve element <NUM>. The second annular member <NUM> is fixed to the partition wall <NUM> by being inserted, with interference, into the end of the second through hole <NUM> on the side of the pressure chamber <NUM>. The second annular member <NUM> is formed of a sintered alloy.

The members constituting the relief valve <NUM>, which are the second annular member <NUM>, the second valve element <NUM>, and the second valve spring <NUM>, are arranged in this order from the side of the pressure chamber <NUM> toward the reservoir chamber <NUM>. The second annular member <NUM> constitutes a valve seat on which the second valve element <NUM> of the relief valve <NUM> is configured to be seated. The second step <NUM> in the second through hole <NUM> constitutes a valve retainer retaining the second valve element <NUM> and the second valve spring <NUM>.

The partition wall <NUM> is fitted to the inner periphery of the plunger <NUM> in a dimensional relationship where no interference exists. By setting no interference between the outer periphery of the partition wall <NUM> and the inner periphery of the plunger <NUM>, it is possible to strictly control the size of the first gap portion <NUM> (leakage gap) with high dimensional accuracy. One end of the return spring <NUM> is in contact with the end of the partition wall <NUM> on the side of the pressure chamber <NUM>. The partition wall <NUM> is pressed against a shoulder <NUM> on the inner periphery of the plunger <NUM> by the force which the partition wall <NUM> receives from the return spring <NUM>.

In this chain tensioner <NUM>, it is possible to reliably prevent falling off of the relief valve <NUM> due to, e.g., vibrations of the plunger <NUM>, or the pressure which the relief valve <NUM> receives from the oil in the pressure chamber <NUM>.

Specifically, the relief valve <NUM> is inserted, from the side of the pressure chamber <NUM>, into the second through hole <NUM>, which is a stepped hole having a larger diameter on the side of the pressure chamber <NUM>, and a smaller diameter on the side of the reservoir chamber <NUM>. Since the direction of the pressure applied to the relief valve <NUM> from the oil in the pressure chamber <NUM> is the same as the direction in which the relief valve <NUM> is inserted into the second through hole <NUM>, which is a stepped hole, it is possible to reliably prevent falling off of the relief valve <NUM> from the second through hole <NUM> due to. e.g., vibrations of the plunger <NUM>, or the pressure which the relief valve <NUM> receives from the oil in the pressure chamber <NUM>.

Also, in this chain tensioner <NUM>, since, when mounting the check valve <NUM> and the relief valve <NUM> to the partition wall <NUM>, the direction in which the check valve <NUM> is inserted into the first through hole <NUM>, and the direction in which the relief valve <NUM> is inserted into the second through hole <NUM> are the same direction (direction from the side of the pressure chamber <NUM> toward the reservoir chamber <NUM>), it is not necessary to change the direction of the partition wall <NUM> when mounting the check valve <NUM> and the relief valve <NUM> to the partition wall <NUM>. Therefore, it is possible to assemble the chain tensioner easily/efficiently.

Also, in this chain tensioner <NUM>, since the second annular member <NUM> functions as a valve seat of the relief valve <NUM> (member on which the second valve element <NUM> is seated), and the second step <NUM> of the second through hole <NUM> functions as a valve retainer of the relief valve <NUM> (member retaining the second valve element <NUM> and the second valve spring <NUM>), it is possible to minimize the number of components constituting the relief valve <NUM>.

While, in each of the above embodiments, the coil diameter of the return spring <NUM> is set such that the return spring <NUM> comes into contact with only the partition wall <NUM> and not the plunger <NUM>, the coil diameter of the return spring <NUM> may be increased such that, as illustrated in <FIG>, the return spring <NUM> comes into contact with both the partition wall <NUM> and the plunger <NUM>.

Also, while, in the above embodiment, a member formed of a sintered alloy or a resin is used as the first annular member <NUM>, a metal plate may be used as the first annular member <NUM> as illustrated in <FIG>. The first annular member <NUM> of <FIG> includes a cylindrical portion <NUM> fitted to the inner periphery of the first through hole <NUM> at its end on the side of the pressure chamber <NUM>; an end plate portion <NUM> integral with the end of the cylindrical portion <NUM> close to the reservoir chamber <NUM>; and holes <NUM> axially extending through the end plate portion <NUM>.

<FIG> illustrate the fourth embodiment of the present invention. The fourth embodiment is different from the third embodiment only in the structure of the relief valve <NUM>, and otherwise the same in structure as the third embodiment. Therefore, only the features of the fourth embodiment different from those of the third embodiment are described below.

As illustrated in <FIG>, a relief valve <NUM> is inserted in a second through hole <NUM> comprising a stepped hole directly formed in the partition wall <NUM>. The second through hole <NUM> is a stepped hole having a larger diameter on the side of the pressure chamber <NUM>, and a smaller diameter on the side of the reservoir chamber <NUM>. The relief valve <NUM> is retained in the second through hole <NUM> by a second annular member <NUM> inserted and fixed in position, in the end portion of the second through hole <NUM> on the side of the pressure chamber <NUM>.

As illustrated in <FIG>, the relief valve <NUM> includes an annular second valve seat <NUM>; a second valve element <NUM> movable between (i) a closed position where the second valve element <NUM> is in contact with the annular second valve seat <NUM> and (ii) an open position where the second valve element <NUM> has been moved from the closed position toward the reservoir chamber <NUM> (see <FIG>) (to the right in <FIG>); a second valve spring <NUM> biasing the second valve element <NUM> from the side of the reservoir chamber <NUM> (see <FIG>) toward the pressure chamber <NUM>; and a valve sleeve <NUM> retaining, as a single assembly, the second valve seat <NUM>, the second valve element <NUM>, and the second valve spring <NUM>.

The valve sleeve <NUM> is a cup-shaped member including a tubular portion <NUM> surrounding the second valve element <NUM> and the second valve spring <NUM>; and an inwardly extending flange portion <NUM> formed at one end of the tubular portion <NUM>. The valve sleeve <NUM> can be formed of a metal or a resin. The inwardly extending flange portion <NUM> supports the second valve spring <NUM>. The second valve seat <NUM> is press-fitted to the other end of the tubular portion <NUM>. The second valve seat <NUM> is formed of a sintered alloy.

As illustrated in <FIG>, the valve sleeve <NUM> is inserted in the second large-diameter portion <NUM>, while retaining, as a single assembly, the second valve seat <NUM>, the second valve element <NUM>, and the second valve spring <NUM>, and the valve sleeve <NUM> is prevented from being pulled out of the second large-diameter portion <NUM> by the second annular member <NUM>. The second annular member <NUM> is fixedly press-fitted in the second through hole <NUM>.

In the chain tensioner <NUM> of this embodiment, since the relief valve <NUM> includes a valve sleeve <NUM> retaining, as a single assembly, the second valve seat <NUM>, the second valve element <NUM>, and the second valve spring <NUM>, it is possible to first assemble, as a single assembly, the components constituting the relief valve <NUM> before mounting the relief valve <NUM> into the second through hole <NUM>, and then to mount the single assembly into the second through hole <NUM>.

Claim 1:
A chain tensioner comprising:
a tubular cylinder (<NUM>) of which one axial end is a closed end and the other axial end is an open end;
a tubular plunger (<NUM>) axially slidably inserted in the cylinder (<NUM>), and having an open insertion end inserted in the cylinder (<NUM>), and a closed protruding end protruding out of the cylinder (<NUM>);
a partition wall (<NUM>) that axially divides a space surrounded by the cylinder (<NUM>) and the plunger (<NUM>) into (i) a pressure chamber (<NUM>) of which a volume changes as the plunger (<NUM>) axially moves, and (ii) a reservoir chamber (<NUM>) of which a volume remains unchanged even when the plunger (<NUM>) axially moves;
an oil supply path (<NUM>) through which oil supplied from outside the cylinder (<NUM>) is introduced into the reservoir chamber (<NUM>);
a first through hole (<NUM>) axially extending through the partition wall (<NUM>);
a check valve (<NUM>) that allows only movement of oil from a side of the first through hole (<NUM>) close to the reservoir chamber (<NUM>) to a side of the first through hole (<NUM>) close to the pressure chamber (<NUM>);
a leakage gap that allows leakage of oil from the pressure chamber (<NUM>) when the plunger (<NUM>) axially moves such that the volume of the pressure chamber (<NUM>) decreases; and
a return spring (<NUM>) biasing the plunger (<NUM>) in a direction in which the plunger (<NUM>) protrudes out of the cylinder (<NUM>),
wherein the oil supply path (<NUM>) is connected to an outlet end of the leakage gap such that oil that has leaked from the pressure chamber (<NUM>) through the leakage gap returns to the oil supply path (<NUM>),
characterized in that the chain tensioner further comprises:
a second through hole (<NUM>) axially extending through the partition wall (<NUM>) in parallel with the first through hole (<NUM>); and
a relief valve (<NUM>) disposed at the second through hole (<NUM>), and configured to release oil from the pressure chamber (<NUM>) into the reservoir chamber (<NUM>) when pressure in the pressure chamber (<NUM>) exceeds a predetermined pressure,
wherein the first through hole (<NUM>) is formed at a position displaced from a center of the plunger (<NUM>) to one of two opposite sides of the center of the plunger (<NUM>), and
wherein the second through hole (<NUM>) is formed at a position displaced from the center of the plunger (<NUM>) to the other of the two opposite sides of the center of the plunger (<NUM>).