JPH0754943A - Auto tensioner - Google Patents

Abstract

(57) [Summary] [Purpose] To obtain a structure in which the installation space does not increase and the belt is securely prevented from slackening. A base end portion of a swing member 38 is swingably supported by a fixed shaft 37, and a pulley 43 is attached to a tip end portion of the swing member 38.
Rotatably supported. In order to press the pulley 43 against the belt 69 by the torsion coil spring 49, elastic force in the rotational direction is applied to the swing member 38. When the swing member 38 is rotated by the elastic force of the torsion coil spring 49, the damper device 55
The plunger 59 and the receiving block 58 supported by the swinging member 38 are separated from each other. Therefore, the pulley 43 is rapidly displaced. When rotating in the opposite direction, the receiving block 58 pushes the plunger 59. In this state, the displacement of the pulley 43 is slowed down by the action of the damper device 55.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The automatic tensioner according to the present invention is a timing belt for an automobile engine, or a belt for driving an auxiliary machine such as an alternator or a compressor (hereinafter, these are collectively referred to as "belt"). ) To give the proper tension.

[0002]

2. Description of the Related Art Since a camshaft of an OHC type engine or a DOHC type engine is rotationally driven in synchronization with a crankshaft, a drive device using a belt 1 as shown in FIG. 6 is widely used. In FIG. 6, 2 is a drive pulley that is driven to rotate by the crankshaft of the engine, 3 is a driven pulley fixed to the end of the camshaft, 4 and 4 are for guiding the belt 1, or are water pumps. Guide pulleys or driven pulleys for driving auxiliary machines such as 5 are belts 1
This is a pulley for applying proper tension to the.

The pulley 5 is pivotally supported by a rocking member 7 which rocks around a pivot 6. A spring 8 is provided at the end of the bracket 15 fixed to the swing member 7, and the pulley 5 is elastically pressed toward the belt 1 so that the dimensions of the belt 1 and the like due to temperature changes and the like. The tension of the belt 1 is always kept constant irrespective of changes and vibrations accompanying the operation of the engine.

By the way, the belt 1 is included in the auto tensioner.
In addition to the function of applying tension to the belt, a function of holding down the belt 1 is required. That is, in the camshaft drive device shown in FIG. 6, when the drive pulley 2 is rotated, a part of the belt 1 located between the pulleys 2 to 5 is
It tends to vibrate in a direction perpendicular to the traveling direction. If this shake is allowed as it is, the amplitude of the shake becomes large (vibration grows), a large vibration is generated in the driving portion by the belt 1, or in the case of a toothed belt, so-called tooth skipping occurs. Cause problems.

For this reason, for example, Japanese Patent Laid-Open No. 58-65 has been used.
As described in Japanese Patent No. 357 and No. 63-180759, a damper device is incorporated in an auto tensioner, and the vibration of the belt 1 is suppressed by the damper device. FIG. 7 shows an example of an auto tensioner incorporating such a damper device.

A rocking member 10 and a slide bearing 11 are provided around a fixed shaft 9 fixed to the front surface of a cylinder block of an engine.
It is swingably supported by. The front half 12 of the swing member 10 is eccentric with respect to the fixed shaft 9. Then, the pulley 5 is provided around the tip half portion 12, and the rolling bearing 1
It is rotatably supported by 3. Further, a bracket 15 is externally fitted and fixed to the base half portion 14 of the swing member 10. Then, by engaging the end portion of a tension spring (not shown) with the engaging portion 16 formed at the end portion of the bracket 15, the swinging member 10 has an elastic force in the direction of rotation around the fixed shaft 9. Is given. Above pulley 5
The outer peripheral surface of is elastically pressed toward the belt 1 (FIG. 6) based on this elasticity.

In the space 17 existing between the outer peripheral surface of the fixed shaft 9 and the inner peripheral surface of the rocking member 10, a plurality of fixed side flat plates 18 whose inner peripheral edges are non-rotatably supported with respect to the fixed shaft 9. , 18
And the outer peripheral edge of the swing member 1 on the inner peripheral surface of the swing member 1.
A plurality of rocking-side flat plates 19 and 19 supported so as to be incapable of rotating with respect to 0 are arranged alternately, and the space 17
A viscous fluid such as oil is filled in the inside through the oil supply hole 20. After filling the viscous fluid, the oil supply hole 20 is closed with a steel ball 21. Reference numeral 32 is a restraint plate fitted and fixed to the fixed shaft 9 in order to prevent the flat plates 18 and 19 from separating from each other.

The side surfaces of the plurality of fixed side flat plates 18 and 18 and the side surfaces of the plurality of rocking side flat plates 19 and 19 face each other with a very narrow gap therebetween. Due to the presence of the fluid, the sudden displacement of the flat plates 18 and 19 from each other is prevented by the shearing force applied to the viscous fluid present in the gap, and as a result, the tension of the belt 1 partially increases sharply. In this case, the pulley 5 will not immediately retract.

Further, in order to prevent the slack of the belt 1 more reliably, the pulley 5 is swiftly followed when the belt 1 is slack, and the pulley 5 is not retracted when the belt 1 is tensioned. An automatic tensioner that holds down the belt 1 is also disclosed in, for example, JP-A-63-167163, JP-A-2-72252, and JP-A-2-80839.
It is conventionally known as described in JP-A-3-24346, JP-B-2-16846, JP-A-2-98243, JP-A-4-39349 and the like.

The auto tensioner described in each of these publications is provided with a one-way clutch between the viscous damper and the swinging member (Japanese Patent Laid-Open No. 63-167163), or a ratchet mechanism (Japanese Laid-Open Patent Publication No. HEI-2). No. 80839, Japanese Utility Model Laid-Open No. 2-16846), a checker valve is incorporated to provide a damper device that functions only in one direction (Japanese Patent Laid-Open No. 2-72).
No. 252, No. 3-24346, No. 2-9
No. 8243), a damper device having an inclined surface for facilitating the flow of a viscous fluid in only one direction may be provided (actual open flat 4-3).
9349).

Further, JP-A-62-274143,
Japanese Utility Model Laid-Open No. 60-52458 discloses a structure in which an oscillating member and a damper device are separately provided and separately attached to the front surface of a cylinder block of an engine to form an auto tensioner. FIG. 8 shows an example of such an automatic tensioner.

A swing member 10 supporting the pulley 5 is pivotally supported by a shaft 22 on the front surface of the cylinder block or the like. The cylinder 24 that constitutes the damper device 23 is fixed to the front surface of the cylinder block or the like separately from the swing member 10.
A hollow piston 25 is freely displaceable in the viscous fluid enclosed in the cylinder 24. A compression spring 26 is provided between one end surface of the piston 25 and the rear end surface of the cylinder 24, and a base end portion of a push rod 27 is connected to the other end surface. The tip end portion of the push rod 27 is abutted against a part of the swing member 10 and the swing member 10
In addition, the elastic force of the compression spring 26 can be transmitted. Therefore, the pulley 5 is pressed against the belt 1 based on the elasticity of the compression spring 26. In addition, the actual exploitation sho
As described in Japanese Patent Publication No. 8, in addition to the compression spring 26 in the damper device 23, a spring may be provided in the swing member 10.

On the other hand, the pair of chambers 28 and 29 existing on both sides of the piston 25 communicate with each other via an oil passage 30 provided at the center of the piston 25 and the push rod 27. A check valve 31 is provided at the open end of the oil passage 30 so that the check valve 31 opens only when a viscous fluid flows toward the chamber 28 provided with the compression spring 26.

In the case of the auto-tensioner shown in FIG. 8 constructed in this way, the push rod 27 is rapidly displaced in the projecting direction (upward direction in FIG. 8) and slowly in the retracting direction (downward direction). When the belt 1 is slackened, the pulley 5 is swiftly followed, and when the belt 1 is tensioned, the belt 1 is held down without retracting the pulley.

[0015]

SUMMARY OF THE INVENTION The present invention provides an automatic tensioner that can be installed in a narrow installation space at a relatively low cost while ensuring the followability when the belt is slack.

That is, in the case of the conventional structure shown in FIG. 7, since the pulley 5 can be displaced only slowly in any direction, when the belt becomes loose, the pulley 5 cannot follow this and the tension of the belt is increased. It may be temporarily lowered.

Further, the above-mentioned JP-A-63-167163, JP-A-2-72252, JP-A-2-80839, JP-A-3-24346 and JP-A-2-16846.
In the case of the auto tensioner described in Japanese Patent Laid-Open No. 2-98243, Japanese Patent Laid-Open No. 4-39349, etc., the one-way clutch suffers from fretting wear, resulting in insufficient durability (Japanese Patent Laid-Open No. 63- No. 167163), the internal structure is complicated and the manufacturing is difficult, and the manufacturing cost is high (Japanese Patent Laid-Open No. Hei 2).
-72252, 2-80839, 3-
No. 24346, Japanese Utility Model Laid-Open No. 2-16846, No. 2
No. 98243), the characteristics tend to change over time due to wear of parts, and even if the belt is loosened, some resistance from the viscous fluid is received, so the followability of the pulley is not always sufficient (Japanese Patent Laid-Open No. Hei 2). -72252, 3-2.
No. 4346, No. 2-98243, No. 4-
No. 39349).

Further, in the case of the auto tensioner shown in FIG. 8, since it is necessary to separately mount the swinging member 10 and the damper device 23 on the front surface of the cylinder block or the like, the installation work is not only troublesome, but also the installation work is difficult. The space becomes large, and the engine incorporating this auto tensioner becomes large.

On the other hand, Japanese Utility Model Laid-Open No. 2-56950 discloses that
In the structure shown in FIG. 8, the member provided with the shaft 22 and the cylinder 24 forming the damper device 23 are integrated, and the elastic force of the torsion coil spring pushes the pulley toward the belt to reduce the size. And an invention for improving the assembling property are described. However, in the structure described in this publication, the distance between the shaft 22 and the pivot that pivotally supports the pulley is short, and the displacement amount of the pulley that presses the belt is larger than the displacement amount of the push rod of the damper device. I can't take it.

By increasing the distance between the shaft 22 and the pivot, the displacement amount of the pulley is increased, and as an autotensioner that presses the pulley toward the belt by the elastic force of the torsion coil spring, for example, actual flat blade 2-54946. Although the structure described in Japanese Patent Publication is known, in the case of this conventional structure, since the pulley steadying is simply achieved by frictional resistance and internal loss of rubber, the belt steadying effect is incomplete. Is.

The autotensioner of the present invention solves any of the problems described above.

[0022]

An automatic tensioner of the present invention comprises a fixing member, a fixing shaft provided on the fixing member,
A swing member whose base end is rotatably supported around the fixed shaft, a pivot shaft provided at the tip end of the swing member and parallel to the fixed shaft, and rotatable around the pivot shaft. And a pair of locking portions provided at both ends of the coil portion, the coil portion being arranged around the fixed shaft, and the pair of locking portions being fixed to the fixing member and the fixing member. A torsion coil spring that applies an elastic force to the rocking member in the direction of pressing the pulley toward the belt by locking the rocking member to the rocking member, a fixed arm piece provided on the fixing member, and the rocking member. The rocking side arm piece provided on the member is provided with one end thereof to support the base end portion thereof, and the other of the fixed side arm piece and the rocking side arm piece is provided with a damper device having its tip end portion opposed thereto. And
This damper device is provided between a cylinder cylinder in which a viscous fluid is sealed, a piston axially displaceably fitted inside the cylinder cylinder, and between the piston and the cylinder cylinder. A spring for urging the piston in one direction, a plunger that increases the amount of protrusion from the cylinder tube in accordance with the displacement of the piston based on the elastic force of the spring, and a passage that communicates both axial end surfaces of the piston. A check valve for opening and closing the passage, the check valve having a function of closing only when the piston is displaced against the elastic force of the spring. In addition, the fixed shaft exists outside the outer peripheral surface of the pulley in the diametrical direction.

[0023]

In the operating state of the autotensioner of the present invention constructed as described above, the oscillating member oscillates on the basis of the elastic force of the torsion coil spring, and the oscillating member is rotated about the pivot axis. The freely supported pulley is elastically pressed toward the belt. Since the pulley is pressed against the belt, the swing of the swing member is limited. Therefore, with the displacement of the piston that constitutes the damper device supported by one of the fixed-side arm piece and the swing-side arm piece, the plunger moves to the other of the fixed-side arm piece and the swing-side arm piece. When abutting against each other, the tip end portion of this plunger is pressed against the other arm piece by the elastic force of the spring that constitutes the damper device.

When the belt is loosened from this state, the rocking member rocks based on the elasticity of the torsion coil spring, causing the pulley to follow the movement of the belt. At this time, the displacement of the plunger is slightly delayed, so that the tip of the plunger is separated from the other arm piece. Therefore, when the belt is loosened, the damper device does not become a resistance against rotating the swinging member so that the pulley follows the movement of the belt.
The pulley is made to quickly follow the movement of the belt to prevent the tension of the belt from decreasing. Due to the elasticity of the spring, the plunger protrudes from the cylinder with a little delay from the movement of the swinging member until its tip end abuts against the other arm piece. In this way, when the plunger is pushed out of the cylinder based on the elasticity of the spring, the check valve in the damper device opens, so that the piston and the plunger are displaced relatively quickly, and after a very short time, The tip of this plunger abuts the other arm piece.

On the contrary, when the tension of the belt increases, the swing member tends to rotate against the elastic force of the torsion coil spring. In this state, since the other arm piece is pressed against the tip of the plunger, the plunger has to be pushed into the cylinder against the elastic force of the spring in order to rotate the swing member. At this time, since the check valve of the damper device is closed, the displacement of the piston and the plunger can be performed only slowly. As a result, the displacement of the pulley supported by the swinging member is only slowly performed by the action of the damper device, and the belt is restrained by the pulley, so that the vibration of the belt can be restrained.

[0026]

1 to 4 show a first embodiment of the present invention. The fixing member 33 has a mounting hole 34 in the middle portion, and is fixed to the front surface of the cylinder block or the like by a bolt (not shown) inserted in the mounting hole 34. A base end of an annular fixing shaft 37 is internally fitted and fixed in a circular hole 36 formed at one end (upper end in FIGS. 1 and 2) of the fixing member 33.

The base end portion (lower end portion in FIGS. 1 and 2) of the swing member 38 is rotatably supported around the fixed shaft 37. That is, the cylindrical portion 39 formed at the base end portion of the swing member 38 is provided around the fixed shaft 37, and the slide bearings 40 and 41 are provided.
Is fitted through. Then, the bolt 35 (FIG. 3) inserted into the fixed shaft 37 is screwed into a screw hole formed in the front surface of the cylinder block or the like to prevent the rotation of the fixing member 33 together with the bolt inserted into the mounting hole 34. I am trying.

A circular convex portion 42 forming a pivot parallel to the fixed shaft 37 is formed at the tip of the swing member 38, and the pulley 43 is rotatably supported by the convex portion 42. The inner ring 47 of the rolling bearing 44 is fixed. That is, the bolt 45 is passed through the center hole of the inner ring 47 and the seat plate 48, and then the nut 46 is tightened to suppress the periphery of the center hole of the inner ring 47, so that the inner ring 47 is prevented from coming off from the convex portion 42. In this case, the pulley 43 is rotatably supported on the tip of the swing member 38. In this state, the fixed shaft 37 is located radially outside the outer peripheral surface of the pulley 43.

A coil portion 50 of a torsion coil spring 49 is arranged around the cylindrical portion 39. The torsion coil spring 49 is provided with a pair of locking portions 51a and 51b at both ends of the coil portion 50, and one locking portion 51a is locked in a locking hole 52a formed in the fixing member 33. The other locking portion 51b is inserted into the locking hole 52b formed in the swing member 38 via the slide bearing 53. The torsion coil spring 49 causes the swing member 38 to move.
In addition, the elastic force to rotate in the clockwise direction in FIG. 1 about the fixed shaft 37 is applied.

Further, a base end portion of a damper device 55 is supported on a fixed side arm piece 54 provided at a position apart from the fixed shaft 37 by a part of the fixed member 33. On the other hand, a rocking side arm piece 56 is provided at a part of the rocking side member 38 at a position apart from the convex portion 42, and a receiving block 58 is provided in a recess 57 formed in the rocking side arm piece 56. It is fixed in place. The damper device 55 is attached to the end surface of the receiving block 58.
The tips of the plungers 59 constituting the above are opposed to each other.

The damper device 55, as shown in FIG.
A viscous fluid such as oil is sealed inside the cylinder cylinder 60, and a piston 61 is inserted inside the cylinder cylinder 60.
It is fitted so that it can be displaced in the axial direction (vertical direction in FIG. 4). A compression spring 62 is provided between the piston 61 and the rear end surface of the cylinder cylinder 60 to press the piston 61 in a direction projecting from the cylinder cylinder 60. The plunger 59 has its base end portion brought into contact with the piston 61. Therefore, when the piston 61 is displaced due to the elasticity of the compression spring 62, the plunger 59 is displaced.
The amount of protrusion from the cylinder cylinder 60 increases.

A circular hole 63 is formed in the center of the piston 61.
Through holes 64, 64 are formed in the hollow plunger 59 with its distal end closed toward the base end.
64 forms a passage that connects both axial end surfaces of the piston 61. The ball 65 is pressed against the opening end of the circular hole 63 by the elastic force of the second compression spring 66 to form a ball valve type check valve 67 that opens and closes the passage. The check valve 67 is closed only when the piston 61 is displaced against the elastic force of the compression spring 62.

Furthermore, in the case of the embodiment shown in the drawings, the stopper pin 68 prevents the torsion coil spring 49 from being elastic,
By disabling the swinging member 38 about the fixed shaft 37, the work of hanging the belt 69 around the pulley 43 can be easily performed. That is, in a state where the swing member 38 is swung against the elastic force of the torsion coil spring 49, the small hole 70 formed in the swing member 38 and the small hole 71 formed in the fixing member 33 are aligned with each other. The stopper pin 68 is inserted into both the small holes 70 and 71.

In this state, the pulley 43 supported by the swinging member 38 is not displaced by the elastic force of the torsion coil spring 49, so that the belt 69 is attached to the pulley 43.
You can easily carry over. When the stopper pin 68 is pulled out after the belt 69 is stretched, the pulley 43 is pressed by the belt 69 based on the elasticity of the torsion coil spring 49.

In the use state of the automatic tensioner of the present invention constructed as described above, the rocking member 38 rocks based on the elasticity of the torsion coil spring 49, and this rocking member 38.
The pulley 43, which is rotatably supported at the tip of the, is elastically pressed toward the belt 69. Pulley 43 is belt 69
When the rocking member 38 is pressed against the rocking member 38, the rocking of the rocking member 38 is limited, and the rocking side arm piece 56 provided on the rocking member 38 is prevented from further displacement. Therefore, when the piston 61 constituting the damper device 55 supported by the fixed side arm piece 54 of the fixing member 33 is displaced by the elastic force of the compression spring 62, the protrusion amount of the plunger 59 from the cylinder cylinder 60 increases,
The tip of the plunger 59 is pressed against the receiving block 58 supported by the tip of the swing-side arm piece 56.

When the belt 69 loosens from this state, the swinging member 38 swings in the clockwise direction in FIG. 1 about the fixed shaft 37 based on the elastic force of the torsion coil spring 49, and the pulley 43 of the belt 69 moves. Follow the movement. At this time, since the displacement of the plunger 59 is slightly delayed, this plunger 5
The tip of 9 and the receiving block 58 are separated from each other. Therefore,
When the belt 69 is loosened, the damper device 55 does not resist the rotation of the swinging member 38 so that the pulley 43 follows the movement of the belt 69, and the pulley 43 moves the belt 69. To quickly follow
This prevents the tension of the belt 69 from decreasing. Further, since the distance between the fixed shaft 37, which is the rotation center of the swinging member 38, and the rotation center of the pulley 43 is sufficiently large, the displacement amount of the pulley 43 can be made sufficiently large and the slack of the belt 69 can be prevented. Even when it is large, the pulley 43 can sufficiently follow this slack.

Due to the elasticity of the compression spring 62, the plunger 59 projects slightly from the movement of the swinging member 38, and projects from the cylinder cylinder 60 until its tip end abuts against the receiving block 58. As described above, when the plunger 59 is pushed out of the cylinder cylinder 60 based on the elasticity of the compression spring 62, the check valve 67 in the damper device 55 opens, so that the piston 61 and the plunger 59 are displaced relatively quickly. After a very short time, this plunger 5
The tip of 9 collides with the receiving block 58.

On the contrary, when the tension of the belt 69 increases, the rocking member 38 resists the elasticity of the torsion coil spring 49,
The fixed shaft 37 tends to swing counterclockwise in FIG. In this state, since the receiving block 58 is pressed against the tip of the plunger 59, in order to rotate the swing member 38, the plunger 59 and the piston 61 are opposed to the elastic force of the compression spring 62 and the damper resistance. Then, it has to be pushed into the cylinder cylinder 60.

At this time, the ball 65 constituting the check valve 67 built in the damper device 55 is pressed against the opening of the circular hole 63, and the passage communicating with both sides of the piston 61 is closed. Therefore, the piston 61 and the plunger 5 are
The displacement of 9 is only performed slowly. As a result, the displacement of the pulley 43 supported by the swinging member 38 is only slowly performed by the action of the damper device 55, and the belt 69 is suppressed by the pulley 43.
The vibration of the belt 69 can be suppressed.

Next, FIG. 5 shows a second embodiment of the present invention. In the case of the present embodiment, contrary to the case of the above-described first embodiment, the cylinder cylinder 60 constituting the damper device 55 is supported by the swing side arm piece 56, and the receiving block is provided at the tip of the fixed side arm piece 54. Supports 58. Other configurations and operations are similar to those of the above-described first embodiment.

[0041]

Since the autotensioner of the present invention is constructed and operates as described above, the installation space does not increase in spite of the structure having excellent durability and reliability, and the belt tension is always optimized. Can be kept in a good state. Also, because the amount of pulley displacement can be increased, even when the amount of belt slack is large,
The pulley can sufficiently follow the movement of this belt.

[Brief description of drawings]

FIG. 1 is a front view showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a part cut away and viewed from the right side of FIG.

FIG. 3 is an exploded perspective view of the first embodiment of the present invention.

FIG. 4 is an enlarged cross-sectional view of a damper device.

FIG. 5 is a front view showing a second embodiment of the present invention.

FIG. 6 is a front view showing a belt drive device of an engine provided with an auto tensioner.

FIG. 7 is a sectional view showing a first example of a conventional structure.

FIG. 8 is a sectional view showing a second example of the same.

[Explanation of symbols]

 1 Belt 2 Drive Pulley 3 Driven Pulley 4 Guide Pulley or Driven Pulley 5 Pulley 6 Axis 7 Swing Member 8 Spring 9 Fixed Shaft 10 Swing Member 11 Sliding Bearing 12 Tip Half 13 Rolling Bearing 14 Base Half 15 Bracket 16 Engagement Stop 17 Space 18 Fixed side flat plate 19 Swing side flat plate 20 Oil supply hole 21 Steel ball 22 Shaft 23 Damper device 24 Cylinder 25 Piston 26 Compression spring 27 Push rod 28, 29 Chamber 30 Oil passage 31 Check valve 32 Suppression plate 33 Fixing member 34 Mounting hole 35 Bolt 36 Circular hole 37 Fixed shaft 38 Swing member 39 Cylindrical part 40, 41 Sliding bearing 42 Convex part 43 Pulley 44 Rolling bearing 45 Bolt 46 Nut 47 Inner ring 48 Seat plate 49 Torsion coil spring 50 Coil part 51a , 51b Locking part 52a, 52b Locking hole 53 Sliding bearing 54 Fixed Side arm piece 55 Damper device 56 Swing side arm piece 57 Recessed portion 58 Receiving block 59 Plunger 60 Cylinder cylinder 61 Piston 62 Compression spring 63 Circular hole 64 Through hole 65 Ball 66 Second compression spring 67 Check valve 68 Stopper pin 69 Belt 70,71 small holes

Claims (1)

[Claims] 1. A fixed member, a fixed shaft provided on the fixed member, a swinging member whose base end is rotatably supported around the fixed shaft, and a tip end of the swingable member. The fixed shaft is provided with a pivot shaft parallel to the fixed shaft, a pulley rotatably supported around the pivot shaft, and a pair of locking portions at both ends of the coil portion. And the pair of locking portions are respectively locked to the fixed member and the rocking member, so that the rocking member is given an elastic force in a direction of pressing the pulley toward the belt. The base end portion of the torsion coil spring is supported by one of the fixed side arm piece provided on the fixed member and the swing side arm piece provided on the swing member. The other side arm piece and a damper device having its tip end opposed to the other side arm piece,
This damper device is provided between a cylinder cylinder in which a viscous fluid is sealed, a piston axially displaceably fitted inside the cylinder cylinder, and between the piston and the cylinder cylinder. A spring for urging the piston in one direction, a plunger that increases the amount of protrusion from the cylinder tube in accordance with the displacement of the piston based on the elastic force of the spring, and a passage that communicates both axial end surfaces of the piston. A check valve for opening and closing the passage, the check valve having a function of closing only when the piston is displaced against the elastic force of the spring, and the fixed shaft of the pulley. An auto tensioner that exists on the diametrically outer side of the outer peripheral surface.