Abstract:
A tensioner comprises shaft members that mate with each other by way of thread portions. A first shaft member is rotatable with respect to a casing and is restrained from moving in its axial direction. A second shaft member, which is restrained from rotating with respect to the casing, is movable in its axial direction. A torsion spring applies torque in a first direction to the first shaft member. A torque switching member, which can switch frictional torque in association with the rotation of the first shaft member, is provided between the first shaft member and the casing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation Application of PCT Application No. PCT/JP99/06700, filed Nov. 30, 1999, which was not published under PCT Article 21(2) in English. 
    
    
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 10-339685, filed Nov. 30, 1998; and No. 11-328865, filed Nov. 18, 1999, the entire contents of both of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a tensioner for appropriately maintaining the tension of a force transmitting member, such as an endless belt or endless chain, in a power transmission mechanism that uses the force transmitting member. 
     A force transmitting member, such as an endless belt or chain, is used in a power transmission mechanism that transmits rotary motion to cam shaft in an engine of an automobile, for example. In some cases, a tensioner is used to keep the tension of the force transmitting member appropriate. FIGS. 21 and 22 individually show sections of a conventional tensioner. This tensioner is provided with a casing  1 . A first shaft member  2  and a tubular second shaft member  3  are inserted in the casing  1 . The casing  1  is provided with a flange portion  1   b  that has a mounting hole  1   a  for fixation on an apparatus such as an engine. An external thread portion is formed on the outer surface of the first shaft member  2 . An internal thread portion is formed on the inner surface of the second shaft member  3 . These external and internal thread portions mate with each other. A rear end portion  2   a  of the first shaft member  2  is inserted in a fitting hole  9  that is formed inside the casing  1 . The end face of the rear end portion  2   a  is in contact with the inner surface of the casing  1 . A torsion spring  4  is provided around the first shaft member  2 . One end  4   a  of the torsion spring  4  is anchored to the first shaft member  2 , while the other end  4   b  is anchored to the casing  1 . If the spring  4  is twisted, the repulsive force of the spring  4  generates torque that causes the first shaft member  2  to rotate. The first shaft member  2  is rotatable with respect to the casing  1 . 
     The cylindrical second shaft member  3  penetrates a sliding hole  5   a  that is formed in a bearing  5 . As shown in FIG. 22, both the outer peripheral surface of the second shaft member  3  and the inner peripheral surface of the sliding hole  5   a  are noncircular. Thus, the second shaft member  3  is allowed to move in its axial direction with respect to the bearing  5 , and is prevented from rotating. If the first shaft member  2  is rotated by means of the repulsive force of the torsion spring  4 , therefore, the second shaft member  3  generates an axial thrust without rotating. For example, the repulsive force of the spring  4  acts in a direction such that it causes the second shaft member  3  to project from the casing  1 . A moderate tension can be applied to the aforesaid force transmitting member, the belt or chain, by applying this thrust to the force transmitting member. If the second shaft member  3  pushes the force transmitting member, a reactive force from the force transmitting member acts on the shaft member  3 . The shaft member  3  moves in its axial direction to a position such that this reaction force (input load) balances with the thrust of the shaft member  3  that is generated by means of the torsion spring  4 . Thus, the conventional tensioner has a linear characteristic such that the input load is proportional to the movement of the second shaft member  3 . 
     The tension of the force transmitting member, the chain or belt, continuously changes depending on the operating conditions of the engine, for example. Since the conventional tensioner has linear characteristics, however, it cannot easily cope with a wide variation in input load. 
     The following is a description of the relation between the force (thrust) of the tensioner which pushes the force transmitting member and a displacement amplitude σ of the tensioner. The stiffness of the tensioner can be represented by the movement (i.e., displacement amplitude σ) of the second shaft member relative to the load received from the force transmitting member. Although a tensioner with great thrust and high stiffness can resist a heavy input load, its displacement amplitude a is small. If the thrust of the tensioner is made smaller, in contrast with this, a heavy input load cannot be coped with, although the displacement amplitude σ can be made greater. The displacement amplitude σ becomes smaller if the stiffness of the tensioner for a large engine displacement is enhanced. Thus, a high-stiffness tensioner must inevitably be designed to function within a narrow range of displacement amplitude σ, that is, the degree of freedom of the tensioner design is low. 
     The object of the present invention is to provide a tensioner capable of coping with a large variation of amplitude despite its high stiffness, thereby dealing with a wide range of input loads. 
     BRIEF SUMMARY OF THE INVENTION 
     A tensioner of the present invention comprises: a first shaft member rotatably inserted in a casing so as to be restrained from axial movement and having a first thread portion; a second shaft member having a second thread portion mating with the first thread portion, axially movable with respect to the casing, and restrained from rotation; a torsion spring for generating torque capable of rotating the first shaft member; and torque switching means for changing the turning torque of the first shaft member in accordance with the rotational angle of the first shaft member. 
     The torque switching means can use a torque switching member that is adapted to generate a small frictional torque when the rotational angle of the first shaft member is narrow and to generate a large frictional torque when the rotational angle is wide. 
     In the tensioner of this invention, the load applied to the second shaft from a force transmitting member such as a belt or chain causes the first thread portion and the second thread portion to rotate the first shaft member. As long as the rotational angle after the start of rotation of the first shaft member is narrow, the torque switching member generates a small turning torque. If the rotational angle of the first shaft member becomes wider, the torque switching member generates a strong turning torque. Thus, a heavy received load can be coped with, and a small amplitude displacement can be followed satisfactorily. The force transmitting member that is used in a large-displacement engine or the like, for example, can cope with wide variations in the received load, and an appropriate tension can be applied to the force transmitting member. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a sectional view of a tensioner according to a first embodiment of the present invention; 
     FIG. 2 is a sectional view of a part of an engine showing an example of use of the tensioner shown in FIG. 1; 
     FIG. 3 is an exploded perspective view of a torque switching member of the tensioner shown in FIG. 1; 
     FIG. 4 is a diagram showing the relation between the axial length of the tensioner shown in FIG.  1  and torque; 
     FIG. 5A is a sectional view of a part of the tensioner shown in FIG. 1; 
     FIG. 5B is a sectional view of a part of a tensioner according to a second embodiment of the present invention; 
     FIG. 5C is a sectional view of a part of a tensioner according to a third embodiment of the present invention; 
     FIG. 6A is a sectional view of a tensioner according to a fourth embodiment of the present invention; 
     FIG. 6B is a sectional view of the tensioner taken along line F 6 —F 6  of FIG. 6A; 
     FIG. 7 is an enlarged view of a part of the tensioner shown in FIG. 6A; 
     FIG. 8A is a sectional view of a tensioner according to a fifth embodiment of the present invention; 
     FIG. 8B is a sectional view of the tensioner taken along line F 8 —F 8  of FIG. 8A; 
     FIG. 9 is an exploded perspective view of a part of a tensioner according to a sixth embodiment of the present invention; 
     FIG. 10A is a sectional view of a part of the tensioner shown in FIG. 9; 
     FIG. 10B is a sectional view of a part of a tensioner according to a seventh embodiment of the present invention; 
     FIG. 11 is a sectional view of a part of a tensioner according to an eighth embodiment of the present invention; 
     FIG. 12 is a sectional view of a part of a tensioner according to a ninth embodiment of the present invention; 
     FIG. 13 is a sectional view taken along line F 13 —F 13  of FIG. 12; 
     FIG. 14 is a sectional view of a tensioner according to a tenth embodiment of the present invention; 
     FIG. 15 is a diagram showing the relation between the axial length of the tensioner shown in FIG.  14  and torque; 
     FIG. 16 is a sectional view of a tensioner according to an eleventh embodiment of the present invention; 
     FIG. 17 is a diagram showing the relation between the axial length of the tensioner shown in FIG.  16  and torque; 
     FIG. 18 is a sectional view of a tensioner according to a twelfth embodiment of the present invention; 
     FIG. 19 is a diagram showing the relation between the axial length of the tensioner shown in FIG.  18  and torque; 
     FIG. 20 is a sectional view of a tensioner according to a thirteenth embodiment of the present invention; 
     FIG. 21 is a sectional view of a conventional tensioner; and 
     FIG. 22 is a sectional view of the tensioner shown in FIG. 21 taken in the diametrical direction. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment of the present invention will now be described with reference to FIGS. 1 to  5 A. In the description of the embodiments to follow, like numerals are used to designate common components. 
     A tensioner  10  shown in FIG. 1 is used in a power transmission mechanism  101  of an automotive engine  100  shown in FIG. 2, for example. The power transmission mechanism  101  transmits rotary motion of the engine  100  to a camshaft  103  by means of an endless force transmitting member  102  such as a timing belt or chain. The tensioner  10 , which is mounted in a given position on the engine  100 , generates thrust mentioned later, thereby pushing the force transmitting member  102  in the direction indicated by arrow V. 
     The tensioner  10  comprises a hollow casing  11 , a first shaft member  12 , and a second shaft member  13 . Thread portions  16  and  17  of these shaft members  12  and  13  engage each other in a threaded manner, thereby forming a shaft assembly S. The shaft assembly S is inserted in the casing  11 . The casing  11  is formed having a cavity portion  14  that extends in the axial direction of the casing  11  and in which the shaft assembly S is to be inserted. The front end portion of the casing  11  has an opening, through which the second shaft member  13  advances and retreats. A tapped hole  15  is formed in the rear end portion of the casing  11 . A bolt  15   a  for sealing the interior of the casing  11  is screwed into the tapped hole  15 . 
     The external thread portion  16  is formed on the first shaft member  12 . With respect to its axial direction, the first shaft member  12  includes a region  12   a  in which the external thread portion  16  is formed and a torque adjusting portion  12   b.  The second shaft member  13  is cylindrical and has the internal thread portion  17  on its inner peripheral surface. The external thread portion  16  engages the internal thread portion  17 , thereby forming the shaft assembly S. Usually, these thread portions  16  and  17  are designed to have a wider lead angle than conventional threads have. For example, multiple threads such as triple threads are used for them. 
     A torsion spring  18  is provided around the shaft assembly S. The torsion spring  18  extends in the axial direction of the shaft members  12  and  13 . One end portion  18   a  of the torsion spring  18  is anchored to the first shaft member  12 , while the other end portion  18   b  is anchored to the casing  11 . The rear end portion of the first shaft member  12  is formed having a slit  19  that extends in the axial direction of the shaft member  12 . The one end portion  18   a  of the torsion spring  18  is inserted in the slit  19 . A bearing member  20  is fixed to the front part of the casing  11 . The other end portion  18   b  of the torsion spring  18  is fixed by means of the bearing member  20 . The bolt  15   a  is removed from the tapped hole  15 , an operating member W such as a screwdriver is inserted into the hole  15  from the outside of the casing  11 , and the distal end of the operating member W is plugged into the slit  19 . If this is done, the first shaft member  12  can be rotated by means of the operating member W. If the spring  18  is twisted after the first shaft member  12  is rotated in a first direction (e.g., clockwise), the spring  18  stores elastic energy (initial torque) that urges the shaft member  12  to rotate in a second direction (e.g., counterclockwise). 
     The bearing member  20  is fixed to the front end portion of the casing  11  by means of a snap ring  21 . The bearing member  20  is formed having a noncircular sliding hole  20   a  through which the second shaft member  13  is passed. The diametrical cross section of the second shaft member  13  has a noncircular shape corresponding to the sliding hole  20   a.  Although the second shaft member  13  can axially move with respect to the casing  11 , therefore, it is prevented from rotating. A cap  22  is provided on the front end of the second shaft member  13 . As shown in FIG. 2, the second shaft member  13  directly or indirectly abuts on the force transmitting member  102  through the cap  22 . 
     If the shaft member  12  is rotated in the first direction by means of the operating member W, the torsion spring  18  is twisted. The spring  18  stores elastic energy that urges the first shaft member  12  to rotate in the second direction. On the other hand, the second shaft member  13  is prevented from rotating by the bearing member  20 . If the first shaft member  12  is rotated in the first direction by means of the operating member W, therefore, the second shaft member  13  moves in a direction such that it is drawn into the casing  11 . 
     When the first shaft member  12  rotates in the second direction by means of the elastic energy stored by the spring  18 , its torque acts on the second shaft member  13 . Since the rotation of the second shaft member  13  is prevented by the bearing member  20 , however, the second shaft member  13  is subjected to thrust in a direction such that it projects from the casing  11 . On the other hand, a load Z that is delivered from the force transmitting member  102  to the second shaft member  13  acts in a direction such that the second shaft member  13  is pushed back into the casing  11 . Accordingly, torque is generated such that the first shaft member  12  is rotated in the first direction. Forces that resist this torque include frictional torque generated between the first shaft member  12  and the casing  11 , the repulsive force of the torsion spring  18 , etc. As the second shaft member  13  moves to a position where those resisting forces balance with the aforesaid input load, a moderate tension can be applied to the force transmitting member  102 . 
     The tensioner  10  of this embodiment is provided with a torque switching member  30  between the casing  11  and the first shaft member  12 . As shown in FIG. 3, the torque switching member  30  includes a first shaft receiving member  31  and a second shaft receiving member  32 . In this specification, the shaft receiving member sometimes may be referred to simply as “receiving member.” The torque adjusting portion  12   b  of the first shaft member  12  is provided with an end member  33 . An end portion of the torque adjusting portion  12   b  is inserted in the end member  33 . The shaft member  12  and the end member  33  are fixed to each other by means of a pin  34 . The end member  33 , which rotates integrally with the shaft member  12 , constitutes a part of the first shaft member  12 . The end member  33  is formed having a projection  35  that projects toward the first shaft receiving member  31 . The end member  33  may be formed integrally with the first shaft member  12  on an end portion of the shaft member  12 . 
     The first receiving member  31  is in the form of a cylinder having given inside and outside diameters and includes a bottom portion  31   b.  As shown in FIG. 5A, the end portion of the first shaft member  12  is rotatably inserted in the first receiving member  31 . An end face  12   f  of the first shaft member  12  rotates in contact with the bottom portion  31   b  of the first receiving member  31  with a contact diameter D 1 . The first receiving member  31  is formed having a recess  36  that receives the projection  35  of the end member  33 . The recess  36  has a given length with respect to the circumferential direction of the receiving member  31 . The projection  35  can move (rotate) in the circumferential direction of the receiving member  31  within the range of the circumference length of the recess  36 . When the projection  35  moves in the recess  36 , the first shaft member  12  and the receiving member  31  never rotate integrally with each other. In other words, the first shaft member  12  can race with respect to the first receiving member  31  within the angular range indicated by E in FIG.  3 . 
     If the projection  35  moves within the range of E in the circumferential direction of the recess  36 , the projection  35  abuts against an inner surface  36   a  or  36   b  of the recess  36  in the circumferential direction. When the projection  35  abuts against the inner surface  36   a  or  36   b,  the first shaft member  12  rotates integrally with the receiving member  31 . 
     The second receiving member  32  is fixed to the casing  11  in a manner such that it is press-fitted into a circular hollow  37  in the casing  11 . The receiving member  32  is in the form of a cylinder having given inside and outside diameters and includes a bottom portion  32   b.  The first receiving member  31  is rotatably inserted in the second receiving member  32 . As shown in FIG. 5A, the bottom portion  31   b  of the first receiving member  31  and the bottom portion  32   b  of the second receiving member  32  touch each other substantially throughout the surface. These receiving members  31  and  32  can relatively rotate in a manner such that they are in contact with each other with a contact diameter D 2 . 
     The first shaft member  12  is supported in the hollow  37  of the casing  11  by means of the first and second receiving members  31  and  32  that mate with each other. Accordingly, the first shaft member  12  can smoothly rotate without unexpected movement. The receiving members  31  and  32  are formed having connecting holes  31   a  and  32   a,  respectively, in positions corresponding to the slit  19 . In applying the aforesaid initial torque to the torsion spring  18 , the distal end of the operating member W (shown in FIG. 1) can be fitted into the slit  19  through the connecting holes  31   a  and  32   a.  Depending on the material of the casing  11 , the end portion of the first shaft member  12  may be inserted directly into the circular hollow  37  in the casing  11  without using the second receiving member  32 . This particular feature is applicable to all the following embodiments. 
     The first shaft member  12  can rotate in both the first and second directions with respect to the first receiving member  31 . Thus, the projection  35  moves between the inner side surface  36   a  or  36   b  of the recess  36  as long as the shaft member  12  rotates within the aforesaid range of E with respect to the first receiving member  31 . In this case, only the shaft member  12  rotates with the second receiving member  32  and the first receiving member  31  kept stopped. Thus, the end face  12   f  of the first shaft member  12  rotates in contact with the bottom portion  31   b  of the first receiving member  31  with the contact diameter D 1 . Accordingly, a relatively small frictional torque corresponding to the contact diameter D 1  is generated. 
     If the first shaft member  12  rotates further, the projection  35  engages the inner side surface  36   a  or  36   b  of the recess  36 . Based on this engagement, the first receiving member  31  rotates integrally with the shaft member  12 . Thus, the bottom portion  31   b  of the first receiving member  31  rotates in contact with the bottom portion  32   b  of the second receiving member  32  with the contact diameter D 2 . Accordingly, a relatively strong frictional torque corresponding to the contact diameter D 2  is generated. 
     FIG. 4 shows the relation between turning torque that is generated when the second shaft member  13  is subjected to the input load and the axial length of the tensioner  10  of the first embodiment. When the first shaft member  12  starts to rotate under the input load, the projection  35  moves in the recess  36  in the initial stage of the rotation. As this is done, the shaft member  12  rotates with the contact diameter D 1  with respect to the first receiving member  31 , so that a relatively small frictional resistance is generated. If the shaft member  12  rotates in the first direction, the repulsive force of the torsion spring  18  increases. However, the repulsive force is small as long as the torsion of the torsion spring  18  is small. Accordingly, the shaft member  12  rotates with a relatively small torque T 1 , thereby applying a small push force V to the force transmitting member  102 . 
     If the input load from the force transmitting member  102  increases so that the second shaft member  13  is further pushed back into the casing  11 , the projection  35  abuts against the inner side surface  36   a  of the recess  36 . Thereupon, the first shaft member  12  and the first receiving member  31  start to rotate in a body. In this case, the friction diameter changes into D 2 , the frictional torque increases, and the repulsive force of the torsion spring  18  also increases. Thus, the shaft member  12  starts to rotate with a strong turning torque T 2  at a point P 1 , as shown in FIG. 4, thereby applying a relatively strong push force V to the force transmitting member  102 . 
     When the received load decreases, as when the received load increases, the shaft member  12  rotates with the friction diameter D 1  to generate a small turning torque as long as the rotational angle of the shaft member  12  is narrow. If the rotational angle of the shaft member  12  becomes wider, the projection  35  abuts against the inner side surface  36   b  of the recess  36 , whereupon the shaft member  12  and the receiving member  31  rotate with the friction diameter D 2 . Thus, a strong turning torque is generated. 
     According to this first embodiment, the stiffness of the tensioner against a heavy received load can be improved without failing to secure a relatively great amplitude displacement by switching the contact diameter for the rotation of the first shaft member  12  between D 1  and D 2  according to the rotational angle. Accordingly, this tensioner can cope with input loads ranging from light ones to heavy ones. If the load applied to the tensioner  10  from the force transmitting member  102  in an engine or the like, for example, is light, therefore, the second shaft member  13  can satisfactorily follow a small amplitude displacement, so that the tension of the force transmitting member  102  can be kept at an appropriate value. 
     FIG. 5B shows a second embodiment of this invention. In this embodiment, a first receiving member  31  is formed having a taper surface  38  of which the thickness increases toward its center. Thus, a contact diameter D 1  for contact between a first shaft member  12  and the first receiving member  31  is made further smaller than the aforesaid contact diameter D 1  according to the first embodiment. 
     FIG. 5C shows a third embodiment of this invention. In this embodiment, a recess  49  is formed in the central portion of the lower surface of a first receiving member  31 . Thus, the first receiving member  31  touches a second receiving member  32  with a contact diameter D 2  in an annular end face around the recess  49 . By doing this, the contact diameter D 2  can be kept fixed even if the receiving member  31  is worn to a certain degree. Although both FIGS. 5B and 5C show only those portions which are needed in explaining the contact diameters D 1  and D 2 , other portions are constructed in the same manner as in the first embodiment. 
     FIGS. 6A to  7  show a fourth embodiment of this invention. One end portion  18   a  of a torsion spring  18  of this embodiment extends in the diametrical direction of a first receiving member  31 , and penetrates a recess  36  of the first receiving member  31 . In this case, the one end portion  18   a  of the spring  18  can move in some measure in the recess  36  with respect to the circumferential direction of first receiving member  31 . In this allowable range for the movement, the first receiving member  31  is stationary even though a first shaft member  12  rotates. If the rotational angle of the shaft member  12  becomes wider, the one end portion  18   a  of the spring  18  engages an inner side surface  36   a  or  36   b  of the recess  36 , thereby causing the first receiving member  31  to rotate integrally with the shaft member  12 . Since the one end portion  18   a  of the spring  18  of this embodiment fulfills the same function with the end member  33  of the first embodiment, the number of components of the tensioner  10  can be reduced. 
     FIGS. 8A and 8B show a fifth embodiment of this invention. In this embodiment, one end portion  18   a  of a spring  18  and an anchor piece  39  are inserted in a slit  19 . The anchor piece  39  extends in the diametrical direction of a first receiving member  31 , and both end portions  39   a  of the anchor piece  39  are situated inside a recess  36  of the first receiving member  31 . The first receiving member  31  never rotates in the allowable range for the movement of the end portion  39   a  of the anchor piece  39  in the recess  36  when the first shaft member  12  rotates. If the rotational angle of the shaft member  12  becomes wider, the end portion  39   a  of the anchor piece  39  abuts against an inner side surface  36   a  or  36   b  of the recess  36 , whereupon the first receiving member  31  rotates with the shaft member  12 . Thus, the anchor piece  39  fulfills the same function with the end member  33  of the first embodiment. In these fourth and fifth embodiments also, the turning torque can be changed by switching the rotational contact diameter of an end face  12   f  of the first shaft member  12  between D 1  and D 2 . 
     FIGS. 9 and 10A show a sixth embodiment of this invention. In the case of this embodiment, a second receiving member  32  is formed having a pair of recesses  40 . Projections  41  are formed on a first receiving member  31 . The projections  41 , which are situated inside the recesses  40 , can move within the range of the length of the recesses  40  with respect to the circumferential direction of the second receiving member  32 . As shown in FIG. 10A the bottom surface of the first receiving member  31  is formed having a taper surface  42  of which the thickness increases toward the center. By doing this, a contact diameter D 2  for contact between the first receiving member  31  and the second receiving member  32  is made smaller than a contact diameter D 1  for contact between the first receiving member  31  and a first shaft member  12 . 
     In the case of this embodiment (FIGS.  9  and  10 A), the first receiving member  31  rotates with the shaft member  12  in the allowable range for the movement of the projections  41  in the recesses  40  when the first shaft member  12  rotates. At this point in time, the contact diameter is D 2 , and a generated frictional torque is relatively small. If the rotational angle of the shaft member  12  becomes wider, the projections  41  abut against inner side surfaces  40   a  or  40   b  of the recesses  40 , whereupon the rotation of the first receiving member  31  stops, so that only the shaft member  12  rotates with the large contact diameter D 2 . A frictional torque generated in this case is greater than one that is obtained when the shaft member  12  rotates with the contact diameter D 2 . Thus, also in this embodiment, the turning torque of the first shaft member  12  can be changed in two stages. 
     FIG. 10B shows a seventh embodiment of this invention. The fundamental arrangement of this embodiment resembles that of the sixth embodiment. In the seventh embodiment, however, a portion  43  that is thicker than its surrounding region is formed in the center of a bottom portion  32   b  of a second receiving member  32 . Thus, a contact diameter D 2  for contact between a first receiving member  31  and the second receiving member  32  is made smaller than a contact diameter D 1  for contact between the first receiving member  31  and a first shaft member  12 . In this seventh embodiment, as in the sixth embodiment, therefore, the turning torque can be changed in two stages. 
     FIG. 11 shows an eighth embodiment of this invention. The fundamental arrangement of this embodiment resembles that of the sixth embodiment (FIG.  10 A). In the case of the eighth embodiment, however, a taper surface  42  is formed on the bottom portion of a first receiving member  31  so that a first contact diameter D 1  and a second contact diameter D 2  are substantially equal. 
     In the case where D 1  and D 2  are equal, as in the case of this embodiment, the properties of the surface of a contact portion between the shaft member  12  and the receiving member  31  and the properties of the surface of a contact portion between the receiving members  31  and  32  are differentiated so that the respective frictional torques of the two contact portions are different. The turning torques T 1  and T 2  can be differentiated by varying, for example, the type of plating for the two contact portions, surface hardness, or material of the contact portions. Thus, the value of the turning torque can be adjusted by suitably treating or modifying the surfaces of the contact portions. This technical idea is also applicable to the first to seventh embodiments described above. 
     FIGS. 12 and 13 show a tensioner of a ninth embodiment of this invention. The tensioner of this embodiment comprises a first receiving member  31  in which an end member  33  is inserted for rotation, a second receiving member  32  in which the receiving member  31  is inserted for rotation, and a third receiving member  45  in which the second receiving member  32  is inserted for rotation. The third receiving member  45  is fixed to the bottom surface of a casing  11 . 
     AS shown in FIG. 13, a projection  35  formed on the end member  33  penetrates a recess  36  formed in the first receiving member  31 . The projection  35  can move between inner side surfaces  36   a  and  36   b  of the recess  36  with respect to the circumferential direction of the first receiving member  31 . The first receiving member  31  is formed having a projection  46  like the projection  35  of the end member  33 . The second receiving member  32  is formed having a recess  47  that is penetrated by the projection  46 . The projection  46  can move between inner side surfaces  47   a  and  47   b  of the recess  47  with respect to the circumferential direction of the second receiving member  32 . When a shaft member  12  rotates, the projection  35  moves in the recess  36  as long as the rotational angle is narrow, so that the first receiving member  31  and the second receiving member  32  are stopped. Since the contact diameter of the shaft member  12  is then D 1 , the turning torque is minimal. If the rotational angle of the shaft member  12  increases by a certain degree, the first projection  35  first abuts against the inner side surface  36   a  or  36   b  of the recess  36 . Thereupon, the first receiving member  31  rotates with the shaft member  12 . As long as the rotational angle of the first receiving member  31  is narrow, the second projection  46  moves in the recess  47 , so that the second receiving member  32  never rotates. Since the contact diameter is then D 2 , the turning torque is medium. If the shaft member  12  rotates further, the projection  46  abuts against the inner side surface  47   a  or  47   b  of the recess  47 . Thereupon, the second receiving member  32  also rotates with the shaft member  12 . Since the contact diameter is then D 3 , the turning torque is maximal. Thus, in the tensioner of the ninth embodiment, the turning torque of the shaft member  12  can be changed more finely in three stages. In this embodiment also, the type of plating for the individual members, surface hardness, or material may be varied in order to differentiate the respective frictional torques of the aforesaid three contact portions. 
     FIG. 14 shows a tenth embodiment of this invention. A tensioner  10  of this embodiment is provided with a connecting spring  50  that constitutes a clutch mechanism. Further, a torsion spring  18  is provided around a first shaft member  12 . On the other hand, the torsion spring  18  of the tensioner  10  of each of the foregoing embodiments is provided covering the first shaft member  12  and a second shaft member  13 . However, the torsion springs  18  of any of the embodiments have the basic function of applying torque to the first shaft member  12  in common. The repulsive force of the torsion springs  18  described in connection with these embodiments acts in the direction to push out the shaft member  13  from the casing  11 . Depending on the direction of the input load, however, the repulsive force of the torsion springs  18  may be made to act in the direction to push back the shaft member  13  into the casing  11 . 
     The tensioner  10  of this tenth embodiment is also provided with a tubular second receiving member  32  that is fixed in the casing  11 . The receiving member  32  has a bottom portion  32   b.  A tubular first receiving member  31  having a bottom portion  31   b  is rotatably inserted in the receiving member  32 . An end portion of the first shaft member  12  is rotatably inserted in the first receiving member  31 . A hole  32   d  is formed in the center of the bottom portion  32   b  of the second receiving member  32 . A protrusion  31   d  to be inserted into the hole  32   d  is formed in the center of the bottom portion  31   b  of the first receiving member  31 . The protrusion  31   d  projects into a tapped hole  15  through the hole  32   d.  A slit  31   c  is formed in the distal end of the protrusion  31   d.    
     One end portion  18   a  of the torsion spring  18  is anchored to the first receiving member  31 . The other end portion  18   b  of the torsion spring  18  is anchored to the casing  11 . The connecting spring  50  is provided between the inner peripheral surface of the torsion spring  18  and the outer peripheral surface of a torque adjusting portion  12   b  of the shaft member  12 . One end  50   a  of the connecting spring  50  is anchored to the first receiving member  31 . The other end  50   b  of the connecting spring  50  is anchored to the first shaft member  12 . Torques that are generated as the torsion spring  18  and the connecting spring  50  are twisted have the same direction. 
     An operating member W such as a screwdriver is inserted into the tensioner  10  of this tenth embodiment (FIG. 14) through the hole  15 , and the distal end of the operating member W is fitted into the slit  31   c.  Then, the respective ends  18   a  and  50   a  of the springs  18  and  50  are individually rotated for a given number of times in a first direction by turning the operating member W. The first shaft member  12  is connected to the first receiving member  31  by means of the connecting spring  50 . If the receiving member  31  is rotated in the first direction, therefore, the connecting spring  50  causes the first shaft member  12  to rotate in the first direction. This rotation causes the second shaft member  13  to move in a direction such that it is drawn into the casing  11 . Simultaneously with this rotation, the torsion spring  18  is twisted in a direction such that it stores a repulsive force, whereupon it is given initial torque. 
     If an external load to push the second shaft member  13  is applied to the tensioner  10  of the tenth embodiment that is given the first torque, the load is transmitted to the first shaft member  12  via thread portions  16  and  17 , whereupon the first shaft member  12  rotates. As long as the received load is so light that the connecting spring  50  is twisted only slightly, the first receiving member  31  never rotates if the shaft member  12  rotates. In this case, an end face  12   f  of the shaft member  12  rotates with a contact diameter D 1  with respect to the bottom portion  31   b  of the first receiving member  31 , so that a small frictional torque is generated. 
     If the received load increases so that the rotational angle of the shaft member  12  widens, the first receiving member  31  is coupled to the shaft member  12  as the twist of the connecting spring  50  increases. Thereupon, the receiving member  31  and the shaft member  12  rotate. In this case, the first receiving member  31  rotates with a contact diameter D 2  with respect to the second receiving member  32 , so that the turning torque increases. 
     FIG. 15 shows changes of the turning torque of the tensioner  10  of the tenth embodiment. When the first shaft member  12  rotates for a narrow rotational angle (or with a light received load), a turning torque T 1  based on the contact diameter D 1  is generated. When the second shaft member  13  further moves in the axial direction as the received load increases, the shaft member  12  and the receiving member  31  are coupled to each other at a point P 2  of FIG. 15 by means of the connecting spring  50 . In this case, a relatively strong turning torque T 2  is generated on the basis of the contact diameter D 2 . 
     If the increased received load is reduced so that the shaft member  12  rotates in the opposite direction, the contact diameter changes according to the rotational angle, so that the turning torque can be changed. 
     As seen from FIG. 15, the turning torque T 1  in the first stage and the turning torque T 2  in the second stage are continuous with each other, and there exists no step portion Q such as the one shown in FIG.  4 . Thus, according to this tenth embodiment, the change of the turning torque is mediated by the elastic action of the connecting spring  50 , so that the continuity between T 1  and T 2  can be obtained. According to the tenth embodiment arranged in this manner, compared with the foregoing embodiments, the fluctuation of the turning torque can be smoothed. 
     FIG. 16 shows a tensioner  10  of an eleventh embodiment of this invention. This tensioner  10  comprises a protrusion  12   c  formed on a first shaft member  12  and rubber members  51  provided on the protrusion  12   c.  The protrusion  12   c  and the rubber members  51  are situated inside a recess  36  that is formed in a first receiving member  31 . The protrusion  12   c  and the recess  36  constitute a clutch mechanism that connects the first shaft member  12  and the first receiving member  31 . A cylindrical second receiving member  32  having a bottom portion  32   b  is fixed to a casing  11 . A cylindrical first receiving member  31  having a bottom portion  31   b  is rotatably inserted in the receiving member  32 . An end portion of the first shaft member  12  is rotatably inserted in the first receiving member  31 . The protrusion  12   c  is formed on the peripheral surface of the first receiving member  31 . The rubber members  51  that function as elastic members are attached individually to the opposite side faces of the protrusion  12   c.  The rubber members  51  face inner side surfaces  36   a  and  36   b  of the recess  36 , individually. Further, a torsion spring  18  is provided around the first shaft member  12  and the first receiving member  31 . One end portion  18   a  of the torsion spring  18  is anchored to the first receiving member  31 . The other end portion  18   b  of the torsion spring  18  is anchored to the casing  11 . The tensioner  10  of this eleventh embodiment, like the tenth embodiment, is provided with a protrusion  31   d  having a slit  31   c  for initial torque, a hole  32   d,  etc. 
     If the tensioner  10  of the eleventh embodiment is subjected to load in the direction to push a second shaft member  13 , the load is transmitted to the first shaft member  12  via thread portions  16  and  17 , whereupon the first shaft member  12  rotates. As long as the rotational angle of the shaft member  12  is narrow, the protrusion  12   c  moves in the recess  36 , so that the first receiving member  31  never rotates. In this case, the shaft member  12  rotates with a contact diameter D 1 , so that the turning torque is relatively small. 
     If the received load increases so that the rotational angle of the shaft member  12  widens, the rubber members  51  engage the inner side surface  36   a  or  36   b  of the recess  36 . This engagement causes the rubber members  51  to be compressed as the receiving member  31  and the shaft member  12  are coupled to each other. Thereupon, the receiving member  31  and the shaft member  12  rotate. Thus, the first receiving member  31  rotates with a contact diameter D 2  with respect to the second receiving member  32 . Accordingly, the turning torque increases. 
     If the increased received load is reduced, the repulsive force of the spring  18  causes the first receiving member  31  to rotate in a second direction, and the first shaft member  12  also rotates in the second direction. Thus, the second shaft member  13  moves in a direction such that it projects from the casing  11 . In this case, the turning torque can be also changed between a smaller turning torque T 1  for the contact diameter D 1  and a greater turning torque T 2  for the contact diameter D 2 , depending on the rotational angle of the shaft member  12 . 
     FIG. 17 shows change of the turning torque of the tensioner  10  of the eleventh embodiment. As seen from FIG. 17, the turning torque T 1  in the first stage and the turning torque T 2  in the second stage are continuous with each other with P 3  between them, and moreover, the torque T 1  in the first stage is represented by a downwardly convex curved line. The characteristic of this torque T 1  can be obtained as the rubber members  51  are compressed between the protrusion  12   c  and the inner side surface  36   a  or  36   b.    
     FIG. 18 shows a tensioner  10  of a twelfth embodiment of this invention. The tensioner  10  of this embodiment has a protrusion  12   d  and a recess  36  to be penetrated by the protrusion  12   d,  besides aforementioned tenth embodiment (FIG.  14 ). If the rotational angle of a first shaft member  12  is narrow, the protrusion  12   d  can move between inner side surfaces  36   a  and  36   b  of the recess  36 . The protrusion  12   d  and the inner side surfaces  36   a  and  36   b  of the recess  36  constitute a clutch mechanism. This clutch mechanism regulates the angular range in which the shaft member  12  and a receiving member  31  can rotate with respect to each other. 
     One end portion  18   a  of a torsion spring  18  is anchored to the first receiving member  31 , while the other end portion  18   b  is anchored to the casing  11 . One end  50   a  of a connecting spring  50  is anchored to the first receiving member  31 , while the other end  50   b  is anchored to the first shaft member  12 . 
     The protrusion  12   d  is formed on the peripheral surface of the first shaft member  12 . The recess  36  is formed in an end portion of the first receiving member  31 , covering a given length with respect to its circumferential direction. The protrusion  12   d  is situated in the recess  36 . Thus, the angle at which the shaft member  12  and the receiving member  31  can rotate relatively to each other is regulated according to the circumferential length of the recess  36 . 
     If a load in the direction to push a second shaft member  13  from the outside is applied to the tensioner  10  (FIG. 18) of the twelfth embodiment, the load is transmitted to the first shaft member  12  via thread portions  16  and  17 , whereupon the first shaft member  12  rotates. If the received load is light, that is, if the rotational angle of the shaft member  12  is narrow, the protrusion  12   d  moves in the recess  36 . Accordingly, the first receiving member  31  never rotates, and only the shaft member  12  rotates. In this case, an end face  12   f  of the shaft member  12  rotates with a contact diameter D 1  with respect to a bottom portion  31   b  of the first receiving member  31 , so that a small frictional torque is generated. 
     If the received load increases so that the rotational angle of the shaft member  12  widens, the twist of the connecting spring  50  increases, and the protrusion  12   d  abuts against the inner side surface  36   a  of the recess  36 , whereupon the receiving member  31  rotates with the shaft member  12 . In this case, the first receiving member  31  rotates with a contact diameter D 2  with respect to a second receiving member  32 , so that a strong frictional torque is generated. FIG. 19 shows change of the turning torque of the tensioner  10  of the twelfth embodiment. The contact diameter changes from D 1  to D 2  at a point P 4  in FIG. 19. A step portion Q between T 1  and T 2  can be reduced with use of the connecting spring  50 . 
     FIG. 20 shows a tensioner of a thirteenth embodiment of this invention. This tensioner has a third receiving member  60  and a second connecting spring  61 , besides aforementioned tenth embodiment (FIG.  14 ). First and second receiving members  31  and  32 , torsion spring  18 , and first connecting spring  50  share the same constructions and functions with those of the tenth embodiment. 
     In this thirteenth embodiment, a first shaft member  12  is rotatably inserted in the third receiving member  60 . The third receiving member  60  is rotatably inserted in the first receiving member  31 . The second connecting spring  61  is provided between the inner peripheral surface of the connecting spring  50  and the outer peripheral surface of the shaft member  12 . One end  61   a  of the second connecting spring  61  is anchored to the third receiving member  60 . The other end  61   b  of the second connecting spring  61  is anchored to the first shaft member  12 . The direction of the repulsive force that is generated as the torsion spring  18  is twisted is coincident with the direction of the repulsive force that is generated as the connecting springs  50  and  61  are twisted. 
     An operating member such as a screwdriver is inserted into the tensioner  10  of this thirteenth embodiment (FIG. 20) through a hole  15 , and its distal end is fitted into a slit  31   c.  Then, the torsion spring  18  and the connecting springs  50  and  61  are individually rotated for a given number of times in a first direction by turning the operating member. When the first receiving member  31  rotates in the first direction, the twist of the connecting springs  50  and  61  increases, so that the first shaft member  12  rotates in the first direction. This rotation causes a second shaft member  13  to move in a direction such that it is drawn into the casing  11 . Simultaneously with this rotation, the torsion spring  18  is twisted in a direction such that it stores a repulsive force, whereupon it is given initial torque. 
     If an external load to push the second shaft member  13  is applied to this tensioner  10 , the load is transmitted to the first shaft member  12  via thread portions  16  and  17 , whereupon the first shaft member  12  rotates. As long as the received load is so light that the rotational angle of the shaft member  12  is narrow, the twist of the connecting spring  61  is so small that the third receiving member  60  never rotates. In this case, an end face  12   f  of the shaft member  12  rotates with a contact diameter D 1  with respect to a bottom portion  60 b of the third receiving member  60 , so that a relatively small frictional torque is generated. 
     If the received load increases so that the rotational angle of the shaft member  12  widens, the twist of the second connecting spring  61  increases, so that the connecting spring  61  causes the shaft member  12  and the third receiving member  60  to rotate with each other. In this case, the third receiving member  60  rotates with a contact diameter D 2  with respect to the first receiving member  31 , so that a medium frictional torque is generated. 
     If the received load further increases so that the shaft member  12  rotates further, the twist of the first connecting spring  50  increases, whereupon first receiving member  31  is also rotated via the connecting spring  50 . In this case, the first receiving member  31  rotates with a contact diameter D 3  with respect to the second receiving member  32 , so that the frictional torque is maximal. 
     If the increased received load is reduced so that the tensioner  10  is actuated in the opposite direction, the repulsive force of the torsion spring  18  causes the first shaft member  12  to rotate in a second direction. When the received load is reduced in this manner, just as when the received load increases, the contact diameter changes in three stages in accordance with the rotational angle of the shaft member  12 , so that the turning torque can be changed gradually. 
     In the tensioner of this thirteenth embodiment, the turning torque of the shaft member  12  can be changed more finely in three stages. In this embodiment also, the type of plating for the individual members, surface hardness, or material may be varied in order to differentiate the respective frictional torques of the aforesaid three contact portions. Although the turning torque is changed in three stages in either of the ninth and thirteenth embodiments, it may alternatively be changed in four stages or more. 
     As is evident from the above description, the tensioner of the present invention can be suitably used in a power transmission mechanism that uses an endless belt, endless chain, etc., such as an automotive engine, for example. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.