Abstract:
A tensioner having a backstop device which allows free rotation of the pivot arm in a first direction but controls rotation of the pivot arm in a second, opposite direction. The backstop device permits a predetermined, limited amount of free rotation in the second direction and thereafter, employs a braking device to prevent rotation in the second direction if the torque that acts on the pivot art is less than a predetermined threshold.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates in general to automotive belt tensioners and, in particular, to a timing belt tensioner in which the position of the arm backstop is controlled by a one-way clutch and a functional brake. 
       BACKGROUND OF THE INVENTION 
       [0002]    Automotive belt tensioners are well known in the art and have been used to regulate tension in various belt systems, e.g., timing belts. In general, a belt tensioner includes a movable support structure that rotatably supports a portion of a belt in an engine or other mechanical system. The rotational position of the arm/pulley sub-assembly of a belt tensioner normally self-adjusts to compensate for increases or decreases in belt path length due to the thermal expansion or contraction of the engine and/or belt wear and stretch, thereby regulating tension in the belt. Additionally, the entire tensioner assembly is normally manually adjustable relative to the engine block so that the tensioner can be adjusted to the proper position on the engine regardless of the engine build tolerances. 
         [0003]    One common type of conventional belt tensioner includes a fixed structure and a pivoted structure, which generally consists of an arm/pulley sub-assembly that is pivotally mounted on the fixed structure. A coil spring surrounds the pivoted member, and the ends of the spring are respectively connected to the fixed structure and the pivoted structure so as to bias the pivoted structure toward a position of maximum belt take-up. The spring biasing force decreases as the pivoted structure moves from a position of minimum belt take-up to a position of maximum belt take-up. Although the spring force varies within the range of movement provided, substantially constant tension is maintained on the belt. U.S. Pat. No. 4,473,362 discloses such a tensioner. 
         [0004]    Additionally, timing belt and chain tensioners normally have stroke limiters. A stroke limiter customarily includes a pair of fixed stops which prevent rotation of the pivot arm beyond predetermined distances from the normal pivot arm position; a first stop limits arm rotation towards the belt and is commonly referred to as a “free arm stop,” and a second stop limits arm rotation away from the belt and is commonly referred to as a “backstop.” The backstop is normally positioned such that even if the pivot arm is rotated against it, there will not be enough slack in the belt for the belt to rise above the teeth in any of the sprockets in the drive and “jump over” or disengage from the teeth. In other words, the backstop is designed to prevent tooth skip, which tooth skip would otherwise cause timing errors between the various sprockets and, consequently, equipment errors and damage. 
         [0005]    The common practice of placing the backstop at a certain distance from the nominal pivot arm position is not feasible for tensioners which provide no initial manual installation adjustment and with which the rotation of the pivoted structure is intended to compensate for engine build tolerances. In other words, with such tensioner configurations, the tensioner arm does not have any fixed nominal position and, therefore, there is no fixed backstop position, either, thus making it necessary to adjust the backstop position during the initial tensioner installation either manually or, preferably, automatically. In addition, the increased life expectancy of modern engine components results in longer belt life and belt stretch, and hence generally greater adjustment ranges are required for the pivoted structure of the tensioner during the life of the tensioner. Therefore, if manual service adjustments are to be avoided, it becomes even more important for the backstop position to be self-adjusting. 
         [0006]    Several known tensioner designs provide such self-adjustment of the tensioner backstop. For example, U.S. Pat. No. 4,145,934 discloses a wedge which is pushed against the arm eccentric (lever) so that the arm cannot rotate away from the belt once the tensioner arm has been biased towards the belt by the tensioning spring. Similarly, U.S. Pat. No. 4,351,636 discloses a tensioner that is similar in principle, but with a ratchet-and-pawl assembly instead of a wedge. Another ratchet-and-pawl type tensioner mechanism is disclosed in U.S. Pat. No. 4,634,407. In each of these patents, however, the tensioner arm is unable to rotate away from the belt once it has rotated towards the belt; thus, such configurations do not allow for belt tension control during thermal expansion of the engine block. 
         [0007]    U.S. Pat. No. 4,583,962 discloses an improvement over such designs. In particular, it discloses a mechanism which allows a limited amount of return stroke of the arm towards the backstop to accommodate thermal expansion of the engine. The tensioner of this patent utilizes a spring clutch-type one-way device and an arc-shaped slot configured to permit the arm to rotate backwards. Similarly, U.S. Pat. Nos. 4,822,322 and 4,834,694 disclose tensioners in which the one-way mechanisms are constituted by conventional, one-way (roller) clutches, and tensioner arm return stroke is controlled by arc-shaped slots. Furthermore, U.S. Pat. No. 4,808,148 discloses a tensioner in which, rather than a slot-limited reverse stroke, a resilient biasing element (e.g., an elastomeric spring) is provided between the ratchet-and-pawl assembly and the stationary mounting member. 
         [0008]    The above-mentioned tensioner designs all suffer from the limitation that the backstop can not move back, away from the belt, once it has moved towards the free arm position or when operating under other than optimum, hot engine running conditions. Because the backstop may move beyond the optimum position during cold starts and/or as a result of severe engine kick-backs, the tensioner arm will frequently contact the backstop, thereby causing noise, damage, and/or premature failure of the components. Furthermore, tensioners of this type do not permit the belt to be re-installed or replaced. 
         [0009]    U.S. Pat. No. 4,923,435 discloses a tensioner with viscous material disposed between the arm and a one-way clutch mechanism. This particular design does not, however, guarantee that the tensioned belt will not jump a tooth. Because the viscous material allows the tensioner arm to rotate if the belt load is applied continuously (which can occur particularly when the engine is forced to rotate backwards due to the car rolling backward without the engine running), the viscous material does not function as a positive stop, but rather only as a rotational damper. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention overcomes these limitations of the prior art by providing a tensioner in which the backstop automatically “finds” its proper operational position by “following” the tensioner arm as it pivots toward the free arm position; which maintains that operational backstop position under nominal or transitory (e.g., vibratory) belt loads; and which permits the backstop to be moved backwards manually (e.g., during installation) and to move backwards under sufficiently or prolonged high belt loads. 
         [0011]    According to one aspect of the invention, a timing belt tensioner has a pivot shaft which is secured to the engine block; a pivot arm pivotally mounted on the pivot shaft; a torsion spring operatively mounted between the pivot arm and a fixed structure (e.g., the engine block) so as to bias the pivot arm in a belt take-up direction (i.e., toward a free arm position); and a backstop device. The backstop device includes a generally cylindrical stop sleeve, a frictional brake (supported, e.g., by a generally cylindrical clamp holder), and a one-way clutch. The stop sleeve and the pivot arm preferably are cooperatively configured to permit a predetermined, limited amount of rotational movement of the pivot arm relative to the stop sleeve. The one-way clutch allows relatively free rotational movement of the stop sleeve (and hence the pivot arm) towards the free arm position, but engages the stop sleeve to the frictional brake when the stop sleeve rotates away from the belt, i.e., towards the minimum belt take-up position, with the frictional brake effectively “anchoring” the one-way clutch to a fixed anchoring point, e.g., to the pivot shaft or directly to the engine block. Alternatively, a hydraulic coupling, e.g., one using a viscous material, may be implemented in place of the frictional brake. 
         [0012]    The frictional brake resists arm movement towards the minimum belt take-up position caused by belt forces at a level such as that resulting from the crankshaft being turned backwards, but “releases” and allows the backstop to rotate toward the minimum belt take-up position when the pivot arm is rotated manually, e.g., during installation. The configuration of the tensioner according to the invention permits easy installation, simplifies construction, and hence reduces manufacturing and installation time and cost. 
         [0013]    According to another aspect of the invention, rotary apparatus for selectively transmitting rotary power or torque includes a pair of rotary members which are axially interconnected directly with each other and which are rotatable relative to each. A one-way clutch spring is disposed in overlying relation with the pair of rotary members and rotationally interlocks (i.e., prevents relative rotation between) the pair of rotary members when one of the rotary members rotates in one direction and allows the rotary members to rotate relative to each other when it rotates in the opposite direction. One of the rotary members may have a lip which constrains opening of the turns of the clutch spring, and the clutch spring may have one or more turns of a larger diameter than the rest of the turns to provide a certain amount of free stroke before the clutch spring rotationally interlocks the rotational members. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention will now be described in greater detail in connection with the drawings, in which: 
           [0015]      FIG. 1  is a partial front elevation view illustrating an automobile internal combustion engine with a timing belt assembly including a tensioner; 
           [0016]      FIG. 2  is a section view of one embodiment of a tensioner according to the invention; 
           [0017]      FIG. 3  is a perspective view illustrating the stop sleeve and clamp holder shown in  FIG. 2 ; 
           [0018]      FIG. 4  is a perspective view illustrating the brake clamp shown in  FIG. 2 ; 
           [0019]      FIG. 5  is a section view of another embodiment of a tensioner according to the invention; 
           [0020]      FIG. 6  is an assembly view of the tensioner shown in  FIG. 5 ; 
           [0021]      FIG. 7  is an assembly view of another embodiment of a tensioner according to the invention; 
           [0022]      FIG. 8  is a cross-sectional view illustrating the configuration of the upper, clamp sleeve and its assembled relation to the lower, bottom sleeve shown in  FIG. 7 , the cross-section being taken along a cutting plane passing through the tenons of the upper, clamp sleeve and being shown looking toward the pivot arm along lines  8 - 8  in  FIG. 7 ; 
           [0023]      FIG. 9  is a section view of another embodiment of a tensioner according to the invention; 
           [0024]      FIG. 10  is a section view of yet another embodiment of a tensioner according to the invention; and 
           [0025]      FIG. 11  is a cross-sectional view of the tensioner shown in  FIG. 10 , taken along lines  11 - 11 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    A timing belt system for an internal combustion engine is illustrated in  FIG. 1 . A toothed, pulley sprocket  112  is fixed to the crankshaft  113  of the engine, and an internally toothed belt  114  is driven by the sprocket  112 . The toothed belt  114  is trained about (and hence drives) a second, externally toothed sprocket  116 , which sprocket  116  is fixed to (and hence causes to rotate) a cam shaft  118  of the engine. A tensioner  10  according to the invention is mounted in tensioning relation with the belt  114 . 
         [0027]    As illustrated in  FIG. 2 , the tensioner  10  generally consists of a pulley  12  that is mounted on a ball bearing assembly  13  which extends circumferentially around a pivot arm  20 , and the pivot arm  20  is eccentrically pivotally mounted on a pivot shaft  16 , e.g., by means of a journal. In other words, the pulley  12  rotates around its own axis of rotation  15  extending through the center of the ball bearing assembly  13 , and the pivot arm  20  pivots (with the pulley  12  pivoting with it) around the longitudinal axis  16   c  of the pivot shaft  16 , which is generally spaced from and parallel to the axis of rotation  15  of the pulley  12 . 
         [0028]    The pivot shaft  16  has a bore  16   b  extending longitudinally through the center of it, and an installation bolt (not shown) passing through the bore secures the tensioner assembly to the engine. The pivot shaft  16  is attached via a press fit to a base plate  30  which, in its preferred configuration, has a center extrusion  31  to improve the press fit between the base plate and the pivot shaft. 
         [0029]    A torsional coil spring  18  surrounds the lower portion (as illustrated) of the tensioner and is operatively mounted between the arm  20  and the base plate  30 , with one spring tang  18   a  extending into a corresponding slot  22  in the arm  20  and the other spring tang  18   b  extending into a slot  33  formed in the upwardly extending outer skirt  32  of the base plate  30 . During assembly of the tensioner  10 , the arm  20  is rotated relative to the base plate  30  before the arm  20  is brought into its final axial position, thereby preloading the spring  18  so as to bias the arm  20  rotationally towards the free arm position. A thrust washer  14  is located between the body of the arm  20  and the flange  16   a  of the pivot shaft  16  and reduces friction between these parts when the arm  20  rotates. 
         [0030]    A backstop device  40  is installed about the pivot shaft  16 , between the arm  20  and the base plate  30 . In one embodiment, the backstop device  40  consists of a generally cylindrical stop sleeve  50 ; a directly interconnected, generally cylindrical clamp holder  60 ; a frictional brake member in the form of a clamp  70 ; a clutch spring  80 ; and a bushing  100 . The frictional clamp  70 , which is illustrated in greater detail in  FIG. 4 , is held within the clamp holder  60  in frictionally rotational engagement with the pivot shaft  16 , and the clamp holder  60  and frictional clamp  70  are cooperatively configured so as to rotate relative to the pivot shaft  16  together, i.e., as a single unit. 
         [0031]    In one preferred form, the clamp holder  60  is directly interconnected with the stop sleeve  50  by means of external, flange-type protrusions  56  and circumferentially extending external groove section  54  on the stop sleeve  50 , which protrusions and groove mate with an internal groove  68  and an internal ring  67 , respectively, on the clamp holder  60 . The end of the stop sleeve  50  on which the protrusions  56  and the groove  54  are located is divided into several narrow, flexible, finger-like portions  59  by means of axial slits  53 , as shown in  3 . Because the finger-like portions  59  are radially flexible, the protrusions  56  will be forced inwardly so as to pass through the internal ring  67  of the clamp holder  60  when the two parts are assembled together and then will spring back to their original position. The interengagement of the protrusions, ring, and respective grooves will lock the stop sleeve  50  and clamp holder  60  axially together while at the same time permitting them to rotate with respect to each other. The stop sleeve  50  and the clamp bolder  60  fit together with a clearance fit so as to permit relatively free rotational movement between these two components. The stop sleeve is made of a flexible material such as nylon to facilitate such inward flexing and outward spring-back. 
         [0032]    The bushing  100  is inserted inside the stop sleeve  50  to prevent the protrusions  56  from collapsing inwards during the operational life of the tensioner. The bushing  100  will also increase the overall rigidity of the stop sleeve  50 , especially when it is resisting the rotational arm movements. 
         [0033]    The clamp  70  is designed to grip the pivot shaft  16 , via pads or brake shoe-type elements  71 , with a predetermined level of force to substantially or “selectively” secure the clamp  70 , and hence the clamp holder  60 , to the pivot shaft  16 . The clamp  70  is designed to grip the pivot shaft  16  with sufficient force such that the level of torque required to overcome the frictional resistance force between the pad elements  71  and the pivot shaft  16  and cause the clamp  70  (and hence the clamp holder  60 ) to rotate, sliding frictionally relative to shaft pivot  16 , is 1) higher than the level of torque caused by belt forces which result under conditions otherwise favorable to tooth skip, but 2) which will allow the clamp  70  (and hence the clamp holder  60 ) to rotate when subjected to torque loads which are higher than the designed holding torque of the clamp  70 . Preferably, the clamp  70  is made of corrosion-resistant material with high yield strength such as 17-4 stainless steel, which permits a large degree of deflection of its spring element before reaching the required pre-load force. Alternatively, it is also possible to make the clamp out of high-strength carbon or tool steels and to apply a corrosion-resistant coating to the part. 
         [0034]    In one preferred form, the clamp  70  will resemble the letter “C,” with a pad or brake shoe-type element  71  attached to each of the ends, as shown in  FIG. 4 . When the clamp  70  is made in this form, it is preferable for the clamp  70  to have a tab  72  to help position the clamp and secure its engagement with the clamp holder  60 . The frictional clamp  70  fits into groove  73  extending partially circumferentially around and partially radially into the clamp holder  60 . The pads  71  fit through apertures  74  extending from the bottom of the groove  73  all the way through the wall of the clamp holder  60  (one aperture on either side of the clamp holder) and hence are able to grip the sides of the pivot shaft  16 . Additionally, the tab  72  fits within a small slot (not visible) defined between ribs  75  (only one visible in  FIG. 3 ) which are formed within the groove  73 . The tab  72  thus helps position, and retain the position of, the frictional clamp  70  within the clamp holder. 
         [0035]    Over at least a portion of the sections by which they are interconnected, the stop sleeve  50  and the clamp holder  60  have cylindrical exterior surfaces  52  and  62 , respectively, which have the same diameter. The clutch spring  80  is mounted over the cylindrical surfaces  52  and  62  with a press fit. One end of the clutch spring  80  is formed into an axial tang  82  that is inserted into an axially extending hole  51  formed inside an axial tongue  50   a  extending from the stop sleeve  50 , and the tang  82  causes the clutch spring  80  to rotate with the stop sleeve  50 . The clutch spring  80  and the two cylindrical surfaces  52  and  62  perform a one-way clutch function: the clutch spring  80  will slidingly rotate relative to the clamp holder  60 , with almost no resistance, when the stop sleeve  50  rotates in one direction relative to the clamp holder  60  as the pulley  12  and pivot arm  20  rotate toward the belt  114 , but the clutch spring  80  will constrict and lock all three parts (the stop sleeve  50 , the clamp holder  60 , and the clutch spring  80 ) together when the stop sleeve rotates in the Opposite direction as the pulley  12  and pivot arm  20  rotate away from the belt  114 . In particular, the coiling direction of the clutch spring  80  is selected such that the stop sleeve  50  (which rotates with the pivot arm  20 , as addressed in more detail below) can rotate freely relative to the clamp holder  60  in the belt take-up direction (i.e., toward the free arm position), but the clutch spring  80  will constrict so as to clamp down rigidly on the cylindrical surfaces  52  and  62  when the stop sleeve  50  is rotated (by the arm  20 ) towards the minimum belt take-up position, thereby preventing the stop sleeve  50  from rotating relative to the clamp holder  60  in the minimum belt take-up direction. 
         [0036]    The forces acting between the pivot arm  20  and the stop sleeve  50 , as well as those acting between the stop sleeve  50  and the clamp holder  60 , may tend to force the clamp holder  60  axially toward the base plate  30 . Therefore, the bottom surface of the clamp holder  60  is preferably formed so as to provide a good thrust bearing surface. 
         [0037]    The spring clutch may be configured and arranged in various ways, depending on the arrangement of the coil spring  80 . It will be appreciated that the torque-transmitting capacity of the clutch spring depends on the number of coils engaged on each surface. Provided there is sufficient axial space for several coils, it is acceptable to position the clutch spring  80  evenly or nearly evenly overlying a portion of each of the stop sleeve  50  and the clamp holder  60 . If axial space is limited, however, one of the cylindrical surfaces can be made shorter, and engagement of the clutch spring to the particular element can be made using a tang, as exemplified in  FIG. 2 . Even in that situation, however, it is still preferable to provide at least a couple of coils over each of the cylindrical surfaces to reduce the amount of force to which the tang is subjected. Additionally, in order to improve control over the sliding rotation of the clutch spring towards the free arm position, it is preferable to provide the stop sleeve with a ring-shaped lip  50   c , which extends far enough to cover at least one turn of the clutch spring  80 . The ring-shaped lip  50   c  prevents the coils of the clutch spring from opening excessively before the spring starts to slide relative to the clamp holder  60 . 
         [0038]    The pivot arm  20  has a cavity  21  formed therein into which the axial tongue  50   a  of the stop sleeve  50  fits. Although the cavity  21  may have a tight fit with the tongue  50   a , it is preferable for the cavity to be slightly larger than the axial tongue  50   a  so as to permit a slight degree of free rotational movement of the pivot arm  20  relative to the stop sleeve  50 . In order to minimize wear on the backstop device  40 , it is recommended that this degree of rotational “play” be at least approximately the same as the rotational degree of arm movement caused by thermal expansion of the engine and/or the arm vibration caused by engine dynamics. This angular range will vary from one engine configuration to another and generally is on the order of 20° to 50°. The pivot arm  20  also has a hex hole  23  formed therein, which hex hole  23  is accessible to a corresponding tool such as an Allen wrench (not shown) or any other convenient lever- or handle-type device which can be inserted into the hex hole  23  through an opening  14   a  in the thrust washer  14 . 
         [0039]    An alternative configuration of a tensioner according to the invention is shown in  FIGS. 5 and 6 . The configuration is similar to that shown in  FIG. 2 , but with some variation in the configuration of the backstop device  140  and the provision of an installation clip (not shown), which can consist of any mechanical device that is capable of rotationally locking the pivot arm relative to the stationary components of the tensioner). In particular, in the backstop device  140 , certain features of the stop sleeve  150  and the clamp holder  160  are reversed (as compared to the previous embodiment) to permit the axial spring tang  182  at the end of the clutch spring  180  to be secured within hole  161  formed in the clamp holder  160  (rather than in the stop sleeve  50 , as shown in  FIG. 2 ). Consequently, because it is preferable for the cylindrical, outer surface  152  of the stop sleeve  150  to be longer than the corresponding cylindrical, outer surface  162  of the clamp holder  160  so as to accommodate the appropriate number of spring coils thereon, it becomes necessary to form the external protrusions  166  and external groove  164  on the clamp holder  160  and the mating or interengaging internal ring  157  and internal groove  158  on the stop sleeve  150 . Accordingly, the axial slits  163 , which define flexural fingers  169  to facilitate assembly of the components, are formed in the clamp holder  160 , as shown in  FIG. 6 . Similarly, the lip  160   c , which restricts excessive opening of the clutch spring  180 , is formed in the clamp holder  160 . Because side forces acting on the clamp holder  10  occur much more seldom than on the stop sleeve  150 , and because the clamp holder  160  needs to rotate relative to the pivot shaft  16  only under high torque conditions, it is possible to forego providing any bushings between the clamp holder  160  and the pivot shaft  16 . 
         [0040]    Additionally, in this embodiment, the clamp holder  160  and frictional clamp  170  are configured such that the frictional clamp  170  fits within the groove  173 , which is formed as a slot extending diametrically all the way across the clamp holder  160 , with the pads  171  being exposed to engage the side surfaces of the pivot shaft  16 . Tab  172  protrudes outwardly from the frictional clamp  170  (rather than inwardly, as in the embodiment illustrated in  FIG. 4 ) and fits within radial slot  176  extending perpendicularly to the groove  173  so as to properly position and retain the frictional clamp  170  in the groove  173 . The frictional clamp  170  is inserted into the clamp holder  160  by pushing the frictional clamp sideways into the groove or slot  173  until the tab  172  is aligned with the slot  176 , and the frictional clamp is then moved radially so that the tab  172  engages into the radial slot  176 . 
         [0041]    The installation clip facilitates installation of the tensioner onto the engine. In particular, the clip is inserted into corresponding holes in the pivot arm  20  and some stationary component or components of the tensioner  10  (e.g., the base plate  30  while the pivot arm  20  is being turned close to or all the way to the outmost backstop position, normally while the tensioner is on the assembly line during manufacture. While the installation clip is inserted, the pivot arm  20  can not rotate away from the initial, factory-set position until the installation clip is removed. 
         [0042]    Yet another configuration of a tensioner according to the invention is shown in  FIGS. 7 and 8 . The overall configuration is similar to the configuration shown in  FIGS. 5 and 6 , but the frictional clamp  270  and the clamp holder  260  and configured differently. In particular, the frictional clamp  270  is configured more like a split ring than the frictional clamps  70  and  170  in the above-described embodiments, and the frictional clamp  270  is configured such that it makes frictional contact with the tensioner pivot shaft to a greater circumferential extent than the two previously described embodiments do. For example, the frictional clamp  270  preferably makes contact over approximately 270° around the circumference of the pivot shaft. 
         [0043]    Preferably, the frictional clamp  270  is made from stainless steel spring wire. Although the dimensions of the spring wire of the frictional spring clamp  270  will, of course, vary depending on the level of torque against which the frictional clamp is required to hold, for reference purposes, the spring wire from which the frictional clamp  270  is made, as illustrated, has a square cross-section that is 3 mm×3 mm. It has been found that the frictional clamp  270  is generally easier to make, stronger, and provides more consistent torque-resisting performance than the above-described frictional clamps  70  and  170 . 
         [0044]    Additionally, the clamp  270  performs slightly differently while rotationally sliding than the clamps shown in  FIGS. 4 and 6  do. While the tang  271   a  is being pushed by the upper, clamp sleeve  260   a , it will cause the clamp  270  to open slightly, thereby reducing the clamping force and frictional resistance to turning. In other words, the frictional brake can be constructed to open at least partially under those conditions in which the frictional holding is expected to be released. As a result, variation in the coefficient of friction will have a reduced effect on the torque at which release/sliding of the frictional brake occurs. 
         [0045]    In order to accommodate the frictional clamp  270 , the clamp holder  260  is formed from two components, namely, an upper, clamp sleeve  260   a  and a lower, bottom sleeve  260   b . As more clearly shown in  FIG. 8 , the frictional clamp  270  fits within the “pocket” that is bounded by shoulder surface  283  and the peripheral wall  284  of the clamp sleeve  260   a.    
         [0046]    Three posts or tenons  285   a ,  285   b , and  285   c  are formed extending from the peripheral wall  284 , and the associated portion of the peripheral wall  284  is “beefed up” accordingly. The portion of the peripheral wall  284  on which the tenon  285   c  is formed has a hole (not visible) extending through it, and that hole transitions—at the lower surface of the peripheral wall  284  (as the clamp sleeve  260   a  is oriented in FIG.  7 )—into a slot  286  that is formed in the radially outer surface  287  of the tenon  285   c . The hole and the slot  287  are configured to receive all or nearly all of the length of the axially extending lower tang  282  of the clutch spring  280  in a manner suitable to retain the lower tang  282 , which rotationally fixes the clutch spring  280  relative to the clamp sleeve  260   a  (and, accordingly, relative to the clamp holder  260  once it is assembled together). 
         [0047]    Two notches or grooves  288   a  and  288   b  are also formed in the peripheral wall  284  of the clamp sleeve  260   a , with one on either side of the tenon  285   c  that is configured to receive the axial tang  282  of the clutch spring  280 . As best shown in  FIG. 8 , the notches or grooves  288   a  and  288   b  receive the tang-like end portions of the frictional clamp  270 . One of the notches  288   a  is relatively narrow so as to receive the corresponding end  271   a  of the frictional clamp  270  with a snug fit, which holds the end  271   a  of the frictional clamp  270  securely in place, whereas the other notch  288   b  is relatively wider so as to accommodate assembly variances. 
         [0048]    It would be desirable for the portion of the clamp sleeve  260   a  that the clutch spring  280  engages to have a relatively high coefficient of friction to facilitate gripping of the clutch spring  280  to the clamp sleeve  260   a . On the other hand, it would be desirable for the portion of the clamp sleeve  260   a  which rotationally engages the stop sleeve  250  to have a relatively low coefficient of friction in order to facilitate relative rotation of the two parts. Additionally, the flexural fingers  269  of the clamp sleeve  260   a  should be sufficiently resilient that they do not break when the stop sleeve  250  and the clamp sleeve  260   a  are pressed together. In view of these various considerations, the clamp sleeve  260   a  might be made by co-molding different materials, each having the desired coefficient of friction and flexibility to achieve these goals, or the clamp sleeve can be made from a material (such as nylon 46) which is selected to meet simultaneously all three of these considerations as well as possible. 
         [0049]    The bottom sleeve  260   b  generally serves three primary purposes. First, it serves to enclose the frictional clamp  270  within the clamp holder  260  in proper position; second, it “ties” the three tenons  285   a ,  28   b , and  285   c  together so that the combined unit of the frictional clamp  270  and the clamp sleeve  260   a  can better withstand torsional loads (imparted to the clamp sleeve  260   a  through the axial tang  282  of the clutch spring  280  and by the wrapping of the clutch spring  280  around the cylindrical outer surface of the clamp sleeve  260   a ) than would be the case if just the single tenon  285   c  were receiving such torsional loads; and third, the bottom (as oriented in  FIG. 7 ) surface  289  of the bottom sleeve  260   b  functions as a thrust bearing which allows the backstop device  240  to rotate relative to the base plate  230  while bearing against the base plate  230  under axial loads that may be generated in the tensioner. To the end of having the bottom surface  289  function as a thrust bearing surface, the bottom sleeve  260   b  preferably is formed from a material such as unfilled nylon, which has a relatively low coefficient of friction μ. 
         [0050]    As further illustrated in  FIGS. 7 and 8 , the bottom sleeve  260   b  has a generally cylindrical peripheral wall  290  and a ring-shaped end “wall”  291 , the exposed portion of which provides the bottom surface  289  that functions as a thrust bearing surface. The peripheral wall  290  bulges radially outward over a slight portion  292  thereof to provide space into which the tang-like clamp ends  271   a  and  271   b  can fit when the clamp holder  260  is assembled. Furthermore, a slot  293  extends axially through the ring-shaped end wall  291 , and the tenon  285   c  (with the axial tang  282  of the clutch spring  280  positioned in the slot  286  of the tenon  285   c ) extends into the slot  293  (but not beyond the bearing surface  289 ) when the clamp holder  260  (and the remainder of the tensioner components) is assembled. Two more slots (not visible) are formed as blind holes in the upper surface of the ring-shaped end wall  291  and are positioned to receive the other two tenons  285   a  and  285   b  when the components of the clamp holder  260  are assembled together. 
         [0051]    The peripheral wall  290  of the bottom sleeve  260   b  has a bead  294  extending circumferentially around the inner surface of the peripheral wall  290 , near the upper edge of the peripheral wall  290  (in the illustrated orientation). The bead  294  engages a narrow, circumferentially extending locking groove  295  which is formed around the peripheral wall  284  of the clamp sleeve  260   a . Thus, once the frictional clamp  270  is properly positioned in the “pocket” of the clamp sleeve  260   a , the clamp holder  260  is assembled by pressing the clamp sleeve  260   a  and the bottom sleeve  260   b  together until the bead  294  snaps into the locking groove  295 , with the tenons  285   a ,  285   b , and  285   c  seating in their respective slots in the ring-shaped end wall  291  of the bottom sleeve  260   b.    
         [0052]    The bottom sleeve  260   b  also has an upper, ring-shaped lip (not visible) around the upper (as oriented in  FIG. 7 ) edge of the peripheral wall  290 , which lip is similar in construction to the lip  50   c  shown in  FIG. 2  or the lip  160   c  shown in  FIG. 5 . The ring-shaped lip is sized to accommodate the lower coil or coils of the clutch spring  280  when the tensioner is assembled and thus prevents the coils of the clutch spring  280  from opening excessively before the spring starts to slide relative to the clamp holder  260 . 
         [0053]    The tensioner (and, in particular, the backstop device  40 ,  140 , or  240 ) operates as follows. During initial installation of the tensioner and the belt, the tensioner is normally mounted to its proper location on the engine using a bolt (not shown) extending through the bore  16   b  of the pivot shaft  16  and threaded into the engine. If the tensioner has an installation clip  11 , the arm  20  will already be in the minimum take-up position to facilitate mounting of the belt. Otherwise, if the tensioner does not have an installation clip (especially during re-installation of the tensioner during field service, when the clip may no longer be available), the person installing the tensioner must move the pivot arm  20  into or close to the minimum belt take-up position in order to install a belt over the tensioner pulley  12 . This can be done by inserting an Allen wrench into the hex hole  23  in the pivot arm  20  and rotating the pivot arm  20  towards the minimum belt take-up position. If there is some rotational “play” between the pivot arm  20  and the stop sleeve  50 ,  150 , or  250  due to rotational clearance between the axial tongue  50   a ,  150   a , or  250   a  and the cavity  21  formed in the pivot arm  20 , the only initial resistance to rotation the installer will feel is that provided by the main spring  18 . Once any “play” has been taken up, the installer must also rotate the stop sleeve  50 ,  150 , or  250 . Because the stop sleeve  50 ,  150 , or  250  is partially rotationally fixed to the pivot shaft  16  via the frictional clamp  70 ,  170 , or  270 , the installer must also overcome the rotational resistance created by the friction between the clamp  70 ,  170 , or  270  and the pivot shaft  16 . This frictional resistance is designed to be high enough to resist the belt force-induced torque caused by reverse rotation of the engine but low enough to allow the installer to rotate the pivot arm  20  backwards. Therefore, the installer will be able to move the pivot arm  20  all the way to the minimum belt take-up position, at which point the belt is installed over the various sprockets and pulleys. 
         [0054]    After the belt is positioned over the various pulleys, the pivot arm  20  (and hence the pulley  12 ) must be allowed to pivot freely towards the belt to provide proper belt tension. If the tensioner has an installation clip  11 , the installer simply removes the clip  11 . If the arm and pulley assembly was manually rotated to the minimum take-up position, the installer releases pressure on the tool (Allen wrench) and lets the main spring  18  cause the pivot arm  20  (and the pulley  12 ) to rotate toward the belt. Once the pulley  12  is resting firmly against the belt, the tool can be removed to complete the manual installation process. In either case, the main spring  18  can provide the necessary arm movement toward the belt (and consequent belt tension) because the backstop device does not create any significant resistance to rotation of the pivot arm  20  toward the free arm position. 
         [0055]    Ordinarily, the tensioner occasionally will be subjected to certain running conditions of the engine which will increase belt loading on the pulley and hence apply torque to the pivot arm  20  in the reverse direction, i.e., the direction away from the belt. Two of such running conditions are cold starts following normal thermal expansion of the engine and backwards rotation of the engine. In each of these instances, if there is any rotational “play,” the pivot arm  20  will rotate backwards toward the backstop until the relevant end surface of the cavity  21  in the pivot arm  20  contacts the axially extending tongue  50   a ,  150   a , or  250   a  of the stop sleeve  50 ,  150 , or  250 ; otherwise (i.e., when there is a tight fit between the axially extending tongue and the cavity), the pivot arm will not rotate relative to the stop sleeve. Subsequently, backward rotation of the pivot arm  20  (i.e., rotation away from the belt) will be prevented because the pivot arm will be linked to the frictional clamp via the stop sleeve, spring clutch, and clamp holder, and the frictional clamp provides enough friction to resist such backward rotation to prevent tooth skip while allowing for manually forced rotation of the frictional brake and, consequently, the tensioner arm to permit easy installation or reinstallation. 
         [0056]    In the embodiments described above, a spring clutch provides the necessary one-way function. However, any known one-way device (e.g., one-way roller clutches, ratchet-and-pawl, etc.) could be used to interconnect the stop sleeve to the frictional brake. Similarly, rather than a clamp, the frictional brake could be of any known construction which creates a braking force by means of friction. 
         [0057]    Alternatively, as illustrated in  FIG. 9  and instead of a frictional brake, the tensioner could be constructed using a hydraulic device designed to create enough resistance and have enough holding power to prevent reverse movement of the backstop and consequent rotation of the arm away from the belt (which rotation of the arm can allow tooth-skip to occur). Such hydraulic device could any previously known hydraulic device or, for example, a viscous coupling assembly like that disclosed in co-pending application Ser. No. 09/547,108 (filed on Apr. 11, 2000, the disclosure of which is incorporated by reference) while still obtaining the benefit of various features of the invention. With such a coupling assembly  340 , viscous material  370  is provided between one member  381 , which is fixed to the outer surface of a bottom portion of the pivot shaft  16 , and a second member  396 , which can rotate relative to the first member  381 . (The second member  396  is shown as being of unitary construction for illustration purposes but may be of two-part, upper- and lower-half construction as illustrated in the above-referenced application Ser. No. 09/547,108). When torsional loads are applied suddenly to the pivot arm  320  and transmitted to the second member  396  via stop sleeve  350  and clutch spring  380 , the viscous material  370  prevents the second member  396  from rotating relative to the first member  381  (due to its viscosity), but when torsional loads are applied gradually and/or continuously, the viscous material  370  permits the second member  396  to rotate relative to the first member  381 . 
         [0058]    Stop sleeve  350 , which is somewhat shorter than the ones described above, is connected to the second member  396  in a similar manner as that described above. In particular, the interconnection between the two components axially locks them together but permits one component to rotate relative to the other. Clutch spring  380  clutches the stop sleeve  350  to the second member  396  in generally the same manner as it clutches the stop sleeve to the clamp holder in the various embodiments described above. 
         [0059]    By providing rotational “play” between the pivot arm  320  and the stop sleeve  350 , as described above (e.g., by making the cavity  321  into which the axial tongue  350   a  of the stop sleeve  350  fits larger than the axial tongue  350   a ), the amount of vibration the hydraulic device must endure is reduced. This prolongs the service life of the hydraulic device. 
         [0060]    A still further embodiment of a tensioner according to the invention is illustrated in  FIGS. 10 and 11 . In this embodiment, the “stop sleeve”  450  is formed as an integral part or extension of the tensioner pivot arm  420 . In this embodiment, the clutch spring  480  is effectively connected directly to the tensioner pivot arm  420 , with the axial tang  482  of the clutch spring  480  extending into cavity  421  formed in the tensioner pivot arm  420 . (In an alternative configuration, the opposite, bottom end of the clutch spring  480  may be affixed in appropriate fashion to the “clamp sleeve”  460 ). 
         [0061]    As illustrated more clearly in  FIG. 11 , the frictional brake  470  is formed as a cylinder surrounding a lower portion of the pivot shaft  16 , with a sector  472  of the cylinder being removed. The “clamp sleeve”  460  is coaxial with and fits over the external surface of the friction brake  470 . The clamp sleeve  460  has a key  462  which extends radially inward from the internal surface of the clamp sleeve  460 , and the key  462  fits within the removed sector  472  of the frictional brake  470 . Thus, the frictional brake  470  is forced to rotate around the pivot shaft when the clamp sleeve  460  has rotated by a sufficient amount to contact the walls  473 ,  474  of the cut-out sector  472  and applies sufficient force. 
         [0062]    It will be understood, of course, that the clamp sleeve  460  is caused to rotate relative to the tensioner pivot shaft  16  by the pivot arm  420  transmitting torque to it through the clutch spring  480 . If the number of coils wrapped around the “stop sleeve”  450  (which is essentially an extended core of the tensioner arm  420 ) is high enough, and/or if the clutch spring has the axial tang  482  positioned securely within the cavity  421  as illustrated, the clutch spring  480  will continuously follow movement of the tensioner pivot arm  420 . 
         [0063]    It is preferred, however, for the tensioner to have a slight amount of “free stroke” to allow the tensioner arm  420  to rotate freely by a certain amount (corresponding to thermal expansion of the engine and/or dynamic oscillation of the belt drive). Such free stroke can be effected by either or both of two alternative features, both of which are illustrated. First, by making the key  462  of the clamp sleeve  460  narrower than the cut out sector  472 , the pivot arm  420  will be provided with a certain amount of rotational play. Additionally or alternatively, by forming the clutch spring  480  with a couple of larger diameter turns, as shown, a certain amount of rotational play will be built into the assembly. This is because once the pivot arm  420  starts to rotate the clutch spring  480 , the larger diameter turns of the clutch spring  480  must constrict into contact with the underlying surfaces before the clutch spring can create any major turning torque towards the clamp sleeve  460 . As yet another alternative (not illustrated), rotational play can be provided by reducing (or even totally eliminating) the number of clutch spring coils on the “stop sleeve”  450  (arm core) and forming the cavity  421  as a circumferential arc so that the tensioner arm  420  can rotate a certain amount before the arm forces the clutch spring to follow the arm rotation. 
         [0064]    Although in the embodiments of the invention described above and illustrated herein the backstop device resists rotation by the backstop device making frictional engagement with the pivot shaft  16 , the tensioner may also be configured such that the backstop device resists rotation by frictional engagement with a fixed portion other than the pivot shaft  16 , such as the base plate  30  or even the engine itself (e.g., if no base plate is provided). These and other embodiments are deemed to be within the scope of the following claims.