Abstract:
A tensioner for an endless belt comprising a stationary structure on which a pivotal tension arm with an idler pulley is mounted. A spring between the tension arm and the stationary structure acts to bias the pulley against the belt, thereby applying tension. The invention includes a one-way device that is operable in response to the extent of pivotal movement of the tension arm in the forward direction to establish different positions at which pivotal movement in the return direction is limited. A damping system is used to restrict pivotal movement of the tension arm as a result of sudden short-time vibratory movements or dynamic vibrations in cold conditions.

Description:
FIELD OF INVENTION 
     This invention relates to a timing belt tensioner having a one-way mechanism which allows a controlled movement of the tensioner arm or arm stops towards the tensioner free arm position. 
     BACKGROUND OF THE INVENTION 
     There are several tensioner designs where the tensioner arm movements toward the free arm position are controlled with an one-way mechanism. U.S. Pat. No. 4,145,934 describes a wedge which is pushed against the arm eccentric (lever) so that the arm cannot rotate outwards once the tensioner arm is biased towards the belt by a tensioning spring. U.S. Pat. No. 4,351,636 describes a tensioner similar in principle except that the one-way wedge is replaced by a ratchet/pawl assembly. Another ratchet and pawl mechanism is described in U.S. Pat. No. 4,634,407. 
     Each of the above-mentioned tensioners described a one-way mechanism, which does not allow the tensioner arm to rotate away from the belt once the arm is allowed to move inwards toward the belt. 
     U.S. Pat. No. 4,583,962 offers an improvement to these designs by describing a mechanism which allows a limited return stroke of the arm towards the backstop required by the thermal expansion of the engine. The detail design of this patent describes a spring clutch type one-way device and an arc shape slot wherein the arm is free to rotate backwards. 
     U.S. Pat. No. 4,808,148 describes a design wherein the slot controlled reverse stroke is replaced by a resilient biasing element such as elastomeric spring located between the ratchet and pawl assembly and the stationary mounting member. 
     U.S. Pat. Nos. 4,822,322 and 4,834,694 describe tensioners wherein the one-way mechanisms are conventional one-way (roller) clutches and the arm return strokes are controlled by arc shaped slots. 
     U.S. Pat. No. 4,923,435 describes a tensioner wherein the arm can have a return stroke controlled by a viscous clutch between the arm and the one-way mechanism. 
     All of the above-mentioned known one-way devices, even those with a controlled return stroke of the arm, have a fundamental problem in allowing the one-way mechanism to move toward the belt (free arm position) whenever engine conditions are such that this can happen. In very cold engine conditions, especially those below freezing temperatures, the engine behavior can be quite erratic. Consequently, the tensioner arm can move towards the free arm position considerably more than in normally running engine conditions, either due to the engine vibrations, stiff valve train/camshaft bearings or stiff or frozen tensioner pulley bearings. This excessive arm movement results in the stops in the one-way mechanism being rotated together with the arm too far toward the free arm direction which eventually causes the tensioner arm to hit the backstop once the normal hot running conditions of the engine are reached. Since it is not normally possible to increase the amount of return stroke allowed for the arm before hitting the backstop without simultaneously increasing the possibility for the tooth skip, all of the known one-way mechanisms used to control the tensioner arm movement can create the above-mentioned failure conditions in the engines operating in cold climate conditions. 
     SUMMARY OF THE INVENTION 
     Accordingly, there exists a need for a tensioner which will solve the problems identified above. 
     An object of the present invention is to solve the problems enunciated above. In accordance with the principles of the present invention, this objective is obtained by providing a tensioner for an endless belt which comprises a stationary structure. A pivotal structure is mounted on the stationary part for pivotal movements in forward and return directions about a pivotal axis. A pulley is rotatably carried by the pivotal structure for rotational movement about a rotational axis parallel to the pivotal axis. A spring is provided which acts between the structures to bias the pivotal structure to pivot in the forward direction corresponding to a movement of the pulley into belt tensioning relation to a belt with which the pulley is rollingly engaged. A one-way device has structure constructed and arranged to be operable in response to the extent of pivotal movement of the pivotal structure in the forward direction to establish different positions at which pivotal movement of the pivotal structure in return direction is limited. A damping mechanism is provided having structure constructed and arranged to restrict pivotal movement of the pivotal structure in one direction as a result of sudden short-time vibratory movements or dynamic vibrations in cold conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be more fully appreciated from the following detailed description when considered in connection with the accompanying drawings, in which the same or like reference numerals designate the same or corresponding parts throughout and in which: 
     FIG. 1 is a top plan view, partially broken away, of the tensioner apparatus of the present invention illustrating the tensioner assembly and the pulley assembly; 
     FIG. 2 is a cross-sectional view taken along section lines  2 — 2  of the tensioner apparatus shown in FIG. 1; and 
     FIG. 3 is a perspective view of the one-way device, the damping unit and the stop interface between the one-way device and the arm. 
    
    
     DESCRIPTION OF THE INVENTION 
     The tensioner apparatus  1 , illustrated in FIGS. 1 and 2, comprises a conventional tensioner  10 , a one-way device  20  and damping unit  40 . The conventional tensioner unit  10  comprises a pulley  11 , mounted on a ball bearing assembly  12  mounted about a pivotal structure (tensioner arm)  13  which has an offset bore for pivotally mounting on pivot shaft  14 . Arm  13  is biased towards a belt (not shown) by a spring  15  extending between the arm  13  and base plate  16 . 
     The one-way device  20  comprises a ratchet-wheel  21  and a pawl  22 . Pawl  22  is a generally arcuate member having an extended tip  23  at one end and pivotally mounted at a pivot post  24  intermediate opposite ends of the arcuate section. The pivot post  24  is fixed to a stationary part or structure of the tensioner, such as a base plate  16  or an extended part of the pivot shaft  14 . At opposite ends of the arcuate section are catch teeth  26  and  27 . Tip  23  extends outwardly from the arcuate section of pawl  22  to engage the damping unit  40 . 
     Ratchet wheel  21  comprises a ring having a plurality of teeth on the inside surface. The teeth extend about most of the inside surface. The ring has a wider section which has an arcuate slot  25 . At opposite ends of slot  25  are bumpers  30  and  31 . Base plate  16  has a circular counter bore which receives the ratchet wheel  21 . Pawl  22  is mounted within the ratchet wheel  21  such that as the pawl  22  pivots one or the other of the catch teeth  26  and  27  engages the inner teeth of the ratchet wheel  21 . 
     The damping unit  40  comprises a piston  41 , a cylinder  42  filled with fluid  43  and a seal  44 . Piston  41  is hingedly attached to the tip  23  of pawl  22  at pin lever  28  at the distal end  29  of tip  23 . 
     The mechanism  60  comprises a push button pin  61  arranged to slide in and out inside guide bushing  62 . Push button  61  engages tip  23  to enable a return stroke for the one-way device  20 . 
     The conventional arm  13  has a downwardly facing cup shaped body  17  with a downwardly extending stop post  18 . The bottom end  19  of the stop post  18  extends into slot of the ratchet-wheel  21  as shown in FIG.  2 . 
     The functionality of the present invention is now described referring to the above-mentioned figures. Prior to the installation of the tensioner to the engine, the arm  13  and the ratchet wheel  21  are rotated to installation position, namely, to the position where the pulley  11  is spaced as far away from the belt as possible and the tensioner spring  15  is wound up to its maximum torque. In FIG. 3 this installation position is in a clockwise direction as is allowed by the design of the arm  13 , the ratchet-wheel  21  and the pawl  22 . If required, the arm  13  can be locked into an installation position by an installation pin  50  that extends through the arm  13  into the base plate  16 . After the tensioner  10  and the belt has been installed on the engine, the installation pin  50  is removed. At this moment, the tensioner spring  15  will rotate the arm  13  into an operational position, in FIG. 3 counter-clockwise. The extended post  18  of the arm  13  will rotate within the stop slot  25  of the ratchet-wheel  21  until it hits the “free arm” end of the slot. In order to reduce noise and impact forces, free arm bumper  30  and hot stop bumper  31  are preferably made out of some relatively soft elastomer material. 
     As soon as the arm post  18  contacts the bumper  30 , it responsively rotates the ratchet-wheel  21  in counter-clockwise direction. The rotation of the wheel  21  will urge the catch tooth  26  out of the contact with the wheel  21  by rotating the ratchet-pawl  22  about the pivot pin  24  in counter-clockwise direction. The rotation of the pawl  22  will move the extended tip  23  towards the cylinder  42 . The piston  41  will move towards the bottom end  45  of the cylinder  42  simultaneously compressing the seal  44  which also acts as a compression spring. Both the compression of the seal  44  and the displacement of the cylinder fluid  43  from one side of the piston to the other create a counter force restricting the lever action of the pawl  22 . 
     The tensioner spring  15  through the arm  13  creates a torque which rotates the wheel  21 . When the spring force is large enough to overcome the above-mentioned counter forces, the wheel  21  will rotate until it pushes the catch tooth  26  all the way out of the contact with the wheel. At this point, the wheel  21  continues to rotate until another tooth of the wheel engages the second catch tooth  27  of the pawl  22  which has been rotated into a tooth stop of the wheel by the lever action on the pawl  22 . As a result of the self-locking geometry between the second catch tooth  27  and the lever pin  24 , the rotation of counter-clockwise rotation of the wheel  21  is not possible to push the second catch tooth  27  out of the contact and the rotation of the wheel is stopped after the rotational movement which corresponds to the angular width of one tooth sector. However, provided the axial spring force created by the seal  44  is large enough to overcome the hydraulic flow and/or viscous damping forces created by the cylinder fluid  43 , the piston  41  is pushed outwards from the cylinder  42  pushing the extended tip  23  of the pawl  22  to move the catch tooth  27  out of the contact with the wheel  21 . At this point, the first catch tooth  26  engages the wheel  21  and the further rotation of the wheel  21  can again start forcing the first catch tooth  26  out of contact with the wheel  21 . Consequently, whenever the belt length is such that it allows the arm  13  to rotate far enough towards counter-clockwise direction and this arm rotation lasts long enough to force the hydraulic damper piston  41  to retract sufficiently, the ratchet-wheel  21  can skip another tooth. However, if the arm movement is very fast as is the case during dynamic vibrations of the engine, the arm movement cannot force the ratchet-wheel  21  to skip the tooth. Similarly, when the engine and the hydraulic fluid  43  inside the cylinder  42  is very cold, the increased viscosity of the fluid  43  makes it very difficult to move the piston  41  and the ratchet-pawl  22  connected to it. Consequently, even aggressive arm movement caused by cold engine conditions, cannot force the ratchet-wheel to skip the tooth. In other words, the device according to the present invention will allow the rotation of the ratchet-wheel  21  and the stops  30  and  31  attached to the wheel during normal engine running conditions, but will not allow ratcheting action during erratic running conditions such as aggressive vibration and/or cold starts of a “frozen” engine. 
     In case there is a need to remove the belt from the engine during a service call of the engine, the tensioner arm  13  can be rotated away from the belt with the following procedure. By pressing the reset push button  61 , the button engages the end  29  of the extended tip  23  of the pawl  22 . The pawl  22  can be rotated counter-clockwise until the first catch tooth  26  comes out of the contact with the wheel  21 . By simultaneously rotating the arm  13  clockwise, the stop post  18  will contact the stop bumper  31  forcing the wheel  21  to rotate together with the arm  13 . The rotation of the wheel  21  will push the second catch tooth  27  out of the contact with the wheel, simultaneously moving the first catch tooth  26  again into contact with the wheel. During this operation, the ratchet wheel  21  together with its stop bumpers  30  and  31  has been rotated clockwise by one tooth. By repeating the pressing of the reset push button  61  and turning the arm  13  clockwise as many times as required, it is possible to bring the arm and pulley assembly out of the contact with the belt. If the arm/pulley assembly needs to be locked into the out-of-belt position, the above-mentioned procedure must be repeated until the arm  13  is brought back so much that the installation pin  50  can be reinserted to lock the arm  13 . 
     The mechanical construction of the present invention is by no means limited to that shown in FIGS. 1 to  3 . For example, the stop bumpers  30  and  31  could an integrated part of the arm  13  or these bumpers and could be replaced by single or multiple flexible connection between the arm and wheel, such as a pair of springs. Similarly, the rigid extension post of them arm could be made flexible and the bumpers could be omitted. 
     The one-way device can be any known device which only allows movement in one direction, such as commercially available one-way roller clutches, rotational or linear ratchet mechanisms with one or several catch tooth, etc. 
     In case the damper unit is a hydraulic device, it is preferred that the hydraulic fluid has a low viscosity index allowing it to increase its damping characteristics substantially when cold. However, the damper unit does not have to be a hydraulic device. It could be substituted by any known frictional damper design, or it could be a device whose damping characteristics would be changed by a thermal switch such as a bimetal or wax thermostat actuated toggle. The connection between the damper unit and the one-way device could be any known mechanical construction, which allows either rotational or sliding movement or both between the parts, e.g. pivot pins, journal or ball bearings, thrust plates or cups, etc. 
     The detail design described above has a bellow type sealing element for the hydraulic cylinder, which can also act as a return spring. However, a separate spring of any known construction, such as compression, extension, torsional or wave washer-type metal or elastomer springs, could be used to keep the ratchet pawl in contact with the ratchet wheel or bar. Furthermore, any spring type component could also be replaced by weights which will add sufficient gravitational forces to bias the tensioner components into right direction. 
     Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the scope of the invention.