Abstract:
A tensioner for a closed loop power transmission system for an internal combustion engine having a drive shaft terminating in a sprocket and at least one camshaft, each terminating in a sprocket, with a single continuous chain wrapping around all of the sprockets. The tensioner contains a pair of elongated tensioning arms, each one in slidable contact with one of the two strands of chain that traverses between the driving sprocket and the driven sprocket(s). Each tensioning arm contains a wear face that remains in constant slidable contact with the strand of chain to which it is adjacent. An adjusting arm connects one of the ends of the tensioning arms. The adjusting arm has a ratchet means that adjusts for the backlash in the system and takes up any slack in the chain.

Description:
FIELD OF THE INVENTION 
     The invention pertains to the field of chain tensioners. More particularly, the invention pertains to a tensioner for two strands of chain that contains a ratcheting device. 
     DESCRIPTION OF RELATED ART 
     A tensioning device, such as a hydraulic tensioner, is used as a control device for a power transmission chain, or similar power transmission device, as the chain travels between a plurality of sprockets that are connected to the operating shafts of an internal combustion engine. In this system, the chain transmits power from a driving shaft to a driven shaft, so that part of the chain is slack and part of the chain is tight. Generally, it is important to impart and maintain a certain degree of tension on the chain to prevent noise, slippage, or the unmeshing of teeth as in the case of a toothed chain. Prevention of such slippage is particularly important in the case of a chain driven camshaft in an internal combustion engine because the jumping of teeth will throw off the camshaft timing, possibly causing damage to the engine or rendering it inoperative. 
     However, in the harsh environment of the internal combustion engine, numerous factors cause fluctuations in the tension of any given portion of the chain. For instance, extreme temperature fluctuations and differences in the relative rates of thermal expansion coefficients among the various parts of the engine can cause the chain tension to vary between excessively high or very low levels. During prolonged use, wear to the components of the power transmission system can cause a steady decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tensions. For example, the reverse rotation of an engine, occurring during stopping of the engine or in failed attempts at starting the engine, can also cause significant fluctuations in chain tension. For these reasons, a mechanism is desired to remove excessive tensioning forces on the tight side of the chain while, at the same time, ensuring that adequate tension is applied to the slack side of the chain. 
     Hydraulic tensioners are a common method of maintaining proper chain tension. In general, these mechanisms employ a lever arm that pushes against the chain on the slack side of the power transmission system. The lever arm pushes toward the chain, tightening the chain when the chain is slack, and it must remain relatively immoveable when the chain tightens. 
     To achieve this, a hydraulic tensioner  1 , as shown in prior art  FIG. 1 , typically contains a rod or cylinder as a piston  2 , which is biased in the direction of the chain by a tensioner spring  3 . The piston  2  is housed within a cylindrical housing  5 , having an interior space which is open at the end facing the chain and closed at the other end. The interior space of the housing contains a pressure chamber  4  which is connected to a reservoir or exterior source of hydraulic fluid pressure via channels or ducts. The pressure chamber  4  is typically formed between the housing  5  and the piston  2 , and it expands or contracts when the piston  2  moves within the housing  5 . 
     Typically, valves are employed to regulate the flow of fluid into and out of the pressure chamber. For instance, an inlet check valve such as a ball-check valve opens to permit fluid to flow into the pressure chamber  4  when the pressure inside the chamber has decreased as a result of the outward movement of the piston  2 . When the pressure in the pressure chamber is high, the inlet check valve closes, preventing fluid from exiting the pressure chamber. Closing the inlet check valve prevents the piston chamber from contracting, which in turn prevents the piston from retracting, thereby achieving a so-called “no-return” function. 
     Many tensioners also employ a pressure relief mechanism that allows fluid to exit the pressure chamber when the pressure in the chamber is high, thus allowing the piston to retract in a regulated manner in response to rapid increases in chain tension. In some tensioners, the pressure relief mechanism is a spring biased check valve. The check valve opens when the pressure exceeds a certain pressure point. Some tensioners may employ a valve which performs both the inlet check function as well as the pressure relief function. 
     Other mechanisms employ a restricted path through which fluid may exit the fluid chamber, such that the volume of flow exiting the fluid chamber is minimal unless the pressure in the fluid chamber is great. For instance, a restricted path may be provided through the clearance between the piston and bore, through a vent tube in the protruding end of the piston, or through a vent member between the fluid chamber and the fluid reservoir. 
     A hydraulic tensioner used with a tensioner arm or shoe is shown in Simpson et al., U.S. Pat. No. 5,967,921, incorporated herein by reference. Hydraulic chain tensioners typically have a plunger slidably fitted into a chamber and biased by a spring to provide tension to a specific strand of chain. A lever, arm or shoe is often used at the end of the plunger to assist in the tensioning of the chain. The hydraulic pressure from an external source, such as an oil pump or the like, flows into the chamber through passages formed in the housing. The plunger is urged outward against the arm by the combined forces of the hydraulic pressure and the spring tension. 
     When a force is applied to move the plunger in a reverse direction (retracting into the housing) away from the chain, typically a check valve will restrict the flow of fluid out of the chamber. In this way, the tensioner achieves the no-return function, i.e., movements of the plunger are easy in one direction (outward and away from the housing) but difficult in the reverse direction. 
     Blade tensioners are commonly used to control a chain or belt where load fluctuations are not so severe as to overly stress the spring or springs. A ratchet with a backlash mechanism may be added to tensioners to limit the effective backward or untensioned travel of the tensioning device. 
     Prior art  FIG. 2  shows a conventional blade tensioner. The blade tensioner  10  includes a blade shoe  11  having a curved chain sliding face and numerous blade springs  21 , preferably made of a seasoned metallic material to impart spring-like tension to the blade springs  21 . The blade springs  21  are arranged in layers on the opposite side of the blade shoe  11  from the chain sliding face, and exert a biasing force on the blade shoe  11 . The ends of each spring-shaped blade spring  21  are inserted in the indented portions  14  and  15 , which are formed in the distal portion  12  and proximal portion  13  of the blade shoe  11 , respectively. 
     A bracket  17  is provided for mounting the blade tensioner  10  in an engine. Holes  18  and  19  are formed in the bracket  17 . Bolts or other secure mounting means are inserted into holes  18  and  19  for securing bracket  17  to the engine. A sliding face  16  is formed on the distal portion of the bracket  17  and slidably contacts the distal portion of the blade shoe  11 . A pin  20  secured on the bracket  17  supports the proximal portion  13  of the blade shoe  11  so that it may pivot with the changes in the position of the chain. 
       FIG. 3  shows a chain tensioning device that has a pair of arms  202 ,  203  which are joined by a pivot  204 . The arms  202 ,  203  are urged apart so that arm  203  applies tensioning force to a chain (not shown) by means of a spring  206  loaded cam block  205 . To prevent collapse of arm  203  during load reversals of the chain, a catch disc  209  and rod  208  are arranged to prevent return movement of the spring loaded cam block  205 . 
       FIG. 4  shows a tensioner that uses a ratchet device in a chain drive power transmission system. The power transmission system includes a drive shaft  302  having a sprocket  303  that uses a continuous loop chain  306  to drive the sprocket  305  of a driven shaft, such as a camshaft,  304 . The ratchet tensioner  301  contains a tensioner housing  307  having a hole  312  for receiving a plunger  308  and a ratchet pawl  317  pivotally mounted about shaft  316  to the tensioner housing  307  and biased by a ratchet spring  318 . The plunger  308  has teeth on one side of its outer perimeter to engage the ratchet pawl  317 . The plunger  308  is biased outward from the hole  312  toward contact with tension lever  310  by the introduction of pressurized fluid into the hollow section  313  and by the force of the plunger spring  314 . The tensioner lever  310  pivots on support shaft  309  and has a sliding face  311  that contacts and applies tension to the slack side of the timing chain  306 . The rearward movement of the plunger  308  back into the hole  312  is limited by the one way engagement of the ratchet pawl  317  with the teeth on the plunger. 
     SUMMARY OF THE INVENTION 
     The present invention is a tensioner for a closed loop power transmission system of an internal combustion engine. The power transmission system includes a drive shaft terminating in a sprocket and at least one camshaft, each in turn terminating in a sprocket, with a single continuous chain wrapping around the sprockets. The tensioner contains a pair of elongated tensioning arms, each one in slidable contact with one of the two strands of chain that traverses between the driving sprocket and the driven sprocket(s). Each tensioning arm has a first end, a second end and a mid point and may be pivotally mounted to the engine housing at either the first end, the mid point or at some location therebetween. Each tensioning arm contains a wear face to maintain slidable contact with the strand of chain with which it is operably engaged. 
     The second end of each tensioning arm is pivotally connected to the other tensioning arm by an adjusting arm. The adjusting arm includes two pairs of rigid elongated straps that adjustably overlap with each other substantially in the middle of the length of the adjusting arm. The overlapping ends of the first pair of elongated straps terminate in hook shapes to provide a seat for a first end of a lengthening coil spring. The overlapping ends of the second pair of elongated straps also terminate in hook shapes that insert through slots formed in each of the first pair of elongated straps. The hook shaped ends of the second pair of straps provide a seat for a second end of the lengthening coil spring. Each strap of the first pair of elongated straps contains a rack of teeth that operatively meshes with a rack of teeth on each strap of the second pair of elongated straps. The lengthening coil spring urges the overlap of both pair of elongated straps so that the adjusting arm continues to shorten in response to increasing slack or wear conditions experienced by the chain. The meshing teeth provide a “no-return” feature by engaging the teeth in only one direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art blade type tensioner. 
         FIG. 2  shows a prior art hydraulic tensioner. 
         FIG. 3  shows an alternate prior art tensioner. 
         FIG. 4  shows a prior art ratcheting tensioner. 
         FIG. 5  shows a front elevational view of the tensioner of the present invention in operative engagement with a closed loop chain drive system. 
         FIG. 6  shows an isometric view of the ratcheting device in the adjusting arm of the present invention. 
         FIG. 7  shows the adjusting arm without the ratchet coil spring. 
         FIG. 8  shows two strap segments of the adjusting arm. 
         FIG. 9  is a cross sectional view of one of the tensioning arms containing a blade spring. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 5 , the tensioner  110  of the present invention is operatively engaged with a closed loop power transmission system of an internal combustion engine. The power transmission system contains a driving sprocket  102  and at least one driven sprocket  104 ,  104 ′. Power from the engine&#39;s drive shaft is transmitted from the driving sprocket  102  to the driven sprockets by means of a chain  100  or drive belt. Most commonly used with internal combustion engines are chain drives. Proper tension must be applied to the chain  100  at all times in order to prevent the jumping of the sprocket teeth by the chain during slackening of any portion of the chain during operation or as a result of increasing wear of the components over time. 
     The tensioner  110  includes a tensioning arm  112  that is operatively engaged with the outer surface of one of the strands of chain between the driving sprocket  102  and one of the driven sprockets  104 . The second tensioning arm  112 ′ of tensioner  110  is operatively engaged with the outer surface of the other strand of chain between the driving sprocket  102  and a second driven sprocket  104 ′. It should be understood that the tensioner  110  of the present invention is also capable of being used in a closed loop power transmission system that has only one driving and one driven sprocket. 
     Each tensioning arm,  112  and  112 ′, may be pivotally secured to the face of the engine housing (not shown) by pivot mounting means  106  and  106 ′ at respective first ends  107  and  107 ′ of each tensioning arm  112  and  112 ′, as shown in  FIG. 5 . The pivot mounting means  106  and  106 ′ allow their respective tensioning arms  112  and  112 ′ to pivot in response to changes in the tension of the chain  100 . Alternative pivot mounting points may also be used, such as pivot mounting means  106   a  and  106   a ′, which may be located substantially at a mid-point along the longitudinal length of each of the respective tensioning arms  112  and  112 ′. The mounting of tensioning arms  112  and  112 ′ need not be symmetrical in that tensioning arm  112  may be pivot mounted to the engine housing at  106  while tensioning arm  112 ′ may be pivot mounted at  106   a ′. Alternatively, tensioning arm  112 ′ may be pivot mounted at  106 ′ while tensioning arm  112  is pivot mounted at  106   a . Also, tensioning arms  112  and  112 ′ may concurrently be pivot mounted at  106   a  and  106   a ′, respectively. 
     An alternative embodiment of tensioner  110  includes having one of the tensioning arms  112  or  112 ′ securely mounted to the engine housing so that it cannot pivot in response to changing chain tension conditions. For example, tensioning arm  112  may be securely mounted to the engine housing at both locations  106  and  106   a . The other tensioning arm  112 ′ is allowed to pivot about a single pivot mount  106 ′ in response to changing chain tension conditions. 
     Each tensioning arm  112  and  112 ′ contains an elongated rectangular shaped chain wear face  105  and  105 ′, respectively, that is semi-rigidly mounted along the length of each tensioning arm facing the strand of chain  100  with which it is operably engaged. Each chain wear face  105  and  105 ′ terminates in hooked ends that wrap around the ends of the tensioning arm on which it is installed. Each chain wear face  105  or  105 ′ is the contact surface with the strand of chain with which its corresponding tensioning arm is engaged. 
     Each chain wear face  105  and  105 ′ has a first end  105   a  and  105   a ′, respectively, and a second end  105   b  and  105   b ′, respectively. Each first end  105   a  and  105   a ′ is joined to its corresponding second end  105   b  and  105   b ′ by a middle portion that acts as the chain sliding face  105   c  and  105   c ′. This is best shown in  FIG. 9 . Each chain sliding face  105   c  and  105   c ′ is in sliding contact with a different strand of chain  100 . Each first end  105   a  and  105   a ′ and each second end  105   b  and  105   b ′ of its respective chain wear face  105  and  105 ′ are curved underneath and around towards the center of the corresponding wear face. Each chain wear face  105  and  105 ′ is longer than its corresponding tensioning arm  112  and  112 ′ such that each curved first end  105   a  and  105   a ′ receives the respective first end  107  and  107 ′of the corresponding tensioning arm  112  and  112 ′and each curved second end  105   b  and  105   b ′ receives the respective second end  109  and  109 ′ of the corresponding tensioning arm  112  and  112 ′, thereby loosely securing each chain wear face  105  and  105 ′ to its corresponding tensioning arm  112  and  112 ′. The chain wear faces  105  and  105 ′ are preferably made of a material that is semi-flexible at engine operating temperatures, in order to allow them to bow out to conform to the changing tension conditions of the chain  100 . Optionally, additional biasing means may be provided by one or more blade springs  114  located between the tensioning arm and the rear surface of each of the chain wear faces  105  and  105 ′. A gap clearance exists between each of the first ends  105   a  and  105   a ′ of the chain wears faces  105  and  105 ′ and the first ends  107  and  107 ′ of tensioning arms  112  and  112 ′. As well, a gap clearance exists between the second ends  105   b  and  105   b ′ of the chain wear faces  105  and  105 ′ and the second ends  109  and  109 ′ of tensioning arms  112  and  112 ′. As at least one blade spring  114  biases each of the chain guide elements  105  and  105 ′ out and away from its respective tensioning arm, the. combined gap clearances between the ends of tensioning arms  112  and  112 ′ and the ends of the chain wear faces  105  and  105 ′ is gradually eliminated until the chain wear faces  105  and  105 ′ can not bow out any further. At this point, the tensioning arms  112  and  112 ′ alone cannot provide further chain tensioning. 
     The second ends  109  and  109 ′ of each of their respective tensioning arms  112  and  112 ′, are connected to each other by an adjusting arm  130 . Referring.to  FIG. 6 , the second end  109  of tensioning arm  112  is secured to first ends  138  and  138 ′, respectively, of a first pair of elongated straps  134  and  134 ′ by pivot joint  132 . The second end  109 ′ of tensioning arm  112 ′ is secured to first ends  140  and  140 ′, respectively, of a second pair of elongated straps  136 ,  136 ′, by pivot joint  132 ′. Elongated straps  134 ,  134 ′,  136  and  136 ′ may be made of any rigid material, such as, for example, steel, aluminum, alloys thereof, or non-deformable synthetic resinous composite materials. 
     Referring to  FIG. 7 , the second ends  142  and  142 ′, of their respective first pair of elongated straps  134  and  134 ′ each terminate into a substantially 180° hook shape. The second ends  150  and  150 ′ of their respective second pair of elongated straps  136  and.  136 ′also each terminate into a substantially 180° hook shape. Second end  150  is slidably engaged through a longitudinal slot  146  in elongated strap  134  and second end  150 ′ is slidably engaged through a longitudinal slot  146 ′ in elongated strap  134 ′. Second ends  142  and  142 ′ create a seat to secure a first end of coil spring  160  ( FIG. 6 ). A second end of coil spring  160  is secured by the seat created by second ends  150  and  150 ′. The resting state of coil spring  160  is longer than its length when installed in the adjusting arm  130  in order to provide a force to bias the respective second ends  142  and  142 ′ away from second ends  150  and  150 ′. The elongating force of coil spring  160  urges the first pair of elongated straps  134  and  134 ′ to overlap with the second pair of elongated straps  136  and  136 ′, when required, in response to increasing slack or wear conditions exhibited by the chain  100 . 
     Referring to  FIG. 8 , various elements of the adjusting arm  130  are removed to better show certain features of ratcheting means  155 . Located on the inner surface  137  and in proximity to the second end  150  of elongated strap  136  is an inner rack of teeth  170 . Although not shown in this figure, the mirror image elongated strap  136 ′ also contains the same elements as does elongated strap  136 . Specifically, on the inner surface  137 ′ and in proximity to the second end  150 ′ of elongated strap  136 ′ is a rack of teeth  170 ′. Located on the outer surface  135 ′ and in proximity to the second end  142 ′ of elongated strap  134 ′ is an outer rack of teeth  172 ′. Also not shown in this figure, the mirror image elongated strap  134  contains similar elements as are present on elongated strap  134 ′, that is, on the outer surface  135  and in proximity to the second end  142  of elongated strap  134  is an outer rack of teeth  172 . When the adjusting arm  130  is fully assembled, the inner rack of teeth  170  of elongated strap  136  mesh with the outer rack of teeth  172  of elongated strap  134  and the inner rack of teeth  170 ′ of elongated strap  136 ′ mesh with the outer rack of teeth  172 ′ of elongated strap  134 ′. The teeth are designed to index in only one direction in response to the force of the coil spring  160  urging the second ends  142  and  142 ′ of the first pair of elongated straps  134  and  134 ′ away from the second ends  150  and  150 ′ of the second pair of elongated straps  13  and  136 ′. Consequently, the distance between the tensioning arms  112  and  112 ′ will decrease in response to an increase in slack or excessive wear conditions exhibited by chain  100 . As the distance between the tensioning arms  112  and  112 ′ decreases, a relatively constant tensioning force on chain  100  is maintained. 
     In order to insure that the inner racks of teeth  170  and  170 ′ remain securely engaged with their corresponding outer racks of teeth  172  and  172 ′, the coil spring seating surfaces of the second ends  150  and  150 ′ are angled toward the central axis of the adjusting arm  130 . Concurrently, the coil spring seating surfaces of the second ends  142  and  142 ′ are angled outward away from the central axis of the adjusting arm. When the compressed coil spring  160  is seated between second ends  142  and  142 ′ and second ends  150  and  150 ′, its natural tendency to return to its elongated resting state generates a force on both the angled portions of second ends  142  and  142 ′ and the angled portions of second ends  150  and  150 ′ to insure that the corresponding enmeshed racks of teeth do not jump out of engagement with each other until desired in response to changing chain tension conditions. 
     The tensioning arms  112  and  112 ′ may only employ wear faces  105  or  105 ′ to provide tensioning in the direction of a slack or worn chain. In conjunction with the ratcheting means  155 , the minimal force applied by the wear faces alone may be sufficient enough to tension certain chain drive transmission systems. This embodiment may provide the desired tension for certain power transmission systems. However, other drive transmission systems may have different tension requirements.  FIG. 9  shows a cross section of tensioning arm  112  in which an alternate embodiment is shown. A blade spring  114  is added within a recess in the body of the tensioning arm  112  to provide additional force for urging the wear face  105  into forcible sliding contact with the chain  100 . Similar embodiments would include more than one blade spring, either stacked on top of one another in a single recess or placed in separate recesses along the length of the tensioning arm  112 . It should be understood that tensioning arm  112 ′ may also incorporate at least one blade spring, if desired. The design parameters of each specific chain drive system may necessitate a tensioner  110  in which both tensioning arms  112  and  112 ′ contain blade springs, or one in which only one of the tensioning arms would contain blade springs. Also, neither one of the tensioning arms  112  or  112 ′ might contain a blade spring. 
     Backlash is the backward or untensioned travel of a tensioning device. The combination of the amount of force provided by the wear faces  105  and  105 ′ and the indexing movement of the ratcheting means  155  of the invention controls the amount of backlash that occurs in the operation of a closed loop chain driven power transmission system. The gap created between the body of the tensioning arms  112  and  112 ′ and the under side of their respective wear faces  105  and  105 ′ is limited by the amount of gap clearance, previously discussed, between the ends of the wear faces  105  and  105 ′ and the corresponding ends of the tensioning arms  112  and  112 ′. The total amount of the combined gap from both tensioning arms defines the backlash in the power transmission system. Backlash determines the timing variation in the driven sprocket(s) and must be kept to a minimum. When slack in the chain cannot be absorbed because the maximum gap between the wear faces  105  and  105 ′ and their corresponding tensioning arms  112  and  112 ′ has been reached, the coil spring  160  of the ratchet means  155  provides the required force to index the meshed racks of teeth by at least one tooth and in only one direction. The indexing of the teeth increases the overlap between the pairs of elongated straps of the adjusting arm  130 , and biases the second ends  109  and  109 ′ of the corresponding tensioning arms  112  and  112 ′ toward each other. The reduced distance between the second ends of the tensioning arms  112  and  112 ′ reestablishes forceful contact of the wear faces  105  and  105 ′ with their respective strands of chain  100 . The unidirectional movement of the teeth prevents the adjusting arm  130  from returning to its previous elongated state which would result in an inability to tension the chain due to the loss of or a reduction in forceful contact between the wear faces  105  and  105 ′ and their corresponding strands of chain. 
     Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.