Abstract:
In a chain tensioner having a plunger biased to protrude from a housing by a spring, a pair of wedge-shaped cams are received in tapered recesses in the end of the housing, cooperate with racks formed on opposite sides of the plunger, and are urged against oblique seats formed by the tapered recesses by a biasing spring. The cams and racks produce a ratcheting action maintaining chain tension by preventing retracting movement of the plunger. The cams and racks disperse the load imparted to the plunger by the chain, and reduce localized wear, allowing the housing to be made from a light weight material such as aluminum. Accuracy requirements associated with conventional pawl-type ratcheting tensioners are avoided.

Description:
FIELD OF THE INVENTION 
     This invention relates to a chain tensioner for maintaining proper tension in a power transmitting chain. The tensioner has utility, for example, in maintaining tension in the timing chain of an internal combustion engine, where the chain transmits power from a crankshaft sprocket to one or more camshaft sprockets. 
     BACKGROUND OF THE INVENTION 
     In a conventional tensioner, a plunger, protruding from a housing, exerts a force on the back of a pivoted tensioner lever having a shoe in sliding contact with the slack (return) run of a chain in order to maintain tension in the chain. The plunger of the tensioner engages the lever at a location remote from the pivot axis of the lever. 
     A conventional ratchet type tensioner  500 , as shown in FIGS. 9 and 10, comprises a plunger  510 , slidable in a hole  531  formed in a housing  530 . The plunger protrudes from the housing, and is biased in the protruding direction by a spring  520 . A pawl  550 , pivotally supported by a pin  540  on the housing  530 , is biased by a spring  560  so that its pawl teeth  551  engage teeth  511  formed on the outer periphery of the plunger. 
     When the plunger  510  protrudes from the housing  530  as a result of loosening of the chain, and is thereafter subjected to an impact force from the tensioner lever, the ratchet mechanism blocks retraction of the plunger by engagement of the pawl teeth  551  with the ratchet teeth  511  on the plunger. 
     When the ratchet operates to prevent retraction of the plunger, the ratchet pawl  550  applies a sideways force to the plunger, which results in eccentric loads, indicated by arrows F 1  and F 2  in FIG. 10, exerted by the plunger on the hole  531 . Especially when the housing  530  is formed of aluminum, these loads can result in significant localized wear. 
     In the conventional ratchet-type tensioner  500 , when the plunger  510  is subjected to excessive impact force from the tensioner lever, the impact force is concentrated on the ratchet teeth  511  and the pawl teeth. The impact force produces significant wear of the tips of these teeth, and failures due to excess wear or breakage of the teeth can occur. 
     Furthermore, in the conventional ratchet-type tensioner  500 , the control of tension in a traveling chain depends on engagement between the ratchet teeth  511  and the pawl teeth  551 . To attain smooth engagement of these teeth, high accuracy in the diameter and mounting position of the pivoting pin  540  are required. Troublesome problems have been encountered in the production of the pin, and also in the production of a pawl with an accurately positioned pin-receiving hole having the proper diameter. 
     Among the objects of the invention are the solution of the above-described problems, and the provision of a chain tensioner which can disperse and reduce the load applied to the plunger, which exhibits superior wear resistance, and which can reduce the requirement for high accuracy in production of components and assembly thereof. 
     SUMMARY OF THE INVENTION 
     The chain tensioner in accordance with the invention is similar to a conventional chain tensioner in that it comprises a housing having a plunger-receiving hole with an opening, a plunger slidably fitting into the plunger-receiving hole and protruding from the opening thereof in a protruding direction, and a spring biasing the plunger in the protruding direction. 
     The chain tensioner differs from the conventional chain tensioner in that it comprises toothed racks formed on opposite sides of the plunger, and a pair of wedge-shaped cams on opposite sides of the plunger, each cam being disposed adjacent one of the respective toothed racks, each the cam having teeth Mockingly engageable with teeth of the adjacent rack to prevent retracting movement of the plunger relative to the cams, and each cam having an oblique surface for engagement with a seat. The oblique surface of each cam faces outwardly away from the plunger, and the shape of each oblique surface is such that the oblique surface has an outer end disposed farther than an inner end thereof along the protruding direction of the plunger. Oblique cam seats are formed in the housing on opposite sides of the plunger, and the oblique surfaces of the cams conform to, and are in engagement with the oblique cam seats and slidable thereon to an extent such that the teeth of the cams can be brought into and out of locking engagement with the teeth of the racks. A spacer is disposed on the housing adjacent the opening, and a second spring, engaged with the spacer and the cams, urges the cams in a direction opposite to the protruding direction of the plunger so that the cams are urged into locking engagement with the toothed racks. 
     In a preferred embodiment, the plunger and housing define an oil chamber within the housing, and a hydraulic valve mechanism is provided on the housing for allowing one-way flow of oil from an external supply of oil under pressure into the oil chamber to apply an additional force biasing the plunger in the protruding direction. 
     In a preferred embodiment of the invention the plunger is rotatable in the housing, so that engagement between the rack and the wedge-shaped cams can be forcibly released. 
     If the plunger-biasing spring is interposed between the flange and the spacer, it is more easily installed, and can be used to hold the spacer against the housing. 
     In an alternative embodiment, the plunger-biasing spring is disposed in the plunger-receiving hole and is interposed between an end of the plunger located within the hole and an end of the hole. In this case, the spacer is preferably screwed into housing. 
     The chain tensioner of the invention may be either the inner mounting type or an outer mounting type. 
     The teeth on the plunger racks and the cams can take various forms so long as they are capable of exhibiting a ratcheting action allowing the plunger to move in the protruding direction but preventing retraction thereof. The protrusion biasing spring causes the plunger to move rapidly in the protruding direction when the chain becomes loose, thereby reestablishing proper tension. 
     The cam biasing spring, which is loosely fitted onto the plunger, biases the wedge-shaped cams against their seats, which, in turn, exert a radially inward force on the cams so that they are urged into engagement with the plunger. However, when the plunger moves in the protruding direction, it draws the cams with it in the protruding direction, and at the same time its teeth, in cooperation with the teeth of the cams, urge the cams radially outwardly, holding the cams against their seats until the teeth disengage. At the instant when the teeth become disengaged the plunger will move forward by one tooth of the rack, and the cams are immediately reengaged with the rack, to prevent retracting movement of the plunger, and maintaining proper chain tension. 
     The use of cams on opposite sides of the plunger distributes the force applied by the chain to the plunger, thereby avoiding concentrated loads on the housing. Moreover, the cams distribute the force symmetrically, avoiding eccentric loads as in the case of the conventional tensioner. 
     Furthermore, in the chain tensioner according to the invention, even when the plunger is subjected to excessive impact force from the chain through a tensioner lever, the plunger is prevented from retracting so that proper chain tension is maintained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view showing a chain tensioner in accordance with a first embodiment of the invention installed in an internal combustion engine to maintain tension in a timing chain; 
     FIG. 2 is a partly cut away perspective view of the chain tensioner shown in FIG. 1; 
     FIG. 3 is an exploded perspective view of the chain tensioner shown in FIG. 1; 
     FIGS.  4 ( a )- 4 ( c ) are sectional views of the chain tensioner shown in FIG. 1, illustrating stages in the operation thereof; 
     FIG. 5 is a sectional view illustrating the distribution of load in the chain tensioner of FIG. 1 when a force is applied to the plunger; 
     FIG. 6 is a partly cut away perspective view of a chain tensioner in accordance with a second embodiment of the invention; 
     FIG. 7 is a cross-sectional view of the chain tensioner shown in FIG. 6; 
     FIG. 8 is a schematic view showing a chain tensioner in accordance with a third embodiment of the invention, illustrating the relationship of the tensioner with a tensioner lever and a transmission chain; 
     FIG. 9 is an sectional view of a conventional ratchet type tensioner; and 
     FIG. 10 an elevational view showing a conventional chain tensioner installed in an internal combustion engine to maintain tension in a timing chain. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The chain tensioner  100  of the first embodiment, illustrated in FIGS. 1-5, is an “outer attachment” type chain tensioner, which is attached from the outside of an engine. The tensioner is used to suppress the vibration generated as a timing chain TC travels over a crankshaft sprocket S 1  and camshaft sprockets S 2  of an engine and also to maintain proper tension in the chain. 
     In the chain tensioner  100 , a spring-biased cylindrical plunger  120  protrudes toward the timing chain TC, which travels past a tensioner housing  110  attached to a wall E of the engine block wall. The tensioner plunger presses against the back surface of a tensioner lever TL, which is pivoted on the engine block, so that a shoe surface of the tensioner lever TL is held in sliding contact with the slack side of the timing chain TC to maintain tension in the timing chain. 
     A tensioner guide TG, fixed to the engine block, guides the timing chain TC so that side run-out of the timing chain TC is prevented. 
     As shown in FIGS. 2 to  4 , the chain tensioner  100  includes a housing  110  having a plunger-receiving hole  111  into which a plunger  120  is inserted. The plunger protrudes from the hole, and a spring  130 , which surrounds the plunger and is in engagement with a flange formed at the outer end of the plunger, biases the plunger  120  in the protruding direction. Two wedge-shaped cams, which are in the nature of cam chips, are engaged respectively with two toothed racks separately formed on opposite sides of the plunger  120 . The wedge-shaped cams are disposed in tapered recesses formed in the front end of the housing on opposite sides of the front opening of the plunger-receiving hole  111 . These cams are in sliding engagement with obliquely tapered cam seats  112  constituted by walls of the recesses, as shown in FIG. 2. A cam biasing spring  150  surrounds the plunger with clearance, biasing the cams  140  against the obliquely tapered seats so that the cams are urged radially inwardly into engagement with the plunger. The biasing spring  150  is held between the cams and a spacer  160 , which is held against the end of the housing by spring  130 . 
     A hydraulic valve mechanism  170  is provided at a rear end portion of housing  110  for receiving oil under pressure from an external oil supply source and accurately controlling the protrusion biasing force of the plunger  120 . This hydraulic valve mechanism  170  comprises a ball seat  171  press-fit into the housing  110 , a check ball  172  engageable with the ball seat  171 , and a retainer  173  for retaining the check ball  172 . The valve mechanism permits the flow of oil into a high pressure oil chamber  180  defined by plunger  120  and hole  111 , but blocks reverse flow of oil from the high pressure oil chamber  180 , so that tension is imparted to, and maintained smoothly in, the chain by plunger  120 . 
     In the operation of the chain tensioner  100 , when the timing chain TC becomes loose, the plunger  120 , biased by the protrusion biasing spring  130 , immediately moves in the protruding direction. 
     FIG.  4 ( a ) shows the condition of the tensioner before the plunger protrudes, FIG.  4 ( b ) shows the tensioner as the plunger is moving in the protruding direction, and FIG.  4 ( c ) shows the plunger moved forward by one tooth. 
     When the plunger  120  moves in the protruding direction to maintain tension in the timing chain, the racks  121  on the plunger  120  draw the cams  140 , which are initially in the position shown in FIG.  4 ( a ), in the protruding direction against the force applied to them by the biasing spring  150 . As the cams are drawn in the protruding direction, a camming action exerted by the teeth of the racks, which are in the shape of ratchet teeth causes the cams to slide radially outward while in engagement with the tapered seats  112 . The cams simultaneously rise and move outward, while sliding on the tapered seats, to a position where they become disengaged from the rack teeth on the plunger, as shown in FIG.  4 ( b ). 
     When the cams  140  rise on the tapered surfaces of the tapered cam sliding seats  112  and the engagement with the racks  121  by the plunger  120  is relieved, the plunger  120  moves forward by one tooth. Spring  150  then immediately pushes cams  140  downward, that is, in the direction of retraction of the plunger, as shown in FIG.  4 ( c ), and the engagement between the cam teeth and the racks blocks retraction of the plunger  120 . 
     As shown in FIG. 5, the force F received by the plunger  120  is split into forces f 1  and f 2 , which are applied to the seats on opposite sides of the plunger. Thus the load applied to the housing  110  is distributed so that concentration of the load is avoided. Moreover, as mentioned previously, the load is distributed symmetrically so that eccentricity is avoided. 
     When a traveling timing chain TC loosens, the plunger  120  is moved forward sequentially, one tooth at a time, to impart proper chain tension to the chain. At the same time, reverse movement of the plunger is prevented so that proper chain tension can be maintained. Although the housing  110  is made of aluminum in the present example, localized wear of the kind which occurs in a conventional housing is avoided. Moreover, a significant reduction in the overall weight of the tensioner can be achieved. 
     The invention affords other significant benefits. Since a simple cam support structure is used, in which a pair of cams  140  are inserted into tapered recesses, the problem of achieving the high manufacturing accuracy and high mounting accuracy required in the case of a pivot pin and pin hole for pivotally supporting a pawl are avoided. Moreover, the difficulties encountered in manufacturing and assembling the ratchet pawl, and achieving accuracy therein, are also avoided, and the working accuracy and working load in the part working and the assembly working can be reduced. 
     Furthermore, by using the protrusion biasing spring  130 , interposed between a front end of the plunger  120  and the spacer  160 , to hold the spacer  160  in abutting relationship with the front of the housing, the cams  140  can be easily inserted into their recesses, and a simple device configuration, and easy assembly thereof, can be realized. 
     Because the two racks  121  are situated on opposite sides of the plunger  120 , the plunger can be rotated in the housing by 90 degrees to release the racks from the cams  140 . Accordingly, assembly can be carried out easily, and the engagement between the racks and the cams can be released easily for maintenance or for checking the timing chain. 
     The chain tensioner  200 , according to a second embodiment of the invention is shown in FIGS. 6 and 7. This tensioner is also an outer attachment type tensioner, having the same structure as that of the first embodiment. Accordingly, reference numbers in FIGS. 6 and 7 exceed the reference numbers designating corresponding components in FIGS. 1-5 by 100. 
     In FIGS. 6 and 7, a protrusion biasing spring  230  is interposed between the rear end of the plunger  220  and the bottom of plunger-receiving hole  211 . The spacer  260  is screwed to the front end of the housing. That is, it may be secured to the front end of the housing  210  by screw fasteners, or alternatively, it may be threaded onto the front end of the housing. Many of the features of operation and advantages of the second embodiment are substantially the same as those of the first embodiment. 
     The tensioner  200  can be miniaturized by reducing the distance from the front end of the plunger  220  to the front end of the housing  210 . Moreover, the tensioner can be arranged nearer to the chain, so that its response to changes in tension of the timing chain TC can be improved. Additionally, in this embodiment, the plunger  220  is prevented from separating from the housing by the fact that the spacer  260  is independently secured to the housing. Therefore disengagement of the plunger from the housing is prevented. 
     In FIG. 8, tensioner  300 , which is a third embodiment of the invention, has the same basic structure as in the first embodiment. Here, components are designated by reference numbers that exceed by 200 the reference numbers for corresponding components in FIGS. 1-5. 
     The chain tensioner  300  differs from the tensioner of the first embodiment principally in that mounting holes  313  are provided for mounting the chain tensioner  300  on an engine block inside the engine, so that this tensioner is a “inner attachment” type tensioner. Otherwise, the features of operation and advantages of the third embodiment are substantially the same as those of the first embodiment. 
     Although the chain tensioners have been were described as used with a timing chain TC they can also be used with a timing belt. 
     Important advantages of the invention may be summarized as follows. 
     The tensioner not only maintains proper chain tension but also distributes the force exerted on the tensioner plunger by the chain through both cams to the cam seats on opposite sides of the plunger. Accordingly, localized wear of the housing, of the kind generated in a conventional tensioner, is avoided. The invention also results in an overall reduction in the weight of the tensioner. In particular, because localized wear is reduced, the housing, which accounts for most of the weight of a conventional tensioner, can be formed of aluminum, and a significant weight reduction can be realized. 
     Another important advantage is that the cam support structure, in which a pair of cams are inserted into tapered recesses, avoids the problems associated with the high accuracy requirements in the case of a ratchet-type tensioner utilizing a pawl. Moreover, even if a rack tooth or a pawl tooth is broken, exchange and maintenance can be carried out quickly and easily. 
     The cam and rack mechanism in accordance with the invention can be used advantageously in a tensioner having a hydraulic valve mechanism for introducing oil from an external oil supply in order to control the protrusion biasing force accurately. Accordingly, proper chain tension can be imparted and maintained in a smooth manner. 
     The invention has the further advantage that the racks and cams can be easily disengaged by rotating the plunger. Accordingly, the tensioner can be assembled easily, and can be disassembled easily for maintenance of the tensioner or for checking or maintaining the timing chain. 
     In the cases where the protrusion biasing spring is interposed between the front end of the plunger and the spacer, handling of the protrusion biasing spring is easy, and the spacer may be held against the front of the housing by the spring without the need for threads or fasteners. 
     In the second embodiment, in which the protrusion biasing spring is interposed between the rear end of the plunger and the bottom of the plunger-receiving hole, the distance from the front end of the plunger to the front end of the housing can be reduced, and the tensioner can be arranged nearer to the chain for improved responsiveness to changes in tension in the chain. 
     In the second embodiment, since the spacer is independently secured to the front end of the housing, the plunger can be more reliably prevented from disengagement from the housing.