Abstract:
The drive-chain tensioner is automatically self-adjusting. A chain-slipper is guided to follow wear-movement of the chain. A cam is mounted on the follower. If slack in the chain increases, the cam is rotated by a cam spring to a position in which a larger radius of the cam lies under the follower. The cam provides a solid abutment that prevents the chain from compressing the tensioning springs and becoming slack when drive is reversed. For compactness and good load distribution, the chain is supported on a saddle between two cams. The cams have ratchet teeth.

Description:
This invention relates to chain tensioners, of the kind used for taking up slack, due to wear, in a transmission-drive chain or belt. 
     BACKGROUND TO THE INVENTION 
     In a typical case such as a transmission drive chain, the degree of tension or slack in the chain is adjustable by a person un-clamping a slipper member, moving the slipper member to a new location where the chain is tighter, and then re-clamping the slipper member. This adjustment is not automatic, in that the chain gets progressively slacker until the person effects the adjustment. 
     In other known types of tensioner, a spring presses the slipper against the chain, whereby tension is maintained in the chain as the chain wears, due to the resilience of the spring. 
     In a case where the chain acts uni-directionally, i.e only in forward-drive, a spring-biassed slipper pressed against the slack-run of the chain can be adequate. But in the case where the chain acts sometimes in forward-drive, and sometimes in reverse-drive, a spring-biassed slipper pressed against the slack run of the chain is not enough, because the slack-run becomes the tight-run in reverse. 
     Providing two slippers, one to each run, and spring-biassing them together by means of a floating spring clamp, can serve in those cases. However, in that case it is hard to achieve the right compromise of spring forces over the required range of movement: particularly since sudden reversals of load can hurl the tensioner suddenly from side to side. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     By way of further explanation of the invention, exemplary embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
     FIG. 1 is a side-elevational, partly-sectioned, view of a three-axled ATV (all-terrain vehicle), showing the transmission drive chains. 
     FIG. 2 is a corresponding side elevation of the drive chains, in which a chain tensioner embodying the new invention is included. 
     FIG. 3 a  is a close-up of the chain tensioner shown in FIG. 2, showing the tensioner in a new-configuration. 
     FIG. 3 b  is the same view as FIG. 3 a,  but shows the tensioner in a worn-configuration. 
     FIG. 4 is a front elevation of the chain tensioner of FIG.  2 . 
     FIG. 5 is a plan view of the tensioner of FIG.  2 . 
     FIGS. 6 and 7 are views on arrows  6 — 6  and  7 — 7  of FIG.  5 . 
    
    
     The apparatuses shown in the accompanying drawings and described below are examples which embody the invention. It should be noted that the scope of the invention is defined by the accompanying claims, and not necessarily by specific features of exemplary embodiments. 
     In FIG. 1, one of the main drive roller chains  15  is kept in tension by an automatic tensioner  17 . FIG. 2 shows the range of movement which the tensioner has to accommodate between the line  19  of a chain in the new-configuration and the line  20  of the worn-configuration, is shown in FIG.  2 . The range corresponds to (a little over) the length of one link of the chain. Thus, if the chain becomes slacker than the line  20  of worn-configuration, the remedy is to take one link out of the chain. 
     FIG. 3 a  shows the condition of the tensioner in the new configuration, and FIG. 3 b  shows the condition of the tensioner in the worn configuration. 
     The tensioner includes a ratchet-cam  23 . A cam-shaft  24  carries a slipper  25  (made of nylon), against which the chain directly engages. 
     The ratchet-cam  23  is provided with means for exerting biassing forces thereon, being a biassing-force BF-T, which exerts a rotational torque on the ratchet-cam, and a biassing-force BF-U, which exerts an upward force on the cam. 
     As the chain wears, the configuration line of the chain moves from the new line  19  to the worn line  20 . The ratchet-cam  23  has teeth  26 , which engage a cross-bar  27 . As the chain slack increases, the cam  23  is allowed to move progressively upwards, under the action of the biassing force BF-U, which thereby maintains-tension in the chain. 
     As this upward movement of the ratchet-cam continues, the tooth  26 N, being the tooth which is in engagement with the cross-bar in FIG. 3 a , breaks contact with the cross-bar. Now, the biassing force BF-T causes the ratchet-cam  23  to rotate anti-clockwise, and the ratchet-cam rotates until the next tooth  26 N+1 engages the cross-bar  27 . After that, the engagement of the tooth  26 N+1 with the cross-bar prevents the biassing-force BF-T from rotating the cam further, until increasing slack in the chain again causes the cam to click over to the next tooth. 
     When the chain is driving forwards normally, the tensioner is in the slack-run of the chain. In that case, the force between the chain  15  and the nylon slipper  25  is minimal. But when the chain is driving in reverse, the tension in the chain can become very high; therefore, the contact force between chain and slipper is correspondingly high, especially (because of the geometry of the layout) as the chain becomes worm. This high contact force between the chain and the slipper is reacted by the engagement of the flat  28  of the tooth against the top surface of the crossbar  27 . 
     The designer should see to it that the geometry of the teeth, the cam-shaft  24 , and the cross-bar  27  are such that the ratchet-cam does not tend to slip back, once a tooth has clicked over the cross-bar. 
     The magnitudes of the biassing forces is important. On the one hand, adequate tension must be maintained in the chain to stop the slack-run of the chain from slapping. On the other hand, if the biassing force BF-U were too large, the chains might wear at too rapid a rate. Plus, and perhaps more importantly, too large a BF-U force would exaggerate the gap between the flat  28  and the cross-bar  27 , whereby, upon a sudden imposition of heavy tension in what was the slack run of the chain, due to drive-reversal, the flat  28  would be smacked against the cross-bar  27  with a noticeable knock. When the biassing-force BF-U is light, this gap is not forced to its maximum. A biassing force BF-U in the region of 5 or 10 lbsf has been found satisfactory, given a total travel of a little over 2 inches, in 12 ratchet steps. 
     Of course, ATV&#39;s are subjected to abusive motions, whereby the tendency for the chains to bounce and slap is quite marked. A particularly heavy bounce of the chain might cause a tooth of the ratchet-cam to click over prematurely. If that happens, the extra tension would dissipate gradually as more wear occurs, and knock is all the less likely to occur as the flat  28  is pressed tightly against the cross-bar. 
     As shown in FIG. 4, two ratchet cams  23  are welded to the cam-shaft  24 , and the nylon slipper  25  lies between the two cams. The cam-shaft is located between left and right guide channels  30 L, 30 R, which are welded to the left and right chassis members  32 L, 32 R. The cam-shaft floats vertically within the guide-channels, under the action of biassing springs. 
     The upwards force BF-U (shown diagrammatically in FIG. 4) on the cam-shaft is derived from left and right springs  34 L, 34 R. Both springs are shown structurally in FIG. 5, and the left spring  34 L is shown in FIG.  6  and the right spring  34 R is shown in FIG.  7 . The total upwards biassing force BF-U is provided by the sum of the upwards forces due to the two springs  34 L, 34 R. 
     The biassing torque comes only from the right spring  34 R. This spring, as shown in FIG. 7, presses upwards on the cam-shaft by virtue of its central coils being wrapped around the cam-shaft. The torsional bias arises from the engagement between one end of the spring  34 R and the tag  36  on the ratchet-cam. 
     The springs are so arranged that the springs press the cam-shaft  24  against the sides of the guide channels  30 L, 30 R to only a minimum extent over the whole range of travel of the cam-shaft. 
     Both springs of course require enough stressed wire within the spring to provide adequate biassing force over the whole travel range. The left spring  34 L, which is responsible only for half the upwards bias BF-U needs only a few coils, as shown, whereas the right spring, which is responsible not only for half BF-U, but also for all of BF-T, requires many more coils. (If the springs lie nearer the other of the chassis members, the springs would be reversed.) 
     In some embodiments of the invention, the ratchet teeth  26  on the cam  24  may be so formed as to provide the same depth of step between the first and second teeth as between the eleventh and twelfth teeth. The length of the flats  28  between the teeth varies, as shown, in order that the angular increment required to click a tooth is the same throughout the range of angular movement of the cam. 
     A nylon slipper  25  has been shown for rubbing against the chain. A passively-rotating idler-sprocket could alternatively be provided. The advantages as described arise from the design of the tensioning device, as described, rather than what is tensioned, whereby the term chain or drive chain as used herein should be construed as also including drive belts. 
     The apparatus as described herein may be compared with chain tensioners that operate on a single-direction basis. 
     Single-direction tensioners operate in a comparatively more benign environment, in that a single-direction tensioner is never subject to reverse loading. The slack run of the chain remains the slack run at all times, and the heavy forces arising in the tight run are never experienced by the single-direction tensioner. By contrast, a both-direction tensioner has to cope, upon drive-reversal, with the effects of the full tension of the chain. Furthermore, the both-direction tensioner has to cope with shock loading. Also, reversal of direction can be quite violently abusive, particularly in a case such as an ATV, as described herein. 
     On the other hand, the main purpose of most single-direction tensioners, for example of the type as commonly found in association with the timing chain of an automotive engine, is to provide a constant tension in the chain. The speed of the chain might vary during operation from zero to high linear speeds, but the speed is never reversed. Demanding as this requirement can be, the requirement for supporting heavy loads in the tensioner, and particularly for supporting heavy shock loads, does not arise. 
     In a single-direction tensioner, the idea is to maintain a steady spring force against the chain. If the tensioner is to include an automatic adjuster, the designer often is concerned to provide enough built-in resilience in the tensioner that the chain does not “bottom”, which might cause a sudden increase in chain tension, and consequent shortening of service life. On the other hand, in a both-direction tensioner, the chain inevitably always bottoms against the tensioner, when reversed. 
     In a both-direction tensioner, the abutment against which the chain bottoms in reverse cannot be allowed to be resilient. Indeed, the maintenance of chain tension in the apparatus as described, by means of spring resilience, only happens in the forwards direction. In reverse, what has now become the slack tun of the chain is (in most cases) not tensioned. 
     While single-direction tensioners (including e.g those for timing chains in auto engines) have a tradition of automatic self-adjustment for maintaining chain tension and for curbing the higher amplitudes of vibration of the slack run of the chain, both-direction tensioners have no such tradition. Conventionally, to compensate for chain wear, the tradition has been that both-direction tensioners have been manually adjustable. 
     The difference in manner of operation between single-direction and both-direction tensioners leads to a number of differences that must be borne in mind by the designer. First, and major, is the requirement that the apparatus has to cope with abusive shock loading, and with reversals of load. On the other hand, at least on ATVs, the really violent abuse occurs when the chain is basically not moving (e.g when the driver is trying to extricate a stuck vehicle); so the designer of the chain drive needs to emphasise good pure-tension properties, as well as good wear properties. 
     In a single-direction tensioner, often the designer is concerned to balance the force of an adjusting spring with the force of the chain tensioning spring, the two springs being arranged in series. In a both-direction tensioner, the chain-tensioning spring cannot be allowed to be in series with the adjustment spring (and indeed, in reverse, there is no chain-tensioning spring). 
     Because the adjustment spring and the chain-tensioning spring are not in series, the two springs can each be designed without having to be compromised by the need. to accommodate to the other&#39;s requirements. In the apparatus as described herein, the chain-tensioning spring does not bias the cam, either to promote or inhibit adjustment; and the cam-adjuster (torsion) spring does not affect the force with which the slipper or follower presses against the chain. In fact, the cam spring provides a torque on the cam, which is reacted against the cross-bar by the edge-of-tooth of the currently-engaged ratchet tooth; the adjuster spring is isolated from affecting the chain tension because the edge-of-tooth surface lies more or less parallel with the load line that supports the tension in the chain. 
     Similarly, the chain tension is isolated from affecting the adjuster, because the flat-of-tooth surface of the currently-engaged ratchet tooth engages the flat of the cross bar, and sits more or less flat-on, i.e at right angles, to the load line. The two functions—chain tension support in reverse, and adjustment—are quite separated, in the apparatus as depicted herein. 
     Space being tight around the transmission chain of an ATV, It is important to fit the tensioner apparatus into a small space envelope. One of the more constrained dimensions of the envelope is the dimension in the (vertical) plane in which the adjuster travels to take up the slack in the chain. The apparatus as described herein has a large adjustment travel distances, but the envelope in this plane is kept to hardly any more than the chain requires anyway as it becomes progressively slacker due to wear. 
     This efficient use of the available envelope has been achieved by placing the two ratchet cams one either side of the chain. The chain slipper is supported by the cam shaft or spindle, which straddles across (rigidly) between the two cams. 
     To ensure that the load from the chain is divided between the two cams, the cam spindle is free to float in the tilting or tipping sense (i.e in the roll-sense with respect to the ATV itself). The load from the chain lies between the two cams, and, because it can rock or tip, the cam spindle does rock, until both cams touch firmly against the cross-bar. 
     Not constraining the cam assembly against rocking is useful not only for equalising the load between the two cams, but also from the manufacturing standpoint. It would be quite difficult to mount the assembly of the two cams (the assembly being rigid in itself) rigidly to the frame (i.e rigid in the sense of constraining the assembly against tipping) while permitting the required up/down movement. 
     In the designs as shown, the cam spindle is well-constrained against rocking or tilting of the cam assembly in the yaw sense. However, it does not matter so much that the cam spindle cannot tilt in the yaw sense. The yaw constraint might result in only one of the cams having its edge-of tooth surface touching the cross-bar, but that does not matter when compared with lack of constraint in the roll-sense, which means that both flat-of-tooth surfaces can share and divide their loads onto the cross-bar. 
     Placing the cams to the sides of the chain can be expected to result, as shown, in a long-travel adjuster in a compact space envelope. But, again as shown, this placement has not involved any compromise in the strength of the ratchet teeth and other components. As can be seen, the components are chunky and sturdy—as of course they need to be in a both-direction tensioner. The two cams, which carry the slipper between them, provide a good solid platform for supporting the chain loads. 
     Another aspect the designer should consider is this. The heavy forces arising from the chain in reverse need to pass through abutting surfaces. The designer should prefer that the abutting surfaces should be simply at right angles to the direction of the heavy forces, and in the apparatus as depicted herein, this is the case. However, it might occur to a designer to try to secure the movement needed to procure automatic adjustment by aligning the ratchet teeth at an angle; some designs of automatic adjuster have been based on turning a screw thread, to take up slack, by the use of angled ratchet teeth. However, in such a design, when the teeth are under load, the fact that the teeth lie at an angle means that the designer must provide some means whereby the ratchet teeth can be prevented from rotating. That is to say, an angled-tooth design involves applying a force to the ratchet teeth (when adjustment is required), and then applying another heavy force to stop the ratchet from moving (when adjustment is not required). Such designs start to resemble intricate clockwork, which, apart from being fragile, is not appropriate on an ATV. In the apparatus as described herein, the heavy forces from the chain in reverse are reacted by components engaging and abutting flat-on. During reversal, the flat-of-tooth surface smacks against the flat of the cross-bar, both abutting surfaces being at right angles to the direction of the heavy force. 
     As far as over-adjustment is concerned, the following points can be mentioned. In any chain tensioner, if the chain is over-stretched in forward drive, the resulting extra slack in the slack run of the chain might be taken up by the adjuster as if it were wear. Such over-adjustment is detrimental, because the chain remains in induced tension, which causes further wear to take place more rapidly, at least until the extra tension has been dissipated. 
     However, when the adjuster is based on a ratchet, there are only a few specific times, in the life of the adjuster, where over-adjustment is likely to occur. Thus, when a tooth has just clicked over, it would take a very large over-stretch, just then, to make the adjuster click over again. With a ratchet adjuster, in most of the conditions of wear of the chain, a mildly abusive over-stretching of the chain will not cause the ratchet to click over. Only when the ratchet is near to clicking anyway will over-stretching cause click-over, and over-adjustment. 
     This may be contrasted with an adjuster of the type that takes up slack continuously, i.e steplessly. In that case, whenever over-stretching of the chain occurs, i.e at any time, it will cause over-adjustment. With a stepless adjuster, over-stretching always leads to over-adjustment, whereas, with a ratchet adjuster, the likelihood of over-adjustment due to over-stretching the chain waxes and wanes depending on the disposition of the ratchet teeth. 
     The fewer the number of teeth, the fewer the periods when over-stretching of the chain might cause over-adjustment. Providing a ratchet with a travel made up of, say, ten or twelve teeth, gives a good compromise between good slack take-up while avoiding over-adjustment. 
     Providing just a small number of ratchet teeth also means that each flat-of-tooth surface can be large, which is important because the forces are heavy and abusive. And as mentioned, with a stepped ratchet, the flat-of-tooth surface can be made tangential to the cam-spindle, whereby the force on the flat-of-tooth surface, even though large, imposes no tendency for the surface to be displaced laterally. 
     The total lift of the cam comes from the aggregate of the lifts of the steps between the teeth. In a stepless cam, the lift would come from the fact that points on the cam abutment surface lie at an angle to the tangential. A stepped ratchet can have a high lift, in aggregate, even though the abutment surface of each tooth is tangential; but a stepless cam cannot have a high lift, since that would mean the abutment surface would be at a large angle to the tangential, which might be enough to give rise to spurious side loads at the abutment. 
     The total adjustment travel capability of the apparatus should be in line with other servicing requirements. On an ATV, the total lift preferably should be equal to one, or two, links of the chain; i.e the designer should provide that when the adjuster runs out of travel, the remedy is to take one or two links out of the chain.