Abstract:
A hydraulic tensioner for a drive system, including a flexible, inextensible drive element, has a bi-directional travel limiter associated with a hydraulically biased tensioning cylinder. The travel limiter is bi-directional because it limits not only loading of the tensioning cylinder against a tensioner shoe in a direction tending to tighten a drive element, but also resists movement of the tensioner shoe in a direction tending to allow slack in a drive element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a tensioning device for maintaining desired tightness within a flexible inextensible drive element in a system such as a camshaft drive for an internal combustion engine. 
     2. Disclosure Information 
     Devices incorporating flexible inextensible drive elements such as fiber-reinforced belts or metal linked chains have traditionally employed tensioning devices. In order to be effective, such devices must damp excess motion of the chain or belt without imposing unreasonably high hub loads upon either the driving member, such as a crankshaft of an engine, or other driven members such as idlers or camshafts. Unfortunately, if tension is not adequate, the chain or belt may skip a tooth, causing unacceptable engine operation or, in extreme cases, engine damage. If, on the other hand tension within the belt or chain is too great, excessive wear will occur within engine bearings such as crankshaft main bearings or camshaft bearings. 
     It is known to construct a chain tensioner, for driving a camshaft of an internal combustion engine, with the tensioning force being provided by a piston which is pressurized by engine lubricating oil. Such devices usually employ a controlled leakage factor to exert a force of appropriate magnitude upon a tensioner shoe in the direction tending to tension the chain properly, while damping out excess oscillations of the chain. Such tensioners also typically include a ratcheting device which prevents the tensioner shoe from releasing from the chain beyond a predetermined amount when oil pressure drops off, as when the engine is shut down. Unfortunately, such known devices are prone to overextension of the ratcheting tensioner device during startup operating at lower ambient temperatures, which may cause the ills described above as accompanying excess tension in the chain. 
     It would be desirable to provide a tensioner which not only prevents excessive falloff in chain tension when the engine is shut down, but which also prevents overtightening or overtensioning of a chain or belt during operation of the engine. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, a hydraulic tensioner for a drive system having a flexible, inextensible drive element, includes a tensioner shoe in contact with the drive element, a hydraulically biased tensioning cylinder for loading the tensioner shoe so as to maintain a desired tension in the drive element, and a bi-directional travel limiter, operatively associated with the tensioning cylinder, and with the tensioner shoe, for not only limiting loading of the tensioning cylinder against the tensioner shoe in a direction tending to tighten the drive element, but also resisting movement of the tensioner shoe in a direction tending to allow slack in the drive element. 
     According to another aspect of the present invention, a bi-directional travel limiter includes a lockable plunger extending parallel to an axis of motion of the tensioning cylinder, with the bi-directional travel limiter having a cam mechanism for locking the lockable plunger. The cam mechanism, according to an aspect of the present invention, includes a pivoted cam having a resilient biasing element for positioning the cam so as to prevent the lockable plunger from moving in a direction tending to allow slack in a drive element, as well as a hydraulically actuated biasing element for positioning the cam so as to lock the lockable plunger, thereby preventing the tensioning cylinder from moving more than a predetermined distance in a direction tending to tighten the drive element. 
     According to another aspect of the present invention, a tensioning cylinder and a hydraulically actuated biasing element are driven by pressurized oil from a hydraulic pressure source, such as an oil pump in an engine lubrication system. 
     According to another aspect of the present invention, a hydraulically actuated biasing element includes a piston slidably mounted within a bore connected with a hydraulic pressure source, with the piston having an end portion for contacting the pivoted cam, so as to rotate the pivoted cam to a position in which the lockable plunger prevents the tensioning cylinder from moving more than a predetermined distance in a direction tending to tighten the drive element, which may be either a metal chain, or a composite elastomer belt. 
     According to another aspect of the present invention, a hydraulically actuated biasing element includes a piston slidably mounted within the bore connected with a source of pressurized oil having a greater viscosity when cold and a lesser viscosity when hot. In this manner, the bi-directional travel limiter will limit the loading of the tensioning cylinder against the tensioner shoe in a direction tending to tighten the drive element when an operating parameter of the hydraulic tensioner exceeds a predetermined value. 
     It is an advantage of a hydraulic tensioner with an overtravel limiter according to the present invention that inappropriate tensioning of a drive chain, which may otherwise occur when lubricating oil is extremely viscous, as during a cold weather startup, will be prevented. 
     It is another advantage of a hydraulic tensioner according to the present invention that damage resulting to an engine&#39;s crankshaft and camshaft and other ancillary equipment, particularly bearings, will be prevented because excessive tension of a flexible, inextensible drive element extending between a crankshaft and a camshaft will be prevented. 
     It is yet another advantage of a device according to the present invention that this device may be employed in conjunction with known tensioning equipment to prevent damage due to excessive tension arising from over-extension of a tensioner. 
     Other advantages, as well as features of the present invention, will become apparent to the reader of this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of an engine crankshaft, camshafts and drive system according to an aspect of the present invention. 
         FIG. 2  is an enlarged view, partially schematic, of a portion of the hydraulic tensioner shown in  FIG. 1 . 
         FIG. 3  is similar to  FIG. 2 , but shows the inventive tensioner during cold startup operation. 
         FIG. 4  illustrates an alternative embodiment according to an aspect of the present invention, in which a direct-acting, hydraulically driven pawl prevents unwanted drive element tensioning. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in  FIG. 1 , hydraulic tensioner  10  for a drive system is used with a flexible, inextensible drive element such as a chain or belt,  14 . In the arrangement shown in  FIG. 1 , two camshafts,  18  and  22 , are driven by a crankshaft,  26 . Chain or belt  14  is trained over sprockets or pulleys attached to both crankshaft  26  and camshafts  18  and  22 . Tension within flexible, inextensible drive element  14  is maintained by a nonadjustable guide shoe  30 , as well as by a tensioner shoe  34 . Tensioner shoe  34  is pivotably attached to the engine by a pivot,  36 . Tensioner  10  pushes tensioner shoe  34  into drive element  14 , so as to maintain the required tension within drive element  14 . Tensioner  10  is furnished with engine oil under pressure from pump  42 , which, in a preferred embodiment, is the engine&#39;s lubricating oil pump. Those skilled in the art will appreciate in view of this disclosure however, that other types of oil pumps could be used for the purpose of providing hydraulic oil under pressure to hydraulic tensioner  10 . 
     Moving now to  FIGS. 2 and 3 , tensioner  10  is shown as including a hydraulically biased tensioning cylinder, configured as piston  52  housed reciprocably within bore  48 , and having a check ball  56  and bias spring  66  at the inlet to bore  48 , to prevent unnecessary bleed back of oil within bore  48 , and also having a spring,  50 , for biasing piston  52  into tensioner shoe  34  in a direction tending to tension drive element  14 . 
     In addition to piston  52 , a bi-directional travel limiter also exerts various forces upon tensioner shoe  34 . The bi-directional travel limiter includes a lockable plunger  64 , having a number of teeth  60  formed thereupon. Lockable plunger  64  has a head,  65 , which selectively engages piston  52  and also which engages tensioner shoe  34 . When lockable plunger  64  is rendered immovable by the remainder of hydraulic tensioner  10 , which will be explained below in conjunction with  FIG. 3 , lockable plunger  64  will not only prevent tensioner shoe  34  from moving backwardly in a direction tending to allow more slack within drive element  14 , but will also prevent piston  52  from moving outwardly from bore  48 , and thereby applying too much tension to drive element  14  through the force imposed by tensioner shoe  34  upon drive element  14 . If piston  52  collapses too far, head  65  of plunger  64  will contact tensioner shoe  34 , thereby preventing excessive slack in drive element  14 . 
     Tensioner housing  38 , in addition to hydraulically biased tensioning cylinder  44 , includes an additional bore  84 , which is exposed to oil pressure from oil pump  42 . Piston  80  is housed within bore  84 , and engine oil pressure tends to force piston  80  from bore  84  so that end portion  88  of piston  80  impinges upon a pivotable cam,  68  ( FIG. 3 ). When piston  80 , and more specifically, end portion  88  of piston  80 , impinges upon pivotable cam  68 , cam  68  will pivot about cam pivot  72 , and teeth  70  of cam  68  will engage teeth  60  provided on lockable plunger  64 . This engagement of piston  80  with pivotable cam  68  may occur when an engine having the present system is operated at initial startup at a low ambient temperature, because such operation is often characterized by very high engine oil pressure. As a result, the viscosity of the oil is great and extremely high lubrication oil pressures are the rule. These pressures could cause piston  52  to place excessive stress upon shoe  34  and drive element  14 , but for the intervention provided by the present inventive mechanism. In any event, when the engine warms up, a return spring,  92 , which is also housed within bore  84  pulls piston  80  back from its engagement with pivotable cam  68 , and thereby unlocks lockable plunger  64  ( FIG. 2 ). This will allow lockable plunger  64  to move forward and into contact with shoe  34 . At the same time, piston  52  will be allowed to move forward and, backward movement of lockable plunger  64  will be prevented by resilient biasing element  76 , and pivotable cam  68 , which prevent travel of lockable plunger  64  in a direction tending to cause more slack in chain or belt  14 , while allowing lockable plunger  64  to move outwardly, so as to desirably maintain tension within drive element  14  when the engine has been shut down. 
       FIG. 4  illustrates an alternative embodiment according to an aspect of the present invention, in which a direct-acting, hydraulically driven pawl,  100 , which is slidably mounted within a bore,  96 , formed within housing  38  prevents unwanted drive element tensioning by immobilizing lockable plunger  64  whenever the pressure of lubricating oil within bore  96  exceeds the pullback force provided by spring  104 . 
     The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and fall within the scope of the invention. Accordingly the scope of legal protection afforded this invention can only be determined by studying the following claims.