Abstract:
A hydraulic lash adjuster has a plunger assembly the outer end of which can be moved inwardly until the inner end abuts a seal and closes a high pressure chamber, thus preventing further movement. This accommodates any necessary movement of a valve towards its closing position to ensure proper closure. Consequently, net-shaped cams wherein the base circle is not necessarily accurately concentric may be used. A leaf spring ensures opening of the chamber when the base circle of the cam is next reached.

Description:
BACKGROUND OF THE DISCLOSURE 
     This invention relates to hydraulic lash adjusters for taking up slack in a valve train, and to valve train assemblies which incorporate hydraulic lash adjusters. 
     A typical structure of this type is shown schematically in FIG.  1 . The valve train assembly  2  comprises a rocker arm  4  and a hydraulic lash adjuster  6 . One end  8  of the rocker arm  4  engages the stem  10  of a valve  11 . The other end  12  of the rocker arm is mounted for pivotal movement on the lash adjuster  6 . 
     The rocker arm  4  is provided with a roller  14  mounted on an axle  16  carried by the rocker arm  4 . 
     A cam  18  mounted on a cam shaft  15  has a lobe  17  which can engage the roller  14  and thus pivot the rocker arm  4  anti-clockwise as shown in the drawing. This depresses the valve stem  10  against the force of a valve spring (not shown) and thus opens the valve. As the cam continues to rotate, and the base circle  19  of the cam profile again engages the roller  14 , the valve spring returns the valve and the rocker arm  4  to the position shown in FIG.  1 . 
     As is well known, a hydraulic lash adjuster has an oil-containing chamber and a spring arranged to enlarge the chamber and thus extend the lash adjuster. Oil flows into the chamber via a one-way valve, but can escape the chamber only slowly, for example via closely-spaced leakdown surfaces. 
     Accordingly, the lash adjuster  6  of FIG. 1 can extend to accommodate any slack in the valve train assembly, such as between the cam  18  and the roller  14 . After it is extended, however, the oil-filled chamber provides sufficient support for the pivoting movement of the rocker arm  4 . 
     It is important for the base circle  19  of the cam  18  to be concentric with respect to the axis of rotation of the cam shaft  15 . Any slight eccentricity (“run-out”) could cause the valve to close later than it should, or open during the movement of the base circle past the roller  14 . The cam  18  is often formed by sintering and may not have, in its initial state, particularly accurate dimensions. Accordingly, it is conventional, before assembly, to grind either the outer surface, including the base circle  19 , of the cam  18 , or to grind the inner diameter which is fitted to the cam shaft  15 , to ensure accurate concentricity of the base circle  19  relative to the axis of rotation of the cam shaft  15 . 
     Although the arrangement described above works well during normal running conditions, problems can arise in certain circumstances. For example, in order to prevent problems when starting the engine from cold, it has been proposed to use a technique whereby the valves and cylinder head are caused to heat up very quickly. Referring to FIG. 2, the rapid heating of the head  20  of the valve  11  causes the head  20  to expand relative to the valve seat  21 . This expansion results in the valve moving downwardly against the force of the valve spring, as shown on the right of FIG.  2 . This process creates positive lash, which is accommodated by expansion of the hydraulic adjuster as the camshaft rotates. However, as the cylinder head  22  and the valve seat  21  then heat up, their expansion allows the valve  11  to move back upwardly, thus creating negative lash (which will be subsequently exacerbated due to expansion of the valve stem). This negative lash can be accommodated by shrinking of the lash adjuster. However, because the heating process is taking place rapidly, and the shortening of the lash adjuster is limited by the rate of leakage of oil from the high pressure chamber, the lash adjuster does not shorten sufficiently quickly. This problem is exacerbated because the oil is still cold and therefore viscous, thus reducing the leakage rate. This results in valves remaining open (shown in dotted lines in FIG.  1 ), causing starting problems. 
     There have been proposed lash adjusters which provide “lift loss”, that is, which are capable of shrinking to a certain extent before the sealed high-pressure chamber prevents further movement. See for example U.S. Pat. No. 6,039,017. Thus, there is a degree of lost motion of the lash adjuster before the valve starts to open. This lost motion is recovered by a spring after the valve has closed. Using such a lash adjuster, a small degree of negative lash can be quickly accommodated by the lost motion of the lash adjuster, thus making it more certain that the valve will close. 
     There are also lash adjusters which incorporate a seal to prevent leakage of oil from the high-pressure chamber, and in which the chamber valve is arranged such that it is normally open (known as “sealed-leakdown” adjusters). See U.S. Pat. No. 5,622,147. This would permit a small amount of shortening of the lash adjuster before the valve closes as a result of the hydrodynamic force of the oil flowing out of the chamber. However, the amount of lift loss produced is somewhat uncertain, and will depend significantly on oil viscosity and hence temperature, as well as other factors. Also, this form of lash adjuster can sometimes encounter problems when a hot engine is stopped with a valve partially open. The pressure of the valve spring on the lash adjuster causes the high-pressure chamber to remain sealed, so that, if the engine cools and negative lash is created, oil cannot flow out of the chamber and the lash is therefore not accommodated. 
     It would be desirable to provide a lash adjuster of the sealed-leakdown type in which such problems are at least mitigated. 
     BRIEF SUMMARY OF THE INVENTION 
     Aspects of the present invention are set out in the accompanying claims. 
     In a first aspect of the invention, the high-pressure chamber is sealed by a sealing means engaging both the body of the lash adjuster and the plunger as the plunger moves inwardly, thus preventing further inward movement. The arrangement is such that as the cam turns, and returns to base circle, and the pressure on the plunger decreases, the plunger and sealing means separate, preferably assisted by a biasing means such as a leaf spring. Accordingly, the pressure in the chamber is relieved whenever the base circle of the cam is reached. Because the chamber is open, the plunger assembly can be pushed inwardly by a certain amount to guarantee valve closure before the chamber is again closed. 
     According to a preferred aspect of the invention, it has been perceived that use of a hydraulic lash adjuster which provides lift loss (preferably, but not necessarily, an adjuster according to the first aspect of the invention) means that the base circle radius variation of the cam no longer has to be minimized by grinding, allowing the use of net-shaped cam shaft technology instead of more expensive ground cams. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows a conventional valve train assembly; 
     FIG. 2 illustrates differential expansion of engine components in an engine of known type; 
     FIG. 3 is a longitudinal cross section through a hydraulic lash adjuster according to a first embodiment of the invention; 
     FIGS. 4 to  7  show respective components of the hydraulic lash adjuster of FIG. 3; 
     FIGS. 8 and 9 are enlarged views of part of the hydraulic lash adjuster of FIG. 3 illustrating different states encountered during operation of the lash adjuster; and 
     FIG. 10 is a longitudinal section through a hydraulic lash adjuster according to a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 3, this shows a lash adjuster  30  according to a first embodiment of the invention. The lash adjuster has a cylindrical body  23  formed with a longitudinal blind bore  24 . A plunger assembly  26 , which in this embodiment is a one-piece assembly but could alternatively be formed of two or more parts, is mounted for sliding motion inwardly and outwardly of the bore  24 . The plunger assembly  26  and blind bore  24  define between them a high-pressure oil chamber  28  at the lower end of the lash adjuster  30 . 
     The plunger assembly  26  is formed with a relatively narrow waist  31  so that a low-pressure oil chamber  32  is formed between this waist and the bore  24 . Oil from the associated engine can enter the chamber  32  via an aperture  33 . 
     The lash adjuster  30  is provided with an annular polytetrafluoroethylene (PTFE) seal  34  (also shown in cross-section in FIG.  4 ). The cylindrical outer surface of the seal  34  is an interference fit in, and sealingly engages, the bore  24 . The upper surface of the seal  34  can sealingly engage a circumferential outer sealing surface  36  on the bottom of the plunger assembly  26 . 
     A spring  40  engages the upper, central part of a cap-shaped retainer  42  (shown in plan view in FIG.  5  and side view in FIG.  6 ), and forces the retainer  42  into engagement with the plunger assembly  26 , the retainer  42  engaging the center of the base of the assembly  26 . The upper part of the retainer is located within the annular seal  34  and the circumferential outer part is located under the annular seal. 
     The spring  40  pushes the seal  34  and the plunger assembly  26  outwardly of the bore  24 . In this state, oil can flow from the low pressure chamber  32  around the side of the plunger assembly, through a gap  44  between the sealing surface  36  and the seal  34  and into the high pressure chamber  28 . The outer diameter of the lower part of the plunger assembly  26  is sufficiently smaller than the diameter of the bore  24  to allow oil readily to flow therebetween. Accordingly, the plunger assembly can move outwardly to take up slack in the valve train. Any significant outward movement of the plunger assembly will also result in the seal  34  being shifted in the same direction by the outer part of the retainer  42 . 
     The lash adjuster  30  is also provided with a leaf spring  46 , shown in plan view in FIG. 7, disposed between the lower end of the plunger assembly  26  and the upper surface of the retainer  42 . See also the enlarged views of FIGS. 8 and 9. The lower surface of the plunger assembly  26  is provided with a circular recess  48 , which is deeper at the radially outer part thereof. The leaf spring  46  has four arms  50  which are located under the recess  48 , and the outer ends of which are located over the PTFE seal  34 . 
     FIG. 8 shows the state of the lash adjuster when the lobe of the cam is applying force to open the valve. The plunger assembly  26  is depressed, engaging the PTFE seal  34  so that the high pressure chamber  28  is closed and further inward movement of the plunger assembly  26  is thus prevented. In this state, the arms  50  of the leaf spring  46  are deflected upwardly by their engagement with the PTFE seal  34 . 
     When the base circle of the cam is approached, the plunger assembly  26  is allowed to move outwardly under the force of the spring  40 . This of course can occur only if oil is allowed to flow into the chamber via the gap  44  (FIG. 9) which is at that stage created between the sealing surface  36  of the plunger assembly  26  and the seal  34 . Various forces combine to ensure this movement occurs, including the resilience of the arms  50  of the leaf spring  46 , the force of the spring  40  and the force holding the seal  34  against the wall of the bore  24  (which may be a combination of friction and stickiness caused by migration of PTFE into the wall). Such forces have to be sufficient to overcome the pressure holding the seal  34  against the sealing surface  36 , and then any hydrodynamic forces of the oil escaping the chamber  28 , which would tend to move the seal  34  upwardly. The spring  46  is particularly desirable in this connection, as it tends to peel apart the seal  34  and the sealing surface  36 . However, the exact force exerted by the leaf spring  46  is not critical. 
     Accordingly, during operation, it is ensured that the high pressure in the chamber  28  is relieved after the valve has closed, thereby creating lift loss so that the plunger assembly  26  can move inwardly before the valve starts to open, and outwardly after the valve has closed. If the lash adjuster needs to shrink rapidly in order to accommodate the closing motion of the valve, this is accommodated by virtue of the pressure on the plunger assembly  26  causing the assembly to move to a position intermediate the states shown in FIGS. 8 and 9, thus guaranteeing closure of the valve. If negative lash persists, the seal  34  will be gradually pushed down by the plunger  26  and the spring  46 , thereby eventually restoring the intended maximum amount of lift loss. 
     The lash adjuster of FIG. 3 is intended to be used with a rocker arm such as that shown at  4  in FIG.  1 . The lash adjuster could form the pivot of the arm, and the cam could operate on the rocker arm at a location between the lash adjuster and the valve stem (as in FIG.  1 ), or various other configurations (known in themselves) could be used, for example having the lash adjuster disposed between the rocker arm and either the valve stem or the cam. 
     FIG. 10 shows a second embodiment, in the form of a direct-acting bucket tappet  120  incorporating a hydraulic lash adjuster  30  and arranged to move a valve stem  10  in response to the rotation of a cam  18 . This embodiment has components corresponding to those of the FIG. 3 arrangement, with like components bearing like reference numerals, and operates in the same way. The arrangement differs from the FIG. 3 arrangement only insofar as the components are configured in a per se known way for use with a bucket tappet which has the low-pressure reservoir  32 . 
     The cams  18  used to operate the valves of the above arrangements have been formed by a sintering operation (but could alternatively have been formed by other means, such as hydroforming or hot- or cold-forming). However, no additional grinding operations have been performed on either the outer surface of the base circles or the inner surfaces of the cams. Accordingly, the base circle of each cam is not necessarily accurately concentric with respect to the axis of rotation. The cam  18  is thus net-shaped. However, because of the use of the hydraulic lash adjusters described above, the base circle radius variations of the cam no longer have to be minimized by grinding, because any non-concentricity of the base circle will be accommodated by inward movement of the outer end of the plunger assembly  26 , thus avoiding incorrect valve opening. (The term net-shaped is generally understood, and used herein, in the sense of having a shape and dimensions which are at least substantially the same as those resulting from the initial forming of the object. This does not exclude the possibility of small changes in dimensions which are a consequence of, for example, surface-treatment for the purpose of smoothing, as distinct from dimensional changes (e.g. by grinding) for the purpose of altering the function performed as a result of those dimensions.) 
     Although significant grinding is avoided, it may be desirable for the outer surface of the cam to be treated for the purpose of smoothing the exterior of the cam. This may be of particular value in the embodiment of FIG. 10 when the cam operates on a direct-acting bucket tappet, rather than on a roller. 
     In all the arrangements described above, because movement of the outer end of the plunger is allowed, the valve opens later and closes sooner, in relation to the rotation of the cam, than in prior art arrangements. In order to compensate, the profile of the cam is altered as compared with prior art arrangements. A further alteration to the profile may be made in order to extend the ramp of the cam lobe to ensure that the movement of the outer end of the plunger assembly  26  takes place at a controlled velocity to reduce impact forces. 
     In the above embodiments, the gap  44  defines the maximum amount of lift loss. This in turn is specified by the dimensions of the seal  34  and the retainer  42 . The leaf spring  46  preferably has a thickness substantially equal to the thus-defined gap size (although if the spring is slightly thicker, this simply means it will remain in a partially-flexed condition). The gap, and hence the amount of lift loss, should: (a) equal or exceed the maximum amount of negative lash created by the differential thermal expansion of the various engine components, plus, if a net-shaped cam is used, the lash created by the maximum expected amount of run-out of the base circle, i.e. the maximum amount by which the base circle radius varies; and (b) be less than the amount which would cause excessive valve closure speeds. (It will be appreciated that provision of lift loss means that valve opening and closing will take place over a smaller arc of cam rotation, and thus at increased speed.) 
     In one preferred embodiment the size of the gap  44  is in the range of 0.1 mm to 0.3 mm, and more preferably in the range 0.15 mm to 0.25 mm. 
     The spring  40  of the embodiments described above biases both the plunger assembly  26  and the seal  34  outwardly, although it does not bias these components towards each other and so does not inhibit opening of the chamber. It would alternatively be possible to have separate biasing means for the plunger assembly  26  and the seal  34 ; in this case, preferably, the biasing means for the seal is limited in the extent to which it can move the seal towards the plunger (e.g. by inter-engagement of the separate biasing means and the plunger, or suitable selection of the strength of this biasing means with respect to that of the leaf spring  46 ) so that it does not inhibit the restoration of the gap  44 . 
     The invention has been described in great detail in the foregoing specification, and it is believed that various alterations and modifications of the invention will become apparent to those skilled in the art from a reading and understanding of the specification. It is intended that all such alterations and modifications are included in the invention, insofar as they come within the scope of the appended claims.