Abstract:
A two-step roller finger follower having a high-lift follower portion that rotates relative to a low-lift follower portion about a pivot shaft, including a lost-motion compression spring disposed in a linear bore formed in the high-lift portion to exert force against an curved pad on the back side of the valve pallet of the low-lift portion. The spring is retained and guided in its bore by a spring retainer having a planar bottom for engaging the curved pad. Preferably the retainer is a cup positioned in the spring bore such that the stroke of the cup is limited, to prevent leak-down of the associated hydraulic lash adjuster. Driving the spring by a linear-acting retainer in a linear bore causes the spring to be compressed linearly, resulting in a highly stable and predictable spring rate.

Description:
TECHNICAL FIELD 
   The present invention relates to roller finger followers for actuating the valves of internal combustion engines; more particularly, to two-step roller finger followers for controllably activating and deactivating engine valves; and most particularly, to a two-step roller finger follower having a guided lost-motion compression spring. 
   BACKGROUND OF THE INVENTION 
   Two-step roller finger followers (RFF) for controllably activating compression valves in a variable valve actuation train in an internal combustion engines are well known. An RFF extends between a hydraulic lash adjuster (HLA) and the stem of a valve. Engagement with a cam lobe of an engine camshaft causes the RFF to be pivoted about the HLA and thereby to depress the valve stem, opening the valve. A two-step RFF mechanism allows an engine valve to be operated by two different cam lobe profiles, one with the mechanism locked and the other with the mechanism unlocked. When the mechanism is unlocked, the RFF portion that is not directly in contact with the valve stem and the HLA, known in the art as the high-lift follower, typically is provided with a spring means, known in the art as a “lost-motion” spring, to keep that portion in contact with the cam. A typical lost-motion spring is disposed in compression between the high-lift follower and the remainder of the RFF, known in the art as the low-lift follower. Thus, when the high-lift follower engages the cam lobe, all lash is removed from the RFF and force begins to be exerted by the spring against the HLA. If the force of the lost motion spring is too small, the high-lift follower may not be able to stay in contact with the cam under all engine operating conditions. If the spring force is too large, the force of the lost motion spring may overcome the force of the internal spring in the HLA causing the HLA to leak down and become undesirably compressed and depleted of oil. 
   In some prior art two-step RFFs, a torsional lost-motion spring is disclosed. See, for example, U.S. Pat. No. 6,769,387. Experience has shown that a torsional lost-motion spring can have excessive variation in its free angle, resulting in excessive variation in the installed load, making it difficult to balance the force of the torsional lost motion spring from being too large a force and too small a force. Further, a torsion spring exerts substantial friction in use, resulting in undesirably large hysterisis, again affecting the installed load. 
   It is known to employ compression lost-motion springs. See, for example, US Patent Application Publication No. US 2003/02003/0209216. A disadvantage of compression springs as disclosed in this publication is that the springs are not guided. Because the opposing spring seats follow rotational rather than linear paths, the springs can flex as well as compress in use, resulting in unstable spring dynamics and uncontrolled spring rates. 
   Compression lost-motion springs have been found to have significantly less load variation and less friction than torsional springs. However, actually implementing compression springs for this purpose is difficult because of the non-linearity of the actuating path and the limited space available in a typical two-step RFF structure. 
   What is needed in the art is a two-step roller finger follower having an improved arrangement of a compression lost-motion spring wherein frictional losses are minimized, spring compression is substantially linear rather than rotational, and spring length and diameter are maximized. 
   It is a principal object of the present invention to reduce frictional hysterisis and improve RFF working life cycle. 
   SUMMARY OF THE INVENTION 
   Briefly described, a two-step roller finger follower in accordance with the invention includes a high-lift follower portion that rotates relative to a low-lift follower portion about a pivot shaft. A lost-motion compression spring is disposed in a linear bore formed in the high-lift portion and exerts force against a radiused pad on the back side of the valve pallet of the low-lift portion. The spring is retained and guided in its bore by a spring retainer having a planar bottom for engaging the radiused pad. In an alternate embodiment the retainer is a cup positioned in the spring bore such that the stroke of the cup is limited to prevent load from being applied on the hydraulic lash adjuster when the cam is on base circle. Driving the spring by a linear-acting retainer in a linear bore causes the spring to be compressed linearly, resulting in a highly stable and predictable spring rate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a cutaway isometric view of a two-step roller finger follower in accordance with the invention, showing a first embodiment of a follower spring; 
       FIG. 1   a  is an elevational cross-sectional view of  FIG. 1  showing an alternate arrangement of the lost motion springs; 
       FIG. 2  is an elevational cross-sectional view of an RFF in accordance with the invention, showing an alternative embodiment of a spring-retaining cup for limiting spring travel to prevent HLA leakdown; 
       FIGS. 3 and 4  are elevational cross-sectional views of an RFF in accordance with the invention, showing a second alternative spring-guiding mechanism; 
       FIG. 5  is an elevational cross-sectional view of an RFF in accordance with the invention, showing a third alternative spring-guiding mechanism; 
       FIG. 6  is an elevational cross-sectional view of an RFF in accordance with the invention, showing a fourth alternative spring-guiding mechanism; 
       FIG. 7  is an elevational cross-sectional view of an RFF in accordance with the invention, showing a fifth alternative spring-guiding mechanism; 
       FIG. 8  is a view of the underside of the high-lift follower shown in  FIG. 7 , showing a non-cylindrical bore; and 
       FIG. 9  is a detailed perspective view taken in region  9  of  FIG. 7 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a first embodiment  100  of a two-step roller finger follower in accordance with the invention is formed generally in accordance with the two-step RFF prior art. Such a two-step RFF is suitable for use in a variable valve activation system of an internal combustion engine  102 . The view shown in  FIG. 1  represents a section cutaway along a vertical symmetry plane for description purposes such that only one-half of the RFF is present. Thus, where appropriate, the described elements should be considered as having matching but not shown counterparts in the full RFF. 
   A high-lift follower  110  including a cam-follower surface  111  is disposed in a central opening  112  in a generally box-shaped low-lift follower  114 . High-lift follower  110  pivots within opening  112  about a pivot shaft  116 . A roller shaft  118  mounted in low-lift follower  114  supports a roller  120  for following a low-lift lobe of an engine camshaft (not shown). Low-lift follower  114  includes a socket  122 , for pivotably mounting RFF  100  at a first end  124  thereof on a hydraulic lash adjuster (not shown), and a pad  126  at a second end  128  thereof for actuating a valve stem (not shown). A latching assembly  130  disposed in low-lift follower  114  selectively latches high lift follower  110  in position to actuate the valve stem in response to the high-lift cam lobe base circle and eccentric, or selectively unlatches high-lift follower  110  to follow the high-lift cam lobe base circle and eccentric in lost motion. Curved slot  132  in high-lift follower  110  accommodates roller shaft  118  during the pivoting motions of high-lift follower  110  about pivot shaft  116 . All of these relationships are known in the RFF prior art and need not be further elaborated here. 
   Referring still to  FIG. 1 , a blind bore  134  is formed in high-lift follower  110 , opening adjacent curved shoe  136  formed in low-lift follower  114 . In a currently-preferred embodiment, the surface  137  of shoe  136  is curved such that a radius of shoe  136  is parallel to the axis  139  of bore  134  at all positions of high-lift follower  110 . Preferably, the surface of shoe  136  is cylindrical and thus has a constant radius, although a varying-radius non-cylindrical surface is fully comprehended by the invention and may be preferred in some instances to compensate for a non-linear spring rate. A first lost-motion compression spring  138  is compressively disposed within bore  134  and is retained therein by a cup-shaped spring retainer  140  having a preferably planar surface  147  on end  142 , that rides on shoe  136 , and cylindrical sidewall  149 . Retainer  140  is slidably close-fitting within bore  134  such that the motion of retainer  140  is reciprocal and linear with lost-motion action of the RFF. Further, spring  138  is relatively close-fitting within retainer  140  and is centered in bore  134  by a concentric smaller-diameter bore portion  135 . 
   Because shoe  136  makes continuous tangential contact with end surface  142 , preferably over less than the full diameter of surface  142 , as end surface  142  rotates along shoe surface  137  all thrust against shoe  136  is in a direction parallel to the axis  139  of bore  134 . Thus, the compressive force on spring  138  is co-linear with axis  139 , and there is no bending moment imposed on the spring, as opposed to the cited prior art. 
   The use of a curved, and preferably cylindrical, radius on surface  137  makes a line contact with end surface  142  and helps to minimize contact stress in end surface  142  in comparison to a prior art spherical bottom surface of the spring retainer. Also, this arrangement maximizes the length of the lost-motion spring in comparison to prior art spherical bottoms wherein an undesirably large portion of the potential spring space is consumed by the spherical bottom. 
   An advantage of a spring arrangement as shown in  FIG. 1  is that second spring  138 ′ may be disposed within spring  138  to augment the force capability thereof, thus increasing the force density capability within a single bore  134 . Preferably the two springs are counter wound to prevent binding; this allows the springs to mutually support and center each other. Second spring  138 ′ may be a low-rate spring and first spring  138  a high-rate spring, or vice versa. 
     FIG. 1   a  shows the position of high-lift follower  110  relative to low lift follower  114  when high lift follower  110  is on the base circle portion of the cam lobe. As shown, the free lengths of the springs may be sized such that only low-rate spring  138 ′ is in contact when the high-lift follower is on the base circle portion of the cam lobe, thus preventing leakdown of the HLA. As shown, the free length  150  of high rate spring  138  is selected to be less than the length  152  of spring cavity  154  when the high-lift follower is on the base circle portion of the cam lobe so that high-rate spring  138  comes into effect only when the follower moves onto the eccentric portion of the cam lobe. Since the compression springs in accordance with the invention operate linearly and their operating lengths are less than they would be if they were disposed between roller shaft  118  and latching assembly  130 , the range of operating spring forces can be selected to prevent undesirable HLA leak down of the HLA. While  FIG. 1   a  shows outer spring  138  to have its free length controllably selected as discussed above, it is understood that the free length of the inner spring may be controllably selected instead. 
   Another means for preventing HLA leakdown is to limit the outward extent of travel of spring retainer  140 . Referring to  FIG. 2 , bore  134  is provided with a reverse shoulder or step  141  between a larger diameter portion  134   a  and a smaller diameter portion  134   b . Retainer  140  includes an annular groove  180  and a spring clip  182 . When the retainer and spring clip are first inserted into smaller diameter portion  134   b , the depth of annular groove  180  permits spring clip  182  to be compressed inwardly to a diameter that fits within smaller diameter portion  143   b . Then, when the retainer and spring clip pass through smaller diameter portion  134   b  into larger diameter portion  134   a , the spring clip expands and thus cannot return into smaller diameter portion  134   b , thus limiting the stroke of the retainer to the length of the larger diameter portion. The axial position of shoulder  141  is selected such that, at the permitted outward travel extreme of retainer  140 , the high-lift follower surface  111  does not make contact with the base circle portion of its respective high-lift camshaft lobe, thus preventing further expansion of the lost motion springs and undesirable leakdown of the HLA. 
   Referring to  FIGS. 3 and 4 , in a second embodiment  300  in accordance with the invention, bore  334  is formed such that spring  338  is nearly full-fitting diametrically. A spring retainer  340  comprises a head portion  342 , for supporting spring  338  and for contacting shoe surface  137 , and an axial stem portion  343  extending into a guide counterbore  345  formed in high-lift follower  310  that guides retainer  340  during reciprocation thereof between locked position ( FIG. 3 ) and lost-motion position ( FIG. 4 ). 
   Referring to  FIG. 5 , in a third embodiment  400  the spring-guiding mechanism is similar to that shown in embodiment  300  except that the guide for stem portion  443  is a separate female guide element  445  inserted into bore  434  and having a central bore  447  for receiving stem portion  443 . 
   Referring to  FIG. 6 , in a fourth embodiment  500  the spring-guiding mechanism is similar to that shown in embodiment  400  except that head portion  542  is provided with a ball surface  580  for being received in a mating ball socket  582  in shoe  536 ; and female guide element  545  is similarly provided with a ball surface  584  for being received in a mating ball socket  586  formed in high-lift follower  510 . The spherical centers of ball surfaces  580 , 584  lie on the axis of spring  538 , head portion  542 , and stem portion  543 . This arrangement allows the spring force to be exerted linearly on the spring as in the previously-described embodiments. 
   In providing for a compression spring within a bore in a high-lift follower in accordance with the invention, space constraints are severe in providing a spring of adequate spring rate. If the bore is large, to accommodate a large-diameter spring, the follower can be structurally weakened. Thus there is a practical limit on the diameter of a bore. In a typical high-lift follower, the bore may have a maximum diameter of about 7 mm. If the bore is long, to accommodate a long spring, the follower can be similarly weakened. In embodiments  100  and  200 , the spring diameter is constrained to about 6 mm by the necessary wall thickness of the cup-shaped spring retainer  140 , 240 , resulting in a spring diameter sacrifice of about 14%. In embodiments  400  and  500 , the length of the spring is constrained by the presence of guide elements  445 , 545  at the inner end of the bore  434 , 534 . 
   Referring now to  FIGS. 7 through 9 , a fifth embodiment  600  is shown wherein a compression spring  638  is able to occupy the full length and full diameter  639  of a bore  634  and yet be guided in accordance with the invention. Bore  634  includes not only a cylindrical portion  634   a , as in the previously-disclosed embodiments, but further includes opposed channel portions  634   b  extending bore  634  along the length of high-lift follower  610  in a direction where additional space can be made available without compromising the structural capability of the follower. A spring guide  640  comprises a bottom portion  642  having a bottom surface and first and second guide rails  643  formed to conform to the cross-sectional shape of channel portions  634   b . Preferably, channel portions  634   b  are stepped  645  and each of guide rails  643  is provided at an inner end thereof with a resilient latch  647  which expands over step  645  during assembly of the RFF to retain spring guide  640  within bore  634 . Thus the travel of spring guide  640  is limited by latches  647  in the same way as the travel of spring guide  140  is limited by spring clip  182  in embodiment  100 . 
   While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.