GB2354287A - Roll-resistant rocker arm for overhead-cam i.c. engine - Google Patents

Abstract

A rocker arm assembly of the "roller finger follower" type for use in a valve train includes a rocker arm 46 having a socket 60 defined in a first end 48 by which the assembly is pivotably supported by the rounded end of a lash adjuster 44, and a recess 62 defined in a second end 50 with which to engage a valve stem 28. The socket 60 in the first end 48 of the rocker arm 46 is defined, in part, by a pair of diametrically-opposed flats 68. Each flat 68 is generally parallel to a respective reference plane (70, fig.2) that includes the geometric centres 72,74 of the socket 60 and recess 62 and, preferably, the flats are parallel with one another. Each flat 68 overlays a preferably cylindrical section of the lash adjuster 44 proximate to its rounded end 64. In the event of a cold engine start or other circumstance characterised by a tendency for the rocker arm 46 to roll about a longitudinal axis roughly defined by its first and second ends 48,50, the flats 68 engage the cylindrical section of the lash adjuster 44 to thereby limit the extent of such rolling.

Description

2354287 ROLL-RESISTANT ROCKER ARM FOR OVERHEAD-CAM INTERNAL COMBUSTION

ENGINE The invention relates to valve trains incorporating rocker arm assemblies of the "roller finger follower',-type suitable for use in an overhead-cam internal combustion engine, wherein each rocker arm engages a valve stem while being pivotably supported by a lash adjuster.

The prior art teaches overhead-cam internal combustion engines wherein the reciprocating motion of the engine's intake and exhaust valves is controlled using a plurality of cam-operated rocker arm assemblies, with each rocker arm assembly including a rocker arm that is pivotably supported by the engine's cylinder head at a first end. Typically, the first end of the rocker arm is supported by the cylinder head using a ball-and-socket arrangement, wherein a general ly-hemi spherical socket defined in the underside of the rocker arm's first end receives the rounded upper end of a hydraulic lash adjuster.

The second end of the rocker arm is adapted to engage the tip of a given valve stem, preferably via a recessed surface defined in the underside of the rocker arm's second end. The recessed surface is preferably contoured to prescribe the desired axial movement of the valve stem and, hence, of the valve upon pivoting movement of the rocker arm. A cam-responsive surface, usually defined by the radial -outermost surface of a roller carried by the rocker arm at a location intermediate its first and second ends, engages the cam to thereby control the pivoting movement of the rocker arm.

Significantly, during normal engine operation, a plunger spring urges the lash adjuster upwards relative to the cylinder head, possibly assisted by pressurised lubricating fluid delivered to the base of the lash adjuster. In this manner, the lash adjuster operates to slightly elevate and otherwise upwardly bias the first end of the rocker arm to ensure positive and continuous engagement of the roller with the controlling cam and, hence, the proper reciprocation of the rocker arm's associated valve.

Unfortunately, under certain circumstances, the lash adjuster does not elevate the first end of the rocker arm to the desired pivot point above the cylinder head, for example, during preassembly of the valve train on the cylinder head (prior to the installation of the cylinder head atop the cylinder block), or upon a "cold start" of the engine (where the engine's relat ively-more -viscous lubricating fluid is unlikely to elevate the lash adjuster's ball plunger to its nominal height above the cylinder head) In response, the rocker arm may roll in a transverse plane, i. e., in a plane generally orthogonal to the nominal is longitudinal axis of the rocker arm, as defined by the socket and recess in the rocker arm's first and second ends. By way of example only, such rolling may cause deleterious contact between a circumferential edge of the roller with the cam or, in a worst case, the rocker arm may disengage from either the lash adjuster or the valve stem.

The prior art has attempted to prevent such rolling of a rocker arm assembly in an overhead cam engine through use of wider rollers, for example, those whose contact patch exceeds perhaps about 10 mm in width. Unfortunately, wider rollers correlatively provide an attendant and undesirable increase in the inertial mass of the pivotable rocker arm assembly, as measured about the pivot point defined at the rocker arm's first end, while costs associated with the manufacture of such wider rollers generally increase with increased roller width. Additionally, the use of plural intake or exhaust valves per cylinder reduces the amount of space available for each rocker arm assembly, particularly when variable valve timing is achieved through the technique of cam switching, wherein adjacent mechanically- linked rocker arm assemblies engaging respective cams operate to control the movement of a given valve.

Under another approach of the prior art, the second, valve-actuating end of the rocker arm is received within a recess having a pair of opposed parallel-spaced side surfaces which together define a "valve tip grabber" capable of resisting rolling of the rocker arm in a transverse plane through engagement of the prongs with the shank of the valve stem. In a variation on this approach, U.S. Patent No. 5,806,477 teaches use of a separate connector mounted on top of the valve stem which has a pair of parallel, diametrical flats with which to engage a complementary, squareshouldered recess defined in the rocker arm's second end. Unfortunately, the use of such elongated, downwardlyextending prongs or additional connectors on the rocker arm's second end likewise undesirably increases the inertial is mass of the entire assembly. The presence of such prongs or additional connectors also undesirably increases the costs associated with manufacture of the resulting valve train.

What is needed, then, is a rocker arm and associated valve train for an overhead-cam internal combustion engine which resists rolling in a transverse plane while overcoming the deficiencies of the prior art.

Under the invention, an elongated rocker arm, adapted for engagement with a valve stem while being pivotably supported by a rounded upper end of a lash adjuster, includes a first end having a plurality of lower surfaces, including a pair of diametrically-opposed flats, which define a socket adapted to receive the rounded upper end of the lash adjuster, and a second end including a recess adapted to engage the valve stem. The socket and recess each have a geometric centre. Each flat is generally parallel to a respective reference plane, each of which includes the geometric centres of the socket and recess. Each flat also overlays a respective portion of the lash adjuster below its rounded end such that, in the event that the rocker arm is urged to roll in a transverse plane, i.e., about an axis roughly defined by the rocker arm's first and second ends, one flat quickly engages its respective portion of the lash adjuster to thereby resist any further rolling movement of the rocker arm.

While the embodiments of the invention contemplate any suitable range of included angles defined between the two flats up to about 90 degrees, the flats preferably define an included angle of no greater than about 50 degrees and, most preferably, the flats are generally parallel to one another. In the event that the flats are generally parallel with one another, the flats preferably overlay a generally- cylindrical section of the lash adjuster proximate to its rounded end, with the flats preferably being se'parated by a distance that is slightly greater than the nominal diameter of the cylindrical section. In this manner, the invention beneficially takes advantage of the precision- machined is cylindrical surface often found on existing hydraulic lash adjusters proximate to their rounded ends and, hence, the invention may he practised without altering the design of such lash adjusters.

A rocker arm assembly incorporating a rocker embodying the invention is characterised by a reduced inertial mass, as measured about the geometric centre of the socket at the rocker arm's first end, when compared to the prior art designs described above.

The lower surfaces defining the rocker arm's socket preferably also includes a frustospherical surface that defines a frustospherical contact ring engageable with the rounded end of the lash adjuster. The contact ring is advantageously disposed at an angle relative to an external surface of the rocker arm, for example, in order to facilitate the manufacture of the rocker arm using powdered metallurgy. In a preferred embodiment, each flat intersects a respective diametrical portion of the contact ring, with the contact ring further blending into adjoining conical surfaces that are tangent to the frustospherical surface thereof.

The rocker arm)referably further includes an internal passage providing fluid communication between the socket and an upper surf ace of the rocker arm. The internal passage advantageously provides a path through which engine lubricating fluid, directed into the rocker arm's socket through an internal channel defined in the lash adjuster, is routed to an upper surface of the rocker arm, from which it is readily further directed toward the cam-responsive surface by a suitable deflector, perhaps as defined by a surface of an attached clip.

Further in accordance with the invention, a valve train for an internal combustion engine includes a cam supported on the cylinder head for rotation about a cam axis, a valve supported on the cylinder head for reciprocal movement along an valve axis, and a lash adjuster on the cylinder head, wherein the lash adjuster include a shank with a rounded upper end. The valve train further includes a rocker arm assembly including a rocker arm having a first end engageable with the end of the lash adjuster, a second end engageable with the valve, and a camresponsive surface intermediate the first and second ends for engagement with the cam. The f irst end of the rocker arm has a plurality of lower surfaces, including a pair of diametrical flats, defining a socket which has a geometric centre and which receives the end of the lash adjuster. The second end of the rocker arm includes a recess which has a geometric centre and whi-,h engages the valve, e.g., the tip of the valve's associated valve stem. Each flat is generally parallel to a respective reference plane that includes the geometric centres of the socket and recess, and each flat overlays a respective portion of the lash adjuster proximate to the lash adjuster's rounded upper end.

While the embodiments of the invention contemplate any suitable range of included angles defined between the two flats up to about 90 degrees and, preferably, no greater than about 50 degrees, the flats are most preferably generally parallel to a common reference plane that includes the geometric centre of the socket and the axis along which the valve is reciprocated. In a preferred embodiment, where the lash adjuster includes a generally- cylindrical section Droximate to its rounded upper end, the flats are preferably separated by a distance that is slightly greater than the nominal diameter of the cylindrical section. 5 The present invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a partial cross-sectional view of an overhead cam internal combustion engine whose valve train includes an exemplary rocker arm assembly in accordance with the invention; Figure 2 is a side elevational view of the exemplary rocker arm assembly of Figure 1; Figure 3 is a top view of the rocker arm of the exemplary rocker arm assembly; Figure 4 is a cross-sectional view of the rocker arm along Line 4-4 of Figure 3; Figure 5 is an enlarged partial longitudinal crosssection of the modified socket of the rocker arm taken within Circle 5 of Figure 4; and Figure 6 is a transverse cross-section of the modified socket of the rocker arm taken along Line 6-6 of Figure 3.

Figure 1 contains a partial cross-sectional view of an overhead-cam internal combustion engine 10 whose valve train 12 includes a plurality of exemplary rocker arm assemblies 14 of the "roller finger follower',-type in accordance with the invention. Generally, the engine 10 includes a cylinder block 16 defining a plurality of combustion chambers 18 (only one of which is shown in Figure 1 for clarity) A cylinder head 20 mounted on the cylinder block 16 includes a port 22 (which may he either an intake or exhaust port) in intermittent fluid communication with the combustion chamber 18 upon operation of a valve 24 relative to a valve seat 26 defining the opening of the port 22 into the combustion chamber 18. The valve 24 includes a generally-cylindrical valve stem 28 which projects through a complementary aperture 30 formed in the cylinder head 20. The valve stem 28 extends through a coiled compression spring 32 seated atop the cylinder head 20 which operates, in conjunction with a retainer ring 34 secured to the valve stem 28 proximate to its tip 36, to bias the valve 24 to a normally5 closed position against the valve seat 26.

An overhead cam shaft 38 supported for rotation on the cylinder head 20 by suitable bearings (not shown) includes a plurality of eccentric cams 40 (only one of which is illustrated in Figure 1 for clarity). Each cam 40 controls the operation of a respective valve 24 by linearly displacing the valve stem 28 along its axis 42 1rith the aid of the exemplary rocker arm assembly 14, which is itself supported for pivotable movement on top of the cylinder head 20 by a hydraulic lash adjuster 44, in a manner described further below.

Referring to Figures 2 and 3, the rocker arm assembly 14 generally includes a rocker arm 46 having a first end 48 and a second end 50, by which the rocker arm's nominal longitudinal axis 52 is roughly defined. While the rocker arm 46 may be formed of any suitable material by any suitable method, in a preferred embodiment, the rocker arm 46 is formed of a powdered metal. A slot 54 is defined in the rocker arm 46 intermediate its first and second ends 48, 50. A roller 56 is mounted for rotation within the slot 54 about an axis generally perpendicular to the nominal longitudinal axis 52 of the rocker arm. The peripheral surface 58 of the roller 56 defines a cam-responsive surface on the rocker arm assembly 14 for engagement with the assembly's respective cam, as illustrated in Figure 1.

Referring to Figures 4-6, the rocker arm 46 includes a plurality of lower surfaces which collectively define a socket 60 in the underside of the rocker arm's first end 48, and a recess 62 in the underside of the rocker arm's second end 50. The socket 60 is generally adapted to receive the rounded end 64 of the lash adjuster's upwardly -projecting, cylindrical shank 66 such that the first end 48 of the rocker arm 46 is supported for pivoting movement about the lash adjuster's rounded end 64. The recess 62 is generally adacted to receive and engage the tip 36 of the valve stem 28.

More specifically, and as best seen in Figures 3, 4 and 66, the socket 60 is defined, in part, by a pair of diametrical-opposed flats 68. Each flat 68 is generally parallel to a common reference plane (generally indicated by a centre line 70 in Figure 2) that includes the geometric centre 72 of the socket 60, the geometric centre 74 of the recess 62, and the axis 42 along which the valve stem 28 reciprocates. Thus, in the exemplary embodiment illustrated in the Drawings, each flat 68 is generally parallel to the plane in which the rocker arm assembly 14 is intended to pivot during normal engine operation and, necessarily, is in parallel relation with the other flat 68.

As best seen in Figure 6, in accordance with the invention, the flats 68 are separated by a distance which is only slightly greater than the nominal diameter of the lash adjuster's cylindrical shank 66. The flats 68 are also of sufficient depth within the socket 60 such that each flat 68 overlays a respective portion 76 of the cylindrical shank 66 to ensure that, upon slight rolling of the rocker arm assembly 14 about its nominal longitudinal axis 52, at least one flat 68 engages the shank 66 of the lash adjuster 44 to thereby limit such rolling to a predetermined level.

As best seen in Figure 5, in accordance with another feature of the invention, the socket 60 is tilted slightly to a tilt angle cc relative to an additional surface 78 on the underside of the rocker arm 46 adjacent to the socket 60 when viewed in longitudinal cross-section, as seen in the enlarged partial section illustrated in Figure 5. The tilted socket 60 facilitates manufacture of the rocker arm 46 using powdered metal technology. While any 'Suitable tilt angle oc may be used, the tilt angle is preferably less than about 500 and, most preferably, is less than about 250. In the exemplary arm assembly 14 illustrated in the Drawings, the tilt angle is about 160.

Additional features of the socket 60 include a tilted, wide-mouthed lower section 80; an intermediate, tilted frustospherical section or contact ring 82 having a nominal radius roughly approximating the nominal radius of the lash adjuster's rounded end 64, with which to nominally engage the rounded end 64; and tilted frustoconical upper section 84 which communicates with the upper surface 86 of the rocker arm 46 (as best seen in Figure 4) through an internal passage 88. In the exemplary rocker arm assembly 14 illustrated in the Drawings, the contact ring 82 and the upper section 84 are each circumferentially continuous about the socket 60. Thus, in the exemplary assembly 14, the contact ring 82 engages the rounded end 64 of the lash adjuster 44 at a tilt angle oc of about 16' relative to the bottom surface 78 of the rocker arm 46.

As best seen in Figure 1, the passage 88 advantageously defines a portion of a path 90 along which engine lubricating fluid 92 supplied to the lash adjuster 44 is routed up through the lash adjuster's shank 66 and through the rocker arm 46. A clip 94, preferably formed of an elastically resilient material, is retained about the first end of the rocker arm 46. The clip 94 defines a ramped surface 96 extending over the upper surface 86 of the rocker arm 46 proximate to the passage 88. The ramped surface 96 of the clip 94 operates to deflect lubricating fluid 92 exiting the passage 88 towards the interface between the cam 40 and the rocker assembly's roller 56. A longitudinal trough 98, best seen in Figure 4, is preferably also defined in the upper surface 86 of the rocker arm 46. The trough 98 bridges the passage 88 and the slot 54 to further improve the flow of lubricating fluid 92 from the passage 88 to the cam/roller interface.

While the exemplary embodiment includes a hydraulic lash adjuster 44, it will be appreciated that the invention contemplates use of any suitable lash adjuster, including "mechanical", i. e., spring-biased lash adjusters.

Claims (1)

1. An elongated rocker arm adapted for engagement with a valve stem (28) while being pivotably supported by a lash adjuster (44), the rocker arm (46) comprising:

a first end (48) having a plurality of lower surfaces, including a pair of diametrically-opposed flats (68), defining a socket (60) adapted to receive the lash adjuster (44), the socket having a geometric centre (72); and a second end (50) including a recess (62) adapted to engage the valve stem (28), the recess having a geometric centre (74), wherein each flat (68) is generally parallel to a respective reference plane (70), each reference plane including the respective geometric centres (72,74) of the socket (60) and the recess (62).

2. A rocker arm assembly as claimed in claim 1, wherein the flats are generally parallel to one another.

3. A rocker arm assembly as claimed in claim 1, wherein the plurality of lower surfaces defining the socket further includes a frustospherical surface engageable with the end of the lash adjuster.

4. A rocker arm assembly as claimed in claim 3, wherein the frustospherical surface is disposed at a first angle relative to an additional surface of the rocker arm adjacent to the socket.

5. A rocker arm assembly as claimed in claim 4, wherein the frustospherical surface is circumferentially continuous about the socket, and wherein each flat intersects a respective diametrical portion of the frustosDherical surface.

6. A rocker arm assembly as claimed in claim 3, further including an internal passage providing fluid communication between the socket and an upper surface of the rocker arm.

7. A valve train for an internal combustion engine, wherein the engine includes a cylinder block defining a combustion chamber, and a cylinder head on the cylinder block including a port in fluid communication with the combustion chamber, the valve train comprising:

a cam supported on the cylinder head for r-otation about a cam axis.

a valve supported on the cylinder head for reciprocal movement along an valve axis, is a lash adjuster on the cylinder head, the lash adjuster including a shank with a rounded upper end; and a rocker arm assembly including an elongated rocker arm having a first end, a second end, and a cam-responsive surface intermediate the first and second ends for engagement with the cam, wherein the first end of the rocker arm has a plurality of lower surfaces, including a pair of diametrical flats, defining a socket which receives the end of the lash adjuster, the socket having a geometric centre, wherein the second end of the rocker arm includes a recess which engages the valve, the recess having a geometric centre, wherein each flat is generally parallel to a respective reference plane, each reference plane including the respective geometric centres of the socket and the recess, and wherein each flat overlays a respective portion of the lash adjuster.

B. A valve train as claimed in claim 7, wherein the flats are generally parallel to one another.

9. A valve train as claimed in claim 8, wherein the shank of the lash adjuster includes a cylindrical section, and wherein each flat overlays a respective portion of the cylindrical section.

10. A valve train as claimed in claim 9, wherein the cylindrical section has a nominal diameter, and wherein the flats are separated by a distance slightly greater than the nominal diameter of the cylindrical section.

11. A rocker arm as claimed in claim 7, wherein the plurality of lower surfaces defining the socket further includes a frustospherical surface engageable with the end of the lash adjuster. is 12. A rocker arm as claimed in claim 11, wherein the frustosph-erical surface is disposed at a first angle relative to an additional surface of the rocker arm adjacent to the socket.

13. A rocker arm as claimed in claim 11, wherein the frustospherical surface is circumferentially continuous about the socket, and wherein each flat intersects a respective diametrical portion of the frustospherical 25 surface.

14. A valve train as claimed in claim 11, further including an internal passage providing fluid communication between the socket and an upper surface of the rocker arm.

15. A rocker arm for an internal combustion engine substantially as hereinbefore described with reference to the accompanying drawings.