US3618573A

US3618573A - Variable cam and follower assembly

Info

Publication number: US3618573A
Application number: US828474A
Authority: US
Inventors: Clifford H Allen
Original assignee: TRW Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1969-05-28
Filing date: 1969-05-28
Publication date: 1971-11-09
Anticipated expiration: 1988-11-09

Abstract

An adjustable cam and follower assembly effective to vary timing, amount of lift and duration of lift of engine valves and the like wherein the cam has an axially and radially varying contour and the follower has a large radius cam-contacting surface. The cam contour is generated by a grinding wheel of equal radius to the cam follower radius. The cam follower is free to tilt on said contour and maintain straight or curved line contact throughout the entire range of adjustment of the assembly. The cam contour extends across a wide axial plane while the cam follower surface is cylindrical with its surface generated from a centerline, and is free to tilt about a longitudinal axis for maintaining line contact with the cam while at the same time is free to rock about a second axis to follow the lift function of the cam.

Description

United States Patent [72] inventor Clifford 11. Allen Chesterland, Ohio [21] Appl. No. 828,474

[22] Filed May 28, 1969 [45] Patented Nov. 9, 1971 [73] Assignee TRW lnc.

Cleveland, Ohio [54] VARIABLE CAM AND FOLLOWER ASSEMBLY 5 Claims, 5 Drawing Figs.

[52] U.S.Cl 123/90.18, 123/90.27, 123/90.42, 123/90.44, 123/90.5 [51] 1nt.Cl F011 1/34, F011 H18 [50] Field of Search 123/90.

Primary Examiner-A1 Lawrence Smith Attorney-Hill, Sherman, Meroni, Gross & Simpson ABSTRACT: An adjustable cam and follower assembly effective to vary timing, amount oflift and duration oflift of engine valves and the like wherein the cam has an axially and radially varying contour and the follower has a large radius cam -contacting surface. The cam contour is generated by a grinding wheel of equal radius to the cam follower radius. The cam follower is free to tilt on said contour and maintain straight or curved line contact throughout the entire range of adjustment of the assembly. The cam contour extends across a wide axial plane while the cam follower surface is cylindrical with its surface generated from a centerline, and is free to tilt about a longitudinal axis for maintaining line contact with the cam while at the same time is free to rock about a second axis to follow the lift function of the cam.

VARIABLE CAM AND FOLLOWER ASSEMBLY BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to adjustable cam and follower assemblies having relatively wide cam surfaces which vary both axially and radially and maintain line contact relationship with a curved follower surface that is free to tilt about its own longitudinal axis while rocking about a second axis passing through a fixed pivot point. The invention will be hereinafter specifically described as embodied in internal combustion engine valve-operating means, but it should be understood that the invention is not limited to this particular use and that the cam and follower assemblies of the invention are generally useful for varying timing, extent of movement, and duration of movement in mechanical linkages.

2. Description of the Prior Art Speed-sensitive adjustable cam and follower assemblies for internal combustion engines are known in the art as, for example, in the Sampietro U.S. Pat. No. 2,980,089 dated Apr. 18, 1961. In the Sampietro patent, however, the cam and follower only have point contact relationship and high-speed, high-load usage results in rapid wear, skiving and galling of the contacting surfaces, thus soon destroying the efficiency of the engine. This wear has forced the return to fixed or nonadjustable cam and follower combinations which will maintain line contact relationship but which cannot provide the highly desirable variation in valve operation for different engine conditions. Fixed cam internal combustion engines, therefore, have valve settings providing what is known as free-flowing valving, invariably resulting in overlapping of intake andexhaust valve events. During the scavenging of burned residues from the engine cylinder, the exhaust valve closes late to permit the inertia of exhaust gases at high speed to continue to flow from the cylinder. To insure free flow of the intake mixture, the intake valve is opened early to provide adequate flow area for highspeed operation. Unfortunately, there is only one speed at which a fixed arbitrary timing is satisfactory. At low speeds the compromised valve-timing will allow burned residues to be drawn back into the intake manifold and at high speeds unburned residues will be discharged with attendant contamination of the atmosphere.

SUMMARY OF THE INVENTION According to this invention there is now provided a cam and follower assembly which is adjustable as in the aforesaid Sampietro U.S. Pat. No. 2,980,089, but which includes a relatively wide contour having line contact relation with a curved follower contour to avoid the problem encountered with point contact engagement between cam and followers.

In the present invention, the cam surface is tapered in such a way that there exists two planes normal to the cam axis in which a curved, preferably cylindrical, pivoting follower will have different lift functions. One lift function is suitable for the actuation of the poppet valve at high engine speeds while the other lift function is suitable for poppet valve actuation at low engine speeds. Speed-sensitive shifting mechanism is provided to vary the alignment of the cam and follower for varying the timing, duration and extent of valve lift.

The follower of the assembly of this invention has a spherical pivot so that it may align its cam-engaging surface along the axial shape of the cam regardless of the slope angle and regardless of the changing in slope of the cam contour as the cam rotates.

The line contact between the follower and the cam can be either curved or straight depending upon whether the contact occurs on the base circle, the nose or the fiank of the cam. The follower has a large radius cylindrical cam-contacting surface, this radius being equal to the radius of the grinding wheel used to generate the cam contour. This insures full line contact with the follower since in areas on the cam contour where the cam contact lines are curved these lines are partially wrapped around the grinding wheel cylindrical surface in a helical fashion, presenting difi'erent pressure angles at different axial locations. A follower having a substantially different radius would be incapable of contacting the cam along the full length of the grinding wheel contact line. The result would be localized contact and high-bearing stresses between the cam and follower surfaces.

The cam follower surface is cylindrical so that the cam itself can be ground across its full axial width by a cylindrical grindstone of equal radius without an infeed but having its axis tilted in accordance with the specified functions for the cam by means of a tilting mechanism provided for shifting the centerline of the wheel.

It is then an object of this invention to provide a variable timed cam and follower assembly with a curved follower surface engaging the cam along a line contact.

Another object of this invention is to provide a variable timed cam and follower assembly including a follower adapted to tilt in a plan parallel to the axis of the cam and having a cylindrical cam-engaging surface forming a line contact with the cam.

Another object of this invention is to provide valve-operating mechanism adapted to vary the timing, duration and lift of a valve upon relative shifting of the cam and follower longitudinally of the cam axis.

A specific object of this invention is to provide a valveoperating means with a variable timed cam and a cam follower adapted to tilt about an axis normal to a plane which is parallel to the cam axis and having a cylindrical surface in line contact with the cam.

Another specific object of this invention is to provide an overhead camshaft arrangement for automotive engines having wide cam surfaces of varying contour to change the valve timing and followers underlying the camshaft having cylindrical top faces engaging the cam surfaces and mounting means accommodating tilting of the followers about axes normal to planes which are parallel to the camshaft axis.

Other and further objects of this invention will become apparent to those skilled in the art from the following detailed description of the annexed sheets of drawings which, by way of illustration only, show one embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary sectional view of an overhead cam engine equipped with the cam and follower assembly of this invention;

FIG. 2 is a fragmentary sectional view taken along the line I II-II ofFIG. l;

FIG. 3 is a fragmentary top plan view of a speed-sensitive adjustable cam and follower assembly showing an axially shifting camshaft and followers for each of the cams on the shaft;

FIG. 4 is a bottom plan view of one of the followers of FIG.

FIG. 5 is a diagrammatic fragmentary side view of one of the cams showing the relative positions of the follower on the cam as the camshaft is shifted; and

FIG. 6 is a diagrammatic perspective view showing generation of the cam surface and the coacting cylindrical follower surface.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. I the reference numeral 10 illustrates a valve-inhead engine equipped with the variable timed valve-operating mechanism 11 of this invention. The engine 10 has a cylinder head 12 with a valve port 13 controlled by a poppet valve 14 having a head 15 coacting with a valve seat 16 in the head. The valve 14 has a stem 17 slidable in a guide I8 carried by the head 12 and urged to closed position by a spring 19 which is compressed between the head and a spring retainer 20 which is locked on the valve stem 17 near the tip end thereof.

The valve-operating mechanism 11 includes a camshaft 21 with a plurality of axially elongated or wide cams 22 each acting on a rocker arm or follower 23.

Each rocker arm 23 has a cylindrical pad 24 projecting from the top thereof, a forwardly extending nose portion 25 with a hemispherical recess 26 in its bottom face opening into a longitudinal channel 260 receiving the valve stem 17, and an elongated tail portion 27 with a hemispherical recess 28 in its bottom face at the rear end thereof.

A pivot support 29 carried by the engine head 12 has a hemispherical head 30 seated in the recess 28 and providing a fixed pivot support for the rocker arm 23. The pivot support is maintained in seated engagement in the socket provided by the recess 28 by means of a hydraulic or spring lash adjuster 31 acting on the support 29 and accommodating expansion and contraction of the linkage to take up clearance and prevent backlash.

The tip end 17a of the valve stem 17 projects beyond the retainer 20 into the channel 260 just below the hemispherical recess 26 in the rocker arm. A ball member 33 is seated in the recess 26 and has a flat face 34 slidably seated on the tip end 17a of the stem.

As illustrated in FIG. 2, the rocker arm 23 can rock or tilt around its longitudinal axis which is defined by the line L in FIG. 1 to maintain full line contact between the cylindrical pad 24 and the cam. However, the pivot 30, while accommodating this rocking or transverse tilting of the arm 23 holds the arm for rocking longitudinally about an axis which passes through the pivot 30 and is parallel with the cam axis. The tip end of the valve stem 17 projecting into the channel 26a prevents lateral displacement of the follower off of the valve tip 17a.

It will be understood that the cam shaft 21 has its cam 22 riding continuously on the pad 24 and therefore the rocker arm 23 is held in its assembled position on the pivot axis 30 and on the ball member 33.

As shown in FIG. 1, the rocker arm 23 rocks longitudinally along line L with the fixed pivot P provided by the head 30. On the other hand, the rocker arm 23 has its cylindrical cam-contacting surface struck from radius R longitudinally of the arm 23 and externally transversely across the top of the arm about midway between its ends so as to be free to tilt or rock transversely of this line L maintaining a contact line between the cylindrical surface 24 of the rocker and the cam. The contact line will skew in accordance with the axial inclination of the cam so that line contact will be maintained regardless of the axial inclination ofthe cam 22.

The camshaft 21, as shown in FIG. 3, is driven from the engine as through a gear 35 and a speed-sensitive flyball mechanism 36 as shown in the Sampietro U.S. Pat. No. 2,980,089 shifts the camshaft 21 axially of the rocker arms 23 so as to provide for the variable timing by engaging the cylindrical surfaces 24 at different axial areas of the cams. If desired, the flyball mechanism 36 could be replaced with a two-position diaphragm actuator sensitive to intake manifold pressure for shifting the camshaft from a low-speed to a highspeed position at a predetermined intake manifold pressure and then back to the low-speed position when the manifold vacuum drops below the predetermined amount.

As illustrated in FIG. 5, for example, the axial shifting of the camshaft 21 aligns the rocker arm 23 from the solid-line position to the dotted-line position while the cylindrical pad 24 continues to maintain a full line contact with the cam surface. As illustrated, the rocker arm shown in solid-line position engages the cam 22 at a normal plane designated No. 1 while in the dotted-line position the rocker arm engages the cam in a normal plane designated as No. 2. The lift characteristics of the cam at the two planes differ by the dimension H so that at plane No. I a substantially higher lift of the valve would occur than at plane No.2.

As illustrated in FIG. 6, the cam 22 has a base circle portion 220, side flanks 22b and a nose 220. As also illustrated the cam has a substantial axial dimension to provide a wide cam surface around its entire periphery. At the base circle portion 220, the cam has a fixed radius along its width and contact lines between the cam and cam follower 23 are parallel with the camshaft axis. However, along the flanks 22b and at the nose 22c the cam surface is inclined both radially and axially to provide the variable lift function of the assembly and contact lines are not parallel with the cam axis and are not necessarily straight. These cam surfaces 22b and 22c can be formed by a large-radius cylindrical grinding wheel engaging the full width of the cam and having an axis of rotation 35 with a point 36 lying in the plane No. 3 and a point 37 lying in the plane No. 3a. A radius 38 struck from point 36 traces the periphery 39 of a grinding wheel lying in the plane 3 while the radius 40 from point 37 traces the periphery 41 of the grind wheel in the plane 3a.

Planes 1 and 2 corresponding with FIG. 5 are shown in FIG. 6. The plane 1 is referred to as the high-speed normal plane for efficient engine operation at high speeds where the timing, valve lift and duration is such as to insure optimum flow of intake and exhaust at high engine speed while the plane No. 2 is designated as the low-speed normal plane of the cam where the valve operation is such as to insure the best intake and exhaust flow condition at low engine speeds. The cam axis is shown at 47. It will be understood that cam axis 47 shifts radially relative to the axis 35 of the grinding wheel so that the periphery of the wheel will form the desired radial shape of the cam surface. Further, the grinding wheel axis 35 can tilt to vary the inclination of the cam in planes parallel with the cam axis along its flanks 22b and 220.

The cam follower surface has the same cylindrical radius as the grinding wheel and is illustrated at 24 so that axis 35, points 36 and 37,

radii

38 and 40 and

peripheries

39 and 41 refer to the cam follower as well as to the grinding wheel. The follower surface 24 can be visualized as that portion of the cylinder lying between lines 56 and 57.

The axis of roll or tilt of the cam follower surface 24 at plane 1 is illustrated at 42 and at plane 2 is illustrated at 43, as shown in FIGS. 5 and 6. The axis of tilt of the grinding wheel is also in plane 1 and is at 42. Thus, the

points

36 and 37 are obviously not at fixed locations but vary in accordance with the degree of tilt about the tilt axis 42. Plane 3 is normal to the grinding wheel axis but tilts relative to the cam axis and relative to plane 1 in accordance with the degree of tilt about the axis 42. Thus, plane 3 intersects the cam axis at 51 while plane 1 intersects the cam axis at 50. The line of intersection of planes 3 and 1 is at 42. Plane 3a intersects the cam axis at 53 while plane 2 intersects at 52.

PLanes

2 and 3a intersect at 43.

It should be pointed out that the high-speed follower tilt axis as well as the grinding wheel tilt axis, both being the same axis at 42, does not remain fixed but rocks in plane 1 about the pivot 30 shown in FIG. I and in FIG. 6 in accordance with the cam lift. The same is true of the low-speed follower tilt axis 43, which is the reason lines 42 and 43 are not shown parallel in FIG. 6 even though they lie in parallel planes 1 and 2.

The line 54 is a line normal to the cam surface in plane 3 which passes through both the grinding wheel axis 35 at 36 and the cam surface at 61, while the line 55 is also nonnal to the cam surface and intersects the surface at 60 and the axis at 35 and 37. Line 54 intersects

tilt axis

42 and 44, while line 55 intersects follower tilt axis 43 at 45.

The axis of the cam 22 between the planes 1 and 2 is designated at 47, while the cam surface has a line contact with the follower 23 and also with the grinding wheel along the line 49. The rocker 23 will roll or rock about its longitudinal axis line L, which may appear at 42 or 43, and will maintain its line contact with the cam surface even though the line is skewed from the positions of plane 1 to plane 2. The grinding wheel and the follower tilt about the tilt axis 42 so that a line which is normal to plane 1 and designated as 46 and which passes through point 36 on the axis 35 is separated from axis 35 by the distance l-I measured along line 55. H is the difference in follower lift when the follower tilt axis is in plane 2 compared to the lift when it is in plane 1 or between the low-speed and high-speed positions of the follower. Line 46 may be construed as the axis location of the grinding wheel if there were no axial tilt, as in a conventional nonvariable lift cam.

The cam surface is built up from a series of contact line elements 49 which lie side-by-side around the periphery of the cam, and each of these contact line elements is parallel to the cam axis only when no lift occurs about the pivot point or, in other words, on the cam base circle 22a.

Proceeding from one element to the adjacent element, the slope of each element relative to the cam axis changes along the cam flank 22b in a continuous and gradual manner according to a mathematical function relating the instantaneous valve lift and velocity in the high speed normal plane 1 to the valve lift and velocity in the low speed normal plane 2 satisfying the equation where V is the lift velocity, in./deg., and Z is the distance from center of the follower radius measured along the line of action to a point adjacent to the cam axis, the latter point being located by a line drawn from the cam axis perpendicular to the line of action. This is the distance from point 36 to 58 in plane 3 and from 37 to 59 in plane 30. The angle d is the cam pressure angle.

A line drawn normal to line 54 and lying in plane 3 is drawn from 58 to 51 and is known as the eccentricity or offset of the line of action. A normal line drawn from 59 to 53 is the eccentricity or offset of action line 55.

a3 is the pressure angle measured in plane 3 between a radial line connecting the cam axis and follower axis at

points

51 and 36 respectively and the line of action 54, and is related according to the above equation to the velocity and lift of the cam function in the high-speed position, while a3 and 01 have different values everywhere on the cam except at the base circle 22a and at the nose 22c where a =0=a since V =0=V This difference in pressure angle along the cam flank 22b accounts for the curvature in the contact line 49 when on the cam surface 22b.

The high-speed plane 1 containing the pivot axis 42 about which the instantaneous contact line elements 49 may rotate as they change slope and as they collectively define the cam surface has the pivot axis so located that the pivoting of the contact line elements does not in any way destroy the accuracy of the functional displacement of the cam follower. The pivot axis of tilt line 42 connects the center of the follower ball pivot 30 and the center of the ball member 33 thereby eliminating secondary to" displacement of the follower which would render control of the high-speed displacement more difficult.

The lift displacements of the pivoted follower 23 when the latter is in the low-speed plane 2 are the sum of the lift displacement in the high-speed plane 1 plus or minus the tilt or roll displacements H of the follower surface centerline as shown in FIGS. 5 and 6. The normal production dimensional tolerances applied to the high-speed displacements are added to the tolerances which must be allowed in the tilt or roll function of the follower to produce greater tolerance in the displacement at the low-speed normal plane. Since dynamic forces and vibrations are considerably less important at low engine speed, this construction is entirely acceptable.

To maintain the desired accuracy of the high-speed displacement, the tilt of the grinding wheel axis 35 must be centered on the tilt line 42. Grinding wheel tilt thereby controls the differences in lift displacement of the cam from the highspeed plane 1 to the low-sepeed plane 2. As mentioned before line 42 rocks in the longitudinal plane and so this action must be taken into account during the grinding process.

If desired, the cylindrical surface of the pad 24 may be either a nonrotatable sliding surface as illustrated or a rotatable roller follower although a nonrotatable surface is much more practical considering that the grinding wheel radius which lends itself to most practical production processes is in the neighborhood of 9 to l 1 inches.

It will further be understood that the pivoted rocker arm follower 23 could be replaced with a barrel-reci rocating follower having a spherical bottom for the cam fol ower surface.

The spherical surface should have a radius equal to he grinding wheel which, since it is large, will tend to minimize contact stresses.

From the above descriptions it should therefore be understood that this invention now provides a cam and follower combination where a curved surface of a follower maintains line contact with a cam of appreciable width having a contour which varies radially and axially to provide variable timing. The follower is free to tilt about a tilt axis which moves from the plane of highest displacement of the cam to the plane of lowest displacement in response to a command which is determined by the engine speed.

l claim as my invention:

l. A cam and rocker arm follower combination adjustable to vary timing, duration and extent of lift which comprises relatively axially shiftable cam and follower means, the cam means having axially and radially inclined flank and nose portions along its width, the follower being a rocker arm having a cylindrical surface between its ends engaging the cam, and means mounting the follower for tilting motion transverse to said cylindrical surface.

2. The assembly of claim 1 including speed-sensitive means effecting the relative shifting of the cam and follower means.

3. An adjustable valve linkage for internal combustion engines comprising an axially shiftable overhead camshaft having a plurality of wide cams spaced along the length thereof, each cam having a base circle, side flanks and a nose between the side flanks, said nose and flanks being inclined along the width of the cam and increasing in radial displacement from the base circle to the tip of the nose, a rocker arm underlying each cam having a cylindrical pad riding on the cam, a fixed pivot for one end of each arm accommodating rocking of the arm transversely of the camshaft, valves having stems acted upon by the other ends of said arms, and said cylindrical surfaces of the pads of the rocker arms struck from radii centered on a line which is parallel with the axis of the camshaft only when the pad is in contact with the base circle portion of the cam.

4. The assembly of claim 3 including means accommodating longitudinal sliding of the rocker arms relative to the valves while preventing lateral sliding of the rocker arms in a direction parallel to the cam axis.

5. A cam and follower assembly which comprises a cam shaft, a cam on said shaft having an axially extending surface with an axially and radially varying contour to provide variable timing, lift and duration along its length, a rocker arm having a curved surface intermediate its ends engaging said cam surface, and cam shaft being axially shiftable relative to said rocker arm to vary the axial position of the cam on the curved surface of the rocker arm, and means accommodating tilting of the rocker arm transversely of its curved surface to maintain line contact between the curved surface and cam.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No- 573 Dated November 9, 1971 Inventor-(5) H.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 45, "PLanes" should be --Planes--; Column 4, line 56, "60 and the axis at 35 and 37" should be --60 and the axis 35 at 37--; Column 5, line 64, "sepeed" should be --speed-; Column 6, line 59, "and cam shaft" should be --said cam shaft--.

Signed and sealed this 30th day of May 1972.

(SEAL) Attest:

EDWARD M.FLETCI-IER,JR. ROBERT GOTISCHALK Attesting Officer Commissioner of Patents

Claims

1. A cam and rocker arm follower combination adjustable to vary timing, duration and extent of lift which comprises relatively axially shiftable cam and follower means, the cam means having axially and radially inclined flank and nose portions along its width, the follower being a rocker arm having a cylindrical surface between its ends engaging the cam, and means mounting the follower for tilting motion transverse to said cylindrical surFace.

5. A cam and follower assembly which comprises a cam shaft, a cam on said shaft having an axially extending surface with an axially and radially varying contour to provide variable timing, lift and duration along its length, a rocker arm having a curved surface intermediate its ends engaging said cam surface, said cam shaft being axially shiftable relative to said rocker arm to vary the axial position of the cam on the curved surface of the rocker arm, and means accommodating tilting of the rocker arm transversely of its curved surface to maintain line contact between the curved surface and cam.