US2708917A - Valve tappet - Google Patents

Description

F. C. PEARSON VALVE TAPPET May 24, 1955 Filed April 12, 1950 m lg mwhw 4. w i

United States Patent VALVE TAPPET Frank C. Pearson, Flushing, Micln, assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware Application April 12, 1950, Serial No. 155,457

Claims. (Cl. 123-90) This invention relates to tappets employed in poppet valve operating mechanisms of internal combustion engines or the like and particularly to means for preventing rotation of tappets in operation.

The invention is applicable, for example, to conventional valve operating mechanisms incorporating a rotary cam slidably engaging a rotatably and reciprocably guided tappet in thrust transmitting relation with the valve. Conventionally, such tappets have an externally cylindrical body slidably fitting the guide and an end face having a substantially fiat portion in direct slidable engagement with the operating surface of the cam. When the tappet is propelled linearly by the cam it is also subjected to side thrust, the amount of which is preferably held to the lowest limits obtainable under the particular design requirements of the engine. The tappet may also be offset longitudinally of the cam axis with the result that rotation is imparted to the tappet by the cam in operation. While in some instances such rotation advantageously promotes uniform wear distribution and hence a free working relation between the tappet and its guide and between component parts of automatic self-adjusting tappets, it frequently happens that the tappet will rotate at a greater speed in operation than is desired, particularly in the case of tappets having cam engaging end faces which are subject to spalling. This spalling condition has been found to be an operating weakness of considerable importance in the case of cast iron tappets which have their lower ends chill-hardened.

With the above in mind it is the principal object of the instant invention to provide means to prevent rotation of the tappets in such operating mechanisms. It is a further object to provide a tappet having rotation restraining means which does not otherwise materially affect tappet operation.

These and other objects, which will be apparent from a reading of the following specification, are obtained by inclining the cam contact face of the tappet at an angle slightly out of perpendicular to the linear motion of the tappet.

In the drawings:

Figure l is a sectional elevational view of a portion of an internal combustion engine having a valve operating tappet in accordance with my invention.

Figure 2 is an enlarged fragmentary vie-w taken from line 2-2 of Figure 1.

Figure 3 is an enlarged fragmentary view similar to- Figure 1.

Referring now to the drawing in detail, the numeral 1 designates generally the frame of an internal combustion engine of the overhead valve type having a cam 2 rotatably driven by the cam shaft 3. The cam 2 has an operating surface 4 (Figure 2) of substantial width upon which rests the cam follower or tappet 5 which is in thrust transmitting relation with the poppet valve 6 through a push rod 7 and rocker 8. The structural details of the particular tappet 5 illustrated are not important to the instant invention, it being sufficient to state that this tappet is intended to be representative of the Wellknown hydraulic self-adjusting type which serves to maintain close contact between the cam 2 and the valve 6 when the valve :is in seated position as shown in Figure 1. 9 is a bore formed in the engine frame 1 and constituting a guide for the tappet, accommodating both its rotation and reciprocation in response to rotation of the cam 2.

While in Figure 2, I have shown the cam 2 centered with respect to the tappet, in which position no rotation would be imparted to the tappet by the cam providing uniform contact pressure exists at all times between the cam surface 4 and tappet face 10, it frequently happens due to slight variation in surface finishes, manufacturing tolerances, and operational strains occurring in the engine frame and cam shaft, that a tappet so located will nevertheless spin in operation. Obviously, where the tappet and cam are relatively offset longitudinally of the cam shaft 3, there will be a definite rotation imparted to the tappet at all times. While such rotation is sometimes desired to a limited extent, as for example to maintain a free working relation between the telescoping parts of hydraulic tappets, it has been found that excessively rapid spinning of the tappet is conducive to unsatisfactory wear characteristics of the tappet end face 10 in operation. To provide suflicient hardness for the tappet face 10, it is conventional practice to supelficially harden the lower end of the tappet body. This superficial hardening in the case of cast iron tappets is obtained by the well-known chill-hardening method, which produces a surface subject to spalling under certain load conditions encountered in valve operation. This spalling condition is agravated by excessive spinning of the tappet, those tappets which spin the most being found to spall more than tappets which rotate only slowly, and a minimum tendency to spall occurs with tappets which do not spin.

As means for preventing such tappet rotation I provide the tappet end face 10 with a slight inclination to the transverse plane, as indicated by the angle A. Due to the fact that the tappet 5 tends at all times to take up lash in the valve operating train the tappet end face 10 is constantly urged into contact with the cam surface 4, and since the end face 10 varies in elevation with respect to the cam surface 4 by reason of the angle A the tappet will tend to seek its lowest level which will correspond to a set angular position of the tappet on the cam. Al-

though angle A must be large enough to positively insure that the tappet will not rotate in operation, it is important that this angle not be so large as to materially increase the tappet side thrust in its guide or delay the rate of valve opening as dictated by the particular design requirements of the engine. While at first thought it might seem that angle A would have to be relatively large (several degrees), I have found that in applying my invention to the valve operating mechanisms of conventional automotive engines, an angle A as small as three minutes will satisfactorily stop rotation of tappets which do not have a designed offset longitudinally of the cam, and that the angle may be increased to 13 minutes without noticeably increasing the tappet side thrust or affecting the valve timing.

While the invention has been described as applied to tappets of the automatically self-adjusting type and has perhaps its greatest advantages in such applications, obviously fixed-length tappets may be similarly restrained from rotation without encountering excessive side thrust or materially affecting the valve timing. The angle A in such cases may need to be increased somewhat over that required by self-adjusting tappets, particularly where relatively large valve operating lash values are used.

I claim:

1. In a poppet valve operating train, a rotary cam having a generally convex operating surface of substantial width, a poppet valve, a rotatably mounted tappet of the automatic-lash-take-up type in driving relation there-be tween having a substantially flat end face in direct abutment with the cam ope-rating surface, said end face being inclined at an angle of 3l3 minutes to a plane transverse to the valve opening movement of the' tappet whereby rotation of said end face is restrained by the tendency of the tappet to take up lash in the valve train.

2. A tappet for transmitting linear motion to a poppet valve from a rotary cam, said tappet having a cylindrical guided surface and a substantially flat cam contact face, said face having a 3-13 minutes inclination to a plane normal to said guided surface.

3. A tappet of the automatic self-expanding type for transmitting end-wise movement to a poppet valve in re sponse to a driving rotary cam, said tappet having an external surface adapted to journal in a tappet guiding bore and an end face adapted to slidably abut the cam, said end face being inclined at an angle of 3-13 minutes to a plane normal to its said external surface whereby expansion of the tappet in operation will tend to maintain said end face parallel to the rotary axis of the cam.

4. A tappet for transmitting linear motion to a valve from a rotary cam, said tappet having a cylindrical guided surface and a substantially fiat contact face, said face having the smallest inclination to a plane normal to said guided surface that is necessary to prevent continuous rotation of the tappet about the tappet axis and that will cause the tappet to oscillate and adjust itself to engage the cam squarely across the fiat contact face.

5. A tappet for transmitting linear motion to a poppet valve from a rotary cam, said tappet having a cylindrical guided surface and a substantially flat cam contact face, said face being inclined to a plane normal to said guided surface at an angle, the maximum of which has its tangent equal to or slightly greater than the dynamic coefficient of friction between the said contact face and the said cam.

OTHER REFERENCES Mechanical Engineers Handbook, Lionel S. Marks, McGraw-Hill, 1916, pp. 232-235.

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