The present invention relates to a tappet made of light metal such as Al alloy and a method of manufacturing the same.
Recently, tappets used in a direct acting type valve operating mechanism in an internal combustion engine are made of Al alloy to lighten the valve operating mechanism and to increase allowable rotation speed of the engine. Al alloy tappets have lower strength and wear resistance than conventional steel tappets, so that wear resistant materials are mounted or coating layers are formed on the upper surface of the tappets which contact a cam and on the outer circumferential surface which contacts a cylinder head.
FIGS. 8 and 9 show one example of a conventional tappet in which the upper surface which is slidably engaged with a cam is made to provide wear resistance. In FIG. 8, a groove la is formed on the upper surface of an Al alloy tappet body 1, and a wear resistant steel shim 2 is detachably engaged in the groove la. The shim 2 may be fixed by press-fit or casting.
In FIG. 9, a wear resistant cam receiving disc 3 is fixed on the upper surface of the tappet body 1 made of Al alloy by soldering.
In addition to the two examples, particles such as ceramics are dispersed on the whole surface of the tappet body to increase total strength and wear resistance.
In the tappet as shown in FIG. 8, effective surface area of the shim 2 which contacts the cam becomes smaller. Therefore, to obtain a desired lifting amount of. a valve, it is necessary to increase an outer diameter of the tappet body 1 and to mount a larger diameter shim 2. However, such structure increases the whole size of the tappet against lightening, and in addition, a cylinder head in which the tappet is mounted becomes larger to limit flexibility of planning in the engine. High accuracy mechanical processing must be made to the groove la, thereby increasing cost.
In the tappet in FIG. 9, the cam receiving disc 3 is fixed on the whole upper surface of the tappet body 1 to avoid decrease in effective surface area, but owing to connection of different materials, it is troublesome to solder them, thereby increasing cost.
Both the conventional tappets comprise two members, so that it is necessary to provide different size shims 2 and cam receiving discs 3 to make management of parts troublesome and not to attain lightening. Ceramic particles are dispersed on the whole tappet body, but it is difficult to disperse the particles uniformly, so that cost increases.