Abstract:
A method of manufacturing and Al alloy tappet, used in an internal combustion engine, includes supplying wear resistant particles on a surface of a light metal tappet body, the surface slideably contacting a cam, kneading said particles with a surface layer of the surface of the tappet body to imbed them and then changing the surface layer after kneading to a flat surface.

Description:
This application is a divisional application of U.S. patent application Ser. No. 08/955,946 filed Oct. 22, 1997, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     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. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing disadvantages, it is an object of the present invention to provide a tappet in an internal combustion engine and a method of manufacturing the same to increase wear resistance on the cam-contacting surface without attaching a shim or a cam receiving disc, thereby lightening it and saving cost. 
     According to one aspect of the present invention, there is provided a tappet in an internal combustion engine in which wear resistant particles are dispersed in a surface layer of the surface of the tappet made of light metal, the surface slidably contacting a cam. 
     Therefore, wear resistance of the surface which slidably contacts the cam can be increased without a shim or cam receiving disc. 
     According to another aspect of the present invention, there is provided a method of manufacturing a tappet in an internal combustion engine, the method comprising the steps of: 
     supplying wear resistant particles on the surface of a light metal tappet body, the surface slidably contacting a cam; 
     kneading said particles with a surface layer of the surface of the tappet body to embed them; and 
     changing the surface layer after kneading to a flat surface. 
     Therefore, the tappet which has wear resistant surface which contacts the cam can be easily manufactured at low cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages of the invention will become more apparent from the following description with respect to embodiment as shown in accompanying drawings wherein: 
     FIG. 1 is a central vertical sectional front view of one embodiment of a tappet according to the present invention; 
     FIG. 2 is a central vertical sectional front view which shows the step for putting particles on the upper surface of a tappet body in a method of manufacturing the tappet according to the present invention; 
     FIG. 3 is a central vertical sectional front view which shows the step for kneading the particles; 
     FIG. 4 is a central vertical sectional front view which shows how to knead by moving corrugated edge of a punch horizontally; 
     FIG. 5 is a central vertical sectional front view which shows the step in which the surface layer is melted again and solidified to form a flat surface; 
     FIG. 6 is a central vertical sectional front view which shows the step in which the surface layer is pressed and rolled by the punch to form a flat surface; 
     FIG. 7 is an enlarged vertical sectional view which shows that material is filled in pores in the surface layer; 
     FIG. 8 is a central vertical sectional front view of a conventional tappet; and 
     FIG. 9 is a central vertical sectional front view of another conventional tappet. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a tappet according to the present invention. A tappet body  4  is made of Al alloy by cold forging to form a cylinder the upper end of which is closed. Spray coating layer is formed on the outer circumferential surface of the tappet body  4 . 
     A wear resistant metal chip  7  which contacts the end of a poppet valve used in an internal combustion engine is put in a groove  6  formed in the middle of the lower surface of a top wall  4   a  of the tappet body  4 . Wear resistant hard particles  8  which has a diameter not more than 10 μm are uniformly embedded. The particles  8  include ceramic powder such as Al 2 O 3 , SiC, NbC, SiN, BN, CrC and TiB 2  which have higher melting points than Al alloy, self-lubricating metal powder such as Mo and Si, and intermetallic compounds such as CrSi 2  and MoSi 2 . 
     The particles  8  dispersed in the surface layer of the top wall  4   a  in the foregoing embodiment make the whole surface which includes sliding surface with a cam rigid to increase strength, thereby increasing wear resistance without mounting a shim or cam receiving plate in conventional methods. 
     FIGS. 2 to  5  illustrate a method of manufacturing the above tappet in order of steps. In FIG. 2, on the upper surface of the top wall  4   a  of the tappet body  4  made of Al alloy by cold forging, the particles  8  are weighed at a desired amount and put at uniform thickness. Then, in FIG. 3, a punch  9  which has a corrugated edge  9   a  on the lower surface is pressed onto the upper surface of the top wall  4   a , and the edge of the blade cut into the upper surface of the top wall  4   a , so that the particles  8  are embedded in the surface layer. In FIG. 4, the punch  9  is rotated in normal and reverse directions around an axis little by little, and presses the particles  8  several times while the corrugated edges are horizontally moved. The corrugation  10  is moved at random, so that the particles  8  are kneaded and embedded deeply into the surface layer of the top wall gradually. The upper surface of the punch is not limited to a corrugated form, but other forms may be allowed to knead the particles substantially. 
     After the kneading step, the surface of the top wall  4   a  is melted again by heating means such as a gas burner, laser beam and plasma arc, and solidified to form a flat surface as shown in FIG.  5 . The particles  8  which have higher melting point than that of the tappet body  4  dispersively remain without melting. 
     Then, the tappet body  4  is subjected to T6 treatment as fixed in JIS (Japanese Industrial Standards). Thereafter, spray coating layer is formed on the outer circumferential surface, and a chip is put in the groove  6  and totally finished to form a tappet as shown in FIG.  1 . 
     Flattening treatment after the kneading step in FIG. 4 may be achieved by pressing and rolling the currugation  10  with the punch  11  which has a flat lower surface. To facilitate rolling, the surface of the top wall  4   a  may be suitably heated and softened. 
     When the surface of the top wall  4   a  is made to a flat surface by the remelting or the punch  11 , pores may be formed on the surface layer, When the pores are fine, oil-keeping capability of lubricating oil is increased to decrease wear in the top wall  4   a  or cam. Thus, the pores may be kept. 
     Relatively large pores increase frictional resistance and decrease density or strength. As shown in FIG. 7, self-lubricating filled material  13  such as MoS 2 , high molecular weight polyethylene, acetal resin and fluorine resin known as Trademark “Teflon” may be immersed or filled in the pores  12 . In the foregoing step, T 6  heat treatment and mechanical processing to suitable portions may be carried out before embedding the particles or right after molding the tappet body. The tappet body  4  may be formed by warm or thermal forging, or Al casting other than cold forging. 
     The foregoings merely relate to embodiments of the invention. Various modifications and changes may be made by person skilled in the art without departing from the scope of claims wherein: