Method of making a rocker arm
This application is a continuation, of application Ser. No. 021,859, filed Mar. 4, 1987, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to rocker arms for internal combustion engines, particularly of the kind having a portion made of ceramics in order to attain good wear resistibility. 
     2. Description of the Prior Art 
     A rocker arm partly made of ceramics is known in the art and has been used in some LPG engine equipped vehicles. In such a rocker arm, only a portion for contact with a cam is made of ceramics so as to efficiently improve the wear resistibility. Use of ceramics in the prior art rocker arm is therefore directed to the durability, reliability, maintenance, etc. of the engine and has no direct relation to the engine performance such as engine power, etc. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a rocker arm which comprises an arm section extending throughout the overall length of the rocker arm and having a portion for engagement with the cam and a portion for engagement with the valve, the arm section being made of ceramics. 
     The above structure is effective not only for improving the wear resistibility but also for improving the engine performance. 
     It is accordingly an object of the present invention to provide a novel and improved rocker arm for a internal combustion engine which can improve the engine performance as well as the wear resistibility. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic elevational view of a valve operating mechanism for an internal combustion engine, to which a rocker arm according to an embodiment of the present invention is applied; and 
     FIGS. 2 and 4 are views similar to FIG. 1 but showing modifications of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, a rocker arm according to an embodiment of the present invention is generally indicated by 10 and shown as being constituted by an arm section 12 made of ceramics and a bearing section or pivot section 14 made of an aluminium alloy. The arm section 12 extends between an end of a valve stem 16 and a cam 18 to engage the same at the opposite ends thereof. The pivot section 14 is formed with a bore 14a in which a journal section of a rocker shaft 20 is rotatably received, i.e., the rocker arm 10 is mounted at the pivot section 14 on the rocker shaft 20 for oscillation. 
     More specifically, the arm section 12 has vertically opposed upper and lower sides and is adapted to engage at the lower side with the end of the valve stem 16 and the cam 18 and to install at the upper side the pivot section 14. The pivot section 14 is positioned between the opposite ends of the arm section 12, i.e., the rocker shaft 20 is positioned between the end of the valve stem 16 and the cam 18. 
     With the above structure, the rocker arm 10 can be lighter in weight and larger in mechanical strength as compared with the aforementioned prior art rocker arm partly made of ceramics. By this, the inertia mass of the rocker arm 10 can be smaller than that of the prior art rocker arm. 
     FIG. 2 shows a modification of the present invention. By this modification, the rocker arm 22 is entirely made of ceramics, i.e., the rocker arm 22 is constituted by the ceramic arm section 22a only. The arm section 22a is formed with a bearing or pivot portion 22b in the form of a part-cylindrical depression on the upper side thereof. The arm section 22a is adapted to install at an end thereof a lash adjuster 24 by way of which the rocker arm 22 is put into contact with the end of the valve stem 16. Except for the above, this embodiment is substantially similar to the previous embodiment. 
     FIG. 3 shows a further modification of the present invention. By this modification, the rocker arm 26 is entirely made of ceramics, i.e., made of nitride silicon by injection molding, and formed into a shape different from that of the rocker arms 10, 22 of the previous embodiments of FIGS. 1 and 2. In other words, the rocker arm 26 is constituted by a ceramic arm section 26a only. The arm section 26a has at an end a depression 26b for receiving therein an end of a pivot 24a of a lash adjuster 24 so as to oscillate about the pivot 24a. The arm section 26a also has on the upper side thereof and in the place intermediate between the opposite ends thereof a portion 26c for contact with the cam 18, i.e., the cam 18 is positioned between the lash adjuster 22 and the valve stem 16. 
     The all ceramic rocker arm 26 of this embodiment is produced by injection molding and then processed by barrel finishing. After that, the portion 26b of the rocker arm 26 for contact with the pivot 24a of the lash adjuster 24 is ground by using the valve contacting portion 26d and the cam contacting portion 26c as reference or datum surfaces. By this method, only the contact portion 26b needs to be ground actually, thus making it possible to simplify the manufacturing processes of the rocker arm 26. 
     In FIG. 3, indicated by the one-dot-chain line is the shape of the comparable prior art rocker arm made of metal. As will be seen from the comparison between the two shapes in FIG. 3, the all ceramic rocker arm 26 of this invention can be thinner or more slender than the metal rocker arm and therefore can be considerably lighter in weight while providing a good wear resistibility particularly at the portions for contact with the lash adjuster 22 and the valve stem 16. 
     Since the rocker arm 26 can be thinner than the comparable prior art metal rocker arm, it becomes possible to reduce the weight as much as 20%. In the meantime, by the performance test of a 2-liter four cylinder gasoline engine which is provided with the rocker arms 26 of this invention together with all ceramic valves, it was found that the fuel consumption during idling is improved as much as 5% and the maximum rpm becomes higher as much as 5% as compared with the engine provided with the rocker arms of the aforementioned partly ceramic type. 
     FIG. 4 shows a further modification of the present invention. The rocker arm 28 has at an end thereof a semispherical projection 28a for contact with the recessed end of the pivot 24a of the lash adjuster 24. Except for the above, this embodiment is substantially similar to the previous embodiment of FIG. 3. 
     From the foregoing, it is to be understood that the maintenance can be improved considerably since the portions of the rocker arm for contact with the associated parts are made of ceramics which has an excellent wear resistibility. 
     It is further to be understood that by the use of ceramics in the foregoing manner the rocker arm of this invention has a large Young&#39;s modulus and is thus superior in rigidly to the aforementioned prior art rocker arm. 
     It is still further to be understood that the rocker arm of this invention can be thinner and lighter than the prior art rocker arm since the rocker arm of this invention is larger in rated strength (i.e., ratio of strength to weight) than the prior art rocker arm which is mainly made of metal, resulting in that the rocker arm of this invention can follow the operation of the cam more efficiently and accurately than the prior art rocker arm and therefore the engine can be operated at a speed higher than ever, i.e., the maximum speed of the engine can be set larger than ever. 
     It is yet further to be understood that the rocker arm of this invention can contribute to improvements in the engine performance particularly when used together with the ceramic valves since the weight of the ceramic valve can be less than half of that of the metal valve and therefore the ceramic valve is all the more affected adversely or unfavorably by the metal rocker arm or rocker arm mainly made of metal due to its large inertia mass.