A tappet for use in an internal combustion engine

Abstract: In a method of manufacturing a tappet for an internal combustion engine, powdered metal "P" filled in a cavity 22 of a die 21 is compressed by lower and upper punches 23 and 25 to form a body and a top wall of the tappet, and is sintered. During compression molding, frictional resistance of powdered material increases density of the upper and lower ends of the tappet, thereby improving strength and wear resistance.

Description: BACKGROUND OF THE INVENTION The present invention relates to a tappet for use in a direct acting type valve operating mechanism for an internal combustion engine, and in particular, to a tappet in which the density at the upper and lower ends to which larger stress act is increased, thereby improving strength and wear resistance. A direct acting type valve operating mechanism in an internal combustion engine is generally constructed as shown in Fig. 2. In a guide bore 2 formed in a cylinder head 1, there is provided a roughly H-sectioned tappet 3 having a circumferential wall 3a and a disc-shaped top wall 3b, the tappet being slidable in an axial direction of the bore 2. In the center of the circumferential wall 3a of the tappet 3, there is provided a stem 4a of an engine valve 4, the axial end of the stem 4a being engaged with the middle of the lower surface of the top wall 3b. At the axial end of the stem 4a, a spring receiver 5 is engaged via a pair of cotters 6, and an engine valve 4 is energised upwards by a valve spring 7 under the lower surface of the spring receiver 5 in the cylinder head 1. A disc-shaped shim 8 is provided in a recess on the upper surface of the upper wall 3b, and a rotary cam 9 contacts the upper surface of the shim 8. When the cam 8 rotates, the engine valve 4 is driven up and down via the shim 8 and the tappet 3 to open and close inlet and exhaust ports of the cylinder head 1. The foregoing tappet 3 is generally formed by cold forging of soft steel such as low carbon steel. Deviated load "F" acts to the tappet 3 at the beginning of pressing by the rotary cam 9, and by bending moment, the upper and lower ends of the tappet are subjected to larger stress "F1" and "F2" which act horizontally by the cylinder head 1. The upper and lower ends of the tappets need high compressive strength and wear resistance. However in the conventional method of manufacturing the tappet 3 from soft steel which is suitable for cold forging, it is difficult to increase strength of the upper and lower ends locally, but what has totally average strength is only manufactured. To overcome the disadvantage, it is suggested to make a tappet by cold-forging of high strength material such as high carbon steel and high chromium steel, but there is disadvantage that such material has low moldablity and products have cracks to involve unstable quality because the material is rigid. The European patent application EP-A-0399400 describes a process for manufacturing a sintered metallic product, this process comprising the following steps : filing a cavity with a first metallic material which pores are occupied by a second metallic material, laying at least one part of said sintered substation with a predetermined amount of a third pulverised noble metallic material, sintering said predetermined amount of said third material until a required degree of porosity is obtrude. The objective of this process is to manufacture a tappet which is not subject to internal tensile stresses. This document is not relied upon the afore mentioned disadvantage. The application JP-A-58 107 401 describes an apparatus for powder molding in which the dimensional preciseness is maintained after sintering. The objective of this invention is different from the objective of the present application. SUMMARY OF THE INVENTION It is an object of the present invention to overcome the disadvantage and to provide a method of manufacturing a tappet for an internal combustion engine to increase strength of the upper and lower ends of the tappet by simple means. According to the present invention, there is provided a tappet for use in an internal combustion engine of a vehicle, said tappet being manufactured by compressing powdered metal by upper and lower punches and sintering it, said tappet comprising a top wall and a body, said tappet having higher density at said top wall and upper and lower ends of said body, and lower density at an intermediate portion between said upper and lower ends of said body, thereby improving strength and wear resistance. At the top wall and the upper and lower ends of the body which are liable to be subjected to large stress in the tappet, strength and wear resistance are increased. The intermediate portion of the body has lower density or larger porosity, thereby increasing oil-having capability and increasing wear resistance thereof. BRIEF DESCRIPTION OF THE DRAWINGS The feature and advantages of the present invention will be apparent from the following description with respect to drawings wherein : Fig. 1 is a schematic sectional view which shows a compression molding machine for carrying out the present invention and a process for molding powdered material ; and Fig. 2 is a sectional view which shows a known direct acting type valve operating mechanism in an internal combustion engine. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Fig. 1 is a schematic sectional view which shows main part of a compression molding machine for carrying out the present invention. There is provided a cavity 22 which has an inner diameter corresponding to an outer diameter of a tappet to be molded, and a lower punch 23 is slidably inserted in axial direction. At the upper end of the lower punch 23, there is provided a smaller diameter portion 24 to form a cylindrical body of the tappet. An upper punch 25 on the die 21 has a diameter equal to and the same axis as the lower punch 23, and moves in an axial direction between a position which is spaced from the die 21 up wards and a position where it is fitted in the cavity 22. At the lower end of the upper punch 25, there is provided a smaller diameter portion 26 to form a recess in which a shim fits. To mold the tappet under compression, the lower punch 23 goes down to a predetermined height in the cavity 22, and the upper punch 25 goes up. A required amount of powdered metal "P" is filled into the cavity 22 on the smaller diameter portion 24 of the lower punch 23. As shown in Fig. 1 the upper punch 25 goes down, while the lower punch 23 goes up to compress the powdered metal "P" so that compressed powdered metal which has nearly the same form as the tappet to be manufactured is molded in a space formed by the wall of the cavity 22, by the upper end 24 of the lower punch 23 and by the lower end 26 of the upper punch 25. In the compressed powdered metal 27 after molding, owing to frictional resistance of powder to each other during compression, higher density is made at a top wall 27a and the upper and lower ends of a body 27b close to the upper and lower punches 23 and 25, while lower density is made in the middle of the body 27b. After the molded compressed powder 27 is pulled out of the die 21, it is sintered to form a tappet having higher density at the upper and lower ends of the body 27b, thereby increasing strength and wear resistance of the upper and lower ends of the tappet to which higher stress acts during operation of an engine. In the embodiment, to form the recess in which the shim fits on the upper surface of the tappet, the smaller diameter portion 26 is projected on the lower end of the upper punch 25, but a flat surface may be formed at the lower end of the upper punch 25 to provide a tappet having no such recess. The foregoing merely relate to an embodiment of the present invention, but various changes and modifications may be made by person skilled in the art without departing from the scope of claim wherein :

Claims: A tappet (27) for use in an internal combustion engine, of a vehicle, said tappet (27) being manufactured by compressing powdered metal (P) by upper and lower punches (23,25) and sintering it, said tappet (27) comprising a top wall (27a) and a body (27b), said tappet having higher density at said top wall (27a) and upper and lower ends of said body (27b) and lower density at an intermediate portion between said upper and lower ends of said body (27b), thereby improving strength and wear resistance.

Inventors: HIROSE MASAHITO, NAKAMURA KAZUFUMI, HIROSE, MASAHITO, NAKAMURA, KAZUFUMI, Hirose, Masahito, c/o Fuji Oozx Inc., Nakamura, Kazufumi, c/o Fuji Oozx Inc.
Applicants: FUJI VALVE, FUJI OOZX INC.
Application year: 1995
Main classification: F01L1
IPC codes: B22F5, F01L1, B22F3