Nitrided valve lifter and producing method therefor

Abstract

A sliding surface (2) of a valve lifter (1) or a shim (3) is subjected to a gaseous nitriding to have a surface hardness of Hv 660 or higher and a thickness of a compound layer (6) of 1 to 5 μm. The nitrided layer on the surface is a dense compound layer (6) with a porosity of 5% or less.

Description

TECHNICAL FIELD [0001] The present invention relates to a valve lifter which is a valve driving component, subjected to nitriding, of an internal combustion engine, and a producing method thereof, and further relates to a combination of the valve lifter and a cam. BACKGROUND ART [0002] Referring to FIG. 10, in a direct drive valve actuating system 10 of an internal combustion engine, a valve lifter 1 for converting a rotating motion of a cam 11 into a reciprocating motion of a valve 12 is subjected to a sliding contact in a reciprocating motion with a cylinder block 14 and a shim 3 or an upper surface 2 (cf. FIG. 1) of the valve lifter coming into contact with the cam 11 is subjected to a sliding contact under a high load and an impact load, so that an improvement in a wear resistance and an impact resistance is indispensable. On the other hand, the surface roughness of sliding surface of the cam 11 of which a material has a wear resistance and an impact resistance properties has to...

AI Technical Summary

Benefits of technology

"The present invention provides a valve lifter with a high-wear resistance nitriding compound layer on its surface and a method for producing it without surface roughness and deformation. This layer can be achieved without requiring a polishing process. Additionally, the invention provides a valve lifter that can be used with a cam that has not been polished."

Problems solved by technology

However, in the sliding component and the producing method thereof as disclosed in JP-A No. 2002-97563 employs a prior nitriding process, whereby a compound layer having a brittle surface layer with deteriorated surface roughness is formed relatively thick (5 to 15 μm) in the nitriding process, and relatively large deformation is occurred in the processed articles.

Consequently it is necessary to improve the surface roughness while leaving the compound layer in a subsequent step, and there is required a very difficult and unstable polishing process of making a thin compound layer further thinner.

However such process forms a porous layer and is associated with drawbacks not only of forming a brittle-ε-phase (Fe2-3N) but also of causing a large deformation in a processed article and a significant deterioration in a surface roughness.

Also since a uniform polishing is difficult to achieve for example because of an unevenness in the hardness, the porous layer may still remain after the polishing.

In the upper surface of the valve lifter in a sliding contact under a high load and an impact load, the state, in which the porous layer is remained on the upper surface, results in a peeling or falling off of it, thus leading to a trouble.

However, since the compound layer is composed of an ε-phase (Fe2-3N), a dense γ′-phase (Fe4N), and a mixed ε+γ′ phase, and it is difficult to do a uniform polishing because of a phase distribution and a hardness variation, there tend to be given a portion where the compound layer is completely removed and a portion where the compound layer still remains within a same polished surface, so that a compound layer with uniform thickness cannot be obtained.

Consequently there are encountered such drawbacks that the wear resistance varies and an effect of reducing the frictional torque cannot be obtained.

Also even in case a uniform compound layer is obtained by a polishing in keeping with a surface undulation as disclosed in JP-A No. 2002-97563, an improvement in the surface roughness is still insufficient.

Also the aforementioned polishing process is very costly.

Regarding the cam in sliding contact with the valve lifter, on the other hand, the ground surface roughness of the sliding surface (of the cam) is relatively rough so that it causes deterioration in a surface roughness of the brittle compound layer of the upper surface.

Thus, expensive equipments for paper-lapping machine, etc. and long machining time resulting in the high cost are required in order to reduce the frictional torque from the beginning of the operation and to avoid an unstable boundary lubricating condition.

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