Rocker arm made from high chrome cast iron

Abstract: A rocker arm is made by casting from high chrome cast iron with a microstructure composed of a matrix of martensite and carbide precipitates occupying 30 to less than 45% of the matrix area, and then superfinished by grinding to have a surface roughness of Ra 0.05 to 0.2. The precipitates have an average diameter of 20 micron or less and an average globularity of 40% or more in which the globularity of each precipitate is calculated as a ratio of the real area to the circumcircular area.

Description: The invention relates to a rocker arm that is made from high chrome iron by casting and finished by grinding, and more particularly to a surface quality for the rocker arm being unaggressive to the mating member. As described in JP A Sho 63-303030, rocker arms made from high chrome cast iron are available for use in valve mechanism in an automotive internal combustion engine. The high chrome cast iron characteristically forms hard precipitates of (Fe, Cr)₇C₃ and the like, which direct to the casting surface. The rocker arm is improved in abrasive resistance due to the hard precipitates. However, the rocker arm could be aggressive so as to abrade the mating member or cam in excess, if improperly finished by grinding. GB A - 2 205 108 discloses a rocker arm made from high chrome cast iron, said rocker arm comprising a microstructure composed of a matrix of martensite and carbide precipitates. EP-A -0 371 760 relates to a high strength high chromium cast iron containing fine particle precipitated hard carbide. Valve rocker arms are made from this material. The precipitated hard carbide has an average particle size of 20 µm or less in a martensite matrix with hardness of HV 500 or more and has a ratio from 30 % to 45 %. The precipitated hard carbide has a spheroidal ratio of 40 % or more. The rocker arm is finished by grinding to protect the mating member against intolerable abrasion. When it is ground to have a surface finish coarser than proper values, the cam will wear still soon. On the other hand, when it is ground to have a surface finish finer than the proper values, the grinding will be very costly although the cam member wear less. The rocker arm should have the optimal surface finish that is obtained at a reasonable grinding cost and unaggressive to the mating member. But, it is difficult to determine the optimal surface finish. When similar cams are mated with the respective rocker arms which are the same in surface roughness but made from different high chrome cast iron materials, they will differ in abrasion. This means that the optimal surface finish is concerned with the microstructure of the high chrome cast iron constituting the rocker arm. The present invention as claimed is intended to provide an improved rocker arm that is made from high chrome cast iron and finished by grinding to have a surface quality, by which a mating member is protected against abrasion beyond a tolerable limit. It also solves how to to determine the optimal surface finish required by the rocker arm on the basis of the microstructure of high chrome cast iron constituting the rocker arm. In accordance with the present invention, the rocker arm is made from high chrome cast iron by casting and finished by grinding to have a surface roughness ranging 0.05 to 0.2 µm Ra in which the surface roughness Ra is defined as center-line mean roughness by Japanese Industrial Standard B 0601. The high chrome cast iron has the composition, by weigh of C: 2.5-3.7%, Si: 1.0-2.0%, Mn: 0.5-1.0%, Cr: 15-20%, Ni: 0.3-0.7%, P: no more than 0.3%, S: no more than 1.0%, Fe and impurities: the balance. It further contains one or more of W, Mo, V, Nb, Ta, Ti and B, if necessary. The chrome cast iron has a structure composed of martensite matrix with a hardness of Hv 700 or more and hard carbide precipitates occupying 30 to less than 45% of the matrix area. The precipitates have the average diameter smaller than 20 micron and the average globularity of more than 40%, in which the globularity of each precipitate is calculated as a ratio of the real area to the circumcircular area. In general, an amount of abrasion is proportional both to the surface roughness (Ra) and to the sliding condition (PV) between a cam and a mating cam follower. In the case of the rocker arm having similarly shaped carbide precipitates in the microstructure, the finer or smaller its surface roughness Ra becomes, the less the cam will wear. On the other hand, in the case of the rocker arm having the same surface roughness, the carbide precipitates will be more aggressive against the cam as the average globularity becomes smaller. Accordingly, in order that the individual cams are similar in wear, the surface roughness Ra of each rocker arm should be determined depending on the average shape or globularity of the carbide precipitates in the microstructure. In the case of the rocker arm made from the aforementioned high chrome iron, when the surface roughness Ra is larger than 0.2 µm, there is the possibility that it abrades the cam beyond a tolerable limit. On the other hand, when the surface roughness Ra is smaller than 0.05 µm, although the cam surely wear within a tolerable limit, the grinding cost will become very high as compared with the reasonable. In conventional, the rocker arms were mostly finished to have a surface roughness ranging more than 0.2 to 0.8 µm Ra with the result that the mating member wore sooner than a tolerable limit, or sometimes finished at an excessive cost to have a surface roughness of less than 0.01 µm Ra, since no optimal surface roughness of the rocker arm has been unknown. From the foregoing, the optimal surface roughness is determined on the basis of the microstructural properties of the carbide precipitates in the high chrome cast iron constituting the rocker arm in accordance with the present invention. This offers advantages that not only the rocker arm but also the cam are highly improved in abrasion resistance when mated with each other, and that the rocker arm is always finished by grinding at a reasonable cost. Besides, it is easy to determine the optimal surface finish that is given to the rocker arm made from high chrome cast iron. In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following examples are given. It will be appreciated, of course, that these examples are merely exemplary of the invention and not to be taken as a limitation of the invention. Rocker arms were molded as one piece from high chrome cast iron having the composition as shown in Table 1. The rocker arms were heat-treated after casting and then photomicrographically analyzed. The analysis showed that they had an acicular and globular carbide precipitate mixture being of 41% globularity, by average, 18 micron diameter, by average, and 35% content, by area. They were classified into 6 samples, 3 Examples and 3 Comparisons on the basis of the surface roughness Ra, after being finished under several conditions having different grinding and/or feeding speeds. Each of sample rocker arms was mated with the respective cam and tested in a test engine as follows: Test Engine: Straight 4 cylinder OHC-type 2000 cc gasoline engine Driving: Motoring Valve Spring Load: 20% more than normal Camshaft: made from chilled iron Lubricating Oil: 7.5W-30 reproduced oil at market Engine Speed: 600 rpm Operating Time: 600 hr The test results are shown in Table 2. This table shows that Comparisons 1 and 2 abrade the respective cam members by 57 to 75 micron while Examples abrade the cam members by 10 to 20 micron and that the cam member wear much less when mated with the inventive rocker arm. The table also shows that Examples wear by 8 to 12 micron while Comparisons 1 and 2 wear by 18 to 20 micron. This means that the inventive rocker arm is also improved in its own anti-wear property. In the case of Comparison 3 having an extremely fine surface roughness of 0.01 µm Ra, the rocker arm and the respective cam wear by 8 micron and 9 micron. Comparison 3 surely improves the rocker arm as well as the cam in abrasion resistance but can not excel Example 3 in which both the rocker arm and the cam wear by 8 micron. Besides, it is disadvantageously difficult and costly to finish the rocker arm to the surface roughness of 0.01 µm Ra. Therefore, the value of Ra is limited within a range of 0.05 to 0.2 µm in the present invention. Composition of high chrome cast iron (%, by weight) TC: 3.5 Si: 1.4 Mn: 0.7 P : 0.10 S : 0.05 Ni: 0.5 Cr: 18.0 Mo: 0.5 W : 3.0 Samples Grinding Ra (micron) Amount of Wear (micron) Rocker arm Cam Example 1 superfinish with CBN stone 0.18 12 20 Example 2 superfinish with CBN stone 0.11 9 12 Example 3 superfinish with CBN stone 0.05 8 10 Comparison 1 ordinary finish with GC stone 0.85 20 75 Comparison 2 ordinary finish with GC stone 0.33 18 57 Comparison 3 superfinish with CBN stone 0.01 8 9

Claims: A rocker arm made from high chrome cast iron and finished by grinding, characterised in that said rocker arm is superfinished to have a surface roughness of Ra 0.05 to 0.2 µm, Ra defined as a center-line mean roughness that said rocker arm comprises a microstructure composed of a matrix of martensite and carbide precipitates constituting 30 to less than 45% of the matrix area, and that said precipitates have an average diameter of 20 micron or less and an average globularity of 40% or more in which the globularity is calculated as a ratio of the real area to the circumcircular area of each precipitate. The rocker arm as claimed in claim 1, wherein said high chrome cast iron has a composition by weight of C, 2.5-3.7%; Si, 1.0-2.0%; Mn, 0.5-1.0%; Cr, 15-20%; Ni, 0.3-0.7%; P, 0.3% or less; S, 0.1% or less, the balance being Fe and inavoidable impurities. The rocker arm as claimed in claim 2, wherein said cast iron optionally contains at least one of W, Mo, V, Nb, Ta, Ti and B in an amount of 3 to 10% by weight. The rocker arm as claimed in claim 2, wherein said matrix has a hardness of Hv 700 or more.

Inventors: KANO MAKOTO, KAWAMURA OSAMU, TAKAHASHI TERUO, TANIMOTO ICHIRO, TSURUTA SEIJI, KANO, MAKOTO, KAWAMURA, OSAMU, TAKAHASHI, TERUO, TANIMOTO, ICHIRO, TSURUTA, SEIJI
Applicants: ATSUGI UNISIA CORP, NIPPON PISTON RING CO LTD, NISSAN MOTOR, ATSUGI UNISIA CORPORATION, NIPPON PISTON RING CO., LTD., NISSAN MOTOR CO., LTD.
Application year: 1990
Main classification: F01L1
IPC codes: C22C37, F01L1, F02B1