Metal treatment – Process of modifying or maintaining internal physical... – Carburizing or nitriding using externally supplied carbon or...
Reexamination Certificate
1999-12-20
2001-09-25
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Carburizing or nitriding using externally supplied carbon or...
C148S217000, C148S226000
Reexamination Certificate
active
06294029
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of treating and smoothing a surface of a sliding member and a counter member such as a shim and a cam for driving a valve of an engine.
2. Description of the Related Art
Typically, intake valves and exhaust valves of an engine are driven by cams on camshafts. Such a cam moves keeping in contact with a cam follower such as a tappet at the top of the valve and a shim or a rocker arm attached to the tappet. Since frictional resistance between these cam and cam follower is one of causes of engine output loss, it is preferred to make a sliding surface of the member as smooth like a mirror surface as possible so as to lower its frictional resistance against the counter member. It has been proposed to finish a member that slides keeping in contact with a cam to a sliding surface with a surface roughness of less than 0.3 &mgr;mRz (in terms of ten point average roughness) and apply a coating of a material having a hardness higher than 100 Hv such as titanium oxides to the sliding surface by arc discharge ion plating. One of such techniques is known from, for instance, Japanese Unexamined Patent Publication No. 7 - 118832, entitled “Sliding Member and its Production. According to the technique,” a sliding surface of a member is formed with droplets of titanium oxide thereon by coating a titanium oxide on titanium stuck on the sliding surface so as to form fine protrusions thereon. While the sliding member slides with its sliding surface kept in contact with a cam surface at the beginning of use, the fine protrusions, on one hand, polish the cam surface to a mirror surface and on the other hand are removed by the cam surface from the sliding member. Another technique that is described in Japanese Unexamined Patent Publication No. 9 - 302454, entitled “Pre-Treatment of Carburizing Quenched Material And Manufacture,” a sliding member is polished to a surface with a roughness between approximately 6 and 25 &mgr;mRy (in terms of maximum roughness) by shot peening and subsequently treated by carburizing and quenching. By means of this technique, while the sliding member slides with its sliding surface kept in contact with a counter member at the beginning of use, an abnormally carburized surface layer on the sliding member is removed, so as to smooth the sliding surface of the sliding member due to an adaptation effect between the two members.
In the former smoothing technique described in Japanese Unexamined Patent Publication No. 7 - 118832, although control of a current density can govern the droplet density to some extent, it is difficult to realize stable control of the droplet density. If the droplet density is low, it is hard to smooth sufficiently a sliding surface of a counter member, and, conversely, if it is high, scuffing or light seizing is apt to occur in the sliding surface. Because the size, height and position of droplet are hardly controllable, manufacturing variations of the sliding member are great. In addition the arc discharge ionic plating causes an increase in manufacturing costs of the sliding member. On the other hand, the alternative technique described in Japanese Unexamined Patent Publication No. 9 - 302454, while the running-in of a sliding member, which is primarily performed with an attention of removing an abnormally carburized surface layer on the sliding member at the beginning of use, can lower the surface roughness of the sliding member, however, since the surface roughness of the sliding member before the carburizing and quenching treatment is significantly high, the smoothness of a sliding surface that is provided by the alternative technique is limited. Furthermore, it is hard for a sliding surface of the counter member to be smoothed because the alternative technique does not take into consideration the surface roughness of the sliding member.
The term “surface roughness (&mgr;mRa)” as used in the specification shall mean and refer to an arithmetic average roughness Ra measured in &mgr;m. When taking X-axis and Y-axis in the direction of the average line of a roughness curve sampled by a standard length l, and in the direction of the average line of a roughness curve, respectively, and expressing the roughness curve by a function y=f(x), the arithmetic average roughness Ra is given by the following expression:
Ra
=
1
/
l
{
∫
o
l
&LeftBracketingBar;
f
(
x
)
&RightBracketingBar;
ⅆ
x
}
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method of treating and smoothing a sliding surface of a sliding member such as a cam follower by which the sliding member with sliding frictional resistance reduced relatively to a counter member such as a cam is provided at low costs.
The foregoing object of the present invention is achieved by applying surface treatment to a sliding member, such as a cam follower installed in a valve drive system of an engine, cooperative to slide on a counter member, such as a valve drive cam, with the surface kept in contact with a surface of the counter member which is finished by pressurized particle containing fluid blasting to a sliding surface with surface roughness ranging approximately 0.01 to 1.0 &mgr;mRa so as to provide the sliding member with surface higher than surface hardness of the counter member. Specifically, the surface treating method of the present invention comprises the steps of pre-finishing the sliding member to a surface with surface roughness lower than surface roughness of the counter member, blasting pressurized fluid containing particles harder than the surface of the sliding member on the surface of the sliding member to finish the sliding member to a surface with surface roughness ranging approximately 0.01 to 1.0&mgr;mRa, and hardening the surface of the sliding member to surface hardness higher than surface hardness of the counter member.
According to the surface treating method, the sliding member is pre-finished by, for instance grinding and polishing, to a relatively smooth surface with surface roughness lower than the surface roughness of the counter member. When applying pressurized fluid containing particles harder than the surface of the sliding member to the surface of the sliding member by shot blasting, the surface of the sliding member with relatively low surface roughness is formed with a number of minute projections that are separately and approximately uniformly distributed over the surface of the sliding member. If the lower limit surface roughness 0.01 &mgr;mRa is exceeded, it is difficult to smooth sufficiently a sliding surface of the counter member during running-in operation and, on the other hand, if the upper limit surface roughness 1.0 &mgr;mRa is exceeded, it is difficult to smooth the sliding member. Before or after forming the projections, the sliding member at the sliding surface is hardened. Since the projections are harder than the counter member, they polish and smooth the surface of the counter member during running-in operation. At the same time since the projections, which are minute and distributed separately from one another, are below in hardness than 1500 Hv although harder than the counter member, they are easily rent apart from the sliding member and smoothed during running-in operation. As a result, the sliding frictional resistance between the sliding member and the counter member becomes very low. In addition, since it is possible to form and distribute the projections uniformly over the surface of the sliding member through a simple treatment, a certain drop in sliding frictional resistance between the sliding member and the counter member is realized constantly at low costs.
The sliding member is preferably pre-finished to a surface with surface roughness lower than 0.2, &mgr;mRa with an effect of forming more uniform projections by the pressurized particle containing fluid blasting and providing a more smooth surface of the sliding member through sunning-in operation.
The surface of the s
Sakate Nobuo
Shibahara Masahiko
Brooks & Kushman P.C.
Coy Nicole
King Roy
Mazda Motor Corporation
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