The present invention relates to a plain bearing, and more particularly to a plain bearing in which two pieces of bearing metal each having a semicylindrical shape are combined together.
Heretofore, multilayer plain bearings of the type that a sliding surface of each bearing metal piece with backing metal is coated with a soft overlay, have been known as the plain bearings used for a bearing in a big end portion of a connecting rod and for a main bearing in an internal combustion engine which rotates at high speed and on which a heavy load is imposed. The purpose of forming the overlay is not only to improve the conformability but also to provide a surface which can embed foreign particles which would enter between the sliding surface of the plain bearing and the shaft so as to prevent seizure. Herein, it is generally known that increasing the thickness of the overlay enhances the embeddability and conformability but deteriorates the load carrying capacity and fatigue resistance. Thus, the thickness of the overlay is made to be in the range of 10 to 30 micro meters (.mu.m) in, for example, an engine such as an automobile engine.
Herein, in the plain bearing which is composed of two pieces of semicylindrical bearing metal combined together and assembled into a housing, one of the two pieces of bearing metal which is imposed with a heavier load on an inner surface thereof will be referred to as a bearing metal piece located on a main-load side, while the other bearing metal piece which is imposed with a lighter load on an inner surface thereof will be referred to as a bearing metal piece located on a non-load side. There have conventionally been disclosed various types of plain bearings paying attention to a difference in load which is imposed on respective surfaces of these two bearing metal pieces.
JP-U-58-144123 (Prior Art 1) discloses a plain bearing in which one bearing metal piece located on a main-load side is coated with an overlay on an inner surface thereof, while the other bearing metal piece located on a non-load side is not coated with an overlay on an inner surface thereof. It says in this publication that the above construction makes it possible to provide a low-cost plain bearing.
Further, JP-U-62-28922 (Prior Art 2) discloses a plain bearing in which two pieces of bearing metal are each coated by plating with an overlay having a greater thickness than usual, the whole sliding surface of each of the bearing metal pieces being subjected to an eccentric surface finishing so that the overlay thickness is equal in the center axis direction of each of the bearing metal pieces but different between a center portion and opposite end portions of each of the bearing metal pieces in the circumferential direction thereof. It says in this publication that the above construction makes it possible to provide a plain bearing which is excellent in foreign matter embeddability, load carrying capacity and fatigue resistance.