Metal working – Method of mechanical manufacture – Prime mover or fluid pump making
Reexamination Certificate
2000-08-24
2002-09-24
Lee, Kevin (Department: 3753)
Metal working
Method of mechanical manufacture
Prime mover or fluid pump making
C029S888040, C092S071000
Reexamination Certificate
active
06453554
ABSTRACT:
This application is based on Japanese Patent Application No. 11-239364 filed Aug. 26, 1999, the contents of which are incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to a piston for a swash plate type compressor, and more particularly to a die-cast piston for such a compressor, and a method of producing such a piston by die-casting.
2. Discussion of Related Art
A piston for a swash plate type compressor is conventionally produced by forging or die-casting. Described more specifically, a blank for the piston is formed by forging or die-casting, and the blank is subjected to appropriate machining operations, to produce the desired piston. The swash plate type compressor piston may be a single-headed piston or a double-headed piston. The single-headed piston includes a head portion and a neck portion which are slidably movable in a cylinder bore formed in a cylinder block of the compressor. The neck portion is a generally U-shaped portion having a base section and a pair of substantially parallel arm sections which extend from the base section. The double-headed piston includes two head portions on the opposite sides of the neck portion. Since the single- or double-headed piston is reciprocated within the cylinder bore, it is generally required to reduce the weight of the piston. For this reason, there has been proposed a piston formed of an aluminum alloy and designed to have a wall thickness as small as possible. On the other hand, the base section of the neck portion of the piston is subject to repeated application of a bending moment during a reciprocating movement of the piston. To assure the intended durability of the piston, therefore, the reduction of the wall thickness is limited. While the piston produced from a forged blank has a comparatively high degree of strength, the piston by produced by die-casing inevitably has a lower strength.
SUMMARY OF THE INVENTION
It is therefore a first object of the present invention to provide a die-cast piston for a swash plate type compressor, which piston has a sufficiently high degree of durability while having a reduced weight. A second object if the invention is to provide a method of producing such a lightweight, highly durable die-cast piston.
The first or second object may be achieved according to any one of the following modes of the present invention, each of which is numbered like the appended claims and depends from the other mode or modes, where appropriate, to indicate and clarify possible combinations of elements or technical features. It is to be understood that the present invention is not limited to the technical features or any combinations thereof which will be described for illustrative purpose only. It is to be further understood that a plurality of elements or features included in any one of the following modes of the invention are not necessarily provided all together, and that the invention may be embodied without some of the elements or features described with respect to the same mode.
(1) A die-cast piston for a swash plate type compressor including a cylinder block having a cylinder bore formed therein, the piston comprising a generally cylindrical head portion slidably movably received in the cylinder bore, and generally U-shaped neck portion having a base section and a pair of substantially parallel arm sections which extend from the base section, the die-cast piston being characterized in that the base section has an inner surface including at least one as-cast surface area formed in a die-casting process.
In the die-cast piston constructed according to the above mode (1) of this invention wherein the inner surface of the base section of the neck portion includes an as-cast surface area or areas, the base section has an increased degree of durability or strength. Generally, a die-cast article or product has a chilled layer adjacent to its surface, and the chilled layer left unmachined after the die casting process is effective to improve the strength of the die-cast article. The chilled layer is formed by rapid cooling and solidification of a molten mass of iron in a casting mold, at a portion of the molten mass in contact with the inner surface of the mold which defines a mold cavity. The chilled layer is characterized by a discontinuous change in the crystallization ratio of the primary crystal or &agr;-phase (proeutectic) and the eutectic silicon with respect to each other. Since the chilled layer has high values of hardness and strength, the presence of the chilled layer adjacent to the inner surface of the base section of the neck portion is effective to increase the bending strength of the base section, particularly, the durability of the base section. The inner surface of the base section, which is located close to the outer circumferential surface of the swash plate of the compressor, is conventionally subjected to a machining operation, which results in the removal of the chilled layer. In the piston according to the present invention, however, the chilled layer adjacent to at least a portion of the inner surface of the base section is left unmachined, so as to provide at least one as-cast surface area, so that, like a forged piston, the present die-cast piston has sufficiently high degrees of durability and strength owing to the chilled layer while having a significantly reduced weight. Although the inner surface of the base section is preferably left as-cast over a surface area as large as possible, the inner surface may be required to be more or less machined at some area, for the purpose of removing a rib or casting fins. A die-cast blank that is subjected to machining and other working operations to produce the piston is usually provided with a reinforcing rib or ribs to assure accurate and efficient machining of the blank. The casing fins are inevitably formed on the blank, at a parting plane of the mold at which the mold halves are butted together to define the mold cavity. Where the inner surface of the base section is partially machined, the surface area to be machined is desirably minimized to maximize the total as-cast surface area for maximizing the durability and strength of the base section of the neck portion.
(2) A die-cast piston according to the above mode (1), wherein the inner surface includes a central machined surface area and a pair of as-cast surface areas, the central machined surface area being located at an intermediate portion of the inner surface as viewed in a direction perpendicular to a centerline of the piston which passes a center of the generally cylindrical head portion, the central machined surface area being formed as a result of a machining operation in the intermediate portion of the inner surface, and wherein the pair of as-cast surface areas are located on opposite sides of the central machined surface area as viewed in the direction.
The die-cast blank which is processed to produce the die-cast piston is generally provided with reinforcing rib or ribs for the purpose of preventing thermal strains of the blank during heat treatment thereof and reducing elastic deformation during machining operations on the blank. For instance, a rib is formed so as to extend between a pair of arm sections which extend from a base section of a neck portion of the blank in a direction perpendicular to the centerline of the blank which passes the center of the head portion, so that the rib connects the base section and the two arm sections. The rib is formed so as to extend from a central portion of the inner surface of the base section, which central portion is located an intermediate portion of the inner surface as viewed in the direction perpendicular to the centerline. The rib is eventually removed by a machining operation, so that the die-cast piston does not have the rib. Accordingly, the central portion of the inner surface of the base section of the blank must be subjected to a machining operation to remove the rib, so that the inner surface of the base section inevitably has a centr
Fukushima Shigeo
Hoshida Takahiro
Kato Takayuki
Takamatsu Masato
Kabushiki Kaisha Toyoda Jidoshokki Seisakusho
Lee Kevin
Woodcock & Washburn LLP
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