This application is based on Japanese Patent Application No. 11-247112 filed Sep. 1, 1999, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a piston for a swash plate type compressor, and more particularly to techniques for reducing the weight of the piston.
2. Discussion of the Related Art
As a piston used in a swash plate type compressor, there are known a single-headed piston and a double-headed piston. The single-headed piston includes a head portion which is slidably fitted in a cylinder bore formed in a cylinder block of the compressor and an engaging portion formed integrally with the head portion for engaging a swash plate. The double-headed piston includes two head portions on the opposite sides of a single engaging portion. Since the piston is reciprocated within the cylinder bore, it is desirable to reduce the weight of the piston. It is generally required to reduce the weight of the single- or double- headed piston used in a swash plate type compressor of fixed capacity type wherein the angle of inclination of the swash plate with respect to a plane perpendicular to the axis of the drive shaft of the compressor is fixed. The single-headed piston is usually used in a swash plate type compressor of variable capacity type wherein the angle of inclination of the swash plate is variable to change the discharge capacity of the compressor. When the single-headed piston is used in the variable capacity type swash plate compressor, it is particularly required to reduce its weight in order to achieve a stable operation of the compressor and reduce the noise of the compressor during its operation.
JP-A-10-159725 discloses a technique of reducing the weight of the piston. Described in detail, this publication discloses a method of producing a single-headed piston with a hollow head portion, by closing an open end of a cylindrical body portion of the piston with a closure member. The piston produced according to this method has a considerably reduced weight. However, the weight of the piston cannot be reduced to a satisfactory extent which permits the piston to be used in the swash plate type compressor of variable capacity type wherein the drive shaft is required to be rotated at a relatively high speed to achieve high operating performance of the compressor.
The present invention was made in the light of the background art described above. It is a first object of the present invention to provide a piston for a swash plate type compressor having a significantly reduced weight.
It is a second object of the present invention to provide a method of producing a blank used for manufacturing the piston of the invention.
The first object indicated above may be achieved according to any one of the following forms or modes of the present invention, each of which is numbered like the appended claims and depend from the other form or forms, where appropriate, to indicate and clarify possible combinations of technical features of the present invention, for easier understanding of the invention. It is to be understood that the present invention is not limited to the technical features and their combinations described below. It is also to be understood that any technical feature described below in combination with other technical features may be a subject matter of the present invention, independently of those other technical features.
(1) A piston for a swash plate type compressor including a head portion which includes a cylindrical body portion whose open end is closed by a closure member, and an engaging portion which is formed integrally with the head portion on the side remote from the open end and which engages a swash plate of the compressor, wherein: the body portion includes a hollow cylindrical section whose inner circumferential surface is provided with a plurality of axially extending reinforcing projections, each of which protrudes from the inner circumferential surface in a radially inward direction of the hollow cylindrical section and which extends in a direction parallel to a centerline of the hollow cylindrical section over a substantially entire axial length thereof, the closure member being fixed to the body portion such that the closure member is held in abutting contact with an end face of each of the reinforcing projections on the side of the open end of the body portion.
The piston used in the compressor receives a pressure of a compressed gas at its end face which partially defines a pressurizing chamber in the compressor. In the piston constructed according to the present invention, the end face of the piston is provided by one of opposite major surfaces of the closure member (outer end face), on which a considerably high pressure of the gas acts in a direction in which the closure member is pushed into the hollow cylindrical section during manufacture, of the piston. In view of this, it is preferable that the closure member be fixed to the hollow cylindrical section such that the closure member is received and supported by the hollow cylindrical section at the other major surface (inner end face) opposite to the above-indicated outer end face.
If the wall thickness of the hollow cylindrical section is reduced to a value in a range of 1xcx9c2 mm for reducing the weight of the piston, the closure member cannot be received and supported by the hollow cylindrical section with high stability. For fixing the closure member to the hollow cylindrical section with their axes being aligned with each other, at least an axial portion of the closure member needs to be fitted in the hollow cylindrical section. For the following reasons, the reduction of the wall thickness of the hollow cylindrical section, however, makes it difficult that the closure member is fitted in the hollow cylindrical section with its inner end face being securely received and held in abutting contact with the hollow cylindrical section. When the hollow cylindrical section includes a large-diameter axial end portion formed on the side of its open end, which large-diameter axial end portion has an inside diameter larger than that of the other axial portion of the hollow cylindrical section, the closure member is fixed to the hollow cylindrical section such that the closure member is fitted in the large-diameter end portion such that a shoulder formed between the large-diameter end portion and the adjacent axial portion is held in abutting contact with the closure member. According to this arrangement, the closure member is held in position by the shoulder of the hollow cylindrical section while the axes of the hollow cylindrical section and the closure member are aligned with each other. When the wall thickness of the hollow cylindrical section is relatively small, the maximum inside diameter of the axial end portion is limited, so that the radial dimension of the shoulder is inevitably small. Where the large-diameter axial end portion has a low degree of concentricity with the hollow cylindrical section, the radial dimension of the shoulder is considerably small at a local circumferential portion thereof. In this case, the closure member fitted in the hollow cylindrical section cannot be sufficiently supported by the shoulder at the local circumferential portion. Accordingly, the closure member may undesirably be pushed into inner axial portion of the hollow cylindrical section by the pressure of the compressed gas acting thereon.
In the piston constructed according to the present invention wherein the hollow cylindrical section is provided with a plurality of axially extending reinforcing projections formed on its inner circumferential surface, the weight of the piston can be sufficiently reduced by reducing the wall thickness of the hollow cylindrical section at its circumferential parts in which the reinforcing projections are not formed, while permitting the closure member to be fixedly supported by the reinforcing projections. Accordingly, the present arrangement is effective to not only reduce the weight of the piston by reducing the wall thickness of the hollow cylindrical section, but also prevent the closure member from being pushed into the hollow cylindrical section upon exposure to the pressure of the compressed gas.
By suitably determining the cross sectional shape, location, and number of the reinforcing projections, the rigidity and mechanical strength of the hollow cylindrical section can be significantly increased, as compared with those of the hollow cylindrical section which has a constant wall thickness. This is based on a fact that the rigidity and mechanical strength of a relatively thin-walled structure can be increased when the structure is provided with reinforcing ribs. In this respect, the present arrangement is effective to improve the mechanical strength of the head portion of the piston while reducing its weight.
The head portion of the piston whose hollow cylindrical section is provided with the axially extending reinforcing projections as described above can be easily produced by die-casting. When the head portion of the piston whose hollow cylindrical section having a constant small wall thickness is produced by die-casting, the cast thin-walled structure has a relatively large circumferential surface area. It is, however, difficult to die-cast such a thin-walled structure since a molten metal does not easily flow through a mold cavity which has a relatively small radial dimension corresponding to the small wall thickness of the hollow cylindrical section and which has a relatively large diameter, whereby the mold cavity may not be uniformly and entirely filled with the molten metal. Accordingly, there is a limit to reduce the weight of the head portion of the piston by reducing the wall thickness of the hollow cylindrical section, due to the above-mentioned difficulty in the process of die-casting. In contrast, when the head portion of the piston whose hollow cylindrical section has the reinforcing projections is formed by die-casting, the molten metal comparatively easily flow through circumferential portions of the mold cavity, which portions have a relatively large radial dimension corresponding to the radial dimension of the reinforcing projections, so that the mold cavity can be uniformly and entirely filled with the molten metal, resulting in easy die-casting of the head portion of the piston. Accordingly, the lightweight piston can be produced efficiently by providing the reinforcing projections (thick-walled circumferential portions) on the inner circumferential surface of the hollow cylindrical section, rather than by reducing the wall thickness of the hollow cylindrical section to a constant small value.
(2) A piston according to the above mode (1), wherein the plurality of reinforcing projections are equally spaced apart from each other in the circumferential direction of the hollow cylindrical section
(3) A piston according to the above mode (1) or (2), wherein each of the reinforcing projections protrudes from the inner circumferential surface in the radially inward direction of the hollow cylindrical portion by an amount which is not greater than 300% of a wall thickness of the hollow cylindrical section, and each of the reinforcing projections has a circumferential dimension as measured in the circumferential direction of the hollow cylindrical section, which is larger than the amount of protrusion from the inner circumferential surface, each of the reinforcing projections providing a thick-walled circumferential portion having a wall thickness larger than a nominal wall thickness of the hollow cylindrical section.
(4) A piston according to the above form (1) or (2), wherein each of the reinforcing projections is in the form of a rib which protrudes from the inner circumferential surface in the radially inward direction of the hollow cylindrical section by an amount which is larger than a circumferential dimension of the rib as measured in the circumferential direction of the hollow cylindrical section.
The hollow cylindrical section may have both of the projections according to the mode (3) and the ribs according to the above mode (4).
(5) A piston according to any one of the modes (1)-(4), wherein the closure member has a plurality of fitting protrusions formed on its inner end face, for engagement with the body portion so as to prevent relative rotation of the closure member and the body portion.
In the piston according to the above mode (5) of the present invention, the relative rotation of the closure member and the body portion is prevented by the engagement of the fitting protrusions of the closure member with the body portion, facilitating the machining operation which is effected during manufacture of the piston from a blank. For instance, a closing member which gives the closure member may have a holding portion formed on its outer end face remote from the fitting protrusions, so that the blank is held by a suitable chuck at the holding portion of the closing member. When the blank held by the chuck is rotated to perform the machining operation thereon, the closing member and the body portion are effectively prevented from being rotated relative to each other by the engagement of the fitting protrusions of the closing member with the body portion. The holding portion may be cut away from the closing member after the holding portion has achieved its function.
(6) A piston for a swash plate type compressor, including a hollow cylindrical head portion and an engaging portion which is formed integrally with the head portion and which engages a swash plate of the compressor, wherein the piston includes a plurality of axially extending reinforcing projections which are formed on an inner circumferential surface of the hollow cylindrical head portion, so as to extend in a direction parallel to a centerline of the hollow cylindrical head portion over a substantially entire axial length thereof.
The piston according to the above mode (6) is substantially the same as the piston according to the above mode (1), except that the piston according to the mode (6) does not include the feature that the end faces of the reinforcing projections are held in abutting contact with the inner end face of the closure member. This mode (6) aims to improve the rigidity and mechanical strength of the hollow cylindrical section itself. It is noted that the piston according to this mode (6) may employ the technical feature according to any one of the above modes (1) through (5).
The second object indicated above may be achieved according to the following mode (7) of the invention.
(7) A method of producing a blank used for manufacturing a piston for a swash plate type compressor, as defined in any one of the modes (1)-(6), comprising the steps of: preparing a casting mold consisting of two mold halves which define a parting plane at which the two mold halves are spaced apart from each other and butted together and which have respective molding surfaces; inserting, into the casting mold, a pair of slide cores which are slidably movable in a direction perpendicular to the parting plane, each of the slide cores having an outer circumferential surface whose configuration follows that of an inner circumferential surface of the hollow cylindrical section of the piston, the outer circumferential surface of each of the slide cores cooperating with the molding surfaces of the mold halves to define a mold cavity therebetween; injecting a molten metal into the mold cavity to form the blank for the piston; retracting the slide cores out of the casting mold; and moving the mold halves apart from each other at the parting plane to remove the blank formed in the mold cavity.
This mode (7) of the present invention permits easy production of the blank for manufacturing the piston defined in any one of the above modes (1) through (6).