1. Field of the Invention
The present invention relates to a free piston for use in an electromagnetic reciprocating compressor and, more particularly, to an improved one-way air valve to be fitted on the partition wall of the free piston.
2. Description of the Related Art
Electromagnetic reciprocating compressors are generally used as air-supply devices in massage apparatuses, spraying devices, and aeration devices incorporated in sewage disposal facilities. Each electromagnetic reciprocating compressor comprises a cylinder and a free piston inserted in the cylinder and capable of reciprocating. The free piston is moved in one direction by a spring, and in the opposite direction by an electromagnet. In other words, the piston is driven back and forth within the cylinder by a drive means having the spring and the electromagnet, thus compressing air and subsequently supplying the compressed air to a device where the compressed air is needed.
A conventional electromagnetic reciprocating compressor is disclosed in Japanese Patent Publication No. 57-3226. FIG. 1 shows a portion of a free piston incorporated in the electromagnetic reciprocating compressor of the general type. As a illustrated in this figure, the piston is inserted in a cylinder, both slidably and rotatably. The partition wall, i.e., a part of the piston head, partitions the inner space of the cylinder into a pressure chamber and a open chamber. The pressure chamber is formed between the distal end of the piston head and the inner wall of the cylinder head. The open chamber is formed between the proximal end of the piston head and the inner wall of the cylinder. An air hole is formed at the partition wall of the piston head. This hole is usually closed by a disc-shaped valve made of rubber and attached to that surface of the partition wall which opposes the cylinder head. The disc-shaped valve is mounted to the partition wall by a backing plate and screws as is shown in FIG. 1, or by a retainer holding the valve and a ring fastening the retainer to the partition wall.
As is generally known, the air hole formed at the partition wall, and the disc-shaped valve constitute a one-way valve of a free piston for use in an electromagnetic reciprocating compressor. The one-way valve opens when the piston moves away from the cylinder head. When the valve thus opens, the air is introduced into the pressure chamber from the open chamber. Hence, every time the piston moves backward, fresh air flows into the pressure chamber.
As is shown in FIG. 1, a recess is formed in the front surface of the head of the free piston, and a substantially conical hole is cut in the rod-shaped portion of the piston. The recess, the conical hole, and the inner wall of the cylinder head usually form an air-compressing chamber. Air is compressed in this chamber as the piston moves toward its upper dead point. The air thus compressed within the chamber flows from the cylinder through the exhaust valve mounted on the circumference of the cylinder. As the piston further moves toward the upper dead point, it closes the exhaust valve, and the residual air is compressed within the air-compressing chamber. When the pressure of the air within the chamber rises to a predetermined value, the compressed air functions as a bumper, preventing the piston from moving further to abut against the cylinder head.
The conical hole, which is formed in the rod-shaped portion of the piston, helps to prevent the piston from being deformed while the piston is being molded by die-casting. The conical hole has such a size as to adjust appropriately the resonance frequency of the piston to that of the spring biasing the piston in the forward direction. In some cases, the conical hole need not be formed in the rod-shaped portion of the piston.
The one-way air valve is composed of a relatively great number of parts. These parts are complex and difficult to machine, and much time is required to assemble the one-way valve. Further, since the conical hole opens to the recess of the piston head, the air-compressing chamber is too large to raise the air pressure within it to a value sufficient to prevent the piston from abutting against the cylinder head.