In a piston-type compressor which has a plurality of cylinders

　　In a piston-type compressor which has a plurality of cylinders, usually, a valve plate device is interposed between a cylinder head having a suction chamber and a discharge polyurea Grease chamber and a cylinder block having cylinder bores. This valve plate device has a valve plate having a suction hole communicating between the suction chamber and the cylinder bore and a discharge hole communicating between the cylinder bore and the discharge chamber, a suction valve disposed on a surface of the valve plate facing the cylinder block for opening and closing the suction hole, and a discharge valve disposed on a surface of the compressor valves plate facing the cylinder head for opening and closing the discharge hole.

More concretely, when a piston is in a suction stroke, the suction valve opens the suction hole of the valve plate and the discharge valve closes the discharge hole of the valve plate, and when the piston is in a discharge stroke, the suction valve closes the suction hole of the valve plate and the discharge valve opens the discharge hole of the valve plate, thereby preventing back flow of gas. The lubricant oil having adhered to the valve plate enters into a portion between the valve plate and the suction valve, and the suction valve is made difficult to be opened by a sticking force due to the surface tension, the adhesion force, etc. of the lubricant oil.

As a result, in order to open the suction valve, it is necessary to add a force excessively by an amount of the sticking force due to the surface tension, the adhesion force, etc. of the lubricant oil in addition to a sticking force due to a rigidity of the suction valve itself, and in order to obtain this force, it becomes necessary to enlarge a pressure difference between the inside of the cylinder bore and the inside of the suction chamber, and whereby, the piston must perform an excessive work.

Where, as for the discharge valve side, in order to prevent an excessive compression that may occur by a condition where the discharge valve becomes difficult to be opened by the lubricant oil, Patent document 1 discloses a structure wherein a gas filling concave portion is formed on a portion of the valve plate facing the discharge valve.