Reciprocating compressors are widely used in refrigerators, for example (Patent Literature 1). FIG. 12 is a longitudinal cross-sectional view of the main part of a typical reciprocating compressor. A reciprocating compressor 200 includes, as main elements, a closed casing 101, a compression mechanism 103 disposed in the closed casing 101, and a motor 105 disposed in the closed casing 101 to drive the compression mechanism 103.
The compression mechanism 103 has a cylinder 112, a piston 114, a connecting rod 118, a shaft 120, and a bearing 122. The shaft 120 has a main shaft portion 124 and an eccentric portion 125 provided on the upper part of the main shaft portion 124. The main shaft portion 124 includes a journal portion 126 located inside the bearing 122, and a portion 127 projecting downwardly below the bearing 122 and fixed to the rotor of the motor 105. The eccentric portion 125 and the piston 114 are connected by the connecting rod 118. The power of the motor 105 is transmitted to the piston 114 through the shaft 120 and the connecting rod 118. As the piston 114 reciprocates in the cylinder 112, a refrigerant is compressed.
The load of the compressed refrigerant acts on the shaft 120 in the direction of an arrow A through the connecting rod 118 and the piston 114. The journal portion 126 is long enough to support large loads. The longer the journal portion 126 is, however, the more friction losses between the shaft 120 and the bearing 122 tend to increase. Since reciprocating compressors are characterized in that they undergo significant changes in the magnitude of the load during one cycle, the longer journal portion 126 may produce opposite effects. That is, the longer journal portion 126 works effectively when a large load is applied, but the longer journal portion 126 causes an increase in friction losses when a small load is applied.
In order to solve this problem, conventionally, a reduced diameter portion 128 with a smaller diameter is formed in the main shaft portion 124. This reduced diameter portion 128 achieves reduction of friction losses between the shaft 120 and the bearing 122 without impairing the ability of the bearing 122 to support the shaft 120.