a compressor has a plurality of cylinder bores defined around a rotary shaft in a cylinder block and a plurality of double - headed pistons accommodated in the respective cylinder bores . the pistons compress refrigerant gas in a plurality of front and rear compression chambers defined in the bores in the front and rear of the respective pistons according to the rotation of a cam plate mounted on the rotary shaft . the compressor has bearing that receive thrust loads applied to the rotary shaft during operation of the compressor . the pistons create a net thrust load determined by the difference between the maximum of the sum of loads applied to the front sides of the pistons by the refrigerant gas in the front compression chambers and the maximum of the sum of loads applied to the rear sides of the pistons by the refrigerant gas in the rear compression chambers . the pistons transfer the net thrust load to the rotary shaft in a predetermined direction .