Oil-free screw compressor

An oil-free screw compressor comprising:

• a screw rotor accommodated in a rotor chamber of a casing of the compressor;
• a bearing configured to support a rotor shaft of the screw rotor;
• a shaft seal device arranged in a shaft seal space of the casing, the shaft seal space being formed between the rotor chamber and the bearing, the shaft seal device including an oil seal arranged at a position closer to the bearing and a gas seal arranged at a position closer to the rotor chamber;
• an atmosphere communication space formed between an oil seal portion formed in any one of the oil seal and the rotor shaft and a gas seal portion of the gas seal, the atmosphere communication space communicating with an atmosphere communication hole;
• and
• an oil return groove formed on the rotor shaft, the oil return groove including a return inclined surface inclined so as to expand in diameter radially outward from a gas seal-side end to an oil seal-side end of the return inclined surface,
• wherein the oil seal portion is a viscoseal including a spiral groove portion,
• wherein an inner peripheral surface of the oil seal includes an inner peripheral boundary portion that defines a part of the atmosphere communication space, on a side closer to the bearing in the atmosphere communication space, and
• wherein the oil seal-side end of the oil return groove is arranged in a vicinity of the inner peripheral boundary portion and at a position closer to the bearing than the inner peripheral boundary portion.