Abstract: A method of controlling the air output of a screw compressor, wherein the average air output of the compressor during operation is adapted to air consumption by throttling the flow of suction air or by connecting the compressor alternately to full air output capacity and idle operation capacity. In order to reduce the power consumption of the compressor, the air output is controlled first by throttling the flow of suction air and by simultaneously measuring underpressure in the suction conduit of the compressor. Thereafter the compressor is controlled so as to operate alternately at full air output capacity and idle operation capacity, and the pressure rise rate in the air-pressure network is measured when the compressor operates at full output capacity and the pressure fall rate is measured when the compressor operates at idle operation capacity.

Abstract: A cog-belt-driven power transmission device comprising a belt pulley having a cogging on the outer periphery and a belt having a corresponding cogging on the inner surface. In order to suppress the noise produced during rotation of the belt pulley one or more discharge channels for air entrapped between the coggings are made in the cogging of the belt pulley and/or the belt, said discharge channels connecting the cog gaps to the atmosphere.