Abstract: A screw compressor is connected to a motor to be driven by the motor even during periods of low compressed air consumption. During such periods, the screw compressor is at least partially unloaded to make it easier and less costly to drive the compressor. The unloading is performed by removing air from the compressor. Preferably, that is done by communicating the air inlet of a small capacity vacuum device with the air outlet of the screw compressor. Suction from the vacuum device is transmitted to the air outlet of the screw compressor to suck air out of the screw compressor to reduce the engine horsepower needed to rotate the screw compressor. The vacuum device can also be used to boost the air volume and/or the air pressure. The system can be used in a drilling rig which drills holes in the ground. The screw compressor can be unloaded during start up of the motor by briefly driving the vacuum device by pressurized liquid from a pre-pressurized hydraulic accumulator.

Abstract: A percussive rock drill bit includes a steel bit body and carbide button inserts mounted in a front cutting face thereof. The bit body includes a passage intersecting the front cutting face for conducting a flushing/cooling medium to the front cutting face. The flushing/cooling medium travels to an outer periphery of the bit body and travels upwardly within flushing grooves formed in the outer periphery of the bit body. Portions of the bit body situated between successive ones of the flushing grooves constitute wings of the bit body. Some of the button inserts constitute gauge inserts. Only a gauge insert is mounted in each wing. At least some, possibly all, of the gauge inserts are offset circumferentially from a center of the respective wing toward a trailing end of the wing.