A mobile drilling rig includes a platform mounted on drivable ground supports. Drilling equipment is disposed on the platform and is operated by a power system that includes hydraulic pumps, a screw type air compressor, and a motor for driving the pumps and the compressor. A gearbox is common to the hydraulic pumps and the air compressor and includes an intermeshing gear arrangement for transmitting an inputted power from the motor to the pumps and the compressor. Compressed air from the compressor travels to an air reservoir in which compressor oil is separated from the air. The compressor oil is conducted to an oil inlet of the gearbox and is circulated through the gearbox to an oil outlet of the gearbox which communicates with an air inlet of the compressor.