In order to reduce windage loss in a gear box, this paper presents a method to realize low air pressure in the gear box. The gear pair is operated like a gear pump with a tip cylinder fixed at mesh approaching region. The paper focuses on temperatures of the air in the gearbox and equipment components to find out sources of heat generation in the test rig. Air adiabatic compression between gear teeth is noticed to be the mam cause of heat generation. Then, we designed a pocket at the side block to release the air between mating gear teeth. As a result, all equipment components show an obvious drop in temperature.