Improving the Fatigue Strength of Helicopter Gears by Heat Treatment

The US Army, Aviation Applied Technology Directorate has established a goal to improve the power density and life of helicopter transmissions. If the fatigue lives of the currently used gears can be improved by a standard manufacturing process such as heat treatment, costly redesign can be avoided. This paper reports on the work conducted under a US Army Phase I SBIR program, with the goal being a 25% improvement in the bend fatigue of carburized Pyrowear 53 steel gears. This program used a combination of process simulation and experimental materials testing, with the quenching process being the primary variable being examined as a means of achieving the project goal. The DANTE heat treat simulation software was applied to compare predicted residual stress levels for conventional oil quenching and intensive quenching of notched test pieces. In conjunction with the simulations, actual test pieces were machined and heat treated, and then tested in three point bending to simulate stresses in the root of a gear caused by bending. This paper presents results of the simulations and bend fatigue tests.