With the introduction of High Hot Hardness (H³) gear steels into aircraft transmissions, there is a growing need to evaluate gear related material properties at high temperatures. Knowing these characteristics would allow the design and implementation of gear boxes on aircraft that are capable of operating at high temperatures, which would in turn reduce the weight of transmission lubricant and on-board lubricant cooling systems. This would directly and favorably impact the payload of the aircraft.

The Gear Research Institute has been in the forefront of evaluating H³ gear steels. Shown below is a set-up, on a current project, sponsored by an aircraft engine and transmission manufacturer, to characterize the bending fatigue strength at temperatures around 300 degrees F. The first photograph shows the set-up immersed in hot oil and the test in progress, while the second photograph shows the set-up with the oil drained out. GRI has conducted these tests at even higher temperatures, utilizing other heating mediums.