Rotary Geared Actuators (RGAs) are used on a number of new fighter aircraft, including the F-35, where they are used for the leading edge flap actuator system (LEFAS) and the wing fold actuator system (WFAS), as well as the weapons bay door. All of these actuators are constructed in essentially the same way, with a combination of cadmium (Cd)-plated high strength steel for the ring gears and gearbox and a Cd-plated high strength steel or nickel-cobalt-chromium-molybdenum alloy pins. This gear design has been a source of a large number of stress corrosion cracking (SCC) failures on the F-18 wing fold.

The objective of this program was to validate the use of S53 corrosion resistant steel in place of the Cd-plated gears, which would eliminate the need for Cd plating and should also reduce the incidence of SCC.

Technology Description

S53 was developed by computational metallurgy under SERDP project WP-1224 by QuesTek Innovations LLC and was demonstrated for use on landing gear under ESTCP project WP-200304. The material was developed specifically for landing gear and is mechanically equivalent to 300M high-strength steel, but with stainless steel qualities. The ultimate strength is 280 ksi with corrosion resistance approaching 15-5 PH stainless steel. It is much stronger than any stainless steel available today.  In principle, S53 is a valid replacement material for the major components of the RGAs used in LEFAS and WFAS units.

Demonstration Results

Initial evaluations concluded that S53 has promise as a gear material for applications that do not require a carburized surface. Since it is a stainless steel, S53 becomes austenitic with excess carbon, but there may be other ways to create a hard case. S53 does not need the additional corrosion protection of a sacrificial coating, and has good corrosion performance with a chrome-free adhesion promoter, a chrome-free primer, and topcoat. In the fully hardened state, the mechanical properties of S53 are close to those of 4340M. In rolling/sliding gear testing, S53 shows significantly less damage and causes less distress to counterface materials than does 4340M. Although this testing was limited, it shows that S53 is likely to be a superior material for gears that do not experience high enough Hertzian stresses to require carburization.  From the standpoint of producibility, S53 undergoes higher growth on heat treating than is seen in 4340M and similar steels. Although this growth is isotropic, it will require significant testing to ensure that S53 gears can be designed and fabricated accurately. S53 is more expensive than low carbon ultrahigh strength steels such as 4340M, because of its use of high alloying elements and a more expensive heat treat regimen. However, the material does not require sacrificial coatings for corrosion control.

Implementation Issues

As the F-35 RGA designs matured, it became clear that S53 could no longer meet the mechanical requirements, and the speed of the F-35 Low Rate Initial Production program made it necessary for the original equipment manufacturers (OEMs) to default to a combination of bushings and coated steels to meet program milestones. Since this meant that S53 would not be implemented in the program, this project was terminated early.