Space shuttle orbiters use geared actuators to control the rudder/speed brake and body flap panels that are used for flight control and steering. Therefore, the gears used to position the panels are safety-critical components. Inspection of gears from flight units revealed unexpected damage to the gear working surfaces. Working as part of the NASA Safety Engineering Centerís (NESC) Independent Assessment Team, a project was executed to determine whether or not gears with defined levels of damage are fit for service.
The first two steps of the project were to establish the root cause of the damage and to replicate the damage on gear test specimens. Experiments were done using NASA Glenn Research Centerís Spur Gear Test Rigs, which were specially modified for this project. The rigs were arranged to give a controlled dithering (back-and-forth) motion to a set of gears representative of those used on orbiters. The test gears had damage fully representative of that found on the flight hardware. The damage on the working surfaces of the gears was identified as a form of fretting damage. Thus, it was established that the observed fretting damage on flight hardware was caused by many repetitions of a small-amplitude dithering motion with relatively small loads.
Gear tooth with fretting and pitting damage tested to failure using the single-tooth bending test method.
Long description of figure.
The third step of the project was to investigate the damage tolerance of gears having fretting damage. Test gears were subjected to fretting motions to impart fretting damage and then subjected to motions and loads representative of flight control requirements. After the damaged gears were exercised to simulate flight requirements, the residual strengths of the test gears were determined using a newly devised and commissioned single-tooth bending fatigue test rig. Both the damaged test gears and new gears were tested to failure to determine and compare the fatigue lives and failure modes. Several tests were done since gear bending fatigue is a probabilistic phenomenon requiring multiple tests to properly establish fatigue lives. The testing demonstrated that the bending fatigue lives of the undamaged and damaged gears were similar. Most importantly, both the new gears and the damaged gears failed at the base of the gear tooth, away from the fretting damage that was located on the contacting-flank portion of the gear teeth (see the photograph). We concluded that the fretting damage did not reduce the low-cycle-fatigue lives of the gears.
This gear testing devised and executed at Glenn demonstrated the damage tolerance of gears with fretting damage. The project and the demonstrated results were a key part of the NESC Rudder/Speed Brake Gear Margins Independent Technical Assessment supporting the safe return to flight of the space shuttle.
Find out more about this research:
Glennís Mechanical Components Branch:
NESC: http://nesc.nasa.gov/index.cfm?linkfrom=home (external site)
U.S. Army Research Laboratory at Glenn contacts:
Dr. Timothy L. Krantz, 216-433-3580, Timothy.L.Krantz@nasa.gov; and Dr. Robert F. Handschuh, 216-433-3969, Robert.F.Handschuh@nasa.gov
Glenn contact: Dr. Bradley A. Lerch, 216-433-5522, Bradley.A.Lerch@nasa.gov
Author: Dr. Timothy L. Krantz
Headquarters program office: NESC
Programs/Projects: NESC, Orbiter Rudder/Speed Brake Gear Margins Independent Technical Assessment
Last updated: October 16, 2006
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