399 Resolution for Fretting Wear Cont amination on Cryogenic Mechanism 

Charles S. Clark*   

Abstract 

The Near infrared camera (NIRCam) instrument for NASA is one of four science instruments to be  installed into the Integrated Science Instrument Module of the James Webb Space Telescope (JWST) which is intended to conduct scientific observations over a five year mission. The NIRCam instrument incorporates multiple mechanisms that perform specific tasks as part of the observatory ground testing, instrument commissioning, and on-orbit science and diagnostics—all of which must operate between 293 and 37 K and be tested to typical launch and space environment standards.  Two of these mechanisms, the pupil imaging lens assembly (PIL) and filter wheel assembly (FWA), use common bearing mounts designed for operation at ambient and cryo temperatures. Modifications to the existing bearing mounts were developed to address fretting damage and associated contamination between the bearing race inner diameter and fixed shaft interface. Comparative proto-flight level vibration testing of four (4) new design configurations was performed alongside a control configuration similar to the original design. To ensure the trial tests simulate worst case environments, the setup went through the equivalent of five 3-axis vibration tests, one single cryo cycle, and a post cryo-cycle vibe test. The final vibe test was run in a 5% relative humidity environment as requested by the customer to reflect latest thinking of the actual JWST launch environment. This paper presents details of the investigation, re-design trades, and trial testing that demonstrated that a titanium shaft with a diamond-like-coating along with a Nitronic-60 sleeve was the preferred configuration of the bearing mount design for both the FWA and PIL units.   Introduction 

The near infrared camera (NIRCam) is one of five instruments aboard the James Webb Space Telescope (JWST) observatory which will be conducting deep space scientific observations from the second Lagrangian point of the Sun-Earth orbit at a passively cooled temperature of 37 K. The NIRCam instrument will process the light from the JWST 6.5-meter primary mirror at wavelengths of 0.6 to 5.0 microns. The NIRCam instrument is designed with two optical benches constructed of beryllium that are mirror copies of each other as shown in Figure 1. Each of the two NIRCam optical benches is configured with optical elements that divide the science beam into a shortwave and longwave path. The filter wheel assembly (FWA) and the pupil imaging lens assembly (PIL) are two different types of mechanisms being developed for the NIRCam instrument to support on-orbit calibration of the JWST observatory as well as conduct mission scientific observations. There are four FWA assemblies and two PIL assemblies installed on the NIRCam instrument, with two FWAs and one PIL located on each optical bench. As shown in Figure 1, one FWA is installed on the longwave path, and one FWA and one PIL are on the shortwave path.                                                    * Lockheed Martin Space Systems Company - Advanced Technology Center, Palo Alto, CA 

Proceedings of the 41st Aerospace Mechanisms Symposium, Jet Propulsion Laboratory, May 16-18, 2012