91 Failure Analysis and Recovery of a 50- mm Highly Elastic Intermetallic NiTi Ball  Bearing for an ISS Application  

Christopher DellaCorte*, S. Adam Howard * and Lewis E. Moore III† 

Abstract  

Ball bearings used inside the ISS Distillation Assembly centrifuge require superior corrosion and shock  resistance to withstand acidic wastewater exposure and heavy sp acecraft launch related loads. These  requirements challenge conventional steel bearings and provide an ideal pathfinder application for 50 -mm  bore, deep- groove ball bearings made from the corrosion immune and highly elastic intermetallic material  60NiTi. During early ground testing in 2014 one 60NiTi bearing unexpectedly and catastrophically failed  after operating for only 200 hr. A second bearing running on the same shaft was completely unaffected.  An investigation into the root cause of the failure determined that an excessively tight press fit of the bearing outer race coupled with NiTi’s relatively low elastic modulus were key contributing factors. The  proposed failure mode was successfully replicated by experiment. To further corroborate the root cause  theory, a successful bearing life test using improved installation practices (sel ective fitting) was  conducted. The results show that NiTi bearings are sui table for space applications provided that care is  taken to accommodate their unique material characteristics.  

Introduction    Bearings, gears and other mechanical components can be made from a wide variety of materials depending upon application requirements that typically include load, temperature, environment and cost [1, 2] . Often, one overriding performance requirement dictates that a specific material be selected. In  cases where multiple requirements preclude certain classes of materials, practical des igns can become  difficult.    For instance, if a bearing must operate at high temperature but also be lightweight neither pl astic nor  steel would suffice. In such cases, the designer might have to  turn to higher cost ceramics. Alternatively,  the application requirements could be altered through system design changes to accommodate a lower  cost material. A common engineering solution for the aforementioned high temperature example is to employ active cooling which enables the use of low -density plastics or non- ferrous alloys.  Such  accommodations themselves add cost and complexity and in many aerospace applications are not practical.    Another example where conventional bearing materials fall short are applications that simultaneously require extreme corrosion resistance and tolerance to severe static loads (shock events). Taken  separately, these attributes can be found in austenitic stainless steels or superalloys (corrosion resistance) and ceramics or high carbide tool s teels (shock load resistance). Taken tog ether, these  attributes cannot be provided by existing bearing materials [3] .   Over the last decade NASA has been working to develop a new class of bearing materials based upon Nickel- Titanium to address these and other unmet challenges encountered in aeronautics and space  mechanism bearing applications [4- 6]. As an example, 60NiTi (60 weight% Ni, 40 weight% Ti) is hard,  highly corrosion resistant, non- magnetic, electrically conductive, wear resistant, readily machined in the  annealed state and exhibits  good tribological properties. 60NiTi is also superelastic exhibiting a moderate                                                     * NASA Glenn Research Center, Cleveland, OH   † NASA Marshall Space Flight Center, Huntsville, AL  

Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016