Effect of Test Atmosphere on Moving Mechanical Assembl y Test Performance 

D. J. Ca rré* and P. A.  Bertrand*  

Abstract 

Satellites and many of the moving mechanical assemblies (MM As) on board  operate under the lowpressure conditions of the  orbital  envi ronment. In  order to asse ss the lifetim e pe rform ance of the se  device s, it is desirable to  test them under the same con ditions they will e xperien ce during mi ssion  operation. Howeve r, ma ny times contracto rs test MMAs i n a nitroge n gas enviro nment at ~1 atm  pressure, assumin g the t est results will be comp arabl e to th ose obtaine d in a vacu um te st du e to the   inertne ss of the nitrogen gas. We  were  concerned that impu rities i n nitroge n gas, such as ox ygen eve n at  ≤1-ppm  level s, would  lead to different chemist ry and rates of rea ction compared to  vacu um condition s,  where the ox ygen levels are lo wer by  at least a fact or of 10 00. In order to test this hypothesi s, we  performed a set of experiment s using our in-house, thrust -bearing we ar-test facility with  hydrocarbon oil s  contai ning a n aryl pho sphate ester additi ve mixture. We  used a load that re sulted in a Hertzian stress of  1.52 GPa, at the high end of typica l ball-be aring stresse s. We used te st time to failure to asse ss  perfo rman ce differen ces. The wear te sts lasted at le ast a facto r of 5 times lo nger in nitro gen gas th an in   vacuum. Sin ce the tempe rature s of the vacuum and nitrogen test s were mat ched, this impli es that the re  are chemistry differen ces occurring  at th e metal surfaces. We conclude from our test results that nitro gen  gas at ~1 at m pressure i s not a reali stic substitute for vacuum condi tions for lif e testing of MMAs that will  operate und er orbital lo w-pressu re conditions. 

Introduc tion 

The external pressure in which i nterplanetary spa cecraft and Earth  satellite s operate is typicall y <1 x 10-4  Pa. Many of the moving  mechanical a ssemblie s (MM As) are vented to the exte rnal environm ent so  that  the extern al and inte rnal MMA pre ssures a re equili brated. It is desirable to perfo rm life t ests of these  MMAs under the l ow-pressure conditions in  whi ch they will operate [1]. However,  due to budget  constraints, contra ctors often te st the MMAs in a nea r-ambient pre ssure, nitroge n gas envi ronment. The   assumptio n is that the  relatively inert n itrogen  gas will give re sults that are com parable to th ose obtain ed  in a lo w-pressure (va cuum) environm ent. We have  held the belief that there  are differen ces in chemistry  betwe en the two enviro nments, as well as obviou s therma l differe nces due to th e presen ce or absence of   conve ction. F or exa mple, i n nitro gen gas with an  oxygen content o f ~1 ppm, a  low im purity level, there  is  approximatel y a thousa nd times more oxygen than there i s in a l ow-pressu re test at a pressure of 1.3 x  10-4 Pa. This could lead to signifi cant differen ces in th e rate s of su rface rea ctions, whi ch could adversely   affect pe rformance a nd invalidate the  results of th e nitrogen gas testing. In addition, the  source of the   nitroge n ga s coul d play a pro nounced role. Blo w-off gas from  a liquid -nitrogen stora ge tank typically  contai ns more oxygen tha n 1 pp m. The  presence or absen ce of water in th e nitrogen gas could al so pl ay  a role i n surface reactions. To o ur kno wledge, p rior to ou r work, no system atic te sts that compa re liquidlubricated M MA perform ance in vacuu m and nitrog en gas h ave been repo rted. There hav e been re ports  in the literature of stand ard tests of  specific lub ricants that show differen ces in test life between vacu um  and nitrog en gas [2] and betwe en dry air and  nitrogen gas environment s [3]. Also, studie s have  been  reported th at sho w that solid-lub ricated MMAs p erform significantl y better in nit rogen gas than in va cuum  [4,5]. 

Hydro carbon oils u sed to lubricate M MAs in spacecraft typically contain a nti-wear additives to red uce the  wear of the i nteracting m etal su rfaces. In previou s work, we h ave sho wn that the extre me pre ssure  additive, lea d naphthe nate, disa ppeared q uickly from the  lubricant i n ball beari ngs under vacuum  conditions. We determi ned that this was due to che mical reactio ns on the su rface of the met al part s [6].   357                                                  * The Aero space Corp oratio n, El Segund o, CA  

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