depe nding on the ap plication. This i s especially  evident whe n designing a mechani sm intende d to  function in di stinctly different m odes. Passive isola tion mode re quires ma ny ord ers of magnitude lo wer  parasiti c torq ue pe rform ance than i s required to be supplied in t he drive n or safe mo des. In orde r to  achieve req uired me chanism strength and stiffnes s while still maintaini ng de sired p erforman ce,  component selectio n, arra ngem ent an d manufa cturing mu st be evaluated giv en the more stringe nt  requirements.   

Significa nt redesign and repacka ging of the drive and safe m ode m echanisms occurred during the co urse  of this developme nt effort. The first d esign iteration was comp rised of drive and safe mode drum s  mounte d to the Rol amite cylinde rs via large diam eter rollin g element bea rings. Ho weve r, the large - diamete r bearings com bined with the drive and safe  mode tape prel oad re sulted in an analytical mode l  with un acce ptably larg e friction. A mo re acce ptable  solutio n was foun d that  reduced the  friction to  accepta ble levels. The d rums a re currently mounte d on smalle r diamete r shafts with hi gher p recision  bearings. On e desirable/ non-criti cal sa fe mode syst em require ment was re laxed in ord er to redu ce  required tape  preload an d therefo re red uce be aring friction even furthe r. The re quired packa ging location   changed for both the drive and safe mode mecha nisms from each end of the Rolamite cylinde r to within  the cylinde r itself. The rep ackagi ng wa s gene rally st raightforward, but underscor es the nee d to provide   signifi cant vol ume for me chanisms at the  conceptual level whe n developing a n ew syste m. 

Rollin g elem ent beari ngs in spa ceflight mechani sms of th is size are tradition ally con sidered to contri bute  negligi bly to overall sy stem friction. Quantifying th e total available friction bu dget from all  compo nents  early in th e design process helps to esta blish a valid design path and reduce the chan ce of ne cessary   redesign. 

Initial testing  of the enginee ring u nit has shown a preferre d angula r position and the refore exhibit s  signifi cant sp ring rate. An  investigation  revealed tha t the Elgiloy tapes were slightly curved  in the free  state. The m anufa cturing pro cess and packa ging of the materi al is such that t he tape s are cold-rolled to  the desi red thickne ss and  then spool ed for shipm ent. The rollin g pro cess im parts high su rface resi dual  stresses that lead to yieldi ng of the material d uring spoolin g. Two oppo sing equally curve d tapes id eally  still maintain a con stant strain ene rgy state on the Rola mite. Howe ver, tapes with differen ces in curvatu re  were found o n the unit up on clo ser inspectio n causing the unit to see k a lowe st energy state. Further  yielding of th e materi al may have be en the so urce o f the curv ature differe nce. An  altern ate tape m aterial  such as 17-7 PH SS ha s been id entified  as a potentially better mat erial choice for thi s application an d will   likely be in corporated on th e next hard ware b uild. 

Small strai n state differen ces in a tap e driven sy stem  are typically not con sidered to significantl y influence  perfo rmance. In this case, a balan ced strain state is cr itical to sy stem function ality and therefo re mu st be   characterized early in the design proce ss in orde r to bracket acce ptable materi al choi ces and geometry   tolera nce. Flex circuit tape s of multiple condu ctors laminated in Ka pton are often utilized a cross kin ematic  joints in order to reduce parasitic torque. Similarl y const ructed tapes with Be ryllium-copper conductors  have bee n desi gned for the Rolamit e in orde r to pass the req uired power and d ata harne ssing.  Prelimin ary testing of the  enginee ring unit with su ch tapes ha s indicated that  even at very low strain,  signifi cant creep occu rs. Visco-el astic loss testin g will be performe d to further qua ntify flex circuit  perfo rmance. Alternate fle x circuit de signs are currently under i nvestigatio n such as fiberglass laminatio n  and di rect liquid polyme r coating. 

In both of the  above cases, requi reme nts for e ach ne w ap plication should be considered whe n designing  with normally comm on or heritag e com ponent s since past accepta ble perfo rmance may not be the cu rrent  case. 

As is typical of spa ceflight hard ware developme nt, many trade s and co mpromi ses have o ccurred du ring  the co urse of this effort i n order to i ncorporate operational fun ctionality. Both the drive and safe m ode  mechani sms we re de signed in ord er to compile the first pe rform ance estimate. Since the Rolamite joint   must ope rate in several  diverse mo des, even g reater scruti ny was foun d to be require d of each  component a nd asse mbly.   

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