A sliding drawer type canister was designed to contain long duration exposure facility experiments which require vacuum storage before and after space exposure. The elastomeric seals require high closing loads which are generated through camming levers and transmitted through a spring loaded pressure plate. Lubrication was provided by various dry surface coatings. Higher than expected friction required some redesign after which the assembly functioned well and provided good sealing

Werner, K. E.

English

NASA Technical Reports Server

, NSO  8518
DRAWER DRIVE FOR f3PACE _GH.U.TT.L.E_VA.C..UUMCANI,GTER1
Kenneth E. Werner
General Electric Comp_ly
Re-entry Systems Division
SUMMARY
A sliding drawer type canister was designed to contain LDEF experiments
which require vacuum storage before and after space exposure. The elastomeric
seals require high closing loads which are generated thru camming levers and
transmitted thru a spring loaded pressure plate. Lubrication was provided by
r variou_ dry surface coatings. Higher than expected friction required some
redesign after which the assembly functioned well and provided good sealing.
INTRODUCTION
_. A conceptual study for a Vacuum Exposure Control Canister (VECC) for NASA's
Space Shuttle Long Duration Exposure Facility (LDEF) resulted in the sliding
drawer concept depicted in figure 1. The experiment is carried in a tray or
_ drawer which upon closing, slides into the Vacuum chamber and the "drawer-front"
seals against the face of the chamber. The entire assembly is designed to
_ occupy one third of an LDEF experiment frame and to a 15.2-cm (6-in.) depth.
The assembly measures 83.8 x hl.3 x 15.2 cm; the chamber is 37.5 x 37.6 x 9.5 cm;
_ while the experiment drawer measures 35.6 x 34.0 x h.8 cm deep. The application
requires that the drawer may be opened and closed several times in the
-, preparation, exposure, and evaluation of an experiment. This requirement
_ eliminated the use of metallic or ceramic sealing elements in favor of
elastomeric sealing elements, despite the knowledge that such seals could not
_ maintain a hard space vacuum in the chamber. It was also decided that the
_' prototype design phase would include a full scale evaluation of three candidate
-I:_ "Gasko-Seal" designs as manufactured by Parker Seal.
_,,
_ SPECIAL DESIGN REQUIREMENT_
requirements were speci'_l to the LDEF and VECC usages.
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00000008-TSA06
https://ntrs.nasa.gov/search.jsp?R=19800015027 2020-03-21T03:08:21+00:00Z
Oeneral Requlrement_
Tho unit mu_t moot tho _pace Shuttle _nvlronmont which w_ found to bo
typical of aerospace fllgh_. The mOSt s_gnifleant single requirement its
that the mechanism l,usti'unetlonat urn,blenteondltlons, in ,_hard vacuum
at i g, and after lon< periods _n a hard vacuum at zero _:. Thl,qmoans th,_t
all slidln1_,l,embcr;_mu,_tbe provided w_th lubrication tha5 will be .guutained
fo_'lon_,})e_,io(Isin s_nee v_%_]uu/_.The othoi'C,onerlil ]'eqtt[_'(_Is_e_5_l_ID_c'tlrt{_
the dc_it_,l,is that nOnm,_gncticmt]ter!al_be uncd in th_ m,>ehanlzm_uld
structure.
S_ecifio Re_ulrements
Space
The total volume for the VECC sounds large; however the upper 9.5 cm of the
total height of 15.2 cm iS occupied by the vacuum chamber Or is swept out by the
drawer motion. The mechanism must operate in the lower 5.7 cm. The space under
the chamber must house the motor With gearing and the _echanism in the closed
position along with all electrical connectors and wiring, the programme_,
support structure and an experlment data wi,,etrough.
Seals and Loads
The candidate seals are shown in figure 2 along with the face plate
configuration. The seals, Variants of Parker Seal's "Gasko Seal" design, all
are embedded directly into the seal plate, rather than being fabricated into a
separate carrier piece as is often practiced. The integral design approach
eliminates an extra leak path. The single and double seals were carried thru to
tests, while the double lipped single Seal was dropped on the recommendation !
that this seal offered no sealing advantage and sometimes is troubled by gases
trapped between the lips. All the seal configurations were to use Parker
v720-75compound, which is a variant of DuPont "Viton"fluorocarbon and is
considered to provide the best _acuum sealing.
The required force to close the seal is a function Of seal geometry and the
seal material characteristics. The gasko seal design requires that the seal iS
compressed so that it occupies a Very high percentage of the volume available in
the seal groove. The Viton is a very tough material which resists being
compressed into a confined groove. Whereas the preliminary design of the drawer
drive was based on a more typical closing load of 613o N/m (35 ib/in) of sea],
the Viton gasko-seal combination requires a load of 20,315 N/m (116 ib/inch).
This significant increase of design load required a re-thinking of the drive
design. Although a low power drive tar_eted at 25 w_h_ w_ desirable, thi_
requirement was not hard. A firmer limitation was the space available for the
motor. This condition is somewhat alleviated by a loose requirement for closing
and opening time. Initially the design was targeted toward the 3-4 minute
range, but any time within half an hour was acceptable. Energy was more
critical than time.
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............ 00000008 TSA07
,i¸
SEAL EVALUATION
The program plan Was to eva_uato the seals _t th_ aarlloot possible date
,, while the design and fabrloatlon of the remainder of the unit wor_ in progress.
L, Aooordln_ly th_ ohambor_ f_oo p1_o, and pressure plato aooombllos wore deolgnod
_ first. Face platOs aO shown in fi_Uro 2 Wore fabrloatod with blank rases and
il, shipped to the soQI m_nufaoturor whore the grooves were out and seals molded.
The ohambOr was desienod and fobrloatod with a thlok bottom whloh Oould be later
modified to mount and aooommodato the drawOr drive elements. The pressure plate
_ assembly inOludlng the staCk_ of BellOvlllo washer springs wore designed.Individual sprln_s were loaded to a recommended 67_ of defleotlon to the flat
state. The Washers Were stacked in parallel to provide the necessary load and
• these stacks Were used i_ series to provide a deflection Judged necessary for
good load distribution. The total Seal load for the single seal is °L_,;'3(N
(4100 lbs) and for the double seal 36,_(h N (8_oo 1_,:_). Nomin_d ]oe_i_l_ I_r _h_
three spring stacks had been eStabliShed in the early deslgn release. An
examination of deflections in the face plate revealed that the ends deflected
well in excess of the allOwable .008 cm (,003 inch) flatness to assure good
sealing. Side stacks Of springs were introduced to closely match the
distribution of the applied sprln_ load with the seal loading, A breadboard
pressure plate was made up for seal evaluation testing. Figure 3 shows the
spring stack arrangements used and the tie bolted test setup, The bellev_li_
sprin_s were lubricated with dry film MoSo to avoid binding between individual
washers. The tie bolts were tightened to-provide the computed spring
deflections. The face plates - chamber face gap was measured before loading and
!i was found to eonl?o_i_o the _e_l heii_h%l;olerm_ee.O61 to .O66 em (.O2h to .O26
: in). When loaded, the design fully satisfied the seal m_muf:_e%urer's
reoommendatlons that the seal plate faces shall be closed within .008 em (.003
in).
Heli_ leak rate testing is shown in figure 4. Six tests were run using
two single seal face plate."_nd one double seal plate. Leak rates were obtained
as follows:
Leak Rate
SCC/Sec (He)
Run No. Seal Plate S/N Single Seal Double Seal
51 2 4.28 x 10-5
52 2 5.38 x 10-5
53 1 5.17 x 10-5
54 1 4.13 x 10-5
DI I 3.56 x IO-5
D2 1 5.74 x 10-5
Although the double seal tests took up to 50 minutes to arrive at steady
leak rates _:howno :_ii_ni[Lie_u_tdil'C(_renee.]_ bh_ :q)!Lll(':,bi()n,tl_; ;b,':,Hv
state leak rate is the significant criterion. AccordinRly, all subsequent work
_ ..... (),I()()]],)was with the single seal which required a ,,_ i_ v:%h,_rLh:_,:,
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00000008-TSA08
36,4'(4 N (82_00 lb) el_:idilg lo_d. Thos_ toflt_ _t_lldatod tho In'_ozuro plato and
f_prJng ztl_ck do01gn t_ud d_mon_ratod soal lo,_k rato_ _ompat!blo with l_rOd_,:'t:Lom_.
I T _1IvlECIIiiN _._M DEZI(IN
The do_il;n llDp_o_l,eh wz.tfJ driven by the duty c_{c:le. The dry,wet lfiili]t travet.ilo
;l,tc_ full longlDh, 33.] cm (13.1 :l,nodiel_), _tt I, v_ry nominal lof_d]ni"_,ov_!rCOililil_
>. only friction Lind. 'I,I1 the fln&l ,439 cm (,173 lli_]hlJl]), tile ;lOt_<l 'lnet'o_u_oll to 1pile
S final vtilue of 18,237 N (4100 lb). Tlli::_nul_14eiltl3tl drive of lllOdo_t ]ootld
c&pabilLty to t'r_lw-n't_e the dt,_tt_liOO tilld which 'tliun fil:ioi, i_tei_ t_ lo_ld lliu].t].lll,yinl;
llieCh&lflifsm to develop the high to, rminal load. '['he |)fl_l[-c dl_ive t,'Oli'_l[ltit oP I._ i_,oli,l '
motor drlvin!, an Acilie lelid sc1%_w enl;_gint; a tt'uIniloli hilt. The nut dr,tv'e:_ the
_ tray thru a pair of actuating al'InS as sllown in Fi/_ure 5. '' ":_Pll_<i_, al'llll] [_'e hel!.d in
= dlseng_ged position by latches. As the drawer al)[Iro_ch,21_tile clo_ed position
tile oper_tin_,_ arnls pas_ thru s_i openin_ in _ reaction plate, _:d then :ire
J]:'_ unl_tched so that furtiier movement of tlie trunnion nut rotates the actuator
_i armS. The trunnion nut and slide blocks slide in the arms, lad a ca111surface on
each actuating al_m bears against a reaction surface on the reaction plate. The
_:![ actuator arms are notated "to the pressure plate by support fittings which pullthe pressure plate home. Trade offs include gear ratio, lead screw pitch _nd
di_neter, Acme nut vs. ball nut, and lever ratio. A ball nut was rejected as
being too bulky and t_necessary. Low friction coatings promised high efficiency
Y
_;_. of the drive. A load limited 7/16-12 Acme lead screw was used after analysis
-_ of shaft resonance frequencies showed that heavier shafts still resonated Within
the 60-300 Hz vibration requirement. A 218._ :i commercial gear motor with a
! nominal 13 RPM no load speed and a stall torque of .71 N'm (i00 oz:in. ) was
chosen to match the load_ and provide a closing or opening time of approximately
12 minutes. The chosen motor performance was characterized by test 'and found to
-_t" closely match catalog data. It was found that this motor offered good
ii_ performance while the cost of purchasing it and modifying it for space operation
was much less than for purchasing an aerospace motor.
The actuating arms were designed to give the maximum ratio consistent with
__-: the geometry constraints, which proved to be 5:1. This trade off involved the
_qi': Hertz stress at the caroming surface. The radius of the surface was kept as
=_ large as possible, but at 2.54 cm (i.0 in.), the Hertz stress was 1_275,000 kPa
' (].85,000 psi) for the double sepal and 896,000 kPa (130,000 psi) for the single
seal. This stress in combination with the requirement for non-magnetic
materials posed a problem which was solved by fabricating the arms _md tile
reaction plate of A286 steel.
In the vertical pl,%ne the support fittings pull on the pressure plate at a
line below the center o:['the sealing3 forces, creating, a moiltelltof 2083 N'm
(18,450 ibf.ln ). This moment is reacted out thru the Ire]_"<'ure' pl/Ite _'i,l"lliS:iS
shown at H1 and R2 in figure 6. These arms were originally envisioned as
lightweight support ,'rodguiding members, but to carry the larfi,?moment, they
were beefed up and provided with reaction surfaces. The top of the far end of
each arm slides on the bottom of the vacuum ch_unber, and a shelf at the pressure
plate end eng._i_,cs_tmatinF, shelf on the reaction plate. This engagement occurs
_u:Jt prior to seal closure.
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00000008-TSA09
_n _dd_t_on to t_o drawer d_vo, _r1_or _ypo mooh_n_m_ wore d_n_d _nd
bullt to operate a_adoa whloh protect the oi_atomorlo _oal_ from _trong _olar
radiation Wh_le the drawer i_ in the fully Open po_Itlon durln_ _pao_ axpon_r_,
Those appear in Fi_, I°
A programor wee also developed to control the oporatlon of the unit. Thla
not diaeusaOd further in this paper.
MATEa73LS AND ,FR_9¢ION, COATINGS
The structure used aluminum tO a maximum degree. Nuts, Gap screws, and
Various pins, slide blocks and small slippers are of 300 series CRES. The hl_h
stress members, i.e., the trunnion nut, drive screw, the actuator arms and the
reaction plate are all Of A286. This alloy provides _he necessary sLren_h in
a nonma_netlc m_teri_l, and it is the lea_t exj,ensiv__d mo_t r_:tdily
available material meeting these requirements.
For the VECC to be used conveniently, it is required that all sliding
surfaces be self lubricated. The need to operate after extended space exposure
further eaphaslzes the need for relatively permanent lubricated surfaces. Dry
film HoS2 and a number of proprietary surface treatments were used. These
were various grades of "Tufram", "Nedox" an_ "Hi-T-Lube" developed b_ General
Magnapla_e Corporation. Tufram is a form of anodize impregnated with Teflon.
Nedox is a form of nickel plate impregnated with Teflon, and.Hi T - is a dry
film lubricant developed for temperatures above 587 K (6CO°F). The design was
reviewed with the intent of choosing finishes so that in each sliding contact a
relatively hard surface bears against a relatively soft surface.
A few of the major parts and the finishes used are as follows:
[.
Lubricant Surfaces
Component Material Surface Thickness
cm
Drive Worm A286 Hi-T-LUBE .0013
Trunnion Nut A286 NEDOXSF-2 .0018
Slider Blocks 30_ CRES NEDOX SF-2 .0013
Actuator Arms A288 Hi-T-LUBE .0013
Reaction Plate A286 NEDOX SF-2 .0013
Guide Slipper 7075-T65i TUFE_MH-2 .0025
aluminum
Rail 6061-T_ TUFRAM L-_ .0038
aluminum
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i
00000008-TSA10
ThO drlvo porformanae was pradlotad on thO baals of a 70% laad screw
o_fiotanoy which Is approximate_y oqulvalent to a coefficient or friction of
0.15. A torque margin of 1.01 wa_ avallabio for tha double coal and 3.02 for
the alng!o seal.
Adapting the motor fop apace operation wao a ooparato effort. The
manufacturer provided silver graphite brushes and then tho gearbox wao
lubricated Wlth a litgh vacuum grease containing MoS_.
SYSTEM TEST_,
The complete VECC was assembled and operation of various elements
attempted. The seal shades were adjusted to accommodate ao_n_metrlotolerance
build up. The drawer suspension slippers and rails appeared to function well.
The first serious trouble developed when trying to unlock the actuator arm
latches. They did not unlock; rather when the adjustment screws contacted the
reaction plate, the drive motor stalled. After some tlnksrlng with adJustmOnt,
it became evident that the trouble was more fundamental. Geometrically, the
latch lOokS good and wlth the "low" nominal friction expected in all parts it
should work very well. Accordingly, it breezed thru all the entire design
review gamut without a careful force analysis. In actual operation, the low
friction surfaces are effective, but friction is still of a significant value.
Thls fact coupled with the moment arm ratio of the trunnion nut to the latch pin
approximating 12:1 made Lhe force on the latch f_n so hitchthat the latch could
not slide off the pin. The latches were replaced by a _i_II_l_ latch arm as shown
in figure 7, which operates satisfactorily.
The general level of friction proved higher than peedicted including the
drive screw friction. In cycling the unit, the drive motor was dragged down to
near stall before the actuator arm contacted the shut off switch. This
demonstrated that the available motor torque was marginal. Fortunately, a
higher torque motor was available, which could easily fit into the design and
this was later incorporated.
Leak testing of the chamber when closed by the VECC drawer drive produced
leak rates equal to those of the earlier tie bolt tests. The unit was cycled
thrU more than 50 complete openings and closings, demonstrating the durability
of the design and the seals. The unit has since successfully Dassed
environmental testing to Space Shuttle low,is, and :_ver_lJ utl]ts},aveb_e)_b_;[it
for use on LDEF.
l
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00000008-TSA] ]
CHAMBER EXPERIMENT TRAY
Figure I.- Sliding tray Vacuum Experiment Control Canister.
! I_ 37.6 -'i
| | I ...... J I
t
;. ot" _. , ,9 o
' I ---'- ----- "-]b" 1 I |
' lit _ k___k__ _k_.._J__J _L fli,_ I', i .... 1............... _.............. "r.... ,
i ,
i , k__ .L. _ 1........ L....
_, _L=J_ T--_ i ......-h---_-i
i :it k SEE VIEW A
I=" C SECT,ION D_-D
N, VIEW A VIEW B VIEW Ci ,
-- --Figure 2.- Face plate. Dimensions are in centimeters.
7 285
_ _._:: : :: =:_ __:-:. c_._;--c: _-: :_5. _:_ _. ;=_ _--_-; -.,............ -_--- ............................
00000008-TSA12
2O6
d
00000008-TSA13
iFigure 4.- VECC leak rate test aonfigUratiOn (cannister enclosed for helium £i11).
................ \_._ _._jt_/CHAMBER
................ .... [/FACE PLATE
MICRO I / I,._ACTUATORI ._-.x- _..SWIT H _ ARMS ' //X I ....I[rT_uNN,o_ _c%_J."3,,.<, _',"%I ,
- '°" _ _°'_z_,'l ! Z "''°''°Z " _-" ,I
_"o _',-d; !/ .... j_ f
, SUPP_nT ,, SUPOnTI
"___\_ ' 'I 'I I
• 1 ............. IL_J_.4 I <- J_ I
-[ 1l .J ,_ _1__
LOCKED POSITION OPENPOSITION
Figure 5.- Fray drive design.
287
00000008-TSA14
1FA_ PREtt_Uft_
PLATE P/LATE
SEAL. FORCE
CHAMI!IER ........
R2
]____-____-_ _-___ r=====_ CLOSING
___ ........ ..__==_____=:&...... :___4R,z_-_=="G
RAIL blF. B \ C D E
• PLATE ARM \
CHAMBER I \ \
I__ I/" _. \ _ REACTIONI -" L.
"LLT__ li<,..... ,_\ _PLATE / "",._ P._SSUREt:t:V' Kfr-'l L.J-._"-¢_./ b------_..,-_LAT_ARM
' ""' J
A B C D E


Drawer drive for space shuttle vacuum canister

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