The design and evolution is described of a spacecraft Appendage Tie Down Mechanism (ATDM). Particular emphasis is paid to the mechanical aspects of using dry lubricants to increase the efficiency of acme threads and worm gearing. The ATDM consists of five major components. These are a dc torque motor, a worm gear speed reducer, the tension bolt (or T-bolt), nut capture and centering jaws and the capture nut. In addition, there are several minor components such as limit switch assemblies and an antibackdrive mechanism which couples the drive motor to the worm shaft. A development model of the ATDM in various configurations was under test for some time. In its latest version, it has successfully completed thermal vacuum testing, vibration testing, and extended life testing

Head, R.

Nygren, W. D.

English

NASA Technical Reports Server

IN85-33522
THE DESIGN AND DEVELOPMENT OF A SPACECRAFT APPENDAGE TIE DOWN MECHANISM
William D. Nygren and Raymond Head*
ABSTRACT
This paper describes the design and evolutior_ of a spacecraft Appendage
Tie Down Mechanism (ATDM), with particular emphasis on the mechanical aspects
of usir_ dry lubricants to increase _he efficiency of a_.me threads and worm
_ gearing.
ATDM- DESCRIPTION
The ATDM consists of five major components as shown in Figure 1 and i_,
cross section (Fig. 2). These are (i) a DC torque motor, (2) a worm gear
speed reducer, (3) the tension bolt (or T-bolt), (4) nut capture and centering
jaws and (5) the capture nut. In addition, there are several minor components
such as limit switch assemblies and an anti-backdrive mechanism which couples
the drive motor to the worm shaft.
ATDM - OPERATION
The operational sequence is as follows: First the DC torque motor drives
the T-bolt through the worm gear speed reducer with a ratio of 58 to i. ll_e
worm gear hub is splined, allowing T-bolt axial ddvancement while the worm
gear rotates. The center section of the T-bolt is a ball screw which advances
the T-bolt through its fixed ball nut at a rate of u._08 _ _'n200 in.) per.
revolution. The next step is the capture and centering of the nut assembly
(see Fig. 4). As the T-bolt advances, it drives the t_so capture jaws to swing
closed. This forces the nut to the proper position to allow the acme threads
on the tip of the T-bolt to engage the nut threads.
The capture nut is free to move _xlally in its housing against two ets of
springs. The first is a "soft" stack of wave washers which allows the T-bolt
to fully engage the nut under low thread loading. The second stack is made up
of Belleville washers which start compressing only after the soft stack spring
has completed its stroke. The Pelleville springs reach an axial force of
22 678 N (5100 ib) during one revolution of the T-bolt. The mechanism is then
shut off by limit switches on the nut housing, which monitor nut spring
travel. Figure 3 shows the limlt-swltch mounting on the nut assembly.
The actual loading or tensioning of the T-bolt by t_, nut is accomplished
by the difference in the leads of the ball screw thread and the acme capture
thread on the forward end of the T-bolt. The lead of the ball screw is
mMartin Marietta Aerospace, Denver, Colorado
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I0.508 cm/rev (0.200 in/rev) while that of the 1.905 cm (0.750 in.) double-lead
| aeme screw is 0.846 cm/rev (0.333 in./rev). The T-holt always advance_ out of
the mechanism at a rate of 0.508 cm/rev (0.200 in./rev); however, after the
i acme threae engages the capture nut, the nut advances relative to the bolt at
++i a rate of 0.846 cm/rev (0.333 in./rev). Thus the net compression of the nut
i springs is 0.338 cm (0.133 in.) per revolution of the T-bolt.
I
I DESIGN EVOLUTION
!
"1 As system requirements were refined, the preload that the ATDM was|
supposed to develop was substantially increased. The original preload was
,! defined to be 7115 N (1600 Ib) maximum. After the first development unit was
J built, this figure was increased to approximately 13 344 N (3000 lb), and
finally to 22 678 N (5100 ib). This tripling of output force req.ired that
-_I most attention be focused on mechanical efficiency as the electrical power to
drive the mechanism was "fixed" as was the time available for the entire
operation to take place. The increased capability of the mechanism was
required to take place within the original physical envelope.
To provide this increased capability, the following improvements were
made. First, the gear ratio was changed from 50-1 to its present value of
58-1. No further increase was allowable because of gear tooth strength
considerations and size and time limitations. The next step was to change
from a V-thread to the present ball screw for the driver thread. This was a
major improvement; the apparent coefficient of friction dropped from 0.18 to
O.OO7.
Although this w_s a major improvement, a i_ torque margin under
worst-case conditions of voltage, temperature, and motor Kt torque constant
had yet to be demonstrated.
The original development unlt had used wet lubricant (Braycote 3L-38RP)
for surfdces, and it was calculated during testing that the efficiencies of
these two components+ were around the 30% range because or" a coefficient of
friction of approxlh,ately 0.2 at the mating surfaces. Since the mechanism
efficiency was basically the product of the gear box and acme screw
efficiencies, the overall mechanism efficiency was only about 10%. A decision
was therefore made to switch to a dry lubricant such as molydisulfide for
these sliding s -Paces of the ATDM the worm/worm gear and the acme capture
th:_ads.
The dry lubricant recommended by Materials Engineering was LUBECO 905 as
Martin Maretta Co, (MMC) has previously obtained good results with it. After
successful application of LUBECO 905, the observed coefficient of friction was
•I or lower at both worm/Worm gear interface and at the acme bolt/nut
interface in a vacuum environment, this value was seen to futher decrease to
the point where efflciencies _bove 60% were obtained for the individual
_omponents, approximately a four-fold increase for the overall mech_mism.
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1985025199-177
I DRY-LUBRICANT DIFFICULTIES
!
!
When the mechanism was first lubricated with LUBECO 905 and reassembled,
it failed almost immediately during testing. This caused carefull reanalysis
, of the use of dry-film lubricants. Based on the calculated contact stress _md
wear life required to survive testing, the lubricant should have beent
s_ccessful. However, upon further investigation, the following important
facts were learned.
o No matter how well made and aligned they are, a worm and worm gear
must go through a wear-in riod before a true line contact area is
established across the face of each gear tooth.
o In a similar fashion, male and female screw threads must also be
mated together by a wear-in period before true area contact is
established to support the load.
o All sharp corners must be removed around each worm and gear tooth,
and on all corners of both the male and female acme thread. In
particul_ this must be done at any partial threads which spiral from
minor to major dia meters (or vice-versa) such as occurs at the
beginring and endin_ of both the male and the female threads.
o After the proper contact areas are established, the mating parts
should be burnished together, starting with low loads and working up
to the full flight loads in gradual steps.
LAPPING
_o facilitate a rapid wear-in of the worm/worm gear and T-bolt/nut of the
ATDM, the decision was made to lad the mating surfaces together with a
grinding compound. Since it is desired to force one surface to conform to the
other, both the worm and the T-bolt acme thread were hardened by nltrlding
prior to starting the lapping process. These surfaces were chosen for the
following reasons. It was felt that the T-bolts contoL"_ could be more
accurately manufactured and dimensionally verified than could its mating nut.
The worm was chosen for hardenin£ because it has basically one tooth, which
must wear in each tooth of the worm gear to match it exactly. (Thus the worm
gets 58 times the wear that any particular worm gear tooth gets during
lapping).
After _litriding was completed, the worm/worm gears were serialized along
with their housing to be maintained as a matched set. The gear box was then
reassembled, the output shaft was attached to a brake which could ap_ly 1.13
N-m (i0 in.-Ib) of drag. The worm and _orm gear were then coated w_th a
320-grit Clover grinding compound, and the worm was driven at 2CC rpm until
the desired wear pattern was established across the face of each tooth.
Finally, the 320-grit compot_nd was replaced with a 600-grit compound to leave
a 16-# to 32-rms surface finish for the application of the LUBECO 905.
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1985025199-178
In a similar manner, the T-bolt and cagi_,g nut were i_- _.d together with
the same compounds under a constant axial load of 22-44 N Ib), with the
nut fully engaged with the bolt. A special fixture was cosigned for this
purpose (Fig. 5).
BURNISHING
After lapping, the ".ritical parts were relubrlcated with LUBECO g05. The
gear box was then reassembled and reattached to the braka. This time,
however, the load was gradually increased in approximately ]4 steps to an
output torque of 40 N-m (360 in-lb). At this point the input torque was
measured to be 0.15 N-m (1.3 ir-lb), giving an efficiency of 49.7% under stall
conditions. (This would furth .. improve in vacuum conditions. ) Likewise, the
T-bolt and nut were burnished together at gradually increasing axial load
levels to a final load of 17 78 7 N (4000 ib). Here the torque required to
rotate the bolt was measured as 33 N-m (291 in-lb), giving an efficiency of
73%. (The bolt required a torque of 12 N-m (105 in-lb) to keep it from over
hauling). A coefficient of friction of 0.06 in air can be deduced from these
values and the thread geometry.
ANTI-BACK-D RIVE MECHANISM
The success of the dry film lubricant was not completely without a
drawback because it led to a luther complication.
At the 22678 N (5100-ib) preload level, it was found that the acme thread
would back-drive its low coefficient of friction and large helix angle. This
torque was about 17 N-m (150 in-lb) in air and would possibly double under
vacuum conditions. The conclusion was that the acme thread could overcome the
holding torque produced by the ball nut and so deliver a back-driving torqu_
to the worm gear. Here the low friction coefficient is again harmful as it
allows a normally locking gear ratio to back-drlve. This potential for
back-driving is particularly hazardous during the launch and boost phases of a
flight, because the harsh vibration environment increases the likelihood for
back-drivlrg, at a time when a loss of PTDM preload would be most dangerous.
Of the many possible solutions to this problem, most either caused an
unacceptable constant loss of motor torque or required electrical power and
control lines which were not available. The solution was to make the coupling
between the drive motor and the worm serve as a ratchet which would allow the
motor drive the worm in eithel direction, but not allow the worm to back-drive
the motor. Tne operation of this device ia shown in Fig. 6.
The mechanism consists of' a detent disk attached to the worm shaft, and a
cam disk attached to the motor shaft. These two disks are coupled together by
a pin attached to t,,_ detent disk, which rides in a slot in the cam disk.
This arrangement allow approximately 60° of relative motion between disks.
There is also a Vlier-type plunger between the disks (not shown), which locks
the disks into the configuration shown in Fig. 6a prior to the start of
operations. The final component is the ratchet dog, which in this ca_e ha.=.a
roller to minimize friction.
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1985025199-179
!The operation is as follows: First, as the motor starts running (see Fig.
6a), the roller rides on the combined outside diameter of the two disks and is
held in this configuration by the Vlier pin between them. When the mechanism
approaches its desired preload, the transmitted torque becomes large enough to
overcome the Vlier pin, the disks shift to the configuration shown in Fig. 6b,
and the dog starts ratcheting. Next, if the worm attempts to back off, it can
rotate back only until the roller comes against the now "uncovered" detent
(Fig. 6c). Finally, when the mechanism, is desired to be uncaged the reversal
of the drive motor causes the cam disk to cam the roller out of the detent,
and the back-out can commence as shown in Fig. 6d.
PRESENT STATUS
A development model of the ATDM in various configurations has been under
test for some time. In its latest version, it has successfully completed
thermal vacuum testing, vibration testing, and extended-life testing.
Qualification and flight units are scheduled for testing.
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The design and development of a spacecraft appendage tie down mechanism

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