The Column Longeron Latch (CLL) was designed and developed as the securing mechanism for the deployable, telescoping column of the Hoop/Column antenna. The column is an open lattice structure with three longerons as the principal load-bearing members. It is divided into telescoping sections that are deployed after the antenna is place in Earth orbit. The CLL provides a means to automatically lock the longeron sections into position during deployment as well as a means of unlocking the sections when the antenna is to be restowed. The CLL is a four bar linkage mechanism using the over center principle for locking. It utilizes the relative movement of the longeron sections to activate the mechanism during antenna deployment and restowing. The CLL design is one of the first mechanisms developed to meet the restowing requirements of spacecraft which will utilize the STS retrieval capability

Ahl, E. L., Jr.

English

NASA Technical Reports Server

SECURING MECHANISM FOR THE DEPLOYABLE COLUMN OF
THE HOOP/COLUMN ANTENNA
:k
E1vin L. Ahl, Jr.
24891 |
ABSTRACT
The Column Longeron Latch (CLL) was designed and developed as the
securing mechanism for the deployable, telescoping column of the
Hoop/Column antenna. The column is an open lattice structure with three
longerons as the principal load-bearing members. It is divided into
telescoping sections that are deployed after the antenna is placed in earth
orbit. The CLL provides a means to automatically lock the 1ongeron sections
into position during deployment as well as a means of unlocking the sections
when the antenna is to be restowed. The CLL is a four-bar 1_nkage mechanism
using the over-center principle for locking. It utilizes the relative
movement of the 1ongeron sections to activate the mechanism during antenna
deployment and restowing. The CLL design is one of the first mechanisms
developed to meet the restowing requirements of future spacecraft which
will utilize the STS retrieval capability.
INTRODUCTION
The National Aeronautics and Space Administration currently has a
program for development of Large Space Structure Technology (LSST). Some
future spacecraft such as the Hoop/Column antenna and the Space Station
will be so large that they will have to be either assembled in orbit or
deployed once they are clear of the STS cargo bay. One of the Langley
Research Center (LaRC) programs is the development of concepts for
deployable large space structures.
The Hoop/Column antenna concept, as shown in Figure 1, has been
selected as a focus for development of all system disciplines and is being
designed as a candidate Shuttle cargo. The antenna, possibly as large as
122 meters (400 ft) in diameter, with the electronic feed system suspended
on an 85 meter (279 ft) column, will be stored in the STS cargo bay. It
will be deployed while in orbit and then restowed for STS entry/landing.
The main column for antenna deployment is _ posed of 23 internally-
nested telescoping sections. Each section has three 1ongerons spaced 1200
apart with a CLL located at the end of the longerons. There are a total of
63 identical CLL's in the column. A servo motor located in the center bay
applies a tensile force to cables that are threaded through each set of
1ongerons, as seen in Figure 2, to effect deployment. The same servo-motor
may be used for retrieval by pulling on a single cable that is threaded
through the center of the column and is attached to the top column section.
NASA Langley Research Center, Hampton, Virginia _IINTI_]_'_'-'-_,_i.:.7
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https://ntrs.nasa.gov/search.jsp?R=19830016620 2020-03-22T00:57:41+00:00Z
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REQUIREMENTS
It is required that the antenna column sections be sequentially locked
during extension and unlocked during the restow operation. Each of the
longerons when deployed must carry a compressive load of up to 1114 N
(250 lb). The CLL was designed to fit within the 19.05 mm (0.75 in)
diameter longeron. The design load for the CLL components was taken to be
2304 N (515 Ib). This load was developed from the longeron compressive
force of 1114 N (250 Ib) plus 334 N (75 lb) latch margin in the locked
position to allow for some tolerance buildup and to provide column rigidity.
The 1447 N (325 Ib) force on the latched mechanism and the CLL latching
geometry generates a maximum force of 2004 N (450 lb) at the instant the
mechanism passes the center point. An additional factor of 1.15 was super-
imposed to allow for a factor of safety resulting in the design load capa-
bility for the CLL of 2305 N (515 Ib). Stainless steel (17-4 PH) was
selected for the CLL test components to accommodate the design loads in the
small sized package.
OPERATION
Deployment of column sections initiates the rotation of the actuator
arm that is extended into the housing at the base of each longeron. As
longeron B and C move relative to longeron A, the CLL restraining and
actuator arms rotate as shown in Figure 3. The actuator arm then rotates
into an over-center position, locking the linkage into position. After
latching over-center further rotation is stopped when surface A of the
restraining arm mates with surface B of the actuator arm in Figure 3. In
the action of locking, the stops of longerons A and B merge into posltion
engaging two guide pins, which force alignment and provide shear restraint.
The belleville spring washers and piston housed at the top of each
longeron, see longeron A in Figure 3, are an integral part of the latching
system. As the restraining arm swings toward the locked position, it
drives the piston against the washers causing them to compress. In general,
over-center mechanisms rely on the four-bar linkage to deflect when moving
to an over-center position. The classic approach was unacceptable for the
CLL because the small size caused excessive stress in the pins and linkage.
In the CLL design, the belleville spring washers absorb most of the deflec-
tions and serve to limit the maximum force necessary to latch. The
resulting preload of spring and actuator arms is set to provide a rigid
column for the expected external loads.
The single cable attached to the top column section retrieves the
column. The bottom of each longeron strikes the extended actuator arm, as
seen in Figure 4, unlatching the CLL. This process is continued until all
sections are restowed to their original positions.
I
!58
ANALYSIS
Three analytical models were used in the analysis of the CLL. A kine-
matic model was used to develop the length and pivot locations of the four
bar linkage. The links and pins were sized by closed form mathematical
expressions developed from force and moment diagrams. Finally, a finite
element model of the restraining arm was generated to allow a more accurate
calculation of the stresses in this element, since this was identified as
the most critically loaded link from the previous closed form solutions.
The kinematic model of CLL is shown in Figure 5. During deployment,
link BC actuates the four-bar linkage consisting of BD, DE, EF, and FB.
During restow, link AB actuates the four-bar linkage. Links DB, DE, and GH
are loaded in compression while links EF and FG react bending loads. Links
FH and FB are loaded in tension to react the bending and compression loads.
The linkage is centered when angle I, between links BD and ED, equals 1800 .
Further counterclockwise rotation of DB into an over-center, locked posi-
tion decreases angle I to 165o, point G travels in a positive y-axis direc-
tion .25 mm (.010 in) as angle I changes from 1800 to 165.20 . The
belleville spring washers prevent a sharp increase in the stresses in the
restraining arm during the over-center travel of the CLL. The perpen-
dicular distance to point D from a line joining point E Lo point B equals
.86 mm (.034 in). This distance represents the margin of safety for the
CLL against accidental restow. Point G exerts a maximum compression force
of 2004 N (450 lbs) on the belleville spring washers when angle I equals
1800 .
All components of the CLL were analyzed by closed form solutions.
Forces, moments, free body diagrams and load, shear, and moment diagrams
were generated. As a result of the analysis the pins were required to be
3.17 mm (.125 in) in diameter. The maximum force at point C and A respec-
tively, as shown in Figure 5, to latch and unlatch the CLL was calculated
to be 150 N (33 lb).
The restraining arm was further analyzed by a finite element model as
a check on the closed form stress solution because it is the most highly
stressed link and has a complex shape. The maximum bending stress of 454 x
106 Pa (66,000 psi), shown in Figure 6, occurs when angle I, from Figure 5,
equals 1800 . This stress is within 15 percent of the stress predicted by
the closed form solution. The steady state stress, when the CLL is
latched, is 289 x 106 Pa (42,000 psi). Corresponding deflections at point
A f[om Figure 5 are 127 x 10-_ mm (.005 in) when angle I equals 180o and
/o.L x 10-_ mm (.003 in) when latched.
|
TESTING
Four tests were conducted in developing the CLL. A compression test
of the top housing, piston, and beIleville spring washer assembly was
necessary to develop required preload capability shown in Figure 7. A
compression test was conducted on the total latch assembly to simulate the
159
Ideployed column loading condition, shown in Figure 8. Two tests were
conducted to confirm the column longeron loads necessary to latch and
unlatch the CLL shown in Figure g and 10.
The compression test of the top housing, piston, and belleville spring
washer assembly was conducted to determine the number and combination of
washers required to obtain a piston preload of 1447 N (325 Ib) with the CLL
latched while limiting the maximum piston load to 2004 N (450 Ib). The
combination of 11 belleville spring washers shown in Figure 7 gave the
required load conditions with a piston stroke of 0.58 mm (.023 in).
The compression test of the latch assembly gave the preload and the
strain in the restraining arm. As a load was applied, the distance
between the stop distance A was measured. At a load of 1438 N (323 Ibs),
a gap was obtained, and the test was stopped. The gap signified that the
preload of the latch had been obtained. The steady state strain in the
restraining arm was measured at 1.25 x 10-3 corresponding to 258 x 106 Pa
(37,500 psi). This differed from the finite element model results by only
11 percent.
The latching load was determined by test, shown in Figure 10, to be
850 N (190 lb). This is six times the value predicted by calculations.
The large variance is attributed to a high friction load between the
loading longeron and the test housing assembly, and the friction between
links and pins. However the peak strain in the restraining arm was 1.76 x
10-3, which represents a stress of 363 x 106 Pa (52,800 psi). This stress
is 20 percent lower than the peak stress predicted by the finite element
analysis. The unlatching load was determined by test, shown in Figure g,
to be 530 N (119 Ib), which is over three times the calculated value. This
_ff_rence is attributed to friction between pins and links.
i-
i ..
CONCLUSIONS
The test data verified the analysis with the exception of the latching
and unlatching loads. These differences are attributed to excessive fric-
tion in the test setup. Amodification to the latch assembly test housing
is being made for retesting of the deployment loads. A modification may be
made to the actuator arm to reduce the latching and unlatching loads.
Based on the design, analysis and tests, the CLL design shows good poten-
tial for a solution to the latch and unlatch problem of a deployable space
system where a restow capability is required.
160
m|
F IGURE m •
OF POOR QUALITy'
CENTER SECTION
HOOP/COLUMN ANTENNA
161
|
CONCEPTUAL
DESIGN
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--DEPLOYMENT CABLE
FIGURE • LONGERON
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162
DEPLOYMENT
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LONGERON
DEPLOYMENT MOTION
DEPLOYMENT MOT ION
FIGURE 3. - LATCHING MODE
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ORIGINAL PAG;Z 5_J
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166
MODEL
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FIGURE 7. - COMPRESS]:ON TEST
167
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LOAD
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TEST
ASS'Y
FIGURE 8. COMPRESSION TEST
ASS'Y LOADED CLL
168
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ASS' Y
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ASS'Y
F I GURE 9. - UNLATCHING TEST
169
ORIGINAL pAGE: _S
OF POORQUALITY
F-IGURE IO. - LATCHING TEST
170
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Securing mechanism for the deployable column of the Hoop/Column antenna

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