The Ball Trunnion Capture Latch described was developed and designed to restrain a spacecraft deployable appendage in three translational directions. The latch is capable of supporting an appendage during STS ascent and landing events and is capable of releasing and restowing an appendage distorted in three translational directions by thermal growth. The requirements, design, analyses, and tests conducted on a development unit of the latch are discussed

Adams, D. V.

Alchorn, B.

English

NASA Technical Reports Server

A BALL TRUNNION CAPTURE LATCH 
David V. Adams and Brendan Alchorn* 
ABSTRACT 
The Ball Trunnion Capture Latch, developed under a research and develop- 
ment program conducted by Lockheed Missiles and Space Company, was designed to 
restrain a spacecraft deployable appendage in three translational directions. 
The latch is capable of supporting an appendage during STS ascent and landing 
events and is capable of releasing and restowing an appendage distorted in 
three translational directions by thermal growth. This paper discusses re- 
quirements, design, analyses, and tests conducted on a development unit of the 
latch. 
INTRODUCTION 
The function of the Ball Trunnion Capture Latch is to restrain a space- 
craft deployable appendage through ascent and landing conditions. The latch 
must also recapture and preload the appendage during on-orbit conditions, when 
relative thermal growth may occur between the deployable appendage and the 
spacecraft supporting structure. Since the latch is to be of general utility, 
it must be insensitive to thermal growth in three orthogonal translational 
directions. The latch must be tolerant of the spatial distortion of such an 
appendage, and it must overcome any loads associated with pulling a distorted 
appendage back into place. The latch consists of dual four-bar linkages which 
are actuated by a motor-driven ball screw. In the over-center position, one 
link, called the latch jaw, closes down on a ball (spherical) trunnion, lock- 
ing it between two conical cups. The ball trunnion provides the interface 
between the latch and the deployable appendage. 
REQUIREMENTS 
The latch must be capable of withstanding a limit opening load of 44,500 N 
(10,000 lb) against the jaw when the latch is closed. This load includes any 
opening loads induced by side loads reacting against the conic slopes of the 
two capture cups. Additionally, the latch must supply a ball trunnion preload 
of 4,450 N (1,000 lb) to prevent rattle during typical acoustic conditions 
seen at launch. 
*Lockheed Missiles and Space Company, Sunnyvale, California 
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https://ntrs.nasa.gov/search.jsp?R=19820015476 2020-03-21T16:28:00+00:00Z
The latch must also be capable of pulling a trunnion displaced 0.762 cm 
(0.3 in) in the direction of actuation against a load varying from 0 to 4,450 
N (1,000 lb). This must be accomplished while the trunnion is also offset 
0.254 cm (0.1 in) in both directions LO.359 cm (0.141 in) total] orthogonal to 
the direction of jaw travel, overcoming an orthogonal load varying from 0 to 
9,900 N (2,000 lb, vector sum). The latch must be capable of a minimum 
on-orbit life of 3 years, and single-point failures (excluding structure) must 
be minimized. The latch must operate between temperature extremes of -34°C 
(-30°F) and +88"C (+190"F) under conditions ranging from 0- to loo-percent 
humidity. The envelope of the latch measures 45.7 cm by 22.9 cm by 11.4 cm 
(18 in by 9 in by 4.5 in). An acoustic vibration criterion of 16.7 GRMS was 
selected based on typical responses measured on components of this type during 
launch of Lockheed-built spacecraft. 
DESIGN DESCRIPTION 
A ball-end trunnion caught between two conical cups was chosen as the 
design solution. This design configuration permits capture and restraint of 
the trunnion in three translational directions, while it allows some relief 
from rotational loads (depending on friction). Figure 1 shows the latch in 
the over-center position. The latch jaw is shown closed down on the ball 
trunnion, locking it between the two conical cups. One conical cup is located 
on the latch jaw while the other cup is fixed rigidly to the latch structure. 
A half-cone angle of 45" was selected for both cups, based on a trade-off 
between lock-down capability and capture capability. A narrower cone would 
have reduced the vertical reaction on the jaw but would have increased the 
distance required for jaw travel. The peak load on the ball screw actuator 
would have increased because loading would have begun when the mechanical 
advantage was lower. A shallow half-cone angle would have required higher jaw 
vertical loads to pull the trunnion into place. 
Preload is achieved by placing a shim under the fixed cup. Motive power 
is supplied to the ball screw through a 1:2 speed-increasing gear train from 
redundant motors driving through a planetary differential into a common output 
shaft. 
The link pivots and the ball screw supports are redundant. Each joint 
consists of a pin surrounded by a bushing which is free to rotate on either 
its inner or outer surface. The latch is shown in the fully open position in 
Figure 2. 
ANALYSIS OF LATCH 
Analysis of the latch was conducted in two phases: analysis of ascent 
loads sustained when the latch is closed and analysis of loads occurring 
during latch closure. 
Ascent loads were treated as a static loading case, since the latch is 
closed over the ball trunnion and its internal dynamic reactions are 
insignificant. An analytical model of the capture cups predicts that the 
latch jaw takes loads only in the direction of its travel (+Y direction at 
closure). This is due to the fact that the jaw stiffness in the plane 
orthogonal to the direction of jaw travel (the X-Z plane) is low compared to 
100 
that of the fixed capture cup. Further analysis shows that this assumption is 
conservative, since it results in calculating higher jaw vertical loads. The 
results of these analyses forced a change in the design of the latch. It was 
determined that pin diameters at each of the joints would have to be increased 
to accommodate the 44,500-N (lO,OOO-lb) opening load. 
An analytical model simulating operational performance of the latch was 
developed to verify latch closure in the presence of deployable appendage 
thermal growth. The model simulates latch kinematics, stiffness of the link- 
ages and support structure, bearing friction, and ball trunnion/latch jaw 
pull-in forces. Dynamic forces were neglected, since actuation speed is low. 
The kinematic relationship of the latch linkages is shown in Figure 3. A 
motor-driven ball screw, represented as link DE, actuates the four-bar linkage 
consisting of ground link AB, jaw BC, compression link CD, and tension link 
AD. At the full extension of DE, the jaw angle A6 is 0" and links AD and CD 
align with the Y (vertical) axis. This is the over-center, or closed, posi- 
tion of the latch. 
Figure 4 shows how the stiffnesses of the various linkages, including the 
support structure, are modeled. Links AD, CD, and DE are considered simple 
extensional springs. Structural stiffnesses at joints A and B are considered 
decoupled for simplicity. Joint E, which is the ball screw support, is 
treated as an eccentrically loaded cantilever beam. Flexibility at E is rep- 
resented by the two-dimensional flexibility matrix [fE]. Latch jaw stiff- 
ness is computed by assuming it to be a simply supported beam with an over- 
hanging load. For each spring shown in Figure 4, an effective stiffness is 
computed at the jaw based on the kinematic relationship between a unit spring 
displacement and the corresponding jaw displacement. By the principle of con- 
servation of energy, unit strain energy in each link is equated to unit strain 
energy of an effective spring located at the jaw. An incremental change in 
length is computed for each spring due to the unit strain energy. This change 
in length causes a corresponding change in the position of the jaw. The jaw 
displacement is considered to be the displacement of an effective spring lo- 
cated at the jaw. Knowing strain energy and effective jaw spring displacement 
allows the calculation of an effective stiffness (transferred to the jaw) for 
each element. The effective stiffnesses of all the elements are then added in 
series to determine the total effective jaw stiffness. The total effective 
jaw stiffness changes with the geometry of the mechanism. Figure 5 shows the 
plot of jaw stiffness vs. jaw open angle (0" is the closed position). 
If the effective stiffness of the jaw is known, then the position of the 
ball trunnion during pull-in can be calculated, based on the equations of 
static equilibrium. The simulation of ball trunnionllatch jaw pull-in forces 
assumes a rigidly fixed capture cup, a jaw with a finite stiffness in the 
horizontal (X-Z) plane, and a contact angle (based on the slopes of the cap- 
ture cups) of 45". For simplicity, we also assume that the trajectory of the 
ball trunnion projected onto the horizontal (X-Z) plane would consist of 
straight lines. 
Forces and reactions in the latch linkages are then calculated based on 
the latch jaw load. Torque losses are computed for each joint due to an 
incremental change in position. The torque losses are considered work done by 
the ball screw during an incremental change in length. Since work done by the 
ball screw is equal to force time displacement, and the displacement and the 
101 
I - 
work done are known, then an added force on the ball screw due to friction is 
computed for each joint. The various friction losses are then summed to 
obtain a total force on the ball screw force due only to friction. Frictional 
forces are then added to the ball screw for screw force assuming no joint 
friction to obtain the total ball screw force. Total ball screw force is 
shown vs. ball screw extension for a 13,350-N (3,000-lb) preload case in 
Figure 6, along with test data collected for this case. 
It was thought desirable to check the results of the analytical model 
internally. To do this, an algorithm was added which compares the work done 
by the ball screw to work done on the latch in the form of friction, strain 
energy stored in the compliance of the latch, and work done to pull the trun- 
nion into place. This algorithm helped to point out errors in both the coding 
and the synthesis of the model. Calculations of work done by the ball screw 
vs. work done on the latch typically differ by less than 1 percent. 
DEVELOPMENT TESTS 
A development unit of the latch was subjected to tests in order to verify 
capture, preload, and release capabilities of the latch and to validate the 
latch analytical model. A total of 10 tests were conducted on the development 
unit. The tests conducted fall into three general categories: compliance 
tests, tests of latch operational performance, and vibration sensitivity 
tests. 
Compliance tests were performed in order to calibrate the jaw vertical 
load, measure the effective jaw stiffness, and determine the flexibility of 
the ball screw support structure. Calibration was accomplished by placing 
strain gages on either side of the tension and compression links and then 
pulling up with a known force on a ball fixture locked in the latch. Link 
bending due to friction torque required that the strain gages be wired in a 
moment-compensating circuit. This was an early indication of the significance 
of joint friction. This test allowed the determination of vertical jaw load 
in subsequent tests. Effective stiffness at the jaw was measured by inserting 
shims of various thicknesses under the fixed cup and recording variations in 
jaw vertical load. The results showed an effective stiffness of 8,400 N/cm 
(48,000 lb/in), which contrasts with an expected value of about 350,000 N/cm 
(200,000 lb/in). The difference between the two values can be attributed to 
bending and shearing of the linkage pins, phenomena which were not considered 
in the initial analysis. Later versions of the latch will feature larger 
diameter pins to accommodate ascent conditions and therefore should have 
higher effective stiffnesses. Pin stiffnesses were incorporated into the 
analytical model by merely adding them in series to the stiffness of the 
existing spring elements in the input of the model. 
Two preload tests were conducted by closing the latch over a loose ball 
fixture resting in the fixed capture cup. The preload tests were conducted to 
determine the effect of joint friction without cup friction. Cup friction was 
eliminated, since the ball fixture did not move. Shims were placed under the 
fixed cup to obtain preloads of 6,230 N and 13,350 N (1,400 lb and 3,000 lb) 
and the latch was then closed and opened while recordings were taken of ball 
screw extension and required input torque. These tests indicated that joint 
102 
friction approximately doubled the peak required drive-unit torque. The tests 
also verified the simulation of joint friction torque in the analytical model. 
The Z direction pull-in test was conducted in order to determine the 
ability of the latch to capture, pull in, and release a ball fixture offset to 
the side of the latch. In this test, a ball fixture was attached to the end 
of a long threaded rod with a known axial stiffness (Figure 7). The threads 
on the rod allowed for position adjustment of the ball. The surfaces of the 
capture cups and the ball fixture were coated with solid-film lubrication 
prior to the test. The ball was positioned to rest on the near side of the 
fixed cup if not loaded. The latch was then closed, pulling the ball down in- 
to the cup. Tensile force developed in the rod was indicated by a load cell. 
Deflections of the ball, the jaw, and the fixed cup were recorded along with 
ball screw input torque, ball fixture axial force, and jaw,vertical load for 
various ball screw extensions. Latch capabilities of capture, closure, and 
release were demonstrated. This test yielded information that verified the 
simulation of cup friction and side-direction stiffness of the jaw. The test 
also indicated that side forces on the jaw cause rubbing to occur at the side 
of the jaw, thus reducing the efficiency of the mechanism. This problem can 
be alleviated by treating the rubbing surfaces with solid-film lubrication. 
The X and Y direction pull-in test was performed in order to determine the 
capability of the latch to capture, pull in, and release a ball fixture which 
was offset above (vertically) and forward (horizontally) of the fixed capture 
cup. The setup was similar to that used in the Z pull-in test, except that 
the ball fixture was not attached directly to either threaded rod. Instead, 
the ball was centered on a short rod which was supported at both ends by two 
U-shaped brackets (Figure 8). The brackets were attached to the threaded 
rods. As the latch was closed, the ball was pulled in simultaneously in both 
the X and Y directions. Again, latch capabilities for capturing, pulling in, 
and releasing a ball trunnion were verified. Data gathered in previous tests 
allowed the analytical model to simulate this test with fair accuracy. 
The final test was a measurement of latch sensitivity to vibration. A 
free-floating trunnion ball was clamped into the latch so that a preload of 
4,450 N (1,000 lb) was developed. The latch was then vibrated along three 
translational axes to determine the capability of the latch to remain closed 
and maintain its preload in the presence of vibration. The latch was sub- 
jected to sine sweep and random vibration levels of 16.7 GRMS, thus simulating 
conditions that might be seen during a typical launch. The latch remained 
closed. Typical landing conditions (with thermally induced loads orthogonal 
to the direction of jaw actuation) were not simulated, due to the complexity 
of the required test setup. Response of the latch was termed low (Reference 
1). Preload capability was verified by subsequently releasing the ball fix- 
ture and then reclosing the latch while taking strain gage measurements. 
CONCLUSIONS 
A capture latch capable of restraining, releasing, and recapturing a 
spacecraft deployable appendage in three translational directions was 
developed by Lockheed Space Systems Division. Performance of the latch and a 
computer simulation of latch operation were verified by tests conducted on a 
development unit. Bending stiffness of the joint-connecting pins was found to 
103 
have significant effect on overall latch stiffness; however, larger pins 
necessitated by the static opening load of 44,500 N (LO,000 lb) will alleviate 
this condition. The latch is currently under consideration for use on 
several LMSC payloads. 
ACKNOWLEDGMENTS 
We wish to acknowledge the efforts of Jim Larson for his work on the 
initial design of the latch, Joe Wilson for his contribution to the stiffness 
portion of the analytical model, and Ken Buckley for his supervision in the 
development of the latch design. 
REFERENCES 
1. LMSC Test Report No. TA B152, "Test Report for the Capture Latch Develop- 
ment Test," September 16, 1981. 
104 
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David V. Adams 
Lockheed Missiles and Space Company 
P.O. Box 504 
Sunnyvale, California 94086 
Mr. Adams is an engineering analyst for the Mechanisms and Separation Analysis 
Group at Lockheed. He received his B.S. degree in General Engineering from 
the University of Illinois in 1980. 
Co-author of this paper is Mr. Brendan Alchorn who is also with Lockheed 
Missiles and Space Company at Sunnyvale. 
107 


A ball trunnion capture latch

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