Open Journal of Safety Science and Technology, 2012, 2, 8-15 Published Online March 2012 (
An Investigation on the Dynamic Stability of Scissor Lift
Ren G. Dong1*, Christopher S. Pan1, Jared J. Hartsell1, Daniel E. Welcome1, Tim Lutz1,
Anne Brumfield1, James R. Harris1, John Z. Wu1, Bryan Wimer1, Victor Mucino2, Kenneth Means2
1National Institute for Occupational Safety and Health, Morgantown, USA
2Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, USA
Email: *
Received November 30, 2011; revised January 4, 2012; accepted January 16, 2012
The tip-over of scissor lifts in operation has frequently resulted in the death and/or severe injuries of workers. The ob-
jective of this study is to enhance the und erstanding of its major mechanisms and factors influencing scissor lift stabilit y.
Both experimental and modeling approaches were used in this study. Two series of experiments were performed under
possible tip-over scenarios: curb impact and pothole depression. Based on the dynamic characteristics identified from
the experimental results, a lumped-parameter model of the scissor lift was developed. It was applied to investigate the
effect of scissor structure flexibility on the tip-over potential of the lift, to understand tip-over mechanisms, and to ex-
plore preventive strategies. This study found that the fundamental natural frequencies of the lift were generally in a
range of 0.30 - 2.08 Hz, which are likely related to the tip-over. Increasing flexibility of the lift structure generally in-
creased the tip-over poten tial. The tip-over threshold was also a function of bo th ground slope and tilt speed of the lift.
The results suggest that the lift should not be elevated on largely deformable and/or uneven surfaces such as bridged
wood board or a soft soil base. The worker on the lift platform should avoid any large continuous periodic movement or
forceful action in the horizontal p lane, especially when the lift is fully elevated. Besides the tilt angle of the lift, the tilt
speed should be monito red to help prevent the tip-ov er.
Keywords: Scissor Lift; Mobile Elevating Work Platform; Tip-Over of Scissor Lift; Stability of Scissor Lift
1. Introduction
Scissor lifts are typical mobile elevating work platforms
(MEWPs). Their primary function is to elevate workers,
tools, and materials to a desired working height, while
allowing the operator to control the movement and posi-
tion of the lift. Compared with a ladder, a MEWP greatly
reduces the psychological stress and physical demands
on a worker at elevated height. Therefore, if a scissor lift
is properly d esigned, manufa ctured, maintain ed, and app r o-
priately used, it can increase not only the workers’ pro-
ductivity but also their safety. For these reasons, scissor lifts
with different capacities and elevating heights are in-
creasingly used at many workplaces [1,2]. Unfortunately,
fatal and non-fatal incidents have also happened during
scissor lift operations [3,4 ]. Many of these incidents were
associated with lift tip -over and/or workers fallin g within
or from the platform. Many of them can be prevented by
enhancing safety education, training programs, manage-
ment controls, and equipment maintenance. However, fur-
ther studies are required to enhance the understanding of
the mechanisms of the tip-over events and to develop
more effective engineering and operational interventions
to prevent them.
As reflected in the current standards on the MEWPs
[5,6], the basic mechanisms of the MEWP instability have
been generally understood. Its major factors include travel
speed, ground slope, elevated height, load weight and its
position on the lift platform, wind effect, etc. The stan-
dardized certification tests are u sually conducted using new
MEWPs. The joints in the lift structures could wear out
after the scissor lifts are used for a certain period of time;
the stiffness of the lift could thus be reduced. It is unclear
whether the increased flexibility due to wear increases tip-
over potential. While the standardized analyses are gen-
erally based on static loads, it is also unclear how the
workers’ activities on the platform affect the stability of
the scissor lift. While the tilt angle of the lift is usually
monitored, it is unclear whether the control of the tilt angle
alone is sufficient to prevent the tip-over. It is also un-
clear how to determine the lift stability when the plat-
form accidently hits a structure.
A dynamic model of the scissor lift is required to study
these issues. Because the actual tip-over is likely to occur
as a result of combined internal and external factors, such
a model is also required to examine the combined effects.
For example, the model can be used to simulate the in-
*Corresponding a uthor.
opyright © 2012 SciRes. OJSST
teraction between a worker on the platform and the lift,
and to evaluate and improve the effectiveness of personal
fall protection devices such as harnesses and their anchor
points on the platform. The dynamic model can be also
used to help evaluate and improve tip-over prevention
devices such as pothole guards and outriggers. It may
also be used to help investigate accidents and develop
more effective educational materials. Whereas the mod-
eling analyses of several MEWPs have been reported
[7-9], the literature review for this study did not find a
dynamic model for simulating scissor lift tip-over.
To help develop more effective methods/strategies for
preventin g scissor lift tip-over, the objectiv e of this study
is to enhance the understanding the major mechanisms
and factors influe ncing tip-over.
2. Experiments
Figure 1 shows the scissor lift and the test setup used in
this study. The unloaded weight of the lift is 1170 kg
(2579 lb) and its total rated capacity is 250 kg (550 lb).
The platform is equipped with a deck extension with a
rated load of 113 kg (250 lb). Its maximum elevated
platform height is 5.8 m (19 ft) (measured from the ground
to the floor of the platform). The normal speed of the scissor
lift is 3.2 km/h (2 mph or 0.89 m/s) at an elevated height
less than 2.10 m (7 ft) and 1.05 km/h (0.65 mph or 0.29
m/s) at a higher height.
Five in-house packaged tri-axial accelerometers (Kionix,
Model KXM52-1050) were used to measure the accele-
rations in three orthogonal directions at the center loca-
tions on the following five scissor substructures: the
main frame of the base, the 2nd, 3rd, and 4th scissor
frames, and the main frame of the platform. The signals
of the accelerations were input to an in-house packaged
data acquisition system fixed on the main-platform. A
LabVIEWTM program was developed to record them at a
sampling rate of 128 Hz.
Two experimental scenarios (curb impact and pothole
depression) were considered in the study. In the first ex-
periment, the scissor lift was driven into a curb at its normal
traveling speed for a given height, as defined in the stan-
dard [5]. Two impact orientations (90˚ and 30˚) were
considered. Besides the fully elevated platform height
(5.80 m) with a normal speed of 0.29 m/s, the maximum
height (2.08 m) for the maximum speed (0.89 m/s) of the
lift was also used in the experiment. Both forward and re-
verse traveling impacts were tested. Three trials were per-
formed for each test treatment. The brake of the lift was
applied immediately after the wheel (s) impacted the curb.
Since running into a pothole is also a major cause of
tip-over, the second experiment is designed to examine
the characteristics of the scissor lift in the pothole de-
pression test, which is also defined in the standard [5].
Specifically, one of the scissor lift’s front wheels was
driven into a standardized pothole (0.60 m square and
0.10 m depth) in forward travel operation, as also shown
in Figure 1. The brake of the lift was applied immediately
after the wheel had moved into the pothole. The platform
with the extension represents the highest tip-over poten-
tial; hence, this condition was considered in this experi-
ment. Because the scissor lift was equipped with a pot-
hole protection device, the guardrail of the pothole pro-
tection device hit on the edge of the pothole after the
wheel dropped in to it, which effectively preven ted the scis-
sor lift from tip-over.
3. Experimental Results
The curb impact is equivalent to applying a strong im-
pulse to the scissor lift at its base which excites various
vibration modes. Figure 2 shows examples of accelera-
Figure 1. A pictorial view of the scissor lift and test setup.
Acceleration (m/s
) Acceleration (m/s
Acceleration (m/s
Figure 2. Examples of the accelerations measured in the full
height 30˚ curb impact experiment.
Copyright © 2012 SciRes. OJSST
tions measured on the base, third scissor frame, and plat-
form in the X and Y directions in a 30˚ curb impact when
the lift was fully elevated. There were many high sharp
peaks during the impact, as shown in Figure 2. The vi-
bration displacement history in each direction was esti-
mated by integrating the acceleration history. The results
indicate that the high frequency components played little
role in determining the displacement. Hence, the high fre-
quency components are not important where tip-over is of
concern. The response in the vertical direction (Z) pri-
marily includes random high frequency vibrations; hence,
the response in this direction is not important for the
purpose of this study. As also shown in Figure 2, the ac-
celerations measured on the base quickly decayed to its
noise level after the impact, which suggests the base is very
rigid. The accelerations measured on the scissor structure
and the platform in the longitudinal (Y) and lateral (X)
directions generally show a consistent low frequency
waveform. Unlike the sharp peaks, the magnitude of this
waveform generally increases with the increase in the
measurement height. Moreover, the waveforms measured
at different locations on the scissor structure are approxi-
mately in phas e. These observations sugges t that the d omi -
nant displacements of the platform result from the pitch
and rolling motions of the lift. Such motions could signifi-
cantly affect the CG position of the scissor lift and its
stability. A frequency spectrum analysis was further per-
formed to identify the dominant vibration frequency in
each direction. Table 1 lists the resonant frequencies for
the fully-elevated height of the lift.
Mainly because the speed in the low-height tests is
about three times that of the full-height tests, the impact
peaks observed in such tests were much higher than those
shown in Figure 2. However, the accelerations quickly
decayed within one second after the impact. It was very
difficult to identify the dominant pitch and roll wave-
forms and frequencies. This observation suggests the scissor
lift at the low-elevated height is much more rigid than
when it is fully elevated. The rigid model used in the
standards may be sufficient for the stability analysis of
the scissor lift at the low-elevated height. Therefore, the
modeling study focused on the tip-over at the fully-
elevated height.
The acceleration responses measured in the pothole
depression test were similar to those shown in Figure 2,
except that these resonant frequencies were largely dif-
ferent in the rolling axis (around the Y axis), which are
also listed in Table 1. This is largely because when a wheel
dropped into the depression, at least one of the other three
wheels lifted from the ground. This could reduce the ef-
fective wheel-ground contact stiffness. These observations
further demonstrate that the mass distribution, the lift struc-
tural stiffness, and the wheel-ground contact conditions
could play roles in the determination of the dynamic re-
sponses of the scissor lift.
The results listed in Tab le 1 suggest that the most im-
portant resonant frequencies of the scissor lift are gener-
ally in the range of 0.3 - 2.08 Hz. The major frequencies
of the human movements and actions are likely to be in
this frequency range. Therefore, a resonance of the lift could
be excited if a worker on the lift platform made a con-
tinuous periodical movement or regularly applied a large
force in the horizontal plane. This may be one of the me-
chanisms or contributing factors for lift tip-over.
4. Scissor Lift Model
As above-mentioned, high frequency dynamic responses
to curb impact and pothole depression could be substan-
tial. However, such responses did not lead to tip-over of
the scissor lift. On the other hand, the low frequency roll
and pitch responses could cause significant deformations
of the scissor lift substructures, as reflected from the mea-
surements of the accelerations at several different loca-
tions. The deformations could significantly affect the CG
of the lift, which is directly associated with the stability
of the lift [5,6]. Therefore, the rigid body modes and fun-
damental bending vibration modes of the scissor lift are
important to the tip-ov er event. This further suggests that
a lumped-parameter model can be sufficient for simulat-
ing the basic tip-over behaviors. Because the wheel im-
pacts and the lifting of the wheels from the ground are
non-linear events, it is also necessary to consider them in
the simulation. The software ADAMS/View allowed us
to efficiently create a model that simulated the non-linear
tip-over event [10]; hence, the model of the scissor lift
Table 1. The dominant vibration frequencies in the curb
impact and pothole depression experiments with the lift
fully elevated (at 5.80 m).
Test treatment Impact
angle Bounce
(Hz) Pitch
(Hz) Rolling
30˚ 6.75 1.00 0.67
Curb impact: no e x te n s i o n,
forward travel 90˚ 5.83 1.25 0.75
30˚ 6.75 1.00 0.58
Curb impact: no e x te n s i o n,
reverse travel 90˚ 6.33 1.17 0.50
30˚ 6.17 1.17 0.83
Curb impact: with extension,
forward travel 90˚ 5.08 1.25 0.67
30˚ 5.42 1.08 0.75
Curb impact: with extension,
reverse travel 90˚ 5.33 1.25 0.75
Pothole depression: right wheel drop 1.17 0.30
Pothole depression: left wheel drop 1.00 0.43
Copyright © 2012 SciRes. OJSST
Copyright © 2012 SciRes. OJSST
was created using this program. Figure 3 shows the pro-
posed scissor lift model. To determine the nominal wheels’ contact stiffness, the
deformations of the wheels were measured while the fully
loaded scissor lift was standing on level concrete pavement.
The nominal stiffness of each wheel contact element was
calculated from the measured average deformation (2.5
mm) of the four wheels and the full weight (13,925 N)
loaded on the wheels.
The dimensions, connection points, mass properties, and
center of mass (CM) of each part of the scissor lift were
derived from its corresponding components or assemblies in
the technical drawings. The parameters of the constraints/
connections of the model are listed in Table 2.
Figure 3. An ADAMS model of the scissor lift.
Table 2. Major flexible connections and their major parameters of the model (K: stiffness. C: damping. ID refers to the con-
straint number in Figure 3).
Constraint ADAMS Solid to solid contact
ID Location K (106 N/m) C (103 N-s/m) Force Exponent Penetration Depth (mm)
12a wheel to road 1.39 10.0 1.5 2.5
12b whee l to c urb 1.39 10.0 1.5 2.5
13 pothole protection device to road 10.00 200.0 2.2 0.1
14 outer scissor frame 1 roller to base 1.18 8.0 1.2 0.1
15 outer sciss or frame 4 r o lle r to platform 1.00 5.0 2.2 0.1
Translational Rotational
ADAMS bushing or spring-damper K (106 N/m) C (103 N-s/m) Kr (N-m/deg) Cr (N-m-s/deg)
Y 4.00 100.0 0.05 0.0005
Z 1.18 8.0 0.10 0.0010 16 outer scisso r frame 1 to base
X 0.40 20.0 0.05 0.0005
17 actuator axis 90.00 40.0
The contact stiffness between the pothole edge and the
pothole guardrail (Connection #15) depends on their com-
bined stiffness. To understand the basic pothole response,
the simulated pothole was assumed to be identical to that
used in the experiment (Figure 1), which was a square
hole on a steel plate. While the steel edge is very rigid,
the contact stiffness primarily depends on the guardrail
bending stiffness. A beam model simply supported on two
points at the actual guardrail support locations on the base
was used to estimate the stiffness. Obviously, the stiff-
ness is a function of the contact point when the wheel
moves into the pothole. The estimated minimum stiffness
is 5500 kN/m when the contact is at the middle point of
the guardrail. At the contact range (about 1/4 distance of
the guardrail from the center) observed in the majority of
testing trials, the stiffness was approximately 10,000 kN/m
and it wa s us ed in th e si mu la tio n . Th e av e r ag e co e ff ic ie n t
of friction (0.3 to 0.35) for a steel-to-steel contact was
also used for this contact element [11].
The equivalent stiffness values for the connections be-
tween the scissor structure and the base were determined
based on the mean roll and pitch frequencies for the plat-
form with the extension, which are presented in the next
section. The damping values for these connections were
also determined based on the mean decay rates of the
fundamental pitch and rolling responses measured in the
curb impact. Similarly, the stiffness and damping of the
actuator spring and damper were also estimated from the
bouncing responses measured in the curb-impact tests.
5. Model Evaluations
The validation of the model for the critical rigid body
motion largely depends on the system’s center of mass
(CM) position. To verify the CM of the model, an ex-
periment was also performed to measure the CG of the
scissor lift using a tilt table method. A sufficiently large
tilt angle is required to reliab ly measure the CG; howev er,
because of our limited lab space and the safety concern,
only four elevated heights (measured from the ground to
the floor of the main platform) were considered, as listed
in Table 3. In the measurement, the platform and exten-
sion were not loaded. The CGs for the fully extended plat-
form and non-extended platform were measured. The CG
positions were also calculated from the model (without
the load weights). The percent differences between the
modeling results and the experimental data are listed in
Table 3. The small differences suggest that they agreed
with each other excellently. This suggests that the basic
mass distribution in the model is very reasonable.
For the purpose of this study, the pavement was con-
sidered perfect in the modeling. Figure 4 shows the pre-
dicted responses in the 30˚ forward curb impact with the
full height platform and extension. Compared with those
shown in Figure 2, the model peak magnitudes are not
exactly the same as those observed in the experimental
data but their basic response trends and the acceleration
distributions on the scissor lift are very similar.
In the 90˚ curb impact, the excited major motions are
in the traveling direction (Y). Figure 5 shows the com-
parison of the modeling (right column) and experimental
(left column) responses of the scissor lift without pulling
out the extension. Their basic dynamic features are very
Figure 6 shows the comparisons of the predicted and
Table 3. Percent differences between the measured and mod-
eled centers of gravity (CG) of the scissor lift.
Percent difference (%)
Without platform
extension With platform
Elevated height (m)
0.997 0.2 0.0 4.3 0.0 0.2 3.0
1.524 0.1 0.0 1.9 0.2 0.1 1.4
2.155 0.2 0.2 2.0 0.4 0.2 0.7
3.052 0.2 0.2 0.7 0.7 0.2 0.7
Figure 4. Examples of the accelerations predicted in the mo-
deling for simulating the full height 30˚ curb impact.
Copyright © 2012 SciRes. OJSST
R. G. DONG ET AL. 13
Figure 5. Comparisons of the modeling (right column) and
experimental responses of the scissor lift to the 90˚ curb im-
pact under the test conditions: fully-elevated platform, no
extension, and fully-loaded.
Figure 6. Comparisons of the modeling and experimental
responses of the scissor lift to the pothole depression under
the test conditions: fully-elevated platform, pulled-out ex-
tension, and fully-loaded.
experimental responses of the platform to the pothole de-
pression. The predicted responses were comparable with
those measured in the experiment.
6. Model Applications and Discussions
Whereas this model can be used to simulate and under-
stand the tip-over under many hazardous scenarios, the
current study applied it to examine th e tip-over potentials
under four scenarios and to evaluate or explore the re-
lated prevention methods. Th e influence of the scissor struc-
ture stiffness was considered as an independent variable
in some of these scenarios. The roll and pitch motions of
the scissor structure and platform are primarily controlled
by the vertical stiffness (Z-axis) in Connections #14 and
#16 in the model (Figure 3). To simplify the analysis,
the tip-over potential or threshold was considered as a
function of the vertical stiffness. The related vertical damp-
ing value was assumed to vary proportionally to the stiff-
6.1. Tip-Over Threshold on a Sloped Ground
In principle, the tip-over occurs when the scissor lift is on
a ground with a slope beyond a certain value, which is
termed as tilt tip-over threshold in this stud y. Th e process
used in the simulation for identifying the tilt tip-over
threshold is similar to that used in a tilt ta ble test of a vehicle.
Because the wheel span is the shortest in the lateral (X)
direction, the largest tilt tip-over potential is in this direc-
tion. Hence, its major influence factors were examined in
the modeling study.
If the platform of the lift is elevated from a stationary
position, the tilt tip-over threshold primarily depends on
the elevated height of the platform. The smallest tilt thresh-
old is at the fully-elevated platform. As the first applica-
tion of the model, this study examined the effect of the
lift structure flexibility on this threshold. The results are
shown in Figure 7(a), in which the quasi -static tilt t hreshol d
(slope angle) in the rolling direction is expressed as a
function of the stiffness ratio (the variable stiffness di-
vided by the normal stiffness value, 1180 kN/m, as listed
in Table 2). Increasing the stiffness generally increases
the tilt tip-over angle as the flexible deformation result-
ing from gravity makes the CG of the lift move further
towards the tilt direction. However, once the stiffness is
close to or higher than two times the normal stiffness, the
tilt tip-over angle remains more or less the same. A mar-
ginal change (<15%) from the normal stiffness results in
only a slight change (<1.0%) in the tilt tip-over angle.
The reduction of the tilt tip-ov er threshold could become
significant (>5.0%) when the reduction of the scissor
structure stiffness is more than 60% of its normal stiff-
Using the lift on a soft soil base could be hazardous
but the potential danger may not be perceived at the be-
ginning of the lift operation. The wheels could gradually
and unevenly penetrate into the soil during operation, espe-
Copyright © 2012 SciRes. OJSST
Copyright © 2012 SciRes. OJSST
cially when there is some water in the working area. The
Figure 7. The tip-over thresholds under four hazardous co n d i -
tions. (a) The effect of the structure stiffness of scissor lift
on the quasi-static t ip-ov er thresh old (tilt an gle) . (b) The effect
of tilt speed on the tip-over threshold (tilt angle). (c) The
effect of curb impact speed on the tip-over. (d) The effect of
scissor structure stiffness on the requirement of the guard-
rail height in the pothole depression.
wheel penetrations could in effect form a sloped uneven
support. Then, the activities of the workers on the plat-
form could cause some rocking motions of the lift, which
could further accelerate the formation of uneven support.
Rocking motions could also happen when the lift is on
other deformable surfaces such as bridged wood boards
or metal sheets. If the motion of a worker on the platform
is periodic and its frequency is close to the resonant fre-
quency of the lift, the rocking motion could be signify-
cantly amplified and eventually result in a tip-over inci-
dent. This may be one of the mechanisms of tip-over at
some construction sites. While it is beyond the scope of
this study to simulate these hazardous conditions and the
related tip-over events, the effect of the rock/til t speed on
the tilt threshold was examined, as plotted in Figure 7(b).
As expected, if the tilt speed is low, the threshold is close
to the quasi-static threshold. However, the tilt threshold
is substantially reduced when the tilt speed is greater than
This is because the dynamic energy could increase the
tip-over potential. This observation suggests that moni-
toring the tilt angle alone may no t be sufficient to prev en t
tip-over. Both the tilt angle and tilt speed should be con-
sidered in the design of a preventive device such as a tip-
over monitoring and warning system.
This study also used the model to explore a hypothe-
sized intervention approach. Theoretically reducing the
platform height can lower the CG of the lift and increase
the tip-over threshold. The height reduction may be auto-
matically actuated when the tilt ang le and speed of th e lift
reach certain values. As a preliminary exploration, the high-
est possible downward acceleration (1 g) of the platform
was used in the simulation. The results indicate that this
approach would not work if the action takes place when
the lift goes beyond the threshold shown in Figure 7(a),
because it takes too much time for the platform to be
actuated and lowered to a required safe height. To make
this approach work, the descending action must be actu-
ated before the lift reaches the threshold and the specific
action time depends on the tilt speed.
6.2. Tip-Over Threshold of Curb Impact Speed
The impact speed determines the kinetic energy of the
scissor lift. The kinetic energy is partially consumed due
to the system damping and partially transferred into po-
tential energy. The potential energy includes the gravita-
tional potential en ergy resulting from the lift’s CG elev a-
tion and the elastic potential energy of the lift structures.
In the standardized analysis, only the gravitational poten-
tial energy is considered in the calculation of the tip-ov er
speed thresho ld [5]. Th eref ore, the rigid body assu mption
used in the analysis would be on the conservative side if
R. G. DONG ET AL. 15
th e CG change due to the flexibility of the structures could
be ignored. In reality, both damping and flexibility play
some roles in the impact event. While the damping and
elasticity of the scissor lift structures tend to absorb part
of the impact energy, the CG change due to the flexibility
of the structures tends to increase the tilt effect du ring the
impact. As a result, the flexibility did not substantially
affect the tip-over threshold of the impact speed, as
shown in Figure 7(c). However, the results suggest that
to reduce the tip-over potential, it is better to keep the
scissor structures as stiff as possible.
6.3. Tip-Over Threshold of Pothole Guardrail
Figure 7(d) shows the effect of the structure stiffness on
the pothole guardrail height design. Again, because the
flexibility of the structure mak es the CG of the lift move
further toward the tilt direction, a reduction in the stiff-
ness of the lift requires reducing the pothole guardrail
height from the ground so that the tilt angle can be con-
trolled within the stable limit. The results also indicate that
pothole guardrails should be designed to be as low as
possible, providing that this will not affect the movement
of the scissor lift when its pothole guardrails are not de-
7. Conclusion
The results of this study confirmed that increasing the
flexibility of the scissor lift structure generally increases
the tip-over potential. The tip-over threshold is a function
of both ground slope and tilt sp eed of the lift. The curren t
results also suggest that when the scissor lift system be-
comes slack due to severe wear of structural joints and/or
any structural damage, the scissor lift should not be used
before fixing the problem. The lift should not be elevated
on soft ground and/or uneven surfaces. The worker on the
lift platform should avoid any large continuous periodic
movement or forceful action in the horizontal plane, es-
pecially when the lift is fully elevated. Besides the tilt
angle, the tilt speed of the lift can be measured and used
to help prev ent the tip-over.
8. Disclaimer
The content of this publication does not necessarily re-
flect the views or policies of the National Institute for
Occupational Safety and Health (NIOSH), nor do es men-
tion of trade names, commercial products, or organiza-
tions imply endorsement by the U.S. Government.
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