WALKING A FINE LINE
Mechanical Event Simulation software helps a hand-held computer
manufacturer strike a balance between design optimization and production costs.
||Reliability engineers at Intermec Technologies Corporation, Cedar
Rapids, Iowa, used Mechanical Event Simulation software from ALGOR, Inc.,
to evaluate the suitability of an integrated scanner module design (see
inset) for this mobile hand-held computer. The module is designed as a pod
that attaches to the back of the computer and consists of a laser scanner
for bar code recognition and a micro-switch for activation. Product
testing revealed that the new activation micro-switch would fail to meet
an operational life requirement for field use.
By: Bill Woodburn
Reliability Engineer, Intermec Technologies Corporation
Each day engineers and business managers across all industries are faced with
a common dilemma - weighing design optimization vs. manufacturing costs while
meeting time-to-market demands. As a reliability engineer at Intermec
Technologies Corporation (Intermec) in Cedar Rapids, Iowa, I have dealt with
this dilemma first-hand. Fortunately, designers and engineers can use CAD and
FEA software to shorten design times by reducing physical prototyping. In
addition, the animation of FEA results is a powerful tool for presenting
engineering data to both technical and non-technical personnel about the outcome
of design changes.
A large part of my job is to ensure that Intermecís supply chain system
solutions (mobile hand-held computers and automated data collection devices) can
withstand the wear-and-tear of use in distribution, field service, warehousing,
utilities and transportation industries. Recently, I used Mechanical Event
Simulation software from Pittsburgh-based ALGOR, Inc. (ALGOR), to evaluate the
suitability of an integrated scanner module design for an Intermec mobile
hand-held computer. The analyses helped my company make prudent "design vs.
cost" decisions that balanced gains in product life with production costs.
DEFINING DESIGN ISSUES
Intermecís reliability team performs a full suite of environmental tests,
such as operating and storage temperatures, temperature cycling, humidity and
water resistance, on new product designs. We also perform durability and life
tests such as packaged and unprotected drop, vehicle vibration, key/button life
tests and touch panel durability.
For this particular integrated scanner module, extensive durability and life
tests were performed. The module was designed as a pod that attaches to the back
of the hand-held computer. The pod consists of a laser scanner for bar code
recognition and a micro-switch housed within a durable polycarbonate/ABS plastic
blend. The operator activates the scanner by pressing a large curved button
attached to the housing. The scanner then reads the bar code into the hand-held
During preliminary testing of this module, it was discovered that the scanner
activation button failed to meet the three million depression requirement needed
to achieve an operational life of three to five years. In some instances, the
buttons experienced cracking at the connection points within the housing and
failed at around one million depressions (estimated to be one to one and a half
years of operational use).
The next logical step was to perform a Mechanical Event Simulation on the
original CAD solid assembly to simulate the mechanical failure and then develop
a proposal for design modification based on the findings.
ALGORís Mechanical Event Simulation software was chosen as our best
analysis option because of the physics involved in pressing the scanner button.
A typical linear static stress analysis determines stresses based on a
calculated force applied to an area of the model. For the scanner button, the
force applied could not be accurately determined due to the fact that different
operators will press the button with varying force each time. However, we could
easily measure the maximum displacement of the button when it is fully
depressed. Using this known value, we could specify a time-varying prescribed
displacement in a Mechanical Event Simulation. The software determined the
bending stress results based on the motion and displacement that occurred over
time. We could be confident that the stress results were accurate because we did
not need to make assumptions about the forces at work in this design scenario.
DEVELOPING DESIGN OPTIONS
To begin the Mechanical Event Simulation, the reliability team acquired the
CAD assembly, which was created in CoCreate Solid Designer, and imported the
model into ALGOR using an IGES universal file. We used a combination of
automatic and hand-meshing techniques to create a consistent solid brick FEA
mesh throughout the entire model.
The assembly model consisted of two parts: the large curved button and the
connecting piece of housing. The button model used six heat stakes that protrude
through holes in the housing. During the manufacturing process, the heat stakes
are melted down onto the housing to bind the two components. For the FEA model,
truss elements were used to effectively bind the heat stakes to the housing.
Fully constrained boundary conditions were added to the back wall of the housing
component to represent the portion of the housing that was not modeled.
Once the modeling was complete, we identified the material properties for
polycarbonate/ABS blend polymers, which were provided by Intermecís plastic
molding supplier. We decided to use ALGORís von Mises nonlinear material model
with kinematic hardening to account for possible yielding in small areas of the
part that exhibited cracking during laboratory testing. Finally, we added a
prescribed displacement of .105 inches, the maximum button travel measured from
test units, and defined an event duration of .01 seconds, the approximate time
it takes to press the button, with a capture rate of 10,000 per second. This
rate was chosen to ensure that the analysis would adequately capture the
assemblyís behavior throughout the entire event.
||A Mechanical Event Simulation of the original geometry, shown above,
was performed with ALGOR software to determine the maximum von Mises
stresses that result when the button is activated. Based on these ALGOR
results, the reliability team proposed four geometry modifications to
reduce the bending stresses at the back wall of the housing. The inset
shows the final housing geometry, which features an increase in material
at the intersection with the back wall. The final design balances design
optimization and production costs by satisfying life actuation
requirements with the least amount of cost.
The Mechanical Event Simulation in ALGOR yielded stress results for each of
the 100 timesteps. The maximum von Mises stresses and displacements occurred
near the end of the event (timestep 80) on three narrow connection points at the
back wall of the housing. We used the von Mises stress results as input for
fatigue calculations to determine the Modified Goodman Safety Factor and the
estimated fatigue life. The existing geometry had a safety factor of 0.69. A
Modified Goodman Safety Factor less than 1.0 gives an indication of finite life
while a value greater than 1.0 indicates near-infinite life. The calculated
assembly life was found to be below one million depressions. I was confident in
these results as the calculated value of depressions agreed well with empirical
life test results.
Based on the location of the maximum stresses, the reliability team made a
series of four proposed geometry modifications to reduce the bending stresses.
These included removing one row of heat stakes, centering the material that
intersects with the back wall of the housing and flattening the area of
intersection, thinning and lengthening the intersecting material region and
integrating the button modification with all of the housing modifications.
Safety factors for the four new designs ranged from 0.72 to 1.26. Estimated
assembly life calculations ranged from just under one million to 5.8 million
depressions. ALGORís precision contouring was used to ensure that it was
appropriate to compare results between the original model and the new designs.
This feature gauges the quality of the finite element mesh, revealing any
discontinuities that may affect the accuracy of results. The precision contours
for each analysis revealed similar values. The final proposed design, which
integrated the button and housing modifications, exhibited the greatest
reduction in bending stresses.
The reliability team animated the Mechanical Event Simulation results using
ALGORís built-in animation tools. Previous experience has shown that our
audience, both research and development engineers and business managers, can
more readily absorb the quantitative and qualitative aspects of FEA results when
they can view an animation of the simulated mechanical events. In addition, we
could easily illustrate how the stress distribution changed as the modifications
to the geometry were implemented. By showing the research and development
engineers the animated results, we were able to present them with different
options to consider - what to try and what not to
try. This was the intent of performing the Mechanical Event Simulations with
DEFINING THE FINAL DESIGN
In determining the final design, Intermecís research and development
engineers had to take into account the manufacturing process already in place.
Plastic molding tools had already been created for the manufacturer of the
scanner housing. The reliability teamís best-engineered design would have
required the manufacturing equipment to be retooled at a cost of tens of
thousands of dollars. Whereas, a lesser modification of the placement of
material at the connection points of the button with the back wall of the
housing could achieve a satisfactory safety factor and estimated assembly life
performance, while only costing a few thousand dollars in modifications to the
Decisions for changes in the final design were based on the stress contours
from the original analysis. More material was added to the intersection of the
housing back wall and housing and generous fillets were added to reduce stress
concentrations. The cost of adding material to the part (or removing material
from the tool) was minimal in comparison to the cost and time needed for a more
substantial retooling. In the end, Intermec struck the optimal balance between
design optimization and production costs by choosing a design that satisfied
life actuation requirements with the least amount of cost.
By using ALGORís Mechanical Event Simulation, the reliability team was able
to circulate design optimization ideas quickly across several departments within
the company. Presenting animated results also enabled individuals with all
levels of engineering experience to understand of the outcome and implications
of our stress analysis. In the future, we will continue to use ALGORís
Mechanical Event Simulation to troubleshoot product designs as well as to design
adequate vibration test fixtures for Intermecís reliability testing