Specifications

CT Corsair Final Report May 2, 2014
31
Figure 31. Free body diagram for upper scissor
arm analysis
in the z-direction. The scissor arm system is shown in Figure 29. The upper scissor arms are
shown in green, the lower scissor arms are shown in red and the shock absorbing springs are
shown in blue.
8.2 Purpose of Redesign
The previously manufactured lower scissor arm was incorrectly
designed and over-engineered. The existing upper scissor arms from the
Gyro IPT
TM
consist of welded aluminum plates and internal pin
structures. The existing lower scissor arm is made of 1.5in steel tubing
with welded on pin tabs. The built-in safety factor of this component is
nearly seven times the failing criteria of the upper scissor arm. The arm
adds significant weight to the lifting of the central spring unit, with each
arm weighing approximately 10lbs. The scissor arm also collides with
the motor while in rest position and does not allow for the unit to rest on
its base when turned off, as shown in Figure 30 in the red box. The
redesign uses less material, eliminates interference between
components and was designed based on the failing criteria of the upper
scissor arm to avoid over-engineering.
8.3 Scissor Arm Theory and Free Body Analysis
To perform the finite element analysis on either scissor arm,
it is necessary to understand how the loadings are applied to
the arms, the different failure scenarios that could
potentially break the arms and the different assumptions that
need to be made to design a stable system. A free body
analysis of the entire scissor arm system is shown in Figure
31. For the analysis, the upper scissor arm is the point of
interest. There are two forces acting on this component and
therefore it can be simplified into a two force member,
shown on the bottom of Figure 31.
From the free body diagram, it can be seen that the upper
scissor arm is in compression. The forces acting on the arm
are a portion of the simulator weight and the force of the
lower arm pushing back on the upper arm at the pin joint. To
further simplify the loading case, one pin was assumed to be
fixed while the other provides a compressive load against the
bearing hole in a direction purely along the arm, Figure 32.
Realizing that the load is being concentrated on the bearing
holes rather than the end of the arm itself, a high
concentration of stress inside of the bearing holes was
Figure 30. Old lower scissor arm
installed on system. Notice gap
between upper ring and lower ring
on base indicated by the red box.
Figure 32. Upper scissor arm simplified into a
two-force member