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The first part of this analysis involves measuring the geometry of the engine mount to determine the unit vectors (see Table 7). A three-dimensional equilibrium analysis was used to calculate the forces in terms of a static load (Appendix B). Using a static load to determine fracture weights is a reasonable approximation because only low frequency fluctuations will be applied. Dimensional properties of the circular mount bars were determined by applying principles from the Mechanics of Materials text by Roy Craig [21]. The engine mount truss members have a cross sectional area of approximately 0.25 in2.
Table 7: Unit Vectors of Engine Mount Analysis
|
Member
Number |
i |
j
|
k
|
FBD
# |
|
1 |
-0.0625 |
0.998 |
0 |
1 |
|
2 |
77483 |
0.0894 |
0.6258 |
1 |
|
3 |
0.9638 |
-0.26667 |
0 |
1 |
|
4 |
0.69519 |
-0.5336 |
0.4492 |
2 |
|
5 |
0.8295 |
-0.079 |
0.553 |
2 |
|
6 |
1 |
0 |
0 |
2 |
Figure 75 displays a simplified version of the engine mount geometry. After an equilibrium analysis, the member with the highest stress was member 4, which had a tensile stress 24 times the weight. The highest compressive load was in mount member 5, whose load was calculated to be negative 19 times the weight (Table 8). The static weight necessary to cause the engine mount to fail in tension was calculated to be about 978 lbf.
Figure 75: Engine Mount AutoCAD Front View
Table 8: Force Magnitudes of Engine Mount
|
Member
Number |
Force
Magnitude as Multiple of the Static Load |
Tensile
or Compressive |
FBD
# |
|
1 |
W*(-11.23) |
C |
1 |
|
2 |
W*(-0.035) |
C |
1 |
|
3 |
W*(-0.7) |
C |
1 |
|
4 |
W*(24.27) |
T |
2 |
|
5 |
W*(-18.93) |
C |
2 |
|
6 |
W*(-0.4761) |
C |
2 |
The previous analysis determined which of the members is most likely to fail. First, the ultimate stress of the mount material will be used to determine the tensile load at which it fails. Next, the Euler Buckling equation will be used to calculate the buckling load for mount member number 5. The tensile result yielded that the tensile failure load is 196 lbf. The Euler equation result is not yet available.
A metal saw will weaken the engine mount member that is easiest to fracture. Determining how far to cut will involve some analysis. The method chosen to determine how much to cut is decreasing the cross sectional area of the mount member to a reasonable decreased percentage. First, the cross sectional area of mount member 4 was reduced by 80 % and the failure load calculated to be 196 lbf. Note that there are errors in this approximation because of the moments in a sawed weakened member and the neglected reaction moments.