I am using Inventor 2011 Professional. When I go to the Stress analysis and apply a load, do I need to apply gravity to the assembly to get an accurate result? I have tried running with and without gravity and it is having a dramatic effect on the results. Do I need to always run my simulation with gravity applied or not? I would like to understand the reasoning of the principle behind the force of gravity on an assembly. Thanks, MAt

Gravity is very real and very much a part of a complete analysis. If you want a valid analysis, you MUST consider gravity.

In general, I would expect gravity to become less significant as the load on a part becomes much greater than the weight (i.e. F>>mg). You could prove it to yourself: considering a vertical bar under its own weight, calculate the stresses, then add a downward force at the end and calculate the stresses. You should see that as F becomes much larger than mg, the contribution of mg to the total stress will become small. Thus, gravity will be most significant as the load approaches or is exceeded by the weight (i.e. F~=mg or F<mg). How does the load you applied to the part compare to its weight?