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 like any other load on a structure, and any analysis that ignores its effects is suspect. However, the magnitude of the effects of the gravitational load will be just like the magnitudes of any other loading- it depends on the design of the system being analyzed. If the results of your analysis differ significantly from experimental results or your expectations, there may be an issue with the way you are applying the loads to the structure, be they gravitational or otherwise. The fact that you get significantly different results with and without gravity suggest that in this case, gravitational effects are not trivial.

Also, bear in mind that in a linear analysis the effects of each load add together to give the total result. Therefore if you run the analysis with "gravity only" and again with "other loads only" you can compare the relative magnitude and direction of stresses due to the two load cases, which often helps to understand the "combined" results.