Bending Stress Calculation (Which Area Moment to Use?)

[mbuller32]

I'm trying to compare normal stress theoretical values with SolidWorks simulated values.

Refer to Picture
When I use Ix values, my %difference is large.
When I use Iy values, my %difference is less than 1%.

I thought the correct "I" value to use would be Ix. Am I wrong?

https://files.engineering.com/getfi...-8693-47aa-a2c2-674b515f355e&file=Capture.JPG

 

[3DDave]

The axis referred to in I() is like the pivot axis for the bending. In this case the x=axis is pointing at the load, so there won't be any pivoting. the y-axis is perpendicular to the load.

 

[BAretired]

You are doubly wrong! It is neither Ix nor Iy. It is Iz. And, by the way you have shown your axes incorrectly. The Z axis should be pointing in the opposite direction. (right hand rule)

Edit: I interpreted your green arrow as vertical, but the table shows Iy to be larger than Ix, so Iy is the value to be used (usually Ix is the major axis, but not in this case). The moment 'M' is about the Y axis. The axes should be as shown in the red arrows below.

BA

 

[BridgeSmith]

Moments are labeled based on the axis they rotate about. In this case the moment arrow is wrapping around the Y axis, it's designated as M. Moments of inertia are designated the same way, so that when doing calculations, you use M and I to calculate deflections, etc.

Rod Smith, P.E., The artist formerly known as HotRod10