Video-Use Solidworks to Calculate Area Moment of Inertia Worry Free

Calculating the moment of inertia can be extremely difficult. The good news is the 3D modeling software can simplify this process greatly and make it pain free.

Video Transcript

Welcome to The Mentored Engineer. In this video, we’re going to talk about getting section modulus from 3DCAD. In specific, we’re going to talk about doing it in SOLIDWORKS. All right, now, Well, I’m demonstrating everything here in SOLIDWORKS. You can pretty much get this from any 3D CAD system that is out there.

They pretty much all do this. So, it’s just a matter of finding out how to do it in your specific 3D CAD system. So, if you don’t have SOLIDWORKS, you just might want to look in your the help section of your particular 3D CAD or perhaps YouTube has a video on it somewhere to help you. OK, so we’re just going to go through the functions and show you exactly how to do this. So, I’ve got certain shapes here, the first of which is a round circle.

And to get the section properties on here, I’m going to select the surface I am interested in. And in SOLIDWORKS, you can do this one of three ways. The first one is to go to the command manager and to click on the evaluate tab. And then right here are section properties. And it pulls up a dialog box.

Alright, the second way is to go to Tools and go down to the Evaluate pop-up here and then say Section Properties. Or you can do what I’ve done and program it to be the letter R under tools and customize, and then you can go to the keyboard tabs. All right. So right here you can see you have this pop up. If I didn’t select this face, I could go ahead and select it there and then hit recalculate.

If I’m in assembly, I can select multiple faces as long as they’re all on the same plane. All right, so it tells me a bunch of stuff. The first thing it tells me is the area of that surface, which is 5.3 inches squared. So that’s always a nice thing to have. The next thing it does is tell me where the surface is in my relative coordinate plane as far as where the centroid ends up.

OK, so this is a circle, and I drew it about the origin here. it’s going to put its axis there so it’s zero inches in the x and you’ll look down here in the bottom left hand corner for your x dimension your y dimension and your z dimension all right and it’s saying it’s on center x and y which makes sense

and then it’s six inches from the center of the tube to the end of it so this must be a 12 inch tube let’s see if that’s right oops doesn’t like that while I’m in there all right we’ll figure that out some other time So the next thing it does is it calculates what the I-X-X, I-Y-Y, and I-Z-Z are. Okay, so I-X-X is bending about the X1, so this would be bending the tube this way. The I-Y-Y would be bending it side to side, and the I-Z-Z would be putting a torque end to end on this.

Okay, so since it is a perfectly symmetrical tube, we get an Ixx and an Iyy that are the same of 30.23. Now, the torsional one, as we mentioned in our stress flow class, is going to be the sum of those to get our polar moment of inertia. In this case, it’s 60.47, so that is 30.23 times 2, and then you get some rounding here and there. No big deal. And it says here that specifically the polar moment of inertia at the centroid is 60.47 inches.

Alright. Then we have what the axis is, or the angle between the axis is zero degrees and that’s okay. We’ll come to that and why it’s different in just a second. And then it gives us the principal moments of inertia, which is the same as the Ixx and Iyy. Okay, so this last box in the section properties is kind of useless.

I’ve used it once or twice in my engineering career. It’s not very popular, but it’s the area moment of inertia about the coordinate system that you started the whole part from. And most of the time we don’t care because we’re just looking for a specific section and we’re looking at the centroid of that section. So, in this case, it is just moving the section from the middle of this tube where the origin is out six inches to this surface. So, the formula for this is the existing moment of inertia plus the area of that section times the distance to that plane squared.

So, in this case it was 6 inches squared times the area which is 5.3 inches plus the original 30.23 inches and you get 221.09 inches to the fourth. Alright not very useful. Okay so let us close this one and go into something where we start getting a little bit different dimensions here. Okay, so here’s a rectangular tube with the centroid on our X and Y, just like we would have. We’re going to flip that on.

Once again, we get our area, 3.52 inches, and we find out that our Z coordinate is three inches from the center, so this must be a six inch tube. Alright, and we find out that our moment about the tall dimension, or strong axis is what I like to call it, is 17.44 inches cubed, or inches to the fourth. And then bending in the weak axis is 9.32 inches to the fourth. Alright, we could probably guess that. And when we add those together to get our polar moment of inertia and remember this only works for closed section cross sections, we get 26.76 inches to the fourth, which is the sum of the other two.

Okay, if we look at the other ones, it kind of just goes through the other stuff and we see that our principal moment of inertia are exactly what we have. up above, and then we can get the moment of inertia from our output coordinate system as well. Now let me show you how that changes when we make a simple change. Alright, I’ve changed my configuration here, moving my solid up from the central end of the tube to the bottom.

Alright, let’s see what changes now. All right, same area, same moments of inertia at the centroid. So, it doesn’t matter in the whole real world space of what it exists at. At that point, we just care at that very surface, where’s the centroid? Okay.

And now it gives us information up here that our Y is three inches off. All right, so we’re going to want to take note of that. our polar moment of inertia is here same as what’s up here and you see these numbers have changed and we’ve actually added in some new numbers here of what we would do if we were rotating about different accesses. I don’t want to go into that. It really isn’t helpful, and it doesn’t really show us anything new.

So, we’re not going to talk about those other ones. But you can see it did change everything down here than what it was before. Okay, let’s go on to an unsymmetrical shape. This right here is a standard Z channel. And we’re going to take our section modulus again.

And we’ve got our area. We’ve got our output coordinate system and how much is different. I have it symmetric about the top and bottom here. And now we have other numbers here. Look at this.

1.5 or negative 1.5. and that is calculating what’s going on with this random arbitrary coordinate system I put in here. The other ones, because they were symmetric models, used just the X and Y they have a strong axis and a weak axis so what it’ll what it’ll do is it’ll attempt to find out where the maximum moment of inertia is on this and orient their coordinate system there now we are not concerned with this in most cases What we want to do is always, always, always look up here for our moments of inertia at the centroid in the straight up and down x and y coordinates.

In which case it is 3.7 and 1.0 and a polar moment of inertia of 4.7. Now, it’s also saying, hey, this angle between the coordinate system I put in and the coordinate system you put in is 65.8 degrees. And I can look at this model and say, yeah, that’s probably close to 65.8 degrees. I’m going to look X here to Y there. But yeah, about that dimension.

But then it gives us the principal moments of inertia and that’s the moments of inertia at this maximum here where my YY is 4.4. I’m bending about the farthest tip to tip basically. bending about that one so that they would have the most tension and compression loads. And then the XX really doesn’t have much section because a lot of it’s on or near the neutral axis. Okay, but we don’t really use that much, so I just kind of throw it away.

And then you can see here that our moments of inertia about the output coordinates are much different here. But like I said, I rarely have ever used those. Okay now let’s go to something that’s a little bit more complex and go to a T. Now it’s just a standard structural T here and I’m going to have select my mass or

sorry I’m going to select my area moment of inertia here and I’ve got my area of 8.5 inches squared and I’ve got a Y here of 6.3 inches. Now that’s going to come in handy in just a second. And I’ve got all my moments of inertia here and we’re going to keep those just like they are.

Alright, so what we’re going to do is make a sketch on this surface. Alright, we got our sketch and we’re going to put a point right here at 6.7 and that will tell us our dimension of C. Now if we’re interested in going from the centroid to let’s say this point or the top surface, we can figure out what those are just by using the sketch and putting that in there. So if our coordinate system was further up here at the top surface at this corner anywhere else we could come back and find out where the centroid is from that point and then figure out what our C is for bending okay

and the last thing I want to show you is if you have a coordinate system or a part here where I have another flange in here that I’ve added and it’s just in there for a little bit. I can’t grab it from either end of the model. And all I want to do is figure out what that is. So, what I can do is I usually just come in here at a random spot and make a sketch.

I usually don’t even constrain the sketch because I’m going to delete it pretty soon. And then make a cut through all. Alright, and now I can select my surface. I can go in and look at my moment of, area moment of inertia properties and figure out, well that’s my, my strong axis bending moment. This would be the up and down if I’m trying to bend it in that direction.

I get 84.4 and I know it’s 5.9 inches from the top of the beam. So, at this point, I can go in here, I can make another sketch. Make this 5.9. So, I did move it down about .6 inches adding that little flange in there.

And I can get my distance from the centroid. to the top and bottom, I can get it from any of the other coordinates. So that’s how I use sketches there. Where this really comes in helpful is when you have an assembly, and you are trying to figure out exactly where you need a section at exactly the right spot. And you can go in and just make a cut through all the parts you like.

Calculate that section out real quick, figure out your stress, and then go ahead and delete those functions or just suppress them. A lot of times, you know, just makes it go real quick if you can keep those. So, if you change the section, you do that. You can take as many sections as you want very quickly. Just make sure before you release the part for production, you actually take those out.

So that is how you get section properties in SOLIDWORKS. We hope you do this often and thank you for watching this video.