# Homework Help: Complex Moment of Inertia Calculation

1. Oct 31, 2014

### k2kyo

1. The problem statement, all variables and given/known data
I'm just looking for the proper equation to use to find the moment of inertia of a complex assembly of bodies. I'm not a student and this isn't homework, but it seemed the most relevant place to pose the question.

One of my hobbies is making toys and I'm trying to calculate the moment of inertia for a given section of an assembly as it relates to the whole system. All my parts are round, revolved around the same X-axis. The assembly is mirrored on either side of the Y-axis. Therefor, I know all the centers of mass are y = 0, and I can calculate all the x distances (dx in my drawing).. what I'm unsure of is how to relate that x distance into the moment of inertia formula so that I can know what each body is contributing to the whole.

Taking a section of the drawing.

Body 3)
Area = 146.27 mm^2
Mass = 20.12 grams
Volume = 15545.56 mm^3
Moment of Inertia of the single body (Ix) = 4416.93
d(x) = 12.53

Now using solidworks, I know the result..

Moment of Inertia for body 3 in relation to the whole system (Ixx) = 7576.69

But I don't know how it got there.. am I correct in assuming the variable is the distance x to the center of its mass? If so, how do I integrate that into the equation?

Drawing -
http://www.nothingtoseehere.info/Example_Problem1s.jpg

2. Relevant equations

This is the part I'm looking for :) I don't need any help working the equation, just a point in the direction of what I should use.

3. The attempt at a solution

I can calculate the individual moment of inertia for a given part. What I'm trying to work out is a system where I could swap out body 3 or 4 for any other similar design, and recalculate the moment of inertia for the whole assembly without having to build a new assembly in solidworks. So if I can figure which formula to use, I can use the variables to calculate the Ixx for each section 3 or 4 I want to use, and swap those out.

I hope that makes sense, this problem has been driving me nuts. I use to do this type of stuff in college but I fear it's been too many years.

Any help would be greatly appreciated

Thanks,
Kyle

2. Nov 1, 2014

### Orodruin

Staff Emeritus
3. Nov 1, 2014

### CWatters

I've deleted my reply as I was confused by which axis the object rotates around.

4. Nov 1, 2014