# Moment of Inertia Calculation for Offset Mass

1. Mar 28, 2017

### Pinon1977

Ran into a bit of a dilemma. I'm having some problems calculating the moment of inertia for an offset weight configuration. Please see the attached drawing. Basically this offset weight will be turning around an axis as labeled in the drawing. Any help would be greatly appreciated

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2. Mar 28, 2017

### Staff: Mentor

Can't quite understand your drawing, but can you just find the moment of inertia for each mass individually and then add them up?

3. Mar 28, 2017

### Pinon1977

The drawing depicts a front view and side view of a tower that is rotating about an axis. To answer your question, yes; however I cannot find a standard shape formula for this geometry. The 50lbs is a rectangular steel billet and the frame is steel 3" flat bar.

4. Mar 28, 2017

### Staff: Mentor

5. Mar 28, 2017

### Pinon1977

Do you drive the distance to rotating axis value from the CL of my tower axis or from the center of mass?

6. Mar 28, 2017

### Pinon1977

Derive. Sorry not drive.

7. Mar 28, 2017

### Staff: Mentor

The table gives you the moment of inertia about a given shape's center of mass. Then you can use the parallel axis theorem to find its moment of inertia about your axis.

8. Mar 28, 2017

### Pinon1977

That's where in stuck. Can you provide me an example?

9. Mar 28, 2017

### Staff: Mentor

10. Mar 28, 2017

### Pinon1977

So, for instance, the 50lbs that is 40in away fron the rotating axel would have a MOI of 23.41 kg.m2?

11. Mar 28, 2017

### Pinon1977

?.??.???

12. Mar 29, 2017

### Staff: Mentor

If you want someone to check your calculations, show what you did, step by step. (Do it symbolically as much as possible; call the mass m, and so on.)

13. Mar 29, 2017

### Pinon1977

Yes I do want you to check my work, but u don't know how to put formulas on this message board. I'm using a very old smart phone and alot if these newer features don't work on my phone. I just need to know if In the ballpark.

14. Mar 29, 2017

### Pinon1977

So, I am politely suggesting that my question be transferred to someone who is a little more helpful and knowledgeable on the subject (as well as timely in their responses) Thanks Doc....

15. Mar 30, 2017

### Staff: Mentor

You won't find anyone more helpful or knowledgeable than Doc, you should be thankful he's been trying to help you. And we do not allow text speak on this forum it's YOU not U. Please spell out your words.

If you're trying to use an old phone that's incapable of doing what needs to be done, I suggest that you wait until you have access to a computer, all libraries have them if you do not have one.

Last edited: Mar 30, 2017
16. Mar 30, 2017

### Staff: Mentor

Here's the deal:
(1) anyone who's willing to help you can jump in at any time. Good luck with that!
(2) the fact that you had to be told about the parallel axis theorem and a wiki page on moments of inertia indicates that you are a beginner -- nothing wrong with that! But don't expect someone to decipher your diagram or read your mind.

If you want help, show what you've done, step by step:
Pick an object. Describe its mass, dimensions, and rotational axis. (Use proper units.)
State the formula used to find its moment of inertia about its center of mass and your result.
Give the distance from the center of mass to the axis you really want. State how you applied the parallel axis theorem.

That way, should you have made an error somewhere we can point it out.

17. Mar 30, 2017

### Staff: Mentor

18. Mar 30, 2017

### Pinon1977

I extend my apologies, sir. Here is my situation. I am a PE mechanical engineer that is working on a project (not a HS kid doing homework). Within the scope of this project is a machine that I've patented in which the animation of the machine involves 4 individual, multivarious "masses" moving in a non typical or unusual manner around a center point. I have yet to find a formula or equation that properly illustrates the MOI or kinetic energy of it's collective movements. I can certainly sketch it out for you, but I would rather not post this information on a public forum. Do you have an email I can send a SolidWorks Drawing to? Regards. John

19. Mar 31, 2017

### Pinon1977

20. Mar 31, 2017

### Pinon1977

Acknowledged, however, the "U" was a typo on my behalf (if you read the context in which it was used). The U and I are right next to one another on a keyboard.