Moment of Inertia Calculation for Offset Mass

[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

 

[Doc Al]

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

 

[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.

 

[Doc Al]

Those shapes are not on this list? https://en.wikipedia.org/wiki/List_of_moments_of_inertia

 

[Pinon1977]

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

 

[Pinon1977]

Derive. Sorry not drive.

 

[Doc Al]

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.

 

[Pinon1977]

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

 

[Doc Al]

Read this: Parallel Axis Theorem

 

[Pinon1977]

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

 

[Pinon1977]

?.??.?

 

[Doc Al]

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.)

 

[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 a lot if these newer features don't work on my phone. I just need to know if In the ballpark.

 

[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...

 

[Evo]

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...

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.

 

[Doc Al]

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...

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.

 

[PeterDonis]

don't know how to put formulas on this message board

See the PF LaTeX help page:

https://www.physicsforums.com/help/latexhelp/

 

[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

 

[Pinon1977]

See the PF LaTeX help page:

https://www.physicsforums.com/help/latexhelp/

Thank you. I will see if that will help.

 

[Pinon1977]

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.

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.

 

[Nidum]

If the original diagram is anything like to scale then for anyone of the masses the approximation that Moment of Inertia = Mr² where r is the length of the radius arm from mass CoG to axis of rotation can be used .

This approximation is commonly used and accurate within a few percent when the dimensions of the mass are significantly smaller than the length of the radius arm .

For a very accurate result under the same conditions break the mass into two or three sections each with a different radius arm and sum the individual Moment of Inertia contributions .

 

[Pinon1977]

It's not the mass itself that is troublesome, it's the movement of the mass as it's rotating. As the "tower" rotates 360 degrees continuously, there are two weights that move vertically inside the tower once it approaches the 12 o'clock position. It's within this movement that I would like to know the MOI changes.