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# Moment of Inertia

## Moment of Inertia

(OP)
Hello all, hope you are doing great.

I am building an spreadsheet to calculate the reactions on 3D bolt group and I would like to confirm an information.

I calculated the moment of inertia about the centroid of my bolts in all 3 planes, my question is:

Can I add Ixx from xy plane and Ixx from xz plane ? I have to come up with only 1 Ixx to calculate the constants.

Thanks !

### RE: Moment of Inertia

not really sure what calc you're doing ... can you show the geometry ?

I'm not sure how you get two inertia terms for all three axes ? I'd've thought that there be only one term ... unless the plane of the fasteners is generally inclined (but then you'd transform loads into this plane ... won't you ?).

### RE: Moment of Inertia

(OP)
The geometry is a circle, the diameter of the bolt is 0.18 in^2.

my spreadsheet is enclosed. go to 3D bolt group and formulas and calcs tab

### RE: Moment of Inertia

your attachment is a screen shot.

If you have a bunch of bolts on a PCD, then you have Ixx = sum(A*y^2) = Iyy. You could calc Izz, but I'm not sure why you would (but in any case = 2*Ixx).

### RE: Moment of Inertia

I don't know I'd consider the Z (= 4) term and calc Ixz and Iyz.

You seem to be doing a lot with this. I don't think I've ever seen people include the local I of the bolt (and just use Ay^2).

### RE: Moment of Inertia

(OP)
I see... I got some results that matches with other spreadsheets.

Thanks !

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