![]() ![]() ![]() ![]() You can add them up individually, or combine and reduce the terms to simplify it for larger groups. I don't have time right now to recreate the derivation, but it should just be a matter of rearranging and combining the equations for Ix and Iy given in the thread I linked to above.Įdit: In it's most basic sense, the polar moment of inertia of a bolt group is the summation of d 2, where d is the distance from the centroid of the group to the center of each bolt. The formula I use for the total polar I of a bolt group (that I either derived or found a long time ago) is: The formula and derivation can be found in this thread. The moment of inertia of the bolts themselves about their individual centroids is ignored as being inconsequential.įor the polar moment of inertia, which is what you would use to calculate the force for a bolt group where a moment is about the centroid of the bolt group, is Ix + Iy. The reason the units are mm 2 is that the "I" of the bolt group only considers the "Ad 2" portion of the moment of Inertia calculation (Io + Ad 2), where "A" is set equal to 1 for convenience of the calculations (so you don't have to multiply by the area of the bolt to get stress and divide it back out to get force). ![]()
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