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Not really a HW problem (no diagram), I just need to figure out this number for a design.

I have an axle with two crank arms attached to either end, pointing in opposite directions. The entire thing will spin. I need to find the equivalent moment of inertia of the axle with these cranks attached, but I'm confused over what method i should use. Classes never really go deeper with MOI than an I-beam problem or just looking up bh^3/12, etc.

I have all of the specifications of the lengths of axle, cranks, diameters, etc. So should I try:

A) Using the geometry of the part(s) to find I

B) Analyze the part more to find the work and angular velocity so I can find MOI

With method A) i thought I could just use an s-beam geometry to find it:

http://www.efunda.com/DesignStandards/beams/RectangularSBeam.cfm

the numbers seem to make sense when I did it, but for some reason this seems really wrong since the axle is a "horizontal" s-beam so to speak. (The axle is parallel with the ground)

Would the Iyc value in the link above be justified here? Or do I need to start looking at method B?

Any help is greatly appreciated.

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# Homework Help: Finding Moment of Inertia of axle

**Physics Forums | Science Articles, Homework Help, Discussion**