How Do I Calculate the Moment of Inertia for a Blower Shaft?

[Zarathuztra]

I have a blower within a system, and I'm trying to calculate the system inertia. I'd like to get the moment of inertia for this component as would be seen by the shaft. It's shape has me a little stumped (considered perpendicular axis theorem) on what method to use for finding the inertia. Any direction to help get me started would be much appreciated. See the image attached.

 

[Nidum]

Moments of inertia about a common axis can just be added together . So break the component into individual parts , calculate their individual moments of inertia about the central axis and sum them . The symmetry of the component can be used to reduce the amount of calculation necessary .

 

[tygerdawg]

OR you can take the easy route. Get, or create a reasonably-accurate copy, of the CAD model. Assign a material density to the model components. Use the mass properties function of the CAD package to determine the MMO about any arbitrary axis.

 

[Dr.D]

Looks to me like you need to break this into 8 parts: 6 blades, 1 back plate, 1 central hub, and then calculate the MMOI for each one about its own CM. Then use the parallel axis theorem to transfer to the common axis of rotation.

 

[CWatters]

+1.

I'd start with an individual blade.

1) divide a blade it into standard shapes like this...

2)Then look up the equations on this page..
https://en.wikipedia.org/wiki/List_of_second_moments_of_area
for the "area moment of inertia" of each shape about an edge or centre of area (edge probably easier).
3)Then multiply by the area density to convert it to mass moment of inertia.
4)Then apply the parallel axis theory to modify each equation to "move" the axis of rotation from the edge in 2) to the axis of the fan.
5)Then multiply by the number of blades.
6) Then add the MOI of the other parts like the hub and base (which are easier to calculate or at least find equations for.)

Edit: An alternative would be to calculate the MOI for a rectangular blade and subtract the MOI of the "missing" triangle (after applying the parallel axis theory).