# Calculating Inertia Of Rotors

1. Jun 26, 2012

### Henry C

Hello Everyone,

I'm new hear & am hoping that someone can help me out.

I have two Rotors, one on each end of a shaft with bearings in the centre, both have air cooling fins in the centre of each rotor.
I need to calculate the Inertia of these Rotors, I have looked at several formula's but there seems to be difficulty calculating the inertia due to the cooling fins etc.

I thought I may be able to drive the Rotors on the shaft to a given speed then disconnect the drive & use the time it takes for the rotors to come to a stand still to calculate this.

I will just use a very light oil on the roller bearings for the excersise & also blank off the cooling fins so they have don't have drag etc.

Neglecting Bearing friction, will this method give me fairly accurate results?
I have not been able to find the formula's for this & was hoping that someone may be able to help me out.

An example would be very helpful.

I can weigh the rotors & the shaft to get the mass if this is required.

Thank You

Henry C

2. Jun 26, 2012

### xxChrisxx

Hi there,

What does the thing you are measuring acutally look like?
A quick schematic will do.

Also when you say inertia, I am assuming you mean mass moment of inertia (angular mass).

Last edited: Jun 26, 2012
3. Jun 26, 2012

### Henry C

I have attached a drawing of one rotor, their is one of these at each end of a shaft with roller bearings in between.

They are steel, heavy, & about 460mm diameter.

I need to know the Inertia of these Rotors etc.

Yes, Moment of Inertia

Thanks

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4. Jun 26, 2012

### xxChrisxx

How accurate do you need to be?

The geometry makes it too hard to sensibly calcualte by hand.
Do you have access to a CAD system? If you do a quick model, most will output an inertia value.

I have a feeling that you'll need a spin rig. You really need to measure the torque and time to spin up the rotor, then measure the spin down time to get the prevailing (loss) torque. From that you can calcualte inertia.

5. Jun 26, 2012

### Henry C

I would like to be fairly accurate as they will be used to measure torque.

Is their a site or an explanation of the spin rig proceedure anywhere on the internet prefferably with some formula's etc.

No CAD at this stage!

Thanks

6. Jun 26, 2012

### Henry C

Sorry for the double post
Does this make sense,

Torque = I*a I=moment of inertia a= Linear accelleration

Angular accelleration a=r*x r= radius of shaft x= rad/s^2

T=I*x
I=T/x

So if I have a chord & wrap it around the shaft & suspend a "known weight"-mass on this & release it, I then have the vertical distance it travelled & the time taken.

If I then calculate the Linear accelleration , the Angular accelleration,chord Tension & Torque then I should be able to calculate fairly closley the Moment of Inertia using I=Tx.

Neglecting Bearing Friction!

Is this reasonable?

Thanks

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