Inertia Calculations for Bearings in Rotor Assemblies

[Motorhead]

I am doing rotor interia calculations for some rotor assemblies in a brushless DC motor. In the past I have always ignored any effect due to front and rear bearings. I am beginning to question this assumption. Does anyone know if this a valid assumption, and if not, some insight on how to calculate the values? <br>
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The bearings in question are standard radial bearings in which the inner race is fixed to the rotor shaft and the outer race is fixed to the outer bearing journal.<br>
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Thanks for any input.<br>

 

[Baldy]

Machinist Handbook has simple inertia calcs but I typically use a engineering handbook from a bearing manufacturer such as MRC.<br>
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But I bet you could find some software to help you.<br>
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Good Luck its not an easy one.

 

[ecampos]

Motorhead, I design and build very high speed electric motors, they run anywhere from 10,000 rpm to 60,000 rpm. In doing my analysis of the rotor system, the bearings are always excluded. In most cases where the system inertia is a factor, the bearings are of small diameter and hence low inertia. In my case we use a ceramic ball bearing, and only add the inertia of the inner race to that of the shaft to calculate inertia forces. So unless you have a very high rpm motor than I would ignore the bearings. Also if the bearings are very large than you will need to include their inertia.<br>
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Dear Friends:<br>
In my point of view we can take into consideration as follow: <br>
first look at the rotor inertia I(r) along rotating axis<br>
then look at the inertia I(b)of both inner rings of bearings along that rotating axis as an addition elements to the rotating system. Since inertia is proportional to rotating speed then by looking at the percentage of bearings inertia contribution, I(b)/I(r)the assumption is evaluated. It's better to take a look at the Inertia formula to come up with the general idea of how much a geometric element ( ring, non-axis symmetry for example)contibute to an axisting rotating body to justify our assumption.<br>
I hope I'm right.<br>
Good luck.<br>
Tan Bui