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# How to calculate a max mass of a detail which might be installed at the end of the rotor?

## How to calculate a max mass of a detail which might be installed at the end of the rotor?

(OP)
Hello Dear Engineers!

Could you tell me how to calculate a maximum mass of a detail that might be installed at the end of the rotor?
In my case, it is a fan that I want to install at the end of the rotor.
The rotor is installed vertically in two bearings.

I am looking for some source (book or link) where a similar task is described.

Thank you.

### RE: How to calculate a max mass of a detail which might be installed at the end of the rotor?

The bearing manufacturer most likely has an application handbook.

### RE: How to calculate a max mass of a detail which might be installed at the end of the rotor?

probably any dynamics text will tell you how to do this.

mass is only one limit, "mass imbalance" is probably way more critical, "mass moment of inertia" also.

the strength of the "rotor" is as important and the strength of the bearing, as is the strength of the bearings support structure.

### RE: How to calculate a max mass of a detail which might be installed at the end of the rotor?

As rb1957 said, "dynamics". You'll need to look at the natural frequency of the shaft with the mass of the fan attached. Different fans would have different multiples of RPM that have significant frequency content. Look at the type of fan you want and see what frequencies it produces. I'd guess you'd want a natural frequency maybe 5x the RPM.

### RE: How to calculate a max mass of a detail which might be installed at the end of the rotor?

As others said, bearing loads ( NOTE ! one rotor bearing must accommodate axial thermal expansion ).

Also as other pointed out, a very important calculation is the resonant frequency of the fan cantilevered off the bearings. Fan mass, shaft length and diameter, bearing C-C spacing, and support stiffness all are poised and ready to bite you again and again if not given due respect during design.

Do some Googling to see how fan manufacturers waltz around calculated resonant frequencies on rigid supports, and results using more realistic bearing pedestal system stiffnesses.

The fan may provide some "gyroscopic stiffening" and one or two other effects beyond the basic cantilevered point mass model.

The fan impeller itself requires a careful look. It is quite possible for it to have umbrella and wobbling "modes" just waiting for excitation to bite, and bite hard.

Then an often overlooked vicious detail is the repeatability of the concentricity of the bearings>shaft>fan impeller as a result of interface clearances and manufacturing tolerances. Commercial shafting, and bearings and fan hubs with slip fits can significantly unbalance a fan impeller that was nicely balanced in the shop.

Will this be belt driven? The structure between the fan and drive motor, and once again the bearing>shaft interface are snakes just waiting under your workstation.

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