ISO 1940/1 - Balance Requirements for Rotors, How to balance a overhung rotor?

[Ev-36XPro]

Good morning!

I am reviewing an existing rotor design where Dim b is 0.65" and Dim d is 0.4" (see picture below). The distance clearly violates ISO 1940/1 - Balance Requirements for Rotors (link to doc which references ISO 1940/1). My questions are:
[ol 1]
[li]Since distance d is so small I believe it is amplifying the imbalance. Currently the rotor is balanced as if it was supported on both sides (not cantilevered). Do I need to switch to a cantilevered balancing machine setup? Or do you know a way to calculate how much I need to lower my balance requirements to compensate for the distance between the bearing planes.[/li]
[li]The distance between the bearings is so small because they are side by side, with only a very small shim between them. However, on the opposite side of the cantilevered rotor there is a spline shaft which couples with a shaft from a gearbox. Obviously the gearbox shaft has its own set of bearings, do they somehow factor into the value of b]Dim d[/b]?[/li]
[/ol]

I have very little experience with balancing so your help is greatly appreciated. Thanks!

 

[Strong]

The dimensions you have are small, so I am guessing that the rotor speed is rather high. What is service speed?
ISO 1940 is for Rigid Rotors, so have you made an evaluation that rotor behaves as a Rigid Rotor at service speed?
I consider ISO 1940 as a guideline, so I would be careful requiring someone to meet it, or for me to be bound to meet it when the rotor characteristics (dimensions and speed) are unusual. Rather than making calculations (question 1) I would spin up the subassembly to test and correct balance, and then rub the full assembly. The gearbox bearings and splined shaft may add support and stability, but could also introduce misalignment force and a loose fit that could affect increase vibrations.

Walt

 

[Ev-36XPro]

Hi Walt, thanks for the response. The device is a PMA with a very small air gap between the rotor/stator and a relatively thick shaft so I believe "rigid rotor" is an accurate classification. Here are some more details about the design:
RPM: 25000 rpm
Mass: 0.1 kg

I do not have access to the gearbox so any additional test data is not available. Are there equations to verify if the part is rigid or flexible at speed?

 

[Strong]

Rigid rotor is not defined by what it looks like, but whether operating speed is below 1st critical or bending mode or balance resonance. If operating about 1st critical, then rotor is classified as flexible and ISO 1940 does not apply.

I do not do modeling, but I can caution you to be careful about bearing support stiffness. The common/simple assumption that the stiffness is infinitely rigid may not give good results. Here is a link to software if you have the required budget:

https://dyrobes.com/

Walt