Understanding Motor vibration

[Scott Drysdale]

I just came across an interesting article from an industrial electrical magazine about industrial motor vibration analysis and thought this information should be shared here on MRF.
The iPhone iseisometer app will show three separate vibration axes, i.e., x,y and z. Typically the IPhone is positioned on the MR motor arm such that the z axis is aligned with the motor/rotor or bell axial direction.
One focuses in eliminating the motor radial vibration which appears in the x and/or x and y axes.....
But where does that z axis vibration come from? I have been wondering about this for some time now.

Turns out that vibration on the z axis is, for the most part, a consequence of motor axis or motor bell runout. Runout is the result of either:
1) damage to the motor bell or motor shaft, i.e., bent or...
2) Bad manufacturing and/or QC.

In some cases there is a setscrew that can be loosened so that the bell can be re-positioned more in line with the motor shaft. But if that does not work such that bending of the bell spokes is required to realign the bell to motor axis, maybe it is time to toss that motor.... Above situations can also develop as a result of a prop that has more pitch on one side or does not lie totally flat against the top of the bell.

 

[kloner]

There is such a thing as pre load that is set on good motor assemblies.... it is the first indication you broke a bearing in the slop that develops in the up and down movement.

in my experience and before i understood i'd reset that pre load and lemme tell ya, hell was unleashed so be careful to what level anybody that reads this feels there stuff can be fixed.... if the race or balls are broken, this can make it worse

a really good indicator to motor smoothness is i run my fingernail on a powered up un propped motor when i'm setting motor direction and if any feel rough, i usually replace them there..... if that's not smooth, adding mass to it surely isn't gonna smooth it out

 

[R_Lefebvre]

The Z-axis vibration can also be caused by the blades being out of track. The motor can be perfect, but if the blades aren't made perfect, you'll get tracking issues which are not easily fixable on a multirotor.

Not that any of this really matters. You can spend $1B balancing your motors and props. And then as soon as you start actually flying, they start vibrating anyway due to disymmetry of lift. This is especially true when flying forward. And there's nothing you can do to stop it. Well, a 4 blade prop would help a lot. Or a teetering head like a helicopter (this is a big part of the reason they use them).