11x vibration frequency

[teaaddict]

I have just done some noise measurements of a bench grinder mounted on a pedestal just to try out analysing frequencies using a microphone ( Just because I got my hands on one). I picked the grinder because it is quite noisy and didn’t have a gearbox.
It rotates at 2918 rpm circa 50 Hz however the largest peak is at 11x. I don’t know the number of balls in the bearing this could be 11 and could account for it. The next largest peak is at 7x.

My question is if the stand has a natural frequency of 11x the forcing frequency will this show up as a higher peak than the forcing frequency.

 

[GregLocock]

That depends on a bunch of things - more helpfully, it depends on the damping of the mode, and the spectrum of the excitation function.

A well balanced rotatry machine might have no first order in its spectrum.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[electricpete]

If the ratio of 11x/1x is 0.1 in displacement units, the ratio of 11x/1x for the same vibration would be about 12/1 in acceleration units. In other words factor of 100 change in relative magnitudes of those two peaks.

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[sreid]

It could be the motor slot or bar passing frequency.

 

[Strong]

Ball bearings do not produce integer multiples of shaft speed. Their fault frequencies can be calculated from their geometry and shaft speed. Motor rotor bar and stator slot frequencies are well above 11x shaft speed. One possible source of 11x shaft speed is motor cooling fan, so count the blades.

Walt

 

[Tmoose]

Every now and then a wayward bearing has had a BPFsomething pretty close to an integer, like 3.06, which required more lines and better yet a synchronous phase reference to really identify as NOT 3X.