This is the first time that I did a "bump test" to determine the resonant frequencies of a motor. I did (4) test one on the motor outboard horizontal one on the motor outboard vertical. This was also done on the motor inboard horizontal and vertical. The readings that I got is the two horizontal readings were both at 10 HZ. The outboard vertical was at 40 HZ, and the inboard vertical is at 30 HZ. This motor has a dual shaft. The outboard shaft has an inerata weight attached to it. The inboard shaft has a smaal test head attached to it. Is it possible to have all the different frequencies?
Thank you in advance.
Roger Earley

Hi Terrence
resonance was suspected on a transducer fixing frame. I carried out a bump test on the frame which had been assembled from unistrut. I was very supprised to find that there were two different frequencies present horizontal and vertical i have some pictures of the frame and will elaborate if required, resonance at 1x and 2x were found
regards mike

It is possible to realize different frequency naturals at different positions. We often find different bump test values across our motors in respect to inboard (belt tension increasing stiffness) and outboard end. The thing is, is the bump value within 10 to 20% of a forcing frequency.

JB
I am not sure of what you mean by the bump value within 10 to 20% of a forcing frequency?

I agree with JB and JB correct me if I'm wrong, but if there is any forcing freq. such as a slight misalignment or imbalance or even bearing frequencies, a natural freq. that is within 20% of either side of that particular frequency(bearing,etc.)will add to or cause that particular freq(misalignment,etc.)to increase to undesirable levels. Everything vibrates at a natural frequency, the question remains as JB asked how much is it affecting the frequnecy in question.
Regards
Rafael

Roger, yes it is possible to have two or more natural frequencies show up when doing bump tests . remember when bump testing you are not only exciting the motor frequency but also motor mountning frequency as well. "normaly" the motor readings will be higher freq.in the vertical plane as "most mounts are stiffer in this direction. If i suspect resonance I take readings in several places on the motor and skid and use a strobe light fired from the data collector and look for "Phase changes in a referance mark that I have made on the motor shaft If this moves 180 deg between any positions on the motor / skid then indeed you have resonance present .I first used this in the 80s and still find it is relevant today even with the advances in data collectors . Hope this is of some use . Ian

We have performed numerous bump tests over the years. It is quite common to find different resonances at different locations and / or directions. There are a number of reasons for this, including: bearing and structural stiffnesses, variations in mass distribution and different transmission paths.
Your bigger questions will be: how close are these resonances to the exciting frequencies of the machine, and what can we do to change the natural frequencies.
Regards
Paul

RLE:
Sorry about the lapse in time, to many routes and repairs to complete! Say you have a one times turning speed of 1000 CPM. Your bump test reveals a natural of 950 CPM. The 950 CPM is within 10% of your turning speed therefore the turning speed is your forcing frequency. For most rotating units 10% move of the natural is enough but I have had to go 20% at times. The thing is do you move the natural up with stiffness of down with mass. Additionally, you would want to review the imbalance within the machine.

Rafael you are correct and I agree completely.

As PDRand states this is all dependent on stiffness, mass, and dampening! I would say the amount of dampening is the most important. Anyway, prove what is right. If one times, is not a problem then eliminate all things after this. If anyone frequency correlates to bump test then you have to decide the next step. If the amplitude is not bothersome then leave it, if it is alarming then you will have to decide what to do with it. Just because you have different frequencies in different directions does not mean there is a problem unless they are excited to the extent you believe them to be harmful. I have encountered this one times and two times signiture radially in spectral data. Does the two time dominate the spectrums? If so I have found that stiffening in the axial direction has elemimated this resonance at one and two times. May not be in your case, but sounds like the flexure modes I have encounted. Hope this helps!