When you tune the vibration absorber and bump it. One expects a dip at the running speed (or frequency of interest), not a peak.
Excite a the machine and look at the aborber and you expect a peak. Excite at the absorber and look at the motor and you expect a peak. This is the way the transfer functions should look when you have tuned an absorber.
The bump test on DVA shows 12.5 Hz. Can u Pls elaborate what I need to do further. Will changing the DVA position help. As of now it is fixed along motor NDE vertical direction.
Motor speed is fixed at 1000 rpm or 16.9 Hz. I can see a cyclic movement. Vibs varying from 7 to 10 mm/sec
From the initial post, I believe that you were using a tuned absorber per Fox. The concept is to design the absorber to have a natural frequency at (or close) to the excitation frequency. Then you fine tune the absorber.
Did you tune the absorber? If not you need to tune it. The picture shows an absorber that could be tuned for one direction. If you need to reduce vibration in x-y directions this cannot be tuned for both directions.
Thanks again. I fixed up the DVA along max excitation. In this case vibs were high along vertical 13 mm/sec and hence I fixed up in that direction. Along horizontal vibs were low 1.5 mm/sec
I tried to adjust the position of the weight by moving up and down to see the influence on vibs. No increase or decrease in levels.
I calculated the weight and dimensions based on Fox excel sheet. I have a doubt. The motor body weight was 3.5 tons. Can a DVA with 2.47 kg weight overcome the NF of motor and shift it's frequency.... Or is DVA meant for only low weight rotating machines....
I tried this for the first time.. So... Some doubt....
Calcs look good. (this gets you in the ballpark) When you say highest vibs are in the vertical are you saying the highest vibs are in the direction of the sensor in the top picture. Because that is the direction your absorber will effect. If so, you need 16.9 Hz and the absorber rings at 12.5 Hz. Then you need to slide the weight down to increase the frequency or use less weight.
What are the natural frequencies in the horizontal and vertical directions (in horizontal plane) when impact-response is at/near top of motor? If the natural frequency in the vertical direction is close to 1xSS at 16.9 Hz, then you need to increase the DMA frequency, as others have stated. If you do not see very high vibration of the attached DMA, then it is not working. Lesson-1 learned: it is unwise to make a non-adjustable DMA!
Tuned absorbers have been well discussed here for quite some time. You might want to go to the thread at https://www.maintenance.org/topic/dynamic-absorber and pay particular attention to the comments by "electricpete", specifically those at 2/16/13 @ 1:43 PM where he points out an error in the Randy Fox paper on tuned absorbers.
Thanks members for all your valuable comments. Yes I have read electripete comments and all posts on DVA before I ventuted into this. Since it was a mouning resonance issue I had no other Immediate options. So I gave a try.
Finally I added 140 gm of weight after looking into amplitude and phase which was varying. I could able to bring down the vibs by 50%. Motor NDE vertical before 11 to 13 mm/sec.now 5 to 7.5 mm/sec..all other vibs are OK.
Again thanks for everything who shared their views.
Just a last question to Walt... if I tune the DVA to 16.9 Hz it will vibrate more violently along with Motor which is 3.5 ton weight. How it can nullify the motor NDE vibs. I tried this once and saw vibs on DVA was 25 mm/sec while motor NDE vibs 11 to 13 mm/sec
Still have questions. In the first post you say you are trying to address vibration in the vertical direction and the motor is in an uncoupled condition. Is it truly vertical, up and down? If so, then your DVA will do nothing in that direction. Also if the motor is not coupled and the lift is not set on the pump the structure will have a different natural frequency.
"Just a last question to Walt... if I tune the DVA to 16.9 Hz it will vibrate more violently along with Motor which is 3.5 ton weight. How it can nullify the motor NDE vibs. I tried this once and saw vibs on DVA was 25 mm/sec while motor NDE vibs 11 to 13 mm/sec"
You did not provide the motor natural frequencies. The damper at 12.5 Hz is over 20% from the excitation at 16.9 Hz, so it will not vibrate very much. In general a DMA is most effective when the motor structure natural frequency is close to excitation frequency (16.9 Hz), but you have not indicated that is the case. If the motor natural frequency is actually close to 12.5 Hz, then a DMA is not a good choice for vibration reduction! If balancing the top of the motor provides adequate vibration reduction, then forget about the DMA.
Replying to Ron and Walt
As shown in the photograph the motor is in vertical direction sitting on the stool at the bottom, yes right now all measurements done in decoupled condition
vibrations high along vertical direction as shown by mounting of accelerometer. DVA also installed in the same direction
When i did the bump test along Motor NDE vertical, no 16.9 Hz or closer to was seen
So basically motor body natural frequency is not at 16.9 Hz
Basically its the mounting resonance. That is, When i tight the motor mounting bolts (8 no's) near flange the vibs increases to 13 mm/sec. when i loosened the bolts vibs drops to 1.5 mm/sec
So as suggested by walt using DVA in this case may be a wrong choice as its a pure case of mounting resonance issue and not the motor body natural frequency issue. What we need here is isolation, avoid metal to metal contact between motor flange and stool, may be 3-4 mm industrial rubber gasket will do....
Correct me if I am wrong.
Below is what I call directions. When you say vertical I think up and down. Your DVA looks like it will only help the Horz 90- degrees from discharge. Also if the motor is not coupled the center of mass will not be the same as when you couple it and the pump rotor is now supported from the motor thrust bearing. Changing the mass will change the natural frequency a bit. So you will have to tune it again.
Replying to Ron,
Attached is the photograph showing the direction of measurements (Sensor mounted) and also DVA position. I made 2 guys with weight apx 65 kg each to stand on top of the motor to see the influence of mass. no effect and vibs remain same.