High vibration at vibrating screen drive system

Hello to everybody,

I have been taking vibration data from 12 vibrating screens drive systems for a year.

In three of them we have had severe vibration increments from 4 to 6 mm/s to 20-30 mm/s in horizontal and from 3 to 4 mm/s to 8-9mm/s in axial direction

Spectrum shows 1x from screen frequency (842 CPM) and phase shows like unbalance.

SCREEN 401 TREND AND SPECTRUM M2H

Trend and spectrum SC401

We suggested Maintenance people to inspect the cardan coupling and vibration later was lower in axial direction but still remained high in horizontal (cardan was actually replaced for later inspection)

SCREEN 302 TREND AND SPECTRA M2H AND L2A

Trend and spectrum SC302

If any of you have had a similar case and found a solution please let me know. I would really appreciate that. You can find attached a PDF with graphical info of screen.

Greetings

 

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Original Post

Alvaro,

These motor mounts act as vertical cantilever beams. If your 1X vertical vibration levels are 4-5 times less then your horizantal vibration then consider resonance. Just try to do a bump test or coast down test to see your natural frequencies.

Alvaro,

I've seen that increase and faced with the very much same problem before. With the wear and tear stiffness of the system changes with time, even the tightness of the V-belts effect that. I loosened the belts and vibration levels gone down a little bit. Running speed was close to one of the modes of the system. We lowered the motor and changed the v belts with one size shorter. Problem gone... Your system is slightly different then mine. Motor was on top of lay shaft on our case, yours are pretty much on the same plane. But still, it may be worth to consider this possibility too. I'm sure you know that we should be very careful when it comes to speed changes on screens. Check if it changed with time because of the wear on the sheaves & V-belts. Beside the rigid body mode of the screen we should take care of the flexible modes too like pendulum, cross bending, polar etc.. when it comes to natural frequencies.

If i were you i would start with checking wear on the V-belts & sheaves and then alignment of these. Then i would go with cardan coupling checks like you already did, including the ones between the exciters.(we call these lay shafts)

I've already mentioned bump test.

And finally i would go with phasing check between the exciters. I'm not sure what different suppliers suggest for this but i beleive there should be a sequence between the turns of these 3 exciters. (Just 2-3 degrees between each)

 

 

Hello Alvaro, I suggest you check the springs and check on each support the orbits of the screen. Also perform a modal analysis to see your natural frequencies.
I think that something is loose or is in process of it, and therefore various its natural frequency .. not forget to verify the springs and the misalignment that they generate.
Thank you!

Not quite familiar with banana screens, but would check followings;

1. Exciting force increase by screen motion change. This can be checked with orbit and transverse movement of the screen which can be caused by broken spring(s).

2.  Increased transmission force from u-joint and/ cardan coupling. Check any damage or misalignment.

3. Reduced horizontal stiffness of the motor base frame by loose bolt(s) or cracking. Worth recording natural frequency of the base frame via bump test for future reference. 

Hope this helps.

Hello to everyone,

Thanks for your responses.

In order to solve vibration the problems in these 3 vibrating screens I suggested to make uncoupled tests and found interesting results.

SCREEN 401

Vibration dropped to 3.2 mm/s (spectra signature was ok). I suggested to replace cardan coupling. Vibration kept low (5.6 mm/s) after coupling the new cardan. Equipment is ok now.

SCREEN 302 and 601

Vibration was still high (17 and 24 mm/s horizontal 1x layshaft speed). We suggested to replace layshaft  (there is no way no check clearance in layshaft bearings because Maintenance people have to make this inspection before assembly is done. So they have to replace all the assembly)

Attached you can find trends and spectra.

Let me know your opinions.

 

Greetings

 

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