We have a Lightning Agitataor Model 76Q20
the vibration levels are generally ok except on random occasions get faily high levels around 10+mm/s on the gearbox.
the frequecy of excitation seems to be around 20.5hz which does not correspond to any rotational frequencies. We think it could be the resonance of the output shaft being excited by the conditions of the tank- are unable at this stage to check the output shaft. Any ideas or similar experiences

Dolomite_Mixed.pdf (54 Kb, 32 downloads)

Does this occur with tank levels or viscosity? Have you checked run-outs on the shaft? It appears that there's a foot bearing or steady bearing for the shaft; have all issues been addressed there? Is there any piping/pumping/process going on during mixing stages?

its odd - seems to occur at around 70 - 80% when tank is emptying but not when filling.
Levels could be good and then if material starts to pump into the tank from above - sets up vibration again --but not all the time
what issues do you know about with the steady brg
regards
Glenn

If you could see it swril you'd probly see the vortex isn't center with the shaft and high on the side wall of the vessel. Can you vary speed?
How wallowed is the steady bearing and how 'in-line'. Your setup at top should be within 80% of the output shaft's diametrical bearing clearance with no more than 15 seconds of misalignment.

Could be a steady bearing problem, if you have one. Lots of these dont (even when the drawing shows it). Having said that, it is not unsual to get strange vibrations on filling or emptying.

I also have seen a resonance of the tank on its supports that got excited as the liquid reached a certain level. The slosh frequency of the liquid excited the natural frequency of the tank, the agitator was fine. Found some broken supports that were easily fixed and everything was fine.

What's a "steady bearing"?

Generally a steady bearing is located ~mid-way or thereabouts but is also nomenclature by some for the bottom foot bearing - terms may be misleading but there is a drawing. I've been exposed to both terms from different places so....

Have a look at the drawing Pete, and you will notice that at the bottom of the shaft there is a bracket holding the end of the shaft. These gizmo's often have very undamped criticals of the "mud ball on a willow stick" type (over hung mode, but overhung by 20 feet). I have seen unsupported rotors with displacements at the bottom agitator wheel that could be measured in feet.

You solve that by supplying a bearing at the end of the shaft (or at points along the length) so it won't be over hung anymore. Well actually it is usually a bushing of some kind, lubricated by being bronze or plastic, or carbon since you can't supply lube oil to it.

Depending on product, it may be stellite. Dissimilar metals to avoid gaulding shaft material and product allowing. Few have tried rolling element bearings and some need food grade lubricant if used.

There's a need for super critical alignment due to the short constraints and generally a mechanical seal at top; so, generally 80% of the diametrical clearance of the bearing is all that is acceptable radially and 10 mils per 10 feet should be adhered to if possible.

There is no bottom bearing or steady bearing.

the limit ring at the bottom just limits the shaft from moving too far radially

Yep, I just looked at the drawing again, and that gizmo at the end of the shaft is just a bumper ring to limit the vibration level to inches instead of feet....my apologies, I glanced at it and assumed it was a steady bearing.

Still, could well be a resonance issue of the rotor, has Lightning given you a critical speed for it?

Sometimes you also get a mode that is the drive deflecting the tank roof. Can you bump test the drive with the motor off to see if this is a tank roof mode?