whirling of mill spindle

[ebyk]

high everybody
i work in a wide plate mill plant.our mill stand drive
system is equipped with two big "joint shaft spindles,each 51ton weight and 10700mm length.they have 2 huge spindle supports(see attached drawing). Recently,during routine inspections ,we found that the mill spindles are whirling and their supports have oscillations.
In horizontal plane,amplitude of the oscillation is about 8-10 mm for bottom and 2-5mm for top spindle support(measured in middle bearing of spindle shaft).
In vertical plane amplitude is about 1-2 mm,for both spindle
supports.The frequency of oscillation of spindle support in horizontal plane is nearly twice the spindle rotation speed.
(mill stand max.speed and torque are 120 rpm and 8100KNm,respectively.)
Now main question is that the oscillation is due to "low stiffness" of spindle supports, or it refers to some faults in joint shaft itself?
In other words if we assume no support for such a huge spindle, this type of oscillation is normal or not?and finally if we constrain the horizontal movement of the support by some means,what will happens to spindle itself?

thank you and best regards

 

[IRstuff]

I don't see how that could be considered normal, but is this a new problem, i.e., did your previous inspections not see this? I would think that something wobbling nearly 1/2 inch would have been noticeable. If this is new behavior, one can only assume that something failed.

Have you talked to the manufacturer about this? What is their performance specification?

As a general rule unintended wobbles are bad, since they impose stress on components that are not supposed to see that stress. An unintended stress will therefore make even more catastrophic failure more likely.

TTFN
faq731-376

 

[ebyk]

dear IRstuff
thank you for your reply.This problem is not new , but in commissioning times and about one year after that,we did not notice such problem.we have talked to manufacturer of spindles and its supports.they asked us to check conditions of bearings of cardan joints , middle supporting bearings , driving motor bearings and clearance between mill housing and bearing chocks of the mill work rolls.we are checking the bearings ,and till now ,did not find abnormal conditions.It is better to know that during last year , the wobbling magnitude has remained fix.