Hi,

This is equipment of Steel Industry from Bar Rod Mill unit. Equipment is Mill Stand, Which is having reducer gear. The Input shaft have 24teeth and mating gear has 63teeth. Intermediate having 19,66 teeth and output shaft having 31,31teeth. I didnt have any problem with vibration whenever the higher dia bar is rolled vibration will be 4.0mm/sec peak and during lesser dia bar rolled it will 2.5mm/sec peak. The spectrum is showing a high amplitude dominant peak of 73.1orders of input speed at all bearing points of gearbox.. Neither it matches any of the bearing fault frequencies nor gearmeshing.. Input speed is 1033rpm. What may be the reason?? I checked the bearing fault of stand bearings even it doesnt matched..

 

 

Sany

Original Post

Is it something of concern or just academic interest?  If I do the conversion it is 0.03 in/sec peak, likely not to be of any concern.  Gearboxes are wonderful at creating sum and difference frequencies and a double reduction has many possibilities.

It is the concern because these gearbox bearing will fail at even 2.0mm/sec peak...if in future it fails i have to understand y it failed?? I need to analyse y that peak is occuring

Tell us about the failures you are experiencing.  Vibration may not be an indicator of all types of failure.  

As "BEATNIK" asked, what is the sideband spacing?

I don't see anything  specific to the gearing tooth counts that would present an issue although there is a possibility of a "ghost" frequency that might exist.  

Your input speed is 1033 RPM and your profile says India where line frequency ios usually 50 Hz.  Is this a VFD?    

Hopefully someone with direct steel mill experience can offer some help.  

Last edited by John from PA

Sany,

I think the answer lies in your description that the suspect frequency is present when rolling one bar size and not the other. I would test for a natural frequency in the bar and/or the structure of the gearbox and drive.

Walt

If you have bearing failures that you don't see coming with vibration I'd suggest to do Oil analysis. I have big gearboxes where bearing damage only show up in vibration when the damage is very significant.

 

 

Single failure take place that is Input shaft bearing cage will damage. Sidebands are 370cpm that is not realted any of the frequency. It is VFD.

If its a GHOST Frequency den how to solve..

Before chasing down a ghost frequency I would do what Walt Strong says above.  Quoting...

"I think the answer lies in your description that the suspect frequency is present when rolling one bar size and not the other. I would test for a natural frequency in the bar and/or the structure of the gearbox and drive."

Ghost frequencies are the result of manufacturing defects in the gearing that are introduced when initially manufactured.  Simply put, it takes gears to make gears so if a gear used in the manufacturing of your input shaft had 73 teeth, just for example say in the hobbing machine, then you could see a 73X on the pinion.

Since your data supports something around 1 g peak I would not suspect it to be the cause on an input shaft bearing failure, especially the cage.  In addition to what Walt has suggested I would be checking alignment, coupling condition and any other things related to the input shaft that might be in question.  Bearings are known to be an issue with VFD drives.  Differing bar sizes mean differing loads, so you might also want to have the actual bearing design checked based on the demands of the process.

Last edited by John from PA

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