High 1x Throughout Motor and Gearbox

Hello, 

Relatively new to vibration analysis and have been following this forum for a few months. Thanks in advance for all the great information I've gotten just from reading here.

 

At my facility we have a screw conveyor that is critical to our operation.  It has been a problem child for years before I ever joined the company. Both the motor and gearbox were upgraded about 2 years ago to combat quarterly monthly motor failures.  However, we are still having issues that appear only 1 to 3 months after installation and become a major issue after 6 to 12 months of run time.

 

The motor is C-face mounted to the gearbox with some sort of composite bushing that has been thought to be the culprit of the high vibration over the years. I have attached the most recent data that I have taken.

 

The motor runs about 170F, but where it mounts to the gearbox at the bushing is about 180F, and the gearbox was around 160F.

 

I am looking for the best solution possible for the long term.  We have a scheduled outage in a few weeks and management wants to make sure we don't overload resources changing out more than we need.

 

Thanks again,

Jake

 

 

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Original Post

Can you provide some more details about this machine?  What actually are the components that show failure when you rebuild for instance.

I assume the motor is horizontal.  There seems to be some relatively high amplitudes (predominately vertical) on the motor, maybe even the NDE being higher than the drive end.  Have you done a bump test to see what the overhung resonance might be of the motor?

John,

I have only been able to get a look at the bushing after the motor was changed the previous time, which was about 10 months ago.  The inside of the bushing had some scarring that was like a shallow, broad, pit.

The motor is mounted horizontally and the gearbox is shaft mounted.  

I have not performed a bump test.

A picture is worth a thousand words applies here. Running speed vibration can increase in amplitude due to forcing function increase (i.e. unbalance, bent shaft etc...) or increase in degree of freedom (i.e. clearance between machine and base, shaft and bearings, bearings and housings). I am not sure of your bushing arrangement so cannot speak to that but generally if it was a coupling that had higher temperature than either driver or driven I would look towards misalignment however in that case you would typically see a time waveform with "M"'s or "W"'s and a 2X component. Is the bushing lubricated? What does the arrangement look like? With screw conveyors you need to also be mindful of the fill factor. Long conveyor screws generally require a certain fill to keep the screw lifted otherwise they can run hard on the bottom and introduce a "bend" causing an elevated 1X as compared to higher fill rates. Conveyor speed and filling rates affect this. If you have a taper mounted gearbox arrangement check that it has been installed with proper alignment. Check shaft run out when the gearbox is removed and the conveyor is empty as many conveyors are made with stub shafts that may bend or crack depending upon the construction and loading. Hope this helps.

 

CKYJohn, 

I should be back at this conveyor in the next day or two and will get some pictures. But for now, it is a c-face set up. The motor bolts directly onto the gearbox. The bushing is keyed and goes over the motor shaft and into the gearbox key way. With this set up, I can't see much of a chance of misalignment.

The gearbox is also shaft mounted, therefore there is no base. 

There is no way to lubricate the bushing other than upon installation.  The last time the vibration was this bad and we pulled the bushing, it still have Never-Seez on it. However, it also had a film of dust (almost rust looking) over it where it met the motor shaft.  The mechanics here use Never-seez on it because many times when being replaced, the bushing has been locked on the shaft or in the gearbox.

 

The fill level of the screw is interesting, but this screw runs 24/7 mostly with the same load all the time. There are only a few times here and there where we run reduced rates. 

 

I want to think the bushing is getting loose on the shaft or in the gearbox, but I am not sure what the root cause of this would be and for it to keep happening over and over again. 

 

 

 

OK so the bushing is really a coupling... this point had me confused. If the coupling is getting loose then there most likely is relative movement occurring. You stated there is little chance of misalignment with a spigot fit however I would confirm try to confirm that as an increase in temperature at the coupling compared to either shaft towards the bearings, points directly to misalignment.  On startup of course you may see high torque values and relative torsional movement between shaft and coupling unless you are using a VFD or soft start but this would be instantaneous and not cause a steady state temperature increase. I would also check on the dimensions of the shafts and coupling and ensure you are not starting with a loose fit.

"The motor bolts directly onto the gearbox. The bushing is keyed and goes over the motor shaft and into the gearbox key way. With this set up, I can't see much of a chance of misalignment."

This is Not a good assumption. I have seen misalignment on C-face mounted hydraulic pump (STG lube oil). A loose fit and misalignment could cause premature coupling wear and high 1xSS. Don't look past the elephant in the room!

Walt

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