8 meter long Shaft Refurbishment

[ahsanb]

Hi guys,
We recently replaced main shaft of one of our agitators with a new one. We intend to refurbish the replaced shaft which was in service for about 11 years with 85 percent yearly availability. We want to refurbish the shaft so that in can serve for similar tenure again on our other agitators.

Shaft is in reasonably good condition with considerable wear on its steady bearing location due to many steady bearing replacements over the years. Can you guys advice any precautions we need to take before we proceed with the welding job.

Any advice regarding which type of welding procedure to use and what precautions we must take to deny any chances of shaft getting bent beyond redemption will be highly useful. Do we need to make special arrangements to protect shaft from unwanted effects of heat due to welding.Below is some information regarding shaft material and dimesions.

Shaft Material = SS 304
Length = 8 meters
Diameter = 152 mm
Steady Bearing Sleeve Material = Stellite
Steady Bearing Bush Material = Haynes (HS.6B)

 

[asimpson]

I would be worried about fatigue. Micro-cracks wouldn't be easily apparent. If you feel shaft hasn't suffered in its previous life then this shouldn't be a problem.

 

[unclesyd]

Can you provide a sketch as to the location of these bearings?

 

[dhengr]

Ahsanb:
Do what Unclesyd asked, and add to that such things as all shaft dimesnions; all loadings and stresses on the shaft; rpm’s, hours/year in operation, since we already know about the 11 years; how much shaft wear is considerable and what is reasonably good condition? Your question and limited info. provided is a little like saying “design me a beam, just don’t ask any questions, all I need it a beam.”

Depending upon the amount of wear, I’d consider just cleaning the shaft up and polishing that area, and fairing it out into the main shaft and then making your new bushing fit the new conditions. Depending on the fatigue situation I’d have someone check the shaft for surface condition. Flame spraying will rebuild a shaft dia. if the wear isn’t too great, with min. adverse affects. Welding would be my last choice because of the deformation and potential fatigue consequences.

 

[ahsanb]

Dear All,

Thank you for your interest. I am not able to get you guys the whole shaft picture but I have a picture of steady bearing arrangement and I can tell you that steady bearing lies at about 1.5 meters from shaft's lower impeller end.

Shaft has been worn down to maximum of 2mm and on average about 0.5mm to 1mm at steady bearing location. Steady Bearing sleeve is split and is supposed to be slide fit on to the shaft. Some other parameters are as below:-

Shaft RPM = 58
Working 24/7 @ 85 % yearly availability
Process = Purified Terepathalic Acid Slurry @ 35 barg and 260 degree Celsius
No material compatibility has been faced as yet as SS 304 has worked just fine with it.

Hope this helps.

 

[unclesyd]

A few more questions.

Does the shaft have a foot bearing?

Do you have room to move the steady bearing up or down to clear the worn area?

Can you make the bearing longer so it will be out of the wear area?

i think the problem with the wear being so thin it is going to be hard to apply any thing and keep it on the shaft to finish it to size. We like have a least a 1/8" undercut for any of the thermal coating processes.

 

[ahsanb]

No the agitator has just one steady bearing that is supported with the help of three steady arms attached to the vessel.We can shift the steady bearing location about +/- 20 cm from its centerline position by threading in or out the clevis.

Almost every time we re-assemble the steady bearing, sleeve is installed at a slightly different position and so there are many 2-3 mm deep dimples on shaft where we used to install sleeve anti rotation screw.

We would have never changed the shaft if we had not observed agitator motor's amperes exaggerated variation over the last 8 months, upon consultation with OEM : Mixing Solutions we decided to replace the shaft with a new one even though it still had a highly acceptable TIR of 3mm.

Our experience shows that steady bearing sleeve has never shown consider ductility as it is made of hardened statellite which is very hard.

Based on above history would you guys still recommend no material build up or do u suggest we slightly trim the shaft on steady bearing area and do welding on it.

 

[Tmoose]

So wear sleeve rotates with the shaft? Is the sleeve OD the bearing journal?

 

[ahsanb]

Sorry guys I mentioned anti rotation screw in place of sleeve clamping screw so the sleeve rotates with the shaft because of the clamping screw and also due to the fact that it's split halves are slide fit on to the shaft. Steady bearing bush has a anti rotation screw that is just long enough to connect it to steady bearing housing and keep it stationary and not touch the sleeve surface. Steady bearing bush/sleeve diametrical clearance is 1.0mm with a tolerance value of 9 mm, but it has been a standard practice to change bush or sleeve or even both based on which is worn more when the diametric clearance is 5.0 mm.

So sleeve rotates with the shaft, while bush remains stationary.

 

[unclesyd]

I'm wouldn't be against welding due to the shaft size. The welding process will still have to be controlled as not to put enough heat in one spot to cause the shaft to bend.

I would consult with someone we uses HVOF or D-Gun thermal coatings. When you talk to someone make sure they are well versed in the art.

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