Re: Turbine shaft coatings

Posted by Matt on June 12, 2003 at 00:42:04:

In Reply to: Re: Turbine shaft coatings posted by Gordon England on June 10, 2003 at 12:13:40:

: : experiencing Babbitt bearing material "pick up" when running against a steam turbine shaft journal coated with 13Cr steel (via arc spray). Coating was finish ground and polished and applied within all normal parameters. surface of coating appears normal, ie. no excess porosity and bearing material has not marked the coating surface either.

: : bearings were blued against shaft and checked for clearence prior operation yet we still had bearing seizure.

: Hi Matt

: I would check that your lubrication system is working and is sufficient. Failure occurs when the lubricating film between the two surfaces fails to provide separation of the surfaces. Poor bearing alignment, clearance or surface finish will also exacerbate this situation. Thermal spray 13% Cr steel coatings normally provide very good service against white metal bearings.

: Regards Gordon

Hi Gordon, Thanks for your comments. Further research overnight has produced more questions for me:):

Hobart Tafa Application Data Sheet # dated 25 Oct 1982 describes exactly the problem that we are experiencing however they state that seizure is more common with "Aluminium Alloy" bearing material and they recommend applying the "highest possible surface polish" as a solution. I have requested that our client test and determine exactly what material their bearings are.

Good old Metco Flame Spray Handbook (Vol 1 Wire Process, pages A-175 & A-176) dating back to the '60s discusses the same problems of seizure also!..they also state that seizures are more common with harder bearing material (such as Aluminium Alloy) however their suggested fix for the problem is to apply a phenolic sealer to the coating.

For this recent application we had applied a PTFE liquid sealer and a reasonable polish after grinding. We are now going to prepare some 13%Cr spray samples, one with a ground finish and no sealer-the other with a very high polish and phenolic sealer applied, I will then get them both metallographic analysed for my own education. Am very hopeful that our client will advise that they have used "aluminium alloy" bearing material and we will then know the error of our ways and in future will be able to apply a much higher surface polish and phenolic sealer.

Gordon, thank you once again for your assistance in this matter.

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