Porosity in Babbitt sleeve

[Bmallen77]

I work for a company that makes parts for the aerospace industry. We have a run of shafts we are currently doing that have two separate Babbitt bearing journals on each part.
We noticed when these shafts arrived there was a significant amount of “porosity” or small pinhead sized holes in the Babbitt sleeve.
The Babbitt sleeve is approximately .015” thickness over the top of the journal per side.
Typically we do what we refer to as “puddle filling” these small holes using astm grade 2 Babbitt coil. Using an oxy acetylene torch with a size 00 torch tip. I slowly heat up the area containing the holes and get them to melting point, at which time I dab in a small drop of astm grade 2 babbitt coil.
We then machine off the extra Babbitt and typically after one or two tries we get the holes filled. However in this last batch we are having significant issues getting the holes filled. We will puddle fill the holes, then make a cutting pass and more holes will appear. Can’t figure out a solution. We clean the area throughly, make sure it’s completely dry before doing our puddle fills. We even tried using a flux that was recommended from the company we purchase the Babbitt from. Still not getting better results. Anyone have experience with this issue?

 

[Dan from Oakland]

Your method is solid. How is the babbit applied ? sprayed or cast? If you are doing the casting, my guess is your molds are not hot enough or your shaft is not at a temperature that is keeping the tinned area molten, and you are getting outgassing during the casting process. If you are spraying it, sorry, I have no experience with that, but again my guess would be the shaft is not hot enough.

Kinda unusual that the babbit is on the shaft and not the housing, but..............
one more thought- rough turn the babbit and then ply the torch on the surface to reveal any porosity near the surface, but i would really look at the casting process for a source of the problem. How thick is the pre machined babbit???

 

[Bmallen77]

Your method is solid. How is the babbit applied ? sprayed or cast? If you are doing the casting, my guess is your molds are not hot enough or your shaft is not at a temperature that is keeping the tinned area molten, and you are getting outgassing during the casting process. If you are spraying it, sorry, I have no experience with that, but again my guess would be the shaft is not hot enough.

Kinda unusual that the babbit is on the shaft and not the housing, but..............
one more thought- rough turn the babbit and then ply the torch on the surface to reveal any porosity near the surface, but i would really look at the casting process for a source of the problem. How thick is the pre machined babbit???

Hi Dan, I appreciate your feedback. Unfortunately I do not have all the answers for your questions. The customer we do the parts for requires us to use one of their approved vendors for the Babbitt application. So as far as the process goes I’m unsure if it’s sprayed or cast. I do know that when we get the parts back from the Babbitt vendors shop it is typically very rusty and the look of black soot on the outer surfaces of the shaft.
As far as the thickness of the Babbitt I believe it’s roughly .030”-.035”, so we are removing roughly 50% of the material.
What I can say is I have done the Babbitt repair on dozens of these parts and never has it been this difficult to repair. Typically it will be a few holes per part, and when I heat the area, with the torch to get the Babbitt to melting temperature, the hole may grow a little but a small dab of filler Babbitt and it fills up no problem. With this batch when I get a hole to melting temperature all of a sudden many more small holes will appear around the initial hole I am filling. And you end up chasing them sometimes up to1” to fill all the little porosity spots.
So your statement about the Babbitt on the shaft compared to the housing is really interesting as well. I didn’t realize that was kind of a rare thing until I started doing some research on this problem. Couldn’t find any videos or photos of parts set up with Babbitt on the shaft, always on the housing. So it’s been really difficult getting information on this process and the outside vendor has been very secretive about giving us any information as to their process of applying the Babbitt.
I think we will end up having to use our driver on the next delivery to the Babbitt vendor to do some covert surveillance on their operation to figure out what method they are using for applying.
I did have one possible “theory” but maybe I’m really reaching for this one. Is it possible that the acetylene gas (the black soot when burning) could be contaminating the weld pool? I read that people will use the black acetylene soot on their Babbitt molds to help the Babbitt not stick to the mold. Is there any way oxy acetylene isn’t a good gas for performing the puddle fills?

 

[john.k]

The answer is simple........send the babbit work back......reject shoddy work completely....vendors never get the message until they get reworks clogging up their production ........then they get the message.

 

[dkmc]

A proper babbit job has no porosity. Although a certain youtuber would pronounce that 'good' and say the holes provide 'oil retention'....


I've had good luck with no porosity problems.

 

[Dan from Oakland]

The soot should not be an issue if they are using it to coat the mold. Your torch repairs look solid but if you are doing more and more chasing of porosity and blow holes, the vendor needs to make that good. The shaft needs to be tinned for the babbit to adhere properly, and part of the tinning process involves a flux of some sort, and that is probably where the corrosion is coming from. If there is residual flux on the part and the tinned area is not molten, its possible the cool flux is not floating out with the pour and causing your problem. Can you post a picture of an unmachined shaft with the babbit? What does the surface look like?

 

[Peter from Holland]

Question is if these are existing holes opening up or holes created by your proces
X-ray or perhaps echo will get you a answer I think

Peter

 

[jbs007]

I also found it interesting the babbitt is on the shaft and not the housing. I don't do aerospace work but chrome and grind bearing areas of large shafts that go in a babbitt bearing. Sometimes chrome can have pits but that is a different issue and you are not fixing that with babbitt and you're not tooling it either.

I also agree if the work does not pass inspection, do not accept it. If fixing the babbitt is an acceptable method the babbitt vendor aught to do it.

 

[Peroni]

Bubbles in babbitt usually are the result of pouring the metal too fast. There could also be contamination in the molten metal or the metal was not mixed properly before pouring. I would reject those parts. When properly done there should be no bubbles at all no matter the bearing size.

 

[mottrhed]

Never seen babbitt on a shaft, plenty on bearings, but this is an odd design. I dont see why it wouldnt work, just a ton more surface area to babbitt vs the bearing the shaft rides in. Either way I have nothing constructive to help with the original problem, I also would reject from the vendor if thats being outsourced, especially if you dont get to choose and manage the vendor.