Babbitt Bonding to Bronze 1

[Mendit]

We are having bonding problems when spin casting. The casting of bearing shells by the spin cast process has worked well for us for years,we have not changed our methods or products. We recently undertook s job where the split shells are made from SAE 660 bearing bronze. The casting looks great and passes all bond tests, after stress relieving. We are using ASTM #2 babbitt and Flux and Solder 99.9% tin from Johnson Mfg. Problem, we accidently dropped a couple of bearings on the floor and the cast babbitt and bronze fell apart. Any suggestions where the problem is ? Shell temp 425F, babbitt temp 675F spin speed 630 rpm, water mist cooled during spin.

 

[Carburize]

Are you pre-tinning the bronze before applying the babbitt?

 

[Mendit]

Thanks Carburize for your response
Yes we are with an Tin bath immersion process, cool, clean and re-immerse holding at around 400F till ready to cast

 

[UKpete]

Sorry if this is daft or too obvious, but do you know for sure that the original products wouldn't do the same if dropped.

 

[Tmoose]

Was the failure in the bronze, the babbit, or the tinning?

Bronze faiure - Supposedly the lead is insoluble in the alloy, and exists mostly along grain boundaries, tending to make it weak and susceptible to shock.

Tin or bond failure - If the tin/bronze joint was weak due to inadequate or inappropriate prep I'd expect trouble. Is the pre-tin mechanical prep sanding or buffing, which might tend to smear the lead? Would processes like grit blasting or vapor honing be able to clean without smearing? Maybe a chemical etch after mechanical cleaning would be required.

I'm wondering about the reasons to select a bearing bronze to serve as a housing. The ~7% lead in 660 helps it survive when lubrication is marginal. Are the bearing shells "full floating", that is, spinning in a housing while a shaft is spinning inside the babbit ID, making the bronze's bearing qualities important?

These guys say 660 is solderable, but not weldable, and barely brazeable, due to hot shortness.

http://www.anchorbronze.com/c93200.htm

It would seem your temperatures would probably not be up in the hot short range, but .............

 

[Mendit]

Thanks Tmoose for your valuable reply, On the Lead issue we are using ASTM#2 which is lead free. The failure is in the bonding. I was wondering if a rosin type flux would provide better bonding than the acid types. One of the issues suggested by our team was the "as cast thickness" of the spin cast was too great at 1/4"-5/16" on a 3" bearing. The finished thickness is only 1/16". The babbitt bond and babbitt stresses were to high and when dropped the shock load released the bonding. Any commnets here? The selection of SAE 660 is cost related, this material is most commonly produced in a competative market and therefor costs about 50% of other roto-cast bronzes

 

[Tmoose]

The 660 bronze has (had ?) 7% lead. I was thinking out loud whether the mechanical prep of the bronze housing could produce mechanically smeared lead on the surface before tinning had even begun.

If it was epoxy I'd machine the housing ID with rough threads and grit blast or abrade using clean fine grit abrasives (180 grit or a little finer).