stainless or bronze? 1

[motorworks]

Hi
I have a request to make shaft adapters for a speed reducer.
Theses are in a salt H2O area.ie salt water may come in direct contact.The gear boxes are made of AL with a female stainless input.The staft that is going into the gear box is stainless.Gear boxes have a 1" bore and the shafts are 20mm.So the adapters will be 1" by 20mm by 4" long.
Would stainless be better than 660 bearing bronze.have to make several and guy wants price and advice on both.
Any help would be great
eddie

http://www.geocities.com/motorworksnf/123

 

[unclesyd]

I would not make them from SAE 660

First choice would be Nitronic 50.
Second would be 17/4 PH S/S (H1150)
Last would be 316 S/S

Monel 400 would work.

If you have to stay with bronze go to silicon bronze

316 is going to be quickest and least expensive. But if there is any load you have the possibility of cracking in the long run.

If you are around saltwater the top choices are boat shafting materials used in salt water and are very good.

 

[moltenmetal]

Consider 303SS- much easier to machine than 316, costs the same, and is only modestly less corrosion-resistant. Unless these gearboxes are running hot, I wouldn't personally be too concerned about corrosion. If they do run hot, cracking may become a problem over time. If the result of a failure is a catastrophe, you may want to consider a much more resistant material- but then again, you'll have problems with the gearbox shafts, and I would expect fatigue would be more of a problem in these parts than in the coupler.

 

[boo1]

WE have found 303 sst to perform poorly in he salt water enviroment. unclesyd (Materials) options look sound.

 

[TVP]

Agree with boo1, Type 303 stainless steel performs poorly in salt spray environments, and worse yet when immersed in salt water. Nitronic 50 is a good choice for stainless steel.

 

[moltenmetal]

Would it make sense to use Nitronic for the coupling if the shafts are only 316SS (or perhaps 304SS?). The output shaft will be wetted in a crevice inside the coupling...

Thanks for the advice on 303SS in salt spray- not my personal experience (yet!), but I trust yours. Better to learn from someone else's failures than to wait for your own to happen!

 

[TVP]

304 is not that much better than 303 in terms of resistance to salt water, and 316 can have problems with pitting and crevice corrosion. Nitronic 50 would be the minimum I would pursue. If you choose to go with Type 316 for the shaft, there should not be any problem with Nitronic 50 for the coupling.

 

[moltenmetal]

The shaft and speed reducer input are, according to motorworks post, "stainless steel". I understand that motorworks has been asked to choose a material for the coupling- the other parts are what they are. My question remains: is it sensible to specify Nitronic for the coupling if the other parts are only 316, or possibly even 304?

I can see that 303 is inferior to 316, but are you implying that 303 is worse in salt water than 304? I calculate the pitting resistance equivalence index for 304 to be ~ 18 and for 303 to be ~20, and for 316SS ~24. The 303 has small amounts of sulphur and phosphorus added- do these have an overwhelming influence on the corrosion of this alloy?

 

[TVP]

moltenmetal,

The answer to your question is "YES". In terms of corrosion resistance, 303 has ENORMOUS amounts of S, not small amounts. Member mcguire has posted previously on how much pitting resistance is reduced by S. Check his FAQ.

 

[unclesyd]

Moltenmetal,
I've never seen any correlation with the "pitting resistance" and actual behavior in sea water. 303 S/S normally becomes a rust ball with deep pits in a very short time. 304 S/S starts out very slowly as very small pits that take a long time to completely cover a part in rust. 316 S/S takes a very long time to initiate pitting and is very easily cleaned. 316 S/S is usually the preferred material if you can’t go to other materials. This is because of cost and availability.

In giving a suggestion for material for use in salt water I always like to give a choice and let the cost and availability dictate the end product. Like if you were in my area all of these materials would be readily available at reasonable cost.

I opted for a stronger material as his sleeve will be fairly thin. It just so happens that the material is much better in salt water. But you can use any of my suggestons.

 

[moltenmetal]

Excellent info, guys. Thanks for that- I wouldn't normally consider 0.2% sulphur to be an "enormous amount", but your experience definitely indicates an inferior corrosion resistance for 303 than for 304- despite the 0.6% Mo...