Bent SS316L shaft
Bent SS316L shaft
(OP)
First of all, pardon my metalworking/metallurgy illiteracy, but here it goes: one of our fan´s 4-7/16" diameter by 90" long SS316L shafts was bent right at the middle (about 1 degree) because one of the fan blades broke off and the resulting inbalance "folded" the shaft at the middle, right where the fan side bearing is located. So now, the shaft´s end is off-centered about 1" with respect to its original position. We had to replace the shaft for the moment, but my question is, can this bent shaft be somehow straightened up, and if it can, how should we do it? Will it retain its original properties or will it be weakened up at the middle because of the bending/unbending operation? Also, if we reground it to a lesser diameter or cut it up to smaller pieces to make smaller shafts, ribs, etc.., should we take any special care with the bended area, avoid it, etc? Thanks in advance for your comments.





RE: Bent SS316L shaft
If not long then I wouldn't worry too much. It isn't much of a bend.
You probably could have it straightened. It is done is presses with support blocks and dial indicators.
If you are going to machine it down you would either need to straighten it first, or cut out the bent section. It has to be real straight to grind.
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RE: Bent SS316L shaft
Regarding under sizing the shaft to remove the permanent bend, this is acceptable. Also, if you intend to use the material for other applications as you described this is also acceptable. The shaft material located in the bend area can be used it just received a little more strain in comparison to the remainder of the shaft material.
RE: Bent SS316L shaft
Doing it cold in a press should work. Make sure that he guys that do the work know what they are doing. If they seem uncertian, find someone else.
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RE: Bent SS316L shaft
RE: Bent SS316L shaft
RE: Bent SS316L shaft
On a fan this size it can't really hurt your performance. There is one potential bad situation. You did static balance the fan. If the blades are bent funny it may be out of balance when running because of slightly different pressure on the blades.
I was speaking of using a press to straighten the shaft.
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RE: Bent SS316L shaft
What exactly is the problem if you heat up austenitic SS to make it easier to bend? Our welders and mechanics sometimes want to heat up parts to ease bending or straightening, but I haven´t let them do it yet, at least on critical parts.
RE: Bent SS316L shaft
I never had much success in cold pressing austenitic stainless steel shafts (3” or greater in diameter) because of residual stresses introduced during an equipment failure. We had mechanically pressed one pump shaft, determined it was running true, and it never ran the same after it was installed. We kept chasing vibration and could not balance the pump. When we replaced the shaft, it ran fine.
RE: Bent SS316L shaft
Straightening really takes skill. The key is to restraighten the entire shaft. Most people focus on the major bend, and they forget that it must be bent at every support location. You have to check the straightness by rotating and using dial indicators. You have to do it up and down the shaft at various locations and spans. It is a couple hours work, but a shaft hat size may well be worth it.
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RE: Bent SS316L shaft
Working with a 90 inch stainless shaft and fan blades can't be done in a job shop with inexperienced people and pipe wrenches.
RE: Bent SS316L shaft
RE: Bent SS316L shaft
Regarding the fan blade, I can't see any issue with beating the bent tip back. You'd need to use SS friendly tools or risk damaging anticorrosion properties... but I suppose pickling could fix that. You could even heat it and bend it back... but you may reduce the corrosion protection properties - heating would be hard to control insitu.
Just back on the shaft again, putting the contamination issue to one side, if heat straightening was attempted below the carbide precipitation temperature, would detrimental properties still be induced?
RE: Bent SS316L shaft
Cold bending the blades back into shape is no big deal. Just re-balance afterwards and you should be fine.
I wouldn't even wory about surface contamination. After all this was stiking a steel support brace.
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RE: Bent SS316L shaft
You can straighten the shaft if it has a gentle bend, but not if it is a 'crease' or 'buckle'.
Do not even consider hot bending, do it COLD.
Assuming it is gentle and given the size it is worth a try. Put it in a lathe between centres. Run a dial gauge down it and find the highest point and the points at which the shaft starts to bend. As a precaution use a Chloride free pen/marker. Set the shaft up in a hydraulic press between blocks, high point up and set the blocks at the positions where the bend starts. If the distance between the blocks is less than 10 diameters you will really struggle. Set up dial gauge to measure the imposed deflection from the press. Make sure you protect the shaft at the blocks and the press nose. Press the shaft until the dial gauge reads 0 then go on until it reads half the original bend, i.e. push it right through 0. Leave it for a few minutes and release the pressure. Check the dial gauge. Repeat this until you achieve a near 0 reading on the dial gauge. For a shaft this size a deflection of 0.010" would be accetable.
If the thing will not respond then certainly cut it up and make shafts from it, you will not have to worry about the material in the bent area other than it makes machining very awkward. You sould turn shafts between centres to avoid taking them out of the lathe to do one end and then the other.
The fan blades. Are they solid or hollow section? If solid then OK these can be heated and bent, after all you can weld 316L, as a previous respondant has mentioned just pickle and then don't forget to passivate. It is very easy to set up the right conditions for Stress Corrosion Cracking to get hold. Water vapour with 35ppm chlorides at 50 degree C will crack stainless nicely when stress is applied.
If you damage the blades in the process (splits, creases or cracks) then weld them and grind them smooth and use Dye Penetrant Inspection to check for surface breaking defects. Use TiG welding.
If the blades are hollow section these will be past their best, scrap them.
I would be more worried about why the original blade came off in the first place. What one has done the rest will probably follow.
RE: Bent SS316L shaft
corus
RE: Bent SS316L shaft