Steel shaft distortion 4

[r4c3r]

Dear all please help!


Can differential cooling resulted from drought affect the straightness of our shaft?

We use shrink fit procedure to assemble the core pack on to the shaft.
Core pack is heated up to 200ºC and then shrunk on to the 10” diameter 110” long steel 28Mn6 shaft. The shafts are forged and normalized prior machining.

After assembling the core pack on to the shaft it is left for approx 24hours too cool down. Recently we have noticed that some of our shafts distorted after the core pack has been assembled. I was told by people form welding institute that distortion might be as a result of relieving stresses induced during machining.
Can stresses relieve at less than 200ºC?

When examining last shaft that “bowed” I have noticed that when we have left the core pack and shaft assembly to cool it was exposed to draft air coming from one direction. Do you think that shaft distortion might result from differential cooling due to exposure to draft, or is this not enough of temperature difference to have any affect?

I would appreciate your comments.

 

[Neubaten]

It could happen, but I doubt that the air current has anything to do with the bowing, because you have a lot of thickness to originate enough thermal inertia as to compensate the temperature differential. And 200ºC isn't that much of temperature.


I have some experience with that steel, but quenched & tempered, and it was very temperature-sensitive. We determined a stress relieve treatment (after a forming operation) at just 215ºC.

Shot peened parts of this steel relieved all the induced tensions if heated above 180ºC, rendering the treatment useless.

 

[metengr]

When examining last shaft that "bowed" I have noticed that when we have left the core pack and shaft assembly to cool it was exposed to draft air coming from one direction. Do you think that shaft distortion might result from differential cooling due to exposure to draft, or is this not enough of temperature difference to have any affect?

No, I don't believe this is the cause of distortion either. It is probably residual stresses in the shaft prior to installation of the core pack. The fact is the core pack installation, being an interference fit, can effect pre-existing residual stresses in the shaft. If the core pack is slightly misaligned or not equally making metal to metal contact on the shaft OD surface, this will significantly impact residual stresses in the shaft and straightness.

I would double check the core pack alignment first, and as a second option consider a 225 deg C stress relief thermal treatment on the entire shaft before you install the core pack. The 225 deg C thermal treatment, which is slightly above the 200 deg C exposure to local temperature from the core pack installation, should be performed on the shaft in a vertical orientation and held for 3 hours at temperature. This stress relief treatment would reduce any residual stresses in the shaft from the original heat treatment and machining prior to installing the core pack.

 

[Tmoose]

When shrinking a motor rotor onto a shaft distortion is bound to occur. I believe the mechanism is not stress relief, but the unavoidable non uniform rotor temperatures producing a bowed rotor that clamps and captures the shaft in the bowed state. The larger diameter rotor easily overpowers the small diameter shaft's efforts to remain straight. Important features like Bearing journals and the rotor OD must be left several thousands oversize to ensure the rotor assembly can be finished after the rotor installation.

One way to mitigate, if not solve the problem, is to use a sleeve with an 0-ringed bore inside the laminated rotor so sleeve/shaft shrink interface can be pressurized to release the interference and let the shaft straighten inside the fully cooled assembly. Indramat used to have online data and procedures for "partial motor" assembly, but that seems to have been removed from the public areas of the Bosch Rexroth webpage. All I found was a picture of a rotor (with sleeve) and stator that form a "partial motor."

http://www.boschrexroth.com/busines...technik_bilder_de/indradyn/IndraDyn_H_600.jpg

 

[unclesyd]

One thing you might try is immediately hang the shaft vertically after shrinking the rotor on the shaft. When we had several large shafts where two gears were shrunk on the shafts which were drilled for an eye bolt on the end. Also back when we tacked and unstacked turbine rotors we also hung the shafts from the end. We actually stacked the rotors in a vertical condition. The rotors were always sored by hanging from the end.

On a 10" shaft you can actually draw the shaft one way or the other with your temperatures.

 

[ScottyUK]

Perhaps a daft thought, but I'm a electrical guy who plays with generators so I won't feel too bad if it is:

Is this shaft large enough to develop a thermal bend of the type we see in a generator rotor which is off barring? Gut feeling is that it probably is. Assuming the forging develops a thermal bend, could the shrink fit core pack then cause the thermal bend to become a permanent distortion of the shaft as it cools and the two parts devlop an interference fit?

Apologies if this is what Tmoose is describing.


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If we learn from our mistakes I'm getting a great education!

 

[Tmoose]

Hi ScottyUK,

yes, a shrink fit locking parts in a what was a thermally bowed state is what I was trying to describe.

I think Unclesyd's reference to drawing a 10 inch shaft might be related too.

laying 2 warm hands on one side of a 3 inch diameter 18 inch long SS precision spindle shaft for a few minutes can cause a few "tenths" runout. That's a temporary condition, with straightness returning a few minutes after the hands are removed.