steel shaft straightening

[Tmoose]

API 541 E4 seems to forbid straightening electric motor rotor shafts.
"2.4.5.1.2 - c. No shaft-straightening technique is permitted during or
after fabrication of the rotor."

API RP 687 "rotor repair" does not prohibit straightening a rotor shaft during repair, but warns that "Shaft does not always maintain this restored straightness
while in operation under load and temperature."

MIL-STD-2191 (SH) has some specific straightening procedures for the fanchy materials used for propulsion shafting. A few do not require subsequent stress relieving.

Of course I'm asking because we have a 12 inch diameter, 12 foot long shaft that needed to be straightened. 4140 Q&T, it should not see more than 200 F or so in service.

Are there other standards or guidelines regarding shaft straightening?

thanks

Dan T

 

[metengr]

I do not know of any specific standards that would apply because of the risk involved with heat straightening air hardening low alloy steels. I have heat straightened many carbon and low alloy steel pump and motor shafts using techniques that were developed over (a long) time.

The most important piece of information required for heat straightening motor or pump shafts is to know where the bend originates and type of bend - straight or twist.

If the shaft contains a straight radial bend along the axis of the shaft, obtain detailed total indicated runout readings.

Once the origin of the bend is known, place the shaft on a lathe and locate the bend top dead center, not on the bottom.

Place several dial indicators on the shaft - one at the end of the shaft (bent end) and one 12" away from where the bend originates. Center the dial indicators to zero.

Using a rosebud torch with MAPP gas, spot heat the location where the bend originates at top dead center and keep the spot heat on until the shaft deflection reaches between 2 to 3X the original shaft runout. Once the deflection is reached, remove the heat instantly and cool with air (to 800 deg F) followed by wet rags.

You must only spot heat, no feathering, and the size of the spot heat region should be the size of a quarter. The smaller the better.

As a result of spot heating you will locally harden the shaft because it will reach a temperature well above 1400 deg F. However, this is the consequence of heat straightening. Make sure after heat straightening you perform a surface NDT of the spot heat location.

 

[racookpe1978]

As he pointed out above, this method is NOT a physical or "reverse-bending" of the shft: you are not re-curving it and you should not bend past the straight centerline and then expect the shaft to spring back (the 2% elongation) to its final resting position. Heat bending doesn't create or use the potential energy of the metal's springiness like that, so its less forgiving a method.

More accurate also, if you can do it as described above. Test a few round bars first - then try it on your shaft. Three or four bars at 1.00/ft for dumb A36 round steel are much less expensive than your motor shaft.

 

[swall]

Metengr--that was extremely interesting. My former employer used to straighten automotive axle shafts, which were induction hardened 1050. This done with a hydraulic press, v-blocks and operator.Kind of scary to watch.

 

[arunmrao]

Last week I straightened a 3m long,75mm dia hydraulic pump shaft, on a hydraulic press with Vee blocks ,anchored firmly on a 150 mm I beam,so that there is no slip. We need some luck too and this time I was blessed .

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[Tmoose]

The shaft was straightened from 0.010" to about 0.003" TIR (drawing tolerance) by a 3rd party, reportedly by pressing cold.
I'm inclined to say we should accept it, as i believe even if it moves back toward the original 0.010" figure balance corrections can be made to compensate for the component offset. All the internal clearances are on the order of 1/8 inch.

 

[metengr]

Did you perform surface NDT of the shaft where the force was applied? If not, I would do so before acceptance. This is what I would require as a customer or client.

 

[CoryPad]

Swall,

That procedure is done on millions of automotive axle shafts (induction hardened 1050) and hypoid pinions (case carburized 4320) by my current company.

 

[TVP]

Tens of millions annually of automotive piston rods for suspension struts, induction hardened 1035 or 1045. Some are done using roll straightening machines, lots are done manually with a simple hand-operated press. I hate watching it.