Why do rotating shafts need demagnetizing? 1

[kingnero]

I've heard from different people, in different fields, that some of their rotating shafts need demagnetizing.

The crankshaft of large diesel engines for example. I was asked to guess, but apparently I was wrong. I said, because metal swarf could stick to the shaft, and if it were to collect or get into the main bearings (that are shells, not ball bearings) it could cause accellerated wear.

But, it had to do something with the earth's magnetic field. If the shaft were magnetic as well, there could be some opposing force which could lead to an unbalanced crakshaft.
The explanation was rather vague, not much more as I wrote above.

I've heard similar stories from welding and NDT guys on rotating equipment, but no-one was able to explain the actual/full/correct reason behind this.

Any one in the know here?

 

[redpicker]

It sounds like your confusion is resulting from a poorly worded question.

Rotating shafts need demagnetizing because they have become magnetized and the user has decided that this is detrimental.

How/why do they become magnetized?
It is true that if you take a shaft and rotate it in a magnetic field, it will become magnetized. The earth has a magnetic field that is roughly 0.5 Gauss. If aligned with that field and rotated, a steel shaft will be magnetized to an amount less than the surrounding field, so the shaft may pick up some residual magnetism. In my experience, however, the most common way a shaft gets magnetized is for NDE inspections. Both magnetic particle inspection and electro-magnetic inspection are common methods for inspection of shafts and either has the possibility of resulting in a considerable amount of residual magnetism.

Why do users specify a shaft be demagnetized?
Typically, I have seen a requirement for demagnetizing in response to a previous problem of dealing with magnetized parts. The parts tend to stick to each other. If machining operations are performed, the chips will stick to the parts. In some applications, the presence of a magnetic field will interfere with sensors, or if moving, create stray currents that can cause problems. Some users are just afraid that a magnetized part will cause a problem and will specify demagnetizing just as a preventative issue. Magnetism is invisible, and people tend to make sure that things that they can’t see don’t cause them problems. The usual level of acceptable residual magnetism I have seen is 25 Gauss, which is about the level needed to pick-up a paper clip.

So, yes, rotating in the earth's magnetic field can magnetize a shaft, but it is very unlikely that this will ever bother anyone; the amount of magnetization is just too low. IME, when a lot of inspectors magnetize a part for MPI, they will use the "if a little is good, a lot is better" philosophy, which often results in saturation. This can be difficult to remove and if left in the part, often will cause problems.

If you really want to know why a particular shaft needs to be demagnetized, you will have to ask the person who is requiring it. Don't be too surprised, however, if the best answer you get is "because the spec calls for it", since that is often the case.

rp

 

[TVP]

Excellent response from redpicker (as usual). Just wanted to add that there are applications involving rotating or reciprocating shafts with circulating oil that will specify somewhat lower levels of residual magnetism, more like 5-10 G (3-7 A/cm).

 

[Tmoose]

In turbines etc using eddy-current type non-contact proximity probes to monitor shaft vibration, shaft magnetism (and other metallurgical anomalies) fools the prox probes. Sometimes called electrical or magnetic runout.

http://www.google.com/url?sa=t&rct=...b&cd=14&cad=rja&ved=0CE8QFjADOAo&url=https://

http://www.vi-institute.org/FileDow...rYHYBQ&usg=AFQjCNECmWdQtTpeuEH8BvJoPcSsGahlzg

http://www.ge-mcs.com/download/orbit-archives/2001-2005/3q2005_runout.pdf

http://www.ge-mcs.com/download/orbit-archives/orbit_v27n207_runout.pdf

 

[Thunderbird336]

In turbines etc using eddy-current type non-contact proximity probes to monitor shaft vibration, shaft magnetism (and other metallurgical anomalies) fools the prox probes. Sometimes called electrical or magnetic runout.

My previous employer was a steam and gas turbine manufacturer/repairer, and I believe Tmoose is dead on, his explanation is the same that I heard while I worked there. And we did do Mag-particle Inspection there, as well as Xray inspection. The shafts were required to be demagnetized always, whether new or repaired rotors. That could be because nearly all of the rotors went through op-speed balancing subsequent to machining and inspection. Part of the inspection was runout checks, both with indicators and some kind of electrical runout check that I never saw; I don't know if the magnetism may have been a possible influence on the electrical runout or the probes used in op-speed balancing.

-Gary

 

[Tmoose]

Hi Kingnero,

I think in the case of a crankshaft your concern over ferrous swarf sticking in a place it didn't belong is the better guess.
Better than the earth's magnetic field and the crank tugging on each other in any significant way.

>>If<< the explanation was a magnetized crankshaft caused incorrect readings in a particular type of crankshaft balancer I might have spent some time pondering how.

Seems like if it is an engine manufacturer's maximum magnetization spec the is a decent chance there is an explanation why in the manual too.

Are you the "new guy" at work?
Before I spent much more time coming up with hypotheses for a hazy explanation offered by a straight faced or chuckling coworker (or even a boss) I'd Google "shop pranks". Most folks that worked in a fab or repair shop etc have stories about sending the new guy to the tool crib for a Bucket of steam, left handed monkey wrench, being sent to the heat treat shop to get a babbit headed hammer hardened, etc, etc, and etc.

http://www.practicalmachinist.com/vb/warner-swasey-alumni/shop-pranks-jokes-148820/