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Technical Discussions
Ehsan
Ehsan
06:11 Mar-09-2009
Circular shot current calculation

Dear All,
as per ASTM for circular shot NI=K/(L/D).
but i am not very clear at the time of calculating L/D for many parts Viz Crankshaft/Camshafts/spindles and other difficult geometry components.
your valuable responce are awaited.

Thanks

Ehsan

    
 
 Reply 
 
Michel Couture
NDT Inspector,
consultant, Canada, Joined Sep 2006, 845

Michel Couture

NDT Inspector,
consultant,
Canada,
Joined Sep 2006
845
15:49 Mar-09-2009
Re: Circular shot current calculation
In Reply to Ehsan at 06:11 Mar-09-2009 (Opening).

Ehsan,

Andreas brought a valid point. But one thing I would like to mention to all who do MT is the formulas given in ASTM E709 and E1444 are in my opinion only guidleine or starting point if you will. On factor we never really consider in our calculation is: "permeability".

For this reason, when I taught MT, I used to tell my students the calculations are only a starting point. I would always magnetised my parts using a lower amperage than what my calculation showed me. Always, verify with my pie gauge or QQI to ensure you are getting the proper magnetisation. You will be surprise at how low the amperage is compare to calculations. Other things to consider when doing MT is particle mobility and background.

One other thing, whenever you do MT, try to picture the part as being a straight piece of round stock. You were talking about a crackshaft, well how long would it be if it was straight?

Remember, always verify your magntisation with a QQI or pie gauge.

Cheerio's

I hope this will be of help.

    
 
 Reply 
 
Andreas
NDT Inspector, Specially in the oilfield
Tuboscope Vetco(Deutschland)Gmbh, Germany, Joined Oct 1999, 129

Andreas

NDT Inspector, Specially in the oilfield
Tuboscope Vetco(Deutschland)Gmbh,
Germany,
Joined Oct 1999
129
19:17 Mar-09-2009
Re: Circular shot current calculation
In Reply to Michel Couture at 15:49 Mar-09-2009 .

The E 709 eveen state that is only a guideline and you should verify the field by a "gausmeter" , correct would be a Tesla meter to use correct SI units.
I like the so called "Burmah-Castrol strip" , it shows you the magnitude and direction of the field.
If you see the three indication on the strip ,the field is 2.4 kA/m or greater (2 - 6 kA/m is required by EN specs).If not placed perpendicular (90° ) to the applied magnetic ,then no indication will be shown.

    
 
 Reply 
 

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