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11-27-2011 09:33 PM

Kind Reagrds

Nev

14 REPLIES

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11-28-2011 08:40 PM

It might seem a little inconveneient, but it is simple once you know what you normally use. And I have repeatedly seen systems that claim to look after all of this for you to get it wrong once it comes to FEA!

Hope that helps.

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11-29-2011 08:59 AM

Mark.

N/A

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11-29-2011 04:51 PM

Thanks,

Mike

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11-29-2011 05:01 PM

sherman.mark@siemens.com

Mark.

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11-29-2011 05:30 PM

It sounds like you probably have everything correct, and results would thus be showing be in psi. However, please note that if you have any gravity loads, then consistent units would have your acceleration as inches/sec2 and density would be in "snails/inch3". This is because pound is NOT a unit of mass (never was, never will be), it is a unit of force. A "snail" (sometimes "slinch") is the amount of mass which accelerates at 1/in/sec2 when a 1lb force is applied. A snail weighs about 386.1 lbs force on earth (32.1 ft/sec2 * 12in/ft).

Thus, to summarise if you are using inches (but note that you can get Femap to scale the geom to metric when you import it!!!) and lbs, then pressures should be applied in psi, and stress results will be in psi. Deflection results will be in inches, masses and densities should use snails and snails per inch3. A gravity load would be 386.2 in/sec2.

Hope that helps!

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11-30-2011 03:56 PM

Thanks again

Nev

N/A

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01-20-2012 02:38 PM

Femap is "unitless" - but this means you must use a consistent set of units. eg. SI = N, m, kg, Pa. Most engineers prefer mm. If you also prefer to use N, then your consistent set of units would be N, mm, mm/sec2 (for accel), MPa, and tonnes (because F=ma). Thus, if you prefer to use N and mm, then material density must be in Tonnes/mm3. For example, steel would be 7.8e-9. SolidEdge might be using something different.

It might seem a little inconveneient, but it is simple once you know what you normally use. And I have repeatedly seen systems that claim to look after all of this for you to get it wrong once it comes to FEA!

I can (almost) understand

So, if my preference is to work with: inches, pounds-force, pounds-mass, and gravities, how do I convert

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01-22-2012 01:53 AM

But to answer your question, a consistent set of imperial units if you like to use inches is: inches, pounds force, slinches (pound is never a real mass, and is not consistent with F=ma for your other chosen units) and inch/sec2 for gravity/acceleration. The by-product of this is pressures in psi, stress results are in psi, densities in slinch/inch3 . As most reference sources do not provide information in such consistent imperial units, then conversion is required from those sources. Conversion factors are readily accessible via google search.

If you are adamant about using lb-mass, then your choices are: (a) don't use seconds for time; or (b) don't use inches for length or (c) use PARAM WTMASS in your Femap/Nastran runs, which I would recommend only for highly experienced FEA analysts. It internally multiplies entered "pseudo-mass" units (like pounds) by a factor (often 386 lb-mass/slinch or 32 lb-mass/slug) so real consistent mass units are actually used.

I hope that helps.

N/A

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01-22-2012 10:51 AM

Now, 1 lb = 1 lbm * 1 g (386.09 in/sec²). My "point" is that ALL of these values are merely "masks of convenience." I understand that SI-KGS units are more convenient for calculation and are the generally accepted international standard. That is fine, but the unit Newton (and everything that derives from it) is really poorly "meshed" for real world applications. (I can go on my diatribe about German, French, and Japanese engineers blowing up test equipment back when I was the chief mechanical engineer on the program that developed automotive airbag restraint systems when calculating pressure loads in Pascals, if you wish.) The "issue" is presenting INFORMATION in the manner and format with which the USER is most comfortable!

MY question is, when a MODEL is imported in inch-based units, what are the REST OF THE UNITS that are DISPLAYED by Femap?

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