American units 2

[VzHache]

Hi guys,

I have to built a FE Model using the following units:
length -> in
force -> lbf
time -> s

I am not very used with american units and I am looking for the corresponding unit of mass (lbf s²/in ??) and its conversion with lb.

I found that 1 lbf s² /in = 386.1 lb.

Is that correct?

Thank you.

 

[GregLocock]

I'll try and write it down... but may well get it wrong.

1 g is 32.2 f/s/s

1 lbf = force that one g exerts on one lb mass

one lb mass is accelerated at 32.2*12 (=386) inches per second per second, by 1 lbf

so...

1 lb mass = 1 lbf/(386 in/s/s)

which is what you said, so I think you are good to go. When in doubt convert it all back to proper units (grin)


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[VzHache]

Thanks,

I did the calculation you mentionned and it confirms my results.

Bye.

 

[GBor]

2.2 lbf = 1 kg
1 in = 2.54 cm
1 s = 1 s (hey, at least we Americans got ONE unit right )

Many people have NO IDEA how to do what you just did, but your conversion is correct.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group
Magnitude The Finite Element Analysis Magazine for the Engineering Community

 

[KENAT]

What's all this about American units, the Brits invented most of them!

It was slugs that confused me at University.

Like Greg said, for complex problems it's sometimes easier to work in 'native' units and convert to the other system at the end.

 

[cbrn]

Hi,
as Kenat says, if you don't feel totally comfortable with coherent unit systems other than metric, then the best you can do is to express/build (it depends on how the CAD "interprets" the "units") the model in metric units, do all the FEA in metric units, and then express the whole in imperial units: simply do the equivalences mm -> in and MPa -> lbf/sqin. The stress/strain/force field obviously remains the same !!! Most FE programs have a function to "scale" the stress results (more generally: the "summable quantities"), so you can have the plots "expressed" in imperial units only in postprocessing, if you need to include them in reports.

Regards

 

[xerf]

There are many online converters.

http://www.onlineconversion.com/

http://www.unitconversion.org/

I suggest you work the model in Metric and convert the results afterwards. Or you can work them once in both English and Metric and see if the units are ok.

 

[JStephen]

Another approach would be to non-dimensionalize the problem as much as possible.

If all you are doing is a static analysis, then the time shouldn't enter in, and it would just be inches and lbs and psi for everything.

If it's any more involved than that, you need to make sure the units are all consistent regardless of whether it's metric or not- that doesn't just automatically eliminate all problems.

 

[rb1957]

you're right JStephen, the time dimension is consistently seconds, the problem is (i think) the model is in inches, but the length dimension (in the units, and buried in the definition of "g") is feet.

btw, they're "imperial" units, not "american" ... tho' america is the last hold-out !

 

[GregLocock]

They are Imperial units because they were established in the distant past by Emperor Palpatine, in a galaxy far far away.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[cooperDBM]

Imperial units are different from American, or U.S. Customary, units (i.e. tons, gallons, etc.). They both evolved from the old English units but the British adjusted their system in the early 19th century to create Imperial units. At that time the Americans wanted nothing to do with what Britain was doing. There's no practicable difference in any linear measure.

Here in Canada we use Imperial units when not being metric.

 

[csd72]

As greg said

Imperial Units = platoons of stormtroopers

Meanwhile back at the ranch.

Unlike in metric, there is no difference in imperial between a Force and a weight, both are in pounds. So there is no multiplying by gravity.

csd

 

[cooperDBM]

The pound being used for both mass and force in Imperial can be very confusing for anyone converting to or from metric. Clearly mass and force can't always be the same depending on acceleration. A pound-force (lbf) is one pound-mass at the earth's surface (roughly sea-level). Above or below this level a pound is not a pound. For imprecise engineering, like structural (on earth), there's no practicable difference.