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1-Newbie

## Pro/M Modal Result does not match w/ FEA book example(?)

Hello Pro/M gurus!

Can anyone help me verify the results of my Pro/M modal analysis on the following:

Cantilever beam 12.0 inches long, width is 2.0 in, height (or thickness) is 0.5 in.

Properties are: Young's modulus = 30,000,000 psi
Poisson's ratio = 0.30
Mass density = 7.22e-4 lbm/cu. in.

The above modal setup was an example from the book: Building Better Products with Finite Element Analysis by V. Adams and A. Askenazi. (see page 478).

The result given in the book for the first natural frequency is 114.5 Hz (using formula).
The result given as obtained from MSC/NASTRAN is 114.0 HZ.

The result that I got from Pro/M for the first mode is 2243 to 2264 Hz (I tried the analysis on the idealized model and the solid model. Convergence was to within 1%).

Need your help to verify if I made a mistake in my setup. I am using Pro/M 2001.

Jerome Simbajon

5 REPLIES 5
1-Newbie
(To:jeromes.)
Your mass units are miss-given or reported in the problem statement, off by
the well known factor of 384.

(gravity constant (ft/sec)) x (in/ft)

32 * 12 = 384

So the density you'll need in the default Pro/E units is 7.22e-4 x 384,
then you'll get ~116Hz with a default mesh.

1-Newbie
(To:jeromes.)
Marty,

Thanks a lot for the big help!

(I was expecting Pro/M to do that conversion internally since it provides many unit options
when we input the density, modulus, etc.)

Thanks again!
Jerome Simbajon

Marty McCaslin <martyeng@mindspring.com> wrote:
Your mass units are miss-given or reported in the problem statement, off by the well known factor of 384.

(gravity constant (ft/sec)) x (in/ft)

32 * 12 = 384

So the density you'll need in the default Pro/E units is 7.22e-4 x 384, then you'll get ~116Hz with a default mesh.

1-Newbie
(To:jeromes.)
Marty, Jerry, and gang

I simply can't resist............. Apologies in advance.

You guys need to 'get with the program' and start using METRIC!!!!!!!!!!!!
Then these problems simply disappear.

And while your at it stop calling them "English units". We don't use them
in England. Well, we drive down roads marked in miles and we drink pints
of beer, but all engineering and science is metric. Best to call them

Pains me to say so, but the French led the way hundreds of years ago and
they got it right. You don't hear that kind of admission from an
'Englishman' very often.

---
David Reid
Optima Design Services
davidr@optima-design.co.uk
1-Newbie
(To:jeromes.)
Yes, metric is my default method David (millimeters, kg, sec) actually, and
I love beer in any size!

But there are always books, examples and stray users who wonder down the
wrong path, like some of my customers, and those we have to steer straight
to metric. I agree.

Marty

1-Newbie
(To:jeromes.)
David, Marty,

Thanks a lot for the advice. I was stuck with that problem for more than a day since I was expecting Pro/M to do some internal conversion relating the Young's modulus input (unit: psi or lbf per sq. in) to the mass (unit: lbm) since Pro/M provides us the units options when we input those variables.

Jerome

Marty McCaslin <martyeng@mindspring.com> wrote:
Yes, metric is my default method David (millimeters, kg, sec) actually, and I love beer in any size!

But there are always books, examples and stray users who wonder down the wrong path, like some of my customers, and those we have to steer straight to metric. I agree.

Marty

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