cancel
Showing results for
Did you mean:
cancel
Showing results for
Did you mean:

Highlighted
Newbie

## Deformation with a Moment

I am simply loading a steped shaft with a moment and I get unrealistic deformation. Am I doing something wrong to get such a deformation or is this a bug with Pro/Mechanica?

View with deformtion

http://www.geocities.com/cindyordon/deformed2.jpg

View without defomation

http://www.geocities.com/cindyordon/undeformed2.jpg

Don Pitcher

Tags (2)
10 REPLIES 10

## Deformation with a Moment

Don,

Randy Speed
President, CEO
Speed Consulting, LLC
www.speedconsulting.com

## RE: Deformation with a Moment

I realize the units are at the default setting as I wanted to make a quick demonstration of the deformation problem.

I just want to understand why the deformation looks so bizarre. Does anyone know why?

## RE: Deformation with a Moment

Hello Don,

The reason the displacement plots specifically in a torsional load application is due to the fact that the actual nodal displacements happen along a straight line. In the results window, when you scale the disp results by a factor greater than one these linear displacements are subsequently multiplied. Therefore, the rotational displacement will be come distorted and appear to swell the shaft versus rotating it.

Hope this helps,

Steve Seymour

## Deformation with a Moment

In the link below, the "Scale" is 3.4E+11 ! You can reduce the scale. e.g. What does Scale = 1 look like?

You are plotting Stress. What do the values look like when you plot a fringe plot of Displacement?

Don Pitcher <-> wrote: I am simply loading a steped shaft with a moment and I get unrealistic deformation. Am I doing something wrong to get such a deformation or is this a bug with Pro/Mechanica?
View with deformtion

## RE: Deformation with a Moment

Steve,

Your explanation does help. Now I will be able to sleep at night. Thanks

Ray,

I set the deformation to the default of 10%. The values on the displacement plot are good but the visual deformation is the same as the picture I showed. I did not set the scale to 1 because the actual deformation is quite low and I would not expect to see it at that scale.

Thanks for all the help!

## Deformation with a Moment

That is exactly right. Static analysis = small deflection theory. Rotations are the end of result of small translations - when you exaggerate them (scale) you get the "bizarre" look.

Randy

Working on a model trying to utilize the fastener tool?

From what I can gather it looks pretty handy, how reliable of results
have you all been getting with it?

My situation is this.
Bracket with a moment and shear load on a piece of MDF.
MDF mounted to the wall at 16" Centers.
? How many fasteners required to hold on to the wall, what effect does
this have on the MDF board stress?
I have done testing to validate the results.

Now... question on these features.
1) Set Up
On first trials I tried to create volumes the same size of my fasteners,
mesh kept crashing, eventually realized that this feature does this for
me. Is it prudent to set up some smaller regions on your parts to force
mesh generation?

I saw that pre-load required contact areas. Is it prudent to break up
into a few contact areas (that match between parts with volume regions),
or just over the whole surface to surface between parts?
To mesh, how much space to use to avoid the mesher just "tying" the
parts together. If I set the AutoGem Create Links Where Needed off can
I have the parts mating and no links will be created?

3) Any hints clues that help with accurate results with these?

Thanks,
Erik

## RE: Deformation with a Moment

To better understand the limitations of visual deformation in Mechanica, I performed another analysis using a moment load on a shaft. I tried to model an application of a torsion bar spring commonly used on cars and trucks.

I used a 1 inch diameter steel shaft that is 50 inches long. I placed a 5000 lbin load at one end and completely constrained the other end.

For ease of seeing visual deformation, I placed a 1/8 inch slot at the side with the load.

View with deformation

http://www.geocities.com/cindyordon/deformed3.jpg

View without deformation

http://www.geocities.com/cindyordon/undeformed3.jpg

With the scale at 1, the visual deformation still swells the shaft diameter at the load end.

I agree with the numerical results that are displayed. But… Due to the swelling of the shaft, am I correct to believe that no matter what the scale is set at, deformation from a moment is not <u>visually</u> accurate in Mechanica?

## Deformation with a Moment

Don,

This is an effect of small displacement theory, which ALL finite element
analysis uses, and the effect is not limited to Mechanica. Don't worry about
the visual appearance, check your results against the classical method. You
must be using a very soft material, since you have a deflection of 0.23".
Carry out a sanity check using the torsion formula:

T/J = t/r = Gq/l

Also check your units - Pro/Engineer default units are not suitable for
finite element analysis, change to IPS or better still metric.

Regards,

Rod

Rod Giles
Technical Director

www.elite-consulting.com