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dandersonjr1-Visitor
1-Visitor
June 30, 2011
Question

How to apply a force/moment in Mechanica?

  • June 30, 2011
  • 16 replies
  • 6538 views

Good Day All,


Trying to apply a force & moment in mechanica to a surface on a part.
Do I have to apply this using Advance, Total Load, Uniform to get mechanica to apply the moment?
When I use a Simple Force/Moment Load mechanica ignors the Moment for some reason.
To close to the holiday and my brain needs a vacation.



Any help would be appreciated.
Thanks
Don Anderson

16 replies

R
randyspeed1-Visitor
1-Visitor
June 30, 2011
Donald,

You must use TLAP (Total Load At Point) to apply moments to solid models. This is because rotation degrees of freedom do not exist in solid elements.

Randy Speed
President & CEO
Speed Consulting, LLC
(214) 213 4440
(214) 853-9221 fax
www.speedconsulting.com

    J
    JonathanHodgson13-Aquamarine
    13-Aquamarine
    June 30, 2011
    Hi Donald,



    Total Load At Point will certainly work (I don't know about Total Load
    Uniform).



    3D FEA elements don't respond to moments applied to their nodes - the
    nodes only have translation DOFs. By using TLAP, Mechanica translates
    both explicit moments, and moments due to an offset application point,
    into a force distribution.



    Use the Preview button to see how the load will actually be applied.



    HTH,

    Jonathan


    C
    ChrisKaswer16-Pearl
    16-Pearl
    June 30, 2011
    Don,



    Don't forget that it is not possible to apply a moment to 3D solid
    elements (I'm guessing the surface you wish to load is not a shell
    element). Having said that, you can apply a "Total Load At Point"
    (TLAP) and select the surface (3D elements) it would be spread upon.
    This will apply loading to the surface that will represent the load and
    moment that would exist. This technique is normally used for
    transferring loads from MDO to Mechanica. You need to create a point
    which represents the location the load is generated from (usually a
    mechanism joint center). Provide the component loading and moments and
    then select the surface. Mechanica will spread the force out over the
    surface.



    I hope this helps,



    Chris





    Christopher Kaswer

    Principal Design Engineer

    Covidien

    Research and Development, Surgical Devices

    60 Middletown Avenue

    North Haven, CT 06473

    (203) 492-7167 (office)

    (203) 492-4011 (fax)

    www.covidien.com






    P
    ptc-2122881-Visitor
    1-Visitor
    June 30, 2011
    Don,

    It is very difficult to control the local force distribution when using TLAPs to apply a moment to a 3D face.
    I suggest paving the solid faces with shells of negligible stiffness. You can then apply the moment to the shell or underlying surface. You may want to exclude the load application region from stress convergence checking.
    Good luck.

    Joe Colich
    Boeing - STL
    D
    dandersonjr1-VisitorAuthor
    1-Visitor
    June 30, 2011

    So say the end surface of the part sees a force that is applied by another part.That loadload onthe other part is applied 33"away from the mating face and the load applied is 250lb.


    Would it be applied to the part as I have done in the attached image and part file, wildfire 4. This is just a quick model for discussion.


    Thanks,
    Don Anderson

    D
    dandersonjr1-VisitorAuthor
    1-Visitor
    June 30, 2011




    Rotation degrees of freedomdo not exist in solid elements .


    I'm usingTLAP with just the force going forward.
    I will have to play with the shelled suface technique some more.
    I was hoping not to have to add any additional features to the model for applying the load.
    It appears that either way I have to add something and think it will be easier on others to figure out if I use TLAP
    (Total Load at Point) This way I just have to add one datum point to the model.


    The moment inthe load I was usingwas not needed as you pointed out and cancelling my foce I was applying. (thanks Chris)


    Thanks again for the help and have a great Fourth of July Weekend.


    Don Anderson

    R
    randyspeed1-Visitor
    1-Visitor
    July 5, 2011
    We ran into this force distribution issue last week, where the TLAP applies forces not normal to the surfaces. I could think of only one workaround, and that was to apply the force directly, normal to the surfaces, after a little statics hand calculations.We did not use the TLAP, we used force on surface. It actually worked out fairly easily for us.

    Randy Speed
    President & CEO
    Speed Consulting, LLC
    (214) 213 4440
    (214) 853-9221 fax
    www.speedconsulting.com
    A
    Anonymous
    July 5, 2011
    Joe,



    In your opinion do you believe this lack of control could stem from the mesh
    distribution on the applied face?

    Meaning, if the forces are applied at nodes and if a more uniform mesh
    existed on the solid 3D face do you

    believe that that would provide more control over the distribution?



    I could envision a very coarse mesh on the end of a shaft let's say where a
    moment being applied with TLAP

    causing problems.



    Curious what others think too.



    Steve






    R
    randyspeed1-Visitor
    1-Visitor
    July 5, 2011
    Even though the load does end up on the elements, MECHANICA applies the TLAP load as a "distributed" load onto geometry, that is to the "surface", and not to the points or elements. So I would be surprised if the element mesh is related to TLAP load distribution.

    Expanding on Joe's comment that "... it is difficult to control the local force distribution" using TLAP, I have found this to be true specifically when you are trying to represent two parts in contact where the issue of normal and tangential directions comes into play (direction really, not so much distribution). Theoretically (with no friction) the forces should be normal to the surface. This is not what we get. We get some normal forces along with some overlapping tangential forces.

    The simplified example of the shaft end with applied moment coaxial to the axis of the cylindrical shaft (pure torsion) is a classic example of TLAP functionality. The forces are 100% applied tangentially to the plane of the cut shaft. This type of load application is correct when coming from a weld, or some other type of connection in real life where the part stiffness is continuous. However when the user desires to replace a part within the assembly (which would otherwise transfer forces to the meshed part via contact,) with a TLAP, normal forces are desired (not tangential forces).

    In summary, my observation is that there seems to be no control as to normal/tangential portions of the applied force resulting from a TLAP. In the end however, the TLAP does indeed "balance" the applied forces - which is a good thing. For our contact substitution problem, we abandoned the TLAP and used old fashioned statics to apply the load normal to the surfaces to get around this issue and all was good. I suppose that tangential applied loads would be acceptable if the localized area of load application were far enough away from the area of interest, or if there were significant friction forces present.

    Randy Speed
    President & CEO
    Speed Consulting, LLC
    (214) 213 4440
    (214) 853-9221 fax
    www.speedconsulting.com

    A
    Anonymous
    July 5, 2011
    I guess I was envisioning the application as having the moment axis normal
    to the surface (tangential).

    I agree with you of using normal surface forces when applying a
    torque/moment to the keyway of a

    shaft where the applied surface is parallel to the moment axis or other
    similar applications.



    I guess I will have to wait for Joe's response, because I am a little
    confused on the distribution issue.

    While I agree with you that the load is distributed on the geometry, there
    still has to be some level

    of interpolation behind the scenes in order to apply equivalent point forces
    at the nodes of the

    elements. My thought train was more elements.more nodes.more even
    distribution of surface

    forces.



    Steve


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