Applying bearing constraints 1

[grimmble]

Hi,
Im new to the forum and also fairly new to Pro/Mechanica. I have searched the forum and already found some useful tips but am still struggling with a particular problem.
Basically I am trying to run some FEA on a shaft and gear assembly. The shaft will be supported in radial bearings and the gear will be mounted in the centre of the bearings and keyed to the shaft. The problem I am having is with constraining the points where the shaft will be mounted in the bearings. I want to constrain the the shaft to prevent translation in the y and z directions but still allow the shaft to rotate. I have tried constraing the surfaces where the bearings will be placed but as soon as I fix the shaft for translation it can no longer rotate dispite allowing rotation.
If anyone could offer some advise on how best to apply constraints that would simulate mounting the shaft in bearings it would be much appreciated.

 

[JohnAndrews]

Split the model at the bearing surfaces using volume regions, create a datum point at the centre of these regions on the shaft axis. Make a rigid connection between the point and the bearing surface. Constrain the point as required.

 

[grimmble]

Many thanks JohnAndrews for the reply. I have followed your advise and it has worked perfectly. I had been struggling with this for a while. The figures I get now correspond with the manual calcs that I have performed and the shaft assembly moves as I would expect when viewing it animated.
I wish I had posted up here sooner.
Thanks again

 

[NathanZ]

I'm actually having a similar problem with bearing constraints in Pro/m, but i'm not sure I follow your advice, JohnAndrews. Split the model how? Are you basically trying to simulate spider elements, which mechanica doesn't seem to have?

I'd appreciate some further explaination of your method.

Thanks a lot!

 

[seymours2571]

NathanZ,

An alternative to John's method would be to use constraints in a polar coordinate system. You will have to create the polar csys in mechanica, then afterwards when specifiying out your constraints you will just reference the new poloar csys. You will then have options to constrain radially (r), rotationally (theta), and axially (z).

You will have to use either surface or volume regions to simulate the bearing surfaces.

Also, I believe that polar constraints are only applicable for solid elements.

The benifit of using the polar constraints is more accurate representation of the stresses and displacements in the bearing surface region. Rigid connection will tend to over constain portions of the model.

Good luck,

Steve

 

[NathanZ]

I like the idea of using polar constraints. I think I remember something about that now from my FEA class in college. What's confusing to me is that when constraining in polar coords, it asks for Dr, Dtheta, Dz, Rr, Rtheta and Rz. Isn't Rz the same as Dtheta? I'm not sure if i'm setting up my constraints right in polar coords.

As far as surface regions for the bearing surfaces; I don't think that applies in my situation. My bearing surfaces arn't "part" of another surface that I need to sepearte out with surf/vol regions... they're independent surfaces on their own. Basically, the bearing surfaces are just holes that a pin fits through, so the inside surface of the hole is my bearing surface.

 

[seymours2571]

Yeah, I agreee, I have had a little trouble wrapping my head around the idea of Rr, Rtheta, and Rz.

One bug I have found though is that if there are only polar constraints in the model (i.e. no constraints in either the global or local cartesian csys) then the results will be presented incorrectly in mechanica's results viewer. To fix the issue, if you have a polar constraint in which Dz is constrained, then apply a constraint in the global carteisan csys at a point on that surface, but constrain only the z direction (i.e. leave 5 other dof's free). This will then produce the correct displacement results in mechanicas post processor. The issue is detailed somewhere in the PTC knowledgebase, I just remember where off hand.

Oh, and true, your particular case may not require you to split a surface.

Steve

 

[seymours2571]

Oops. I forgot to mention that I would just leave the Rr, Rtheata, and Rz as free. Honestly, I don't think it matters and that may be why PTC does not address Rr, Rtheata, and Rz directly.

The reason is that solid elements only have translational DOF's. Therefore, if you try to apply any rotational constraints (either cartesian or polar) mechanica will just ignor it.

Hope this helps.

steve

 

[mloew]

What is wrong with the built-in bearing loading functionality in Mechanica?

Best regards,

Matthew Ian Loew

http://www.cometsolutions.com

 

[NathanZ]

I don't think that rotational constraints are necessarily "ignored" by mechanica. Every FEA package i've seen has translational and rotational constraints, and they make sense in cartesian coords. I'm just not sure how mechanica really handles the rotational constraints in polar coords when they really don't seem to apply, or would conflict with the translational constraints.

As far as using the mechanica bearing loading, we're talking about constraints, not loads. The built in mechanica bearing LOADING works just fine.

I'm still working on this problem and still haven't come up with a reasonable solution, sadly.

 

[mloew]

Nathan,

Sorry about the confusion between the loading and constraints. I was daft. I may have a solution for you, however. I've typically modeled the bearing itself with elements that allow for a single point in the middle to act as a ball joint (think about two cones cutting out the cylinder so the hole is no longer hollow). An alternate technique is to fill-in the hole completely and use two constraints on the axis of the 'bearing' one constraint 123 and the other 12 (assuming that z is the axis if the bearing).

Using 'real' elements over rigids has the added advantage of not over-constraining the model as well.

Let me know if this makes sense.

Best regards,

Matthew Ian Loew

http://www.cometsolutions.com

 

[seymours2571]

Nathan,

For solid elements in Mechanica the rotational constraints are ignored because they are not applicable. While a user may select a surface or edge to constraint, the constraints are translated to the nodes of the elements. Nodes only have translation not rotation. Thus, the rotational constraints are ignored in Mechanica.

This is documented in the PTC knowlegebase.

While using a polar coordinate system during the constraint application process may provide constraints that are radial and tangential (sp?), for small displacements those radial and tangential dispalcements are actually transalational.

Good Luck,

Steve