Hi all and thanks in advance for helping out. It's so great to have a forum like this, a trouble shared is a trouble halved and all that. Anyways, I have a kart (http://imgur.com/KhEdGVf) that rolls on four casters and I want to investigate how the legs of this kart will splay outward when it carries a load. The casters are connected to metal plates welded onto the bottom of the kart's frame; I call these "caster pads".

What I'd like to do is fix one caster pad and allow the other caster pads to deform outward. Ideally this would be done as follows:

- Constrain all three translations of a single point at the centre of one caster pad, while allowing rotations.

- Constrain a point on each of the remaining caster pads to translation along a horizontal plane, again allowing rotations.

I can acheive a something close to this by fixing one caster pad *surface *( ie. all translations and rotations) and allowing the other caster pad *surfaces* to roller/slide horizontally. However, I don't this it's realistic to constrain the caster pad surfaces against rotation from horizontal. A workaround I've used is to shrink the size of the constrained surfaces so that they approximate points, but this obviously causes a high stress spike around those constraints.

So, which this in mind I have a few questions:

1: I don't seem to be able to constrain a point on the solid mesh while restraining *translations *only; the constraint symbol that results when constraining a point indicates rotations are prevented for that point. Now, a point can't really rotate, can it? Is fixing translation of a point while allowing rotations of the geometry around that point possible?

2: The advanced restraints options only allow me to constrain a point on a plane and have it move in either a positive *or *negative direction in x or z, not both. Any idea why this is? I ask because I'm not certain which direction my caster plates might play out along although I can guess.

3: Adding the constraints the way I've described using the advanced restraints menu (and ignoring the issues raised by 1 and 2) just causes my solver to fail and make me sad. Any general thoughts or comments that may help? Perhaps having all the vertical load supported by points only isn't workable as the weight is supported by zero area?

Thanks again everyone!

solid element DOF, here's a couple articles that discuss it. they aren't in depth but should get the point across.

http://www.eng-tips.com/viewthread.cfm?qid=153522

http://www.finiteelement.com/feawhite2.html

if you really really want to restrain a point in translation only and allow rotation, you could make it a point on a shell. in other programs i have seen the ability to control all 6 dof for any node. however, i don't know if it respects rotational DOF when you are working with solid elements. no guarantees because I was evaluating the software and am not an expert in it. (abaqus in the case)

your boundary condition was telling the software to move the vertex 1m in the 2 planar directions and but stay on the plane. if you had left the 2 planar directions empty, it would have been a normal restraint and it would have stayed in the plane. but unlike "real life" it can't lift off the plane. as long as your load is completely downwards, it should be ok. the only way to get a true version of what you want would be to model a cone and put the cone end on a surface with contact.