Frank,
Wish we could get your model - it is too tuff to try and guess what is the real problem trying to just follow verbal descriptions. And I fully admit Mechanism is not as user friendly and self explanory as we could wish it to be, so in many cases when users are actually sure that they have ot do with a bug it appears to be a design error.
Though it might be still a bug - things happen, but on my statistics cases of mechnism lock are mostly due to design issue.
Creo follows so called "rigid body theory" in moving components around upon kinematic dragging or running analysis. This theory assumes that assembly along with component structure (fully represented in Model Tree) has a parallel "body structure". Only body structure constitutes mechanism drag, DOF or Lock behaviour.
Sometime ago Mechanism Tree was added to try and help understand bodies content. It can be used as a static analysis (which bodies I have now) as well as "what changed" analysis : you add one more component to assembly in a way that you desire, and comaring body tree structure "before and after" you can see if it reduced or increased number of bodies.
Here some info and tips about body calculation:
1. component B fully constrained to component A will be added to the body where A belongs. If A belongs to Ground Body then B will be added to Ground as well.
2. component C added to assembly as "not fully constrained" (be this by constraints or by connections) will create a new body.
3. If a new component D is fully constrained to A (which is in Ground Body) and in addition fully constrained to C (make 2 different constraint sets) this will result in adding C to same Ground body. Formally C continues to show "not fully constrained", but D later "locks" it to Ground in body calculation.
4. Connection definition : each connection can be ONLY created between parts that belong to 2 different bodies. Example : when you add component E by Pin connection, you should be sure that both Axis and Plane references on assembly side belong to same body (possibly to different components, but inside same body). So if you constraint E.prt to A.prt (both of Pin references) you almost granted get a clean connection. If you however reference A and B you must be sure that they both belong to same body - else Creo will not allow to select the second reference.
This is done to ensure that upon attempt to move things around each connection will not "fall apart" since it is not between 2 rigid bodies, but between 3 or more. Common user error is that upon creation of certain connection it really connects 2 bodies. However later some component is added in a way that it closes the loop and makes them the same body ... and user gets these annoying errors "connection is no longer valid" or like this.
Or alternatively, Pin created between A and (B ; C) turns invalid if B and C stop being a single body (say, redefine C to not fully constrained tyope).
Simple body tree analysis will show you that both components now belong to that same one body, hence connection turns invalid.
5. One more advanced item : in most cases all features of same part belong to same body. But it is not always the case. For example, features created with full external references (say Copygeom) will belong to the same body as reference part belongs. This is done in order to avoid creation of connections between such external parent-childs (which will immediately create external references). So if you have A.prt and B.prt, and B.prt has surface copied from A.prt - you will not be able to create connection using this surface from B and any of the geometries of part A (as they belong to same body). Again here - upon connection creation Creo will be there to prevent such connection. But it can easely happen that upon creation valid geometries were selected, and later ... rerouted / replaced etc - and you get your connection invalid.
6. In a particular case of Csys to Csys (or Fix Location) nothing specific should happen - else this is really a bug. If you add Csys to Csys this is a fully constrained way, hence the newly added part should be added to the same body where assembly side Csys belong. Fix Location (if used) is a shortcut that fixes a new component to assembly, strictly adding it to the same body as assembly itself. Try to look in Body Tree - you will see what i talk about.
7. Differences between Connections and Constraints respective Body Calculation :
- in general "not fully defined" constraints will behave the same as similar connection from kinematics point of view. Sure they will not allow addition of all other Mechnism stuff (motion limits, mechanical constants, friction etc).
- constraints are not restrictive to selection of references from 2 bodies only, and they will not "fail" in a way Connection does if bodies change unfaworably - they will just behave in a new context.
8. There is still a way to "fully fix" A.prt to B.prt and still keep them 2 different bodies (for a single purpoce of contact forces calculation - forces may reside only between 2 different bodies). Such connection between 2 components is called "Rigid", it leaves zero DOF between them.
Hope this will be of a little help, at least to get some ideas of how Creo operates bodies in mechanism. It is far from perfect and even further from "user friendly" - but it has quite direct logic behind.
As to the missing help around this topic ... I hope we will put some more either on learning connector, or as a part of tech Support topics, since such level of details is way beyond regular Help imo.
Let me know if some portion above is way too unclear, or if you have any specific case questions - will try to help.
Best,
- Vlad
