Insufficient Constraints HELP

[slingblade]

I am modeling a cylinder and solid piston inside it with a shear pin through one side of the cylinder and roughly half way through the piston.


The load is applied on the end of the piston and transfered through the shear pin to the cylinder which is fixed.


The model was a piece of cake but I can't run analysis without the "insufficient contraints" errors.


My cylinder is fixed at the defaul position, the cylinder and shear pin are a sub assembly and are connceted to the cylinder using General->Insert -- connections at the cylinder bore/piston and shear pin/cylinder pin hole.


Any pointers on why I would get this error? I can run the analysis without the piston/pin subassembly by applying the load to the end of the cylinder. This isn't useful exept that I am ussuming the problem lies with the piston/pin connections.

 

[miya]

Is the OD of the pin smaller than the ID of the cylinder hole? If there is a gap, then Mechanica will assume that they are two different bodies. This will mean that the two bodies can actually pass through each other in Mechanica, hence your "insufficent constraints" where one body is free to float resulting in rigid body modes. It does not matter what Assembly Connections you place in Pro/E, Mechanica will not use those connections.

Possible remedies:

1. Change the diameters of the pins, holes, etc. so that they match up. You will not want any gaps nor any clearances. Any place where the components have faces that mate up, Mechanica will treat that as a "bonded" connection, just as if it was a continuous material.

2. Insert "Contact Regions" between the outside surfaces of the pins and the inside surfaces of the holes. This will provide better load transfer in your results, however, you have now introduced a non-linear analysis where you run time will increase, and mesh density around your contact region will affect your local stress results, etc. Better to stick with #1 above unless you truely are looking for a contact analysis.

Hope that helps.
Ray.
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Ray Miya

 

[slingblade]

I've never used "Contact Regions" before. I need the piston and the cylinder to be considered separately and for all of the force from the piston to be transferred to the cylinder through the shear pin. That makes me think that option #2 is the way to go.<?:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" />


Ill attach a screenshot of the assembly. The shear pin is left out in the image but the area circled in red is the place on the shear pin that I'm interested in.


Any ideas on a "Contact Region" primer? Is a Contact Region the same thing as a simulation volume region?


I'm using WF 2.0 by the way.


Thanks for the help..











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