I have been trying for some time to model a simple four-bar mechanism and then make it undergo à simple static evaluation to know the reaction forces and moment at each joint. In order to validate hand calculation. But for some reason (which i don't understand) is that all the reaction forces that I calculated by hand are the same in solidwork but the moment are not. I had my calculations checked three time by three different person and they are good. I would like to know if this result is normal or not. my files will be attached to the post

Thanks in advance.

Matthieu Tanguay

Hi Matthieu,

There are 2 possible factors to account for the moments not matching correctly.

1. Joint locations not where you expect

2. Redundancies

Joint location can be arbitrary when you are dealing with concentric and coincident mates (since there are infinite possible location positions). It is important that you use the analysis tab when editing the mate to specify the exact location. Otherwise the moments will not reliably match.

Because you are defining concentric and coincident mates for each linkage connection, both of thee try top restrict rotations out of plane. This is called redundancy. The solver only allows one mate to restrict a specific degree of freedom, so any other mates that try to restrict that DOF are ignored. So it can be pure luck if the concentric or coincident mate is actually locking motion in that DOF.

The best approach is to use hinge joints for 3 of the pivots and then a coincident point - line mate for the last position (point at center of the hole on one part and axis on the hole for the second part). This gives zero redundancies. To be safe, I still prefer to manually set the joint location, but with the hinge joint, the origin should be at the intersection of the rotation axis and the coincident faces.

If you make these corrections, you will get the correct loading.

Please include you hand calcs if you would like me to evaluate the model to them. I cannot send you back a model since you have the educational version.

Cheers,

Ian