A rigid body has 6 degrees of freedom. Do you want the cylinder to move in all 3 translations and all 3 rotations?

The way you drew the springs, perhaps you only want the cylinder to translate in 2 directions: X and Y.  If that is the case, add constraints to the structural model to fix the other 4 degrees of freedom. A simple way to add a General Joint. That has integral springs so you can delete the 4 springs you added.