# Pin - the problem with contacts **URL:** \ **Category:** Project Support\ **Created:** [August 11, 2016, 7:47am UTC](https://www.simscale.com/forum/t/pin-the-problem-with-contacts/37071 "2016-08-11T07:47:31Z")\ **Posts on this page:** 1\ **Showing post:** 6
**Author:** ![rszoeke](https://www.simscale.com/forum/user_avatar/www.simscale.com/rszoeke/32/14_2.png) [@rszoeke](https://www.simscale.com/forum/u/rszoeke)\ **Post date:** [August 12, 2016, 7:31am UTC](https://www.simscale.com/forum/t/pin-the-problem-with-contacts/37071/6 "2016-08-12T07:31:26Z")
Nice work @BenLewis! Regarding the symmetry boundary condition, I would like to add two comments: - generally I would try to replace a symmetry constraint with an equivalent **fixed value** constraint whenever possible. The reason is that a symmetry constraint always creates a linear relation between the DOFs of the nodes involved: \vec n \cdot \vec u = 0, where \vec n is the element normal and \vec u the displacement vector. If now any DOF that is not **exactly** (and in numerical analysis usually nothing is exact) normal to the symmetry plane is constrained by another boundary condition or contact, the DOF will be over-constrained. By using a fixed value constraint and defining the mentioned DOF as **unconstrained** you won’t have a problem. Obviously this technique will not work if the symmetry plane is not orthogonal to any of the coordinate axes. - In this particular analysis the symmetry happens not to be a problem. The problems when having symmetry and a slave surface of a physical contact sharing nodes are mostly observed when **friction** is involved and a **Lagrangian** contact formulation is used or in the above mentioned case where the mesh is too _coarse_. Best, Richard --- _[View the full topic](https://www.simscale.com/forum/t/pin-the-problem-with-contacts/37071)._