# Parallel contact between two shell surfaces **URL:** \ **Category:** General Questions\ **Tags:** results\ **Created:** [December 21, 2022, 10:48am UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624 "2022-12-21T10:48:44Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Resende](https://avatars.discourse-cdn.com/v4/letter/r/e47c2d/32.png) [@Resende](https://prepomax.discourse.group/u/Resende)\ **Post date:** [December 21, 2022, 10:48am UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624/1 "2022-12-21T10:48:44Z")
Good morning everyone. I made a model that has one tube inside the other. In the outer tubes there is a force that approximates the contact between both. I’m only using shell elements (S6). The two surfaces are the same element size. The tubes are centered. The model is not converging. Please, could someone help me? I’m using: - surface behavior (Hard), Friction (0.3) - Contact pair (Node to surface), small sliding (No), Adjust (No) - Static Step, Solver (Spooles), Nlgeom (off), Incrementation automatic (initial time increment = 0.1) Link with files: [Disp6.pmx - Google Drive](https://drive.google.com/file/d/1IU8mqEJJSwFWKMUxB39jWd0wC-dP-NGu/view?usp=sharing), [Captura de tela 2022-12-21 073918.png - Google Drive](https://drive.google.com/file/d/1VVJuCVb2T2329_K202K2MbyzgjFqw2uZ/view?usp=sharing) ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [December 21, 2022, 11:03am UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624/2 "2022-12-21T11:03:20Z")
You can post pictures directly here. But it would be best if you could share the .pmx file. The problem is likely related to the initial gap between the cylinders and rigid body motions caused by this gap. ---
**Author:** ![Resende](https://avatars.discourse-cdn.com/v4/letter/r/e47c2d/32.png) [@Resende](https://prepomax.discourse.group/u/Resende)\ **Post date:** [December 21, 2022, 11:13am UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624/3 "2022-12-21T11:13:16Z")
I’m new here and I can’t upload. But I am sending the download link If I put a spring between surfaces with little stiffness, do you think it will work to improve the contact? -------- File -------- ![Captura de tela 2022-12-21 073918](https://global.discourse-cdn.com/free1/uploads/prepomax/original/1X/547796235d221f6aadc91d56c294685d33fc7d8e.jpeg) File .pmx [File .pmx]([Disp6.pmx - Google Drive](https://drive.google.com/file/d/1IU8mqEJJSwFWKMUxB39jWd0wC-dP-NGu/view?usp=sharing), [Captura de tela 2022-12-21 073918.png - Google Drive](https://drive.google.com/file/d/1VVJuCVb2T2329_K202K2MbyzgjFqw2uZ/view?usp=sharing)) ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [December 21, 2022, 11:51am UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624/4 "2022-12-21T11:51:38Z")
All right, I didn’t notice the first link. You could, for example, try adding a separate step (before the current one) in which contact will be established by pressing the outer cylinder (with prescribed displacement in cylindrical coordinate system) towards the inner one. ---
**Author:** ![Kuhl](https://avatars.discourse-cdn.com/v4/letter/k/e0b2c6/32.png) [@Kuhl](https://prepomax.discourse.group/u/Kuhl)\ **Post date:** [December 22, 2022, 11:27am UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624/5 "2022-12-22T11:27:24Z")
Be carefull though with succesive steps in combination with shell elements. I have experienced weird behaviour when trying to solve multiple steps in a shell model as is discussed in this thread: [Load Case Creation - General Questions - PrePoMax](https://prepomax.discourse.group/t/load-case-creation/133/9) Maybe using Amplitude instead of separate step would be a workaround? ---
**Author:** ![ANYS](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/anys/32/7737_2.png) [@ANYS](https://prepomax.discourse.group/u/ANYS)\ **Post date:** [December 24, 2022, 8:08pm UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624/6 "2022-12-24T20:08:42Z")
Hi, The gap between shells is smaller than the sum of the half pipe’s thicknesses. Pipes overlap. Try adjusting the thicknesses first. Gap: 6.548mm Thickness 1: 3.68 mm /2 =1.84mm Thickness 2: 9.5 mm /2 = 4.75mm Thickness 1 + Thickness 2 = 6.59 mm \> Gap: 6.548mm ![screenshot.42](https://global.discourse-cdn.com/free1/uploads/prepomax/original/1X/3698eb69254eece1906fca7b3490194d51dbb3ee.jpeg) ![screenshot.39](https://global.discourse-cdn.com/free1/uploads/prepomax/original/1X/7768577dbb3013377bede05d61a1957abf1249a1.jpeg) ---
**Author:** ![synt](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/synt/32/4658_2.png) [@synt](https://prepomax.discourse.group/u/synt)\ **Post date:** [December 24, 2022, 11:43pm UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624/7 "2022-12-24T23:43:20Z")
but, it seems the values of geometry and thickness are small enough in tolerance? geometry : 6.59-6.548 = 0.042 (mm) tolerance : 1.84\*2.5% = 0.046 (mm) ---
**Author:** ![ANYS](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/anys/32/7737_2.png) [@ANYS](https://prepomax.discourse.group/u/ANYS)\ **Post date:** [December 25, 2022, 2:35pm UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624/8 "2022-12-25T14:35:08Z")
Hi Kuhl, This is a simplified version that works. Start from here and try to recover your original model set up introducing changes one by one. You will realize what was failing and how each change affects the result. @Matej. Changing the Friction makes prepomax to crash. > **[Disp6(1).inp](https://wetransfer.com/downloads/2acb645a7dc7b17d7e65f07e7d9f8a9520221225143320/d3325b)** > > 1 file sent via WeTransfer, the simplest way to send your files around the world ![screenshot.44](https://global.discourse-cdn.com/free1/uploads/prepomax/original/1X/ba7aa84741a76bfc4f69a6eb1b8c5a790227f944.jpeg) ---
**Author:** ![Matej](https://avatars.discourse-cdn.com/v4/letter/m/a698b9/32.png) [@Matej](https://prepomax.discourse.group/u/Matej)\ **Post date:** [December 25, 2022, 5:03pm UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624/9 "2022-12-25T17:03:15Z")
> [@ANYS](#): > > Changing the Friction makes prepomax to crash I would need some more info about that. A .pmx file would be the best, and the description of the steps that led to the crash. I could not reproduce this behaviour on my own. ---
**Author:** ![ANYS](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/anys/32/7737_2.png) [@ANYS](https://prepomax.discourse.group/u/ANYS)\ **Post date:** [December 25, 2022, 5:13pm UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624/10 "2022-12-25T17:13:53Z")
Sure. Try to change the friction stick slope. > **[Disp6(1).pmx](https://wetransfer.com/downloads/955303a176836f732737e8e987202c8020221225171336/bbd01e)** > > 1 file sent via WeTransfer, the simplest way to send your files around the world ---
**Author:** ![Matej](https://avatars.discourse-cdn.com/v4/letter/m/a698b9/32.png) [@Matej](https://prepomax.discourse.group/u/Matej)\ **Post date:** [December 25, 2022, 5:16pm UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624/11 "2022-12-25T17:16:03Z")
Got it. I was changing the friction value. Thank you. ---
**Author:** ![Matej](https://avatars.discourse-cdn.com/v4/letter/m/a698b9/32.png) [@Matej](https://prepomax.discourse.group/u/Matej)\ **Post date:** [December 25, 2022, 5:28pm UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624/12 "2022-12-25T17:28:45Z")
It is fixed in the source now. ---
**Author:** ![Matej](https://avatars.discourse-cdn.com/v4/letter/m/a698b9/32.png) [@Matej](https://prepomax.discourse.group/u/Matej)\ **Post date:** [December 25, 2022, 5:30pm UTC](https://prepomax.discourse.group/t/parallel-contact-between-two-shell-surfaces/624/13 "2022-12-25T17:30:20Z")
I know it is possibly late but I would definitely try to use the symmetry if applicable. It is especially beneficial in cases involving contact since it prevents the rigid body motion and reduces the model size.