# Bolt Preload for Piping connection **URL:** \ **Category:** General Questions\ **Created:** [April 18, 2023, 6:25am UTC](https://prepomax.discourse.group/t/bolt-preload-for-piping-connection/835 "2023-04-18T06:25:32Z")\ **Posts on this page:** 7\ **Page:** 2
**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:** [May 17, 2023, 7:11am UTC](https://prepomax.discourse.group/t/bolt-preload-for-piping-connection/835/21 "2023-05-17T07:11:15Z")
I’ve already replied to the first 2 of your questions on YouTube so I’ll just comment on that last issue. It might be caused by the way those rings were modeled in FreeCAD, they were not meant to be merged so there can be some small misalignment causing incorrect compound part creation. ---
**Author:** ![datsnl](https://avatars.discourse-cdn.com/v4/letter/d/8e7dd6/32.png) [@datsnl](https://prepomax.discourse.group/u/datsnl)\ **Post date:** [May 17, 2023, 12:06pm UTC](https://prepomax.discourse.group/t/bolt-preload-for-piping-connection/835/22 "2023-05-17T12:06:56Z")
Hi Matej, Do you think the problem is what you thought it was for not getting correct contact pressure? ---
**Author:** ![Matej](https://avatars.discourse-cdn.com/v4/letter/m/a698b9/32.png) [@Matej](https://prepomax.discourse.group/u/Matej)\ **Post date:** [May 17, 2023, 12:08pm UTC](https://prepomax.discourse.group/t/bolt-preload-for-piping-connection/835/23 "2023-05-17T12:08:19Z")
The mesh of involved sub-parts is merged if you create a compound part. In this case, there is no need for contact. In such cases, the contact stresses are not meaningful values. You can try to see if the parts are connected (merged) using the View-\>Exploded View tool. ---
**Author:** ![datsnl](https://avatars.discourse-cdn.com/v4/letter/d/8e7dd6/32.png) [@datsnl](https://prepomax.discourse.group/u/datsnl)\ **Post date:** [May 18, 2023, 12:21am UTC](https://prepomax.discourse.group/t/bolt-preload-for-piping-connection/835/24 "2023-05-18T00:21:34Z")
@Matej I tried to just make compound of (bolt + shank + nut) for each bolt and the ChannelSection and LSection are separate. Then as per @FEAnalyst Youtube video, I added tied constraint between bolt head and upper surface, nut face and bottom surface. I added PreTension for 4 bolts but I am getting failed result after nearly 600seconds and using 8cpus. ![image](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/2/2ed67d2237c7b5a15bbba1b5b9828b4e7d8df2d6.png) File is attached here [P5701178.zip](https://prepomax.discourse.group/uploads/short-url/49YgEG1DEqCcXQrvCywBiaXO7E4.zip) (8.0 MB) What am I doing wrong? ---
**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:** [May 18, 2023, 8:22am UTC](https://prepomax.discourse.group/t/bolt-preload-for-piping-connection/835/25 "2023-05-18T08:22:13Z")
It doesn’t converge because of the contact between L section and channel section. If you change the interaction to tied, it will work. ---
**Author:** ![datsnl](https://avatars.discourse-cdn.com/v4/letter/d/8e7dd6/32.png) [@datsnl](https://prepomax.discourse.group/u/datsnl)\ **Post date:** [May 18, 2023, 1:09pm UTC](https://prepomax.discourse.group/t/bolt-preload-for-piping-connection/835/26 "2023-05-18T13:09:08Z")
Yes I tried and it converged in 2.3seconds. But when I change interaction to tied, I lost contact pressure which you can only get in surface to surface contact connection. Is there a way to achieve contact pressure as per this post [CalculiX-Examples/thermal.inp at master · calculix/CalculiX-Examples · GitHub](https://github.com/calculix/CalculiX-Examples/blob/master/NonLinear/Bolt/thermal.inp) Thanks Dharmit ---
**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:** [May 18, 2023, 1:23pm UTC](https://prepomax.discourse.group/t/bolt-preload-for-piping-connection/835/27 "2023-05-18T13:23:33Z")
Tied contact has to be of a surface-to-surface type and you can obtain the contact pressure with it. [Previous page](https://prepomax.discourse.group/t/bolt-preload-for-piping-connection/835.md?page=1)