# Having some trouble with Preload - bolt stress too low **URL:** \ **Category:** General Questions\ **Created:** [February 24, 2023, 2:26pm UTC](https://prepomax.discourse.group/t/having-some-trouble-with-preload-bolt-stress-too-low/740 "2023-02-24T14:26:56Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Mechgru](https://avatars.discourse-cdn.com/v4/letter/m/958977/32.png) [@Mechgru](https://prepomax.discourse.group/u/Mechgru)\ **Post date:** [February 24, 2023, 2:26pm UTC](https://prepomax.discourse.group/t/having-some-trouble-with-preload-bolt-stress-too-low/740/1 "2023-02-24T14:26:56Z")
Hi everyone, I’m trying to understand the Preload load type better. I have a simple example of with two bolts. The thread is connected to the part with a tie and the other interfaces are all surface-surface contacts (Type Hard, friction coefficient 0.1). I followed the YouTube tutorial by Matej Borovinšek and finally got both preloads to work, BUT the stress in the bolt shank appears to be very low. The preload is 45 kN and the shank cross section is 51.672 mm^2, giving a nominal stress of 870 MPa. The S11 stress at the surface of the shank is between 32 and 38 MPa. Also the stress pattern in the shank is like the one shown in the [Pre-tension - Bolt](https://www.youtube.com/watch?v=7ucoBTqCcWE) tutorial _before_ the boundary layer is created at the interface. The contact force at the underhead interface is only 2135 N, which corresponds to a nominal stress in the shank of 41.3 MPa, similar to the reported value. What am I doing wrong? Does the thickness of the boundary layer need increasing? I have uploaded a plot of the bolt stress. Regards, Rod ![PrePoMax_GQ_rg-01](https://global.discourse-cdn.com/free1/uploads/prepomax/original/1X/17e23c055120e9f797aa24673b665de3adab2028.png) ---
**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:** [February 24, 2023, 4:40pm UTC](https://prepomax.discourse.group/t/having-some-trouble-with-preload-bolt-stress-too-low/740/2 "2023-02-24T16:40:30Z")
Can you share the .pmx file ? ---
**Author:** ![acal](https://avatars.discourse-cdn.com/v4/letter/a/e79b87/32.png) [@acal](https://prepomax.discourse.group/u/acal)\ **Post date:** [September 30, 2025, 9:54pm UTC](https://prepomax.discourse.group/t/having-some-trouble-with-preload-bolt-stress-too-low/740/3 "2025-09-30T21:54:04Z")
Hi Mechgru, I have a similar issue with a model used to check the correctness of preloads in bolts and flanges with contact. The pre-tension does not reach the prescribed force, and stays much lower. Have you solved this issue ever since? ---
**Author:** ![Mechgru](https://avatars.discourse-cdn.com/v4/letter/m/958977/32.png) [@Mechgru](https://prepomax.discourse.group/u/Mechgru)\ **Post date:** [October 1, 2025, 7:36am UTC](https://prepomax.discourse.group/t/having-some-trouble-with-preload-bolt-stress-too-low/740/4 "2025-10-01T07:36:54Z")
Hi Acal, I found the solution to the problem, I wasn’t following the tutorial correctly. The Boundary layer has to be created on _BOTH_ sides of the cut-face. Here’s the result with the correct mesh: ![Screenshot From 2025-10-01 08-36-01](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/2/2597ab78335cc3b08d5cd00d5cc86b1e549a4573.png) Regards, Mechgru ---
**Author:** ![Matej](https://avatars.discourse-cdn.com/v4/letter/m/a698b9/32.png) [@Matej](https://prepomax.discourse.group/u/Matej)\ **Post date:** [October 1, 2025, 8:14am UTC](https://prepomax.discourse.group/t/having-some-trouble-with-preload-bolt-stress-too-low/740/5 "2025-10-01T08:14:55Z")
For pre-stress, prismatic elements must be used; therefore, a boundary layer must be applied when tetrahedral elements are used. If you are able to use hexa or wedge elements in the boundary layer, the boundary layer is not needed. ---
**Author:** ![acal](https://avatars.discourse-cdn.com/v4/letter/a/e79b87/32.png) [@acal](https://prepomax.discourse.group/u/acal)\ **Post date:** [October 1, 2025, 11:39am UTC](https://prepomax.discourse.group/t/having-some-trouble-with-preload-bolt-stress-too-low/740/6 "2025-10-01T11:39:21Z")
Thank you Mechgru!! Now the model behaves as expected, EXCELLENT!