General Mechanical

General Mechanical

Bolt Simulation High Stress!

    • Leroy Benjamin
      Subscriber

      Hi,

      I am doing a FEM calculation of my setup and trying to find if the bolts I am using can withstand the forces. 
      at the moment I have applied a very low load and still the stresses are high in one circumferential line of my bolt (please see image).

      I have applied bolt pretension as I have seen in different Ansys videos. I am using an M10 bolt. 

      any idea what is causing the high stresses?

      thanks!

       

       

    • SaiD
      Ansys Employee

      Hi,

      1. Are you applying the bolt pretension and the loads (remote force) in the same step or in two different steps? You need to have at least 2 steps when bolt pretension is involved. Step 1: apply the bolt pretension, Step 2: apply other loads on the assembly (it imitates the fact that in real life, we always tighten our bolts first and then subject the assembly to other loads).
      2. Applying bolt pretension cuts the bolt into two parts and pulls them towards each other, creating a tensile force in the bolt. The plane that cuts the bolt into two should not be located in a region where bonded contact is defined because this would mean that the two halves are not allowed to move towards each other. Check the location and ensure it is not in the region where bonded contact is defined.

      You may find the following course (especially lesson 2) helpful: Modeling the Bolt and Preload | Ansys Innovation Courses

      Hope this helps.

       

    • Leroy Benjamin
      Subscriber

      Hi,

      thanks a lot for the reply:

      1. yes I have used 2 steps to apply pre tension as outlined in the course you said. 
      2. I have defined individual Co ordinate system for each bolt and as you can see from image below it is not close to bonded contact region. 
      3. are there any other courses for bolts? I have been stuck on this issue for weeks now. Any help will be appreciated. 

      Thanks!

    • SaiD
      Ansys Employee

      Hi,

      There is a learning track with 4 courses: Pre-loaded Bolted Connections - ANSYS Innovation Courses You might find something helpful there.

      Could you post a cross-section view of the assembly? Does the horizontal pipe end near the region where the high stresses are observed on the bolts? I wonder if there is a sharp corner contacting the bolt that could be causing this. Maybe you could also post a picture with the mesh.

    • jonl
      Subscriber

      I don't think the stresses at the threads will be accurately predicted by a cylinder with bonded constraint. You can instead use the "Working load" in the bolt tool to check the average forces in the bolt (assuming the joint is not sliding and shearing the bolts).

    • Leroy Benjamin
      Subscriber

      Hi, 

      thank you both for the reply. As asked I am adding images.

       

       

       

       

       

    • SaiD
      Ansys Employee

      Hi,

      Thanks for sharing the images. I am not sure if this will help, but can you try using "Multizone method" for the bolts to mesh it with hex elements? Even the horizontal cylinder can probably be meshed with hex elements if Sweep (Axissymmetric) or Multizone method are used. 

      Altering the mesh might help fix the problem.

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