Sequential Bolt Pre-tension Study Using Altair

Bolt tightening sequence plays a very important role in proper assembly of parts, as it directly affects the performance of the system. One of the challenges observed in the industry is upon tightening the consecutive bolts, the previous bolts seem to loosen up and also the stresses increase, so it important to follow the right bolt tightening sequence. We are showcasing this by performing bolt pre-tension finite element analysis on two plates bolted at four locations, we are studying the bolt tightening for two sets of sequences, by understanding the bolt forces, stresses, and displacements.

We are using Altair products i.e., HyperWorks for performing preprocessing, OptiStruct solver for non-linear analysis and HyperView for postprocessing.  

FE Model Details:

• Two Plates are modelled as Shell elements with 3mm thickness
• Four Bolts are modelled as Cbar elements with Diameter 20mm and connected to the plate with rigid elements
• Material used is Steel i.e. (E 195000, Nu 0.29, Rho 8e-9)
• Bolt pre-tension analysis performed as four individual non-linear load cases with CNTNLSUB, which continues non-linear solution from previous non-linear subcase
• Pre-tension force of 2000N applied on all 4 bolts
• The plates are constrained on the two opposite sides
• Units (Tons/mm³)
• Refer to the below Figure 1 for details

 Figure 1 Model details

Bolt tightening sequence:

The below Figure 2 shows the bolt tightening sequence for the two cases.

Figure 2.  Bolt tightening sequence for the two cases  

Results:

Case1:

The below (Figure 3) shows the von Mises stress plot for case 1, we can observe that upon performing subsequent bolt-pre-tensioning, the stresses on the previous bolts are increasing across all the 4 bolts, the max stress observed is 510 MPa.

  Figure 3 Case 1 Von Mises Stress plot

  The below (Figure 4) shows the bolt forces of all the 4 bolts for case 1, we can observe that upon performing subsequent bolt-pre-tensioning, the forces on the previous bolts are reducing across the 3 bolts, the first bolt has reduced to 1026 N, the second bolt has reduced to 1176 and the third bolt has reduced to 1839 N, only the fourth bolt is retaining the complete load of 2000 N.

Figure 4 Case 1 Bolt forces

 Case2:

The below (Figure 5) shows the von Mises stress plot for case 2, we can observe that upon performing subsequent bolt-pre-tensioning, the stresses on the previous bolts are increasing across all the 4 bolts, the max stress observed is 502 MPa.

Figure 5 Case 2 Von Mises Stress plot

 The below (Figure 6) shows the bolt forces of all the 4 bolts for case 2, we can observe that upon performing subsequent bolt-pre-tensioning, the forces on the previous bolts are reducing across the 2 bolts, the first bolt has reduced to 917 N, the second bolt has reduced to 1075 N, the third and fourth bolts seem to be retaining the complete load.

Figure 6 Case 2 Bolt forces

 Conclusion:

• For Case 1 the Max stress observed was 510 MPa and only the fourth bolt was retaining a complete load of 2000 N
• For Case 2 the Max stress observed was 502 MPa and the third and fourth bolts retained the complete load of 2000 N

Looking at the above 1 and 2 points, it seems like Case 2 sequencing seems to be better than Case 1.

About Prashant Hiremath

Prashant Hiremath is working with Altair Engineering Inc. for the last 14 years; in his current role he is working as a Technical Manager–HyperWorks and responsible for supporting Altair Simulation Suite of Products. He received his M Sc. in Automotive Engineering from Coventry University, UK. He has 18 years of experience in Finite element analysis with focus on various domains i.e. Crash, Durability, NVH and Optimization. He also has experience with Product Design and Development.