Why Bolt Pre-Tension Setup is Important

When designing a structure which is clamped by bolts, it is important to note that pre-tensioning of the bolts is advantageous for design and implementation purposes. Bolt pre-tension increases the stiffness of the structure and thus causes the shear stress to be transferred mostly between the plates through friction.

When performing an FEA simulation, it is important to remember to apply the bolt pre-tensioning in a correct manner such as to accumulate more accurate results. This document will thus provide a short tutorial on how to set bolt-pretension up properly in ANSYS Mechanical and to show comparisons of how the results differ when setting it up inaccurately.

 

Figure  SEQ Figure \* ARABIC 1 - Simple Beam Experiment

Firstly, let’s get down to the basics and explain the core principle behind pre-tensioning by making use of a classic example, namely a beam in bending as seen in Figure 1 below.

 

 

 If one was to apply only one load at the centre of the beam it would experience pure bending. The deformation would be large and the beam (depending on the magnitude of the load) would snap at the centre. However, when creating resistance by applying a higher force at the ends of the beam – in resistance to the load applied to the centre – the deformation results will ultimately decrease in magnitude. The principle behind this example is the same as bolt-pretension and why bolt pre-tensioning is important. Two scenarios were simulated in order to show the importance of how the setup of the analysis can change the results. In the both scenarios, a 50kN force in tension was applied to the ends of the beam and a remote force of 20kN applied to the centre of the beam. In the first scenario both loads were applied at the same time, namely in one load step. In the second scenario, the tension force as applied in the first load step and the remote force in the second step.

The results are tabulated below and shown in Figure 2 and Figure 3:

Scenario

Total Deformation (Max)

Equivalent Stress

(1) One Load Step

0.9954 mm

157.19 MPa

(2) Two Load Steps

1.0353 mm

141.55 MPa

 

Figure  SEQ Figure \* ARABIC 2 - Total Deformation for Scenario 1

 

Figure  SEQ Figure \* ARABIC 3 - Total Deformation for Scenario 2

 

It is evident from the above-mentioned result set that the deformation from scenario 2 differs by 3,85% which, depending on the application, could have a significant impact on the accuracy of results needed.

In the following example, two plates are bolted to each other. The bolts were given a pre-tension of 20kN and the load applied to the centre of the plates has a magnitude of 30kN. Figure 4 shows how to setup the bolt pre-tension correctly. The tension load should be applied in the first step, and step 2 and 3 should be locked. This insures that the pre-tension is not applied in every load step.

 

 

 

Figure  SEQ Figure \* ARABIC 4 - Bolt Pre-Tension Setup

 

It is a common misconception that the load should be applied in all three steps. Thus, the two cases shown below show the difference between applying the load in all three steps and applying the load only at the last step. For case 1 the load was applied in all three load steps as shown in Figure 5. This is also where the load step can be altered.

Figure 6 shows that the load is only applied in the last/third load step, which resembles case 2. Take notice of the Load vs Time graph which visibly explains how the load will be applied during the simulation.

 

 

 

 

 

 

 

 

 

 

 

 

Figure  SEQ Figure \* ARABIC 5 - Load Entries For Case 1

 

Figure  SEQ Figure \* ARABIC 6 - Load Entries for Case 2

The results of the simulation can be seen in the table below. Attention was given to the total deformation and the gap that is created when the maximum force instance occurs.

Case

Total Deformation [mm]

Gap [mm]

Case 1

1.1209

0.76458

Case 2

1.1153

0.60997

 

It is evident from the results above, that the total deformation difference between the two cases lies at 0,499%, which is not significant, however, the gap difference lies at 20,22%, which is a significant amount.

 

Figure  SEQ Figure \* ARABIC 7 - Structure and Static Setup

However, the above-mentioned example is rather a simpler case. Let’s have a look at an example that is more relevant to industry applications and that has significantly more bolts connecting the structure, shown in Figure 7 below.

 

 

Once again, both cases were tested where the load is applied in all three steps or just in the 3rd step. The case numbers will be as assigned to above.

The table below shows the results for the above-mentioned example.

Case

Total Deformation [mm]

Gap [mm]

Case 1

0.19831

0.14488

Case 2

0.16863

0.11408

 

The total deformation differs by 14,97% and the gap differs by 21,26%. The differences are significantly higher/worse than the simple example that was shown earlier on. This goes to show that applying the load in the wrong load step could have significant consequences for bolted connection designs, depending on size, geometry and magnitude of loads applied

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