Pretensioned Bolt Analysis

Many engineering assemblies are put together using bolts, which are usually pretensioned before application of working loads.

A typical sequence.


Figure 1. Pretensioned Assembly - Application of Pretensioning Loads (Step 1)

In Step 1, upon preliminary assembly of the structure, the nuts on respective bolts are tightened, usually by applying specified torque (which translates into specified tension force according to the pitch of the thread).

As the result, the working part of the bolt becomes shorter by a distance Δ L . This distance depends upon the applied force, the compliance of the bolt and of the assembly being pretensioned.

From the perspective of FEA analysis, it is important to recognize that:
  • Pretensioning shortens the working part of the bolt by removing a certain length of the bolt from the active structure (in reality this segment slides through the nut, yet the net effect is the shortening of the working length of the bolt). Since the bolt stretches, there is a smaller effective length of the bolt material to span the distance from the bolt mount to the nut.
  • Calculation of each bolt's shortening Δ L , due to applied forces f , requires FEA solution of the entire model with the pretensioning forces applied. This is because the amount of nut movement due to given force depends on the compliance of the bolts, of the assembly being bolted and is also affected by cross-interaction between multiple bolts being pretensioned.

At the end of Step 1, the amount of shortening Δ L for each bolt is established and "locked", simply by leaving the nuts at the position that they reached during the pretensioning step.

In Step 2, with the shortening Δ L of all the bolts "locked", other loads are applied to the assembly (Figure 2). At this stage the stresses and strains in the bolts will usually change, while the length of material removed Δ L remains constant for each bolt.


Figure 2. Pretensioned Assembly - Application of Working Loads with "locked" Bolt Shortening (Step 2)

Application Variations

In practice, there may be variations of the application of pretensioning loads and more complex pretensioning sequences than that presented above. For example:

In alternative assembly scenarios, instead of using a nut on top of the bolt, the bolt may be screwed into a base and thus compress the assembly (Figure 3).


Figure 3. Alternative Approach to Application of Pretensioning Loads

In this case, the shortening (removal of material) of the working part of the bolt happens at the thread within the base, rather than at the bolt-nut interface. Yet the final mechanical effect is the same.

Sometimes the pretensioning by torque/force is augmented by "tightening" via specified number of turns. This means that on top of the Δ L , due to pretensioning force, an additional Δ L ' is added according to the number of turns and the pitch of the thread.

In an automated assembly process, usually all bolts are pretensioned simultaneously. Sometimes, however, the tensioning may happen in sequence or in groups. In such cases, while Δ L is "locked" for bolts that have already been pretensioned, consecutive pretensioning force is applied to the next batch of bolts, which then become "locked" for the following step.