Simulating Bolted Assemblies

Among the many ways parts can be assembled together, bolts are useful when parts need to be disassembled for maintenance or repair. They can also be used to connect parts made of dissimilar materials, when welding is more challenging.

From a FEA standpoint, bolts can be modeled in many different ways, from simple links between parts using constraint equations to fully detailed models of the bolts, potentially including full thread details. Whatever the model one will use, simulating bolted assemblies will usually require a multiple steps analysis to mimic the reality. The assemblies are first bolted, and then used in their environment with corresponding loads and boundary conditions. A structural analysis model will let you apply loads in a similar way. Bolt pretensions are applied first, generating stresses and deformations in the model. Then, once bolts have been locked, additional loads will be applied.

Let’s focus on an assembly where bolts are geometrically modeled, yet not with the detailed thread (note that the following would equally apply to bolts modeled as simple beams). What are the challenges you will face in building such a model? What do you need to look at? Well, one of the biggest challenges when dealing with bolts is that you rarely have a single bolt. In reality, chances are you will have dozens if not hundreds of them. How can you efficiently deal with a high number of bolts? Can you easily setup one bolt and replicate the setup for all other ones? How can you efficiently setup your model to reflect all contacts between bolts and the various connected parts?

The video below demonstrates how to efficiently create pretension bolts in a model and how multiple steps analyses can be used to analyze the design of your bolted assembly.

Watch how efficiently ANSYS Mechanical handles bolted assemblies

Once the geometry has been imported, contacts will be automatically detected in the assembly. You just have to make sure the contact between your bolts and the connected parts have the appropriate properties, for example frictionless or with a given friction coefficient. A nice idea would be to group your contacts accordingly, so you can identify them later when reusing your model.

Contacts Between Bolts
Contacts between bolts and connected parts

Then you need to define the pretension on each bolt. The best solution here is to create the loading conditions on a single bolt and then replicate to all other bolts with the same properties. In a few mouse clicks, you will generate the conditions on all bolts in your model.

Definition of Bolt Pretentional Auto Gen Loads
Definition of bolt pretension and automated generation of loads on multiple bolts

As mentioned earlier, your simulation will usually have a minimum of two steps: one to apply the pretension, one to apply other loads while bolts are locked in their constrained state. The first one mimics the assembly of your product, the second one the real behavior of your product under real loading conditions. Whether you define the bolt pretension through a load or an adjustment, you will likely be interested in looking at reaction forces to make sure all loads have been properly applied. Here again, automation will save you huge amount of times – define the results you want to look at on one bolt and automate the creation of results on all other bolts.

Typical Results for a Bolted Assembly
Typical results for a bolted assembly

Contact Pressure Bolt Head Pretention
Contact pressure at the bolt head for pretensioned state and under real loading conditions

Picture of Pretension Results
Picture of pretension results (contour & table) Stresses induced by bolts and pretension results

Stresses Induced by Bolts Pretension Results
Stress induced by bolts and pretension results

If you are interested in further detailed analysis of your bolts, you may want to include the details of the thread. Geometric modeling of the thread would however lead to larger models as capturing the stresses will require a high mesh density. A much better solution would be to model the thread as a contact without the thread geometry available. The contact algorithms would take care of computing accurate stresses as shown below. Contact us if you need further details on this modeling technique.

Simplified Model for Faster Threads
Simplified model for faster yet accurate analysis of bolt threads