Modeling Screwed Connections in Finite Element Analysis (FEA)

Similar to bonded contacts, their main advantage lies in their simplicity and stability. Setting up such connections is done quickly through the engineer and also allows for a fast calculation of the problem. In addition to this, in contrast to bonded contacts, they also allow for a clear force extraction of the simulated screw by inspecting the created reference node.

Albeit they are thus able to give a numerical value for the loads, they still exhibit artificially stiff behavior, potentially falsifying the prediction. This is evident when bearing in mind that the physical properties of the screw are still nowhere included in the model. Thus, joining two components with an M3 screw would yield the same results as an M20 screw. Additionally, it is still not possible to include preload or friction effects in this simulation.

1D bolt representations are created by modeling the screw shaft as beam elements and then connecting them to the surrounding structure via rigid or elastic elements. They can thus be seen as an extension of the RBE method by introducing a non-infinite stiffness between the connecting points. Typical implementations include OptiStruct’s CBAR and CBEAM, and the beam elements in ANSYS Mechanical.

They are beneficial because they support preload definitions as well as defining the stiffness of the screw shafts by accordingly tuning the stiffness of the beam element while still staying efficient for large assemblies, since they are only one-dimensional and thus scale well. This results in a good balance between accuracy and cost.

Still, they lack the ability to resolve these stresses in a three-dimensional manner. For example, the local thread stresses are not depicted, and depending on the chosen interfaces for the RBE elements at either side, it is also quite regular that the forces are not distributed correctly. Additionally, it is necessary to manually calibrate the stiffness of the one-dimensional element, making sure it is in accordance with the stiffness of the entire screw. This becomes especially tedious for bolts with shanks or fit screws due to their changing cross-section, not only increasing labour but also room for error.

Here, the contact of the screw head to the joint component can be set to either bonded or frictional, depending on the problem requirements. Implementations or do for all software, solid meshing together with appropriate contact definitions. Under the hood, pretension is applied by axially sectioning the screw into two parts and applying a relative axial displacement at the created interface, essentially artificially enlarging the screw length and thus creating tensioning forces that represent the pretension. The solver then manually tunes the artificial displacement to match the user-specified pretension. Both Ansys and Altair provide appropriate tools to streamline this process.

The gains of this approach lie in the increased resolution of the connection. 3D stresses, such as the notching effect between the screw head and the screw shaft, or the specific shape of the screw head, can be calculated, providing increased realism for the stresses inside the screw. Likewise, this also offers superior contact pressure predictions, since the screw stiffness is modeled three-dimensionally. The drawbacks of this approach mainly reduce the increased mesh effort, which takes longer to achieve for the simulation engineer, as well as increasing solving times, potentially also making the problem less robust. Additionally, the approach still has its limitations when it comes to the analysis of the thread behavior itself, as it is still idealized as a bonded cylindrical contact rather than depicting each thread individually. ANSYS offers a Bolt Thread Geometry Correction that promises the approximation of the thread stresses. However, this comes at the cost of having to set the element size to below a fourth of the pitch size, making the mesh size typically too small for practical evaluation.

Here lie multiple implementation challenges. Firstly, creating an appropriate mesh is very difficult due to the extremely fine mesh size. It is not essential to set the mesh size to the pitch, but rather it must be significantly smaller, to also be able to account for the created stresses in every crease of the pitches. Even if meshing is achieved, this typically results in significant convergence issues due to the complex contact behavior. Nevertheless, it is the only way to accurately simulate thread stresses, which can be necessary if the governing failure mode is expected to be induced inside the threaded region. However, by following engineering standards such as the VDI 2230, it can be ensured that breakage occurs outside the engaged thread, thus circumventing any need to deploy such complex models in typical engineering problems.

Thus, their disadvantages in extremely high computational effort and infeasibility for full assemblies limit the typical usage to academic studies or model calibration and validation.

Professional joint simulations create significant advantages for any and every company. Early detection of critical joint weaknesses in initial project phases can save many resources, as can the reduction in overdesign and material usage. This avoids late-stage design changes and allows for faster iteration cycles in product development. In later project phases, a company can benefit from improved correlations between calculation, test, and reality, avoiding aimless and uninformative testing campaigns. The reliable result extraction possible for simulations not only enables downstream verification but also generates defensible results for certification and customer reviews. Thus, FEA is a powerful and scalable accelerator for any project.

Bonded contact without explicit screws is sufficient for concept studies, surrogate models, and global stiffness estimation where neither preload, slip, nor separation are relevant. This approach is computationally robust and fast but cannot represent preload, friction, realistic stiffness distribution, or joint failure modes.

FiniteNow helps you select the least complex model that still answers your engineering question, saving time and computational cost. You receive actionable results—forces, stiffness, safety margins—instead of generic stress plots that are difficult to interpret or defend.

Rigid elements enable fast load transfer and clean force extraction at reference nodes, but they introduce artificial stiffness and ignore bolt geometry, material, preload, and friction. As a result, different bolt sizes behave identically in the model, making this approach unsuitable for realistic joint behavior or fatigue analysis.

Beam-based bolt models offer the best balance between accuracy and efficiency for many engineering problems. They allow preload definition, realistic axial and shear stiffness, and scalable modeling for large assemblies. While they cannot resolve local 3D stresses or thread behavior, they are well suited for system-level analysis, force extraction, and correlation with analytical standards such as VDI 2230.

Bolt preload is typically introduced either by applying an axial force directly to a beam-based bolt element or, in 3D solid bolt models, by axially sectioning the bolt and imposing a controlled relative displacement. The solver iteratively adjusts this displacement to achieve the specified pretension force.

Yes. Aligning FEA with VDI 2230 is one of FiniteNow’s core strengths. We extract joint stiffness and bolt loads correctly from simulation and map them directly into the standard, ensuring defensible safety factors suitable for design reviews, certification, and customer documentation.

3D solid bolts without explicit threads are recommended when local stresses in the bolt shaft or under the bolt head are of interest, or when accurate contact pressure distributions are required. They significantly improve realism compared to 1D models but increase meshing effort, model size, and solution time.

Yes. FiniteNow supports the full development cycle—from fast concept-level models for decision-making, through design-phase optimization, to high-fidelity verification models suitable for testing correlation and certification support.

Explicit thread modeling is only necessary when the governing failure mode is expected within the thread itself. Due to extremely high meshing and convergence requirements, fully threaded models are typically limited to academic studies, calibration, or validation work. In most industrial applications, thread stresses are instead assessed analytically using standards such as VDI 2230.

FiniteNow focuses on engineering decision support, not just simulation delivery. We combine deep numerical expertise with standards-based engineering judgment, ensuring your bolt simulations answer the real question behind the model—and not just produce visually appealing results.

Bolt modeling strongly affects joint stiffness. Bonded contacts and rigid elements overestimate stiffness, while beam-based and 3D bolt models provide progressively more realistic compliance. Accurate stiffness prediction is essential for load sharing, fatigue life estimation, and correlation with test results.

No. FEA does not replace analytical standards but complements them. FEA is best used to extract realistic forces, moments, and joint stiffness values, which then serve as high-quality inputs for VDI 2230 calculations. This combination significantly reduces uncertainty compared to purely analytical or purely numerical approaches.

Simplified models can assess global forces and stiffness, while beam-based models allow evaluation of axial and shear loads relevant for fatigue. 3D solid bolts enable local stress and contact pressure analysis. Fully threaded models are required only for detailed thread stress evaluation. Slip, separation, preload loss, and fatigue all require at least beam-based modeling.

No. FiniteNow also offers model reviews and targeted consulting. If you already have an existing FEA model, we can audit your bolt modeling approach, preload definition, contact setup, and result interpretation – often delivering major improvements without rebuilding the entire model.

By identifying the true governing failure modes and realistic load paths, FiniteNow helps eliminate unnecessary conservatism. This often leads to fewer bolts, smaller bolt sizes, reduced flange thicknesses, and lower material usage—without compromising safety.