Internal bolt preload is created using a temperature load on the model and taking advantage of the material Coefficient of Thermal Expansion (CTE). The bolts should be modeled with solids or beam elements. Contact regions should be defined between the interface surfaces. The model should be the same as the Beams-as-Bolts or Solid-with-full-Contact models.
The trick to getting the preload in the bolt is to assign a zero CTE to the mating parts and some real value for the bolt material. A negative temperature load is applied (cool down). This will cause the bolt material to shrink due to the CTE while the other parts remain unchanged because of the zero CTE.
Calculating the temperature change is performed as follows:
The following Table list the recommended Preload (lbs) for various common SAE Grade and fastener sizes.
| Thread Size | Grade 2 | Grade 5 | Grade 8 |