Often the simplest method of applying a bolt preload is to add forces directly to the model. You may apply equal and opposite forces at each bolt location in the model to clamp the joint together. This may or may not include modeling the bolt stud itself to get the contact around the hole diameter. Apply one force in tension to the nut end of the bolt. Apply the second opposite force to a surface region created around tho bolt hole on the part. This region should be the size of the bolt head, nut or washer. You will also define Contact Regions between the two interface surfaces.
This method assumes that the bolt load never changes as other external loads are applied to the structure. Hence, the axial stiffness of the bolt is not accounted for.
Note the high stresses around the bolt head area due to the preload force. You need to be careful in evaluating the stresses in your model using conventional factors of safety. These high local stresses may be well over you normal allowable. It would probably be more appropriate to compare these stresses to the material ultimate or yield rather than using conventional factors of safety.
Note the contact area between the two interface surfaces is approximately two bolt diameters. This is the assumption used with some of the other bolt connection modeling techniques.
The following Table list the recommended Preload (lbs) for various common SAE Grade and fastener sizes.
| Thread Size | Grade 2 | Grade 5 | Grade 8 |