Simplified Modeling of Bolt Threads
When an internal and an external thread are engaging, there will be a complex local stress state. When analyzing such geometries, you can approach the problem at three levels:
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Make a full 3D model of the actual thread geometry and solve the contact problem. This approach is only used in some rare cases, since such a model by necessity becomes very large. Also, the actual stress state in the thread itself is seldom the target of an analysis.
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Ignore the thread completely and just connect the two parts by a union operation or through a Continuity pair. This approach will provide an accurate solution outside a region with a size of a couple of bolt diameters from the bolt hole.
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Use the simplified Bolt Thread Contact condition. With this approach, the stresses will be correct, except at distances where stress concentrations caused by the actual thread geometry are significant. The important part of the solution that can be captured by this simplified contact condition is that the contact pressure between the threads will push the walls of the bolt hole outward. This will cause significant tensile hoop stresses around the bolt hole.
When you use Bolt Thread Contact, you model the face of both the bolt and the bolt hole as cylinders. The actual geometry of the thread is taken care of by the mathematical formulation of the contact condition. The most important parameter is the thread angle, since it determines the direction of the contact forces.
In the Bolt Thread Contact node, you enter general data about the bolt geometry. For each individual bolt, you need to add one Thread Boundary Selection subnode.
Penalty Factor
The contact condition is enforced using a penalty formulation. In practice, this means that a stiff spring is inserted between the two boundaries. In most contact problems, you want to use a high penalty factor, in order to avoid excessive overclosure of the contacting boundaries. In this case, however, there is a certain flexibility which has its source in a slight bending of the threads in real life. This effect can to some extent be accounted for by allowing a certain flexibility in the contact condition. For this reason, the default penalty factor is rather low. You may need to adjust it to suit your conditions.
A higher penalty factor also means that the force is transferred from the external thread to the internal thread over a shorter distance along the bolt axis.
Contact Orientation
In most cases, you know a priori which side of the thread that will be in contact. You can reduce the size of the contact problem by a factor of two by selecting an appropriate value for the Contact orientation. The default is to search in both directions.
If a bolt is pretensioned, the contact orientation will be Up if the bolt orientation is such that the Orientation vector points toward the bolt head. If the bolt geometry, including orientation, is computed using the Automatic option, you will often have to use the Direction adjustment option to have full control over the up and down directions.
Modeling Hints
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You must create one Contact Pair for each bolt and mating bolt hole. The bolt should be the destination side of the contact pair.
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In the Part Libraries, you will find predefined parameterized bolt geometries which also contain a domain intended to be used as drill for the bolt hole.
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The cylindrical surfaces of the bolt and bolt hole should have the same diameter. If you want to decrease the stiffness of the bolt because it is not solid all the way out to the nominal diameter, a good suggestion is to decrease Young’s modulus by a factor based on the stress area of the bolt.
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Since the contacting boundaries are cylindrical, there is a risk that the bolt, instead of computing the gap to the nearest point on the bolt hole, will instead see a point on the opposite side. A good practice is to select Manual control of search distance in the settings for the Contact Pair. Half the diameter of the bolt is an appropriate search distance.
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Since the relative displacements between the bolt and the hole are small, setting Mapping method to Initial Configuration in the settings for the Contact Pair can improve efficiency.
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In the initial state, the two objects being joined may possess rigid body modes, just like in any other contact problem. If so, you can for example add weak springs to maintain stability.