Modeling bolt threads (SOL 402)

To model a threaded connection between a bolt and its casing, you can use the Virtual Thread Contact simulation object.

The Virtual Thread Contact simulation object lets you model a threaded bolt cylinder and its casing by defining the bolt cylinder and bolt casing contact surfaces, bolt orientation, thread characteristics, and contact parameters. The software simulates the threaded connection by generating a contact element that connects the faces of the bolt cylinder and bolt casing and prevents axial movement. The bolt cylinder and bolt casing can be 3D solid meshes of linear or quadratic elements. Both faces must use the same order of element.

Using Virtual Thread Contact to simulate threading works well for small, relative sliding in the connection, but the entire bolt assembly can undergo large displacements and rotations.

Like glue contact, you can define Virtual Thread Contact only at the solution level.

Bolt kinematics

To correctly model the bolt, you must specify two directions:

• The vector that points radially outward from the outward from the face of the bolt cylinder. You establish this vector by selecting the inner (bolt cylinder) and outer (bolt casing) contact surfaces.

• The vector that is normal to the thread surface in the direction that the bolt would be pulled out if it were pulled from the head of the bolt. You establish this vector by selecting a bottom and top center node or point on the bolt cylinder.

Contact surfaces

The contact surfaces consist of the outer surface (inner face of the bolt casing) and the inner surface (outer face of the bolt cylinder). Selecting the correct outer and inner surfaces establishes a vector that points radially outward from the inner surface. Both the outer and inner regions must be cylindrical.

(1) bolt casing; (2) inner surface of bolt hole	(3) outer face of bolt cylinder

A gap or penetration can exist between the inner and outer surfaces. The software removes this during the solve.

Bolt orientation

To establish the orientation of the bolt, you identify the orientation and direction of the bolt axis by selecting a point or node on the bottom face and a point or node on the top face of the bolt cylinder. These points establish a vector that is normal to the thread surface in the direction that the bolt would be pulled if it were pulled from the head of the bolt. The nodes or points must be on the axis of symmetry, and they must not be coincident. The direction of the bolt axis is critical in determining the direction of the contact force.

In addition to establishing the orientation of the bolt, if a bolt undergoes large displacements or rotations, the software follows the direction of the node selected on the bottom of the bolt. Therefore, whether you select nodes or points to establish bolt orientation is dependent on whether you are modeling large displacements or rotations, as follows:

• To model large displacements or rotations:You must first create an RBE2 or RBE3 element on the bottom face of the bolt. When you select the node for the bottom of the bolt, you must select the center node of the RBE2 or RBE3 element. This ensures that the node follows the rotation of the bolt.Creating an RBE2 or RBE3 element on the top face of the bolt is optional, but it is useful for ensuring that the node remains on the axis of symmetry in the deformed geometry for post-processing. Otherwise, selecting a point or node on the top face of the bolt on the axis of symmetry also ensures that the bolt direction is computed correctly. You can use this same method to model small displacements or rotations, but for small displacements or rotations, you can also use points instead of nodes.

• To model only small displacements or rotations, you can create and select points for the bottom and top faces of the bolt. Creating a point at each end of the bolt cylinder is convenient when meshing does not result in a center node in the top and bottom faces.

Thread characteristics

The thread can be conical (not a helix), or all of the threads can be in the same direction. The thread can be single sided (all threads are in the same direction) or double sided (threads are in alternating directions).

• To model Single-Sided Contact, you set an angle value. The contact normals are all in the same direction, but you can represent the bolt in tension or compression depending on the order in which you select the nodes or points to define the bolt orientation.

• To model Double-Sided Contact, you set an angle value and a pitch value.When you set the bolt orientation, you must select the bottom and top nodes in order.

For more information, see Thread Shape in Virtual Thread Contact dialog box, or see the THDOPT parameter on the BVTPAR bulk entry in the Simcenter Nastran Quick Reference Guide.

Friction and contact parameters

You can set a static friction coefficient for the virtual thread contact, and you can create a Virtual Thread Parameters modeling object.

To model flexibility (normal stiffness) of the bolt threads, you can use the following options in the Virtual Thread Parameters modeling object:

• Normal Contact Modulus (NCMOD)

• Normal Behavior Penalty Function (NPENAL2)

• Constant Compliance Factor (CFACTOR1)

The stiffness is normal to the thread.

Post-processing

The output for Virtual Thread Contact is the same as for regular contact. Review the output contact pressure and forces to determine if your model is set up correctly.

Where do I find it?

Application	Pre/Post
Prerequisites	Simcenter Nastran as the specified solverStructural as the specified analysis typeSOL 402 Multi-Step Nonlinear Kinematics as the specified solution type
Command Finder	Virtual Thread Contact
Simulation Navigator	Under the active solution, right-click Simulation Objects→New Simulation Object→Virtual Thread Contact

Modeling bolt threads (SOL 402), Simcenter 3D 2021.1 Series

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