Understanding spider elements in bolted connections

The locations at which the software creates the spider-type connections depend on the bolt's type.

• For the Bolt With Nut type, the software creates spider-type connections to the nodes on the contact surfaces for the head and the nut. In the following graphic (A) shows the bolt, while (B) shows a display of only the finite elements that comprise the bolt. Notice the spider connections at the bolt's head and nut.

• For the Bolt in Tapped Hole type, the software creates spider-type connections to all nodes that lie along the length of the engaged threads in the tapped hole. You use the Effective Thread Length option in the Bolt Connection dialog box to define the length of the engaged threads. In the following graphic (A) shows the bolt in a tapped hole, while (B) shows a display of only the finite elements that comprise the bolt. Notice how the legs of the spider elements connect along that entire effective thread length.

• For the Spider at Junction type, the software creates spider-type connections to the nodes on the specified junction planes. In the following graphic (A) shows the two spider elements at the junction plane. (B) shows a display of only the finite elements that comprise the spider-type connection.

You can control whether the software creates the spider connection to the corner nodes or the corner nodes and the midnodes of the elements in the surrounding mesh. If you select the Connect Spider to Midnodes option in the Bolt Connection dialog box, the software creates the spider connection to the midnodes in addition to the corner nodes. In the following graphic, (A) shows a spider connection to only the corner nodes of the surrounding mesh. (B) shows a spider connection to both the midnodes and the corner nodes.

Defining spider-type connections at an intermediate junction plane

For the Bolt With Nut and Bolt in Tapped Hole types, you can use the Create Spider at Junction Plane option in the Bolt Connection dialog box to have the software create an additional spider-type connection at an intermediate junction plane between the mating bodies.

The following graphic shows an example of a Bolt with Nut type of bolt that was created with the Create Spider at Junction Plane option selected. Here:

• (A) shows the location of the spider connection at the bolt's head.

• (B) shows the location of the junction plane.

• (C) shows the location of the spider connection at the bolt's nut, which is located on the underside of the junction plane.

This graphic shows only the finite element model of the bolt itself. Notice the spider connections at the bolt's head (A), junction plane (B), and nut (C).

Controlling the size of the spider-type connection

The Spider Diameter option in the Bolt Connection dialog box controls the overall diameter of the spider connection.

• For the Bolt With Nut type, you can control the diameter of the connection at both the bolt's head and nut.

• For the Bolt in Tapped Hole type, you can control the diameter of the connection at the bolt's head.

• For the Spider at Junction type, you can control the diameter of the connection at the specified junction planes.

You can define the diameter of the spider as either a percentage of the hole that you used to define the location of the bolt's head/nut or junction plane. You can also define the spider's diameter as a scale factor. In general, the diameter of the spider should be at least as large as the bolt's head and/or nut. For a bolt connection to be sufficiently rigid, the spider elements must connect to a sufficient number of nodes on the bolt's contact surfaces (the head, nut, and any junction planes).

You can use the Circular Imprint command to create circular edges around the bolt holes in the contact surfaces at the bolt's head, nut, or junction plane. By imprinting an additional circle around the ends of the bolt hole, you ensure that the legs of the spider elements created at the bolt's head and nut have sufficient nodes in the surrounding mesh to which to connect. For more information, see Imprinting circular edges on a face.

Tip:

If you use the Absolute Diameter option in the Circular Imprint dialog box to define a new circular edge around a bolt hole, use that diameter as the Spider Diameter value in the Bolt Connection dialog box.

The following graphic shows a bolt hole surrounded by a larger imprinted edge that was created with the Circular Edge command.

• (A) shows the spider element when its diameter is equal to 100% of the diameter of the hole. Notice how the legs of the spider element only connect to the nodes on the edge of the bolt hole.

• (B) shows the spider element when its diameter is equal to the diameter of the imprinted edge (40mm). Notice how the legs of the spider element now connect to the nodes along the edge of the imprinted circle.

Learn more

Modeling bolted connections

Modeling the bolt's shank with a spring element

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Understanding spider elements in bolted connections, Simcenter 3D 2021.1 Series

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