MET 425 Lecture Notes

Text: Building Better Products with FEA,
by V. Adams & A. Askenazi, (Read pp. 255-257, 399-409)

Reference: ANSYS Modeling and Meshing Guide, Chapter 12

Multi-Point Constraints (or Rigid Elements)

• can be used between elements with unmatched DOF's
• can tie together parts with different meshes on their interface
• are coupling of one or more DOF's between several nodes

COUPLED DOF's: the DOF of one node is constrained to move with another node - it is not constrained to have no motion, but must have the identical motion as the "master" node.

ANSYS: Preprocessor > Coupling/CEQN >

• will let you create, copy, and delete sets of coupled DOF's on a model. 
• Also, under "Coincident Nodes...", ANSYS can automatically couple different parts together (if the meshes on the interface align).

CONSTRAINT EQUATIONS: allow a more complex relationship than coupled DOF.  A linear equation relating the displacements at the nodes can be created.

ANSYS: Preprocessor > Coupling/CEQN >

• will let you create, copy, modify, and delete constraint equations.
• Also, under "Adjacent Regions...", ANSYS can automatically connect different parts together when the meshes on the interface DO NOT align.
• under "Rigid Region", ANSYS will create constraint equations to make the nodes you pick move as a "rigid body"
• Constraint equations are only valid in analyses with "small deformations"

ANSYS element MPC184, Multipoint Constraint Rigid Link and Rigid Beam, is valid in "large deformation" solutions

MODELING JOINTED INTERFACES

  Simplifications like Coupled DOF or Constraint Equations are often used to model joints in an assembly.

Rotational Joints (Hinges)

• can happen by mistake: connecting elements with different DOF's
• can by created intentionally: if hinge motion is desired by NOT connecting the rotational DOF's between parts in an assembly

BOLTS, RIVETS, PINS

• can often be modeled with coupling of translational DOF's
• or, if the hole is modeled, with beam elements and rigid "spider"

Detailed Fastener Analysis

• usually in 3D solid models
• need to consider preload in the fasteners
• uses special elements to determine contact location and pressure

Bolt Preload caused by torque (approximation)

• when torquing the bolt head: F = T/(K*D*1.2)
• when torquing the nut:  F = T/(K*D)
  • F = axial force on the fastener
  • T = the applied torque
  • K = friction factor
  • D = major diameter of the fastener

ANSYS and WB element PRETS179, defines a 2-D or 3-D pretension section within a meshed structure  [see ANSYS Basic Guide> Chapter 2. Loading> Section 2.9. Defining Pretension in a Joint Fastener]

Rotational Bearings (journal bearings, ball bearings):

• axial and radial stiffness (Figure 13.12 & 13: beam and "spider")
• or (Figure 13.14, 15, 16: 3D solid model)

Translational Joints (linear bearings, slots, pins):

• may need friction/contact behavior (nonlinear)
• if friction can be ignored, coupled DOF normal to sliding face

WBE 11.0 - "Connections", Types of Joints:

In WB Simulation Help > Using Simulation Features > Attaching Geometry > Joints

• Fixed Joint - Constrained DOF's: ALL
• Revolute Joint - Constrained DOF's: UX, UY, UZ, ROTX, ROTY
• Cylindrical Joint - Constrained DOF's: UX, UY, ROTX, ROTY
• Translational Joint - Constrained DOF's: UY, UZ, ROTX, ROTY, ROTZ
• Slot Joint - Constrained DOF's: UY, UZ
• Universal Joint - Constrained DOF's: UX, UY, UZ, ROTY
• Spherical Joint - Constrained DOF's: UX, UY, UZ
• Planar Joint - Constrained DOF's: UZ, ROTX, ROTY
• General Joint -Constrained DOF's: Fix ALL, Free X, Free Y, Free Z, and Free ALL

Obviously, the proper placement of a joint coordinate system is critical to creating the desired joint behavior.  In the "Details Area," if you click on the joint CSYS, if can be references to other surfaces and it can be oriented differently.