Loads and Constraints

Overview#

Every simulation is a statement about boundary conditions. Constraints say where the model cannot move; loads say what pushes on it. The solver can only answer the question you posed. Most bad simulations are bad because the supports or loads do not represent the real world.

Two principles:

1. A model must be restrained against rigid-body motion (three translations and three rotations in 3D), but only just. Extra restraint adds artificial stiffness.
2. Loads should be applied where and how they act in reality, spread over a realistic area, and in the correct direction.

Where to find it#

• Constraints: Simulation workspace > Setup tab > Constraints panel > Structural Constraints (then pick the Type in the dialog). Bolt Connector and Rigid Body Connector are also in the Constraints panel.
• Loads: Setup tab > Loads panel > Structural Loads (then pick the Type), plus Edit Gravity, Linear Global Load, Angular Global Load, Point Mass commands. Thermal studies use Thermal Loads instead.
• Constraints and loads are listed in the study in the browser, where you can edit, rename, suppress or delete them.
• Structural constraints are not required in thermal, electronics cooling and modal frequencies studies.

Step by step#

Adding a fixed support and a force:

1. Open the study. On the Setup tab, Constraints panel, click Structural Constraints and choose the Type Fixed.
2. Select the face(s) where the part is held (for a bracket, the bolt-contact face or the inside of the bolt holes).
3. Choose which directions to fix with the three axis buttons (all three are selected by default). You can select faces, edges or vertices, but not mix entity types in one constraint.
4. Click OK. A constraint marker appears on the geometry (you can change marker size in Preferences > General > Simulation).
5. On the Loads panel click Structural Loads and choose the Type Force. Select the face where the force acts.
6. Choose the direction: Normal, Angle or Vectors (x,y,z). Enter the magnitude, for example 500 N (an arbitrary value for this exercise).
7. Check the arrow in the canvas points the way you intended.
8. Run Pre-check.

Options reference#

Structural constraint types (Structural Constraints dialog):

Connectors (Constraints panel): Bolt Connector (subtype With Nut or Threaded Hole, optional head and nut washers, optional Axial or Torque preload; available in static stress, modal frequencies, structural buckling and nonlinear static stress) and Rigid Body Connector (ties translational degrees of freedom of multiple bodies; also available in event simulation). Symmetry is not a constraint type: apply a Frictionless constraint (or fix the normal direction) on symmetry faces.

Structural loads (Structural Loads dialog Type list):

Direction types for forces are Normal, Angle (X, Y, Z angles relative to a reference) and Vectors (x,y,z). Other loads in the Loads panel: Gravity (off by default for a new structural study; use Toggle Gravity On and Edit Gravity; standard value 9.807 m/s2 in SI), Linear Global Load and Angular Global Load (only one global load per study), and Point Mass (Auto or Manual, to represent bodies or items not modelled; give it gravity to get a weight). Thermal loads (Applied Temperature, Convection, Heat Source, Internal Heat, Radiation) belong to thermal and thermal stress studies.

Tips and pitfalls#

Warning: General FEA guidance: fixing a single point creates a singularity where stress theoretically becomes infinite. Autodesk does show a single edge as a valid way to make a simply supported bar, so use an edge deliberately for that purpose; otherwise fix a face of finite area or use a Rigid Body Connector across the hole.

Tip: Constrain the bolt hole's cylinder using Pin, with the axial direction free if a bolt would not carry axial load through the face you did not select. A fully fixed hole is stiffer than a real bolt.

Warning: A point force applied to a vertex does not exist in reality. Spread the load across a patch (split the face in the Design workspace, or use a remote force with a rigid connection) so that the local stress is not a numerical artifact.

Tip: A force applied to a curved face with Normal direction varies to stay normal at every point (radial on a cylinder); for curved faces the Direction Type defaults to Angle. Check that this is what you wanted.

Tip: Check the unit system in the load dialog; mixing N and lbf happens easily.

Warning: If the model has less constraint than needed, the solver may report rigid body motion or produce huge displacements. Pre-check only validates that the standard inputs are present, so do not rely on it to catch borderline cases such as a part held only by friction. For static stress, thermal stress and shape optimization you can enable Remove rigid body modes in the study settings, but only if the loads and moments are balanced; otherwise the reaction forces and results are wrong.

Practice exercise#

Take a wall-mounted shelf bracket with two screw holes. Version A: fix both holes completely and apply 200 N down on the shelf face. Version B: use Pin constraints with axial direction free on one hole. Compare peak stress at the hole edges. Expected result: Version B usually gives a different peak stress and a more realistic load path; you learn how sensitive the answer is to the idealization.

Worked example: translating a real mounting into boundary conditions#

The masses, dimensions and the 2 g factor below are illustrative assumptions for the exercise, not Autodesk data.

A 3 kg camera is hung from an aluminium arm bolted to a wall by two M6 bolts, 60 mm apart vertically. The arm projects 150 mm. The camera weight acts at 150 mm from the wall.

1. Load: weight = 3 x 9.81 = 29.4 N downward, but a dynamic factor for bumps is sensible: use 2 g, so 58.9 N.
2. Applying the load: the camera's mass is distributed through a mounting plate. Use a Remote force at the camera centre of mass, tied to the 25 x 25 mm mounting patch, rather than a point force on a vertex. This introduces the 8.8 N m moment (58.9 x 0.15) automatically.
3. Supports: the bolts clamp the arm's back face against the wall. A Fixed constraint over the entire back face is stiff and unrealistic because only the area under the bolt heads clamps. One alternative idealization: constrain the back face with Frictionless (normal motion blocked), and apply Pin constraints on the two holes with the axial direction free and radial and tangential fixed to prevent sliding.
4. Reaction checks: the top bolt should carry a tensile reaction (about M / 60 mm = 8800 / 60 = 147 N in pull-out, plus shear 29 N each), and the back face should carry the compression. If both holes show the same vertical reaction and no pull-out, the model is over-stiff.

Applying loads from real sources#

The Bolt Connector converts a Torque preload plus a Torque Coefficient into an equivalent axial preload force. The common short-nut relation F = T / (K d) is general engineering practice, not Autodesk data: with T = 5 N m, K = 0.2 and d = 6 mm, F = 5000 / (0.2 x 6), about 4167 N. After adding a bolt connector, update the contacts manually: the automatic contact is Bonded, but the contact between bodies held by bolts should usually be Separation.

Direction and coordinate systems#

Force direction options are Normal, Angle (X, Y, Z angles relative to a reference entity) or Vectors (global components). Normal direction on a curved face gives a radial pattern; on a planar face it gives a single direction. When using vectors, confirm the axis orientation in the view cube, then check that the preview arrow points correctly. For a load at an angle, such as a 30 degree pull, components are F cos 30 = 0.866 F and F sin 30 = 0.5 F; verify the total with the reaction sum.

Over-constraint and under-constraint, with signs#

A reliable test: apply a small load (for example 1 N) and check that the deformed shape shows the supports acting like the real ones.

Prescribed displacement and remote loads#

Prescribed displacement is useful when you know how far something moves but not what force it takes: pressing a snap-fit tab 2 mm. Read the reaction force from the constraint after the solve to get the insertion force. A Remote Force acts from a point in space on the selected faces, and a Remote Moment gives the same stress as a Moment on the same faces wherever it is located; check that the selected faces are large enough, since a tiny face behaves as a point and brings singularity back.

Failure modes and fixes#

Practice exercise#

Quantify the effect of the idealization on the camera arm.

1. Build the arm (6 mm thick, 40 mm wide, 150 mm long) with two M6 holes 60 mm apart.
2. Case A: Fixed constraint on the whole back face; Remote Force of 58.9 N at 150 mm.
3. Case B: Frictionless on the back face plus Pin (axial free) on both holes.
4. Case C: Fixed on the two hole cylinders only.
5. For each, record the peak stress, tip deflection and the vertical reaction on each hole.
6. Explain the order of the results in one paragraph. Expect A to be stiffest, C the most flexible, and B the most realistic.

Related#

• Static Stress Study
• Contacts
• Results Interpretation