Inventor FAQ’s


Q1: When do I use Dynamic Simulation?


Dynamic simulation is useful throughout the design process. It can help refine your design:

  • In digital prototyping, make subtle changes that affect the design, and then see the results before you go to physical parts.
  • In failure analysis, take existing products and cycle them through simulations. Apply forces the components encounter and find out when and where they fail.

Use simulation and analysis to help you determine optimum shapes for the types of mechanisms you use.

Q2: What knowledge is needed to use Dynamic Simulation?


Helpful things to know are:

  • Assembly constraints position components in relation to one another. When doing a simulation of your assembly it is possible that the assembly constraints can over-constrain the model for simulation purposes. A typical example is an angle constraint placed as a “motor” to cause component motion when driven. Suppress those constraints before entering the Dynamic Simulation environment. Review the assembly constraints to see which ones act as “drivers” for motion, then suppress them. Leave constraints that maintain joint-type relationships unsuppressed, since they are converted to joints.
  • Automatically Convert Constraints to Standard Joints, on by default, converts mate and insert constraints into standard joints upon entering the dynamic simulation environment. This function speeds up the process of building a simulation.

If you are curious about how the constraints contribute to creating a joint, enter the dynamic simulation environment and review the list of automatically created joints. Then, in the Dynamic Simulation Settings dialog box, turn off the automatic constraint conversion option (it removes the automatically created joints) and construct joints manually. You can delete the manual joints and turn the automatic conversion option back on.

Note: When Automatically Convert Constraints to Standard Joints is checked, the only standard joint available for use is the Spatial joint. You can continue to add non-standard joints.

  • When using 2D Contact joints, we recommend increasing the time steps beyond the default 100/sec by 4X – 10X. Based on the number of steps and position of components, a 2D contact can occur between steps. The increased number of steps ensures that the contact occurs during a step.
  • Tutorials that ship with Inventor assist with understanding the basics. Go to Help Learning Tools  Tutorials  Autodesk Inventor Simulation for a list of available simulation tutorials.


Q3: How do joints and constraints differ?


Constraints are used in the assembly to place components in relation to one another. Inventor provides these basic constraints, some with modifiers:

  • Mate
  • Flush
  • Angle
  • Tangent
  • Insert
  • Rotation
  • Rotation-Translation
  • Transitional

Within the Assembly Environment, it is possible to drag parts or to drive a constraint to review motion. Only geometry is respected, and information such as velocity, acceleration, and loads are not available.

In the Dynamic Simulation environment, we use joints to obtain the results. It is possible to define dynamic parameters in joints such as friction, damping, and stiffness. There are standard joints (revolution, prismatic, spherical, and so on) and advanced joints (contact, rolling, sliding, and so on):

Standard joints are built in one of three ways:

  • Automatic Joint Conversion (default).
  • Manually convert constraints into joints.
  • Manually through a series of selections and input.

Advanced joints are built manually through a series of selections and input.

Q4: Is it possible to use Sub-assemblies?


You can use subassemblies. By default, subassemblies are considered rigid bodies. To create joints between subassembly components you must set the assembly to Flexible.

Right-click the assembly and click Flexible.

Q5: Why are all the components in the grounded folder?


In Inventor 2008, when entering Dynamic Simulation, all components were grounded. as is the case when no joints are defined.

You can look at it this way. In the Assembly environment, the first component is grounded by default. Thereafter, all components are unconstrained unless you apply constraints to them.

In Dynamic Simulation, all components are grounded until you define joints for the components. Joints define degrees of freedom. If all the components are ungrounded, then calculating a simulation is extremely time consuming and yields potentially random results.

A component that is grounded in the assembly is grounded when you go into the simulation environment. If you create an assembly using Inventor defaults, the grounded component is the first one placed in the assembly.

In the Dynamic Simulation environment, when the Automatically Convert Constraints option is OFF, all components are placed in the Grounded folder. As you add joints, you define degrees of freedom and it causes a component to move into a mobile group.

When the Automatically Convert Constraints option is ON (the default) components disperse into their Mobile groups. Components can remain in the Grounded folder based on the joints assigned by the automatic constraint conversion engine

Q6: Why is joint friction ignored during an unknown force simulation?


The Unknown force simulation is a static computation for a succession of positions. There are no velocities in the joints. The friction model of the joint follows a regularized law, depending on the velocity in the degree of freedom (the friction force is equal to 0.0 when the velocity is null). There is no friction in an Unknown force simulation. For the same reason, damping in joints are ignored (depending on velocity). An external load defined with a law based on time, in the Input Grapher, always has the same value for time = 0.0.