Assumptions in Simulation

Depending on your simulation needs, there are different levels of SolidWorks Simulation that account for different levels of reality. With any simulation package, I believe that we are forced to make some assumptions due to the complex nature of reality. For example, it can be shown that all the stars in the universe exert a small amount of gravity on us and everything we design.  If I were designing an engine fixture, I believe it would be reasonable for me to make the assumption that the gravity exerted on my fixture by all these stars can be neglected. If I didn’t make this assumption, this would make the problem very difficult (or impossible) to solve. By making this assumption, I simplify my problem substantially, and I may not notice the difference when I look at my results.

The purpose of this article is to illustrate some (but not all) of the major assumptions that several different simulation packages make. It will be up to you to decide whether or not these assumptions may be reasonable for your applications.

Structural Static (with SolidWorks Simulation)
Assumptions: Deformable body and without acceleration

Static analysis is a particular type a structural analysis that assumes that accelerations are equal to zero. This assumption makes the calculations much simpler and easier to solve. Generally, this assumption means that when forces are applied, the body does not move freely in space. Instead of moving, the forces cause the body to deform allowing the user to see the resulting stresses, strains, displacements, and other results.

Example of a SolidWorks Simulation output:

Static Analysis

Kinematics and Dynamics (with SolidWorks Motion)
Assumptions: Undeformable body and with acceleration

SolidWorks Motion allows the user to simulate movement in assemblies. One of the major assumptions that SolidWorks Motion uses to make calculations easy is that all the bodies in the assembly are rigid. This means that when a force is applied to a body in SolidWorks, the body does not deform in any way. The flexibility of your components is not taken into account. Instead, when forces are applied, bodies are able to move with respect to one another.

Things like springs, dampers, impact, etc. can still be simulated using the various other options in the interface. Also, the user is able to simulate a static analysis on a frame-by-frame basis on their components.

Example of a SolidWorks Motion output:

Motion Analysis

Structural Dynamic (with SolidWorks Simulation Premium)
Assumptions: Deformable body and with acceleration

It can do almost everything that some of the other analyses cannot, but it can be very cumbersome to do some types of problems since it accounts for a higher degree of complexity. In this analysis, bodies can move freely in space and deform.

Example of a SolidWorks Simulation Premium output:

Nonlinear-Dynamic Analysis

In some scenarios, it may not be clear what assumptions you can make. For these scenarios, my suggestion is to try running the simulation using a few different options. If you see a big difference in the results, this should give you some insight into the assumptions you may need to work with on other models as well.

Shaun BentleyShaun Bentley
DASI Solutions

This entry was posted in Tips & Tricks and tagged , , , . Bookmark the permalink.

4 Responses to Assumptions in Simulation

  1. Ana says:

    I’m so exciting your nice work. Very very good article.

  2. Heinrich Meier says:

    Dear Shaun Bentley,

    What linkages do you use in your nonlinear example?
    Everytime I use standard pins there is a reaction momentum
    at the pins blocking free motion.

    Your example seems to be the only working one performing a nonlinear, dynamic studies on a mechanism. Can you tell us how to do it?

    Btw we discussed this in a german forum and we are a bit frustrated at the moment. Thank you so much.

    • Shaun Bentley says:

      I used fixed hinges and applied a rotational displacement of 4pi. Honestly, this method was tricky and required troubleshooting to get it to solve all the way up to 4pi.

      I seemed to notice the same moments that you may be describing. The fixed hinge (to some extent) allows the free tangential movement of the face normal to the rotational axis. For small displacements, this approximates rotation nicely, but when displacements are large, you bump into strange behavior.

      Perhaps a better alternative is to explore the “Remote Loads/Mass” and select the “Displacement (Rigid Connection)” option. With this option, you can connect the cylindrical faces to a point in space using “rigid bars” (as it says in the help file). The point in space should likely be located in the center of the cylindrical pin location. It’s difficult to describe. It may be much easier to send you a model with this applied to it so you might see what I’m talking about.


      • Heinrich says:

        Hello Shaun Bentley,

        Sorry I have not recognized your response as I did not receive a notification email.

        Well I also tried it with the displacement-method. But even with many trials in solver configuration this then causes strange behaviors at the pins between the body which seem to show some similarities to that.

        It would be so great if you could send a small example of the alternative approach you described as I fear to not understand it properly. And we are discussing this now since weeks here:

        my email: ( can you remove it from this post after sending ?)

        Best regards and have a nice day


Leave a Reply

Your email address will not be published.