Restrain Your Models Before Restraining Yourself

Soft Springs vs. Inertial Relief

Have you ever leaned back on the backrest of your chair, and then realize that you didn’t have a backrest? Have you ever leaned on a fence post that you thought was fixed to the ground, and then found out that it wasn’t fixed at all? Though the results of these “simulations” can sometimes be comedic for onlookers, if you try to simulate these types of events using Static analysis (included in SolidWorks Simulation) you will either (1) get the error message shown in Figure 1 below, or (2) it will send your model into outer space. Don’t believe me? See Figure 2 below.

Figure 1 – “No restraints…” error

Figure 2 – Under restrained model on its way to the Sun

In order to resolve this conflict, you can do one of three things:

1)  Restrain your model using Fixtures, Connections, and even External Loads (“Remote Load/Mass” option can behave like a fixture; Figure 3)

Figure 3 – Restraints

2)   Enable “Use Soft Spring to Stabilize Model” (Figure 4)

3)   Enable “Inertial Relief” (Figure 4)

Figure 4 – Study Properties

Use Soft Spring to Stabilize Model
This option tends to be fairly intuitive, but it has a few nuances. When enabled, it basically surrounds the model with soft springs to hold it in place. How soft are they?  The following stiffness value is used for the soft springs:

approximately 1.1 [ounces] per [foot] per [1 million nodes]

or more precisely (if you don't mind SI units):
1e-6 [newtons] per [meter] per [node]

The springs exert enough force that it should stabilize the model if there are any small imbalances, but they typically don’t apply enough force to noticeably influence your results.  An interesting thing to note is that the total amount of force applied by the springs is related to the number of mesh nodes in your model. This means that the finer your mesh, the more error you could see from enabling this option. If you are working with some combination of small forces (maybe a couple of lbs), fine meshes (perhaps a few million nodes), and large displacements (more than few inches) you may not want to keep this option on all the time, but this scenario is fairly rare.

Use Inertial Relief
The inertial relief option tends to cause some confusion for users. I would like to give some cursory information regarding it in this article, but a more detailed explanation can be found in my Youtube video here.

Basically, this option does something very similar to the soft spring option, but instead of applying a balancing force through the stiffness of “springs”, it applies a “gravity-like” acceleration to your model in an effort to balance whatever loads you’ve applied.  The following image (Figure 5) shows the net forces that result from this balancing acceleration on a simple model alongside the resulting displacement plot of an under restrained model.

Figure 5 – Left: Applied force (Purple) and Inertial Relief forces (Black); Right: Structural Response

The following video illustrates Inertial Relief on an unrestrained model with a simple force applied.

Using Inertial relief in conjunction with Fixtures is NOT recommended, since the forces from the inertial relief are applied alongside the stabilizing load that a fixture applies, and thus may drastically influence the results (Figure 6).

Figure 6 – Left: Study without Inertial Relief; Right: Same study with Inertial Relief (200 % difference)

Instead, it is recommended to apply inertial relief when anticipated forces applied by Fixtures are small or zero, and when the model is balanced by External Loads (with few exceptions).  A simple example of this is if you have apply a 1000 lb tensile force on one end of a bar and an equal and opposite 1000 lb tensile force on the opposite end, then inertial relief may be used to stabilize the model (Figure 7).

Figure 7 – Left: External Load Balanced (Inertial Relief recommended); Right: Not External Load Balanced (Inertial Relief NOT recommended)

Perhaps a more frequent use of inertial relief is to balance loads which are imported from SolidWorks Motion.

Overall, my recommendation for any user of SolidWorks Simulation is to run your own simple experiments with these two options with a very simple version of your model.  This should give you the best idea of how these options affect the results that are of interest to you.

Shaun BentleyShaun Bentley
DASI Solutions

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