Simulation tip: How to stabilize your model

To constrain a model the right way can be a challenge in simulation. You want a model without rigid body motion, but you do not want it to be too stiff. The 3-2-1- method is one way to help you solve this kind of problem and get a result with the expected deformations. I have created this step by step guide on how to stabilize your model in SOLIDWORKS Simulation.

Degrees of Freedom

But first some we will refresh our memory about “Degrees of Freedom” and all that it entails.

A simple explanation is that each point on an object can move in three directions, one for each room dimension, in x, y, z direction. This type of DOF is called Translational degrees of freedom. A body also have rotational degrees of freedom. In total, a body has six DOF that all must be fixed to prevent the body from moving.
When it comes to statics, we often talk about how many degrees of freedom a body/object have. But what is a degree of freedom (DOF)?

Some of you that use SOLDIWORKS Simulation have probably encountered the problem of the system being unstable.

Six Degrees Of Freedom (DOF)

According to you and the equilibrium diagram it should be in equilibrium, but Simulation is not interpreting it that way. To get a body in static equilibrium the sum of all the forces and the moments applied on the body must be zero.
However, sometimes small rigid body motion appears which makes the body unstable – it can move – and the result ends up being inaccurate.

I have created this Simulation tip step by step guide on How to stabilize your model.

SOLIDWORKS Simulation tip: The 3-2-1 method

The 3-2-1-method can help us solve this problem. In three steps we will constrain the different degrees of freedom to move.

In the first node all the three translational freedoms are constrained
In the second node two translational freedoms are constrained
And in the last node one translational freedoms is constrained

The sum of all this is six – which is also the sum of the degrees of freedom acting on a static model. If we constrain them, we prevent the rigid body motion.

We use the model below to show the method. A material is applied on the body and two reversed forces on 1000 N each, the sum of the forces acting on the body is zero.

But when running the study, we get the message “the model is unstable”

You can do like this to solve the problem:

In the first point, all three translational degrees of freedom, x, y, z are constrained, but the point is free to rotate as a “ball-and- socket”.

In the second point we constrain the two translational freedoms normal to the line we just “walked” along, the x- and z-direction.

In the third point, we constrain the translational freedom normal to the plane containing point, 1, 2 and 3.

This results in a model that is stable and constrained in all DOF and we only get the expected deformations. With this method we also reduce the risk of stress concentrations that can appear if you constrain your model too much. This is a simple method to use when you want so to simulate a self-balanced system.