Thank you for your suggestions and your models. I've modified the elastoGap model and created a rotary version of it (can be found in the attachments). Like you said, it was pretty easy and straightforward. I got the same results as yours when I simulated the rotary version of your test setup with the rotary elastoGap model.

I've created the double-sided version of the preloaded spring and transformed it into a block. Unstreched spring angles of elastoGapRotary and spring models are written as functions of preload torque and spring constant. This way, desired preload and spring constant values can be inputted in the model parameters section directly.

Test setup:

Results with positive input torque:

Results with negative input torque:

Attachments:

There seems to be some bug in the way that SystemModeler handles pre() in this case. I sent a simple example to the developers to see what they think. I will post more information when I hear back from them.

As an alternative, I recommend you look at the "elastoGap" component and model your shaft in a similar fashion. I recommend a very large spring-damper to model the stop in your shaft combined with a softer spring (the preload) and some inertia. This would be duplicated in both directions. The stop must not be instantaneous or the simulation will likely be too stiff (this is why the large spring/damper is important).

I have attached a model that demonstrates this on a preloaded linear shaft (or plunger). I have an animated version and a non animated version. Both give the same output for the plunger.s (animated) and the mass.s (non animated). The animation helps visualize what is going on. The plunger is preloaded at 1m against its stop by a linear spring. When the external force (the green arrow in the animation) exceeds the preload, the plunger extends. As soon as the force is removed, the plunger returns to its preloaded state.

This approach would likely be more globally stable than the original approach using pre() because Modelica does not do well when there are sudden changes (like hitting a stop or limit). It is best to model them with a some compliance (like elastoGap) to avoid integration problems.

I suggest you take my model and use elastogap as a template to create a rotary version of elastogap. The code in that component is not very complicated. If you create a rotary version of elastogap, you should post it here -- it would be useful for others.

Alternatively, you can use the multibody library to build your preloaded rotary shaft from components (including elastogap) by creating a translational stop at a fixed radius and a rotary spring, an inertia, etc. (less code to write but it could be time consuming)

Regards

Model graphic:

Here is the same model without animation:

Plots of Results from both models:

Attachments: