Friction in shock absorber with variable preload

Luca Cavagna

Hello

I have an issue with defining ftiction in a shock absorber.

I modelled the airspring and damping forces as SpringDamper element.

Then I wanted to include sealing friction. So I created 2 bodies and places a translation joint.

My idea was to use the friction tab panel in the joint to include a friction which depends on the airpring value during compression.

However, there is no way to define as an expression the preload field.

Any idea or workaround?

GTT Adam

Hi Luca!

For modeling friction in shocks, I would recommend you define a force (or force pair) that acts between the two bodies that are currently being used for your SpringDamper element. That way, the friction force can vary throughout the simulation depending on the velocity, direction of travel, and other physical parameters.

If we use the "Sedan_1.mdl" as an example:

1. Create a translational force that will act between the "Vehicle Body" and "Strut tube (lwr strut)", act at the "Spring upr", and oriented using the "Spring lwr". This is the same definition as the "Coil spring" used in the front suspension.
2. For the force's Fz value, create an expression that will create a force from a STEP function, which takes the SpringDamper velocity as the input. Tailor the STEP function to match the friction values and dynamic friction values to mimic Coulomb friction trends

Adding Coulomb friction is also described in this example, found in the help documentation:

https://help.altair.com/hwsolvers/ms/topics/solvers/ms/xml-format_68.htm?zoom_highlight=coulomb

 

Thanks,

Adam Reid

Luca Cavagna

Hello Adam and thanks for the reply.

Actually this is more or less what I did. Added springdampee elements and used expression to define their main properties (adiabatic/isoth air compression and damping due to bernoulli eq. and loss factors). So I added simply an expression for friction. I used ATANH instead of STEP but they do the same job. Thanks