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# Problem with dynamic friction: Solidworks Motion

Question asked by Gabriel Romero on Jun 23, 2018
Latest reply on Apr 11, 2019 by Inho Sung

I have this problem with SolidWorks Motion with the subject of Mechanical Vibrations:

I need to model a mass-spring-damper model using Coulomb Damping (friction in a flat surface) with the following info:

Mass = 2.5 Kg

Spring constant = 26 N/m

Dynamic friction = 0.3 (this is for the damping)

Initial position = 0.320 m

Inicial velocity = 0 m/s

I need to find position, velocity and acceleration at t = 0.150 s.

I have demonstrated that the correct answers are:

Position = 0.3158 m

Velocity = - 0.0555 m/s

Acceleration = - 0.3408 m/s^2

Then I proceed to simulate in SolidWorks Motion this problem, but what I find is that it fails when the

dynamic friction is quite high (like this case) and I get the wrong answer. I also demonstrated that

using a force equivalent to the friction force instead of using dynamic friction works fine. But I really need to

know why it fails using dynamic friction. Here I attached the files:

The assembly "Force" is the system using the equivalent force instead of dynamic friction, which works fine.

The results from the assembly "Force" are in the excel file "DataForce" which are the correct ones.

The assembly "DynamicFriction" is the system using the dynamic friction, which fails to bring the correct answers, and the one I need help.

I will appreciate any help from you guys.