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COMBIN40 As Simplified 1D Friction

Spring that Imitates Static/Dynamic Friction

Sometimes, a simple linear spring is not adequate. In tough jobs where a spring with a built-in gap or force saturation is needed, COMBIN40 is the spring of choice. It could behave similarly to a one dimensional friction contact like in Wikipedia:
Static to Dynamic Friction

In the animation above, the spring is actually two COMBIN40 springs in parallel. When done just right, they could imitate the behavior of static to dynamic friction forces:
Spring Forces

To do it in Mechanical, here's the tip! Under Geometry, you'll have to change the Element Control setting to Manual. This allows the spring element to be modified.
Element Control

Next, two identical springs are added (parallel springs). The snippet for the first spring:
Spring #1 Snippet

The second spring snippet:
Spring #2 Snippet

The snippet has two lines of code. The first line changes the element to COMBIN40, the second line defines the stiffness (K1) and the amount of saturation force. A negative value drops the force to zero after the absolute value is reached. In the above example, 200N spring force drops to zero while the 100N saturated force remains. 

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