A tension-only spring (rubber band or bungee cord)

Grasshopper Kangaroo
1

I would like to simulate a bungee cord. I need a one-way spring: it exerts a pulling force when stretched beyond rest length, but does not exert a pushing force when shortened below rest length. Is there a version of Length(Line) that does this?

2

Bungee cord do have some resistance while compressing it, but as it is really flexible, it will “crumple” instead.
Do the same.
Make your pulling spring be composed of 3 or more (the more the better) chained springs. This way it will be able to pull but will just “crumple” when compressed.

3

Thank you for the suggestion, Riccardo. In my application I don’t need to model the details of “crumpling”, so in the meantime I found what I think is a simpler solution. At Running a symmetrical simulation Daniel Piker writes “(you can also use ClampLength to create springs with non-zero rest length which act only in tension).”

So here is what I think I need to do.

ClampLength exerts no force when the length is between LowerLimit and UpperLimit. When the length is greater than UpperLimit, it exerts a spring-like force that grows linearly with “length-UpperLimit”. So what I need is to set LowerLimit equal to zero, UpperLimit equal to rest-length, and Strength inversely proportional to rest-length.
That statement about Strength comes from Daniel Piker. At How to set tension to a line when drawing centenary with Kangarroo2 he writes “Simply put, the strength is the spring constant k, which can be calculated using E*A/L Where E is the Young’s modulus (in Pa), A is the area(in m^2), and L is the length (in m)”

I would appreciate if someone can confirm that I am understanding ClampLength correctly.

2 Likes
4

See this as well:

PS: I have various things on that matter … but using solely C# … thus they could be kinda a big black box to you (unless you know how to code).

5

Yes, that’s all correct!

6

Thanks, Daniel. The mechanics of ClampLength are working fine in my simulation of a tensegrity structure, but I no longer see the lines in the output. This is because the O output of the solver produces lines for Length(Line) but nulls for ClampLength. Do you have any advice on how to obtain those lines?

7

To get the lines in the output you can connect them to a ‘Show’ component and connect that to the GoalObjects input.