Why can't a three point anchor be calculated?

RigRite works by using vector mathematics to analyze how angles affect forces in a system. There are some things that simply cannot be worked out from angles alone, three (or more) point anchor systems is one of them.

Take the following example. If you were to rig A and C so that they support the load then gently hold B in place there would be no way to know from the angles alone that there was almost no force on B. Likewise, you could hang the load on B alone and gently hold A and C in place. The angles look the same but the loading is very different.
http://rigright.s3.amazonaws.com/3point.png

There are however some things we can tell from the angles. In this diagram we can see the maximum and minimum forces that could be on each anchor given the angles. In RigRite it would probably look something like this:

http://rigright.s3.amazonaws.com/3point-rigrite.png?_=12

What about showing how the forces would be distributed if the system was set up optimally?


Even Loading

It is technically possible to have an equal force on all three anchors, but only when the system is completely symmetrical as in the first diagram above. As soon as one point moves sideways (like the second diagram) it’s no longer possible to load the anchor equally.


Optimal Loading

So we can't achieve even loading, surely we can show how they would be loaded is the system was set up as optimally it can be, right? Sorry, it depends on what you consider optimal. The following are all ways to load the second anchor above. The angels remain the same but the tension and therefore the force on each anchor is different:


  • The most even way to share the force is 62% - 24% - 24% but this has a total anchor force of 110%

  • The lowest total force can be achieved with 49% - 57% - 0% total anchor force is 106% but the third anchor is unused.

  • You can minimise the force any one anchor experiences with 51% - 51% - 5% but the third anchor is still almost unused.

  • 71% - 0% - 43% is also possible but the center anchor is unused.


If you'd like to see three-point anchors in RigRite showing a load range please vote for this feature to let us know. 

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