How do Pink Arms work?

I’ve heard that pink arms are hard to engineer and I see why. I can’t find any information or technical details about how they work. I’ve seen a few CAD models but even with those I haven’t been able to quite figure it out. How do they work?


I would start by understanding how ‘normal’ elevators work. 233 PINK-style arms are sorta just an elevator packaged into one series of tubes (which makes it a lot more complex).

Do you understand how normal elevators work? If you don’t there are a couple CD threads floating around or I could find more material. If it’s just a confusion about how PINK arms package it, I see ClayTownR is replying and they could answer that better.

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A telescoping arm is, generally speaking, just a standard FRC elevator in a smaller package.

There are two families of telescoping arm in FRC: active-extension and passive-extension arms. An active-extension arm extends by means of a motor pulling a rope, chain, or belt up, much like a cascade elevator. A passive-extension arm, such as the telescoping kit sold by WCP, uses constant force springs to extend the arm (and only motor power to retract it).

In the circumstances of this year’s game, I would generally recommend having an active-extension arm. These arms are, to shamelessly misappropriate a mathematical term, isomorphic to cascade elevators. Each stage is pulled up by a rope, chain, or belt. The first stage is actively powered up by a single loop (or pair of half-loops), and the later stages are indirectly pulled up by the same clever rigging that makes cascade elevators work. There’s no reason you couldn’t also do a continuous elevator, but space limitations often interfere with that.

The main challenges in building a telescoping arm are bearing design and energy chain routing. Both require significant thought in packaging and DFM/DFA.

I can give a detailed walkthrough of an arm I’ve designed before if needed - let me know.


Yeah for the most part and I’ve worked with the climber in a box from AndyMark. I figured it was something like that, but because some of them are so compact is was hard for me to piece together.

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Thank you for describing the pink style arm, it’s new to our team.
Why do you recommend an active extension arm? Is it due to the loading when in a horizontal position that can bind the arm?
We have modified our arm to have bearings at both ends and so far the constant force springs seem to still work with a 5 lb load on the end of a 28" extension.
Thanks again for the added details. This is a new one for us and have been bit in the past for not understanding our design limitations.

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I recommend an active extension for this sort of game because the constant force springs are a limiting factor on the speed of extension. If you want to get really good cycle times, you’ll need a really fast telescope - less than 1 second on the extension. If you use constant force springs, the forces required to get that extension are quite large (making the assembly bulkier and also more dangerous to use). I also believe that CF springs are the most dangerous parts of most FRC robots and so should be used sparingly and with some safety-consciousness.

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