Constant-force springs
 

Our team is thinking about using some constant-force springs for our shooter. We are curious if you can double the springs up on the same axle.

To clarify, we are wondering if it is possible to open two springs, place them on top of one another and then let them roll into one package. Has anyone ever done this? How does it work if you have? Can you double the force in almost the same space as one spring?

Thanks in advance.

Re: Constant-force springs
 

Vulcan Spring is actually giving FIRST teams a PDV

http://info.vulcanspring.com/first-robotics-competition

They even have a design guide http://www.vulcanspring.com/conforce...-force-springs

Re: Constant-force springs
 

The Vulcan spring PDV pamphlet included in the KOP has a couple of suggestions for using multiple springs.

I don't see why you couldn't wrap them together, but it would most certainly reduce their life and also their efficiency due to friction between the springs (i.e., you wouldn't double your force, you'd be at 180% or some amount of reduction like that)

Re: Constant-force springs
 

I assume you are thinking of use 4 of the 40.9 pound springs? We called to ask them what the laminating procedure was, and they are supposed to be welded together. We decided that we couldn't do that safely and get a reliable product, so we are ordering pre-laminated ones form them for quite a bit more price and time than the stock ones. We don't have them yet to have any results.

Re: Constant-force springs
 

We tried two 40-lb constant force springs from McMaster-Carr on a prototype shooter, and the exact same design (including the same pull) with surgical tubing threw the ball much, much farther.... I think there's a speed-of-wind-up and the sheer inertia of the things to take into consideration, there.

Also, trying to interweave them proved downright dangerous last year, so we had them on the same shaft, but side-by-side. Be very careful with these things!

Re: Constant-force springs
 

1519's planning on using constant-force springs for our launcher... We used some (fiercely under-powered ones) on one of our intitial prototypes and just assembled the core of what we hope will be our competition launcher (also using constant force springs).

We're using 4 ~16lbf constant-force springs, with the potential to add smaller springs to tweak it (we already added two springs that we think are 7lbf). This gives us a total of 78lbf theoretically, which when we used our robot-weighing scale looked more like 65-72lbf. We have 8-10" of travel on this mechanism. Using human pull-back this seems to give a throw we're happy with (7+ vertical foot rise when fired straight up).

First, I'd be wary of springs that are on the order of 40lbf. We didn't inter-coil our 16lbf springs, but just pulling them back by hand to put them on a spool and bolt them to the launching mechanism was non-trivial enough that I frankly can't imagine doing it with much stronger springs (let alone 2.5x the strength!). I'm also unsure how inter-coiling springs or using larger springs affects the retracting speed... all I know is the little springs are quick enough for us.

Second, Vulcan Springs is providing a PDV for up to 6 springs. I didn't order them personally, but it sounded like it was easy to work with Vulcan Springs to get them. They were shipped promptly (next day?) and only took a few days to get to NH (I don't recall how many days). So, unless you really need the McMaster speed, I'd definitely recommend saving the $20+ per spring and use the PDV.

So far we like the constant force these springs provide!