# Thread: Calculating the Shape of an Arc

1. ## Calculating the Shape of an Arc

Evening all. I'm not an engineer, so forgive any mis-applied labels.

I'm trying to design something that will depress two opposing spring-loaded pins simultaneously. So far, the idea I'm working with is a plate with grooves/arcs in which the pins sit (kind of like a ying/yang shape); when you rotate the plate, the pins slide along the arcs, pushing them together/compressing the spring. My question to you all is, if I need to depress the pins 1 inch, and the plate has a 3 inch diameter, how do I figure out the shape/curve for the arcs needed to move the pin over the length of the arc it travels.

Cheers - Seth

2. Hi Seth,

Welcome to the forum. Just decide on a reasonable rotation in degrees and then use two arcs that will give the lift in that desired angular rotation. The fewer degrees the harder it will be to compress the springs so the more torque the motor will need to do the work. Just select a degree of rotation and an offset from that center to the "cam" grooves.

Kinda like the pic attached, but you are not limited to 180deg, just make the total offset of the rotational travel of the "cam" to be 1" of lift from it's starting point. This design will probably bind as the working pressure from the "cam" will be at 90-deg to the movement of the spring, so not a good thing. Perhaps if you can give us a better idea (sketch or picture) of what you are trying to achieve we can give you a better idea on how to do it.

eedge sprint cams 01.png

3. Thanks Pinkerton. That makes a lot of sense to me, and seems a lot easier/more straightforward than trying to come up with some kind of graduated arc.

Since you're curious, I'm trying to make a rubber-band powered tennis ball launcher. This is for the locking/trigger mechanism. The idea was for the spring-loaded pins to pop into holes drilled in either side of the tube to lock it in place, and then this cam mechanism to push in the pins and release the band with the ball. I have thought about doing it the other way, which is to say two pins facing the inside of the tube, and a catch attached to the band pushes past the pins, locking it. It seems like I could just use the cam idea in reverse to push the pins up and away from each other, rather than down and toward each other. Doing it that way might also solve one problem I was thinking about, which is how the piece with the pins would behave as it travels down the tube after being released. Seems like it would add some unnecessary binding/friction to the whole thing.

Diagram 1.jpg Diagram 2.jpg

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