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Help with springs!

Anyone know how to calculate spring strength?
advice engineering spring

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#1 meteorhound1

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Posted 31 January 2014 - 10:31 PM

Today I managed to scavenge a spring from my robotics team. I was wondering if anyone could help me find the strength of it and if so, maybe some ideas of what I could do with this type of spring. And if anyone knows how I could find the spring constant that would be helpful as well. Pictures and specs below.
Length: ~5 inches
Full Compressed length: ~2 inches (couldn't get an accurate length because the spring kept popping out of the vice when I tightened it.)
Compressed length with an 8lb weight on it: ~4.8 inches
OD: ~1.1 inches
ID: ~.85 inches
# of coils: 13 total, ~3 per inch
Thickness of spring material: ~.11 inches
Comparison to [k26]
fmAj0zD.jpg


Edited by Aeromech, 23 November 2015 - 03:17 AM.

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#2 roboman

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Posted 31 January 2014 - 11:49 PM

Well, you can figure out the spring constant with F=kx, where k is the spring constant (lbs/inch, in this case), F is the force exerted, and x is the amount of deflection. If you divide 8 lbs by .2", you get 40 lbs per inch, assuming your measurements are accurate. That means that at full compression (3" deflection), the spring load is 120 lbs, which is almost certainly too heavy for you to prime without a system that provides a bunch of mechanical advantage.
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#3 meteorhound1

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Posted 01 February 2014 - 12:05 AM

Well, you can figure out the spring constant with F=kx, where k is the spring constant (lbs/inch, in this case), F is the force exerted, and x is the amount of deflection. If you divide 8 lbs by .2", you get 40 lbs per inch, assuming your measurements are accurate. That means that at full compression (3" deflection), the spring load is 120 lbs, which is almost certainly too heavy for you to prime without a system that provides a bunch of mechanical advantage.

Yea i calculated that but it seemed way too high. The spring is really beefy so idk what to do with it. Any suggestions on how to get mechanical advantages?
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#4 roboman

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Posted 01 February 2014 - 12:10 AM

507 Mechanical Movements is a really good resource for that kind of thing. You're gonna wanna go past all of the pulley-based stuff, obviously, unless you want a block and tackle on whatever you end up building. Take a look at the toggle mechanisms for punches, like #140, among other things.

Keep in mind, of course, that whatever you build will have to be beefy enough to stay together under the force of the spring (and the impact when you release it), and your plunger will have to be pretty big to make up for the short stroke.
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#5 Zorns Lemma

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Posted 03 February 2014 - 08:49 PM

Your measurements are most certainly off by enough to make your calculations useless. If you have a set of calipers you can calculate the theoretical spring constant through many different tools like https://www.efunda.c...mp_designer.cfm
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#6 Blue

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Posted 03 February 2014 - 10:56 PM

Seems like you are trying to base an entire blaster around a ~$5 part... by all means go for it if that's your goal but not the route most would take. Also looks like that spring is too beefy for any practical purposes nerf wise. Were there other springs like that one being used as shock absorbers on the robot or something like that?
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