Good source of springs?

[JohnBot]

Hi all,

I'm looking to purchase 2 to 4 springs with the following specs, more or less:

- Compression spring rated at about 15 to 25 pounds
- An inch or less in diameter, if possible. If not, as narrow as possible
- Around 5" or 7" in length, max

Anyone know a good source? I checked out McMaster but that's a little over the top and pricey. Plus I'd like to see it up close, if possible.

I checked home depot and other hardware stores, they have springs, but the specs are often lacking. And I'm not ready to bring them home and test them at this point.

This is going to be for a type of shock absorber. Thanks.

[waltr]

Small quantities at a specific spec will be hard to find.

You could look into winding your own springs for piano wire. Or a local machine shop might be able to wind them for you.

I wonder if there are any lawnmowers that use a spring like you need. Try a lawn mower shop.

[Soeren]

Hi,

Front fork springs from small motorcycles (what do you call the < 50CC mini types btw.) or even from Mountain Bikes would probably be in that neighborhood size-vise (although harder I guess).

[amando96]

yep, cheap walmart mtb forks could do!

[JohnBot]

Hi, I wasn't able to find Mountain Bike springs... but I was able to find die springs online. Lots of options to choose from and I basically got a few 4-inch springs that require about 16 pounds to deflect 1/8 inch. Max weight 200pds. sounded perfect.

http://www.wttool.com/product-exec/product_id/10847

[Soeren]

Hi,


They must really hate whenever this time a year shows its face with a heading like "Die Springs"  


You did see the "Load Required: To deflect 1/8", 16 lbs."?
That tells me that you need more sloppy springs than these 200 lbs types, which will only give you 0.8% deflection at 16 lbs.

[JohnBot]

hhhmmm... I did see the deflection spec. This is basically for a shock absorber to stop the action of a catapult arm. The arm will be hitting the shock absorber (made with these springs) with around 60 pounds of force. So I thought I'd use two of these. That's about 32 pounds of resistance, so to speak... my 60 pounds of force should hit these and deflect them about 1/4 inch, no?

[corrado33]

A catapult arm eh?  I built a catapult once.  It launched a couple balls about the length of 1.5-2 football fields.  It was a awesome, I built an onager though, I don't know what you're building.  This is what mine was based on..



Yeah and your arm would pretty much be hitting a brick wall if you used two of those springs.  I agree you need "sloppier/softer" springs.

[JohnBot]

wow, that pic is great. But that's based on the mechanics of a trebuchet not a catapult, correct? i.e. the floating ring, etc. But the theory is the same I guess.

Brick wall, huh? That's disappointing since my new springs just arrived. Oh well.

My catapult will be powered with surgical tubing and hit a dowel parallel to the to the ground to stop the arm. The dowel will be connected to the springs, creating a shock absorber. Can anyone recommend specs of the correct type of springs you think I need. I guess my calculation was not correct...

[JohnBot]

would one of these do... ?

http://www.hardwareandtools.com/cmpsprg49/

[corrado33]

would one of these do... ?

http://www.hardwareandtools.com/cmpsprg49/

Honestly I was just going off of what soeren said.  And the onager and trebuchet work off of different principles.  The onager uses a spring force (the rope) while the trebuchet uses weight.  (Well I guess both of them work off of the principle of torque...) I believe in olden times, trebuchets were used for long to medium range, and onagers were used for medium to short range.   

Honestly, I would just build it with the springs you have, and see what happens.  The worst that can happen is you break a piece.  I didn't use any springs or dampening equipment to slow down my arm.  With that said, we managed to break a 2x4 cut in half in a couple shots.     But the arm is very easy to replace on those types of catapults.

[MangoBot]

you can buy a variety at home depot, and a trebuchet is when a group of people pulls on strings well another person guides the ammo. The floating ring is a trait of most catapults. You could use materials such as cotton balls for absorbtion

[Soeren]

Hi,

Had forgotten this until it was resurfaced...

But I had this thought about it... If you want to absorb a shock, why not use shock absorbers?
They would need some springs to "reload" (if not built in). Some (if not all) can be adjusted to be harder or softer.

I think it would be the least damaging end stop possible by simple means, but I am not sure if it would matter to shooting power and/or precision in a noticeable way.

Another method would be to have an arrangement of a vertical bar on each side with short perpendicular bars joined by their free ends with a rope lacing and a rope "spring" like the driving force in the "Onager" artwork posted by corrado33 at the bottom of each. That would give a reaction against the catapult arm starting soft and getting gradually harder.

Btw. does anyone know how they dealt with varying humidity vs range and precision with such rope "springs"?
Natural fiber ropes like hemp and linnen change their characteristica enormously going from very dry to soaked, leather, rawhide and sinew have the same issues in that respect.

[waltr]

If you want to absorb a shock, why not use shock absorbers?

I just had a thought on this also.

What about a few layers of a gym mat. These have fairly good shock absorbing properties.

[corrado33]

Another method would be to have an arrangement of a vertical bar on each side with short perpendicular bars joined by their free ends with a rope lacing and a rope "spring" like the driving force in the "Onager" artwork posted by corrado33 at the bottom of each. That would give a reaction against the catapult arm starting soft and getting gradually harder.

I like that idea...

Btw. does anyone know how they dealt with varying humidity vs range and precision with such rope "springs"?
Natural fiber ropes like hemp and linnen change their characteristica enormously going from very dry to soaked, leather, rawhide and sinew have the same issues in that respect.

I'm gunna guess and say a few test shots.    Then again, I really don't know.  I might have to dig my catapult back out for fun.