Help sourcing a spring

[bobgaggle]

Hi all, I'm building a scaled up version of a pin ball plunger. I need a spring of some kind to pull the plunger back into the body. Somewhere in the realm of 9" stroke length. Everyone wants to go with a gas spring to avoid the thing slamming back, but finding one that's light enough is proving tough. Screen door closers are too heavy to comfortably pull, and mcmaster and grainger are scant on lightweight (5lbs or less) gas springs with a long enough stroke. Extension springs are also proving hard to find, load rates climb the longer the extension length and get heavy at the end of the stroke. Not sure if im overthinking all this, any ideas?

 

[sk8rsdad]

Have you considered treating the 9" spring as decorative and using something else, like surgical tubing, to act as the actual spring?

 

[bobgaggle]

Now that's interesting. something stretchy like rubber tube or elastic. I like that. funny because there's actually a fake spring in the design of the thing. were using a recoil air hose that fits around the inner tube perfectly. The actual spring would live inside the inner tube.

 

[venuetech]

I never have tried this but there are a number of videos out there telling how to make your own spring.

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Is just one

Or
https://www.thespringstore.com

 

[kicknargel]

McMaster has Reducible-Force Gas Springs that allow you to bleed off the gas to get the force you want. Obviously a gas spring doesn't get the snappy motion of a pinball plunger.

https://www.mcmaster.com/gas-springs

 

[RonHebbard]

McMaster has Reducible-Force Gas Springs that allow you to bleed off the gas to get the force you want. Obviously a gas spring doesn't get the snappy motion of a pinball plunger.

https://www.mcmaster.com/gas-springs

@kicknargel Mr. Nargel; you may find this link of use: https://www.google.com/search?sourc.........0i131j0i10..11:1j12:16j13:0.78kWOSHy6Fk
Up here north of the walls we have Spaenaur. Spaenaur are like your McMaster Carr on steroids. Spaenaur's clients are companies such as Home Depot. Sparenaur don't normally sell to end users but they are EXTREMELY good when it comes to handing out one or two samples gratis. When I was with the Stratford Shakespearean Festival we had most of Spaenaur's catalogues. Spaenaur's warehouse was only 40 minutes from the festival. If / when we only needed one or two of something we'd contact Spaenaur and have a sample packaged and held for pickup at their pickup counter then one of our drivers would stop by and fetch our "samples" free of charge.
Toodleoo!
Ron Hebbard

 

[venuetech]

Everyone wants to go with a gas spring to avoid the thing slamming back

I think slamming back is about 80% of the action for a pin ball plunger, why get rid of something that is so much part of the game?

 

[JonCarter]

@RonHebbard For any amateur company there's nothing like program credits to get problems of all kinds solved and free stuff of all kinds donated. Most of the time all you need is a prop master/head carpenter with a phone book and chutzpa.

 

[DrewE]

Maybe (besides the fine suggestions already given) you could use a pneumatic cylinder connected to an accumulator that you pressurize to whatever pressure works out well. You'd likely have to pump it up with a bicycle pump or similar before the show, and quite possibly install a shut-off valve if the cylinder leaks more than a teensy bit. Do make sure that whatever materials used here are suitable for compressed air use (e.g. not PVC pipe).

Another option might be to use a screen door or other spring and a cable and pully arrangement to reduce the force. That would also permit you to use a shorter spring.

If this is actually going to shoot a ball, you presumably don't want too feeble of a spring.

 

[jonliles]

Maybe (besides the fine suggestions already given) you could use a pneumatic cylinder connected to an accumulator that you pressurize to whatever pressure works out well. You'd likely have to pump it up with a bicycle pump or similar before the show, and quite possibly install a shut-off valve if the cylinder leaks more than a teensy bit. Do make sure that whatever materials used here are suitable for compressed air use (e.g. not PVC pipe).

Another option might be to use a screen door or other spring and a cable and pully arrangement to reduce the force. That would also permit you to use a shorter spring.

If this is actually going to shoot a ball, you presumably don't want too feeble of a spring.

Whats wrong why using PVC for pnuematics. Even household stuff has a 40psi rating stamped on the side of the pipe. The requirement is to stay under the stamped rating. They make many varieties that are rated for much higher (at smaller diameters)

 

[DrewE]

Whats wrong why using PVC for pnuematics. Even household stuff has a 40psi rating stamped on the side of the pipe. The requirement is to stay under the stamped rating. They make many varieties that are rated for much higher (at smaller diameters)

The danger is not that it can't generally take the pressure (it generally can); but rather, should it fail due to impact or becoming brittle with age or being cold or whatever, it does so by very rapidly breaking into sharp little shards. With water pressure, the moment that happens the local pressure on the shards drops to basically zero (water is non-compressible, and has inertia, so it takes a bit for it to start moving outwards) and these shards just kind of fall down and you have a big wet mess. With compressed air, the air drives them out at very high velocity, posing a genuine risk to your fleshy bits.

That's why PVC cement usage instructions specify not to test the pipe joints for leaks with air pressure, and why OSHA does not permit its use for compressed gasses in above-ground applications.

Some other plastics (at least some forms of ABS pipe, for instance) are entirely suitable for air pressure.

 

[RonHebbard]

The danger is not that it can't generally take the pressure (it generally can); but rather, should it fail due to impact or becoming brittle with age or being cold or whatever, it does so by very rapidly breaking into sharp little shards. With water pressure, the moment that happens the local pressure on the shards drops to basically zero (water is non-compressible, and has inertia, so it takes a bit for it to start moving outwards) and these shards just kind of fall down and you have a big wet mess. With compressed air, the air drives them out at very high velocity, posing a genuine risk to your fleshy bits.

That's why PVC cement usage instructions specify not to test the pipe joints for leaks with air pressure, and why OSHA does not permit its use for compressed gasses in above-ground applications.

Some other plastics (at least some forms of ABS pipe, for instance) are entirely suitable for air pressure.

@jonliles and @DrewE Decades ago, pipe for water sprinkler systems was ALWAYS threaded schedule 40 iron. Now a days, in Canada, fitters are installing a yellow PVC pipe identified as "Harvel (Reg’d) Blazemaster (Trade Mark) Sprinkler Pipe " which is specifically manufactured and approved for the purpose. The yellow pipe is tested in accordance with what @DrewE posted. When initially installed, the system is filled with air to a specific (low) pressure and checked for leaks for 48 hours. If the system has maintained air pressure for at least 48 hours, the air pressure is increased and it must retain a specific, higher, pressure for a further 48 hours. If everything has hung together the system is finally filled with water and all air bled out before it's deemed ready for operation.
Being the inquisitive electrician / non-fitter I am, I queried the fitters and had "fragmentation" explained to me along with water escaping appreciably slower than air.
Toodleoo!
Ron Hebbard

 

[jonliles]

@DrewE is only half correct. The stated OSHA memorandum is from 1989 and revised in 2004. Stated memorandum requires protection from the PVC Shatter. The selection of newer non-rigid PVC (newer engineering) should be acceptable and compliant with OSHA and ASME or which there are quite a few applicable PVC products. And no, rigid PVC or rigid ABS should not be uses in pnuematics.

 

[Michael K]

Some interesting ideas bouncing around here which I do not have enough knowledge to contribute to. However, going back to the original question, if you're aiming for a 5 lb draw at 9 inches, you will need a spring constant of around 0.56 lb/in, which means this spring would do nicely (and being 36" long gives plenty of wiggle room for length and multiple cuts): https://www.mcmaster.com/9662K23