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Reinforced washers?

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Reinforced washers?

Unread postAuthor: saefroch » Wed May 05, 2010 10:02 am

I am still working on improving my pneumatic (will be posted on the showcase soon), which isn't quite complete because of one big issue. Operating it at 200 psi causes the piston to ram back into the adapter I am currently using as a stop with such force that the washer becomes hopelessly bent after a few shots, and causes the rear seal on the piston to malfunction. Basically this means I need to replace the rearmost washer on the piston every five or so shots. The effect is actually so significant that there is a noticeable sound difference between the first shot on a new washer and about the fifth. Suggestions on a stronger washer or some sort of mechanism to prevent any bending? My stop is currently a 3/4"-1/2" NPT adapter.

I will be getting pics up by the end of the day, if so needed.

EDIT: Picture:

Image
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Last edited by saefroch on Wed May 05, 2010 2:18 pm, edited 1 time in total.
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Unread postAuthor: grock » Wed May 05, 2010 1:28 pm

what are you using as a bumper? a better bumper usually stops your piston from destroying itself.
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Unread postAuthor: saefroch » Wed May 05, 2010 2:14 pm

... bumper? No bumper here. I'd love suggestions for one, but be warned it might get shredded.
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Unread postAuthor: metalmeltr » Wed May 05, 2010 2:18 pm

If you stack washers or get thicker ones it would be much stronger than just one thin metal washer.
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Unread postAuthor: iknowmy3tables » Wed May 05, 2010 2:18 pm

yeah make sure you have a good bumper, but If I'm thinking about the wrong part then just use multiple washers
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Unread postAuthor: saefroch » Wed May 05, 2010 2:31 pm

Suggestions for bumper material? I've got maybe two inches that the piston moves back until it is stopped to put a bumper into, and it will have to withstand a lot of force.
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Unread postAuthor: clemsonguy1125 » Wed May 05, 2010 2:56 pm

For a bumper you could cut 4 or 5 layers of thick rubber
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Unread postAuthor: saefroch » Wed May 05, 2010 3:32 pm

I guess I forgot to mention that two 1/4" nuts need to be able to go through the middle of the bumper. Will rubber still work if it's got a gaping hole in it?
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Unread postAuthor: spudtyrrant » Wed May 05, 2010 5:07 pm

saefroch wrote:I guess I forgot to mention that two 1/4" nuts need to be able to go through the middle of the bumper. Will rubber still work if it's got a gaping hole in it?
3/4" rubber washers should work just fine
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Unread postAuthor: clemsonguy1125 » Wed May 05, 2010 5:26 pm

Yeah it will, just cut out some rubber gaskets and super glue them together
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Unread postAuthor: saefroch » Wed May 05, 2010 8:43 pm

Guess that'll be another trip to the hardware store! Gonna have to stack a lot of them to get much cushioning effect, and I'll have to dismantle the gun... I think the threads are crimped together ugh. Oh well, it's worth it not to chew through washers so fast.

Thanks for all the suggestions and help! I'll be posting the cannon on the showcase as soon as I fix the issue.
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Unread postAuthor: Technician1002 » Wed May 05, 2010 9:32 pm

When your piston moves, a force is applied and Newton's laws apply. An object at rest will remain at rest until a force is applied. An object in motion will remain in motion until a force is applied.

The amount of force is directly related to the distance and time the force is applied to make the change in motion. The longer the deceleration range (longer bumper) the longer the time, the lower the force. Without a bumper, the time and distance for deceleration approaches zero, the force approaches infinity. This high force breaks things. Plan on lowering the force by increasing the distance and time the force is applied.

This short deceleration distance is how a blacksmith shapes metal, nails are driven into wood, wood is chopped, etc. Air bags in cars and crumple zones are designed to lengthen the deceleration time and distance to reduce forces on the human body to reduce injury to the soft body.

In summary, you need a properly designed bumper.
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Unread postAuthor: saefroch » Wed May 05, 2010 9:36 pm

This I know. Impact force = mass * change in velocity / time. Simple equation, classic raw egg demo. Increase time, decrease impact force, cushiony substances work beautifully.
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Unread postAuthor: jor2daje » Wed May 05, 2010 9:45 pm

Aswell as adding a bumper you should consider shortening the distance the piston travels back. Aparantly you only need to travel back 1/4 of the piston seats diameter in order to achieve maximum flow, although I usually do atleast 1/2.
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Unread postAuthor: saefroch » Thu May 06, 2010 9:49 am

Well the piston seat diameter is about .6", and if I were to only allow the piston to move back a quarter of that distance the head would probably still be sitting inside the 3/4"-1/2" bushing. Just look at the picture, and the ratio of the depth of the large bushing to the diameter of the front sealing head.

And the distance isn't causing any additional leak out the pilot, that's what the rear seal is for.
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