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Help me with a Mauler Design Change

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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Help me with a Mauler Design Change

Unread postAuthor: sgort87 » Wed Aug 04, 2010 5:30 pm

Okay, so I've been doing a bunch of changes with my Maulers, mainly with the piston. It now features reduced OD in certain parts of the piston - bringing the weight to about half of what it was (about 2oz now). It has a single floating o-ring for WAY better piston movement and sealing, as opposed to 2 o-rings with grooves cut deep enough for the rings to seal when stretched onto the piston. It also uses the o-ring for one-way sealing. Air pushes the ring forward and can leak by to fill the chamber, but when the valve is piloted, the o-ring pulls back and seals perfectly and effortlessly - MUCH better than any check method I've used for pistons before. The back plug also now features a floating o-ring, which makes removing the back much easier.

Now this is where you guys come in. I want to get rid of the screws. My plan is to use an internal snap ring to hold the back in. What I'd like to know from you engineery types, is if the tee would hold up to the forces when a groove is put into it, how big the groove can/should be, and how strong the ring needs to be. And since there wont be screws, do you have any good practical methods in mind for keeping the back plug from spinning when threading or unthreading the pilot assembly?
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Re: Help me with a Mauler Design Change

Unread postAuthor: SpudBlaster15 » Wed Aug 04, 2010 8:44 pm

sgort87 wrote:What I'd like to know from you engineery types, is if the tee would hold up to the forces when a groove is put into it


Depends on the depth of the groove. To figure out how much strength you'll have at the groove, you'll need to find the cross sectional area of the grooved pipe, then multiply it by the tensile strength of the material. This will give you the force required to produce an axial failure of the pipe. The equation will look something like:

F = S<sub>tensile</sub>*(pi*r<sub>outer</sub><sup>2</sup> - pi*r<sub>inner</sub><sup>2</sup>)

The outer radius is obviously the outer diameter of the T fitting, and the inner radius would be the internal diameter/2 + the ring groove depth.

From there, you can determine the force applied by internal pressure in the pilot chamber, and figure out how deep the groove can be before it starts to seriously compromise the strength of the housing.

...how big the groove can/should be...


I'd say as deep as you can make it while maintaining a safety factor of at least 3 or 4 on the PVC's axial failure pressure.

...and how strong the ring needs to be.


With a SCH40 PVC hosuing, I can't foresee a situation in which the steel snap ring/C clip will be the weak point in the system. I'd worry about the strength of the T first, and make sure it will handle the stress effectively.

And since there wont be screws, do you have any good practical methods in mind for keeping the back plug from spinning when threading or unthreading the pilot assembly?


Perhaps you could make 2 provisions in the end plug, and use an angle grinder wrench to hold it in place when twisting the pilot fitting.

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Re: Help me with a Mauler Design Change

Unread postAuthor: ramses » Wed Aug 04, 2010 9:59 pm

SpudBlaster15 wrote:The equation will look something like:

F = S<sub>tensile</sub>*(pi*r<sub>outer</sub><sup>2</sup> - pi*r<sub>inner</sub><sup>2</sup>)



You also need to consider axial stress. I'm not sure if the stresses need to be added, a real mechanical engineer could tell you. Use yield stress, not absolute tensile strength.


BTW, I have that same angle grinder wrench. Great idea to use something similar on this, but it would have to be supplied to the consumer. It's not like you could just tell them to go buy the angle grinder from harbor freight and use the wrench.
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Re: Help me with a Mauler Design Change

Unread postAuthor: SpudBlaster15 » Wed Aug 04, 2010 10:15 pm

ramses wrote:You also need to consider axial stress. I'm not sure if the stresses need to be added, a real mechanical engineer could tell you. Use yield stress, not absolute tensile strength.


That equation is designed to model axial stress, hence the pipe wall cross sectional area calculation. Axial stress is the main consideration here, as I don't believe the groove would significantly alter the cylinder's hoop stress bearing ability.

Using yield strength is typically a good idea, but as rigid PVC is very brittle and doesn't undergo significant plastic deformation before experiencing tensile failure, the ultimate and yield strength values will be very similar. It's still a good idea to use yield strength however.
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Unread postAuthor: ramses » Wed Aug 04, 2010 10:24 pm

oops, I meant hoop stress. :oops:

Hoop stress is likely much greater than axial stress.
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Unread postAuthor: Technician1002 » Wed Aug 04, 2010 11:57 pm

I am not an engineer. I am a tech and work with engineers so the info I have is limited on all the stress maps and material strengths etc.

I wanted to drop in my 2 cents worth and say great job on the improvements to the o rings and such. I didn't know the rings were not floating on the original piston. Using them for a one way valve is a nice touch.

The big issue I find with PVC is it's lack of durability for repeated impacts. The snap ring sounds like a good idea for steel, but I have my concerns when used with PVC. It is a narrow ledge in a shallow groove. It may be fine for a static load, but in this application as a retainer for the plug the piston strikes, i worry about material failure, even with a bumper.

For an example of the issue, I started a small thread on the PVC failure I recently had on my Marshmallow cannon. The failure was far from the piston, and on the other side of an assembly behind a bumper. The cannon has a low mass piston. Over time, the part failed.

I don't have a drawing of a proposal, but i think if the entire valve was placed in a sandwich of 2 plates, and the plates have bolts between them, the valve would be much stronger and the PVC would not experience axial forces on the PVC. The outer steel frame would provide compression In summary the assembly would be held together like an electric motor with bolts that go from one end plate to the other. With the external frame the PVC would have no tensile forces that would expand a crack or fatigue.

This took a year of hard use before it failed. The PVC part has been replaced by a black iron pipe cap. Sorry no photos yet.

My thread on the failure is here;
http://www.spudfiles.com/forums/pvc-is-fragile-t21334.html
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Unread postAuthor: sgort87 » Thu Aug 05, 2010 8:27 pm

So I thought over everything and figured out a way to reconfigure the valve so a snap ring can be put into a deep groove without affecting the tee.

I'll have to extend the 2.5" sch 80 piston track back, use a section of that pipe to house the plug, and groove the back of the pipe to hold the ring. This way I can groove it a 16th or 8th inch and not have to worry a bit about the integrity of the valve.

I'd also be using a sticky rubber sheet affixed to the back plug instead of a wider diameter one sandwiched between the plug and the piston track pipe.

As for the spinning problem, the grinder wrench would work, but then it would allow the piston assembly to swing around after the build is over. I think I'll go with a single set screw threaded into the plug and sticking into the tee/track.

I like where this is going. I'll get you guys some pictures of the recent and future updates.
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Unread postAuthor: grock » Fri Aug 06, 2010 11:10 pm

something i did to make my piston servicable was make use 2 triangularish plates with 3 threaded rods connecting them, to hold the back cap on. the front plate centered on the barrel, and the back plate was more or less centered by the front plate. check it out

unfortunetly, it would raise the price of the valve quite a bit, maybe oyou could offer it as an option, it works pretty well
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Unread postAuthor: jrrdw » Sat Aug 07, 2010 5:33 am

I applaud you on your inovation, your all ways thinking of a better way...but...getting rid of a static mounting system (screws) moving to a pressure rated system (snap ring) is removing safty from the design.

Imagine a 2" PVC plug popping out of the valve at 100 - 125 psi. Wouldn't want to get hit by it...

What about some type of cam/lever locking device? I do understand why you want a internal plug holder. No unsitely fasteners, quicker disassembly etc...
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Unread postAuthor: evanmcorleytv » Tue Sep 27, 2011 7:21 pm

I just bought a mauler a few months ago.. (or at least I received it a few months ago..) I was 80ecorl@gmail.com.. BUT, would I be able to send in MY mauler for modifications? BTW.. I have designs for the cannon.. I just need to earn some more cash..
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Unread postAuthor: mattyzip77 » Tue Sep 27, 2011 7:52 pm

Sounds like a great idea. I was going to buy a mauler from you, however you shut down shop. Any chance the new ones will be for sale??
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Unread postAuthor: MrCrowley » Tue Sep 27, 2011 8:00 pm

mattyzip77 wrote:Sounds like a great idea. I was going to buy a mauler from you, however you shut down shop. Any chance the new ones will be for sale??

This topic is over a year old. Store still shut down :wink:
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Unread postAuthor: mattyzip77 » Tue Sep 27, 2011 8:12 pm

MrCrowley wrote:
mattyzip77 wrote:Sounds like a great idea. I was going to buy a mauler from you, however you shut down shop. Any chance the new ones will be for sale??

This topic is over a year old. Store still shut down :wink:


Son of a biscuit, gosh darn noobs!!! :x Myself included! :lol:
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Unread postAuthor: MrCrowley » Tue Sep 27, 2011 8:14 pm

mattyzip77 wrote:Son of a biscuit, gosh darn noobs!!!

Well that 'noob' actually had a pretty legitimate reason for kicking up the topic. I saw his post before you had replied and decided to leave it.
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Unread postAuthor: mattyzip77 » Tue Sep 27, 2011 8:24 pm

MrCrowley wrote:
mattyzip77 wrote:Son of a biscuit, gosh darn noobs!!!

Well that 'noob' actually had a pretty legitimate reason for kicking up the topic. I saw his post before you had replied and decided to leave it.


Agreed, never said it wasnt. As always, you are right on top of your game Crowley! :happy2:
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