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Hi guys, firstly congratulations on some truly terrifying contraptions, I'm consistently suprised by the quality of work some people can produce from their sheds!
Right, I was hoping for some advice if anyone can see their way to helping me out...
Let me give you the background. I am a mechanical engineering student at the University of Nottingham in England; and as part of the course I have been asked to take a previously made prototype to production quality. The prototype is a golf ball cannon that for various reasons needs to fire golf balls at a speed/distance/spin such that it would mirror a golf shot.
As it stands the prototype works reasonably well but does not have the oomph to mirror anything over a 5 wood. The current setup is a SCUBA tank attached to a regulator which in turn is attached to a 3litre "pony" bottle. This bottle is pressurised up to 10bar and attached to a fast acting solenoid valve which is welded to a steel tube of 1.5" ID. The tube is about 70cm long.
In summary that is:
3l Pony bottle (rated to 10Bar)
90bar rated hose
1.5" diameter 'Fast acting' solenoid valve (apparently it cost nearly £500)
70cm long tube
My problem is that I have all sorts of Health and Safety stuff to worry about so i would rather not change anything behind the pony bottle (all of which has been certified) also i don't have details of how fast acting the "fast acting" valve is. One last issue is that pretty much everything must be bought due to certification issues.
From what I have seen on the forum (and the software recommended on the spudwiki) I think the best way to go is to change the hose to 1.5" pipe, change the solenoid valve to a QEV and increase the length of the barrel.
My questions are such...
Where to get a 1/1.5" QEV from (UK)?
Will a normal QEV be better than a very expense solenoid valve?
Is the pony bottle setup crap enough to warrant me changing it?
Is there anything daft I am missing to improve performance?
Many, many thanks in advance and sorry for being so bloody demanding!
You don't need a pony bottle, just a standard length of (thick pipe for your safety concerns) pipe.
A QEV (piston valve) will be faster than a solenoid, and have more flow. However, you could get a more professional and substantially cheaper product if you made your own barrel sealing tee valve or made the gun in a coaxial configuration.
As far as performance goes, keep dead space (useless volume) to a minimum, reduce the number of turns the gas must take, remove sharp edges in flow, use a tight fitting (or o-ringed) piston, minimize pilot volume etc.
In order to place a spin on the projectile you will need a detent (hop-up, if you're familiar with airsoft) that is springloaded and protudes into the barrel, placing a spin on the projectile as it passes.
Do you have any requirements for rate of fire?
Seen as you live in the UK have you taken legality into consideration? For air guns you have an absolute maximum limit of 12 ft/lb energy before it must be registered as a firearm which I guarantee you WILL exceed. Would it be registered as sports gear (i.e. automatic pitcher etc.), products testing, a special effects cannon or something similar?
You'll have issue finding a 1 1/2" QEV, and the price will be massive. A barrel sealing tee would work well, especially if it can be manufactured from something more professional than PVC.
Have a look around with the search tool (advanced search, not quick search) at things like Solar's Eclipse, other golfball guns, detents, breeches and piston valves. If the configuration can be changed a QDV could be used. Tech will probably chime in about it.
You are shooting golfballes right? then you could just use a 3/4 QEV that would do fine but if money is no object (which im guessing it is) you could use a 1 inch one but you would have to use very low pressures because these guns will easily shoot a golfball 400-500 yards> Im not sure what a pony bottle is but you could just use steel or copper piping. Another thing as you are a mechanical student im guessing you have lathes drill press's ect then i would go with inonickname's idea-make you own they're not realy that hard to make you just need the right parts which can be quite hard in the UK.
I visit occasionally to make unrelated posts.
Depends on what you're trying to do. I've got a 1.5" QEV on my golf ball gun.... I wish it was bigger.
To find the performance issues, you will need to find the parameters of the materials you are working with. As far as the pony tank, the volume is fine. Simple math will show that 3 L of air at 6 bar or more will expand to completely void a golf ball barrel under pressure.
The focus on performance should move to the plumbing between the pony tank and barrel. A factor is many of these launchers is simply how air behaves at near sonic speeds. Any type of bend, edge, restriction, etc and you have a choke point that limits the flow to sonic speeds. Add a couple of these in a row and each one progressively lowers the pressure. Soon you may be delivering air through an elbow into the barrel at nearly sonic speed, but at no pressure. This compresses some in the barrel and backs up behind the golf ball.
The goal should be to deliver air under pressure into the barrel behind the golf ball where it under pressure expands in the barrel. In theory, golf ball launches near sonic speeds are possible.
The first things to look at are the controllable parameters. Are there any bends between the pony tank and barrel? How long is the path? In counting bends, count the interior of the valve.
Second thing to look for is any restrictions to flow. What is the ID of the barrel? What is the ID of all restrictions between the pony tank and barrel. Count everything from the neck of the pony tank, through any pipe, through any valve seats, to the barrel. Can anything be improved?
Third thing to evaluate is the speed of the valve. Vaves have a finite opening speed. If the valve is so slow the golf ball has left the barrel while the valve only made it 1/4 the way open, the above list of restrictions includes the restriction of a partially open valve.
To evaluate your flow, take the launcher out onto a field (to eliminate echos, remove the barrel and record the sound of the launcher. The time of the discharge can be measured by the duration of the high pressure hiss. This will give an overall duration to vent the pony tank out through the plumbing. Shorter is better. A short bang is much better than a long hiss.
Use the free software program Audacity to analyze the recording of the cannon. It can expand the waveform so you can accurately measure the pressure decay.
Firstly thanks guys for the quick and detailed response!
I will post pics of the setup tomorrow when i get back to uni...
Tackling things in order
- Agreed about not needing the pony bottle, but its there and tested so I'm gonna leave it in
- Coaxial would def be the way to go but I have packaging issues and the length of the barrel is already a bit of a problem
- The 'detent' is a nice idea but it needs to be more controllable...I have a (patent pending) design for the spin anyway so its not an issue except for the fact that the system in place takes 20-40% off the energy of the ball.
- Rate of fire is not a problem I'm looking at right now - as you picked up on, this is a fairly dicey legal issue anyway...turning it into a death cannon probably wouldn't go down that well!
- Generally Thanks for the advice! (Do you know of any QDV datasheets or similar?)
- As I said in the previous paragraph, the spin mechanism take off a fair bit of the balls' energy so big valves and high pressures needed I'm afraid!
- The planned production run is 10,000 for this item, I will make one if I need to but for the project I would also have to source and cost a manufacturer to make them in production so I'd like not to take the extra step if i don't have to. Thanks for the reply!
- Contrary to what the ladies may say, bigger IS always better
- The current setup is using flexible hose thats rated at 35 and 90 bar, 35 between SCUBA tank and pony bottle and 90 between bottle and valve (dunno about the ID but they are about 20 and 50mm OD respectively and each about 3-400mm long), I plan on changing the important one to some 30mm ID high pressure hose as i need the flexibility do you thing this will be ok?
- As a aerodynamicist I understand more than most about M > 0.4 compressibility effects, and they suck!
- Love the idea of recording the sound...I was going to use some high speed photography of the valve action but that is a much better idea
- Thanks for the expert advice!
Generally...One area I've really been struggling with in the QEV dept. is how to get a pilot air line going. I need to be able to fire this thing remotely; and for that i am going to be using servos to alter barrel pitch/yaw/power etc but i have no idea how to actuate the QEV - with the solenoid valve its easy i just pass a current through and it fires. How on earth can i do it with a QEV??
For an idea of the required power I figure I need approximately 110-120m/s muzzle velocity to get the required resultant velocity.
Once again thanks all in advance
EDIT: Also, I meant to ask about the losses having the line go into the side of the barrel rather than the end. Does anyone have any experience with the loss of power?
For a fairly simple to manufacturer golf ball launcher, getting the efficiency of the air pulse high permits the use of lower pressures. This results in lower mechanical requirements resulting in much lower manufacturing costs and higher safety. In my present line of air cannons, the Quick Dump Valve line, for safety I am using tanks (pony tank, chamber, accumulator whatever you call it, modifying it for the least restrictive flow from the tank to the barrel as possible and then building as fast as possible valve into it. This unfortunately deviates from "off the shelf" certified components, but for a large production, the certification process should be fairly painless. I am starting working with tanks DOT certified to transport propane and then operating them at less than 1/2 the working pressure.
The speed of the QDV is not difficult to calculate using GGDT. The piston is modeled like a projectile in a barrel as it receives the same acceleration forces as a projectile in the barrel when the valve begins to open. It typically opens the full stroke in the same time a golf ball will move in the barrel about 2 inches. After the first 2 inches of golf ball movement in the barrel, the QDV is fully open for the chamber, accumulator, pony tank, etc to discharge into the barrel with little flow restriction. The permits operating at lower pressure to achieve your required velocities and use less source energy. A hard yank on the rod is not required to trip this high speed valve.
The control rod can be operated by a single action pneumatic cylinder for full semi auto operation under remote control.
A basic overview on the QDV operation and theory is in this video. I hope it helps. To spill the beans, Shureshot is considering a production run of a version of this.
Just to show the flow of a QDV, this is the Audacity waveform showing 100ms of the sample recording. The sample is from 6.39 sec to 6.49 seconds of the recording. As you can see the entire discharge is pretty much completed in the 100 ms. This recording is of the 3 gallon tank with the 2 inch valve, quite considerably larger than the 3 liter pony tank in your launcher. Get a recording and compare performance.
EDIT, I added the MP3 of the tank valve test. Download the zip file and uncompress to play it. SF doesn't permit direct uploads of MP3 files.
Save the high speed photography to analyze the launches.
Remember that a hose to the barrel 1/2 the diameter of the barrel only has 1/4 the area. Due to the pressure rating on the hose, I'm assuming the hose is much smaller than 1/2 the barrel diameter. much smaller. Upgrade to a high flow large diameter low pressure solution.
Just wondering if you did make them what do you think the retail price would be because they could be a real big hit.
I visit occasionally to make unrelated posts.
I personally am not going to go into production due to the legal risks involved. If I did retail them, as a dangerous toy in the hands of drunk rednecks, the largest part of the retail price will be for liability insurance. High speed launchers are dangerous if handled improperly. The scientific community plays with this stuff on a regular basis to develop and test other products such as golf balls.
As a recreational product for general consumers, the liability is too great. It may be the reason Shurshot does not have one in production yet. They have my permission to use the design. They are dealing with the legal issues already with their launchers. They are better equipped to manufacture and retail these.
I could see in production a small one like the Marshmallow cannon going for about $600-1,000 with a t shirt barrel and golf ball barrel. The 3 gallon with a stronger tank (Disposable tanks dent easily) could easily fetch $2,000 to $2500 each retail to the testing community. They would be high value for their high power, repeatability, and durability in a testing environment.
Make the QEVs? I would have no idea but i guess about £3-4 would cover the production cost
@Tech - Great video, it answered just about all my questions. I think I will make a prototype valve myself in the style of the vid although I'm going to make it so the valve is normal to the barrel with the pony bottle in a kind of over/under config. This is to make it a little more compact and keep the servo to open the valve away from the crowded area at the bottom of the barrel. I'll post some pro/e pics in the next couple of days to illustrate the point...
Thanks again for all your help!
EDIT: Also what do you think to having a dashpot damper type arrangement for the piston after it has cleared the gap? I'm thinking it would be good to have a consistent way of slowing down what is effectively a bullet
RE-EDIT: qwerty were you talking about making the cannon or the qev? The cannon is planned to retail at £5000 but it has proprietory kit to spin the golf ball and will have remote controlled servos to control pitch, yaw, pressure and spin from software installable on a laptop - its also going to be carefully calibrated to mimic any type of golf shot required and account for wind...as for selling it - it's a research tool and so we plan on licensing its use to those who actually need it - it's the only way it can get past the health and safety executive
This is true, the area does act as a dashpot already, but due to the size of the opening for the pin on the rod used to close the piston, the bleed port is too big to effectively slow the piston on it's own. The piston does have lots of acceleration in the short travel as full chamber pressure is pushing one side so the "dashpot" doesn't do much slowing until the piston has traveled far enough to raise the pilot area to well beyond the chamber/barrel pressure. At that point there is very little volume left to act as a dashpot, so most of the deceleration energy is taken in the bumper and stopper assy.
Dashpots work well with applications with low volumetric compression. They don't work as well in high pressure uses as the distance remaining after compression is too small. At high pressure the dashpot effect is overwhelmed.
Edit, for a commercial use a magnetic induction ring can be added to the piston to safely slow it down in place of a dashpot. As a non contact brake, the decelleration curve will be highly repeatable with no wear.
To see how it would work, drop a neodymium magnet end wise down a length of 1/2 inch copper pipe. The faster the speed the stronger the brake. The ring of copper can be on the inside of a ring magnet.
I was thinking more along the lines of having the non barrel end of the piston housing effectively open to atmosphere and allowing the piston free travel down the housing as it opens until the gap is fully cleared, then having a spring - damper system to absorb the energy which would stop any kick back or any damage to the end cap. I can tune the spring - damper system to be most effective too. As for the piston design i think i've got quite a neat idea for that...will show you the design when i've done it!
EDIT: BTW as a mechanical engineer the dynamics of machines is a large part of what i do so it's right up my street!
At high speeds, this valve has been tested using a spring. Due to the construction of a coil spring and the speed the piston impacts the spring, the coil compression is not uniform. After smashing a few springs and damaging the valve, I gave up on using metal coil springs behind high speed valves. A pad with a large cross sectional area seems to work best and produces the least amount of piston damage. The forces involved in slowing the piston with a small contact area creates huge contact area forces.
Coil springs were not designed for the sudden shock wave of a piston impact. It breaks springs.
Springs provide unsafe recoil. What is required is safe energy dissipation of a high energy mechanical component. This is why elevators have huge banks of electrical resistors. They are used to dissipate the mechanical energy of an elevator to bring it to a safe stop.
Link to information to the use of resistors on electric motor controls for braking dynamic loads.
Good luck with your cannon. @tech, sureshot sells a golf ball cannon but its a combustion. Also, I think this valve of yours is the most innovative I have myself seen. Oh, and on springs, you may try extension springs as I have used with the striker. They work well and hold up fine even to 5X shots, the problem with my design is the resistence tends to be a pain and if the piston isn't constantly lubricated it will stick, escpecially since the piston o-ring is compressed rather than floating.
Nah it's easy enough to do...just has to be tuned properly...i like the idea of non contact braking just because its funky tech but i think its massive overkill tbh
For the piston im thinking some aluminium or titanium alloy or possibly structural plastic and this kind of shape...i can wrap the piston arms in springs and the block is meant to represent the damper. Once the piston clears the gap it makes contact with the springs and damper which bring it to a stop thus no kick back
EDIT: the first sentence sounded glib...apologies, i just meant that i have designed a 85kg centrifugal clutch before...the spings on that experienced 10s of kNs of force everytime the pads caught...by comparison its not so tough
Last edited by Sam@UoN on Thu Oct 15, 2009 4:57 pm, edited 1 time in total.
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