off-season launcher

[NickE]

Instead of a sprinkler valve, you might be better off with something pressure rated for compressed gases. Sprinkler valves are designed for liquid and if a failure arises, it would likely be catastrophic. For a robot that's going to promote your team in front of people, it's probably better to be safe than sorry.

We're running 6 6" KOP "sticky" wheels on our t-shirt cannon with the center wheel dropped. They work fine on a variety of surfaces.

[IanW]

While it is certainly better to use components that are rated for air pressure, I found in my own (not too extensive) research that large enough solenoid valves (1" diameter) would be prohibitively expensive for some teams. However, if you know of some source that has such valves for cheap, I would definitely look into it.

And before someone says that an catastrophic failure would also be "prohibitively expensive," I guess that's just something teams will have to consider. I suppose the other option is to just wrap the sprinkler valve in layers (and layers (and layers)) of duct tape.

If you're looking to build a t-shirt launcher, then why do what everybody else is doing and make a pneumatic canon? Our team decided to try a different strategy, that was both cost effective, crowd-pleasing, and a far shooter! Say hello to the t-shirt SLINGSHOT! Just get some surgical tubing, a long robot, and....BAM! A self-firing, enjoyable, and can shoot more than t-shirts robot! We made ours out of things we found in our robotics room.

Below is the original version (we've been working on it)!

[Da Kid]

i agree that a catastrophic explosion would be bad but we plan to put the three barrels inside of a large diameter piece of PVC so even if something does explode it will be contained.

again if someone could chime in on tank sizes(main and secondary) that would be very helpful.

and after looking around i found most of the valves are 24 volt ones. our team isn't the richest(we hope more people will sponsor us one they see this robot) is there a place to get cheap 12 to 24 volt converters. i've looked around and it seems most are around $50 that i've seen.

[114Klutz]

Originally Posted by Da Kid

i agree that a catastrophic explosion would be bad but we plan to put the three barrels inside of a large diameter piece of PVC so even if something does explode it will be contained.

again if someone could chime in on tank sizes(main and secondary) that would be very helpful.

and after looking around i found most of the valves are 24 volt ones. our team isn't the richest(we hope more people will sponsor us one they see this robot) is there a place to get cheap 12 to 24 volt converters. i've looked around and it seems most are around $50 that i've seen.

This is an off season project - why not just run 2 batteries in series? The minibot batteries are 12v each.

[OSU_Springer]

Originally Posted by 114Klutz

This is an off season project - why not just run 2 batteries in series? The minibot batteries are 12v each.

Excellent idea. I would caution that care must be taken here to not then wire the subsequent 24 VDC potential into the PD board. I'm sure that the transformer(s) there would not like the new voltage, nor would the downstream components.

To be safe, I would search places like allied electronics and newark for a 2-turn transformer. This would give you your voltage conversion from 12 VDC to 24 VDC. Just make sure that the transformer is rated for the right power (watts) or amperage, depending on how the spec is written.

[Da Kid]

Originally Posted by 114Klutz

This is an off season project - why not just run 2 batteries in series? The minibot batteries are 12v each.

i've searched around and found out that an AC sprinkler valve on a DC circuit can be run on lower voltage since DC isn't as affected by either a resistance or an inductor and thus has more current. 12 volts should be fine.

[NickE]

Originally Posted by Team2374

While it is certainly better to use components that are rated for air pressure, I found in my own (not too extensive) research that large enough solenoid valves (1" diameter) would be prohibitively expensive for some teams. However, if you know of some source that has such valves for cheap, I would definitely look into it.

And before someone says that an catastrophic failure would also be "prohibitively expensive," I guess that's just something teams will have to consider. I suppose the other option is to just wrap the sprinkler valve in layers (and layers (and layers)) of duct tape.

A catastrophic failure would be more than expensive. It has the potential to seriously injure team members or spectators regardless of how much duct tape you wrap around it. Is that really a risk you, your team and your school are willing to take to save a few bucks?

[IanW]

Hence why I put "prohibitively expensive" in quotes the second time.
I would still like to know what valve 254 uses - might make my entire point moot.

[NickE]

Originally Posted by Team2374

Hence why I put "prohibitively expensive" in quotes the second time.
I would still like to know what valve 254 uses - might make my entire point moot.

From our team website:
1" NPT MAC Solenoid Valve at ~17.0 cV Flow Capacity with a 23 ms Response Time.

For the increased reliability, safety, performance and peace of mind, buying a quality valve was well worth the investment.

[OSU_Springer]

Originally Posted by NickE

From our team website:
1" NPT MAC Solenoid Valve at ~17.0 cV Flow Capacity with a 23 ms Response Time.

For the increased reliability, safety, performance and peace of mind, buying a quality valve was well worth the investment.

An ASCO 8210G004 12 VDC solenoid valve is $218 LIST. You can probably get this cheaper if you go to a distributor and explain that you are a school related team. This is a 1" NPT valve, with a full bore orifice. If you step up to the 1 1/2" NPT 8210G022 12VDC solenoid valve, you get a much higher Cv (22.5 vs 13) but your price goes up to $405.

Both of these valves are only rated for 125 PSIG, so be careful not to try climbing above that. The biggest thing with building the air cannon is making sure that you can get an appropriate amount of air into the barrel in as short amount of time as possible. That's why it seems that higher pressures are better. You'll only need a limited amount of pressure if you can get the flow high enough. This of course also depends on your secondary accumulator (tank) volume. Make it too small and you won't have enough air to accelerate the projectile to a proper velocity, make it too large and you are just wasting energy (assuming equal barrel lengths).

A note on compressor usage: Do not expect kit compressors to last very long if you are using them constantly. They are surely not rated for anything higher than a 10-15% duty cycle. This means that they are only rated to run at the pressure rating for 10-15% of time. In other words, 6-9 minutes out of every hour. Running them more than that risks overheating the compressor and burning something out.

[Bryan Herbst]

Originally Posted by NickE

A catastrophic failure would be more than expensive. It has the potential to seriously injure team members or spectators regardless of how much duct tape you wrap around it. Is that really a risk you, your team and your school are willing to take to save a few bucks?

Out of curiosity, have you ever heard of an incident involving a sprinkler valve failure?

There are plenty of stories about and warnings against using PVC to store air, but I have never before heard someone claim that using a sprinkler valve would be dangerous.

Sprinkler valves are typically very solid. I wish I had a data sheet on hand for one of them. If someone does, could you read off the ratings?

[Travis Covington]

Originally Posted by Tanis

Sprinkler valves are typically very solid. I wish I had a data sheet on hand for one of them. If someone does, could you read off the ratings?

Typically solid when used for gardening? I don't understand the argument here. Sprinkler valves are rated for water pressure. A plastic sprinkler valve would likely fail in the exact same way a PVC storage tank would. Use a valve rated for air pressure if you are even remotely concerned with safety. Water is incompressible and behaves completely different than air when pressurized. Please refer to the countless other threads here where this has been discussed before. Bottom-line; please don't cut corners when people’s safety is at risk.

[Bryan Herbst]

Originally Posted by Travis Covington

Typically solid when used for gardening? I don't understand the argument here. Sprinkler valves are rated for water pressure. A plastic sprinkler valve would likely fail in the exact same way a PVC storage tank would. Use a valve rated for air pressure if you are even remotely concerned with safety. Water is incompressible and behaves completely different than air when pressurized. Please refer to the countless other threads here where this has been discussed before. Bottom-line; please don't cut corners when people’s safety is at risk.

Yes, they are rated for water pressure. That doesn't mean that they are at a high risk for failure when using air. It simply means that they need to be tested before being used with compressed air.

As for "a plastic sprinkler valve would likely fail in the exact same way a PVC storage tank would:" Not true. PVC used as an air storage tank almost always fails at the end caps or connection points, typically due to the a failure in the cement used to join two parts. If it isn't an endpoint, then it's because there was a crack somewhere along the body. If you are operating at reasonable pressures, a PVC tank will not spontaneously explode.

I have very carefully watched every thread on CD relating to air cannons and PVC air storage. I would never use PVC to store air. However, you may notice that every single team in those threads intends to use (and many have been using) sprinkler valves. None of them have reported any problems, and no one before has ever called them out on the sprinkler valve.

[Travis Covington]

Originally Posted by Tanis

Yes, they are rated for water pressure. That doesn't mean that they are at a high risk for failure when using air. It simply means that they need to be tested before being used with compressed air.

As for "a plastic sprinkler valve would likely fail in the exact same way a PVC storage tank would:" Not true. PVC used as an air storage tank almost always fails at the end caps or connection points, typically due to the a failure in the cement used to join two parts. If it isn't an endpoint, then it's because there was a crack somewhere along the body. If you are operating at reasonable pressures, a PVC tank will not spontaneously explode.

I have very carefully watched every thread on CD relating to air cannons and PVC air storage. I would never use PVC to store air. However, you may notice that every single team in those threads intends to use (and many have been using) sprinkler valves. None of them have reported any problems, and no one before has ever called them out on the sprinkler valve.

I still encourage you to do further research before using a sprinkler valve. While there may not be any documented failures that you have found thus far, the associated risks should not be taken lightly. Explaining in near absolutes how something will not fail is not helping your case here, either. I do not believe you know how all PVC storage tanks fail, and also do not believe you know all of the failure modes of a sprinkler valve. Objectivity is key, and building something safe which your school, students, and team can be certain won't harm anyone is my only concern. I posted initially because it seems like many people on these forums are not taking the safety of these cannons seriously. Please do not take my posts as personal attacks, because they are far from that. Using internet forums, youtube videos, and tech blogs to determine the safety of processes, mechanisms, materials, and hardware scares me and I hate seeing corners being cut with regard to safety in an effort to save a few bucks. Please just be as cautious as you can when proceeding.