Spring Idea

[Captain Galaxy]

I had an inspiration about a new desing. It basically adds a spring to the LPD. I don't know if it's ever been tried before but I think i could be very effective. What do you think?

[aEx155]

I think this has been talked about before in another thread. Basically, a spring wouldn't be able to replicate the amount of force that air can for the same price (remember, air is free).

Adding a spring would just complicate things further, and it would be harder since you would have to support the uneven force of the spring. Also, while you don't have to replace air, you can;t say the same for springs.

[Captain Galaxy]

The design wouldn't be completely spring-dependent. The PC would have a piston similar to the one in the Supercannon II, with a spring behind it. The PC would be completely sealed, so the air on the other side of the piston would also be compressed.

[cantab]

Well I suppose it WOULD boost power compared to using air alone. But you'd be putting extra, uneven, forces on the piston and endcap; you'd need to be careful that they could withstand those forces. And you could gain the same effect by just increasing the air pressure in the air side.

Another drawback to using a spring is it constrains where you can put the piston. With just air, you can have it where you want - holding more air for greater constancy of power, or more water at the cost of greater power drop. With a spring, the spring sets that position.

[Silence]

A spring can help make a compact LPD water gun, since you don't need a massive air chamber. But springs with enough force (10 square inches * 50 PSI = 500 pounds, and that's a conservative estimate) are dangerous, heavy, expensive, and prone to rust.

With a small chamber, where the diameter (and spring force) isn't high and you don't have room for air, then it's possible. Just make sure you keep an eye on practicality.

[Ben]

I started writing something short, but it became much longer.

While a spring water gun in an LPD configuration would work, there's a few reasons why it's not ideal. As previously mentioned, springs can be expensive and air is free. While using both will provide a boost, so will simply increasing the pressure. I considered this earlier but when I started looking at the cost of appropriate springs, I decided it would be simpler to use air.

The main problem with spring LPD design is that it'd get very lengthy unless you use wide diameter pipe (which decreases you pressure). LPD design works because the displacement is small enough to make a small difference in force applied. To do that with a spring you need a long spring, and you can't fold springs like you can with air chambers, so the system will be long by it's nature.

There's no advantage of springs over air pressure aside from the fact that you can store the spring system without having to depressurize the air chamber. Air also offers you the option of selecting what pressure you want to operate at. With springs the procedure to change the operating pressure would be very difficult.

The most reasonable spring LPD system would use a moderate diameter piston. You want a large diameter piston to keep the displacement low but you also want a small diameter piston to keep the surface area low so the force creates as much pressure as possible (pressure = force / area). Something in the middle (let's say a 3 inch piston) gets some of both. As for the spring, the spring will have a low spring constant and very long length so that the total force increase for the displacement is rather small.

Selecting the longest spring McMaster-Carr offers, I can see how the design would work. To get a measly 20 PSI, you need to compress this spring 24 inches. This will give you about 9 inches of travel (which translates into about a liter of capacity). The maximum pressure would be 28 PSI when the spring is compressed 33 inches (nearly full compression). The %drop (a measure of how constant the sytem is--lower is better) would be about 28%, compared against 20 to 30% for LRT (it varies) and 25% for the first LPD system I made. As you can see, this spring isn't much better than one layer of LRT in pressure and the same in %drop. Using two springs is an option, but that would cost $20 and wouldn't get as high pressure as an air-based LPD system.

Obviously McMaster-Carr's selection is far from the most comprehensive, but I'd imagine more appropriate springs would cost a lot more than the $10 one I used above for the calculations.

Constant force springs (tape measure-like) are an option too but the appropriate ones are expensive and you have to use a pulley system to get reasonable pressure.

[Silence]

Originally Posted by Ben

The main problem with spring LPD design is that it'd get very lengthy unless you use wide diameter pipe (which decreases you pressure). LPD design works because the displacement is small enough to make a small difference in force applied. To do that with a spring you need a long spring, and you can't fold springs like you can with air chambers, so the system will be long by it's nature.

That part I disagree with. You can take a 36" spring and compress it into a 12" pipe. When pumping it may compress to 3", as you said, and the force/pressure will only drop 27% as the displacement goes from 33" to 24" (per Hooke's law).

But how does that need extra length? The spring can be compressed right inside the pressure chamber. The tough part is pushing and clamping it into place, as you'll need to exert a lot of force to do that.

[Ben]

Perhaps I was unclear. If you used 2 inch pipe (what I consider normal for this) it'd get lengthy to have any reasonable capacity. 3 inch and 4 inch are acceptable but have lower pressures.

Edit: Actually, it wouldn't be too much longer now that I think about it. It's worth looking in to, but we'll need better springs before proceeding. That and better/safer construction methods, because as you said, you'd have to exert a lot of force to close the chamber.

Edit: I'm looking into it more now. I like the dimensions of the system I described but the pressure is pretty low. At least 60 PSI would be ideal.

Edit: Hmm... I can't seem to find anything longer than that 36 inch spring I mentioned. Maybe two or three of them would be ideal...

Edit again: I've found more springs on McMaster-Carr. I don't know why I didn't consider these ones before. I'll be looking into how feasible they are.

Edit: Nope, none of them seem more feasible than the one I choose before. I wish there was one with a spring constant in the range of 15 to 20 pounds per inch but there's not.

Edit: Changing the piston to a 4 inch one makes the system work well. McMaster-Carr part number 9662K78 will make 750 to 1000 pounds in the 19 to 25 inch range (1.3 liters of capacity), which is good for a %drop of 25% between about 60 and 80 PSI. The only problem is how to safely build the system with 1000 pounds of force... aluminum anyone? Maybe I'll give this a shot after learning machining and CAD better. It's possible, and it'd be nice, but it'd be hard to build and pretty dang expensive. Neat project though.

It'd still be rather large but it gets the pressure I want. Maybe I should just use two or three of the springs I mentioned first though to get something with a safer level of force though....

It's obviously a long way from a complete water gun.

[aEx155]

Originally Posted by Ben

The only problem is how to safely build the system with 1000 pounds of force... aluminum anyone?

You could always use threaded rods and aluminum plates instead of all aluminum (if that's what you ere planning)



The picture should explain it. Basically, you have a metal plate behind the spring, and one that's holding onto a coupler on your pipe. That way, you don't have to worry about an end cap breaking.

EDIT: On a side note, I was thinking of a CPH design for a tall base (EX tree house) that would use a weight on top of a piston falling down a long pipe (6- 8 feet, perhaps). Originally, I ruled it out because it would've taken 800 pounds of weight (1600 if you counted a counter-weight) but seeing as how a spring version would be just as much, it doesn't seem as radical...comments anyone?

[cantab]

Originally Posted by Ben

The only problem is how to safely build the system with 1000 pounds of force... aluminum anyone?

PVC. The forces will actually be just the same as with air pressure.

Originally Posted by aEx155

comments anyone?

Heavier, more unwieldy, and less powerful than a motorised pump.

[aEx155]

Originally Posted by cantab

PVC. The forces will actually be just the same as with air pressure.

I don't think the problem is containing the forces, It's getting them into place, then holding them there. If you used all PVC, then you'd need to hold the end cap, piston and pipe in place while the glue cures. If you used some amount of metal, you would be able to seal it right away or at lease eliminate cure time.

Originally Posted by cantab

Heavier, more unwieldy, and less powerful than a motorised pump.

How would it be less powerful? You're still getting 60 PSI, and it would be a lot cheaper. Also, if it's stationary, how would it be unwieldy? (I should have mentioned that)

To add more on to what I previously said: the pipe would be mounted at the base vertically, with an outlet for water (you could also have an inlet if you needed one) to a hose and trigger. You could fill the tube up from the top, then put the piston and weight in, then open trigger to fire. Just a thought...

[Ben]

Originally Posted by cantab

PVC. The forces will actually be just the same as with air pressure.

I'm worried about the cap at the end where the spring meets the pipe. The pressure will be much higher in spots because that 1000 pounds will be distributed over a smaller area. Something strong like a large washer would help distribute the forces.

Before proceeding I definitely will do the math after making some models to see how feasible PVC would be. If it works, good, that'll save some money.

Originally Posted by aEx

EDIT: On a side note, I was thinking of a CPH design for a tall base (EX tree house) that would use a weight on top of a piston falling down a long pipe (6- 8 feet, perhaps). Originally, I ruled it out because it would've taken 800 pounds of weight (1600 if you counted a counter-weight) but seeing as how a spring version would be just as much, it doesn't seem as radical...comments anyone?

They used a similar system to mine for gold back in the 1800s. I don't know too much about it but it could be made very powerful. The main problem would be setting it up and resetting it but if you have a reasonable way to do that, it should work fine. It'll obviously be stationary but if that's fine by you then it's good. It depends completely on what fits the bill.

A motorized pump would have the advantage of being much more practical if it needs to be moved around. I'd probably go for a motorized pump over a weighted system for that reason but as I said earlier, it depends on what you want out of the system.

[Silence]

aEx, 800 pounds is a lot of weight and you would need a strong support and lots of strength to work with it. A counterweight won't work - you'll need to lift it up in order to work on the original weight anyway. You're right that you'd have perfectly constant force though. At the moment I would say the disadvantages outweigh the advantages, plus you might just want a hose.

An 800 pound weight is unwieldy because of the setup costs. What will you use for the weight - a big bucket of gravel? How are you going to get it up there? What materials are needed to keep it up there? How are you going to lift it each time you need to repressurize? What if something fails and the thing falls? Remember, this is half the weight of a small car, and maybe about the weight of an old small car like the Geo Metro.

[aEx155]

I was going to explain how I was going to make such a system, but, assuming that I would use iron for the piston weight, at 4" diameter, it would've taken 18 feet of it to get that much weight, so yeah...it's WAY to radical when I start thinking about it. Maybe a smaller version could be used, but for now I'm scrapping that idea.

[Silence]

Haha, I didn't expect that...it's incredible how hydraulics and Pascal's principle work. Speaking of which, you could just use a much longer pipe with a smaller weight, but that also becomes impractical fairly quickly.