Two-Stroke Pump

[Silence]

Since nobody has yet followed Ben's suggestion of using a pump that works on both strokes as in Storm guns, I decided to take the initiative with this thread .

Basically, two-stroke pumps (TSPs, though you can't use this to describe a TSP homemade) work by having water and in/out holes on both sides of the pump plunger. When you push in, water enters the farthest compartment and the nearest compartment pressurizes the PC; the opposite happens when you extend the pump. Clearly, although it might be slightly harder to generate more pressure when pushing the pump, one can achieve double the pressurization in a certain amount of time. I'm fairly new to homemades, and I'm surprised that nobody else has done this!


Although such pumps might seem more complex than standard ones, they are not; essentially, they comprise two standard systems back to back in the same pump piston. Their intake and outflow tubes merge into the ones that go to the reservoir and the PC, but the compartments must have 2 check valves each. In the image, you could make the top 2 tubes join in a similar fashion (but in reverse) to the bottom ones if you really wanted to; I just like this because it has a slanted handle, though the front tube will get in the way of the trigger unless the both of the tubes form the grip.

Yes, this may be slightly more expensive, but for exactly twice the pressurization rate, who can complain? Thank goodness I haven't wasted anything by building any good homemades without this yet--my PCgH and other homemades, which I will build in several weeks, will definitely feature two-stroke pumps. I really need to hear glowing remarks from everybody .

[m15399]

Not sure if you've seen this topic, but here ya go (I fixed the pic):
http://s8.invisionfree.com/soakermed...?showtopic=513

How do you plan to keep a seal on the left side? I thought we had a "consensus" on the fact that you couldn't put an oring on the inside of a pipe.

If you look at the above topic, the design I drew doesn't have that problem. It's also a lot longer, but better bigger than nothing. You could make it slightly more compact by using a sideways tee like in your design.

[Silence]

Well, there you go, everybody. m15399 did the work that I should have done and found the thread I read long ago that talked about this. Well done.

m15399's two-stroke pump diagram is okay, but this seems a bit more practical and convenient, as it allows for a design that can replace a standard design easily. His version, unfortunately, is a bit "clumsy"...no offense, I hope.

Later, m15399 pointed out to Ben (I'm assuming Ben hasn't actually built a two-stroke pump, as he would have hit this wall) that if you don't have individual check valves for each compartment, then the water will just go back and forth between each compartment. You must look carefully to see this problem!

It's shame I didn't realize that there was a similar thread at SM until now...but now, at least, there is a thread dedicated to two-stroke pumps themselves. Hopefully, everybody will post pictures of their homemades that use these effective pumps and keep this thread going.

[joannaardway]

Question: Has anyone ever built one of these?

I doubt it. I think that asking for a two stroke pump on a homemade is a lot to ask (also given that humans cannot exert as much force on an out stroke)

I think a multi bore pump is going to be the acme of the art - rather than a twin stroke pump.

[Ben]

I actually designed a two-stroke pump once, but never constructed it. I essentially put an alluminum rod with two seals on two pumps as the typically design is. A hole was drilled in the center of the alluminum rod which attaches to the sleeve that goes over the pumps (somewhat like a tracked pump). A picture would help, but I think everyone will get the picture.

The main problem with a two-stroke pump to me would be the length to me. These things seem as if they'll be too long for a good setup. I have not made a water gun with a two-stroke pump however, so I would be interested in someone else's implementation and their opinion of the design.

[Silence]

@ joannaardway: If anything, a multi-bore/multi-stage pump will be more complex and more difficult to implement. I've put off my plans for that for the present, and I doubt I will return to them.

Clearly, however, humans can't fight too much force on an out-pump; however, it might be a little easier on non-tracked pumps, as the grip is a little easier to hold. I plan to have a pump handle with a hole in it (similar to the MI Defender's main grip), so that on the out-pump, the thumb can help push.

@ Ben: Unfortunately, I don't really see the design you have in mind. However, I will take a guess: did you have two standard pumps back-to-back and with the pump handles fused? If so, then I can see how it might be to long and awkward.

@ everybody: I really see the potential in this design, but first, I must overcome one flaw. Do you (everybody) notice the opening where the pump shaft enters the piston tube? This will require an O-ring, but that leads back to the classic problem of attaching an O-ring to the inside of a narrow PVC tube. Can you say, "Some Difficulty Involved!"?

However, on the bright side, I might be able to solve this problem--due, of course, to the slightly easier circumstances. In this case, the O-ring only needs to be attached right near the edge of the tube; and even more conveniently, one can use a screw-on endcap to cover the tube after sliding the piston's plunger inside. I think you can see where I'm going with this...

Yes, that's right--it should be entirely possible to take an O-ring with a larger-than normal OD and place it at the opening of a male-threaded fitting. Then, you can simply screw on a female-threaded endcap (with a wide hole for the pump shaft to pass through), effectively clamping the O-ring to the end of the tube! With this design in effect, the first homemade two-stroke pump can easily be built.

EDIT: @ Ben: So, is it neccessary to use two check valves for each compartment or not?

[joannaardway]

Two check valves are needed per compartment.

As a bonus, the pump shaft itself reduces the area of the outstroke pump. It pumps less, so less force is need.

I still think that the multi stage thing is going to be easier, but I don't know. I'll come back with reports of ease.

[Silence]

Originally Posted by joannaardway

As a bonus, the pump shaft itself reduces the area of the outstroke pump. It pumps less, so less force is need.

I still think that the multi stage thing is going to be easier, but I don't know. I'll come back with reports of ease.

That's a very nice point about the pump shaft reducing the volume of the outstroke (a better word than out-pump, as a stronger person can outpump a weaker person, and as outstroke sounds more technical )--it never even crossed my mind. Clearly, if a homemade builder wanted to significantly reduce the outstroke volume, then he or she could make the shaft as large as he or she wanted to.

I didn't know you were actually going to try the multistage pump--it's definitely possible, and I'm sure it will be easy to use, but you might experience some trouble while constructing it. Well, not really--but at this point, it does just seem like extra trouble. I really want to hear the results when it's completed .

[Silence]

After some discussion in the "two stroke pump" thread over at SM, Duxburian pointed out that you'll have to use aluminum or laminated/sealed wood or something for the pump shaft instead of the standard unprotected wood; this is because the water in the chamber farthest from the user will soak the wood, increasing the weight and eventually causing rotting.

[DX]

And don't forget that there are two designs that work. The tracked pump variety does not hit the same wall that the non-tracked hits, although the tracked two stroker needs a somewhat longer pump than most are used to.

[Silence]

Yes, the tracked version will also work, as long as the user doesn't mind having the pump stick out 50% farther than the average pump...but I guess that isn't so bad since you don't actually need to reach the whole distance. Both designs are ending up somewhat complicated, but what else can one do? I'm actually starting to like the tracked version, so kudos to Duxburian and m15399, but I still like the relative simplicity of the non-tracked version. Also, remember that the non-tracked version has a potential for reducing the force needed on the slightly more difficult outstroke due to the width of pump shaft, but that isn't an option on tracked pumps. On the other hand, tracked pumps don't need aluminum rods, so I guess the two methods come out just about even. It's up to personal opinion and choice now.

The SM article I linked to is much more thorough concerning the tracked version, so if you're interested, check that out, too.

[st_jimmy]

This is an interesting design. You will have to build it with extreme precision for it to work, though. I'm not sure it would be a good idea to use it on your first good homemade, as it might end up just being a huge hassle.

[DX]

Also, remember that the non-tracked version has a potential for reducing the force needed on the slightly more difficult outstroke due to the width of pump shaft, but that isn't an option on tracked pumps.

I get nit-picky when people make absolute statements. Perhaps it's from all the SAT prep which drilled into my head that most absolute statements are incorrect...

You could make a tracked pump shaft larger. Plus, the size really has little to do with how hard the strokes are. It is mainly the resistance in the pc. All pumps get difficult as the pc fills. Also, a double stroked stock CPS gun would be easier to pump than a double stroked modded CPS gun, even with the same shaft sizes...a weird hypothetical example.

The non-tracked is the more complicated of the two, since it involves more parts and adjustments than the tracked. The tracked is just more work, since you have to physically cut it. And since I don't have a drill press, I have to do that by hand with a small hacksaw. It takes forever, not to mention that the blade isn't exactly straight anymore...

[Silence]

@ st_jimmy: I don't think this would require too much precision--just a little more complexity--but you're right in that it is a long shot for my first homemade. However, I'm confident, and if I get the homemade working, it should truly be fantastic.

@ Duxburian:

Originally Posted by Duxburian

I get nit-picky when people make absolute statements. Perhaps it's from all the SAT prep which drilled into my head that most absolute statements are incorrect...

You could make a tracked pump shaft larger. Plus, the size really has little to do with how hard the strokes are. It is mainly the resistance in the pc. All pumps get difficult as the pc fills. Also, a double stroked stock CPS gun would be easier to pump than a double stroked modded CPS gun, even with the same shaft sizes...a weird hypothetical example.

The non-tracked is the more complicated of the two, since it involves more parts and adjustments than the tracked. The tracked is just more work, since you have to physically cut it. And since I don't have a drill press, I have to do that by hand with a small hacksaw. It takes forever, not to mention that the blade isn't exactly straight anymore...

I'm not sure exactly what point you're trying to make. My statement was perfectly legitimate: (1) It's harder for humans to push with certain force on the outstroke than with the same amount of force on the instroke; (2) in non-tracked pumps, the pump shaft will take up a constant fraction (due to the area) of the volume of one chamber, though that is not the case with tracked pumps; and (3) this "wasted" volume can be used to ease pumping on the outstroke, because Bernoulli's principle states that a smaller area [of the pump] results in a shorter distance that the plunger [of the PC] travels, thus reducing resistance. If there's any problem with the statement, please say so.

Naturally, it is possible to make a tracked pump wider, but there will be even more resistance on both the instroke and on the outstroke. If you use a non-tracked pump, you can make the outstroke's difficulty similar to that of a 1/2" pump. Also, the area of a base of the pump tube is directly proportional to the resistance in pumping, given that the PC is already at a specific PSI. More importantly, at any PSI, you'll need 9/4 more force to push a 3/4" pump than to push a 1/2" pump--regardless of how pressurized the PC is.

The non-tracked pump isn't neccessarily more complicated than the tracked pump is, the only difference being that the non-tracked pump involves clamping an O-ring using a female-threaded endcap (although this hasn't been done before, it should be easy), while you'll have to use tape and everything to add the extra O-ring to a tracked pump shaft. As I have said, it's all a matter of personal preference.

[st_jimmy]

iImeant the seal from the pump to the outside- for it to work that will have to be made VERY well so it wont just leak all of the water out. After all, pumping it into an already pre-charged pc will put great pressure on the pump.