Introduction: Efficient Water Pump
Follow these instructions to make a really simple water pump that's easy to make and works efficiently. We designed this pump for a brief we were given at the Glasgow School of Art to attempt to design and create the most efficient way possible of lifting water up by 60mm. You can apply this design to a number of areas, but it's also just a lot of fun to make and get working.
It began as a more traditional rope pump but soon evolved into what is sometimes called an 'elephant pump', a design based on 1000 BC Chinese designs. This pump works using a rope covered in little discs that is dragged up a pipe. Water gets trapped between consecutive discs and is elevated up the pipe, just like a lift would take you up to your hotel room. At the top of the pipe there is a reservoir where the water comes out, is collected, and is then directed to wherever it needs to go.
Some of the decisions we made when designing the pump were made because of what materials and equipment were available to us. We understand these will not be the same for everyone so we would encourage you to alter the design to suit your needs best. The premise for this ancient pump is simple and part of the fun is modifying it yourself.
We would encourage you to read through the whole process before doing anything because each of the different parts rely on the others heavily.
Step 1: The Rope
The first order of business is your rope. The rope is just that, a rope, with discs tied on that fit into your pipe. You'll be needing a pipe very soon to put these discs in, and it's often easier to make the discs fit the pipe than the other way round so it might be an idea to source one of those first.
Size is crucial here. Our discs were 29.5mm in diameter because our pipe was 30mm. We tried larger and smaller sizes but these each had their own problems. Too large and the discs don't move fluidly in the pipe, so they can't just be a perfect fit. It's actually fine for a small amount of water to escape round the sides, in fact this lubricates the discs. If they are too small, however, more water escapes around the sides. This still works but you must move the rope through the pipe a lot faster to get it to work. As efficiency was our challenge we found 29.5mm, so half a millimeter smaller than the pipe's inner diameter, to be the perfect compromise.
Truth is you can make the discs out of a lot of things. We tried wood initially but it swells as it gets wet and there is a lot of friction with the pipe so we wouldn't recommend that. In the end we settled with acrylic. We cut the discs out on a laser cutter with a 5mm hole in the center for the rope to go through. 5mm is a good diameter for the sort of rope you should be using, just ordinary string gets really soggy and has a tensile strength that's worse than Heskey is at shooting on target. Thick enough rope also means you only need one knot to hold the discs in place as it won't slip through the hole. We used a mixture of 3 and 4mm thick acrylic, this was by accident though but it makes little difference because of something else we had to alter in the discs.
Just having acrylic discs is problematic for two reasons. The first is that they, while denser than water, don't sink fast enough to not get caught coming into the pipe as they are dragged in sideways as opposed to upwards from the bottom of the tank. The second problem is that in the pipe the discs can tilt, which stops them pushing as much water up.
To make the discs sink fast enough we added weight. We did this by gluing a washer on to the disc, trying to keep it central so as to not encourage further tilting in the pipe. This works well and washers are cheap too so that's handy.
The way to stop the discs tilting is to instead of having one disc each time, tie two right next to each other (one with a washer, one without - making a 'washer sandwich'). This makes the discs thick enough that the distance from the one edge to the other on the opposite side of the disc is larger than the diameter of the pipe so it cannot tilt very much at all. One thing that is important is to not glue the two discs together. This is as a more rigid overall piece gets caught a lot more easily. If you leave the two discs separate then they can move around a little bit as they should be tied on slightly loose. This massively reduces the chances of your discs jamming in the pipe or getting caught trying to get into it.
Before you tie the discs on I would recommend reading the next step as it shall heavily influence the spacing between the discs.
So to summarise, we have a washer sandwiched between two discs tied on to a rope at regular intervals.
Step 2: The Drive Wheel
To get the rope through the pipe something has to pull it. There are a few ways you could do this, one being just an ordinary wheel, but we opted for a sort of 'gear'.
This 'gear' pulls the discs out of the pipe and drags them around before dropping them back towards the water.
We drew out a shape for the gear on the computer, then had it laser cut out of acrylic. It was made out of two of the 'gear' shapes with a few discs in the middle to separate them.
We had a couple of problems with our shape. One was that the curve that was designed into the shape to keep the discs in place was both unnecessary and worked too well. We couldn't drop the discs and they kept getting stuck, with the discs on the rope getting caught inside the trap we designed. Just a gap in the gear is enough to pull them through.
The other problem we had was that the edge of each of the 'legs' wasn't smooth enough and so sometimes the discs would get knocked back and not get caught in the mechanism to be pulled around, this usually meant the rope was knocked off centre and once out of sync none of this worked.
An almost complete redesign occurred last minute, the way we did it in the end was with MDF as it was cheaper, but you could still use acrylic like we had originally planned but be aware it is quite brittle (we discovered this the hard way).
What we did was keep the same design of a gear pulling the discs around, but we had enough sheets of this to stick together and make a really wide, solid shape. Using PVA meant this was really strong. To stop the discs slipping out, we added two large MDF wheels either side to trap the rope in.
Step 3: The Pipe
We got a long length of PVC pipe from a local hardware store, they're normally an expensive store but our pipe was £1 and easily 2 metres long. It doesn't need to be that long though, to raise the water 60cm we maybe used 70/80cm of continuous pipe in our design.
Traditional rope pumps usually have a drive wheel at the bottom of the water source, to guide the rope back into the pipe, when you have disks on the rope they can easily get caught coming into the pipe and when it gets jammed thing break and no one is happy. What we did instead of a drive wheel, however, was use a funnel. We attached it onto the bottom of the pipe so it was flush, and there were no opportunities for it to catch. It guided the discs in just nicely, but took a lot of sanding to ensure nothing got caught.
This method does rely on your discs sinking fairly successfully, to ensure that as they are pulled back up they at least are on target for the funnel which can move them into place precisely.
At the top of the pipe, we built a reservoir out of an old part from a machine that had a little spout on it. Anything is really acceptable here, the idea is the open up the pipe to let all the water you've pushed out to escape, and then you can channel it off to where you want it to go however you want. We attached all this to the pipe with silicon sealant and hot glue, it was a bit messy but did the job well.
Step 4: The Gearing
Gears. These were very tricky, probably the easiest place to go wrong is gears. There are a few good tutorials on here for homemade gear boxes, you might want to check them out. A lot of this depends on what you've built, how much force you need and how fast your discs must go in order to be able to move water up a pipe of your chosen diameter. For that reason the numbers are all going to be different. We calculated a much lower gear ratio than we needed though, even when we did the classic engineering trick of double it and some just to be sure. I think we miscalculated the force needed to fight water resistance and frictional forces dramatically. Go for a high gear ratio basically, these rope pumps don't need to be run that fast to pull water through them.
We used a worm drive on the motor shaft, we just found one the fitted well and hammered it on tight. This was plastic, and so were our gears. In the end this just ate itself under the force and got real mashed up. We fixed this in the end by using metal Mecano gears, if you have some of those use them. You can probably purchase small gears cheap from a local hardware store or something as well.
We started trying to laser cut gears from acrylic, but like I said we moved onto Mecano. This is still worth a shot. We were under pretty strict deadlines and the time pressures meant we couldn't fix the problems we were having with custom made gears but I remain convinced they would work if we have the ratios correct.
We attached the gears to the axle that the driving wheel for the rope was on using really strong glue. Make sure everything is glued on at 90 degrees or it'll turn at a wonky angle and everything breaks.
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