How to make the propeller pump
by Dave Keenan, 1-May-2005
This propeller pump is one component of my low-energy chemical-free swimming-pond filtration system. This system and the instructions for building it represent a year of self-funded (or should I say wife-funded) research and development. Three and a half completely different pump/lift-tube designs were built, tried and rejected before this one was settled on.
I understand that while many people will feel confident to construct the undergravel filter, some may be less confident to construct this pump or the more difficult UV head or power supply and control box. There are two options in this case:
Note that it was necessary to design and build this pump myself because I couldnít find any commercial units which combine extra-low-voltage (ELV) with submersibility and axial flow. The same goes for the UV unit. You will find commercial pumps and UV units with two of these properties but never all three. Submersibility and axial flow are necessary for energy efficiency while ELV is essential for safety and legality. You can use 230 Vac submersibles in fish ponds but not in swimming pools.
A fourth property that would be nice is brushlessness. The motor I use has carbon brushes which wear out in a year (or less if the voltage is not significantly reduced below rated). However, every submersible ELV motor I have found (usually as part of a centrifugal solar fountain pump) costs more than 10 times as much as the Rule bilge pump and comes with less than a 10 year warranty or is significantly less efficient. Although I may investigate these further in future if the nuisance (and waste) of replacing the motor every year starts to get to me.
If anyone can figure out how to make a heavy-duty computer fan motor be submersible, please let me know.
From hobby shop (e.g. Hobbyrama):
From general hardware or building supplies (e.g. Bunnings), or plumbing supplies and electrical supplies:
From marine supplier (e.g. Whitworths or Glasscraft):
From Electronics supplier (e.g. Jaycar)
Note that this capacitor is contained in the DC speed control kit, 12 V 10 A (Jaycar Cat. No. KC-5225) that you will need to build the Power Supply and Control box, so rather than just buying this capacitor, you might buy this kit now.
Tools and consumables
Notes regarding the cable
The cable is described above in terms of square millimeters of copper, as is standard for house wiring. Marine and automotive cable however, is usually described in terms of its overall diameter including insulation, e.g. 4 mm or 6 mm. This tells you absolutely nothing about the area of copper and hence the voltage drop per meter per ampere, which is what we care about. For example, 4 mm cable may have a cross-sectional area of copper anywhere from 1.25 mm2 to 5 mm2 ! You may find the square millimetres given on a label on the drum, but more likely you will find something like "2 x 31/0.45" which means it has 2 cores, each having 31 strands of 0.45 mm diameter wire. You can calculate the cross-sectional area of each strand as pi/4 times its diameter squared, then multiply that by the number of strands. If youíre trying to work it out in the shop without a calculator, you can use either 3/4 or 4/5 as a good enough approximation of pi/4 (~0.785).
It is important that any submerged cable be tinned to reduce the corrosion rate, i.e. the individual strands are coated with a thin layer of tin and look silver-grey instead of copper coloured.
First we cut the intake holes in the lift tube and make it able to house the motor and propeller. Cut a length of 65 mm DWV PVC pipe 70 mm shorter than the original maximum depth of the pool, typically 1.2 m. If in doubt, cut it oversized. You can fine tune it later when the undergravel filter is in place and the pond is full of water. Use a sheet of A4 paper wrapped around the pipe to draw a straight line for the cut.
We need to cut eight intake holes around the circumference of the pipe, near one end. Mark at intervals of 27 mm along one edge of the sheet of A4 paper (27, 54, 81, 108, 135, 162, 189, 216 mm). Wrap the paper around the pipe and use it to draw a line 30 mm from one end of the pipe, then mark along this line at the 27 mm intervals. These are the centres of the intake holes.
Cut the eight holes with a 20 mm holesaw. Deburr them. Cut an 11 mm square notch out of the end of the pipe, in line with a point midway between two intake holes. This notch is to fit around the motorís cable-entry strain-relief.
Now we drill the holes for the motor locating screws (not shown in photos). We drill three 3.6 mm (9/64") holes 75 mm from the end of the 65 mm pipe, approximately equally spaced around the pipe, with one of the holes being on the opposite side from the notch (not in line with it).
Now we add the conduit that allows the cable to pass into the undergravel filter trunk without letting water in. This conduit will also serve as a silicone-filled junction box for the cable and its radio interference suppression capacitor. Cut a 60 mm length of 20 mm diameter rigid grey PVC electrical conduit. (The photos show an earlier 100 mm length). Cut an 11 mm square notch in one end of it to match the one in the lift tube. File a flat area about 5 mm wide along the full length of the conduit, in line with the notch, and do the same for a corresponding length on the 65 mm lift tube, in line with its notch. Cut a piece of duct tape about 200 mm long to use as a clamp and have someone else hold it and be ready to use it to clamp the conduit to the DWV pipe after gluing. Liberally coat both flat filed surfaces with PVC jointing cement and press them together, carefully aligning them notch to notch. You only have about 10 seconds in which to correct any errors of alignment. Hold them together with a hand at each end of the conduit, with the conduit uppermost, while your assistant tapes them together tightly across the middle of the conduit. Leave it to set and donít disturb it for at least half an hour, preferably overnight. Certainly leave the duct tape on it overnight.
Now we make the motor packing ring. Cut a 25 mm length of 65 mm DWV PVC pipe. Make two lengthwise cuts in it 30 mm apart so that it can be squeezed into the lift tube and will still leave an 11 mm gap to align with the cable entry notch. This is not to be glued in, only held in place with a bit of silicone that can be cut with a knife when the motor needs replacing. Its slight out-of-roundness and springiness allows a friction fit of the motor. Do not attempt to assemble the packing or motor into the lift tube at this stage.
Remove the 12 volt "Rule 1100" bilge pump from its packaging. Note that this is a very patriotic USA bilge pump with its red, white and blue. We are going to effect a communist revolution and reduce it to only its red bits.
Cut the wires 40 mm from the pump body. Unclip and dispose of the blue strainer. Using a hacksaw carefully cut the outer white part of the bilge pump (the pump chamber) away from the red part (the waterproof motor housing). Cut just on the red side of the welded seam and work your way around and around, never letting the blade penetrate more than 8 mm, until the white part comes away and can be disposed of. Use a file to remove the burring on the red part, caused by the hacksaw, and to chamfer the outer edge of the cut so it will be easier to insert into the pipe later.
Use two flat-blade screwdrivers, one on each side of the stainless-steel motor shaft, to lever off the blue impeller and dispose of it.
Now we drill and tap the brass coupling (which currently has an unthreaded 1/8" hole through it) to cut an M5 thread in one end of the hole.
Take the brass coupling and clamp it in a vise so that you can drill into the end farthest from the small set-screw. To ensure that you do not drill all the way through, but stop at least 3mm short of the set-screw, you might wrap a piece of tape around the 11/64" drill bit, as a depth gauge. With the brass couplings Iím using, that means putting the tape 8 mm from the tip of the drill. Drill out the hole to the appropriate depth, clear the hole of debris then use the M5 tap and handle to cut the thread in the hole. Remember to back the tap out half a turn after every turn in. Stop when the tap gets hard to turn because youíve come to the end of the 11/64" hole. Thereís a tutorial on hand-tapping at http://www.jjjtrain.com/vms/cutting_tools_hand_tap.html.
An alternative to this is to buy another splined coupling with an M5 thread already in it, and join the two together with a short length of brass tubing that fits tightly over the splines. The disadvantage of this is that it puts the propeller further away from the motor, which makes it more susceptible to hitting the sides of the pipe if the motor tilts or wobbles slightly.
Now we join the propeller to the newly threaded brass coupling using an M5 set screw (not the much smaller set screw that is already in the side of the brass coupling). The aim is to have the M5 set screw completely isolated from the water by means of the superglue, without gluing your fingers, and with half its thread in the brass coupling and half in the brass insert of the propeller.
Now we assemble the propeller and coupling onto the motor shaft. Put the M3 teflon washer on the shaft first if you have it. The motor is not designed to take the continuous end-thrust it will be subjected to in this application and so the thrust bearing must consist of the brass coupling against the plastic of the motor housing, unless we can interpose some teflon.
Interesting fact: Teflon on steel has about the same coefficient of friction as greased steel on steel. Few other combinations come close to these. However the synovial (egg-white-like) fluid in your body's joints results in a coefficient of friction ten times better!
Use the appropriately sized Allen key or hex driver to securely tighten the small grub-screw of the brass coupling onto the flat side of the motor's shaft. Ensure that the brass coupling is close to the motor housing but the propeller still turns freely. You might want to put a piece of paper between them while tightening the grub-screw and then remove it.
Thereís a nice story about the propeller: In the course of developing this system I emailed various companies asking about buying small quantities of various exotic components or materials. In almost all cases it was like sending my email into a black hole. In the case of the propeller, the response was completely and delightfully the opposite. My local hobby shop had told me that the Australian importer did not carry the unusually large propeller that I found I needed, so I emailed the manufacturer, "Radio Active Manufacturing" in the UK, http://www.radioactivemfg.com, telling them what I wanted it for. In case you havenít guessed, their name relates to radio controlled models, not that other nasty stuff. They immediately sent me four propellers by airmail, free of charge, and told me that I should find that the Australian importer should carry them in future. Three cheers for Radio Active!
Next we join the heavy duty marine cable and radio interference suppression capacitor to the motor cable in such a way that they will fit inside the 20 mm conduit that we glued to the lift tube.
Warm up your soldering iron. It will need to be at least a 60 watt electric or preferably a butane-powered one to cope with the 5 mm2 cable. If you havenít already done so, cut the motor wires off 40 mm from the motor body. And cut about 4 metres of the 5 mm2 marine two-core cable. Strip about 20 mm of the outer sheath from one end of the marine cable. Then strip about 5 mm of the insulation from the two individual cores and from the two motor wires.
Orient the cable and motor wires so that black meets black and red meets brown. Capacitor orientation doesnít matter (blue in the photos). Please ignore the diode (black and silver) shown in the photos. The diode must be installed at the motor speed controller, not near the motor. This is dealt with in the section on the power supply and control box. Please ignore the part in the instructions for the motor speed controller where it says the diode must be installed near the motor. Far from preventing radio interference, this causes it!
Push the leads of the capacitor a millimetre or two into the middle of the motor wires. Solder these two joints and tin the marine cable cores.
Bend the capacitor up so the motor wires run parallel to and touch the marine cable cores. Solder the motor wires to corresponding marine cable cores.
Check that the connection is mechanically strong by pulling the motor wires (not the motor) and marine cable apart. Visually check that there are no short circuits between red/brown and black. It is difficult to check for shorts with a multimeter once itís all soldered together because the motor winding resistance is only about 0.8 of an ohm.
Making sure the motor is securely held and that everything and everyone is clear of the propeller. Connect a source of 3 to 6 volts DC (e.g. a battery) to the free end of the marine cable, with the red wire positive. Ensure that the propeller spins anticlockwise when viewed from the propeller end (not the motor end).
When the PVC cement between the conduit and lift-tube has had at least half an hour to cure, have a friend feed the free end of the marine cable through the conduit, from the notched end, while you hold the motor and the the marine cable to prevent twisting of the soldered joints near the motor. Just before the joint disappears into the conduit, stop and have your assistant tape the marine cable to the lift tube at about the middle of the lift tube. Fill the gap between the solder joints with neutral-cure silicone and cover all wires and the capacitor with silicone.
Smear some silicone on the inside of the motor packing ring and the inside of the lift tube where the packing ring will go. Wrap the packing ring around the motor, aligning its gap with the black rubber strain-relief, and push both into the lift tube until they are flush with the end, at the same time feeding the capacitor and diode into the conduit. Screw the three 20 mm M4 stainless screws into their holes in the lift tube (cutting their own threads) until they are all pressing on the motor housing just enough to stop the motor from being easily pushed out, but not enough to crack the motor housing. Test this by inserting a finger into an intake hole and gently attempting to push the motor out. Ensure that all three locating screws have the same amount sticking out on the outside of the lift tube.
Fill the conduit with neutral-cure silicone by putting the nozzle in to the middle and withdrawing it slowly as the conduit fills. Do this from both ends of the conduit. Use a 300 mm plastic cable tie to tie the marine cable to the lift-tube near the conduit. Remove the tape near the middle of the lift tube and use another 300 mm cable tie in its place. Set it in a safe place while the silicone cures, preferably overnight. See the photo of the completed unit at the start of this article.