Hi Guys,
So, many hours in the shedshop last night and this morning later, I completely stripped out the old jet drive unit, which was painful to say the least, at the same time I also investigated the source of the vibrations in the old unit, which is when I found the old impeller had slipped forward on the shaft causing the leading edge of the blades to grind against the inside of the tunnel.
The impellers were originally designed to be grub screwed onto the shaft, but that was quickly discarded as they are such a tight press fit onto the shaft and they have never given any bother, I suspect what has happened, is the impeller has been subjected to speeds and loads it can't withstand during testing and as load increases, the impeller expands to the point where it can walk up the shaft, the stuttering on the last run is the impeller hitting the tunnel, stopping, the shaft then rotates separately to the impeller, the friction this causes heats up the PLA and when PLA gets warm it becomes really sticky, at this point the impeller grips the shaft and drive is restored.
Examining the original tunnel body, which is now the oldest part of the entire build, the walls are relatively thin, or more precisely, the same thickness as a Graupner unit, this allows the rather long impeller shaft to flex in the shaft tube, the new unit addresses this and many other issues.
When I set out to redesign the tunnel housing, I had several points in mind:
- The unit as a whole had to be rigid
- Incorporation of an integral motor mount
- Use a shorter impeller shaft
- Integral shaft seal in the housing body
- Use a solid coupling between the motor and impeller shaft
- Be able to print the unit in one piece on my 3D printer
With these points in mind I had to alter quite a few key aspects about the original design, the biggest of which was that it was just to big to fit into the build envelope of my Lulzbot Mini, which has a build volume of 150 x 152 x 158mm, the original Graupner unit is a relatively long and tall unit, so I lowered everything, included a base plate to tie everything together and knocked around 40mm off the overall length.
Some limitations are placed upon me when designing things to be 3D printed, namely, that they are 3D printed and certain shapes are very problematic to print, this is what lead to the motor mount being a bolt on item, but it turned out not to be a problem, as the entire structure is remarkably rigid. During a break at work I machined up a solid shaft coupling from some scrap Brass stock, this sits nicely between the motor and impeller shaft, which is effectively half the length of the old one at 120mm, the dual bearings in the motor support the drive end nicely, with the other end of the impeller shaft being supported by a Bronzelite top hat bush in the stator housing.
Cooling has always been somewhat of an issue on this project, the passive cooling system worked well, by just didn't provide the throughput necessary to keep the motor cool, my active pumped system was a bit of a flop altogether and best forgotten about, but I am very pleased with the latest system, tapping into the positive pressure inside the stator housing, the harder the jet drive works, the higher the pressure and the more cooling is provided, which is evident in the video as cooling water expelled increases proportionately with speed.
Testing on the lake today:
- Test run one, to try and keep the balance of the boat, I had decided to use a 2250mah 3S Lipo, which should give ample power for a test run but be much lighter than the 5000mah 3S batteries I normally use. Firstly some harbor maneuvers were carried out to test the steering, reversing and also to check the cooling system was working; with everything appearing to be in order, I sailed out of the harbor and started to apply throttle... when the Lipo alarm went off. So in the boat came and no sooner had I lifted the boat out of the water than the alarm stopped, I suspect that there is something very wrong with this battery, a fresh 5000mah was swapped in for test two.
- Test run two proved to be very successful indeed, although it was not without incident, the boat suddenly stopped in the middle of the lake, although it did restart again and later examination of the GoPro footage revealed the problem to be loss of pump prime due to rough water, oh and the ESC exploded spraying the inside of the boat with capacitor goo, although it did keep going. Apart from these two minor upsets, everything went to plan, the drive is very smooth and vibration free, the biggest limiting factor in getting the most out of the drive now is the design of the hull, the very basic airboat design I have currently is very easily lifted above the water by bad water, which is really bad for jet drives, but in calm water, this thing really monsters along.
In conclusion:
- The brushless motor is more powerful than the drive can handle at the top end as above 80% something throttle cavitation of the impeller starts to become an issue
- I made it through an entire test without being rescued!!!!
- I need a decent hull and am very tempted to make a twin jet power boat
- A bigger, water cooled ESC will be installed and if I can get a full run next weekend, I will call this a success and move on to something else, namely progressing my Stolly design, or finishing one of my other boats, might build my Springer!
Video from today:
https://youtu.be/1fIXD4WxVa0Alex
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Topic: Water Jet Drives & 3D Printing (Read 99694 times)