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Sep 13, 2019, 08:30 PM
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I think that's exactly correct.

I know (or, I've read, I'm not quite that old) that back in the day, PWC racers would take their brand new skis off the showroom floor, tie them to the back of the truck, and drag them through the gravel. The smooth shiny gelcoat on the fiberglass would slow them down a few MPH on the top end. Back in college I believe the term my professors used was "stiction", but I've forgotten more of what I learned back then than I'd like to admit. I think it's closely related to surface tension, but I could be wrong there too!

It's the same reason golf balls have dimples and are not perfectly smooth.
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Sep 14, 2019, 07:48 PM
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Kayaker's Avatar
Quote:
Originally Posted by MiteyF
It's much better to have to bore out for the shaft than print it to fit! And I must say, I find it hard to believe that you can get away without balancing an impeller spinning at 15-20k rpm. I know on the gas boats if you've got an out of balance prop, it'll shake your driveline apart. I don't think I've ever weighed one of my beryllium or stainless props, but they blades are so thin, I think they weigh even less than this steel/bronze impeller! Either way, I won't be running it without a good balance! Maybe I can crank out a balancing shaft on my lunch break today...
I didnít see any difference after sanding impellers but I do it anyway. One of the impellers in the photo is not finished at all, I havenít tried to run it.


When an air bubble goes through an impeller it runs way out of balance, this is why jet drives with ball bearings at both ends of the shaft run so much better. Impellers with bearings can even run OK with one broken blade. I spin mine to 26K and see no need for balancing.


Shapeways says that printed steel has a size variable of + or Ė 5%. This means getting an impeller to fit inside the wear ring and fitting the shaft in the impeller will very a lot. I have installed four Shapeways printed steel impellers and I have had to drill all four impellers to fit shafts. After drilling they are an easy press fit and would spin on the shafts with some torque. I drag the edge of a file down the shaft to scuff it up. This gives a tighter fit and the impeller needs light tapping with a hammer to put it on. None of these impellers have spun on the shaft so far. This works with Shapeways plastic impellers also. I had one steel impeller that was so loose that it could free spin, adding epoxy and scuffing has fixed it to the shaft. This impeller has about 10 sessions with no problems.
Sep 15, 2019, 03:16 PM
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Thread OP
I debated going with the regular impeller vs the drive dog version, and as you can see I decided on the latter. I drilled my impeller to be on the "loose" side. It slides on easily, but there's still no slop. I've had enough problems getting everything on this pump lined up right, so anything that makes disassembly easier without hurting reliability, I'll take.
Sep 15, 2019, 04:25 PM
Jetdrives R Us
sundogz's Avatar
Quote:
Originally Posted by MiteyF
I debated going with the regular impeller vs the drive dog version, and as you can see I decided on the latter. I drilled my impeller to be on the "loose" side. It slides on easily, but there's still no slop. I've had enough problems getting everything on this pump lined up right, so anything that makes disassembly easier without hurting reliability, I'll take.
You can easily change impellers now without sacrificing the shaft, say to a different pitch or one with non-overlapping blades. What problems have you had 'lining things up'?
Sep 15, 2019, 05:19 PM
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I've tried a few different methods for assembling things.

If I install the motor and coupler, then slide the shaft in and into the coupler, it's hard to get the shaft lined up to start in the stator-side bearing.

If I install the motor and coupler, then place the shaft in the stator bearing and install the shaft/stator as one piece, it's difficult to get the shaft into the coupler.

If I install the shaft/stator into the coupler, then go to mount the motor, it's tough to get the motor aligned.

I think there may be a slight misalignment with the front (motor side) bearing. Everything appears to spin nicely when it's all assembled, so I guess I'll just have to see if it eats up bearings. When I first installed the bearing into the housing, it seemed like a nice fit. That's the problem with FDM parts I suppose, lots of tolerance stacking (literally)
Sep 15, 2019, 10:09 PM
Jetdrives R Us
sundogz's Avatar
If you printed the inlet housing standing on the motor mount everything will be lined up. It may be your coupler - I've had more problems from poorly made couplers than I care to say. If you tighten the coupler on the motor beforehand, it will be off-center due to the grub screw pushing the shaft to one side of the coupler bore. A little extra shaft clearance can be tolerated if you use the following method:

Install a 3mm ring of sealant around the stator wear ring where it meets the flange. I prefer butyl rubber calking for this. Install the shaft/impeller into the stator bearing. The impeller should not touch the stator fins when pushed back and turned. If it does, put a spacer behind the bearing to prevent this.

With motor and coupler in place (all screws loose, washers on the motor mount screws) slide the stator/impeller/shaft into the inlet housing through the seal and guide it into the coupler. Install and tighten the 6 stator fastener screws.

Centering the coupler between the motor and pump shafts (motor held against mount), tighten the coupler grub screw onto the flat of the impeller shaft. Now (holding the motor tightly against the mount) tighten the coupler grub screw on the flat of the motor shaft. Finally, tighten the motor screws.

Using this method puts a bit of preload onto the motor and when things settle in eliminates excessive play in the driveline. The coupler will be off center slightly due to shaft bore clearance but it won't cause detectable vibration unless the clearance was excessive or the coupler was poorly made.
Last edited by sundogz; Sep 19, 2019 at 09:42 PM. Reason: Forgot to secure the stator!
Sep 19, 2019, 09:49 PM
Jetdrives R Us
sundogz's Avatar
Please re-read the instructions above. I had omitted an important step Fasten/secure the stator before continuing to the coupler. My apologies.
Sep 21, 2019, 10:37 AM
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Thread OP
I've installed some intake grates in my pump housing and can't really do much "guiding" of the shaft once it goes in with the stator.

Not sure what you mean by the coupler being off-center, a good coupler (mine seems to have a nice snug fit) shouldn't be off center at all, right?
Sep 21, 2019, 10:59 AM
Jetdrives R Us
sundogz's Avatar
Guide the shaft into the coupler, (wiggle the coupler) as it may be akilter since it's not tightened down.
Most couplers aren't a 'snug' fit, they have some play which will cause it to be off-center (slightly) when the set screw is tightened.
Last edited by sundogz; Sep 21, 2019 at 03:49 PM. Reason: typo - fit, not 'it'


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