Flex shaft thought

[jfrancisco892]

I was talking with my dad today who helps me out with machining and metal model making. I was going to have him machine a flat on the prop shaft on the flex cable for the set screw of the drive dog and he brought up an interesting point. He suggested that with the high rpm setups in boats that the flat for the drive dog can create an imbalance and the shaft should have the same amount machined off the opposite side to keep it in balance. Just wondering if anyone has thought of this before or if anyone does this to the prop shaft through this school of thought. Also could this same concept be applied to motor shafts with the flat for the set screw for the flex coupler? I feel like keeping everything in balance no matter how small might make a difference since the rotational speed is so fast which could amplify any slight imbalance.

[Fluid]

IME too much time is spent on minutea which have little to nothing to do with actual performance. The tiny amount of potential imbalance from a small flat spot is totally eclipsed by the huge vibrations set up by each prop blade hitting the water with great force multiple times each revolution. This is exacerbated by the whipping flex cable gyrating inside a semi-confined stuffing tube. This is a complex dynamic setup not a solid shaft running in fixed bearings.

I applaud your dad's concern but there are bigger fish to fry.



.

[Peter A]

Consider also the grubscrew tightens into and fills most of that area, and the grubscrew is a different weight to the ali or drive dog etc, etc. In reality very difficult to balance out. I'm with Fluid on that one, not worth the wasted time, although the flat spot is usefull for the grub screw to grip on.

[jfrancisco892]

I wouldn't agree that the "grubscrew" fills most of that area because it fills most of the area it occupies in the drive dog. The relative densities of the set screw and the drive dog may be slightly different leading to a slight imbalance in the drive dog itself but that would likely be considerably less than the weight of the material removed on the prop shaft for the flat.

[ray schrauwen]

The closest to balanced shaft and drive dog would be from Ed Hughey with a pinned drive dog imo. Steve used to sell them but, he doesn't list them anymore. Ed will still make some on request if you know how to get a hold of him or someone that knows him.

btw, some of the best flex cable is sold by Hydromarine, That German flex cable is far more rigid than any I've ever used. Cable like that really helps reduce cable whip and shrinkage. I wish I could find some in the west here maybe it's BMW or Porsche speedo cable??

[keithbradley]

The biggest real issue with imbalance concerning couplers and driveshaft setup is in regards to runout, not so much weight. If you're buying any type of Chinese collet I can guarantee that there is some amount of runout...I've tested a number of them and I've never seen a perfect one. This will create vibration to a much higher degree when compared to the material removed for a flat spot.

[T.S.Davis]

Jimmy, you could have your dad make you a drive dog with no set screw and then use Loctite 603 to permanently place it. A longer drive dog might be a good idea if you go that route. It's probably overkill to get that detailed. A stuff tube that's slightly off would have more impact than resonance IMO.

Something I do is set the dog on the shaft with the grub screw. Then put the stub shaft in a drill motor so I can spin the dog against my belt sander to get everything round......ish. Close enough. Closer than just having the grub screw sticking out. I also taper it a bit so that the leading edge is a touch smaller than the struts trailing edge. Then I get progressively finer with sandpaper etc.

[T.S.Davis]

regards to runout,

Many moons ago I had an Octura coupler resonate the bearings completely out of a Lehner 2280 on 32 cells. I was pretty bummed.

[jfrancisco892]

Thanks you for the insight everyone, I appreciate it very much. Thanks for the tips Terry although I don't think I'm gonna have my dad machine me up a drive dog, he does enough for my boating ventures as it is lol.

[Jeff Wohlt]

Just silver braze it on and forget it all...no flat and not grub screw...done! But really...not even worth worrying about.

[urbs00007]

IME too much time is spent on minutea which have little to nothing to do with actual performance. The tiny amount of potential imbalance from a small flat spot is totally eclipsed by the huge vibrations set up by each prop blade hitting the water with great force multiple times each revolution. This is exacerbated by the whipping flex cable gyrating inside a semi-confined stuffing tube. This is a complex dynamic setup not a solid shaft running in fixed bearings.

I applaud your dad's concern but there are bigger fish to fry.



.

if you were running straight shaft in precision ball bearings would you for example run 2 drivedogs, one before and after prop with setscrews 180 to each other? any other issues you would address?

[Fast Guy]

If your dad's a machinist put another flat on the opposite side of the prop shaft and drill and tap the drive dog for a second set or grub screw 180 degrees from the original one. They should balance each other out.