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Flex shaft breaking

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Old 12-30-2018, 04:03 PM
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PickyBiker
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Default Flex shaft breaking

I have built a nice little boat from a purchased hull.
Hull
ESC
Motor
Battery
Rudder

It is pretty fast, but the flexible 3/16" shaft is square on the end and it wallows out the square hole in the prop shaft after just a few runs.

When the square flexible shaft goes into the square hole in the prop shaft, is it supposed to be glued, epoxied or otherwise fixed?

If not, what is a common cause for this type for failure?
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Old 01-01-2019, 07:26 PM
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This would be easier to answer if you could post a few pictures of your set up, shaft and collet. Without seeing anything, I'm suspecting that you have a binding issue or the collet is too big for the shaft. I've been running the same shaft and collet in a scale nitro powered hydroplane for at least 5 years without an issue
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Old 01-02-2019, 06:50 AM
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Old 01-02-2019, 08:21 AM
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I already see at least part of the problem. You're shaft is binding in the strut. Picture 4 shows it the best but, in a nutshell, you need to rebend the stuffing tube to line it up with the front of the strut. The way it is right now, you've got the stub shaft pushing down on the front of the strut and lifting up on the rear. The drag that misalignment is causing is probably what's been damaging your couplers. You many also need to shorten the shaft a bit or relocate the motor to take up the extra length of the shaft
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Old 01-02-2019, 09:58 AM
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Originally Posted by Hydro Junkie View Post
I already see at least part of the problem. You're shaft is binding in the strut. Picture 4 shows it the best but, in a nutshell, you need to rebend the stuffing tube to line it up with the front of the strut. The way it is right now, you've got the stub shaft pushing down on the front of the strut and lifting up on the rear.
Yes I see that. I'll get that fixed.

Thank you!
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Old 01-02-2019, 10:30 AM
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Just got to thinking, all that drag may have been causing your batteries to run down faster as well. You may see longer run times once you get everything taken care of.
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Old 01-02-2019, 01:07 PM
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I run gassers and I would never have that much flexshaft exposed from the tube to the strut.May be causing the flexshaft to wobble and wearing the square drive in the prop shaft.I leave no more than 1/2" between the tube and the ferrule on my 1/4 " flexshaft setup
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Old 01-02-2019, 01:10 PM
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I almost forgot to answer the other part of your question. The square drive shaft is supposed to "float" in the collet, allowing the flex cable to shrink a bit under load. If it is hard mounted, using set screws or by using a collet that can clamp down on the cable, the stub shaft, that being the hard shaft the prop is mounted on, would need to extend roughly 1/4 inch back from the strut to allow for that shrinkage.
Tidnab, I run about the same between the ferrile and stuffing tube on my scale Elam as well. It gives me enough room to get the shaft out without having to move the strut but not so much that it lets the shaft whip
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Last edited by Hydro Junkie; 01-02-2019 at 01:20 PM.
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Old 01-03-2019, 09:22 AM
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Thanks again Hydro Junkie
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Old 01-13-2019, 10:10 AM
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So, any updates?
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