Flex shaft question

[Psycho1]

I noticed several posts recommending a gap between the strut and the drive dog. I assume this is because the flex shaft will tighten up while twisted tighter under torque, making it shorter. What would be a close starting point for the gap with a 3/16" shaft, with a 4074 2000kv motor running on 4 or 6S? Prop will most likely be in the 440-645 range.

[Tails Up]

3-4 mm is recommended.

[Fluid]

How lone is the flex cable, that is what really matters along with the stuffing tube - is it Teflon lined or not. The default gap is the diameter of the cable, so 3/16". This is usually safe.



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[fweasel]

How lone is the flex cable, that is what really matters along with the stuffing tube - is it Teflon lined or not. The default gap is the diameter of the cable, so 3/16". This is usually safe.

I've been using the thickness of the cable as a rule of thumb. What does running a liner have to do with the gap? That's something I have not seen mentioned before (I'm new too).

[Psycho1]

Mine is a 3/16" about 14" long 10 1/2" tube, no liner. will see how things go with 3/16" then try 1/4" and see if it picks up any speed. If no change, then I guess 3/16" is enough to prevent torque pressure/drag at the strut. Thanks for the replies.

[boredom.is.me]

I'm surprised no one has built a test rig for this. As was stated above, length is going to have the largest impact over how much the cable does contract. With that said, I'm personally not satisfied with the "diameter of the flex" rule.

A tub with a transparent side would be all that's needed. Take a measurement of the length of cable beyond the coupler. Take a note of the flex diameter. Then just let it rip with a larger than necessary gap. It will be a little challenging to see the shaft, but marks can be made directly on it for reference.

You should end up with a very basic equation something along the lines of this: Contraction = Length x Ratio
The ratio would be dependent on the diameter of the flex but would simply be: Ratio = Diameter x whatever number

I know there are more variable such as motor torque* and prop size (again another torque value), but above would still provide a bare basic.

I have enough acrylic to build a rig for just a motor/driveline setup, but would need a flex and strut assembly. It would be a ~3" wide channel, about 3 feet in length, with a large hose connection on each end to allow water to circulate. The longer the hose or channel, the cleaner the water should re-enter. Another method would be to dump it into a larger reservoir to dilute turbulence prior to it flowing back into the thrust channel. ...I get worked up easily...

At least that's the way I see it.

[MarkF]

Put a plastic or teflon washer between the drive dog and strut. Then use the cable size for the gap. If you have a 3/16 cable and a 3/16 gap you will know if your fine or not by how much wear is on the washer. If the washer has wear on it, give yourself more gap. If not your fine.

Mark

[JohnZ]

I have a BBY Offshore 26 with a .150 cable and was recommended 1/16" gap but I used +3/32 instead. I don't see how if any if there is a wee bit too much gap.