Flex cable failure

[robbo1]

Well it finally happened? after years of running my flex cable broke. With that to the bottom of the lake went the octura s&b x642 😭 (purchased from ose). Are flex cables prone to failure?when everything has been looked after and driveline all seems spot on, or could there be an underlying problem? Boat is apparition 1600kv on 6s with 3/16 one piece cable with Teflon liner inside stuffing tube. Just to add I?m pretty sure looks as if it broke where flex joins onto the solid part of the cable.

[Fluid]

The two most probable causes are fatigue located where the cable meets the stub shaft. Poor alignment or too great an angle in the stuffing tube will eventually break the cable. The high rpm your setup runs only quickens the fatigue.

The other probable cause is corrosion. If the cable is not removed from the stuffing tube, fully dried and not put back in for the next day at the pond, rust will start on the inner core of the cable. Some folks take the cable out and dry it off - then out it right back into the wet stuffing tube for storage.

Combine corrosion with high loads and poor alignment, and cables will break. I follow the above routine and I cannot remember the last cable I broke...


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[robbo1]

I always remove the cable and spray with brake clean then wd40 and leave it out until I take the boat out next. I also spray inside the Teflon liner with brake clean to remove any moisture and muck. Teflon liner slips pretty well into the end of the strut and stuffing tube has quite a nice curve to it with no binding issues so I cant see any misalignment problem. Strut most of the time is neutral but sometimes I do use some slight angle.... does this angle possibly be causing the stress and fatigue? I never tinned the end of the cable when it was cut to length..... would this have caused some sort of failure? I’m just scratching my head as I don’t want to loose another expensive prop. (Around 70 Aussie dollars shipped)

[Norwest]

Now one more way to make sure of joining cable and shaft. Clean shaft inside and cable end . I then heat up cable end and add solder and then reheat shaft some so cable slides with some ease. Tip; I use a small water line hose to fit tight over cable/coupler end so the whole c/s isn't lost due to coupler letting go. Also ZOZER said once to use SEIZE in coupler if you need to tighten coupler a little more than usual!

[martin]

What did the failed end look like that you still have, was there any sign of corrosion down into the core of the flex.

[martin]

I assume the flex was a brazed joint being a 3/16" flex to 3/16" stub shaft, sometimes the heat from the torch during brazing can compromise a flexes strength. Which over time & use with torque can cause flexes to break.

[Fluid]

Another concern would be any acid flux left after soldering/brazing. If not removed, internal corrosion can start immediately. Ed Hughey once told me he took extra effort to use minimal flux and cleaned it up well on the thousands of cables he made. Perhaps one reason most of mine are still running years later.



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[TRUCKPULL]

An other fault can be the gap between the strut and the drive dog.
If to small when the cable twists up, the dog is now trying to pull the shaft off of the cable.
So that one time you try a larger or higher pitch prop to see what speed you can get. "SNAP"


Larry

[robbo1]

Always run 3mm gap between drivedog and strut, pretty confident that is the norm and can’t see any corrosion. Well I’ve ordered another flex cable drivedog and the octura Teflon bushes for the strut from ose. Another stuffing tube and Teflon liner will also be going in as the stuffing tube is cracking. I suppose we will see how long this one lasts lol.

[robbo1]

On another note what is the purpose of tinning/soldering the end of the flex cable?

[martin]

The tinning on the coupling end is to stop the flex unwinding or crushing down by a collet coupling closing down onto the flex. Its also help full if you are using the horrible set screw type couplings, some like to use tinned flexes others dont bother. Personally i dont bother tinning any more, i simply sand a bevel on the end & always rotate the flex clockwise when pulling out or putting the flex into the coupling. Rotating clockwise prevents the end of the flex catching & unwinding.

[robbo1]

Hey guys back again. I’m in the process of repairing my cat with a new stuffing tube and Teflon liner. I have just read a post on this forum that the stuffing tube is meant to slide into the strut..... is this correct? My tube does not fit in the strut but the Teflon liner does. Could this have created a problem in the failure of the flex? There was about 10mm of liner in the strut on my original setup. If the stuffing tube is meant to go into the strut how do I get it in? I can not thin out the stuffing tube any more than it’s original state as there would be nothing left of it and on the other hand if the stuffing tube is supposed to go into the strut how would I adjust angle?

[robbo1]

Some expertise and opinions would be greatly appreciated … I want to get it right and not find out it’s incorrect once I have done it.

[rol243]

can you drill out approx 1 inch of the strut so the liner can slide in ? this may have caused the shaft to fail but who knows as it could have also just been wear and tear or a bad join.

[robbo1]

can you drill out approx 1 inch of the strut so the liner can slide in ? this may have caused the shaft to fail but who knows as it could have also just been wear and tear or a bad join.

The liner does slide into the strut just not the stuffing tube…. Or is the stuffing tube what you meant?

[785boats]

What I usually do is solder (or CA glue) a short piece of brass tube, the next size down, into the end of the stuffing tube. About half an inch into the stuffing tube & about 1/4" protruding into the strut.
This allows for strut angle adjustment. But it also stops any movement or whipping between the stuffing tube & the strut which will reduce stress & fatigue at the weld in the flex.
Of course the liner does not protrude into the strut now & has to be cut to fit inside the stuffing tube flush with the little insert.

Small tube soldered into stuffing tube.

m_002.jpg

Strut in place.

m_004.jpg

[martin]

As Paul has explained with short step down tube the teflon liner will be slightly shorter, this makes no difference at all to the workings of the flex shaft not having the liner as long as it was before.

[rol243]

Yes i meant to drill the strut so the brass tube liner can slide in approx 1 inch.

[robbo1]

What I usually do is solder (or CA glue) a short piece of brass tube, the next size down, into the end of the stuffing tube. About half an inch into the stuffing tube & about 1/4" protruding into the strut.
This allows for strut angle adjustment. But it also stops any movement or whipping between the stuffing tube & the strut which will reduce stress & fatigue at the weld in the flex.
Of course the liner does not protrude into the strut now & has to be cut to fit inside the stuffing tube flush with the little insert.

Small tube soldered into stuffing tube.

m_002.jpg

Strut in place.

m_004.jpg

I quite like this idea as it gives me adjustability for strut angle and no drilling into the strut. Stuffing tube I’m using is 9/32 with Teflon liner to accomodate the 3/16 flex cable, so does that mean I need the 1/4 tube for the sleeve? I’m used to working in millimetres so can someone please confirm or correct me if that’s wrong.

Also should the prop shaft be running in the strut only? I have just realised the prop shaft is running 10-15mm into the stuffing tube as well. The flex cable I ordered from ose has prop shaft length of 3.125, should I have ordered the 2.75 long prop shaft?

[785boats]

Yes, you need the 1/4" tube to fit inside the 9/32" stuffing tube.

If the solid prop shaft protrudes out of the strut into the stuffing tube, then adjustments will be almost non existent.
You may well need the shorter shaft so that the flex enters the strut,
Save the longer shaft for another boat that maybe has a stinger on it rather than a strut.