Flex shafts breaking. I must be doing something wrong. Help.

[ScarabChris]

These motors, those "CNC Aluminum Outboards" I did build a spacer block to make them longer. Just required a longer than stock flex cable. Boat runs amazing but I break a flex cable after 2 or 3 runs and it's not the same one. They are counter rotating with the proper flex shaft for the reverse rotation. They break about 1/4" below the motor coupler. This time it really sucks because it broke and I lost the shaft assembly including the red anodized prop. Those props weren't cheap like $45 each and they are no longer made.

I am thinking maybe I am not leaving enough gap between the drive dog and lower unit? I have about 1.5mm gap. Think the flex cable is shrinking more than that under load and breaking the cable? And here I switch from the geared Aquacraft motors so I didn't have to deal with discontinued gears stripping out. Now I can't keep the flex shafts together and losing expensive props really sucks.

[Fluid]

The usual cause of flex cable breakage near the coupler is insufficient cable support. There may be too much of a gap between the end of the stuffing tube and the coupler. By taking thrust at the motor you are putting the cable in compression which by itself is fine, but if there is an unsupported porting of the cable it can over-flex, stress and break. If you didn’t extend the stuffing tubes when you installed the spacers, that may be the problem.



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[ScarabChris]

I did think about that as well. The stuffing tube is teflon and there is a about 12mm of exposed cable on the one that broke. In these pictures you can see the tubes and flex shafts. The one with the higher tube is the one that didn't break. The one where you see only the flex shaft is the one that broke, It broke about 1mm in the tube.

I will make new tubes and bring the tube to within one mm of the motor coupler. I have some brass and aluminum tubing and a bender, maybe I can make a brass or aluminum stuffing tube. Just have to get the bend just right. Might take a few tries. Yeah...I think you're right, to much space between the

[Fluid]

Teflon tubing provides very little support. I’m not familiar with those lowers but to provide proper support you will need longer metal tubing surrounding the Teflon - if that is possible.



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[ScarabChris]

The lower unit has 3 very short screws, then the lower splits in half exposing the groove for the shaft and stuffing tube. These are .130 flex cables. There won't be space for a metal tube over the teflon but I think I can make an aluminum stuffing tube and eliminate the teflon one. There is a lubrication port in them, that is the little green hose inside you see. You inject lube there and it forces it into the stuffing tube through a little hole in the stuffing tube. I think I can duplicate this with the aluminum tube.

But at the end of the day, I am confident the breaking is from the tube not getting close enough to the motor coupler. I have new flax shafts coming from OSE and they should be here Friday so I'll do the work and snap a few pics in the process so maybe it will be helpful for people in the future.

[ScarabChris]

Another concern....

Thinking with a gap between the drive dog and lower unit when the motor is pushing the boat all the thrust is pushing on the flex shaft and thus the vertical mounted motor. I would guess that is adding to the breaking issue. Is there a way to ensure the thrust will be able to transfer from the drive dog to the lower unit? Is there a formula for cable shrinkage so we can minimize the gap? Maybe a formula taking into account the thickness of the flex shaft in relation to the length of the flax shaft?

How about in the gap a small ring of silicone tubing this way it can "squash" for the shrinkage but apply some of the thrust to the lower unit?

[Fluid]

If you want to replace the Teflon with metal, use brass not aluminum. Most aluminum tubing is very soft and bends easily in use. Brass works great without Teflon.

Taking thrust at the lower unit is tough to do correctly, OBs which do this almost all use square-ended flex cable with no clamping collet. That lets the cable float and adjust itself for cable windup. There is no ?formula? for cable shrinkage, too many variables like cable construction, rpm, prop size, pounds of thrust, stuffing tube ID, etc. Using a short piece of silicon tubing will do nothing to help, it is too soft and will pop off. Best to take the thrust at the motor like 95% of all inboard models.

Yours is a great example of why the thrust location doesn?t matter much. The folks who say that taking thrust at the motor pushes the boat from that point and acts completely differently than taking thrust at the strut. With that kind of thinking, your OP boat which takes thrust vertically should have the transom leap out of the water when applying WOT. The fact that it does not clearly demonstrates the fallacy of that line of thinking.



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[ScarabChris]

I agree brass is the material easier to work with. But I only have enough of the correct size tubing in aluminum so I'll try that first. Here is the lower unit split open. There is a very precise channel for the flex shaft and stuffing tube. There is no way the aluminum will alter it's angle in there so as long as I can keep it lubricated I think it will hold. I think I can bend the aluminum tube pretty easy using the cover plate side of the lower unit as a template.

In the picture of the full outboard you can see the spacer block and anti ventilation plate I made. I know in this picture it looks sloppy but I have some screws out so it's not buttoned up here but when it is fully assembled it is very tight and true. The spacer block was a MUST to make these F1 style motors work and only added 9.5mm to the flex shaft and stuffing tube. The anti ventilation plate really helped reduce the props cavitation while getting on plane. Night and day difference.

[Fluid]

The idea of using metal tubing is to extend it closer to the coupler, giving support to the cable outside the housing. Simply lining the case with aluminum would be worse than using the Teflon since it will wear out very quickly.


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[ScarabChris]

Yeah the metal tube won't work, at least not aluminum. No matter how hard I tried I could not get the bend without slightly flattening the the tube in the bend making it so the flex shaft can not fit much less spin freely. I will have to use the teflon. But I think I can put a short length of aluminum tube over the teflon tube between the exit of the mid section and the motor coupler. This should stabilize the flex shaft in that space just below the motor coupler.

Here is a dry fit of a new flex cable and teflon tube. You can see how tight it is and how tight the bend is. I may have to buy those scale Evinrude outboards with the gears. LOL

[ScarabChris]

Ok so I tore down the motors and rebuilt them with all new bearings, new flex shafts and found a couple things that could have been causing the breaking. The brushless motor the way it mounts put a decent bend on the flex shaft between the motor coupler and where it enters the mid section. So I made some shims. I wanted to make it so I can lower the motor on the mount and have the flex shaft enter the motor coupler without even moving it. It is all tight now. I am going to test now.

If it gives me any crap I am starting over with two of these. https://www.offshoreelectrics.com/pr...prod=bel-99999

[ScarabChris]

Broke a flex shaft in the first minute but upon disassembly I found this was my fault. I did not solder the motor end of the flex shaft very good so it unwound inside the coupler. I have extra flex shafts so I am going to repair the one that failed and tear down the other one just to be sure I didn't make the same mistake on that side.

If there is a problem on tomorrow mornings test I will be on OSE getting those new outboards. $300 + new brushless motors. Wife will be irritated but oh well. She spends money worse than me. LOL

[Fluid]

I’m a bit confused, a new cable should not unwind inside the coupler unless you used reverse. The torque will tighten the cable, not unwind it. I haven’t soldered a cable in over a decade, but sometimes have used green LockTite. Haven’t had a cable unwind in years, even on 100 mph boats. Do you have the correct cable on the correct motor?




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[ScarabChris]

I believe the solder job I did on the end with the flat spot was not good. I have redone it with new flex cables. I will admit it has been many years since I worked with flex cables so I may be messing it up. Especially with these small cables. I am used to the 1/4" shafts which are much easier to work with. This is what I am hoping will be good. This is much more stout than the one that just broke.

[ScarabChris]

Ok then I think I got it dialed in....for now at least. I got it together last night, ran it for a good 20 mins this morning and it worked great. I redid the broken flex shaft and I pulled the other one out and beefed up the soldering on the motor end just to be sure.

Hauls ass and everything stayed cool. Water comes out in a nice stream on the back of the motors though those aluminum tubes which I did like that to make flushing with fresh water easy. The boat picks up water at high speed, water goes through the ESC's first then that water goes through the motors and out those little tubes.

Anyway, I'm going to check tightness on all the motor coupler and drive dog set screws and run it several times today and if the hold up I can hold off on $300+ for a new pair of motors..for now.