Flex Shaft Questions

[R2315]

Hi everyone,

I'm in the process of adjusting the length on a flex shaft and have a few questions. 1) Should I be leaving a small space between nylon washer before the drive dog and the drive shaft? 2) On the shaft there is a small flat space for the drive dog set screw to fasten into, with the drive dog in that position and the prop on, there is to much room between the end of the prop and the prop nut. I can move it forward, lock the set screw on the unflattened portion of the shaft and cut flex shaft accordingly but I will be loosing 5mm permanently and I'm guessing the drive dog would not be as secure, any suggestions? I don't want to limit myself from being able to put other props that might be longer than a X440 or a M440 that I currently have, anyone know if that might become a issue? Also how much grease are you guys using on or in the shaft?

Thanks,
Ryan

[fweasel]

Yes, you need to leave a gap between the drive dog and the strut. I use the thickness of the flex cable as a rule of thumb. In your first picture, you can loose the washer (really no need for it with a proper gap) and decrease your gap by a few mm.

That prop has a pretty short hub on it so it's very likely your next prop is longer. If you find that the drive dog grub screw walks along the flattened edge of the prop shaft as soon as you begin to tighten the lock nut, you can dimple the flat spot with a small drill bit to give the grub screw something to grab onto. More often than not, if I put on a short prop that leaves too much prop shaft showing on the back end, I just fill the space with a few aluminum spacers before I put the lock nut on.

[Prodrvr]

To add to that, you want some of the threads going past the end of the prop after you tighten the nut. Think about how far the studs are sticking out past the hub of a race car wheel.

[Prodrvr]

Oh and the grease question...I never measured it, but maybe if I did, it would be in between 1/4 and 1/2 a teaspoon. I put a little between my thumb and pointer finger and let it ride on the cable as I screw in the cable with the other hand. Once the cable bottoms in the collet (I have the cable trimmed so I get a 3/16" gap), wipe off the extra grease with some paper towels and that's it. You don't need that much grease in there.

[R2315]

Okay, I have a .150 shaft to 5 mm - 3/16 so I will leave about an 1/8" gap and see if I can find some 3/8" to 1/4" spacers for the end of the prop before the nut.

Thanks for the quick responses, much appreciated!

Ryan

[Fluid]

Depending on the length of the cable, follow the advice above. Leave a gap equal to the diameter of the stub shaft - 0.187". Better too much gap than too little, you DO NOT want the drive dog rubbing on the back of the strut.

.

[Luck as a Constant]

what they said.

[rol243]

don,t forget to solder the motor end of flex shaft so it doesn,t unwind its end.

[R2315]

Okay, thanks everyone!

Ryan

[Fluid]

don,t forget to solder the motor end of flex shaft so it doesn,t unwind its end.

This used to be 'de rigeur' for cables, but I haven't done it for years. With quality cable and collet couplers, a clean cut is all that's necessary today.

.

[rol243]

yes Fluid maybe with quality cables such as aeromarine / u.s. or the great hughey but the majority of cables supplied with rtr boats these days don,t really have what you would call quality cables , made in ***** i have been buying some of these cheaper variety 1/4 inch flex cables and if you don,t silver solder the ends , yes the winding will part as i did with 2 of these cables before i realised they needed attention the smaller .187 flexy is even worse.

[iop65]

you leave a 4,76mm gap for a 4,76mm flex?
I think a 4,76 mm flex has to be about a meter long before it shrinks that much under load
Never solderd the tip of a flex, heating up the steel above a certain temp. makes it brittle,you dont want that either

[R2315]

okay so I'm getting some conflicting info and just want to clarify. I have read a couple of times that I wouldn't need to worry about the flex shrinking under load when it is a two piece like the one I'm using (Octura flex on 5mm - 3/16" Shaft), is that accurate?

[fweasel]

That's not a two piece cable. Yes, there are two components: flex cable and prop shaft, but they are soldered/bonded together as a single piece. An actual two piece cable has a slip joint or dog bone style interface between the two pieces which prevents a flex cable wind-up from affecting the position of the outer prop shaft in relation to the strut.

[rol243]

as mentioned just leave a gap between drive dog and strut end of say 3/16 inch and all will be fine.

[CraigP]

Fweasel, the true 2-piece system still has a problem. When under high torque, that square drive section doesn't want to float, it can lock up. I think it's safe to say that ANY flex drive application should have some safety distance. The only true prop shaft drive that really is safe from windup is a straight shaft. Then you can be on bearings and collars. Even a wire drive can have changes in overall length.

[R2315]

Got it, Thanks all!

[iop65]

Leaving a gap too big :the solid part of your shaft is not using the whole lenght of the first bushing

[iop65]

If a 4,76mm flex would shrink about 4mm ,this would mean about 71cubic mm of steel is "gone"
Hope hou have enough gap between flex and liner for these 71 cubic mm😉

[CraigP]

Good points... iop, that's probably why my attempt to use a SS tube instead of brass didn't work out. The wall on SS is .028" and on brass it's 0.014". I didn't break anything, but the datalogger showed higher power used while GPS reported slightly lower speed. It got pretty hot hitting the throttle on the stand! I changed it out to brass... Lesson learned.

[Prodrvr]

So you used a tube with double the wall thickness and wonder why it didn't work? Am I correct or did I miss something? Just woke up lol.

[CraigP]

Not wondering anymore! At the time I ordered parts, OSE didn't have any brass. I saw the stainless and talked myself into it. What I actually talked myself into was changing a freakin' strut tube! I'm lucky I didn't break something...

[CraigP]

Forest Gump: "Stupid is as stupid does, can I have another chocolate?"

[Prodrvr]

I have some different stainless tubing on the way to me now. I need to do some experimenting.

[CraigP]

The SS is inherently has less friction, surface hardness matches the steel on the flex better. I think you can get it in thin wall. My experience told me not to mess with those diametric clearances. The "slop" needs to be in there... BTW, not running a liner, that was 1/4 tube to 3/16 flex shaft, all OSE parts...

[Prodrvr]

Yes I agree...needs some slop. I don't run liners either.