New to Flex Shafts

[TomG]

New to flex shafts. I previously ran strictly hard shafts with universals.

Do flex shafts change there length as RPM increases. I was told instead of running a thrust washer, I should leave a gap of a 3/16 to 1/4 inch between the back of the strut and and the prop drive dog. If this is true what stops the strut bushing from backing out of the strut.

[urbs00007]

New to flex shafts. I previously ran strictly hard shafts with universals.

Do flex shafts change there length as RPM increases. I was told instead of running a thrust washer, I should leave a gap of a 3/16 to 1/4 inch between the back of the strut and and the prop drive dog. If this is true what stops the strut bushing from backing out of the strut.

the drive dog stops the bushing from coming out. It floats back and forth with no problem. And since the flex cable shortens while being run, the amount the bushing can come out is really not much at all.

[NativePaul]

Do flex shafts change there length as RPM increases. I was told instead of running a thrust washer, I should leave a gap of a 3/16 to 1/4 inch between the back of the strut and and the prop drive dog. If this is true what stops the strut bushing from backing out of the strut.

Firstly welcome to the future, flex drives whether traditional spiral wrapped flex shafts or flexible wire shafts allow you to run much less lift at the prop, which results in you being able to run much more power to the prop before blowing it out, and/or CoGs further forward for more stability.

You are along the right tracks with your first statement, but not quite there. Spiral wrapped flex shafts do indeed shrink under load, but they shrink more with more torque applied, not with more RPM gain. For example the general rule of thumb is to leave a gap of 1 flex shaft thickness between the drive dog and the strut, but here in the UK because we run lighter boats with higher RPMs and much smaller props than is usual in USA, when we run flexies (they are dieing out in favour or wire drives) typically the gaps we leave are much smaller. You may think that a symantic difference with an irrelavent example, but if you ran a gap that was just big enough and you propped up, you would lose RPM, but the flex would wind up more, and that gap would no longer be enough leading to exessive drivetrain losses and quite possibly letting the smoke out of your ESC or motor.

With the strut bushings, traditional bushings whether lead/teflon, sintered bronze or PEEK should be a very tight friction fit in the strut and hold themselves in place firmly. Ball races need to have a bearing retainer applied which can be chemical eg loctite, or mechanical eg an "E" clip. I think you must be refering to floating bushings as found in some proboat RTRs, but I have no expreiance with these so I will leave that for others to answer.

[Fluid]

Floating bushings are very common in the US, they are found on a great many FE, nitro and gas race boats, not just on cheap RTRs. If it works, they will use it.


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[Jeff39]

IMG_3480.JPGAnyone have experience with the Dragon Hobby F1 boats? I'm breaking a lot of flex shafts.