Originally Posted by
TomG
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.
Paul Upton-Taylor, Greased Weasel Racing.
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