Flex cable and its expansion for FE's

[barryaclarke]

I have moved into FE from gas and have a nagging question. In gas, one allows extra room for expansion for the flex cable when it gets hot from running with RPM's around 20,000. In electrics, a .187 flex cable is secured to the motor by a coupler, then the cable runs down to the prop shaft where it is again secured, and then attached to the strut. In this case the strut is an inline aluminum rudder and strut system for catamarans #ose-80085. As I am looking at this, how does the flex cable expand from heating without fracturing something between the motor mount and strut when running RPM's of 30,000 which is 50% faster than gas?

[srislash]

Most of us leave space between the drive dog and the strut with a washer(usually teflon)equal to the width of the cable.We then install a thrust bearing between the coupler and the motor to take load off the motor internals.Or you can run a square shaft system.And it is my understanding that it is done because of flexshaft windup rather than heat.Although I suppose heat could factor in.

[JT13031]

I'm kind of new to FE boats but have spent enough time on here I think I can shed some light. The direction the cable is wound and the way the motor spins and the prop pushing and what not the flex cable is actually twisted up tighter so the prop gets sucked in closer to the strut and theoretically I'm guessing the that by the cable getting twisted tighter it gets smaller

[Rumdog]

The cable doesnt expand from heat in FE, or Gas boats. It will actually shrink in length under load and gain a very minimal amt. of diameter.
run a gap width between the drive dog and strut that is equal to the cables diameter. No telon washer needed as the drive dog shouldn contact the strut anyhow.

[Fluid]

I am not new to FE boats, I've been racing them for decades. Most do exactly the same thing that most gas racers do with their flex cables - leave a one-cable width gap between the strut and the drive dog to allow for cable shrinkage under load. There is no need for a thrust washer in that gap, although it gives some less-experienced guys comfort to use one. Experienced racers usually take the thrust on the motor bearings since axial thrust is what most are designed for (unlike those in many gas engine bearings which are designed for radial loads). Few FE racers take the thrust on the strut. Thrust washers at the motor end of the cable are only needed on cheap FE motors with cheap endbells - but if the washers are offered for sale some guys will use them regardless of need.


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[Kris Flynn]

Hi Jay,

Just to clarify something you mention about cheaper motors and end balls...

I run the older style Neu motors with the spot punched end balls, not the later models with screws. I have a fear of them coming out under load, or maybe they should just be loctited in?

Thanks
Kris

[Fluid]

Good catch, that is what I meant by "cheap endbell", not just 'cheap motors'. Either poor-quality bearings or an endbell not properly attached. Neu fixed that issue years ago, and although not many folks had the endbell push off, it did happen. I last saw it this summer at a race where the first place boat in P hydro lost the final heat due to the endbell coming off. You can send the motor back to Neu for a retrofit, use loctite, or a thrust bearing. The problem with the thrust bearing is that it is very difficult to get the correct pre-load with the lack of radial play on the Neu motors. Without the correct preload, the bearing either does nothing, or it creates a bind and increases friction.


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[Insaniac]

I left about 1/8" gap on my FE-30 and even that wasn't enough. I ended up with a flare on the end of my aluminum strut from the cable winding tight up under load. Leave AT LEAST one cable diameter between the strut and drive dog or you could end up snapping your cable or pulling the end bell off the motor!

[barryaclarke]

You are correct. Not too sure what I was thinking when I said heat. Thanks

[Kris Flynn]

Thanks Jay, I will try loctite next, just ran my SAW Sport hydro for 2 runs yesterday and the endball moved 1/16".

You had explained to me the way to set pre-load on thrust bearings...I just need to get some good quality ones...

Kris