Flex shaft gap?

[viper1]

How much gap should I leave between the strut and drive dog to allow for shortening of the flex shaft under load?

It's a 3/16" one piece shaft, and the flexible part of the shaft is only 222mm (or 8 3/4 inch) long.

Thanks in advance,
GJ

[dana]

Same gap as flex shaft size. So, 3/16 gap

[viper1]

Ok thanks, that seems like a lot, for such a relative short length ?

[dana]

Maybe, but it's just what I always do.

[martin]

If the gap is slightly more than you need it makes no difference, if its not enough you risk breaking the flex or loading the motor more by the dog running against the strut with the flex not reaching full wind up. As said set the gap on flexes at whatever the diameter of the flex is & youll be safe.

[znus]

Check your teflon washer for wear. It should show a little. The prop should push on the strut/stinger, not (through the flex) on the motor shaft. I think 3/16 is too much, especially on a short flexshaft. So, adjust and check that washer for wear.

[dana]

Washer is not necessary for 1 piece flex

[martin]

If your using a 1 piece flex & stub the dog wont come into contact with the washer or stub, if it does then your gap is to small & you risk breaking the flex as it wont of reached full wind up. 2 piece flex & stubs are different as the flex isnt fixed to the stub & the flex floats in the square drive so your able to set the dog close to the washer & the dog runs against the washer as its designed to do with this type of shaft.

[iamandrew]

Yeah i leave the gap and still use a teflon washer between the drive dog and the shaft. Its good to have just incase somthing slips

[osprey21]

set the gap on flexes at whatever the diameter of the flex is

This.

[viper1]

Setle Down, Setle Down ! :-)

I think I opened the good old can 'O worms here !

But I can see both points of view. I followed my gut feeling before I posted this and I do have a teflon washer between strut & dog. Indeed it shouldn't do much, but if the gap would be on the (too) small side, I rather have that rubbing first than metal-to-metal.

New question that evolved from this; What does a thrust bearing look like? I guess the motor bearings won't last long if all axial forces are projected on the motor?

[znus]

Again, the stinger/strut should take the axial force from your DD/prop, not the motor. Do use a washer and check for wear. You can start at 3/16" and work your way inwards. Im guessing you'll stop at less than a tenth of an inch. A short axle doesn't flex that much.

The simplest thrust bearing is just a fat teflon washer, or it can look like this http://www.offshoreelectrics.com/pro...?prod=dh-53136 . Mount your motor coupler flush to it. In a proper (but maybe not Dana-approved ) setup the thrust bearing will take the forces at low speed while the washer will take them at high.

[znus]

Well, it's what happens under load.

[viper1]

The simplest thrust bearing is just a fat teflon washer,

That's funny you say that,
Guess my 'gut feeling' was not that far off, as I also already made a teflon washer there believe it or not !

I actually just did an 'experiment', spanner on the collet and another one on the prop nut (bottomed out), with extreme twisting force (more than motor will ever produce) the shortening was about 1.0 mm max, or .04 inch.
When adding more force (and here I was scared I would brake the shaft, but a good test anyway)Then when finally the shaft slipped in the collet, it screwed itself out a bit, as the shaft works like it is threaded, so automatically increasing the gap.

[znus]

That's funny you say that,
Guess my 'gut feeling' was not that far off, as I also already made a teflon washer there believe it or not !

I actually just did an 'experiment', spanner on the collet and another one on the prop nut (bottomed out), with extreme twisting force (more than motor will ever produce) the shortening was about 1.0 mm max, or .04 inch.
When adding more force (and here I was scared I would brake the shaft, but a good test anyway)Then when finally the shaft slipped in the collet, it screwed itself out a bit, as the shaft works like it is threaded, so automatically increasing the gap.

Yep, there's no way a flex of that length will shorten 4,76 mm (or more) under load. I usually recommend starting at half of the flex diametre, even for longer shafts. But ALWAYS check the teflon washer for wear. I run my own 1/4" one piece shaft (of ~30 cm) with a gap of less than 2 mm. It's a 5682 motor with a 50/3 prop, so it's a pretty beefy setup. Of course a 1/4" shaft is stronger and will flex less.

Funny test with the spanners and I agree - that home made load you put on it will be the most torque it'll ever see. When going full speed, getting airborne and free revving and then back into the water - conditions are at their worst and that's what you gotta prepare for.

The teflon washer in the rear will absolutely not heat up, at all. The square end two piece shafts depend on 1000's of watts going through that washer. It pushes the whole boat forward, which is its duty.

So, for the third time - adjust for a little wear on the teflon washer and your golden