Do Graupner flex couplers shorten when under load?

[Make-a-Wake]

I run several of these flex couplers and wondered if they shorten when loaded up. Im asking because i dont want my straight shaft drive to "pull in" and cause undue friction. Should i leave a small gap between the drive dog and the teflon washer/oil lite bushing? Thanks!

[befu]

Any flex cable will shrink. Lets say you leave 3/16" on 3/16 drive, and the flex cable is 12" long. If the cable shrinks .015" per inch of cable under load, it will shrink by 0.018", or about 3/16"

Now, those numbers are just made up. But, if you run a solid shaft with a flex shaft coupler that is 1.5" of flex cable, under load that drive would shorten by 0.022".

So, yes it will shrink, but it will only be a fraction of what a full length cable will shrink, so if you leave .030 to .060 you should be good. I do this same thing as I run several solid shafts with pieces of flex cable as connectors.

The second portion is due to thrust. As you push on the end of the shaft (prop), the whole cable and motor will slide up until you run out of end play against a bearing surface. A flex cable will also shrink a little due to this, but it accounted for in the gap that is left.

So leave a gap, but much less than a full flex cable.

Long answer, huh

Brian

[Jeff Wohlt]

The coupler, as posted, does not shorten....the cable does! Yes, you need a gap at the prop end between the drive dog and the strut. I like about 3/16". This also depends on the length of the cable as well. You should be good with that amount.

[785boats]

I think he's talking about these couplers Jeff.
http://www.offshoreelectrics.com/pro...?prod=grp-3345

If so, I think the shrinkage would be minimal.

An easy way to ckeck is to hold the motor shaft end of the coupler & try to turn the prop in the reverse direction that it would normaly rotate (to simulate the load) & see how much the gap between the drive dog & the strut changes.
This works for full length flex shafts too.
A word of caution ...wrap a piece of cloth around the prop so you don't cut yourself, as you need to apply as much force as possible without breaking the flex. They don't shrink as much as you think.
Cheers.
Paul.

[Make-a-Wake]

Thanks Gents.................yes that is the one, 65mm long 4mm to 5mm. Im asking because i run one with a Neu Motor that is quite powerful(the cable hasnt come loose inside the ends yet), and when it comes back to shore, the shaft has "pulled" out about about 1/4 inch. I guess its actually pulling the straight shaft a bit back out of the end of the coupler???

Here's the main thing in my mind, i certainly cant see how it runs under load in the water, and i also dont want all the thrust on the bearings of an expensive motor................... the 1/4 to 3/16 inch is ok in everyones opinions so far...............I guess some faith is in order here!

[785boats]

I don't there's any way that the short bit of flex can shrink 1/4" under load.
Is there a flat spot on the shafts (both motor & prop shaft) where the Allen screws bite? And are the screws done up nice & tight on the flat spots.

[Make-a-Wake]

Yes Paul.............there are flat spots and i cant just push the shaft back in.......its tight where its sits when it comes in. I cant figure out why it would pull "out" when the prop is pushing it in.................

[785boats]

Got me beat too then pal. I'm out of ideas.
Sorry.