OPINION on broken shaft

[Speedfreek]

Just would like to know what you guys think, happend to this shaft .
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[NativePaul]

My guess is you ran an out of balance prop and/or prop nut and a crack propagated from the bottom left in picture 3 which allowed the shaft to bend and threw it further out of balance and quickly snapped the end off.

Why are the step downs in shaft diameter there? Deleting them would make for a stronger shaft, or if they have to be there putting a raduis on there inside edges would spread out the stress load a little and make it a bit less failure prone.

[martin]

What is the diameter of the step down & prop size on that shaft ? is it a 3/16" bore prop. As you are running very large powerfull TP 5850 motors on 12s, also as said a step down with no radius will also break easier. It also looked like there was not enough gap between the drive dog & the stinger to allow for flex wind up.

[Speedfreek]

My guess is you ran an out of balance prop and/or prop nut and a crack propagated from the bottom left in picture 3 which allowed the shaft to bend and threw it further out of balance and quickly snapped the end off.

Why are the step downs in shaft diameter there? Deleting them would make for a stronger shaft, or if they have to be there putting a raduis on there inside edges would spread out the stress load a little and make it a bit less failure prone.

Prop was a Andy brown ? I would think it would be balanced , Not my shafts and yes I agree that there should be a radius there

[Speedfreek]

What is the diameter of the step down & prop size on that shaft ? is it a 3/16" bore prop. As you are running very large powerfull TP 5850 motors on 12s, also as said a step down with no radius will also break easier. It also looked like there was not enough gap between the drive dog & the stinger to allow for flex wind up.

Yea 3/16 , I have gone up to the next size now and I machined it so I can have more of a gap too.IMG-20190210-WA0007.jpg

[Johnc]

to me looks like of scarring on that shaft (maybe more grease or more frequent greasing)

[Johnc]

although after thinking about it, it would effect the flex shaft not where it broke, but still in all looks like it should have some grease in there

[srislash]

to me looks like of scarring on that shaft (maybe more grease or more frequent greasing)

That is from the bearing. BTW Speedfreak I run two bearings at each end to carry the load and last longer. Once they wear they could cause a wobble and that may happen quickly with only one bearing. Something I’ve been doing on the step down shafts is run a 1/4” drive dog which locks the prop. Just a theory but I haven’t had a stepdown shaft fail.

[Speedfreek]

That is from the bearing. BTW Speedfreak I run two bearings at each end to carry the load and last longer. Once they wear they could cause a wobble and that may happen quickly with only one bearing. Something I’ve been doing on the step down shafts is run a 1/4” drive dog which locks the prop. Just a theory but I haven’t had a stepdown shaft fail.

This drive does have two bearings at the prop end , I have put the 1/4 drive in there now and I have allowed 6.4 mm of gap too where before it would not allow me to do that , that's what I think happend.

[Speedfreek]

What is the diameter of the step down & prop size on that shaft ? is it a 3/16" bore prop. As you are running very large powerfull TP 5850 motors on 12s, also as said a step down with no radius will also break easier. It also looked like there was not enough gap between the drive dog & the stinger to allow for flex wind up.

Yep that's what I think

[Speedfreek]

So how much would you say a 1/4 drive flex approx 8 in long would contract on eceleration ?

[srislash]

So how much would you say a 1/4 drive flex approx 8 in long would contract on eceleration ?

I would not think it would be much. 8” is fairly short. I know my 6” 3/16” have as little as 2mm gap with no issue. It’s all a matter of experimentation to find it. The little white thrust washers can help with this

[Speedfreek]

I would not think it would be much. 8” is fairly short. I know my 6” 3/16” have as little as 2mm gap with no issue. It’s all a matter of experimentation to find it. The little white thrust washers can help with this

it's 7" long close enough i have 6mm gap now and 1/4 dog drive shaft , it better not brake now these props cost to much to loose.

[MARK FORREST]

whats it out of probably not balanced prop could be a bad bearing too

[Speedfreek]

whats it out of probably not balanced prop could be a bad bearing too

Bearings are three runs old , prop was a Andy Brown.
I might just put in oil-immersed sintered bronze bushings front and rear less headaches , expecialy in salt water...