Propshaft coming away from coupling

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24 May 2020 at 16:27 #2923
Jonny Machon
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@jonnymachon57538
Propshaft coming away from coupling
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24 May 2020 at 16:27 #88343
Jonny Machon
Participant
@jonnymachon57538
Hi guys

Took my boat out today for a test and all fine no vibrations or leaks and runs nicely however under high forward throttle the propshaft is unscrewing itself from the coupling and forcing motor backwards

Any suggestions why this maybe and how I can fix it

Motor is a mtroniks m600

Prop 40mm

Propshaft is raboesch waterproofshaft

Raboesch coupling with inserts

Running 3s lipos

Thanks
24 May 2020 at 16:59 #88344
Colin Bishop
Moderator
@colinbishop34627
You need a locknut screwed up against the coupling.

Colin
24 May 2020 at 17:06 #88346
ashley needham
Participant
@ashleyneedham69188
Jonny. Welcome to the forum.

As Colin says, a locknut is needed at both the coupling end and prop end ideally.

Some shaft inners are plain though and in this case a grub screw is used. A flat filed on the shaft would then be of assistance.

Ashley
26 May 2020 at 09:43 #88414
Malcolm Frary
Participant
@malcolmfrary95515
An unlocked coupler has a motor at one end turning it, and a prop that doesn't want to turn at the other. If it is just sitting on the thread unlocked, it will move on the thread. A correctly applied locknut and/or thread locking compound will prevent this, as will following Ashley's suggestion of locking it in place using the grub screw on a flat filed on the shaft. The flat is a must – grubscrewing on to a thread might well render the thing unmovable later.
8 June 2020 at 22:45 #88849
Dave Cooper 6
Participant
@davecooper6
Hi Jonny

The good news is that the flat doesn't need to be all that big – just enough for the end of the grub-screw to bite on is good enough. As they say above, best to use some 'Loctite' (or, similar thread compound) as well on the grub's thread.

I was a bit tight for space on my little RAF launch, so I took the rudder and prop shaft out to do it. Just a light touch on the bench grinder will do it, or, use a small hand file on the coupling end of the shaft.

If you have a fairly large hull you may be able to do it 'in-situ'. Lock nuts also seem like a good idea, although I didn't find I needed any…

Good luck with the mod'

Dave C
12 June 2020 at 20:07 #89006
captainslog
Participant
@captainslog
This makes me wonder why the shaft threads arnt reverse threaded,as in an angle grinder ( and other things) is there any specific reason why this isnt so? I agree as well, i always use lock nuts but at the motor shaft end only. seems to work ok. dont ask why but it was a long wait on the bank of windless lake on my own and an eventual swim to retrieve.
12 June 2020 at 20:40 #89011
ashley needham
Participant
@ashleyneedham69188
I suspect that as the forces are quite minor, the cost of faffing about with reverse threads is not worth the extra cost. A locknut should be more than enough.

Ashley
12 June 2020 at 22:05 #89014
Colin Bishop
Moderator
@colinbishop34627
And if you have twin screws counter rotating then one will inevitably be affected.

As Ashley says, a locknut is just good engineering practice.

Colin
13 June 2020 at 10:59 #89032
Malcolm Frary
Participant
@malcolmfrary95515
Even with a reverse thread, locknuts will still be needed. On many boats that have reverse, reverse is the boats brakes, useful on occasion for avoiding expensive damage to concrete banks.

An unlocked connection between shaft and either coupler or prop will allow the inertia of one or the other to move whenever force is applied at one end with resistance at the other. Turning one way, something will try to unwind, turning the other way, things will tighten up. Both are undesirable, and it doesn't matter whether cheap standard threads are involved, or expensive special ones. The expensive ones just cost more to replace when they fall off.

As far as I know, angle grinders only turn one way, and I'm fairly sure that they turn to cause a standard thread to self tighten. Tooling to produce left hand threads is more expensive than for normal threads, the captains of industry are likely to go for the lower cost option.

I wonder if the OP got sorted, or whether it was a drive-by posting.

Edited By Malcolm Frary on 13/06/2020 11:05:41
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