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Author Topic: Prop shaft jamming problem. Help needed.  (Read 2440 times)

Fastfaz

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Prop shaft jamming problem. Help needed.
« on: July 29, 2016, 03:12:00 PM »

 Greetings everyone,
     Can anyone hep me out with the following problem which I have never had occur before and am bashing my head against the wall trying to resolve it.
     The boat is my take on an MMM Norton cross with the following spec- twin screw,70mm Kort nozzles, Kort eze prop shafts, 70mm Propshop 4 blade Kort props, twin T12 motors with U/J type couplings.
     Now the problem- everything is now in place shafts are parallel and square to the centre line, Kort nozzles are plumb and square to centre line, motors at the correct angle and straight (motors were run to check alignment prior to final fixing in place). However when I run the motors clockwise no problem whatsoever but when I run them anti clockwise in a few seconds the shaft screws tight and would completely lock up if left running.
     The detail of assembly from the prop through to the motor is thus- propeller,brass locknut, brass washer, into the Korteze shaft, inside the hull brass washer,locknut (end float in shaft), locknut tightened onto the splined brass insert of the coupling, plastic centre of the U/J type coupling, splined brass insert onto the drive shaft of the motor.
     I have checked and double/treble checked the clearance and tightness of everything, I have removed the motors to check the spin off the shafts for binding (all fine), had the motors running forwards and backwards in the vice (no problems and smooth), I've swapped the two shafts over (still the same), I tried a second locknut in front of the first brass nut at the shaft end locking the insert to this and it still jams.
     I tried a completely different motor and it still jammed going anti clockwise, I've changed the coupling to the MMM rubber type it still jams. I can't seem to figure out how this can happen with the insert and brass nut locking themselves even if the inside of the hull end is right at the end of the thread with freeplay. Its driving me mad!
     Help please? all advice greatly appreciated, what I do not understand is that I have built all of my boats prior to this exactly the same way but never had this problem.
       Thanks in advance.
             Regards,
                  Faz. <:( <:( <:(
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Stavros

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Re: Prop shaft jamming problem. Help needed.
« Reply #1 on: July 29, 2016, 04:41:22 PM »

Use some thread lock....if you aint got any pinch some nail varnish or even paint

Dave
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chipmonk

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Re: Prop shaft jamming problem. Help needed.
« Reply #2 on: July 29, 2016, 04:57:35 PM »

Hi Faz, seems like there's movement  there some where. Thread lock should sort it out. It only takes a tiny amount of movement in the lock nuts and oops its locked again. I had a similar problem with a police launch a very long time ago. Since then I only use shafts threaded on the prop end with grub screws on the in board end, no problems since. I know it doesn't help but if all else fails new inner shafts with one plain end?
Cheers Chris M
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Stan

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Re: Prop shaft jamming problem. Help needed.
« Reply #3 on: July 29, 2016, 05:30:44 PM »

HI fast faz do you have any collets on your shafts see picture. Fitting these stops any moment on the shaft. I have to assume when the props are spinning anti clockwise the force is pulling the locknut against the shaft by fitting these collects this kills any moment on the inner shaft either forward or backwards no saying this is the problem but worth a try.

Stan {:-{ {:-{ {:-{ {:-{ {:-{ {:-{may be  :-)) :-)) :-)) :-)) :-))
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morley bill 1

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Re: Prop shaft jamming problem. Help needed.
« Reply #4 on: July 29, 2016, 05:54:02 PM »

this problem only arises with shafts threaded at both ends so I use plain inner ends if you have a mate with a lathe get them turned plain  Bill..
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Fastfaz

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Re: Prop shaft jamming problem. Help needed.
« Reply #5 on: July 29, 2016, 07:07:08 PM »

   Thanks guys for the replies I've got a sneaky feeling that the threaded portion of the shaft is too long i.e. the thread will actually be inside the tube so centrifugal force (outside interference) would turn the threaded part to the right and tighten up the drive which would shorten the shaft hence the lock up.
    I had to walk away from it this afternoon as it was driving me nuts but it will not defeat me (he hopes).
             Thanks again guys.
                  Cheers (beer in hand),
                        Faz.
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morley bill 1

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Re: Prop shaft jamming problem. Help needed.
« Reply #6 on: July 29, 2016, 07:30:05 PM »

knock the bearing out of the tube and shorten the tube by 1/4 inch that will cure the threads been too long  Bill..
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malcolmfrary

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Re: Prop shaft jamming problem. Help needed.
« Reply #7 on: July 30, 2016, 08:21:54 AM »

Any locknuts need to be locked against the item that they are locking, not just tightened.  Use thrust washers between the rotating and static faces, thread locking compound helps.  If the same problem happens at the outer end, you lose the prop.
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Fastfaz

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Re: Prop shaft jamming problem. Help needed.
« Reply #8 on: July 30, 2016, 08:40:18 PM »

   Update.
      After considering arson on the boat I (after help from friends) tried other alternatives none of which worked so I thought I will buy some new couplings at Formby model shop but while I'm there ask  Steve for advice. We checked everything and think that the culprit is the "inferior" red coupling however when we connected a new black coupling the jamming occurred again although it took longer to lock up, we then checked the motor end float and it seems that this is a lot more than the other motor hence the jammimg (both T12's are new and unused prior to this) so the next stage is to Araldite the splined inserts into the coupling and see what happens.
     I am extremely bemused as to how or why this happens on only one motor and shaft of a pair so I will swap the motors to shafts and see what happens. Basically I'm a lot "xxxxx" off with the cost of the shafts/motors and what has happened but hey I will resolve it and I am sure that if the T12 is the culprit Brian will sort something out.
       I will keep you posted (sorry about the pun).
              Cheers,
                    Faz.
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malcolmfrary

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Re: Prop shaft jamming problem. Help needed.
« Reply #9 on: July 31, 2016, 11:07:49 AM »

If the threaded universal is not locked to the shaft, it will try to wind in one direction because of its inertia when the shaft spins.  It will either try to wind towards the shaft outer tube, cutting the shaft end play to zero thus becoming a brake, or it will try to scrunch the UJ, with similar results.  A picture of the area would help.
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Fastfaz

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Re: Prop shaft jamming problem. Help needed.
« Reply #10 on: August 01, 2016, 08:13:33 PM »

   Greetings everyone,
       I am very pleased to say that I have resolved the jamming problem although I am not exactly sure of the culprit. After almost throwing the boat in the bin I decided to completely strip out the motors and mounts then start from scratch re mounting everything, miraculously after realigning and quadruple checking everything we have lift off and all is working as it should do. I checked the symmetry of the hull and the port side is a slightly different shape to the starboard side this together with the way in which the motor sat into the mount I think may have been the cause but I cannot confirm this.
     Thanks to everyone who posted with ideas and a big thanks to John Poll and my mate Alan Beswick.
           Cheers guys,
                Faz.
P.S. Won't be needing the fire extinguisher now! :-)) :-)) :-))
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