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Author Topic: Prop shafts all wrong!  (Read 2939 times)

flag-d

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Prop shafts all wrong!
« on: May 29, 2006, 12:08:41 AM »

 ???
I have a Fairmile Type D MTB/MGB running 4 motors (Graupner Speed 500's) and 4 shafts.  The shafts run in a tube, then a P-frame, then an A-frame, then the prop.  I took hours lining everything up but have still ended up with 4 stiff shafts.  Now seriously considering removing the lot and starting again.  How do I ensure I don't make the same mistake again (4 times!)?

Mike
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Doc

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Re: Prop shafts all wrong!
« Reply #1 on: May 29, 2006, 02:39:58 AM »

Mike,
I would think that sort of 'tacking' the stuffing box/shaft/supports, making small adjustments (wiggle the @#$ things till they seem loose, not tight), then gluing things down would be the way to do it.  Lots of 'assumed' things in that.  The 'key' phrase being don't lock things down solid until you're sure it's right (uh, a little more ~sure~ than the last time, right?).  ...  Which basically, is exactly what you said you'd have to do.  Remove them and do it over... times 4 (total, hopefully not each!).
 - 'Doc

PS - I can hear you saying, "Look at him laughing about this, he thinks it's funny!".  And you're partially right, I am, but only cuz I don't feel so 'alone' now...
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boatmadman

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Re: Prop shafts all wrong!
« Reply #2 on: May 29, 2006, 06:25:06 AM »

In addition to Doc's suggestion, make some kind of jig to hold the shafts (not the tubes or bearing frames) rigid relative to each other, (one for motor end, one for prop end) while you position and tack the tubes and frames in.

Ian
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Shipmate60

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Re: Prop shafts all wrong!
« Reply #3 on: May 29, 2006, 11:11:01 AM »

for multi-shafts I usually solder them together outside the model and fit as one unit.
I measure the hull outlets carefully and transfer these dimentions to a piece of paper, and draw in the outline of the shafts parallel to each other.
Gently clamp in position and soft solder brass oblong section between each shaft, with another couple of strengtheners between them.
As an aside I also solder on a small BA bolt to use as a motor earthing point in a suitable acessable position.
once you are happy the shafts are rigid together i open the hull to accept the shafts.
I draw a centre line inside the hull for reference.
I tape the shafts in the hull then dry fit the "A" Frames to the hull, but VERY loosely.
Invert the hull and check the position relative to the hull of the prop positions.
Measure till satisfied, go and make a coffee, then come back and re-check.
If all is ok run a small amount of superglue in the shaft to hull join and let go off.
When it is fully cured remove inner shafts and fit the outer shafts properly.
I use Epoxy Resin i use for thr hull to shaft joint and suppotr the inner end of the prop tube in a chaped balsa block with P38 body filler.
When the outer shafts fitted as YOU want fit inner shafts and fit the "A" frames in the same way as the outer shafts, but ensuring the inner shaft is a good fit inside them, then Epoxy in.
Sounds complicated, but no more than an hour to fit 4 shafts after they have been soldered together.
I have seen so mant multi-shaft models with the motore each pointing a different way!!
Cant happen using this method!!

BOb
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flag-d

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Re: Prop shafts all wrong!
« Reply #4 on: May 29, 2006, 06:54:38 PM »

Thanks chaps!

I had a nasty feeling that everyone was going to say things like 'measure very accurately', 'line everything up properly', 'keep everything square and rigid' etc.  I'm sure I did all that last time.  One thing that sprang to mind last night: differential thermal expansion.  There do seem times when the hsafts are easier to turn than others.  The hull is GRP and I was wondering if temperature changes might induce a twist or some other strain into the hull which will throw off the already not brilliant alignment and make it even worse?  Your thoughts on this would be appreciated as I could always internally brace the hull.

Mike
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boatmadman

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Re: Prop shafts all wrong!
« Reply #5 on: May 29, 2006, 07:27:56 PM »

think it would need a pretty high temp difference to cause that, as GRP is a pretty good insulator.

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anmo

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Re: Prop shafts all wrong!
« Reply #6 on: May 29, 2006, 08:16:32 PM »

Boatmadman is right, you'd have to be pretty reckless to damage GRP when soldering, and the stuff would give plenty of warning by starting to smelll and smoke before much damage was done. A blowtorch on the other hand would be asking for trouble. I use large crocodile clips as heat sinks between GRP and the joint when soldering, and this has always worked OK for me.
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John W E

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Re: Prop shafts all wrong!
« Reply #7 on: May 29, 2006, 08:54:40 PM »

Hi there Flag-d

Would it be possible to slide one shaft at a time out of the short stern tube and leave it suspended between the P bracket and A frame.? Rotate the shaft, and if its still tight, you will know that the problem lies between the P bracket and A frame.? ?If the shaft is free to move, slide it back through all the way into the short stern tube and rotate the shaft in there - if its free you know that there is a misalignment between the short stern tube and the P bracket.? ?If its a misalignment between these two it will be up to you to which one you want to cut back out, either the stern tube or the P bracket.? Whichever one you choose, if it was me and say, that was where your problem was between the P bracket and stern tube, the procedure I would personally use, would be to remove the stern tube, clean it all up, making sure that the hole in the hull was a good 1/16 bigger all around, so that it was free to move and align itself.? Replace the propellor shaft through the stern tube and through the P & A brackets, making sure that its still free to move, then mask the stern tube in place on the outside of the hull with masking tape, ensuring that the prop shaft is still free to turn.? Then mix some epoxy and just pour a little bit over the top of the stern tube, the masking tape will obviously prevent the epoxy running all the way through the hull and, as it is setting, slowly turn the prop shaft.? ?This will allow the stern tube to align itself naturally, hope you can understand this and it is of some help.

Aye
John E
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boatmadman

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Re: Prop shafts all wrong!
« Reply #8 on: May 29, 2006, 09:33:48 PM »

if you have a reamer the right size for the bushes, use that to clean them out
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Doc

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Re: Prop shafts all wrong!
« Reply #9 on: May 29, 2006, 10:02:45 PM »

Mike,
"Differential thermal expansion" can make some very slight differences.  Not usually enough to make a lot of difference, but certainly possible, and also usually handled by the 'U' joints etc.  GRP doesn't have much affect on metals things through it (the expansion thingy).  Can't say the same for plastics and wood, for instance.
 - 'Doc

PS - Keep that "differential thermal expansionin" in mind!  Makes for a very good excuse.  I plan to use it myself!
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Shipmate60

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Re: Prop shafts all wrong!
« Reply #10 on: May 29, 2006, 11:47:12 PM »

I agree with Bluebird, the first thing to check is where the tightness or misalignment is.
When you know this you can rectify the offending part or parts.
If it is just the "A" frames take them out and re site them.
Take out all the shafts and have a very detailed inspection, can you see anything that doesnt look right?
If it all looks right use the process suggested by bluebird.
I have one minor difference from bluebird though, if you extend the shafts in the outer shaft, check they are all parallel. check by measuring, if they are and to your satisfaction I would concentrate on the exterior hull appendages.

Bob
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flag-d

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Re: Prop shafts all wrong!
« Reply #11 on: May 30, 2006, 08:40:28 AM »

Once again, thank you all!  This really is an excellent forum: so many people so willing to help.

OK, I think I can rule out the differential thermal expansion question (Though I'll keep it in mind Doc.  It does sound awfully good doesn't it?!).  Bluebird, I think your method will be the way ahead.  I did consider using a reamer on the bushes, but I guess that is sort of cheating and may end up giving me rattling shafts (Which sounds painful!), but thanks Boatmadman.

I shall get back to you all when I have sorted it.

Thanks again

Mike
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White Ensign

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Re: Prop shafts all wrong!
« Reply #12 on: May 30, 2006, 08:57:06 AM »

Hi Mike, I have had a similar problem by building my CMB, the only (and best solution) is, to use stencils. I use to make them out of a aluminium or a brassplate. Sign the correct distances for the holes and drill them all together, though you have identical holes at all of them. Then put them over the shafts or tubes, fix them with a drop of superglue (which can be "opened" again with some Aceton) and glue them in the hull. After all is fixed you can take them off and go on with your job. In my opinion the only proper way to install the Shafts and tubes.
Hope this was out of any help,

J?rg
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John W E

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Re: Prop shafts all wrong!
« Reply #13 on: May 30, 2006, 07:52:50 PM »

Hi there,

as an afterthought  ;D  I wonder if you used prop shafts bought over the counter at a standard Model Shop ?   I have done this, from some reputable Model Shops, only to bring them home and find the actual shaft is bent inside the tube.  If you do find that the shafts are slightly distorted i.e. bent, you may be able to straighten them between blocks of wood.  Obviously the blocks of wood have to be hard timber - I used two pieces of planed oak - something like 14 inches long and about 4 inches wide by about 2 inches thick - placed the shaft longways on one piece of wood and placed the other piece over the top parallel with the bottom piece and roll the shaft between the two pieces of wood - by moving the top piece backwards and forwards.
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flag-d

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Re: Prop shafts all wrong!
« Reply #14 on: June 01, 2006, 10:06:17 AM »

Bluebird...What fun I had with the propshafts!  Bought a running gear set for the boat (4 shafts etc etc) and 2 were bent.  Got those replaced, but replacements were too long.  In cutting 1st one down, hacksaw frame snapped (I have such high quality tools!) and I plunged the handle down and nearly got 90 degree bend in the thing!  2nd shaft: cut it to length, then dropped it on concrete floor and bent that one too.  The replacement replacements(!) were then bent less than 10 mins after purchase when son #2 sat on them in the car.  Aaaaaaaarrrrrgh!

Mike
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boatmadman

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Re: Prop shafts all wrong!
« Reply #15 on: June 01, 2006, 10:33:29 AM »

ever thought of flexi shafts?  ;) ;) ;D
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Doc

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Re: Prop shafts all wrong!
« Reply #16 on: June 01, 2006, 04:44:20 PM »

...sails.  Ever thought about using sails instead?
 - 'Doc
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malcolmfrary

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Re: Prop shafts all wrong!
« Reply #17 on: June 01, 2006, 04:54:22 PM »

I sorted my bent shaft with the blocks of wood treatment, but I clamped the straight part between a pair of blocks and then used another block with an appropriate sized hole drilled in it to "stroke" the rest straight.  Took several tries an some time, but it worked.
I agree with Doc.  Sails are Very good.  Paddles are a viable alternative.
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flag-d

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Re: Prop shafts all wrong!
« Reply #18 on: June 01, 2006, 06:25:21 PM »

Hmm, a paddle or sail powered MTB?  Interesting idea...could be a first too!

Mike
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Re: Prop shafts all wrong!
« Reply #19 on: June 02, 2006, 02:47:29 PM »

Mike- use a rubber-string to power the boat. No problem when you sit on it and always flexible enough when you cut it too short....  :D

J?rg
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colin-stevens

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Re: Prop shafts all wrong!
« Reply #20 on: June 02, 2006, 09:48:45 PM »

try assembling all the peices. then use p38 to fix the prop shafts. (i use p38 cos its easily get outable if need be with a router bit. also it dries quickly, reducing the chance of movement) check everything is still easy to turn. fix the rear A frames, p38 again. less is best. check for easy turn. then the middle frame. check for ease of turn. if everything is still easy then fix with epoxy resin glue mixed with micro baloons. worked for me. if things stiffen up you can tell at what point the mistake has happened, grind out the p38 at the mistake and try again.
great fun aint it. not.
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