Model Boat Mayhem
Technical, Techniques, Hints, and Tips => Other Technical Questions... => Topic started by: Hande on March 23, 2016, 06:37:49 pm
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Finally I was brave enough to install the motors.
I use two universal joint couplers in series thinking that will take care of any misalignment my eye can't see.
So: In how deep sh... trouble am I with the following?
When I turn the propeller with my finger, the whole stern-end of the hull moves rhythmically in all directions.
Also, the prop shaft - coupling joint moves sideways. The hull is ABS, so it kind of wobbles with the movement.
My question: How much of such movement would you tolerate? It is very difficult to attain total alignment.
My complaint: Why should we use brass shafts in the first place? They are curved almost by definition. Mine were curved and I tried to bend them straight. Can it be that I just have poor quality (Billings) prop shafts?
I haven't got all my electronics, yet, to see, if this movement is due to my finger pressing the shaft in every other direction while turning and the whole problem is washed away when it's the motor that turns the shaft. I really hope that my concern is premature!!
My build (bit.ly/Neddie_of_Hande) is a two-engine tug (about 90cm long).
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Can you post pictures of the set up, it sounds as if the shaft tubes require further support.
Two uj's in series??
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In one of the threads someone said that the proper way to do it is to have two... I haven't found the tread again, but it made sense to me, although I have seen that many people only use one.
The motors are installed rock-solid.
The shaft tubes sit tight, too.
The overall distance from the motor to the shaft tube seat is very short.
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In one of the threads someone said that the proper way to do it is to have two..
Yes, it makes a Cardan shaft which allows for lateral as well as angular misalignment. (As used on rwd car drive shafts.)
I don't think it's the cause of your problem but I think your joints should be 90° to each other rather than 'inline' as they are. Google Cardan shaft and have a look at some pictures.
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It sounds to me like you have bent shafts, they certainly should NOT be bent by definition, try taking the shafts out and rolling them on a flat surface, a sheet of glass or a mirror is ideal, they should roll easily if they don't they are bent. %%
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Hmmm! Really not convinced with the two uj's idea, I can see it just causing vibration.
It is difficult to judge from your pictures how much support there is to the inboard end of the shaft.
What diameter are the shafts?
Simon
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Cross post with Joe, yes, the shafts should not have any inherent bend.
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The shafts were bent. That much is sure. Brass is so soft that with longer storage and shipping and handling it almost autobends itself. Don't you think steel would be better?
The shafts (4mm) are easily bent with little force. I made an improvement trying to unbend them, but doubt that I could ever make them absolutely straight. After the straightening exercise I tried again: no improvement in terms of moving running gear.
I am grateful for your views and keep following the responses. I decided that I will take no drastic action before I have the motors running. I'll report back at that time. Meanwhile, I will study cardan joint tech {:-{
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I don't think it's the cause of your problem but I think your joints should be 90° to each other rather than 'inline' as they are. Google Cardan shaft and have a look at some pictures.
Can be - I thought it wouldn't matter because each uj turns in all directions without any preference. I chose this configuration, because it looks nicer... :D
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Yes, check the installation running first. I' ll bett it's noisy.
Then improve one thing at a time. Start with the shafts, if they are bent, they are u/ s. Replace them. And yes steel is better though I have many models with brass shafts that are fine. Brass comes in various grades, yours sound suspect. When they are OK, if you still have a problem check the couplings, and come back to us on here.
I'm assuming the tubes and bearings are OK. Are they?
Chas
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Shafts must be STRAIGHT to within a few fractions of a thousandth of an inch (.0001). If they are not, "runout" will cause vibration and wear the shaft tube bearings very quickly. Like someone else said, roll the shafts on a very flat surface. If they go bump, replace them. Trying to straighten a bent shaft is very difficult at best.
And a longer distance between shaft ends will make misalignment between the motor and prop shaft more forgiving. But strive for the best possible alignment; don't depend on your U-joints to correct for misalignment.
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This is the method I used and it worked for me , obtain a piece of 4mm steel rod long enough to go through the tube so that it reaches the motor shaft and comes out into the Kortz nozzle where it touches ther shaft assuming the motor shaft is 4mm there should be no steps between the test rod and the motor shaft ,try a straight edge along the two shafts it should touch both wherever you test it . If you can find a piece of tube 4mm inside dia slide it on the shaft and it should slide from the rod to the motor shaft without resistance . If it was me I would only use one UJ coupling .
You might have to shim the motor mounts , leave the motor mounts a little slack keep sliding the tube back and forth from motor shaft to the rod and tighten the motor down tight and the tube slides back with out any tight spots.
I know you will think it's a bit of messing on and it takes time ,but trying to cut down vibration will stop your build shaking its self apart ...all this worked for me
David
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Thank you for putting me on the right page. O0
I'm off to buying new shafts, now. The Billings shafts should be considered no hope.
The aligning advice with a steel rod is great - so naturally logical :-)
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Can be - I thought it wouldn't matter because each uj turns in all directions without any preference. I chose this configuration, because it looks nicer... :D
Unfortunately it does matter, a Cardan joint if misaligned even by the smallest amount will speed up & slow down as it rotates causing vibration. Putting 2 joints in at 90 degrees to each other will cancel this out (one is speeding up as the other is slowing) Having them "looking nice" will only make it worse.
Paul..
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Have a work with Mark at http://www.modelboatbits.com/
He will happily custom make the shafts you require, and won't rob you on postage to Finland :)
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I've used a tube of rolled card around the coupling, held with a couple of tie-wraps to hold it rigid and straight, which serves as an aligner. Attached shows it alongside the coupling (and the Mick Burke Rolls-Royce brass version!).
Works for me.
DM
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You seem to blame the shafts but have you tried turning them over by hand with the motors and the couplings removed?
They should spin quite freely and straightening the inner shafts isn't too hard if there are tight spots. Once they spin smoothly move onto the couplings.
Craig.
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Hi, just re-read your first post, if the shafts are the originals supplied by Billings, bin them.... I could say they are crap but they aren't that good... They will be a never ending source of problems....
Paul..
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Harsh but true Paul.
Simon
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Hande it sounds like you are on the right track(s) now. One more thought: buy stainless steel shafts. Then you will never need to be concerned about rust inside your stuffing tubes. Have fun.
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I would use stainless steel shafts and not brass, much harder, and less likely to bend. brass shafts in brass stuffing tubes is also a very bad idea. friction will cause the brass to swell, both the shaft and the tube, swelling will lead to the two parts seizing. I have also owned two different billing boats, and on both, the tolerances between the shaft and stuffing tube were so slack that leaking was guaranteed.
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A nice demonstration of the reasons for prop shaft alignment - https://www.youtube.com/watch?v=gmV4qwLfOMY
There should be good clearance between the shaft and tube to keep friction down. The bushes at each end do need to be a good fit to keep the water out (with the help of waterproof lube and and seals that might be fitted). Nothing much wrong with brass shafts in low power situations provided it is the right grade of brass, but stainless steel is much better. Marks Model Bits (mentioned earlier) also does rather nice couplers intended for restricted spaces.
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https://www.youtube.com/watch?v=gmV4qwLfOMY
Awesome! Did NOT know that! :o :o :o