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[taxi]

Having made a bit of a shambles of motor / shaft alignment I ground out motor mount and started all over again only to discover that vibration still exists with motor at full speed despite taking a lot of time with alignment.    The shaft seems to have quite a lot of movement at either end and I do not think that aluminium sheet I used for mounting is thick enough.    The shaft and motor mounting have both been firmly fixed to fibreglass hull using either p40 or  filler.   Motor spins over nice and quiet and I have changed coupling.      Any ideas chaps?

[catengineman]

Hi Taxi,

 Have you run the motor with the shaft disconnected?

If there was no vibration then connect the shaft but leave the prop OFF and try again if there is no vibration then fit the prop back or try a different prop.

If there is too much free play on the shaft to tube (up and down should be almost non existent) the shaft will oscillate = vibration

Richard,

[EXBoat]

Hi Taxi,
Is the skin thickness of the hull enough to take the power of your motor, the hull bottom may need strengthend, the hull could be resonating when the motor is on full power, have you any flexing if you apply pressure to the hull skin in the vicinity of the motor mountings.

[taxi]

I have just run motor and vibration exists both with and without prop.   There is no vibration when motor runs disconnected from shaft but I can see up and down / sideways movement either end of shaft despite using a small amount of grease.    I do not see much need to run at full speed as it is a trawler but what bothers me is the fact that it will be beneath water line.   I do not have a speed controller yet and cannot run motor, a 777 at slower speed and there is no flexing in hull but the motor mounting is a home made from 2mm aluminium which I suspect is on the thin side.

[J.beazley]

Take the shaft out and roll it across a level flat surface.

you will soon see if the shaft is bent, also what type of coupling are you using???

Jay

[catengineman]

Hi, is the shaft you are using an old one ie: secondhand ?

I would think that from the description you have given about the movement then the shaft and tube are not a good fit.
as I said there should not be any noticeable movement in that the shaft should fit "snugly" into the tube.

What voltage are you running the motor with, if 12 volts then drop down (if you have 7.5 handy) the motor will be a bit slower,  then 6 volt or even make up a power supply from 1,5 volt batteries to get different speeds and see what happens.

Richard

[Robert Davies]

It's a question of balance, like when we have our car wheels balanced. See if you can balance the imbalances (!)

Try removing and replacing in turn the coupling and/or props but refit them say 90 degrees out from their original fixing - try it a few times to see if you can minimise the vibes.

Curiously, I've found that during a propulsion refit of my slipway Trent, if the motor/coupling/propshaft are in perfect alignmment I get vibrations, If I skew the motor a few minutes of a degree - no vibes - I'm guessing it may be a torque related issue..... In other projects I've found having the motor VERY firmly mounted to the hull to be a bad thing for vibes - it seems to use the entire hull for 'sounding' - Depending on you situation see if there is anyway of introducing some resilience into the mounting solution.

That's all I can think of now over and above the fine suggestions put forward by the chaps elsewhere in this thread.

-Rob

[Bunkerbarge]

From what you are saying it is obviously the shaft or the coupling.  Take the shaft out and roll it to see if it is bent. 

If it is replace it, if it isn't then you may need to replace the shaft bushes in the stern tube or it could be the coupling itself.  Try it with a differrent coupling and see how it affects the vibration.

This is simply a process of elimination to determine which component is causing the vibration and then changing it.

[Shipmate60]

Taxi,
Check the coupling is drilled dead centre, just lately I have had a few with the hole drilled slightly off centre which causes vibration.
As has been previously stated the shaft should be a snug clearance fit in the bearings.

Bob

[taxi]

Thanks for all the advice chaps.    Todays session in the shed resulted in an improvement but I still don't know the cause.    Shaft is new and straight, coupling also new but I forgot to mention that on my first test run with everything connected up the whole lot locked up when put in the tub due to a slack locknut, whether this did any damage to coupling or not I do not know.   Everything was taken apart today and re assembled with an improvement in vibration and I have also decided to strengthen motor mount .   I will just have to wait and see what hapens tomorrow when the resin sets.

[Bunkerbarge]

Taxi, can you get a good photo  of the set up posted?

We may see something we haven't thought about yet.

[taxi]

Motor mounting doubled up with another 2mm bracket fitted back to back.    Vibration almost gone and I can alter noise by raising or lowering rear end of motor when running.   I think I can now live with it and carry on with the build.

[taxi]

I have since discovered that the 2 washers that I fitted as spacers between shaft locknut and end of shaft have been causing much of the noise.

[Bunkerbarge]

As we say, the problem is sat on the worktop in front of you.  All you have to do it find it.

[PSSHIPS]

Scary ::)

[John W E]

Taxi hi there,

In the earlier years when we used to use this type of motor mounting, which you are using now, the folded tin plate type - they were prone to cracking along the folded edge of the mounting.  This was through, as you are discovering - vibration and slight mis-alignment caused by Dyco Couplings.   These are not the best Couplings in the world, you get good ones and you get poor ones.

Ive included a picture of how I mount motors using 1/8 plywood, this solves the vibration problem in the motor.   If you sit and think about the setup you have, with the motor mounting, you have a thin bit of plate holding the weight of the motor.   As you have already discovered, you can quite easily flex the back of the motor up and down to alter the noise.

The other things is to think of, some of the thrust of the propeller will be transmitted through the shaft directly onto this vertical face of the motor mounting.   This will depend on how much movement fore and aft you have on your propeller shaft and if it is any more than 1/16 of an inch movement fore and aft, you will see it will be bending the mounting of the motor.

Hope this is of some help to try and solve your problem.

Aye
John e
bluebird

[Colin Bishop]

From your picture you appear to be using a single coupling. If there is room a "double" coupling should make a better job of absorbing any minor alignment problems and giving a smoother driveline.

[John W E]

Hi there Taxi

Whilst thinking about your problem it reminded me of a set of couplings I purchased a while ago, and a propshaft.  The thread on the prop shaft hadnt been put on squarely.  Consequently when you screwed the coupling on, it didnt run true to the shaft.  This caused a tremendous amount of vibration.  I was lucky enough to have enough length on the prop shaft to enable me to cut the thread off and drill out the coupling to the same size as the plain shaft, this cured the problem.  Im putting a sketch on here with this posting, and this will show you how to check if both your couplings are running true to the shaft and motor.

First of all, you just need to remove the plastic inserts, leaving the brass coupling pieces on the motor and shaft and then you just need to make up an indicator pointer with a lump of plasticine/modelling clay an old dressmaking pin or a panelling pin.  Set up as per drawing and then rotate your propshaft by hand.

You will then note that if the coupling is running true, the gap between the pin and the coupling should remain constant.  If the gap increases and then decreases, as you rotate the coupling, this will show you that its not running true.   

Hope this is of some help.

Aye
John e
bluebird

[sheerline]

I think Colin Bishops reply is worth considering, we use two couplings in series in the subs as none of these moulded couplings are ever 100% perfect. If you can move your motor forward a bit and fit and extra coupling with a straight splined brass coupler between them, you will probably find it will run just fine. With this arrangement, the motor alignment is not critical. Remember, noise and imbalance consumes power and wears shaft bearings.
Worth a try!
Regards....... Chris

[Bunkerbarge]

I think it is not always clearly understood that a single UJ coupling will not accomodate shafts that are parallel but offset.  They will only accomodate an angular displacement.

A double UJ arrangement or what is known as a dog bone joint will be far more effective.  I had problems with the Revel sub vibrating but I fitted a pair of the Robbe rubber tube type couplings and it now works perfectly.

[Bryan Young]

For a (much) larger and heavier model I am indebted to Brian_c for introducing me to Ford seat motors. These have a spring coupling that can wobble about as much as you want...but still provide smooth vibrationless power. If you are building "big" I suggest you get a few of these motors. BY.