Prop shaft straightness

[SimonD]

I've got a bit of vibration (after having the engine alignment adjusted at the start of the season) and am taking the opportunity to check everything before considering a flexible coupling.

Darglow checked the balance and pitch of the prop (highly recommended by the way - did while I waited at no charge). Now I want to check the shaft straightness. I'm planning to take it to a local machine shop. The shaft is 1" diameter and 55" long. What's the maximum permissible run out?

 

[earlybird]

I think that this sort of measurement is where ignorance is bliss, you'll only worry when you measure it!
However, I'll start the ball rolling by suggesting 0.5-0.7mm TIR at the centre would be quite good.
Over to others.

 

[danielefua]

I've got a bit of vibration (after having the engine alignment adjusted at the start of the season) and am taking the opportunity to check everything before considering a flexible coupling.

Darglow checked the balance and pitch of the prop (highly recommended by the way - did while I waited at no charge). Now I want to check the shaft straightness. I'm planning to take it to a local machine shop. The shaft is 1" diameter and 55" long. What's the maximum permissible run out?

This is an interesting question and I wonder if someone here knows the answer. My guess is that the optimal maximum run-out should be below the one that a good instrument can measure but I understand that mine is what technicians will call an "hand-waving" reasoning! Anyway I would not worry very much because if you go to a good shop, the tool that measures the run-out of your shaft will also straighten it at the same time. A close relative of the instrument used for balancing the tyres.

Daniel

 

[pvb]

I'd think less - say 0.2mm.

 

[superheat6k]

I would look for 0.001" per foot so in your case 0.0045". That's 0.11 mm

You are trying to achieve a coupling alignment within 0.005", so any more run out than this and you will never achieve it. A decent size lathe with centre will easily establish this with a dial indicator, or even better a engine shop with a crank grinder.

 

[xeitosaphil]

I had mine checked at the start of the year in an effort to reduce vibration and usually they put the shaft ( or they did with mine) on a pair of Engineering " V " blocks then measure the deflection in the centre with a" Dial Gauge " by rotating the shaft.
Mine was only 0.006 " so was considered to be ok.

I don't know how they actually straighten one if it is bent so can't help there.

Engineering " V " Blocks http://www.ebay.co.uk/itm/Precision...ks-6-Length-/261993624696?hash=item3d00097478

Dial Gauge http://www.ebay.co.uk/itm/0-1-IMPER...hash=item4641b8d9cb:m:mM_owFHaNO3Nkcjyx5_Nw2g

Picture
https://www.google.co.uk/search?q=m...shaft+with+a+dial+gauge&imgrc=k1_WkHCR7sralM:

Hope this helps

Philip

 

[Tranona]

Lake Engineering in Hamworthy will check it for you. However out of true shafts are not common and vibrations are more commonly cutless bearing wear or misalignment , engine mounts or simply part of the engine touching something.

 

[macd]

I had mine checked at the start of the year in an effort to reduce vibration and usually they put the shaft ( or they did with mine) on a pair of Engineering " V " blocks then measure the deflection in the centre with a" Dial Gauge " by rotating the shaft.

I don't know how they actually straighten one if it is bent so can't help there.

Probably with the self-same V-blocks under a hydraulic press. Used to be a popular solution for bent telescopic fork legs on motorcycles. Also works on Hydrovane rudder shafts, if not kinked. Many small engineering shops can oblige.

 

[SimonD]

Many thanks for the helpful replies.

Looks like the consensus is around 5 thou would be OK. Getting past that up towards 10 thou would be increasingly worrying. I take the point about a bent shaft being less likely than misalignment etc., but it's worth tickinig all the possible causes off the list.

Thanks for the tip on Lake Engineering. They're not too far away; I'll give them a call first.

 

[xeitosaphil]

Many thanks for the helpful replies.

Looks like the consensus is around 5 thou would be OK. Getting past that up towards 10 thou would be increasingly worrying. I take the point about a bent shaft being less likely than misalignment etc., but it's worth tickinig all the possible causes off the list.

Thanks for the tip on Lake Engineering. They're not too far away; I'll give them a call first.

I went through the whole process last summer when ashore, in an attempt to reduce vibration like yourself.

Started with checking the shaft for true, replacing the Cutless bearing, lifting the engine to re adjust all the packing below the engine and re coupling the engine to within 5 thou on all four quadrants with the shaft. Engine now runs true, lot of work but well worth all the effort in my book.

Best of luck with it all

Philip

Forgot to mention, final adjustment should really take place when the boat is back in the water in case there maybe a twist in the hull when ashore?

 

[Trundlebug]

The concensus above is about bang on from my experience.

I was told (by Woodwards in Hull) that the max run out should be about 5 thou - 0.005"
Or, as small as you can get it, whichever is the smaller figure.

A few years ago my prop hit a log and stopped the engine dead from about 1500rpm.
There was some vibration afterwards. Last winter I removed the prop & shaft and replaced both cutlass bearings because they were worn anyway. Sent the prop away for repitching and balancing.

I took the shaft in to work where we measured it on Vee blocks - it was running out about 40 thou, so a long way out.
By the technique described above they managed to straighten it, using vee blocks and a manual fly press, and got it down to 5 thou after several attempts. Thereby saving me around £500 for a new shaft.

Once all back together for the 2015 season the boat has been much smoother.

I would definitely recommend checking the shaft. They do bend, so whilst you've got the prop off anyway it's worth going that bit further and checking the shaft for run out, especially if the prop has hit something.

Usually prop re-pitching companies also offer shaft checking and straightening services - just ask them

 

[SimonD]

Thanks again guys.

The cutlass bearing looks OK and giving the shaft a good heave ho doesn't show any signs of movement. I'll have a close look when I've got the shaft out though.

If the shaft and cutlass bearing are OK, I was thinking of just shoving in an R&D flexible coupling rather than spend hours re-adjusting the mounts (I doubt I have Philip's patience!) as I had the alignment checked at the start of the season and it should therefore be about right. My theory is that this will allow for any slight residual misalignment and allow for alight changes with the boat in the water etc. Sound OK?

 

[Tranona]

Just putting in a flexible coupling may not do any good. Firstly, has it always been like this or has something changed?. Some boats just have rough installations so you need to determine if this is normal. What boat is it? and have you talked to owners with similar installations?

 

[SimonD]

Just putting in a flexible coupling may not do any good. Firstly, has it always been like this or has something changed?. Some boats just have rough installations so you need to determine if this is normal. What boat is it? and have you talked to owners with similar installations?

When I first bought the boat (Moody 35 by the way) it had some vibration around 2000 rpm. Having had the alignment adjusted, that's disappeared, but I have some vibration right through the rev range but getting better as revs increase. I take it from this that the shaft is self-aligning to some extent as it should.

The vibration is not bad (in fact I hesitate to call it vibration) but the shaft is moving enough to let water get past the Volvo shaft seal (which is new and greased). I doubt it's normal, but I can ask on the MOA forum.

 

[danielefua]

...the shaft is moving enough to let water get past the Volvo shaft seal (which is new and greased).

I am afraid that this does not sound quite right. You may call it a non-vibration but it is for sure an oscillation or a wobbling that should not be there.

Daniel

 

[fisherman]

Twice I have found the female register in the output flange on a PRM gearbox to be out by as much as ten thou, once they replaced it free, next time they told me it was within tolerances, and I took it to a local shop to be bushed. The result looked like a bent shaft until I realised the run out got worse nearer to the gearbox.
You can test your shaft by running in gear dead slow and apply a long lever, (broomstick?) the end against something solid in the bilge, touch against the shaft, the length amplifies any movement.

 

[SimonD]

I am afraid that this does not sound quite right. You may call it a non-vibration but it is for sure an oscillation or a wobbling that should not be there.

Daniel

"Wobbling" is a much more accurate description. It's not so bad that the engine is shaking about, but enough to see the shaft moving and, as I say, cause the gland seal to leak a bit.

I like the idea of a broomstick to amplify the movement, but I already know it's there. I did try a dial gauge on the shaft last year, but the results were somewhat inconclusive.

I'm somewhat dismayed that a flexible coupling might not be a 'quick fix'. I thought that's what they were for! If it would do any good, I'd rather fit one now while the boat's out of the water. Can it do any harm (apart from bashing my wallet)?

 

[macd]

I'm somewhat dismayed that a flexible coupling might not be a 'quick fix'. I thought that's what they were for! If it would do any good, I'd rather fit one now while the boat's out of the water.

There's flexible couplings and flexible couplings. The R&D is principally intended to inhibit the transfer of engine noise and vibration from engine to hull. It can tolerate a small degree of misalignment...max 0.5 degrees, if I remember correctly*.

I agree with others than any wobble severe enough to cause a Volvo seal obviously to leak needs attention.

* Which, come to think of it, I'm not sure I do. It may be as low as 0.25 degrees. The R&D website now offers no figure, although ASAP supplies catalogues certainly used to. R&D just specify 10 thou as the maximum range of clearances for their magic red-headed bolt.

 

[danielefua]

"Wobbling" is a much more accurate description. It's not so bad that the engine is shaking about, but enough to see the shaft moving and, as I say, cause the gland seal to leak a bit.

I like the idea of a broomstick to amplify the movement, but I already know it's there. I did try a dial gauge on the shaft last year, but the results were somewhat inconclusive.

I'm somewhat dismayed that a flexible coupling might not be a 'quick fix'. I thought that's what they were for! If it would do any good, I'd rather fit one now while the boat's out of the water. Can it do any harm (apart from bashing my wallet)?

I also agree that a flexible coupling may not solve the problem: they are made to take care of a small misalignment between the shaft and the transmission output; nothing else. First thing you should do is understand exactly which is the cause of the oscillation at the gland seal. For example you could check if the wobbling is produced by the movement of the engine on its mounts and maybe the broomstick trick will help you find out.

Daniel

 

[vyv_cox]

First thing you should do is understand exactly which is the cause of the oscillation at the gland seal. For example you could check if the wobbling is produced by the movement of the engine on its mounts and maybe the broomstick trick will help you find out.

Daniel

It can also be that the hole is not central in the coupling. Try the broomstick method (carefully!) or better use a dial gauge, but move it as far forward as possible, then as far aft for comparison. Sometimes moving the coupling round 90 degrees on its bolts will help.