Propshaft trueness

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Conrad

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Does anyone have any idea what tolerances a propshaft should be true to, I have an inch propshaft about a metre long with approx half a mil bend in the middle measured when rotating against a fixed point. (The engine is a slow diesel, namely a 12hp Yanmar 2GM)

My gut feel is to panic and rush out and buy a new one but I just wondered if any engineers out there would advise otherwise.

Regards

Conrad
 
It's 20 thou in Imperial; I have no idea if this is excessive over 1 metre, I would ask a supplier like Sillette Sonic (just google) or a local supplier. They might even be able to true it up if required.

Edit: one standard I looked up on the web recommends 5 thou (inch) straightness tolerance for a 1 inch shaft 42 inches long. I would still ask Sillette.
 
It rather depends where the bend is, but certainly 1/2 mm is quite a bit out. I think however that if you were to take it to a good engineering workshop they would be able to press it straight for you rather than having to make a new one.
 
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Some people use metric measurements nowadays. Perhaps this is the case here?


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We have been told that the shaft is 1" diameter so it is reasonable to suppose that the "mil" has its normal meaning of 1/1000th of an inch!

It is possible to find other meanings for a "mil" including a Cypriot monetary unit,
10km in Sweden, an angular unit of 1/64000 of one complete revolution and in speech short for millilitre (ie a ml)


0.5 mm may well be 0.0196851" but 1 mil is 0.000100" and half a mil is what has been quoted not 0.5mm
 
When I had my new rudder made with a long stock of 1 inch 316 SS it visibly distorted when the tangs for the blade were welded on . The fabrcator trued it up perfectly with a press and checked with a dial guage and rollers. Still runs true after 15 years.
 
I picked up a rope earlier in the year around my prop and i think it slightly bent the shaft, my shaft is 4ft x 3/4 inch, later in the year i noticed a throb throb throb noise coming from underneath the boat at 3/4 revs,today it was even worse, so put a gauge on the shaft and its throwing about 1 mil out of true and i think its wearing the sterngland oval, no water is coming in but i need to fix it asap,so to save yourself from probs in the future i would sort it out now.
 
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0.5 mm may well be 0.0196851" but 1 mil is 0.000100" and half a mil is what has been quoted not 0.5mm

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1/1000th of an inch is written 0.001" and was also called a 'mil' in engineering in the UK up until decimalisation when it was generally dropped because of possible confusion with the new metric units (millimeter).
However, it is still in use in the USA.
Mixing of units is still a common problem........a former boss of mine insisted on drawing using imperial units, but because it was difficult to get imperial size drills in the UK, all hole sizes were quoted in metric!
 
So far Conrad has kept out of this pedantry although most relevant, perhaps he would put us all out of our misery and say what units he was measuring in presumably using a dial test indicator and not a rule. Although unless in advanced years he is not talking in imperial and although I can remember refering to a thou as a mil in the early sixties in my spell in a toolroom I have not heard it used since.
 
Yes, it is like waiting for the other shoe to drop...not heard of a mil myself, is it used in the US?

Whatever age he is, if the shaft is 0.0005" out of true he must be eagle eyed!
 
Do you know that the run-out is solely due to bend in the shaft? Is the shaft running true at the half coupling and the stern bearing? In general, I don't think that it's a good idea to apply sensitive measuring equipment, such as dial indicators, to yacht stern gear. The results will only cause worry!
 
Hi All

Sorry to take so long to come back. In my circles a mil is a milimetre and a thou is a thousandth of an inch although I can appreciate where the confusion has come from.

For my propshaft it does get worse as there is a 2-3 mil deflection in the tapered section alone so I will have to dip into my pocket to resolve this issue as 2-3 mil will clearly cause the prop to vibrate chronically.

The cause for the bend was my boat being dragged sideways off a beach by an over zealous salvage crew, I was hoping the damage had all been absorbed by the bent prop but no such luck.


Conrad
 
That will give you a bit of a wobble, you may not feel it but it will put additional stress on the bearings.

For a 3-4' shaft you should be looking at a straightness of better than 0.10mm ideally better than 0.06.
Roughly .025mm-0.05mm/foot
 
Isn't some of this discussion purely hypothetical?

After all even if the propshaft is perfectly true before fitting, for it to stay that way once in situ the engine, sterngland and any other brackets/cutless bearings would have to be perfectly aligned to keep it that way otherwise it is going to flex as it rotates. Also the hull would have to be absolutely stiff and the engine vibration free

I realise that a flexible coupling will absorb some misalignment of course but I don't believe it can deal with it all.

Doesn't all this make working to a tolerance of .025mm-0.05mm/foot for a FITTED prop shaft simply too difficult?

I don't pretend to speak as an expert here, merely someone who finds real life keeps getting in the way........
 
Not really, as the bearings have clearance in them, and if you have rubber ones they allow deflection, add to this a rubber mounted engine and what is a slightly bent shaft will vibrate.

Starting point for a smooth set up is a straight shaft, then align it correctly and it will run smooth and wear the bearings less.

We do supply systems to get over this which is a very straight shaft and more bearings /m or foot.
 
We've had our shafts tested and straightened before and we contacted BT Marine about tolerances. If I remember correctly, they recommended no more than 15 thou on the shaft and 5 thou on the coupling face but best to check with them to be sure.

ie. 0.015" shaft
0.005" on the coupling face.

and in metric...

0.381mm shaft
0.127mm on face
 
If your shaft is without a flexible coupling at the gearbox then this amount of deflection means the end of the shaft is probably trying to wobble in excess of 1 MM but being restraiined by the cutlass bearing which is putting exces swash plate type deflections on the gearbox rear bearing so although you may not feel vibration you are heading for a fall and four broken coupling bolts!!!!!

If you have one of those flexible connections mounted between the gearbox and shaft then this will stop and unecessary forces reaching the aft gearbox bearing and all the wobble will do is put ripples in your coffee and rattle your whisky glass to the floor . Mind you the cutlass bearing might complain a bit sooner than if the shaft were straight.

This is a big bend, do you know how you did it????? The doing could have caused more damage elswhere such as broken engine mounts..
 
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