Shaft realignment

[andyorr]

Thinking of fitting a shaft seal to replace stuffing box etc. I have been advised to make sure shaft is aligned correctly. Sounds a daft question but how? Shaft bolts onto a coupling (volvo 2003 on Moody 31) so how do you check alignment?
Help....

<hr width=100% size=1>

 

[Talbot]

Undo the shaft bolts, skin your knuckles, swear, remember that you left the feeler guage at home, swear again, go home trip over cat, swear again, find rusted feeler guage, swear again, go down to halfords and buy a new feeler guage and go to boat, attempt to check distance between sides of the shaft connection, skin knuckles again, drop feeler guage in bilge, swear again and the employ an engineer to do it /forums/images/icons/smile.gif

<hr width=100% size=1>

 

[Monty_Mariner]

When you have a new set of feeler gauges, tie a loop of string to it and attach it to your wrist so you can’t drop it.
Then adjust engine mounting bolts so shaft coupling / gearbox coupling are in line and parallel both vertically and horizontally.


<hr width=100% size=1>

 

[pvb]

DIY guide...

There are various ways of checking shaft alignment, but here’s about the only practical DIY method.

Firstly, if you’re going to fit a new shaft seal, you need to check that the shaft is central in the stern tube - otherwise the new shaft seal won’t necessarily work properly. With the boat out of the water, remove the old shaft seal and check the clearance around the shaft where it enters the stern tube. If it isn’t central, you need to adjust the engine mounts until it is central. In most cases, this will only require vertical adjustments; if horizontal adjustments are needed, it might be worth consulting a marine engineer first. Once it’s central, fit the new shaft seal.

Many Volvo engines use a shaft-gearbox coupling which is bolted together with Allen-key headed bolts. Don’t try to undo these with a normal Allen key. You can buy sockets with Allen keys on them at Halfords, and this allows you to use a regular socket wrench to undo the coupling bolts.

Once the shaft is central in the stern tube, and your new seal is in place, you need to check that the engine is correctly in line with the shaft. Put a little mark or piece of tape on both parts of the coupling where the shaft joins the gearbox output flange, so you can tell how it was originally lined up axially. Then unbolt the coupling, and slide the shaft back away from engine a little so that you can check the coupling mating surfaces, which must be clean.

Then slide the shaft forward again to connect the coupling surfaces. (It should just line up easily, but if it doesn’t then you’ll need to adjust the engine position slightly). Line up the marks. With the coupling flanges touching, measure the gap around the edge of the coupling with a 3thou feeler gauge (sorry, I can only think of feeler gauges in old money). There shouldn’t be a gap at any point bigger than 3 thous. Any gap in excess of 3thou must be corrected by changing engine position, usually the fore/aft tilt.

If there’s an excessive gap at the bottom of the coupling, this would indicate that the engine is tilted too far aft (ie the front is too high). Similarly, a bigger gap at the top of the coupling would indicate that the front of the engine is too low. Adjust the front engine mounts equally to move the front of the engine. Re-measure the gap at the coupling and carry out further adjustments as required.

Repeat these steps until you’re happy that the coupling is in alignment. At this point, the job’s done - unless your shaft is bent! To check whether you’ve just aligned your engine to a bent shaft, rotate the shaft 180 deg relative to the gearbox and check again. If you find it’s now out of alignment, you have a slightly bent shaft and need to consult a marine engineer. If it’s still in alignment, refit the coupling bolts, tighten the engine mounts and locknuts, open beer.

If you want to be really pedantic, you should re-check the alignment with the boat in the water.


<hr width=100% size=1>

 

[SimonD]

Some time ago, PBO had an article on this. The method involved measuring the gap with feeler gauges at 12, 3 6 and 9 O'Clock, then rotating the shaft 90° and measuring again, then another 90° etc. This produces a table of 4 columns (gap at each O'Clock position) and 4 rows (angle of rotation). There is then some arithmetic which tells you whether the shaft is bent and how much the alignment is out.

The article went on to describe how to measure the thread pitch of the engine mounting bolts, the distance between the mounts and the coupling and calculate how many turns are needed to correct the misalignment.

It sounds more complicated than it is and it worked first time for me. Unfortuantely, I can't remember when the article was published and I've lost my copy so that's not much help. Maybe someone else remembers? (Needless to say, PBO has not re-covered anything that useful again, but has told us how to change the oil on numerous occasions!)



<hr width=100% size=1>

 

[andyorr]

Re: DIY guide...

Many thanks. Very thorough - do you write Haynes manuals?!!! There is so much knowledge out there in the ether!

<hr width=100% size=1>

 

[pvb]

Haynes manuals...

No, but I've read one or two! Fortunately, they have pictures too. Best of luck.

<hr width=100% size=1>