Prop shaft wobble

martinriches

martinriches

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I have just fitted a new shaft and cutlass bearing.I also fitted a Python drive[similar to Aquadrive] and a PSS shaft seal.I took the boat out for the first time yesterday and noticed the shaft is wobbling around quite a bit [ 4-5mm at a guess]. The shaft is 25mm dia. and the distance between the cutlass bearing and the thrust block is about 2m. I think the stuffing box must have provided some support now missing with the PSS seal. Does that sound like the problem? would a 25mm shaft unsupported for 2m be causing the problem? I would be very grateful for any answers.


Martin
 
I suggest you contact a prop' supplier as they will have tables giving the maximum unsupported lengths for propeller shafts of any given diameter. You may need to put a short cutless bearing inside the tube behind the PSS seal if that is the problem.
 
Was it a solid stuffing box, or on a flexible hose. The latter would have provided some support and either might have had a bearing inside. 2m unsupported does sound rather long, but I don't have the calculations. I assume that your Python drive has a solid coupling to the thrust block so is not transmitting any engine movement. If so any shaft "wobble" is worrying. As it is a new shaft I also assume that it is not bent, although don't take that for granted!

Solutions would include putting a length of cutless in the inboard end of the tube, or better, easier and cheaper fitting a Volvo shaft seal which not only provides support for the shaft but provides a more secure seal (being a double lip radial seal rather than a face seal).
 
An American book I have states 40 x shaft diameter as a rule-of -thumb for maximum unsupported shaft length. A Dutch brochure on Exalto shafts states 1.75 meter as a maximum for a 25 mm unsupported shaft. On my own boat, I have 1,53 meter shaft length (25 mm shaft) without problems. But I do think there must be noticeable sag in an unsupported 2 m shaft. Another thought: has the prop been balanced?
 
Thanks for all the information. I haven't had the prop balanced and that might be a factor. I am thinking about a plummer block just inboard of the shaft seal that would make it about 1.5m unsupported. How do you balance a prop? can you do it yourself?


Martin
 
Two metres sounds a long length to be unsupported to me. I believe some of the shaft/prop design people could advise on this as it obviously depends on shaft diameter and the like.

It wouldn't have been good practice for your old shaft seal/stuffing gland to have been a key source of support anyway IMHO. There are methods of supporting a long span using interim bearings and the like.
 
It depends on shaft speed, this graph will give you a good guide.
dss-graph.gif
 
Does the shaft wobble when you bar the shaft over? Best checked with a dial gauge but if it's as much as 4-5 mm you would be able to see it by eye. This sounds like a large displacement, so may indicate that the shaft isn't statically aligned with the coupling. With a Python drive there should be virtually zero displacement.

If everything is in line at near zero speed, where have you put your shaft anode? About 1 cm from the cutless bearing is best, that allows sufficient cooling water to flow through the bearing but doesn't impose out of balance forces towards the centre of the shaft.

My shaft is close to the length of yours, with Aquadrive and PSS seal, but does not suffer the same problem.
 
2m for a 25mm shaft is a long way, especially as the true span is from the Python bearing, not the PSS seal.

Certainly your old stuffing box was a fairly good restraint, which is now missing.

You don't mention the size/weight of prop you're using - that has at least as much bearing on the problem (pun inadvertent).

Some try to rectify the problem by inserting a cutless bearing inside the stern-tube - I don't know how successful this proves.
 
Vyv

I will check the alignment of the Python thrust bearing and whether the shaft wobbles when I turn it over by hand.If it is out of line with the coupling then it should be easily remedied. I am hoping for a solution that doesn't involve hauling the boat out and taking everything apart. I found a plummer block with a split bronze bearing that could be fitted with the shaft in situ, so I may fit that anyway, just to give the shaft extra support after checking the alignment. Do you think a bronze bearing would be OK.

Martin
 
The bearing will have to be lubricated, even if it's a good quality leaded tin-bronze or babbitt faced. Does it come with a greaser or does it need an oil reservoir? Seems a pity to revert to a lubricated bearing when you have done away with a packed gland.

The shaft info above suggests that a 25 mm shaft should be OK at up to 2.4 metres. Are you sure about your displacement estimate of 4-5 mm? I would have thought the shaft would knock on the stern tube at a lot less than this.

If the shaft is true at low revs I would be tempted to wait a while until things settle down. If there is no knocking, in forward and reverse, even when turning, I would leave it alone.
 
The more bearings the better, but they will need lubrication.

The Issue is only shaft diamater, speed of rotation and unsuported length. 2m for a 25.00mm shaft will be a problem at anything over 500rpm. For a 1200 rpm shaft at 25mm you need a bearing every 1.2m.

Once this is sorted, if you still have a problem then look at bearing/shaft wear, prop balance, shaft bend, and alignment.
 
<<< The Issue is only shaft diamater, speed of rotation and unsuported length. 2m for a 25.00mm shaft will be a problem at anything over 500rpm. For a 1200 rpm shaft at 25mm you need a bearing every 1.2m. >>>

Are you saying that the graph above is wrong? It suggests, and this tallies with my rotordynamics knowledge, that the displacement will decrease as speed increases from 500 rpm. I would expect this to go on until the first critical is approached.

There's no doubt there are many boats with shaft lengths between bearings of greater than 1.2 metres, my Sadler 34 amongst them.
 
I quoted from the graph?

1200rpm sits between 1200 and 1500 band of rpm. Go to upper end (1500) look at where the line for a 25.00mm shaft crosses and read back to maximum bearing spacing, which is 1.2m.

The graphs shows that with greater rpm more frequent support is needed, this is of course for marine shaft applications that have variable axial thrust loads.
 
Thats how I read it and confirms my gut feeling that 2m unsupported length is too long. Also if I remember rightly the Albin has a very short outer bearing - not a cutless. There was a post about one a couple of months ago.

I have been down this road before, not with a short shaft but with removing a conventional stuffing box complete with bearing (Stuart Turner) and then installing a bouncy 1GM. Solution was to put a short length of cutless in the inboard end of the tube and a Volvo seal. Then of course became virtually rigid which transmitted engine vibration cured by a Bullflex coupling. Three iterations to get it right.

Of course the thrust bearing in the Python should ensure there is no movement of the shaft, which brings one back to the possibility that its coupling is not exactly in line with the outboard bearing. Difficult to check when there is such a large distance to the bearing support. Long term the solution is almost certainly an intermediate bearing, preferrably a cutless in the tube. Get that in line and mate the Python drive coupling up to the shaft.
 
[ QUOTE ]
How do you balance a prop? can you do it yourself?
Martin

[/ QUOTE ]
I've had it done a few times by a specialist prop firm. They have a device similar to a tyre balancer. Then it is a job of taking away some bronze, usually at or near the tip of the blades. IMHO not a good idea to DIY.
 
Thanks for all the answers. looks like I need some extra support and check everything is aligned.
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When checking the static alignment do not assume that the components are perfect. I have seen flanges so badly machined that the faces were untrue... This can be checked by turning one flange of a pair by 180 degrees to the other after alingning and confirming that the alignment is unaffected. If the alignment changes by this simple test then the flange you turned is untrue compared to the shaft. Do this test for each flange at a shaft coupling.

I have come across shaft drive flanges where considerable inaccuracy crept in somehow during manufacture and the flange face was not at 90 degrees to the shaft axis.

This might not be spotted by simple shaft alignement but once the shaft rotates produces the effect you describe.
 
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