tail shaft alignment

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Vegman
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tail shaft alignment

Postby Vegman » Mon Oct 13, 2014 8:30 pm

As we know most journal bearings including stern tube bearings work on a hydrodynamic wedge type principle, such that at full speed the shaft is lifted slightly by the oil pressure and also not longer exactly centered port starboard in the journal.
I am not exactly sure however of what magnitude this shift is
Does anyone know if you have to take this into account when aligning tailshafts to the gearbox or engine coupling? Obviously the alignment is done with the shaft at rest, and from my limited experience I cant recall the hydrodynamic wedge effect ever being taken into account- we just tried to get the misalignment as close as possible to zero.

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JK
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Re: tail shaft alignment

Postby JK » Wed Oct 15, 2014 4:07 am

I never have. If I had thought about it, I would consider the issue miniscule in relation to the size of shafting I was dealing with. We have jack pumps fitted on the last pedestal blocks in line to lift the shaft at low RPMs when the oil film is non-existant.

Rum Baron
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Re: tail shaft alignment

Postby Rum Baron » Wed Oct 15, 2014 9:15 am

The software used for large shaft alignment is extremely complicated nowadays and take into account shaft axial float (less than a mm), heat expansion of gearboxes and bearings and loading conditions, however I am yet to see one that takes the hydrodynamic lift into consideration, probably because there are so many variables involved. I would expect that the effect would be extremely small as all the support bearings would lift a similar amount, hence maintaining the alignment.

I have seen many bearings destroyed by running under speed, usually whilst trailing a shaft. I even heard of one company that produced probes that would screw into the weardown gauge fittings of the A bracket bearings and would tell you the actual lift of the shaft at a particular RPM. I saw this used on one ship that was suffering from extreme bearing wear, turns out their minimum shaft RPM was too slow to maintain the wedge.
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Vegman
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Re: tail shaft alignment

Postby Vegman » Fri Oct 17, 2014 5:36 pm

Thanks Rum Baron and JK- I have seen figures for alignment ofsets to take in to account thermal growth but I also dont recall ever seeing any for hydro dynamic lift.

Re the screw in probes- can you recall the magnitude of the lift that was being recorded?

Also does anyone know any rule of thumb figures for tailshaft alignment when there are no OEM figures available.
I have previously used <5 thou (0.125 mm) for a 6 to 8 " dia shaft.

JFC
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Re: tail shaft alignment

Postby JFC » Sun Oct 19, 2014 8:07 pm

Wow! this is a really good topic, and I hope many of our learned colleagues step up here.

This is a good opportunity to share ur experience and knowledge with the next generation. This approach applies to so many things.....

Anyway I will start my response off with the fact that I have never heard of any kind of tolerance or expectation for tail shaft alignment that took into consideration the oil wedge, and in addition to that, I never had the time to rely on a software program or lasers to analyze my alignment problems. I have been around when they were tried, but my experience has been different on accepting this technology and the slow process when it comes to using it for ships propulsion. I am not saying the technology doesn't work, it is just that there was never time for it in my life as we "had to go".

The only time I recall this being considered was with my steam license training with turbines and those ships........anyway..

Dial indicators and making allowances for sag (underneath measurement), Straight edges, and feeler gauges are the weapons of choice for me from any range of zero tolerance to what ever you want to try and get.

Doing your alignment checks in the water after a few days was crucial. Alignment readings in the dry-dock are a complete and utter waste of time.

Sometimes on larger diameter shafts water lubed shafts, or even on oil lubricated shafts you will want to make an allowance for the wear down of the tail shaft bearing as it may be a long time for renewal and also considering the environment that the ship works in.

Also in addition to the above, shaft bearing loads for a long tail shaft run such as an offshore supply vessel can be most helpful to you, because the shaft line is so long and it is important to know this information.

From my experience, and I had to approve or direct the align and/or re-align large and small tail shafts. Some of them were caused by incorrect fitting of SKF hydraulic couplings, improperly fitted bolts, poor expectation of coupling tolerances, incorrect line boring, and in fact I even had to align a shaft that was damaged in a tow where the torque overcame the shaft brake and the heat of the friction and subsequent damage resulted in setting up a .065" wobble in way of the damaged blue tempered shaft area.

Ran the ship for a year while I scratched my head and finally figured out a way to straighten it out in situ.....

Anyway, the best rules of thumb come with experience and remember - "the amount of misalignment is directly related to the life-cycle of the bearings associated".

In other words, if the the clutch coupling on a small installation claims that it will take up to .015" misalignment and that is why the shipyard or, err, owners bought it........ remember that if you start out with .013" misalignment, you will wear that wee coupling out a lot faster than if you started it off with .001" misalignment......

So alignment is a bit of a experience and judgement call of what you can tolerate as experienced Chief Engineer and /or Superintendent as opposed to what things are considered in your alignment specification.

Class or state is also involved but there are primarily concerned with the fact that you are actually performing the inspection as opposed to what is acceptable alignment, but the more experienced surveyors with be able to "suggest" what may be acceptable or a target.

This is a great topic for this forum. Thanks.

Cheers

JFC

Vegman
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Re: tail shaft alignment

Postby Vegman » Thu Oct 23, 2014 8:05 pm

I actually was an ABS surveyor for a few years.
In my part of the world there wasn't any big drydocks or shipbuilding facilities so as far as tailshaft work went I was mainly dealing with tugs , workboats and motor yachts.
One thing that often surprised me was that we went to such lengths to align engines and shaft accurately, ( less than 0.005") ,but for the water lubricated bearings a weardown of .25 " ( clearance) ( for a shaft less than 9" dia.)was allowable at the special survey.
I guess as you say , they will run with misalignment, they just wont last as long.

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Big Pete
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Re: tail shaft alignment

Postby Big Pete » Sat Oct 25, 2014 9:29 am

I think it is all summed by lining everything up, as well as possible in the circumstances, with the ship afloat in its normal loaded position, preferably giving the Hull a couple of days to "relax" into its natural shape.

Calculating corrections for the lift of shafts due to Hydrodynamic wedge lubrication, the movement of bearings caused by gearboxes and engines heating or even by the Hull cooling in Arctic waters, are incredibly difficult, and in practical terms have to be assumed to be with in the tolerance of the shafting system.

Whowever designs the shafting system should, of course, make sure that the bearings can take the inevitable misalignement, due to these and other factors including wear and damage.

Big Pete.
It is always better to ask a stupid question than to do a stupid thing.


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