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8 comments
Anonymous Poster

Marine Shaft Alignment

05/06/2006 7:45 AM

Mathew writes:
My name is Mathew, working for Marine applicatons as a Marine Mechanic. I was very happy with this web site which answers lot of queries in every aspect of science and technology. I want to clarify my doubt which is deeply rooted in my mind and which is about alignment.

Recently I was aligning a 5" propeller shaft which had a 11" coupling flange with spigot. As usual I put the shaft through the rear end strut bearing which had a clearance of 20 thou with the shaft dia. Inside the stern tube there was one more bearing which also had a 20 tho clearance with the shaft dia. I checked the offset alignment with the Gear box flange, when the whole shaft was resting on the bearings and after doing some minor adjustments I was 99% on the mark. I just pushed the shaft in and it went and seated perfectly in the spigot of the gear box flange without any further pressure. Now it was the time to check the axial alignment. I kept the shaft flange in the spigot of the gearbox and after doing some minor adjustments we got this reading which was 2 thou difference from top to bottom and port to starboard. I had to take the shaft out for some reason and when the shaft flange came out of the spigot, the flange was 10 thou down to the gear box flange.I t surprised me because the shaft was resting on the bearings, when I checked the offset alignment. I shared with my supervisor and he told me that there was 20 thou clearance in the bush and shaft so its 10 thou down! I couldn't understand it but I was told to fasten the couplings. I came down and put the propeller on the shaft and tried to turn the shaft. It was turning freely!

My question is was this an average alignment or a confused alignment? The coupling was a rigid coupling and the engine didn't have any rubber mountings nor the propeller was on the shaft when the alignment was carried out and the stern tube bearing had a gland packing box, with no glands in it as I thought it will affect the alignment.

This is my kind request to you to help me in understanding that my supervisor was right, if he is. I also want to know the effects of (positive and negative) clearance between the shaft and bearings while doing shaft alignment.
In anticipation
Mathew
Marine Applications
Brisbane

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Pathfinder Tags: Alignment marine applications shafts
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Active Contributor

Join Date: Dec 2005
Posts: 18
#1

prop shaft alignment

05/07/2006 8:00 AM

If the packing gland near the propeller is tight (centered) and the bearing with .020 clearance is near the engine, the shaft will need to be lifted .010 for correct alignment when the flange faces are parallel.

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Commentator

Join Date: Jan 2006
Location: Panama
Posts: 58
#2
In reply to #1

Re:prop shaft alignment

05/08/2006 2:05 PM

Hi Mathew. aligment shafts vertically or horizontally requires a lot of visualization. I donĀ“t know if this site has provision for presenting drawings or photos ( wich could be a good idea).Anyways there are many ways to present the readings and outcome of the final aligment but at the end the shaft is going to be in only one physical place. I mean, you can get confused or misunderstand your own readings (I have been doing this for 28 years in both thermal and hydarulic power plants). In adition taking aligment readings at different times of the day on the same spot may give you diferences that you may think comes from your adjusments. You have to consider that the housings and bases where your equipment is sitting will move, vibrate, expand and contract. You can send me photos or drawings to my address so we can take a look at it and give you an advise if necesary. For sure we will learn of this at the same time. llizarraga@pancanal.com

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Associate

Join Date: Feb 2006
Posts: 29
#3
In reply to #1

Re:prop shaft alignment

05/10/2006 7:42 PM

I think, from the info recieved, that the suprvisor is right. It was said that the propeller was not on the shaft, so all of the static forces had not yet been applied. The shaft sags under it's own wieght. Add the weight of the prop and a fulcrum (the cutlass bearing)at one end to the once sagging shaft and you have found that the calculated alignment was correct. Experience will teach you a lot, but by listening you can learn it painlessly. And I hope you commended your supervisor on his fine skill. Is that Brisbane as in Aussie Town?

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Participant

Join Date: Sep 2007
Posts: 4
#4

Re: Marine Shaft Alignment

09/30/2007 7:17 AM

I just removed 1 1/2 inch shaft for inspection and to replace the bearing in the strut. How should I check the alignment between the center of the strut and the center of the transmission coupling? I've heard of "target wire" but not sure how this process works.

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Anonymous Poster
#5

Re: Marine Shaft Alignment

02/29/2008 7:25 PM

you said clearance in the packing gland/stern bearing - youmust center the shaft in this area not let it rest on the bottom - it alloes error - also account for sag.

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Participant

Join Date: Oct 2009
Posts: 2
#6

Re: Marine Shaft Alignment

10/19/2009 9:13 AM

HI, My name is Giuseppe. I am in the first few yrs of my Naval Architect and Marine Engineer with the Italian University.Unfortunately recent pulibished books are very difficult to come by.

I am getting ready to prepare my thesis in subject of Shafting Alignment, i have very limited resouces of material offered by my Professor which dated back to 1960-1970. I have to talk about the alignment procedures, how to determine the bearings position and the reactions that they gives. Than I have to propose a numerical exemple with a software. Could you suggest me some books and software?? Here's my mail: giusep.dv@gmail.com

Sincerely

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Anonymous Poster
#7

Re: Marine Shaft Alignment

01/24/2010 11:38 AM

Matthew,

There have been different schools of thought presented to me on marine propeller shaft alignment over the years and I am sure you have seen as many crazy things as I have. The theory your supervisor seems comfortable with is that the difference in the gear output coupling and the shaft input coupling half is there because you had squared it when doing such a fine job of aligning it. In other words, when you made the coupling "square" with the length of the shaft by moving the engine to it, the shaft then being stiff and difficult to bend "flew" to the center of the bearing nearest the gear output flange much as it will do anyway in motion. Rest assured it is still under the influence of gravity and the end of the shaft nearest the propeller is lying in the bottom of the that bearing in the strut. You have simply centered it in the bearing closest to the gear output shaft.

For instance, if you had used a laser or wire alignment fixture to align the engine making the entire support assembly system for the shaft, and the gear output coupling theoretically straight and square, you would have arrived at the same point with the shaft coupling dropping ten thousands on an engine with solid mounting. Because those fixtures, call them a "shaft simulator" would fit into the struts or cutlass bearings with near zero clearance taking away the .020 play. Then when you install the actual shaft (because no boat or ship propels itself on an alignment fixture) the .020 would show. Half of .020 is what Matthew? And, does your compnay re check the alignment when the vessel is once again waterbourne?

I would love to have solidly mounted engines, however I am afraid that 2600 horsepower on each side would crack the marble countertops!

Bob Miller, Viking Yacht Service Center, Florida USA

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Participant

Join Date: Apr 2011
Posts: 2
#8

Re: Marine Shaft Alignment

04/28/2011 6:33 PM

hi mathew,

this is basha from a mechanical services industry. whenever the alignment is under progress both shafts has to be maitain some distance. here the problem is fist you are corrected the offset alignment and you have push the shaft to check the seating then you have corrected the axial alignment, during axial alignment you may did shiming to correct the axial algnment then you have push the shaft from gear box its shows 10thu down. axial algnment shimming aslo affect the Offset alignment.

the best way is pl fllow the rim and face alignment method or first correct the axial then correct the offset alignment if you want to any details about rim and face aligment i will send you. it very simple, precision and faster.

i use to do alignments as per the original equipment manufacturer tolerances.

best regards

basha

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