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Old 14-10-2011, 05:35   #1
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Must a Prop Shaft Be Perfectly Balanced ?

hello, i'm just putting the boss onto my prop shaft. i had to cut off the old one so i have a slightly ragged end. as it will be spinning up to 1300rpm, does it have to be polished to prismatic perfection, ie a perfectly flat and parallel end to the 'cylinder' of the shaft, or is a few grammes ok?
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Old 14-10-2011, 05:45   #2
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Re: must a prop shaft be pefectly balanced?

I don't fully understand your situation, but perhaps the tolerances listed in ABYC Standard "P-6" (PROPELLER SHAFTING SYSTEMS) will answer your question.
See ➥ http://www.abycinc.org/committees/P-06.pdf
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Old 14-10-2011, 06:52   #3
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Re: must a prop shaft be pefectly balanced?

Any imbalance will give you vibrations, and cuase problems down the line. A Machine shop can turn it flat on a lathe.
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Old 14-10-2011, 07:47   #4
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Re: must a prop shaft be pefectly balanced?

When a shaft vibrates it can destroy the transmission or just wobble itself out of the boat...
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Old 14-10-2011, 07:59   #5
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Re: must a prop shaft be pefectly balanced?

I'm not sure how balanced the anodes are that we bolt to our shafts...just guessing here but doesn't seem like having the end cut-off not exactly 90deg could make that much difference...also what about the set screw that's in the flange that bolts to the trans...?
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Old 14-10-2011, 09:49   #6
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Re: must a prop shaft be pefectly balanced?

Quote:
Originally Posted by James S View Post
I'm not sure how balanced the anodes are that we bolt to our shafts...just guessing here but doesn't seem like having the end cut-off not exactly 90deg could make that much difference...also what about the set screw that's in the flange that bolts to the trans...?
All good points. Also coming from a power boat, we use much higher RPM's. But The degree of imbalance of each part adds up. If the degree of non-flatness is less than the weight of the setscrew it probably will not cause any noticable change.

Still on an internet forum I would hate to say, "OK don't worry", and have this poster write back in a few months saying, Hey I followed your advice and ruined my, (transmission, cutlas bearing, etc...).

Especially when the fix is so simple; lathe the shaft to be sure, and the machine shop can test for trueness, and balance of the shaft to insure years of trouble free use after reinstall.
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Old 14-10-2011, 10:21   #7
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Re: Must a Prop Shaft Be Perfectly Balanced ?

If I understand the original post, he cut the end off and the cut is not perfectly perpendicular? If that is the case then (disclaimer) in my opinion the effect would be minimal.

More critical would be problems from a shaft that is not straight or not perfectly in alignment with the engine/transmission.
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Old 14-10-2011, 10:34   #8
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Re: must a prop shaft be pefectly balanced?

Quote:
Originally Posted by James S View Post
I'm not sure how balanced the anodes are that we bolt to our shafts...just guessing here but doesn't seem like having the end cut-off not exactly 90deg could make that much difference...also what about the set screw that's in the flange that bolts to the trans...?
You will notice the anodes are at the far end but often pretty close to the strut and the bearing.

Very much depends on basic physics - shaft thickness, unsupported length, the amt of the off-center mass, etc..

The less off-balance, the less wear, in any case.

There is no such thing as a perfectly balanced rig, but the accy should be as per May's post, or better.

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Old 14-10-2011, 11:18   #9
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Re: Must a Prop Shaft Be Perfectly Balanced ?

Seems to me this is a better safe than sorry situation. Not to mention that knowing that crooked end is spinning around below me would piss me off.
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Old 14-10-2011, 11:22   #10
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Re: Must a Prop Shaft Be Perfectly Balanced ?

The crooked end is hidden in the coupling, and being slightly crooked is not going to make it wobble--the rest of you guys are perfectionists!
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