Shaft attachment to gearbox

Jegs

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Hi,

Had some work done last year, cutless bearing replacement, & have just noticed, following increased vibration, that the shaft flange which is bolted to the gearbox has been secured with bolts [ thread diam. approx. 0.435" fine 20 tpi thread] with a single nyloc[?] nut to secure them.

No washers have been used; can anyone advise whether this is normal? The bolt thread seems too "fine" to me as I would have expected deeper threads with fewer tpi; am I wrong again?

Hopeful thanks,

John G
 
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Fine thread profiles have higher shear and tensile values than the equivalent diameter course thread bolt made of the same material. Fine threads are less susceptible to vibrating loose than their course thread equivalents. Thats all I can remember about the differences, but there are some more to do with machining and make up risks. A washer below a Nyloc nut would be normal in my opinion.
 
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earlybird

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The dimensions that you quote indicate a 7/16" UNF thread. That's OK, if not the most common. Less agricultural than UNC or Whitworth! Metric is usual in the UK though.
Nylocs are OK too, the reason for the vibration should be investigated. Might be an engine mounting.
Washers would be nicer, but I wouldn't lose any sleep if the fastenings stay tight.
 

Caer Urfa

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Hi,

Had some work done last year, cutless bearing replacement, & have just noticed, following increased vibration, that the shaft flange which is bolted to the gearbox has been secured with bolts [ thread diam. approx. 0.435" fine 20 tpi thread] with a single nyloc[?] nut to secure them.

No washers have been used; can anyone advise whether this is normal? The bolt thread seems too "fine" to me as I would have expected deeper threads with fewer tpi; am I wrong again?

Hopeful thanks,

John G

Hi John

The shaft flange is generally locked onto a key by one or two grub screws, I assume you have checked that they are still there and tight!

I do not have washers with the bolts from the flange to the gearbox on my boat but they do have the nyloc washer in the nut and I assume you have checked yours is tight??

Vibration can be caused by a few things, engine mounts needing replacing, shaft alignment, prop fouled etc, if you have a stuffing box is it getting to hot??

Mike
 

Jegs

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Gentlemen,

Thanks to all of you for taking the time to respond. Amused to note that metric is normal in the UK, they were fitted in France! I suspect that the engineer[?]
simply dropped the engine to the position of the unsupported shaft - about 800mm of 40mm diam. stainless plus flange. I shall investigate further & report any progress.


Merci,

John G
 

pvb

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You say the work was done last year, but vibration has only recently occurred. If that's the case, alignment may not be the problem.
 

Jegs

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You say the work was done last year, but vibration has only recently occurred. If that's the case, alignment may not be the problem.

Indeed not, my first thought was the engine: new filters, check exhaust elbow, new nozzles in injectors [around 500 of your English pounds] & I decided to widen the search & the bolts securing flange to gearbox were not very tight...........heigh ho.

Thank you,

John G
 

pvb

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Indeed not, my first thought was the engine: new filters, check exhaust elbow, new nozzles in injectors [around 500 of your English pounds] & I decided to widen the search & the bolts securing flange to gearbox were not very tight...........heigh ho.

You didn't originally mention that they weren't tight. Just torque them up appropriately, and they should be OK.
 

Jegs

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You didn't originally mention that they weren't tight. Just torque them up appropriately, and they should be OK.


No, because I was concerned with the fastenings & I believe that the bolts were tight initially: hence the question. Sorry for the lack of clarity.

Thank you,

John G
 

SAMYL

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High tensile UNF threaded bolts with nyloc nuts are the bolts of choice for most types of prop shaft joints/universal joints both in vehicles and boats due to their strength and resistance to loosening.
They are not normally used with washers in high vibration situations - just something else to fail.
 
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