Nut autopsy

The thread on my 1 inch shaft is 5/8" UNC, which is almost exactly 16 mm. The pitch of 5/8" UNC is 2.3 mm, the pitch of 16 mm is 2.0 mm. So you could screw on the metric nut up to a point, whereupon it would prove very difficult.
 
lampshuk said:
Thanks, all.
Supplying further data:
This is the only nut, so I can't measure any undamaged threads .

It was locked by a through-drilled split pin. Should be a photo below.

The threaded stud part has been discarded in the process of fixing the prop shaft. Which is, as Tranona suggests, 1" diameter. If I have posted the photo below you will see that it was not in great condition, to put it mildly.

I'm afraid I didn't measure the flats, and I am away from home at the mo, but will make sure to capture it on my return.

Here's the sequence of events that I am struggling with:
The nut was removed in the process of replacing the prop. It came off with the usual application of heat and force. No unexpected drama or trauma.
The new prop was supplied with a large shiny bronze(ish) nut, which spun onto the prop shaft for a few turns, then tightened up. After a few more turns under duress we gave up and concluded it was badly formed in some way so returned it, along with the original stainless steel nut, to the prop manufacturer to match the new nut to the old.
The prop was returned, and another person dry-fitted the new prop, apparently with ease, including fully threading the nut onto the shaft. I have photos apparently showing this but didn't see it in the flesh, so to speak.
At this point, having fitted the prop, we decided that the small amount of play in the cutless bearing warranted its replacement, so the prop had to come off. This was a MAJOR effort, requiring a lot of force even after the initial "pop" of the nut. But it came off.

Cutless bearing replaced ( a real song and dance, but that's another story), time to put the prop back on, but once again the new nut would not thread properly. So as a test I ran the original nut back on. It went on reasonably smoothly to start, but needed increasing encouragement until it bound tightly and needed a lot of force to remove, resulting in the damage to nut and thread.

The prop shaft was removed and taken to an engineering works who fitted a new length of studding to match the new (bronze) nut. This is apparently standard M16.

What's baffling me is how the new nut, having been (apparently) successfully applied once, could fail another time (I am certain I did not cross-thread it) and how could the original nut get so badly damaged being re-applied to its original bolt?

Is it possible that the original stud is imperial (eg 5/8") and sufficiently close in thread form to a modern M16 (say) that they could be confused, resulting in deformation to the stud thread after a couple of applications of the bronze nut, so when the original S/S nut was applied to the (now "modified") stud as a "check" it bound tightly enough to result in the galling (in so many ways) damage?

The job is completed now, but I cannot figure out what sequence of events led to me messing up the thread so badly I needed to repair the prop shaft. So your Holmesian Powers of Deduction are much needed.

View attachment 69989
View attachment 69988
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To what happened? All it takes is a little bit of malforming and SS welds itself to itself. The split pin hole could have been the start of it, the other person putting it together last time? Who knows, but what I do know is that SS will do what it does!
 
I know it's too late now ..... but if you are ever faced with screwing a nut onto a bolt or stud or similar and it goes on a turn or three but then stops, you must never take a spanner to it unless it's a nyloc and has gone right thru to the nylon. By all means remove the nut, give it and the bolt a good clean with a wire brush and apply some grease ..... but if you cannot get the bolt or stud through to the outer face of the nut by hand then there is always something wrong that needs investigation.

A spanner is never the answer unless it is one of the very unusual "squashed" locking nuts but these are rare and very obvious.

Richard
 
My understanding is that original Moody shafts of that era were bronze, supplied by Norris. But of course it might have been replaced.
 
RichardS said:
I know it's too late now ..... but if you are ever faced with screwing a nut onto a bolt or stud or similar and it goes on a turn or three but then stops, you must never take a spanner to it unless it's a nyloc and has gone right thru to the nylon. By all means remove the nut, give it and the bolt a good clean with a wire brush and apply some grease ..... but if you cannot get the bolt or stud through to the outer face of the nut by hand then there is always something wrong that needs investigation.
Click to expand...

Very interesting to this 'bear of little brain', as there's a prop shaft outside, with the burnished prop loose fitted and a Nyloc nut loosely spun on. There's no hole drilled for a split pin.
Should I go find/fit a locking tab washer, or will the 'Nyloc' suffice?
 
zoidberg said:
Very interesting to this 'bear of little brain', as there's a prop shaft outside, with the burnished prop loose fitted and a Nyloc nut loosely spun on. There's no hole drilled for a split pin.
Should I go find/fit a locking tab washer, or will the 'Nyloc' suffice?
Click to expand...

A tab washer is the less favoured option, although with thread lock it will probably be OK. I would not rely solely on a Nyloc nut. A drilled hole is favourite.
 
vyv_cox said:
A tab washer is the less favoured option, although with thread lock it will probably be OK. I would not rely solely on a Nyloc nut. A drilled hole is favourite.
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I agree. Obviously for a tab washer to work properly you need a threaded shaft with a flat and a tab washer of exactly the right size which fits snugly against the flat. I would not recommend grinding a flat into a 360 degree thread in order to fit a tab washer.

I personally prefer a locking nut rather than thread lock. If you don't have much spare thread even a half-thickness locking nut (you might need to grind down a thicker nut) on the prop side of the nyloc is a great solution, provided that the shaft thread protrudes at least a couple of threads beyond the nyloc.

Richard
 
After some discussion on these pages a few years ago I tested Loctite 242 for its effectiveness after prolonged immersion in seawater. This is a medium strength thread lock: high strength ones are available. I found it to be very effective, so while I don't have any particular experience with locking nuts I would say that Loctite adds a good deal of security to the joint.
 
Shafts circa 1980 would have had a castlelated brass nut either BSF or whit if imperial dia
Probably not considerd good practice to mix imp metric but it was very much a cottage industry
 
vyv_cox said:
See post#21. Almost certainly the thread is 5/8" UNC. The shaft is one inch diameter.
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Stand by what I said as that was what was done in our machine shop but then we were clydeside !

Unc and whit are “practically “ interchangeable in smaller sizes
 
scottie said:
Stand by what I said as that was what was done in our machine shop but then we were clydeside !

Unc and whit are “practically “ interchangeable in smaller sizes
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except for 1/2" where the pitch is different
 
vyv_cox said:
The thread on my 1 inch shaft is 5/8" UNC, which is almost exactly 16 mm. The pitch of 5/8" UNC is 2.3 mm, the pitch of 16 mm is 2.0 mm. So you could screw on the metric nut up to a point, whereupon it would prove very difficult.
Click to expand...

Hi, all. Finally got around to measuring the surviving bits.
The nut is 24mm across the flats. Well, 23.85 to be precise, which is close enough to be either M16 or 5/8 since according to baconsdozen the AF distance for 5/8 is 23.81mm

I have a section of the studding that was inserted, and which the new (bronze) nut fits, and can confirm that it is 16mm (well, 15.89) OD.
The thread pitch is 2mm (10 thread peaks in 20mm). I can't measure the pitch of the old nut since my thread gauge doesn't go up to 2mm.
The ID of the M16 nut is a whisker over 14mm, as is the ID of the old nut.
Sorry, my earlier measurement of 16mm does indeed seem to have been dodgy. Apologies for that red herring.

So Vyv's hypothesis looks most likely.

Alol, the engineering firm I used (local to me in Stockbridge) suggested the studding idea as a simpler (cheaper) alternative to re-tapering the shaft. I didn't see them at work, but what they said they would do is simply bore and tap a hole in the end of the shaft, then drive the studding in to it with plenty of loctite. I was a bit reluctant at first, since I would expect it to simply unscrew itself, but they said that between the binding effect of the taper, and the key there would be no risk of an unscrewing. Certainly, we've been out under motor without having the prop spin itself off.

And on that theme, I can confirm that the Varifold lives up to its promise!
 
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lampshuk said:
Hi, all. Finally got around to measuring the surviving bits.
The nut is 24mm across the flats. Well, 23.85 to be precise, which is close enough to be either M16 or 5/8 since according to baconsdozen the AF distance for 5/8 is 23.81mm

I have a section of the studding that was inserted, and which the new (bronze) nut fits, and can confirm that it is 16mm (well, 15.89) OD.
The thread pitch is 2mm (10 thread peaks in 20mm). I can't measure the pitch of the old nut since my thread gauge doesn't go up to 2mm.
The ID of the M16 nut is a whisker over 14mm, as is the ID of the old nut.
Sorry, my earlier measurement of 16mm does indeed seem to have been dodgy. Apologies for that red herring.

So Vyv's hypothesis looks most likely.
Click to expand...

I'm a little confused. :confused:

It sounds like you're saying that it's standard M16 2mm pitch + 24mm AF.

Richard
 
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I'm pretty sure the letters are "TD" and in posts 4 & 5 on the thread the consensus is that the other letters are A4-80 for the steel type.

Buck Turgidson said:
A4-80 is 316 stainless high tensile.

The other letters are manufactures mark. Not size related to my knowledge.
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EG from eBay that has similar letters, but still does not explain what TD means (although looking at that, the steel type on the nut seems to bely the listing title):
CaptureNut.PNG

So following on from Vyv's conclusion, it looks like the original was a 5/8 thread, which we distorted a bit by forcing a large bronze M16 nut onto, then screwed up completely by galling it with a 5/8 S/S one.

Thanks, everyone, for your help. Lessons learned!
 
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