Thin and thick nuts on propshaft: sequence??

[billknny]

It has been clearly stated that in theory, the thin nut goes first, and there are very good engineering reasons for this. It's how I always do it. It makes my engineer heart feel good to do it right.

In actual practice, it doesn't really matter. Nobody ever stripped out the threads on a thin prop nut by putting it on the the shaft behind the large nut. Yes, the aft nut carries much higher thread loading than the front nut. But there is lots of extra strength there. If your prop nuts are backwards, don't sweat it. Just do it right next time.

[slug]

Drilling and pinning a tapered shaft is troublesome

Hard to drill by hand on site when fitting the prop

Future hole alignment under proper torque is hard to achieve

Max props are typically drilled and pinned

Best stick with double nut

I’ve never seen this method fail

[GordMay]

Originally Posted by Squanderbucks

Go to sbmar.com and find the link to Tony’s tips and look at his article on propeller shaft nut sequence.

Interestingly, Tony contradicts what he says “... the good engineering will say ...” with his own version of “... the practical standpoint (sometimes) ...”, adding that “... I think I could write a small book on proper propeller installation, as there are many principles involved ...”

FWIW: I think he should have written that book, rather than this tutorial.

Tony's Tip ➥ https://www.sbmar.com/articles/prope...vs-little-nut/

[Wotname]

Originally Posted by slug

Drilling and pinning a tapered shaft is troublesome

Yes it is

Hard to drill by hand on site when fitting the prop

Yes it is

Future hole alignment under proper torque is hard to achieve

No it isn't but you might have to use a washer

Max props are typically drilled and pinned

Best stick with double nut

I’ve never seen this method fail

Is the double nut a USA thing, is it common in the UK or Europe or NZ? I haven't seen it a lot in Oz.

[kenbo]

Originally Posted by a64pilot

Yes you need a prop puller, but maybe that’s because the nut is holding the prop onto the shaft? That without the nut that over time reverse torque and vibration may let it wiggle loose?
I understand a taper fit, I’ve seen it on Lathes etc although I’m no machinist.
But I’ve also seen things loosen over time from vibration etc.

However just as an example, has anyone ever seen a properly torqued wheel lug nut loosen?
Or a cylinder head bolt? We don’t lock either of those.

Yes I have. Happens when the wheel/hub wasn't cleaned properly of rust or salt corrosion prior to mounting. Can make the same case for improper taper fitment.

[David Mathis]

Will this be the debate that rivals anchoring? After boating for over 60 years and seeing the nuts installed both ways, I’m yet to see a prop come off due to improper nut sequence. I avoid the controversy by using a castle nut.

[mf70]

Thanks for this discussion! My setup came with a castellated nut (2x thick) and the thinner plain nut. I always felt funny putting the thin nut on first, but this discussion and the provided link @ https://www.boltscience.com/pages/twonuts.htm makes it clear why it was set up that way.

[limte]

We used to have a fixed prop and for years installed the narrow nut first, followed by the thicker nut, and never experienced any problems. Several years ago we installed a feathering prop, which did not allow room on the end of the shaft for both a wide and narrow nut, so we have used two bronze narrow nuts for several years. No problems with this either. Perhaps it does not matter all that much about the configuration, as long as the nuts are torqued down and a cotter pin is installed.


Regards,
Bob S

[skipmac]

Originally Posted by David Mathis

Will this be the debate that rivals anchoring? After boating for over 60 years and seeing the nuts installed both ways, I’m yet to see a prop come off due to improper nut sequence. I avoid the controversy by using a castle nut.

But if someone's prop fell off the nuts would be gone so how would you know which way the nuts were installed?

[Dennis.G]

Having recently removed my prop I am pretty darn sure that once you have torqued down on any nut pressing the prop well onto a tapered shaft, you could remove the nut and toss it away and never loose the prop. Would not do that, but it is the taper holding the prop on to the shaft, not the nut.

[evm1024]

I've always put the small nut on first. High school machine shop...

The Bolt Science article has a key conclusion where he says "With the small nut on top ... The results are slightly better than with a single plain nut".

This begs the question: Would you go to sea with only a single plain nut "holding" your prop on the shaft?

The second question is: Would you go to sea with a single plain nut and a split lock washer "holding" you prop on?

Of course we rarely hear of a prop falling off. But we do hear of them falling off.

[Dennis.G]

Originally Posted by evm1024

I've always put the small nut on first. High school machine shop...

The Bolt Science article has a key conclusion where he says "With the small nut on top ... The results are slightly better than with a single plain nut".

This begs the question: Would you go to sea with only a single plain nut "holding" your prop on the shaft?

The second question is: Would you go to sea with a single plain nut and a split lock washer "holding" you prop on?

Of course we rarely hear of a prop falling off. But we do hear of them falling off.

My bet that the best would be a castle nut (with cotter pin), or second choice simply a single nut on threads coated with blue locktite and torqued to proper value. Split lock washers don't work, and double jam nutting seems to only work well on paper and not so well as claimed in real life.

[hirondeller]

I agree with the "belt and suspenders" comment made previously. I also like nylock nuts and I have an assortment in stainless steel, but if you can't find the right thread size....


Fishing line can be used to 'further lock' the second nut: Try winding on monofilament nylon fishing (use line that is thinner than the depth of the thread). Wind the line clockwise as you look at the end of the shaft. As you start the second nut, the metal threads will crush the nylon line when you tighten, this crushed plastic will lock the nut. It may also help keep the nut from 'freezing on' and/or provide some galvanic corrosion protection if the nut and shaft threads are different metals.



I got this technique from an 'off-roader' forum. I have used it successfully on air compressors, autos, boats and an exercise machine (but, so far, not on a prop shaft).


Test this on any bolt and nut and you will see how well it works. It seems to work any time vibration, or, counterclockwise shaft revolutions, are likely to loosen a nut.



What do you think?

[PeterS]

I keep seeing quotes stating "nuts torqued to proper or correct value"?
Does anyone know what the proper or correct value of torque is?

The logic is that enough force needs to be applied to the propeller taper to push it further up the taper than the amount of peak thrust delivered when under way.
Not enough torque is the root cause of nuts coming loose rather than vibration.

The formula for the correct force to be applied to the taper is:
F x Diameter of shaft = TonsF Where F = factor for dry (2.875) or lubricated (2.6)
The torque is calculated from:
(K x T x LbsF.)/12 (where K=factor for dry (0.2) or lubricated (0.15) & T = Nom thread.
Result for say a 3 inch shaft (dry thread & dry taper):
2.875 x 3 = 8.625 Tons Force (19,300 LbsF)
0.2 x 2.25 x 19,300 = 8685 in Lbs/12 = 723 Ft Lbs Torque.

The above can be found referenced in Machineries Handbook and Marks handbook for mechanical engineers, amongst many others.

If the above is practiced nobody will ever see a naked propeller shaft again!!
the 1:16 ratio taper is the last in the series of "self Locking tapers" ISO 1:10 ratio tapers is the first in the series of "non locking tapers", so install the propeller using the above calculated torque setting (proper Torque setting) using the large nut, remove the nut and replace the nuts thin nut first to 180 Ft/Lbs, followed by the main or thick (King) nut at 723 Ft/Lbs. The cotter pin is only to stop loose nuts from falling off the end of the shaft, it is never intended to stop the nut from loosening, it is not called a locking pin.
Do Not under any circumstances be tempted to use thread locker.

I have much to say about keys and tapers but the above is enough for this thread.
thanks all

[a64pilot]

Geezus, 723 ft lbs? What is the thread size of that nut?
Been many years ago but that’s right in the territory that you torque the Jesus nut on a UH-1, and for that you used a special wind up type of torque wrench, one great big nut.

For those that don’t know the whole main rotor assembly on a UH-1 helicopter (Huey) is held on by one nut, and quickly got called the Jesus nut