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akio.kanemoto · 25-01-2010, 15:32

Thank you all for your very detailed replies, although I'm still somewhat confused...

Firstly - to get the simple one out of the way - Nick, I'm using a bedding compound, but am doing it in multiple steps to avoid the sealant moving/squishing out,

1. bedding compound (actually a structural grade sealant) with the plate over finger tight (and clean out excess)

2. wait several days for total cure and torque to about 100 lbs/ft

3. wait several more days for compression if any and torque to the final number as per CF consensus..

4. wait several more days for compression if any and retorque to final number again.

******

Now, for the harder part... while the tables state 131 lbs/ft and most here agree on this lower torque value - why on earth to CNC and Tartan (per links in Gord's post) give 250lbs/ft?

For lubrication - I went to the local industrial bearings/bolts etc place and asked about this and they recommended "Loctite Silver Grade Anti-Seize Compound" - looking at the ingredients, it's full of graphite. So now I've applied this to the threads - some difference in feel, but nothing too dramatic. So my next question is "how do I threadlock something that's lubed"? (no space on thread for second nut, and I'm having enough difficulty ordering standard heavy hex nuts as it is, so I really don't think I'll be getting nyloc nuts or something fancy anytime this century - again, remember I'm in Australia...)

And finally, on clamped load - I've used the calculator on this site: FUTEK Bolt Torque Calculator | Bolt Torque Calculators and used the following assumptions:

Major Bolt Diameter: 0.750 in
Bolt Thread Pitch: 10
Bolt Proof Strength: 80000 psi
Recepticle Length: 0.734 in (height of one heavy hex nut)
Recepticle Strength: 80000 psi
Applied Tensile Load: 700 lb
k Factor: 0.2

Recommended Torque 2429.7 in-lb
(202.48 ft-lb)
(274.56 N-m)
Resultant Tension:16198 lb Maximum Torque 3240 in-lb
(269.97 ft-lb)
(366.1 N-m)
Resultant Tension:21597 lb
Now with these assumptions, the torque is 200 lbs/ft... still way over 130 lbs/ft.

Fiddling the assumptions a little to include bolts while heeling, say multiplying the load by 5, to 3500 lbs per bolt... actually decreases the required torque to about 170 lbs/ft. Obviously it's taking into account the fact that the weight itself will tension the bolt as well.

Having said this, going back to my question above - nothing is taking these numbers to 250lbs/ft as per CNC/Tartan.. why are they that high?

BUT - I'm convinced that the number is higher than 130, the only way to get the torque down that low is to fiddle the friction significantly lower (which in my case, even if you lube it with Superconducting Slippery Goo 2000(tm), with all the burrs on the threads, I really doubt it will go below the standard k=0.2 per a standard new thread)...

Also - if the info helps, there is no way on earth my washers/plates will turn (forwards OR backwards - they are effectively glued in)

**s/v Jedi** · 27-01-2010, 07:27

Akio,

If you work with the bedding compound like you describe, it'll be okay. But cutting some gaskets is little work and will outlast the sealant by a multiple, so I still prefer those. When I talk about Neoprene, most people think about wet suits. But the right material is very hard (Durometer 60 from memory) and thin (1/16"). You still use bedding compound around the threads.

You keep talking about burrs on the threads. I think many here already stated that the threads must be clean but I get the feeling you don't plan to remove them. So, I will repeat: you must remove the burrs, if not everywhere that at least at the spot where the nut will be plus 1/4" both below and above that. There is no way you get proper torque without doing that so better get that Dremel tool out of it's box and put it to work. I find that small files (like used for keys) work even better for a first pass and I always end with Dremel bits that are made out of a hard type of "scotchbrite" material.

You write that there isn't enough room above the nut for a second nut to lock the first one. I doubt that because it doesn't really have to be completely on the threads. Just make sure you hold the first nut in position with a wrench when you tighten the second nut. You will end up with the top nut not completely on the stud... put some sealant in the void.

I agree with the other poster that the lock washers have no place on keel bolts because they are too weak and prone to break.

The lubrication: the big difference will be when you torque the nut, not before that. The best lubricants prevent metal-to-metal contact even at high pressures and that is what makes the difference.

When a boat builder gives torque specs that are different than those from the engineering tables, you have to use the values from the yard. They probably started with the tables but found some trouble which could be corrected with higher torque. You never know, they might even have compensated for burred threads instead of cleaning them. Only the person that came up with those values can tell you the reason.

ciao!
Nick.

um saudade · 27-01-2010, 18:58

Akio,

The calculator you used (FUTEK) is not wrong and there is no consensus here for the lower torque reading. Search torque calcs on the net and you will find several sites that have the same basic outcome for 18-8 stainless.

Nick is correct that the threads need to be clean. If you can’t find a thread file, make one! Again, you will find the how to on the net, or at least an idea of what it looks like. If the threads are not too bad make a poor man’s thread chaser from a ¾ -10 nut. (This does not have to be or should not be stainless; it’s just a tool so get a grade 5) Using a hacksaw blade make at least 4 cuts into the threads to the depth of the threads. Use a needle file to make it wider, say .030 to .040. Use this nut to chase the existing threads on the studs. When it gets turning hard, run it back and forth about 90 degrees a few times and then remove the nut and clean the debris from the threads. Keep doing this until such time as the nut tightens with little or no drag.

Don’t get in there with a power tool, such a dremel or you have the potential to remove too much stock and make the threads weaker that they already are. Any stock removed by any means will mean that you have a weaker connection that it was. Torque calculations are based on tensile strength of the stock, the type of thread (US Standard starts with about a 70% thread fill which is why this thread is a little weak compare to acme or others), the height/bolt size ratio (most nuts for USS are about 1.5x the diameter of the bolt), The actual material that is being bolted together, the size of the washers (in the case of softer materials such as wood or plastic), and so on.

I have no issue with lock washers. Broken washers are the results of used washers being re-used and that is a no-no though we all have been guilty from time to time. But you don’t re-use a lock washer in this type of situation. If the flat washers you are putting down are bedded in a good compound, the compound will in effect bond the flat washer to the FRP and it will not turn!

The only reason that bolts or studs loosen in the first place is vibration. Vibration coming from an external source, say like trying to keep the bolts tight on a vibrating screen for a worst example. The other source is internal, that is that the external torque applied to the bolt is in excess of the torque applied to the bolt when it was tightened, causing it to flex.

If you tighten your keelboats properly, the keel is not going to vibrate so they will not loosen. You will be paranoid and check them over and over until such time when you get more rational and realize that you did it right and it is not going to change! After that you can check it on the same schedule you check every thing else on the boat, that is whatever level you are comfortable with.

I check the keel bolts at every haul-out and they don’t seem to move or deteriorate. But I check.

Um Saudade

akio.kanemoto · 27-01-2010, 20:15

Thanks again for the replies, greatly appreciated - the threads aren't actually THAT bad, but I will try cleaning them up using your ideas.

On the locking front - I understand that some builders use a thickened gelcoat/resin capping - what are your thoughts about putting a lump of epoxy filler (West System 411 for example) over the entire nut and washer such that a. the nut can't turn, b. there is no way water will ever get into the thread and c. it's brittle enough to break away when hit gently with a hammer next time it needs tightening?

um saudade · 27-01-2010, 20:28

It works for me! I'd use 406 or 404 though as they are the hard ones. I think off the top of my head that 411 is the super soft faring filler and it really has no strength. Make sure the threads are super clean before applying and don't use very much. But leave it alone until you are happy that the keel and bolts are not moving and then lock it down with epoxy.

Good Sailing Amigo,

Um Saudade

**s/v Jedi** · 28-01-2010, 06:04

@um saudade: great post, I'm happy to find you here on this forum with your knowledge that easily tops that of me (I'm really just a geek engineer so mechanics was always just something I was forced to learn about... ;-)

What I can add in general, is that I think too many sailors think of the Dremel tool as something to cut or grind or sand stuff with. You really should start to use it for cleaning and polishing too but you will need to buy bits that do not come in the sets that everybody gets.

For cleaning metal fasteners, washers etc. these are what you need:

Amazon.com: Dremel 512E EZ Lock Fine Grit Finishing Abrasive Buffs, 2-Pack: Home Improvement
Amazon.com: Dremel 511E EZ Lock Coarse Grit and Medium Grit Finishing Abrasive Buffs, 2-Pack: Home Improvement
Amazon.com: Dremel EZ402 EZ Lock Mandrel: Home Improvement

For wood/paint these will sand but for steel, they will clean and buff without abrading the metal itself. These are EZ-lock bits so you will need that EZ-lock mandrel as I never saw that included in a kit yet.

For polishing:

Amazon.com: Dremel 423E EZ Lock Cloth Polishing Wheel for Rotary Tools: Home Improvement
Amazon.com: Dremel 520 Polishing Wheel: Home Improvement
Amazon.com: Rubber Polish Point,Pointed: Home Improvement
Amazon.com: Polishing Compound: Home Improvement

You probably already have that polishing compound. The cloth polishing wheel is just like the big ones in a metal/welding shop and also an EZ-lock version so you need that special mandrel for it again (I have one mandrel in use and one spare, no need to buy more than that as they last forever).

ciao!
Nick.

akio.kanemoto · 28-01-2010, 18:08

Quote:

Originally Posted by s/v Jedi

For wood/paint these will sand but for steel, they will clean and buff without abrading the metal itself.

Actually I found this out the hard way.. the plates I had fabricated to replace the washers are 5mm thick 316, and for one of them, the measurement was about 2mm off, so the plate wouldn't go over the bolts.. so I thought I'd pull out my trusty dremel and enlarge the hole a little bit with the coarse grinding stone... anyway, after three stones had turned to dust and my stubbornness finally gave in, the inside edge of the hole was very nice and shiny, with not a millimetre enlargement.