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Mast screws. How tight?

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Turbaani
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Re: Mast screws. How tight?

Postby Turbaani » Wed Jul 15, 2020 3:08 pm

Peter_Frank wrote:
Wed Jul 15, 2020 1:54 pm
I am not in doubt at all, how much to tighten my bolts, as I work with these issues for a living also, different material combinations.

But to share in numbers, just checked, I tighten 6 mm screws in brass around 2 to 3, max 4 Nm.
And the baseplate T nuts never more than 3 Nm, just checked now.

8 Nm is way too much, even for an 8 mm screw in brass T-nuts.
You could do it this hard, if long screws in an aluminium mast yes, but I see no reason to do this, except if they come loose too fast.
Some T-nuts are stainless steel, stronger, but in my experience it is better to tighten for the often used brass ones, as it is more than sufficient, and then you dont have to remember if you got one or another in your board, safer in the long run.


Back to our often used brass T nuts in the board slots, or carbon foils with brass inserts like in my case.

The brass is the soft thing here, that goes bobcus (horrible wrong soon) if tightened too hard, thread is ruined and screw just goes around without force now.
Especially the short flat T nuts for the baseplate, they can not take much.

Getting a torque wrench is overkill, way too expensive and you dont need it, once you got an idea of how much we are talking about here.

I use torque wrenches in my work yes, but there is a lot better way to find out "how much do you tighten" your screws.

Took a picture right now, use an angled Unbrako/Hex/Allen key, and a fishing scale.
Put your hand on the side of the tool to support, and pull the fishing scale till the screw either tightens further, or loosens again - even better, do it both ways so you get an idea.

Image


Now, having the force and the distance, you can calculate the torque:

Torque in Nm = Force in newton * Distance in meter.

In my example it could be I measure 2.1 kg on the fishing scale, and my Distance is 135 mm.

10 Newton is roughly 1 Kg or 2.25 pounds (so you can use your own fishing scale or luggage weight and convert).

Meaning I measure 2.1 * 10 = 21 Newton, and the distance is 0.135 meter.

This gives me a torque of 21 * 0.135 = 2.8 Nm.

You dont need to have a foil with Allenkey screws, you can just go and find ANY other screw in your house or shed, where you can use an Allenkey, as everyone got these.
Because you just have to measure a few times, and then feel with your hand, how tight it is, to know what range we are talking about


Hope this can help those who dont work with bolts or materials and numbers, to know how much they really tighten a screw/bolt :rollgrin:

We can discusss how much we find it good to tighten, we might not agree, but important for many to get a number of how much do THEY tighten, way too much or okay?
Most often not too little is my experience...

8) Peter
Thanks Peter!
Thats is very well explained. I just remembered that I have a fishing scale that has never been in use. Finally it can help me in foiling, amazing!! Lets see how many big fish I need to open my bolts.. :roll:

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Re: Mast screws. How tight?

Postby Onda » Wed Jul 15, 2020 5:47 pm

8 Nm is not too much: https://www.schrauben-normen.de/anziehmomente.html
Sorry, the table is in German, but easily comprehensible anyway.
For stainless steel screws use "Festigkeitsklasse 6.8"
M8 thread needs 19 Nm even!
No questions that less torque can work very well as well. The given values are maximum values, btw the values are only valid for dry thread without any lubrication. If lubricated, the maximum values are significantly lower!

Peter made an excellent explanation! Well done!
However, by using that scale setup you won´t gain the relevant "feeling" for tightening the screw with your hand directly on the key.
I took a suitcase scale, connected a hex key to it and pulled in exactly the same setup and geometry like "in real foil / beach life". I learned that I can just do 6-8 Nm if I use three fingers as close as possible to the "bend" of the key when pulling with almost my full fingers´ force. Hence, I cannot over-tighten my screws.
Don´t know if my text is understandable...

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Re: Mast screws. How tight?

Postby Peter_Frank » Wed Jul 15, 2020 7:00 pm

For long screws (thread) in aluminium, you can tighten quite some more indeed, agree.

But for an M6 screw into a very short brass thread in the baseplate slot in the board, you can as a thumb rule only fasten maybe 25 percent IMO (brass thread 50 to 70 percent of SS), if you want to be sure you dont destroy the brass thread.

That is why I dont tighten more than 3 to 4 Nm in general, with M6 screws.

It is quite a lot for a very short brass thread, if you tighten twice as much :o
And you dont know the origin of the brass nuts either, some can come close to the worst stainless steel threads yes, but in general not much more than half the strength.
Deployed into a sometimes short brass thread, it is risky to over tighten.

I have seen several, windsurfers but also a kitefoiler, tightening "hard" as they believed it was needed, and ruining the brass nut threads.

8) Peter

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Re: Mast screws. How tight?

Postby OzBungy » Thu Jul 16, 2020 2:33 am

I prefer to use a t-handle wrench for hex or torx head screws.

The shaft of the wrench is quite long so it is easy to keep it aligned with the screw.

The t-handle is relatively short. It is easy to apply symmetrical force with a couple of fingers to get the screw nice and tight, but not too tight.

The wrench is quite heavy and the t-handle acts as a flywheel. It is fast to spin the screws in and out. It's faster than an electric screwdriver with no risk of cross threading.

I find L-shaped wrenches quite fiddley to use. It is easy to get the short side off line with the screw, and too easy to over tighten using the long side.

For a while I used a bicycle torque wrench. They are quite cheap and have a fixed torque of 4nm. That is fine for the final tightening, but a bit light when bedding a deep tuttle fitting into the socket.
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Re: Mast screws. How tight?

Postby Peter_Frank » Thu Jul 16, 2020 7:10 am

Very true Oz, I also use short T handles.

Small fast and you dont overtighten.

The illustration (picture) with the L shaped is only to show how to measure the approximate torque one uses, in particular for those who got absolutely no idea.

8) Peter


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