Rivet And Bolt Strength

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Rivet And Bolt Strength

Postby TJDSII6630 » Thu Jul 09, 2020 6:54 pm

Rivet And Bolt Strength

I’m at the end of refitting / repairing a ’74 model and replaced failed fasteners and did a bit of research to ensure I put in the correct replacements.

Actually, I sold the boat and bought an SJ 21. I’m just getting back to do the final edits on this.

I had promised I would post this information and wanted to make good on that prior to moving on.

After searching for specifics on fasteners, and finding only one post mentioning strength, I decided to dedicate a post to fasteners for others who are in the same situation or need the information.

The vision is others will correct my assumptions and errors, and add to or modify this post

If an admin level member will consolidate and edit the comments this post will remain a concise, and a to the point technical page.

Please limit comments to what is technically additive and factual so this data does not get lost in the weeds.

Perhaps the information is already out there and I cannot find it.

Perhaps just list what to use and forgo the explanations.


Definitions
Proof load, in ksi (1000 PSI), is the stress the fastener can take and not permanently deform.
You will see this specked in firearm specifications. If you build your own you want the first shot to be with a 'Proof Load".
With black powder weapons and cannons it is a double charge with a double ball.
Always bring a friend when you make the proof shot. "Here Bill, you take the first shot, I need to go back an get my safety glasses"!


Oh well, Proof load, times the stress area, gives the proof strength in pounds of force.

Clamp load is the tension at the listed torque.

Working Load is the load a part is designed for. You will notice this as a rating on rope and cable as well.

Yield Strength, for those interested, or who will notice if I do not define it, is the point where the fastener permanently elongates 0.2%. Prior to this point it is in the elastic region and acts as a spring, retracting to its original un stressed length.
This is the design failure point in many applications.

Finally, the Tensile or Ultimate Strength is the point the fastener, cable, or rope breaks.

General info: https://www.melfast.com/blog/2016/06/mi ... fasteners/

Galvanic Corrosion
If you put two dissimilar metals together, one will protect the other form corrosion. This is why there is a zinc slug of some sort on the foot of an outboard motor. In this case the zinc is sacrificed to protect the iron and aluminum in the motor.
This is of little concern if your environment is dry and the parts rarely see salt water.

The short answer on using SS rivets is:
A - The surface area of the SS rivets is very small in proportion to the Aluminum
B – Use pant, tape or some other material to electrically separate the two metals.
Tef-Gel is recommended to prevent galvanic corrosion.
I use NoAlox that is used on electrical connections.
The anodized surface of the mast and boom are said to separate the SS hardware from the Aluminum.
C – Coat the rivet and joint with paint or a sealer to keep it dry if you are in salt water a lot.
D – Rinse the salt off after a sail in the ocean

https://www.bssa.org.uk/topics.php?article=89
https://www.albanycountyfasteners.com/b ... -aluminum/


Pop Rivit Sizes to use on Head Stay and Shroud Tang
According to Rudy at D&R the correct rivet for the Head Stay and Shroud Tang is a 3/16 solid SS pop rivet.
Rudy was gracious enough to say shipping would “eat me up”, as I only needed a few, and I should just seek out a local industrial supply source. (Had to put in a good word for Rudy and all he does to help us DaySailer types!)

Solid rivets retain the mandrel and this provides additional strength.

See https://www.twi-global.com/technical-kn ... /pop-rivet

I also installed SS rivets on the jib sheave as I use the jib halyard and the trailer winch to pull the mast forward while attaching the head stay to the stem head.

It took more effort than an Aluminum rivet, but my standard pop rivet gun was able to install the SS rivets.

Removing the SS rivets will be more of a chore. SS does not drill well, so grind off the head and punch the body into the mast.

Everywhere else, Aluminum rivets will be more than adequate if replaced as they corrode.

Reference for Rivets
Info on rivets from various suppliers, and it varies with material and the design of the rivet:

Working load of higher end 3/16” Aluminum rivets Shear - 310 pounds Tension – 500 Pounds
Working load of a 3/16” 318 SS rivet Shear - 1320 pounds Tension – 1610 Pounds This strength is used in the following text on loads.
Working load of a 3/16” SS rivet I bought local Shear - 1000 pounds Tension – 950 Pounds

Proof and Failure point were not given.

Check the specs of what you purchase or have, they vary with material and design. Those that retain the mandrel are stronger than those that do not. One post listed 3/16” Aluminum at 225 pounds in shear.

Install a backing washer, which come with some rivets, if you can get to the back side. This reduces the possibility of “pull through”.

Reference for Machine Screws / Bolt Strength
Fastener Strength Chart: http://www.derose.net/steve/resources/e ... bolts.html

Rigging Strength Reference
1/8 ´1 x 19 Stainless Wire, 316 SS, Breaking strength – 1780 pounds, working load – 40% or 712 Pounds
3/16” 1x19 Stainless Wire, 316 SS, Breaking strength – 4000 pounds, working load – 40% or 1600 Pounds

http://hayn.com/tech/wire_breaking_strength.html

Load Analysis Of The Fasteners
The values given are the possible loads the fasteners can withstand or impose on the other components of the system. The weak link will depend on what material / size is used, how it is installed, and how well it is maintained.

10x24 Machine Screw
A 10x24 machine screw, made from 100 ksi rated material, which is provided by a major supplier, has a proof strength of 1750 pounds.

If you torque the screw to the recommended value of, and I’m averaging between the 120 and 85 ksi materials because I could find these on line, 50 in-lbs dry, as in not lubricated, we have a clamp load, or preload if you will, of 1347 pounds on the screw.
We will use this as the Working Load for this discussion.

As we want to oppose the shroud and stay tension, we cannot torque the screw to this value were we holding two parts together, as the shroud tension would be additive to the tension imposed by the nut.
I used nylon insert nuts and just snugged them down a little bit. This leaves the bolt pretty much unloaded and the shroud can now apply the full working load to it.

(If anyone knows for a fact that this is in error please see this is corrected. And a thorough explanation is given.)

The Weak Link
If 3 each 3/16 aluminum rivets are used (See Rivet Reference above) at the tang and assume they are in pure shear, are still in good shape, and are at their rated working load, the load on the chain plate can be 930 pounds.

With 2 screws on the chain plate to share the load, each will see 465 pounds of tension if we neglect the minimal shear and bending at this point. This means the Aluminum rivets will be the weak link on this system.

Use SS rivets on the tang and the load can go to 1980 pounds per screw. This is above the proof load on the 10-24 100 KSI material screws, which means the SS rivets will not be the weak link of this system. The chain plate will be. Or the Hull!

Also See: https://wilsongarner.com/proof-load-yie ... fasteners/

From the DIEBALL Tuning Guide, the Fore stay tension should be 240-250 pounds and the shroud tension should be 350 to 400 pounds.

At 400 pounds with Aluminum rivets, you have applied 400/3 pounds to each of the 3 rivets.
That’s 43% of the allowable working load to the rivets, and are not even sailing yet.

For 1/8“ wire, we are at 56% of the working load and 22% of the breaking point.


Chain Plate
As found 10-24 SS 1347 psi Working load

https://www.melfast.com/blog/2014/03/wh ... read-ones/

The Proof Load for a screw made from 100 ksi material is 1755 pounds in tension.
The working load will be 1347 pounds in tension.

In this application only snug them down a little bit as any preload is additive to the load from the shroud due to the sails and rocking on a mooring.

The angle of the shroud gives very little horizontal load or bending at the chain plate so we can ignore it and consider this to be all tension.

If the stays are tensioned to 375 pounds, then, and if the screws are not preloaded, you have an additional 2319 pounds you can apply to the shroud.

This assumes the other parts of the system do not fail – hull, chain plate itself, tang, shroud, shackles, and the rivets do not shear or pull through the mast.


Stem Head
The stem head has larger bolts, but I did not see any mention of size on searches.
On my boat it is tapped into a metal bar made of brass.

If we assume they are 10-24, 1000 ksi material, then the same assumptions hold for the chain plates.

In this case we have enough of an angle to consider the dual loading.

If the head stay is loaded to 250 pounds, and if we assume a 30-60-90 triangle, there will be 125 pounds of horizontal force and 216 pounds of tensile force on the stem head. The spreaders will be opposing the 125 pounds of horizontal load at the point the fore stay attaches to the mast.
This is part of and not additive to the 375 pound load mentioned earlier.
This load on the stem head bolts is well within the limits of the 10-24 fastener and SS rivets.

No consideration is given to the effect of the jib halyard but it will reduce the load on the fore stay and put additional load on the shrouds, but how much can you pull the halyard without a winch?

I replaced the Aluminum Rivets with SS as I use the trailer winch and the jib Halyard to pull the mast forward so I can install the head stay more easily.


Safety Factor
The safety factor (SF), or factor of safety, is the additional strength you have over the expected load. I did not mention or apply that in the discussion above but stated the forces that can be applied to reach the working load.

One has to specify if the SF is based on working, proof, yield, failure, or fatigue point.
If you have a working load of 1000 pounds and intend to load the bolt to 250, you have a SF of 4 above the working load to deal with rocking, hard knock downs, hitting tree limbs at the ramp, etc.

If corrosion has removed half of the metal, you are down to 500 pounds working load and a SF of 2.
That’s why you have a SF, to account for loss of strength. When loads are absolutely known, as in aircraft, you will see very small SF above the worst the plane is expected to see. If you fly, take note of the pilot slowing down in rough weather to lessen the loads. At maneuvering speed, the full deflection of the control services can be applied and not over stress the aircraft.

The SF for an elevator is 9 due to the unknowns. Not sure on an escalator, but some folks decided to throw bags of coins on one as fast as they could and over work their buddy at the top. It got the coins to the top, but collapsed a few days later with people on it. Security tapes were reviewed and that is how they found out why it failed.


Fatigue
This discussion does not account for fatigue from repetitive loading due to tacking, rocking at the dock or on a mooring.

https://en.wikipedia.org/wiki/Fatigue_limit

From the diagram one can see that steel, if not loaded too much, can with stand unlimited cycles.

Aluminum will eventually fail at any loading, all be it a gazillion cycles.

You will often see fatigue cracks on the edges of thin aluminum parts.

Look closely at the edges of the control surfaces of WWII aircraft if you can get a chance to get close to them. Most will have, if the parts are original, small cracks radiating from stress points. Also note that there will be a small hole drilled at the end of the crack. This is to spread the stress around a larger area at the end or the crack. If you ever get a crack on an aluminum part, depending where it is and how large, do the same and rock on. Just remember to keep an eye on it.

Fair Winds
Teddy Johnson
TJDSII6630
TJDSII6630
 
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