Custom Fastener Design

[cyph]

Background:
I am designing a custom 10-32 threaded fastener for an underwater vehicle. The fastener head will be exposed on the surface of the vehicle and thus needs to be hydrodynamic (better than button or pan head), hence the need to custom design.

I am trying to determine the strength of the fastener/modify the design for strength in compromise with the hydrodynamics guys. I have done some very basic (I am no FEA guru) analysis in Solidworks Simulation and have been trying to compare it to hand calculations.

The problem:
The calculations for standard fasteners seem to have the max bolt loads derived from the "tensile stress area" but seem to not take into account any stress concentrations. Isn't there a stress concentration at the fillet on the bolt head?

My FEA results seem to match the standard calculation with stress concentration factor in a round shaft with shoulder fillet, which for my geometry I am getting a factor of 2. Is this round shaft shoulder fillet geometry an appropriate approximation for the fastener head/shank geometry?

In general, what is the best way to determine how the head geometry affects bolt strength?

We were also thinking of boring out the fasteners to reduce weight. How will this affect the stress in the bolt?

Thanks!

 

[KENAT]

Why are you saving weight on an underwater vehicle?

I'd look at aerospace type solutions where they care about aerodynamics, should generally read across to hydrodynamics.

Any reason you don't just use countersunk?

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[cyph]

@KENAT:

-Minimizing weight to increase buoyancy (the vehicle needs to be positively buoyant).

-I will Look into aerospace fasteners.

-Trust me I wish I could use countersunk fasteners (and we usually do), but this application requires clamping quite thin walled sections that would not be appropriate for countersinking.

 

[KENAT]

Are you sure that 100° undercut heads can't work?

http://www.diamondfasteners.com/2-5...t-ms-zinc-0206mpu1/commercial-machine-screws/

has 10-24, not 10-32.

Also, how about countersinking the underlying structure and then dimpling the skin, or variations there on?

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[cyph]

@KENAT

For the current application we are using a CFRP shell, so dimpling would not work.

We use undercut often in non-composite applications, but how do you think it would hold up for CFRP shell? How would you undercut countersink CFRP without ripping the fibers out.
I guess then you get into those specialty composite fastening products that are bonded into laminate (something like a g10 shear tube for cored composite plates?)

I am not doing the applied load specs. on these fasteners, but I think that they are fairly highly stressed in both tensile and shear.

 

[KENAT]

How about a fancy stepped 'washer' (or maybe more of a grommet) plus a standard csk fastener?

There's probably a better aerospace fastener out there though, but material compatibility with marine environment may be an issue.

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[hydtools]

Can you use a countersunk washer that would give some transition to ease water flow over the screw head and not have to countersink the shell?

http://www.drillspot.com/products/47656/Generic_1NU84_Countersunk_Washer

Ted

 

[hydtools]

This compares the 82 and 100 degree csk screws

http://www.mechanicsupport.com/screw_countersink_angle.html

Ted

 

[cyph]

@hydtools

I have not seen those types of washers before. I will order some and run the profile by the hydro guys.

But, the height may be too large. Those washer are 7/64'' thick. I was looking into a bolt head that is only 1/32'' thick!! The low profile socket head cap screws had a head 0.098'' thick.

I guess I'm looking into making the bolt head thickness as small as possible where the yield in shear across the fastener head thickness occurs around the tensile failure in the shank. From simple hand calcs. of the shear stress in the thickness of even low profile fasteners (0.098'' thick heads) the stress is about half the tensile stress in the shank (equivalent stress using shear strength~ 50% tensile strength for stainless steel).

 

[unclesyd]

Have you considered rivets or pop rivets. I would give someone like Cherry a call with your requirements. When we were building Al boats they were quite helpful.

http://www.cherryaerospace.com/html/product/blindrivets.html

 

[TVP]

Have you instigated any of the pin & collar fastening systems? Hi-Shear (Lisi Aero) and Alcoa Aerospace are two suppliers that have a variety of lightweight fasteners for composite applications, including low profile/shear heads.

http://www.hi-shear.com/fastener_hst_stds.htm

http://www.alcoa.com/fastening_systems/aerospace/en/product_category.asp?cat_id=216

 

[cyph]

@unclesyd:

Can't use rivets because the application requires easy assembly/ disassembly, which will be done rather frequently.

@TVP:

Those shear pin systems seem nice, but from a quick scan it looks like you need a special nut. My fastener needs to go into a blind tapped hole into some aluminum bar in the current application, and I foresee mostly blind applications in the future for this hydrodynamic fastener I'm looking for.

I appreciate everyone's help. Even if I can find a super-low profile head stock fastener, just out of curiosity; does anyone know about the discrepancy between my results with FEA/ stress concentration hand calculations with the rated strength of fasteners and bolts?

 

[TVP]

Lots of references on this subject. Here are a couple:

http://books.google.com/books?id=JU...tress concentration under head fillet&f=false

http://www.ewp.rpi.edu/hartford/~ba...ress Concentration in Bolt-Nut Connectors.pdf

 

[hydtools]

Here is a quoted study that suggests stress concentration factors >3

Stress concentration factors for the threads and bolt head fillet in a bolted connection have been suggested by Lehnhoff and Bunyard [5] using linear finite element analysis. There were ten models studied. The models included bolted connections where two 20mm thick circular steel plates were bolted together by a single bolt. The bolt diameters used were 8, 12, 16, 20 and 24mm. Two models were made for each bolt diameters. The model used the maximum allowable thread depth based on tolerances for the matric thread profile. The second model used the minimum allowable thread depth. The head fillet was modeled at its minimum radius. The FEA models consisted of parabolic axisymmetric solid elements. Gap elements were used to separate the threads in the bolt/nut region and in the areas of contact between the nut, bolt and members. Element size for engaged threads and head fillet areas were determined by obtaining several solutions for the model with the element size being reduced each time. Friction was included between all surfaces that came into contact. From analysis, it is found that thread stress concentration factors were highest in the first engaged threads and decreased in each successive thread moving towards the end of the bolt. Stress concentration factors in the head fillet were 3.18, 3.23, 3.63, 3.58 and 3.90 for the 8, 12, 16, 20 and 24mm bolts respectively.

Ted

 

[MikeHalloran]

From a structural standpoint, for tying down FRP, a truss head is a much better choice than pan or button head. ... but it's larger than either, which is why it's a better choice.

For frequent removal, a Dzus fastener is a better choice than a screw. Also typically a truss-style head.

Do you need so many fasteners that their drag will severely affect the vehicle's drag?
Can you minimize their number?
Could you streamline them with an add-on, e.g. mold a groove in the FRP, put the fasteners in that, and stretch a big rubber band around the vehicle to fill the groove and cover the row of fasteners?



Mike Halloran
Pembroke Pines, FL, USA

 

[unclesyd]

After checking some old notes and few calls here is an approach that may work in your case.

Build up the area where the hole will be drilled and countersunk to give you reinforcement and prevent delamination.

Make sure the fit up gap between the Al and Composite is O.006" or better. This number has to decrease if the span of the composite increases

Use a 100° flat head self self locking titanium fastener. The button needs to be Nylon for reuse.

If this were an aircraft you could apply a sealer to improve the aerodynamics. Since you plan to remove the fastener at times this may or may not be an option.

http://www.ndindustries.com

 

[KENAT]

Out of interest, how fast, or how quiet, are you trying to go/be that this is an issue?

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[cyph]

@TVP:

I will do some more research including those links when I get some time (in between the thousand other projects I'm working on ). But from a quick scan it seems those sources say there are stress concentrations, and that the shank diameter should be reduced for increase bolt strength. My question is, say the stress concentration at the fillet under a bolt head is K=2, but for standard strength of bolt calculation (for tensile stresses) you use the tensile stress area (At) formula which I believe is a sort of average of the pitch and minor diameters. Then you plug this area into the old P=F/At. Wouldn't this be true for a fastener with a trivial step down from the head diameter to the shank diameter. But, with say a truss head fastener that has a large step down between the head and shank diameters, isn't there a large stress concentration factor that needs to be accounted for at the head fillet? So with a K=2, then the stress at the fillet is twice the P=F/At average tensile stress across the shank cross section?

 

[cyph]

@MikeHalloran:

I will look into some truss heads.

I have used Dzus fasteners much before but sadly they would not work for the geometry I am dealing with.

The CFRP shell is already made, no option of mold re-design. Definitely cannot reduce number, apparently they are significantly loaded.

@unclesyd:
That could work, and thanks for the time you put in with the notes and calls! We were thinking of using this project to develop a hydrodynamic fastener for use in not only this application but as sort of a standard go-to fastener that we use on the outside of other vehicles when countersinking is prohibitive. It seems this build up method you recommend would be time consuming if done frequently as compared to just popping in a few custom fasteners. If we use the custom fasteners often, the per unit cost goes down (I knew there was a reason for taking economics 101 in College.

@KENAT:
Good question, it is a classified project, so I don't even know those parameters... the fun part of working on classified stuff, you get bits and pieces of design parameters here and there, but never the 'big picture'= hard to design when you don't know what you are designing for.

 

[unclesyd]

Off the top of the box.

Turn the head of the 100° upside down to put the taper on top. Make the shank come out of the flat side and to stick the taper in the wind. Instead of a socket use some type spanner wrench. I'll look around as there are several fasteners on the market that are very similar.

In some flow measurements as part of very large project we found a tear or raindrop shape much superior to anything circular. We never tapered the top as we were using the part as a spacer in an electrolytic cell.