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stainless screws

tstates

Member
Ok, just had to ask. I know that bolts and hardware and what not used in aviation aplications have to be approved, etc. I know that I want to change all the fuel tank attach screws and inspection plate screws to stainless to prevent rust in the future. At least for the fuel tanks, which are structural, is it acceptable to use of the shelf stainless hardware? I Know that they, (AN) type, can be expensive, but I do want peace of mind.

I am currently closing up my wings on an RV-10. Fuse will be shipped in one week.

Regards
Thane States
 
tstates said:
Ok, just had to ask. I know that bolts and hardware and what not used in aviation aplications have to be approved, etc. I know that I want to change all the fuel tank attach screws and inspection plate screws to stainless to prevent rust in the future. At least for the fuel tanks, which are structural, is it acceptable to use of the shelf stainless hardware? I Know that they, (AN) type, can be expensive, but I do want peace of mind.

I'm not sure about the relative strength but Van's sells AN stainless versions of the CS screws used on the tank. Quite reasonably priced.
 
For my fuel tank inspection covers I bought some stainless socket head screws from McMaster-Carr. They are Mil-Spec and weren't any more expensive than the regular socket head screws I found at the hardware store.

You might need something different for the -10, but I ordered part number 92200A194, Mil-Spec SS 8-32 1/2" Socket Head Screw.
 
Stainless screws

I used stainless torx screws from Micro Fasteners.
Easier to install and less chance of stripping the head.

http://www.microfasteners.com/catalog/products/SSCFCMXS.cfm

These have the 100 degree head and have worked very well for me.
I used them on my tanks, tips, cowl and spinner. No problems so far with 50+ hours.
Quality is at least as good as the aviation screws and IMHO are stronger. I haven't stripped threads yet even though I use a power driver. Sure makes it easier to remove a bunch of screws.

Mark Dews
N885SM
RV6A
 
Screw Strengths and Hardware Substitution

Guys... check the specifications before substituting hardware!

The MicroFasteners SS screws don't seem to have any material call out -- who knows how strong they are??

Van calls out AN509-8R8 hardware in my old RV-6A plans. These are made of a pretty strong steel with a strength of 125,000 psi. The new MS number for these is MS24694-S5.

The stainless ones referenced from the McMaster-Carr catalog (92200A194) are only rated at 80,000 psi strength.

Similarly, you don't want the similar looking AN507 screws that are only rated at 55,000 psi (and labelled as "non-structural" in the Aircraft Spruce catalog)

If you go with the SS ones from McMaster-Carr, you are using parts that are rated at only 64% strength of the original design.

Your choice, but it is reducing the design margin...

Parts are not parts, and being Mil-Spec doesn't make it stronger, it just makes it a known quality and strength.

Do the later plans call out a different screw??

gil in Tucson
 
Even though the non aviation screws are weaker than AN type,I think that the aluminium will let go long before the screws do. AN or not.

Also stainless screws cause dissimilar metal corrosion under the head so its a good idea to use an appropriate lubricant/sealer here. Or so ive been told.
 
Get the screws already

The proper screws are not that expensive. About 3 cents more apiece according to my Spruce catalog for -8R8 screws. You're building an RV-10 right??You have the potential to carry 3 passengers with you, right??. Screws are cheap in the long run.
 
Very good advice from all. I am very thankfull for the input. I am all about safety, a few bucks more is OK with me. I just want it to look great and perform great. I will charge on and continue this great hobby of building. Should have the fuse kit here in one week, cant wait. Thanks again to all those who offered advice.

Thane
 
quick lesson in screwing from an engineering perspective.

Using commercial grade screws should never be done in a highly loaded joint. In reality it's probably ok - key word "probably". MIL spec. hardware typically implies* that manufacture and quality assurance are verified such that it is essentially guaranteed that the delivered parts actually conform to the specification. Commercial grade hardware can have strength properties that are advertised to be the same strength as a MIL spec. component, but you have typically no idea what the QA standards are (i.e. is it 125ksi nominal where it is allowed to let 50% go through without meeting the specified strength - you never really know).
Corrosion in dissimilar metals should always be considered. Even with a sealer or corrosion inhibiting compound (ACF50 or corrosion X), stainless screws in aluminum structure should be avoided (fuel tanks on the RV). In dryer climates you can probably get away with it, especially if you do some kind of periodic inspection. Cad plated screws in aluminum is good practice, but the heads tend to look crummy after a while, especially if the plating is polished off. Someone should really consider making cad plated stainless screws with the heads polished off (please send me royalites if you use my idea:)).
Depending on the magnitude and kind of load (shear or tension, or both) the strength of the fastener needs to be much greater than the strength of the surrounding materal. "bolt bending" can magnify the stresses at the narrow part of the screw. The failure mode of a screw fastened joint between aluminum sheet (37ksi) or fiberglass (25ksi) will typically bend the screw (125ksi tensile strength) and pop the head off.
Fairings and access panels typically have very little load so you're most likely safe in using just about what ever kind of fastener you want. I buy the most inexpensive screws I can find for the cowl inlet and wheel pants - I loose them or wear out the heads too quickly.

*you have to read the specification to find out; however if the MIL spec says "structural" then the QA standards are going to be high.
 
In the time i've worked on airplanes, I've seen many more problems with the regular cad plate screws rusting in. I've never seen stainless cause any sort of corrosion... Just my 2?.
 
That because they were over torqued. Screws only go in at about 70 inch lbs. I have never seen screws rusted in in all my years of of aircraft salvage and working a/c overhaul bays. I see it could happen if you left it out for 20 years but for the most part they'll all come out. I'm useing aircraft Titanium screws in mine
 
Broken-wrench said:
... I'm useing aircraft Titanium screws in mine
Where are you getting them? Do they come with a Torx head? I really love my Torx head stainless steel screws. Torx rox.
 
No Philips head. Never seen torx titanium!? Failed aviation bussiness auction. I don't like tri wing either if you ever seen one of those.
 
Not on the fuel tanks

There's a reason stainless is not approved for use in structural applications. You can cut a stainless 8/32 in half with a pair of wire cutters. Try that with an AN screw and you'll need a new tool.

An RV wing is designed for 6 Gs. No one knows what the G-rating would be if the part of the wing carrying most of that load was held together with stainless screws. A lot less.

How often do you plan on removing your tanks? Why mess around with something that critical just make it easier to change a fuel sensor 10 years from now?
 
szicree said:
Is this correct?{70 inch-lbs}
Steve,
AC 43.13 chapter 7 has torque tables.
If we are talking 8-32, the torque ranges from 7 to 20 inch-lbs depending on nut type and tension or shear loaded bolts.
-mike
 
az_gila said:
The stainless ones referenced from the McMaster-Carr catalog (92200A194) are only rated at 80,000 psi strength...

...If you go with the SS ones from McMaster-Carr, you are using parts that are rated at only 64% strength of the original design.
gil in Tucson
Gil, as you can see from my post the McMaster screws for which I gave the part number are for the tank inspection plates, not for attaching the tank to the wing. I am not an engineer, but I believe the inspection plate is non-structural and "only" 80,000 psi should be just fine for that purpose.
 
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Tank mounting screws

I was planning to use the micro fasteners torx screws for my tanks, but I'm reconsidering. Not due to strength issues, since there are about million of those screws. I'm pretty sure the reason there are so many is to keep the skin down like a bunch of rivets would do. I'm a bit worried about corrosion of the tank skins, since the stainless steel is at the wrong end of the galvanic table, meaning that the aluminum skin will become the anode. With the cadmium plated screws, the cadmium will be the anode.

Perhaps this is a non-issue, particularly if the tank is painted. Also, there is not a lot of electricity flowing from the tank to the wing spar.
 
szicree said:
Is this correct?
I stand correct i used a Install gun 20LBer , removal 70 Lber Sorry for the bad info. but overtorqued screws were a real bastard to get out. Srew knockers won't work on a RV because the skin is thin
 
I've only been working on aircraft for 3.5 years now, but I've come across several where the screws rusted in. Not overtorqued, but rusted in. YMMV. For inspections plates, it is my opinion (and it would appear that the FAA doesn't disagree on certified airplanes) that stainless screws will not affect to any great degree the condition of the skins they are being screwed into. For the fuel tank install however, I'm assuming (i haven't got there yet) that it calls for shanked screws, which in that case, the cad plated steel are almost twice the strength. However it is my opinion completely that the strength is completely unnecessary for the application, both of the screws (Stainless are the same P/N denoted with a "C" instead of "S" for Cad) are considered structural screws, and I highly doubt that the loads imposed on them would rank anywhere near their 80K PSI rated load.
 
If the screws are rusted in you better look at that plane a little harder because there are probably more things wrong with it than the screws. Anyway.

Why would anyone want to sub aircraft grade screws that are used in thousands of civil, commercial and military aircraft for srews of a unknown strenth and quality to save pennies :confused: ? I don't understand?
 
Tank Screw analysis...

osxuser said:
... For the fuel tank install however, I'm assuming (i haven't got there yet) that it calls for shanked screws ... I highly doubt that the loads imposed on them would rank anywhere near their 80K PSI rated load.
The fuel tank screws are just regular AN screws. The skins are so thin that it wouldn't matter anyway - the countersink transfers the bulk of the load. But I totally agree with your load analysis. If you pulled on one spanwise set (upper or lower) of one tank's screws (30 if memory serves correctly :confused: ) with all 6 Gs of a loaded RV (6 x 1800 = 10,800) that equals 360 lbs per screw (average). Now assume the skins are .032 (I don't remember what they are but that is pretty close). That gives 11,250 psi. shear on each screw. That is a really rough analysis, but it demonstrates that those screws are not highly loaded. I'm with Micky - Torx rox.

Dennis Glaeser
7A Fuselage
 
Other loads

DGlaeser said:
The fuel tank screws are just regular AN screws. The skins are so thin that it wouldn't matter anyway - the countersink transfers the bulk of the load. But I totally agree with your load analysis. If you pulled on one spanwise set (upper or lower) of one tank's screws (30 if memory serves correctly :confused: ) with all 6 Gs of a loaded RV (6 x 1800 = 10,800) that equals 360 lbs per screw (average). Now assume the skins are .032 (I don't remember what they are but that is pretty close). That gives 11,250 psi. shear on each screw. That is a really rough analysis, but it demonstrates that those screws are not highly loaded. I'm with Micky - Torx rox.

Dennis Glaeser
7A Fuselage

Lets see... you have an unfortunate accident.. the fuselage stops very quickly... the wings keep on moving forward .. the root rib support for the fuel tank holds together and the inner fuel tank screws are stressed, while the outer fuel tank screws just go along with the wing flexing forward.
It's easy to see a "rip along the dots" scenario. Are the loads always uniformly distributed as per your analysis? If one screws fails, it can set up a ripple effect, overloading the next one...etc.

Design loads are not always obvious....

But you are the manufacturer.... you get to pick....

gil in Tucson
 
az_gila said:
Lets see... you have an unfortunate accident.. the fuselage stops very quickly... the wings keep on moving forward .. the root rib support for the fuel tank holds together and the inner fuel tank screws are stressed, while the outer fuel tank screws just go along with the wing flexing forward.
It's easy to see a "rip along the dots" scenario. Are the loads always uniformly distributed as per your analysis? If one screws fails, it can set up a ripple effect, overloading the next one...etc.

Design loads are not always obvious....

But you are the manufacturer.... you get to pick....

gil in Tucson
My point is that in that scenario, I believe the tank skins are where the "rip along the dots" will occur, way before the screws fail.
 
SS torx screw calcs

I thought someone did a cal to prove the SS torx screws were more than strong enough to secure the tank to the wing. But I can't find it anywhere.

Can somone point me to it?

Just trying to cover all my bases for the FAA inspection.

Thanks

Frank
 
Whatever type of screw you use, get a set of these...

They work great in those hard to reach places.

7 pc. Bit Wrench Set

You will have to buy the bits to fit your screws (Cap or Torx) but I assure you, it will be money well spent.
 
So does anyone have the calcs?

Still looking for the calcs, saves me a chore..:)

Thanks

Frank
 
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