Structural vs non-structural screws --and-- cad-plated vs stainless

[Zzz]

Shopping for countersunk screws for my sheet metal work, I see reference to structural vs non-structural screws. Van's Aircraft factory, which is just a few miles from my house, has god prices on hardware. In their countersunk screw selection, they specify structural or non-structural. There are no "structural" screws smaller than 8-32.

Is this a big deal since our sheet metal, at least on the fuselage, is most just covering? I would think the engine cowling, being a pressure cowling, might qualify as being "structural."

Next question: Stainless or cad plated for this work? I dislike painting fasteners, because it's always a disappointment if you ever actually use them. The strength difference between cad plated and stainless is negligible for the forces involved in this construction. The only down side to stainless I can think of is the first run through the tight threads of a new nutpate, or so I have read. Every time I do this in my work, it's a non issue.

Thoughts?

[Mark Goldberg]

While I am not the engineer - it would seem even a non structural steel screw would be much tougher than the aluminum sheets it is holding together on your boot cowl and cowling. Stainless is nice looking. I have used them sometimes. The downside of stainless is that the threads do gall more than cad plated screws. Not just the first time you put them into a nut plate. With stainless screws I like to put a dab of anti sieze or BoeLube on the screw threads when I install them. That really helps a couple years later when you try and remove the screw. Mark

[jaredyates]

While I am not the engineer - it would seem even a non structural steel screw would be much tougher than the aluminum sheets it is holding together on your boot cowl and cowling. Stainless is nice looking. I have used them sometimes.

When I asked Bob about this some years ago, his answer was almost exactly the same as Mark's here. Aircraft Spruce has MS24593 screws in a variety of lengths and diameters. I used stainless because they came with our project, but I'm seriously considering swapping them all for cad-plated next time I have the panels off, due to the corrosion concerns of stainless. The short version of the concept of cad-plated screws is that the screw corrodes before the part, which is how I'd prefer for it to happen. The 3/8" length 6-32 seems about right for wing access panels.

[Craig C.]

If I remember correctly, structural screws have a portion of the shank that is not threaded. If you use structural screws, adjust your lengths for the material stackup and nut or nutplate thickness.
Jared's callout should be MS24693 or MS24694. The MS24593 is a hydraulic check valve....would be pretty tough to screw a panel on with them

If you want to chase a spec, try here: www.everyspec.com It's a great and free resource. I use it quire a bit when I'm chasing hardware and Mil-Spec parts.

[Zzz]

The AN numbers are certainly easier to remember...

Did you guys use the countersunk washers? Or just screw right on the dimpled alum?

I hope a future builder appreciates my n00b questions some day...

[Mark Goldberg]

The only place I have used C/S finish washers is on my wingtips. Mark

[whee]

I hope a future builder appreciates my n00b questions some day...

I sure do; keeps me from having to ask them.

[geraldmorrissey]

I converted my certified plane to stainless screws and after living with them for 10 years or so, I would probably not do it again. The biggest issues are the galling and the screws being non magnetic. Drop a screw into a confined area and your in for a days work with bubble gum and a stick. I would think carefully about stainless.
Gerry
Patrol #30 Wings

[Battson]

Get Zinc or Cad plated fasteners I think.
After recent discussions, I have had another look around my plane. I am unhappy with the rate of galvanic corrosion I am seeing. I am going to remove a lot of stainless screws as a result. I don't think this will totally prevent the corrosion, but at least it will slow it down.

[Bcone1381]

Define "Galling"

[Battson]

Think cold-welding.
When two metals rub against each other, depending on the metals, one sometimes becomes stuck to the other at a microscopic level. This eventually jams up the moving parts.

Wiki:
Galling is a form of wear caused by adhesion between sliding surfaces. When a material galls, some of it is pulled with the contacting surface, especially if there is a large amount of force compressing the surfaces together.

Image of galling from a scanning electron microscope:
http://upload.wikimedia.org/wikipedi...lling_lump.jpg

[NorCalScott]

On the subject of cad-plated vs stainless, assuming the tail wires are stainless, is there a functional preference or "un-preference" for using stainless for the clevis ends vs using cadmium plated? I get the cost and aesthetics difference, but since they screw together and after reading this thread I was wondering whether there were any functional considerations favoring one over the other.

[Battson]

Short answer is not really - either is good.

Both ends of the wires only touch other steel parts, so there's no electrical potential. It's the aluminium which suffers - being so far from steel in the galvanic series.

The Cadium might eventually be consumed, but that's to the advantage of the steel which it's protecting. Easy to replace a small part like that.

[Mark Goldberg]

Galling happens when you screw and unscrew threads. It mostly applies to aluminum but also stainless to a lesser degree. The tail wires are kind of set in place once you get your tail surfaces on and in position. You tighten the tail wires and should not have to mess with them again for a long while. Mark

[corefile]

I sure do; keeps me from having to ask them.

Same here!

[Zzz]

Get Zinc or Cad plated fasteners I think.
After recent discussions, I have had another look around my plane. I am unhappy with the rate of galvanic corrosion I am seeing. I am going to remove a lot of stainless screws as a result. I don't think this will totally prevent the corrosion, but at least it will slow it down.

You're seeing this at the interface of stainless screw head and the aluminum sheet? On painted aluminum?

The nutplates are steel so not a big deal.

This is why I've always been a fan of washers no matter what the context. A screw moving against paint is eventually going to remove paint, probably quickly, and it's down to the alloy. So why not provide a compatible or sacrificial bearing surface? The only issue is that many nylon countersunk washers actually raise the screw head up a bit, preventing the benefit of flush fasteners. The stainless flush countersunk washers only raise it up a negligible amount.

[Battson]

You're seeing this at the interface of stainless screw head and the aluminum sheet? On painted aluminum?

The nutplates are steel so not a big deal.

This is why I've always been a fan of washers no matter what the context. A screw moving against paint is eventually going to remove paint, probably quickly, and it's down to the alloy. So why not provide a compatible or sacrificial bearing surface? The only issue is that many nylon countersunk washers actually raise the screw head up a bit, preventing the benefit of flush fasteners. The stainless flush countersunk washers only raise it up a negligible amount.

You're exactly right.
Piston engine planes vibrate a lot, even the well balanced ones. The paint doesn't last long trapped between the screw and the base metal...
Once it's stainless steel on aluminium alloy, all you need is a little water (worse yet salty water) and it's a matter of time until the paint starts lifting around the periphery of the screw.