Trim Adjustment Screw - B12-406

[anotherstevest]

Re: Trim Adjustment Screw - B12-406

There are two aspects of this part of the problem: 1) Identify what is likely to work well - Pretty easy I think, and the info on McMaster Carr is probably sufficient. 2) My preference is to do it in a way that's legit. See: http://www.avweb.com/news/savvyaviat..._197316-1.html

in the "Owner-Produced Parts" section for what's required to be "approved" - it must:

"The only catch is that, to be legal, an owner-produced part must be airworthy. To be airworthy, it must conform to the aircraft's type design. Therefore, if you decide to fabricate a battery box for your Skyhawk, you must duplicate the original battery box as closely as possible, using the same dimensions, materials and construction methods used in the original."

And therefore, it is my strong preference to make the part from the same alloy that was originally used (which has been shown to work "well enough"). So... I would *really* like a part that I can have destructively tested so that I can make use of (and publish) the specific alloy used. PLEASE let me know if anyone can send me one! (and yes I know that many people won't go to this level of trouble, and that "nobody will know" etc.)

Thanks,

Steve.

[Guest]

Re: Trim Adjustment Screw - B12-406

I think I recall that the "B" tail is the same as the L2.

If that is correct then some of the folks here that have the L2 drawing CD should have an actual factory drawing of that part.

So who has that CD that is willing to look? Steve I suspect that you are more likely to find a CD owner willing to look for a drawing than a jack screw owner that is willing to give to you to destroy.

Dave

[M Towsley]

Re: Trim Adjustment Screw - B12-406

Dave, I am trying to remember how exactly the L-2 trim worked but it was different from a BC-12D. It utilized a more direct cables and pulleys system unlike the jack-screw system.

[astjp2]

Re: Trim Adjustment Screw - B12-406

You are way over thinking an owner produced part. We are not supported like Cessna owners get. Its not like you have to send it in for verification that it meets CAR4A to an engineer and have a full blown quality program like Cessna has for their PMA. Turn it on a lathe, using the 360 brass, do a log book entry stating that you built it, an IA inspected it for form, fit, and function, then install it...and the most important thing is to fly it when you get it done. Tim

[anotherstevest]

Re: Trim Adjustment Screw - B12-406

(sorry if this double posts, I got an error last time...)
@astjp2 Really Tim? You wouldn't use 316 or 485 to get decent corrosion resistance? Remember the part isn't exactly safe from the elements and the lead-screw section get lubricated once at assembly and can't be lubricated again unless you disassemble it...

Steve.

[astjp2]

Re: Trim Adjustment Screw - B12-406

Steve, actually there is a lube point in the elevator, but I know that stainless has a tendency to gall so no I would stick to either brass or 4340 and bush the elevator with a thin wall sleeve. Also Brass is much easier to machine, stainless sucks without high end tooling. You can do what you want but I would just duplicate the original as best as I could, with the exception of using modern brass that is used for gears.....360 is the way I would go. Tim

[Hank Jarrett]

Re: Trim Adjustment Screw - B12-406

I think Tim is right. Brass has lower friction and galling and in very corrosion resistant. The strength is lower but the gear teeth have a HUGE profile to take the loads. I doubt the strength difference between brass types will mater much.

Hank

[Guest]

Re: Trim Adjustment Screw - B12-406

Tim, Hank,

I am not familiar with the galling characteristics of stainless but I have to ask this; if it is so bad how do we manage to have stainless nuts and bolts? What am I missing?

Dave

[astjp2]

Re: Trim Adjustment Screw - B12-406

Well every one that I installed at work wound up seizing the nut on the bolt, we had to liberally use antiseize to prevent it. I wont use stainless screws in an airplane anymore either, they either strip or seize. Just because something is available, does not mean its a good product. Tim

[Guest]

Re: Trim Adjustment Screw - B12-406

Interesting. I googled it after reading your post Tim and there are a number of "articles" on it.

I have never had a problem with it but I have only used some stainless screws and they have been threaded in to a steel "nut" part with low to no torque so it may not fit the requirements for resultant galling.

Dave

[Hank Jarrett]

Re: Trim Adjustment Screw - B12-406

Galling and seizing are very different. Galling happens when two parts move relative to each other, often with very little movement, and the surface gets microscopically "rough" jamming the parts up. Seizing happens when two parts DO NOT move relative to each other and usually corrode where the corrosion has a greater volume than the original material and jam the two parts together. Subtle, but very different.
As for stainless screws, the problem is usually that the stainless is more noble than the aluminum and the aluminum it is threaded into corrodes away. On ship board planes it was a devil of a problem, but if the parts are properly coated I haven't had much of a problem with it. Then again I do NOT use SS screws in critical structural parts. I have used them for access covers where if the screw does corrode teh cover, it will be caught and fixed long before failure. You also need to remember that most stainless steel is MUCH weaker than regular steel. A SS bolt WILL NOT carry as much load as an AN or NAS bolt of the same size!

Hank

SS screws are GREAT for upholstery retention! Look great and the loads are insignificant.

[Guest]

Re: Trim Adjustment Screw - B12-406

So hank what do you think about this; I am recovering the Champ and they use #4 PK screws 1/4" long to attach the fabric to the aluminum ribs in the wing and also for the attachment to the steel ribs in the tail feathers instead of wire or rib stitching, the original steel screws were badly corroded and hard to remove when I uncovered it so I am planning on using stainless screws for the recover, no high torque and they are "sealed" in with glue and the fabric fill and enamel top coat.

Is it going to be a problem?

Dave

[cvavon]

Re: Trim Adjustment Screw - B12-406

dAVE THE sTINSON 108-123 ALL USE p/k screws to hold the fabric on all surfaces i bought over 1000 cad plated #4 screws witch was enough to do my plane and a set of -3 wings from Fastenol

[Dick Smith]

Re: Trim Adjustment Screw - B12-406

These 2 links may interest someone learning about galling: https://www.fastenal.com/content/fed...%20Galling.pdf

*https://en.wikipedia.org/wiki/Galling
The concept of "Cold Welding" was a new one to me. Dick

[PA1195]

Re: Trim Adjustment Screw - B12-406

I came across cold welding vs stainless with amateur radio antenna projects. The hardware is commonly used and can be a benefit when the antennas are a couple of hundred feet up a tower for years and potential for corrosion an issue. After some experience commercial manufacturers started recommending an appropriate anti-seize lube as a way to prevent cold welding. Then the problem became fastener torque. The lube increased fastener stress at a given torque over dry threads (use 20-25% less torque is often cited). Stainless dearly loves to shear under stress versus other materials in my experience.

Gary