Monday, March 7, 2016


I've been waiting for a hardware order to come so I could finish putting my control sticks together for the last time. Unfortunately, once again I seem to have ordered the wrong length bolts. However, this time I have enough of various sizes to put together a formula, so (hopefully?) I'll get the right stuff every time from now on. The problem is as follows:

When a person wants to order AN hardware, they have two numbers and two yes/no options for standard bolts. The yes/no options are for a drilled head (for safety wiring) and a drilled shank (for using a cotter pin and a castellated nut.) The first number denotes the diameter, and is in sixteenths of an inch. AN3 is 3/16, AN4 is 4/16, (¼, if you will) so on and so forth. Nice and easy. The second number denotes the length, and the only way to know what it stands for is to have a lookup table. The good news is, lookup tables are readily available online. The bad news is, they only give you two lengths, and those are NOT ENOUGH.

The first number I'll address is *nominal* length. This is the total length of the bolt, (minus the head) which is important to know. Obviously you want to make sure the bolt is long enough to get through the material it's holding with enough room to spare for washer(s) and a nut. In addition, it is sometimes important to know how far the bolt will stick out beyond the nut (as you'll see when I build my control-stick-pushrod-to-cables-bracket.)

"But Matt," the reader may ask, "Why don't you just order a bolt that's a little longer than you think you'll need? You can always just grind it off if the extra length gets in the way." Oh, that it were that simple. Unfortunately, there is another number that's just as important, and it makes precision quite necessary. It's called the *grip* length.

Grip length is the length of the bolt that is *not* threaded. That's right, there's a *lot* of bolt that isn't threaded. In fact, on a 3/16" bolt, only a little over ⅜" is threaded, no matter how long the bolt is. Even that isn't so bad, though; just pick a bolt that has a slightly shorter grip length than the material you're bolting through. Unless, of course, you need a castellated nut with a cotter pin.

Castellated nuts and cotter pins are used when it would be very, very bad for something to come apart. On this plane, the main things I'll have this type of hardware on are the control systems, since they're very important, and they'll be in motion pretty constantly. It's a great system, and pretty hard to mess up. If you get the right length hardware. The length, however, is determined by adding the material thickness to the amount of castellated nut that is below the cotter pin, then subtracting the distance the drilled hole in the bolt is from the end of the threaded portion. This would be feasible, except NONE of these variables are published... at least, not anywhere I found.

If you're still keeping up, you've figured out that neither nominal nor grip length will help with this problem. Therefore, I took some measurements against several different length bolts, both 3/16 and ¼" diameter, and wrote a spreadsheet that gives estimated minimum and maximum material thickness along with nominal and grip length. I've checked it against all of the hardware I have, and it all matches up for me. I'm publishing it here in hopes that I can save some other poor homebuilder a few weeks of guessing and frustration, not to mention extra shipping costs.  Currently it only features AN3 and AN4 drilled bolts, but if I get data from other diameters I'll add that to it.

*The lengths listed for max and minimum material thickness are based on the use of an AN380-2-3 cotter pin. Minimum thickness was based on my personal comfort level of how much bolt needed to stick up and grap the cotter pin, and maximum thickness was taken as soon as it was possible to push the cotter pin through the hole.

**I should mention that I used two different sized bolts for each diameter, and noticed ~.002 inch variations between same diameter bolts. I decided that was an acceptable variation to assume standard hole placement was used throughout the entire lineup of same diameter bolts. However, I do not claim to have tested each length, and therefore my numbers may not be correct in every instance. Your mileage may vary.


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