Matt,
I have not seen any aluminum T-nuts—but I sure did not go looking for them, either!
Therefore I have not tested any, and do not intend to. Here is why.
Four DuBro 4-40 T-nuts (P/N 135) weigh 1.8 grams. If these were made of aluminum, they would weigh 0.62 grams, and you would have saved 1.18 grams. A better place to save that amount of weight would be the landing gear you are using.
The strength of aluminum varies greatly with alloy and temper. You don’t say what the proposed parts are made from or their temper. Without that information, all anyone will give you is a bounding number. So here is mine. One of the companies that specializes in aluminum fasteners uses ASTM 211 materials for nuts. (They do not offer aluminum T-nuts in their catalog, but they might have them as “specials.” They may not go down to size you are interested in, though.) The simplest approximation would be to assume exactly the same geometry and derate the aluminum part based on the ratio of shear strengths. This means the aluminum part would be half the strength of the steel part using likely materials values, such as 1018 for the comparable steel part.
More rigorously, use the shear area of the major diameter and properties for 6061-T6 with a shear strength of around 25ksi. If you don’t do stress analysis, then you are stuck breaking test parts if you want a real answer. If you find yourself stripping the steel .112-40UNC T-nuts that hold your engine down, you are not going to be happy with an aluminum replacement. I use T-nuts when I bury them in structure, so stripping a nut takes work to repair and refinish and is grief I just don’t want. All to save 1.18 grams. Bad choice. If you are using them on the outside of a profile, then there is no repair work. Just keep a few extras in your flight box. Since four of them weigh less than a gram, you won’t notice the extra weight in the toolbox.
Be aware that how threaded fasteners work is that the nut is always of “softer” material than the screw. The nut micro-yields as it is fully torqued, which allows load sharing. Otherwise, all of the tension preload would be in the first thread or two. Since the screw stretches under this same load, you can see that something has to give to result in a useable bearing area. The nut.
If you are going to mix steel and aluminum you need to be aware of galling. A stainless screw in a soft aluminum nut with no lubrication is going to be problematic. I suspect an alloy steel screw with a black oxide finish would be slightly better. This is not a combination used in aerospace—for a dozen good reasons—so you are going to discover each of these things yourself. By the way, LocTite is considered a lubricant when you calculate torques and preloads for fastener sizing. It would probably reduce galling issues with aluminum and save you some grief.
A typical hobbiest never uses a torque screwdriver. Since wrench handles scale with drive size (whether hex, or Phillip, or torx or…..) and the fastener drive size is scaled to the fastener strength, using an aluminum component is going to mislead you…especially when the nut is aluminum. Making stripped threads even more likely. Cross-threading is also going to be a greater issue.
I use custom hardware on racing planes and put 4-40 and 2-56 threads into 6061-T6 aluminum (and some 2024 an 7075 when I have it) all the time. In general, I don’t count on threading an aluminum engine mounting plate for attaching the engine. I use it in a through and through mount—with steel blind nuts on the far side, usually buried under a balsa tripler covered with glass and epoxy. In short, I don’t want stripped threads!
I get the idea of trying new stuff. I also sense in others and sometimes personally feel the frustration of thinking someone wants help, but actually is just floating ideas. The tricky thing is that people want their hobby to be one of discovery. “Hey, I found a great new way to……” But they find out most of the hobby has advanced 40 years beyond the point they want to start from. It is disappointing to be that person who wants to discover new stuff and come to that realization. So my thought is this: go do the tests and the experimenting and feel the discovery. But realize that getting on the web and then asking guys who have already done that very thing—maybe 20 years ago—is tantamount to looking in the back of the textbook for the answer and cheating yourself of your own personal discovery.
I kind of came to this epiphany when I read and began to participate in the discussion on lubricating oils for alcohol fuels. For his own reasons that initially made no sense to people, a forum member wants to find a better/cheaper/more available oil than one of the proven standards that is just as cheap and available as the alternatives. In short, he wants to discover something new and valuable. But instead of doing the experimenting, he polled the forum—in effect, cheating himself of the discovery. And, as an unintended side effect, kind of frustrated the folks who attempted to save him time, money, and his own frustrating journey by giving him the answer. Because he didn’t really want the answers he got—he wanted to experiment and discover something.
It’s your hobby, go where it leads you. But don’t be upset if no one wants to go along that particular path you are taking…because they may have already walked it, or they may have taken prior knowledge and applied it already. Square earth society, you know….
Dave
PS—For reference, I have had no issues with quality using the DuBro T-nuts. I have had to replace a lot of T-nuts in ARFs that I built. Just not impressed with the stamped parts.