Hello Mr. Meatman,
That is a very good question. Technic enthusiast have long thought that the plastic axles were always the weakest link in a Lego creation. In responding to numerous request on web forums, particually Bricklink's, I've made two types of steel axles. The first with the Technic "X" style cut throughout the shaft, which will work great with the current plastic gears, and make the "gears" the weakest link in the system. This, mind you, still strengthens the current system many folds. Many very tough real world devices still use plastic gears at their heart, like Dewalt power drills or RC hobby industry servos.
Secondly, I also offer in my store, another stainless steel axle with the Technic "X" style only on the ends, but flatted along the remainder of the middle, which then will accomodate stainless steel gears. I, by the way, offer metal upgrade gears in 8, 24, and 40 tooth configurations. This will then make the Technic style plastic liftarms holding the axle and gears the weakest link in the system. It still would take quite a lot to break a plastic liftarms. Oh, by the way, "aluminum" Technic style liftarms are also available. The original picture posted by me here is using two 21 unit long black anodized aluminum liftarms.
Use plastic and metal in your creation interchangeably, based on the trade-off of weight over strength.
Thank you for inquiring and please check out my Bricklink store "Brick Machine Shop." In the next couple of weak I plan to upload many more creations.
Feel free to ask for custom parts to be made or custom builds.
This is what I'm working on recently.
Eezo