(I don’t know anything about any of this stuff, except for taking apart toys and clocks as a kid and reading stuff on Wikipedia.)
Review of stuff that is apparently widely known by people who aren’t me ----------------------------------------------------------------------- Normal **spur gears** are easy to cut with a gear hob, but don’t turn smoothly, because as each tooth comes into contact with its mate, there’s a small impact (because the teeth are deformed slightly by the force being transmitted). So car transmissions normally use **helical gears**, where the teeth are twisted into a spiral around a cylinder, so there’s no moment where an entire new tooth engages; it engages smoothly from one end to the other along its length, overlapping in time with its neighbors. Although smoother and less prone to backlash, these have two disadvantages over spur gears: 1. They’re apparently harder to cut, although they’re still made with gear hobs. 2. They create a thrust along their axis when transmitting force, like worm gears or screws. Problem 2 is solved with **herringbone gears**, also called double helical gears, which are two helical gears of opposite handedness manufactured back to back. Not only do herringbone gears not create a thrust along their axis when they’re transmitting force; they actually resist such a thrust if it’s applied. [Forrest Higgs recently wrote on his blog about 3-D printing herringbone gears] [0]. Apparently they’re no harder to make on a 3-D printer than a spur gear. A **planetary gear** or epicyclic gear arrangement consists of an outer “ring” gear with teeth on its inside, a “sun” gear in its center, and at least three “planet” gears filling the space between the sun and ring gears. Typically the planet gears themselves can spin freely, like rollers in a roller bearing, or balls in a ball bearing; but they may be attached to a planet gear carrier. The usual use of this arrangement is as a compact, high-efficiency transmission for providing mechanical advantage. A **thrust bearing** is a bearing that resists an axial force along a shaft without adding too much rotational friction. If you have a helical gear trying to push itself out of place along the shaft, for example, you use a thrust bearing to keep the shaft from sliding out of place. I’m sure I’d have already learned all that stuff if I’d taken some mechanical engineering courses, but I had to pick it up from Wikipedia. I hope it’s not too wrong. The cool thing -------------- Okay, so if you make a planetary gear arrangement out of herringbone gears and let the planet gears spin freely without a planet gear carrier, it forms a thrust bearing with a fairly large bearing surface, perhaps as large as a roller bearing. And because of the herringbone gear’s tendency to resist axial thrust, it functions as a thrust bearing. In fact, it resists motion in five degrees of freedom, leaving only one. It might be hard to assemble, but if you make half of the planet gears out of two separate helical gears, then you can assemble them in situ, with glue, screws, or a threaded tube through the middle of the gear with flanges on both ends. Then it should be easy to assemble. Alternatively, some 3-D printing techniques are precise enough to print the mechanism already assembled. Now, whether it’s *useful* as a thrust bearing depends on whether it would have less friction than some simpler system, and whether you already needed some kind of gearing system anyway. [0]: http://technocraticanarchist.blogspot.com/2010/01/high-speed-alternative-to-belts.html "2010-01-14" -- To unsubscribe: http://lists.canonical.org/mailman/listinfo/kragen-tol
