Saturday, August 14, 2010

make new wheel with rocks?

"So, what kind of rock should I get? Granite? Sandstone? And which quarry should I get it from? I was thinking that granite would last longer but sandstone would ride nicer and would be easier to lob at a dinosaur in case of attack."

Wheels made from rocks are quite a demanding application. Most rocks are very strong under compression (e.g., in a building or wall), but many are relatively weak under tension [] with low elastic [] strength, and therefore they will break relatively easily when a wheel is sheared laterally, such as when rounding a turn (due to forces acting perpendicular to the direction of travel). A way to mitigate this is to make the wheel rather thick, but the disadvantages (weight) are obvious.

Granite [] is probably a better choice than sandstone [] because most sandstones have individual grains that are in contact only over a small part of their area, with the spaces in between cemented together by other minerals that are often quite soft (e.g., calcite). Worse, many sandstones don't have those spaces fully infilled (i.e. the sandstones are porous []), which does increase their elastic modulus [], but makes the material more prone to surface wear (it's easier to rub the mineral grains off the surface -- and it's even worse if water freezes in your neighborhood). Cracks tend to propagate []easily in sandstones. By contrast granite and other intrusive igneous rocks [] are comprised of mineral grains that grew together as the molten rock crystallized and therefore the grains interlock quite tightly with virtually no open spaces between them (i.e. they are holocrystalline [] and often equigranular []). A downside, however, is that some of the more common minerals in many granites (e.g., feldspars [] and micas []) have good mineral cleavage[] (it's not what you think, it's planes of weakness in the crystal structure), and the more coarse-grained granites therefore tend to break more easily (because the cracks propagate along the relatively large, weaker cleavage planes in the large grains). One way around this is to look for a granite with less of the minerals that have cleavage (i.e. less feldspar and mica) and more of the minerals that don't (e.g., quartz []), and to choose a granite that is as fine-grained as possible (then the random orientation of the cleavage planes from grain to grain will mean the cracks can't propagate as far along them before bumping into a grain boundary). As a bonus, quartz has a greater hardness [] than feldspar or mica, so frictional wear will be reduced too. Therefore, a nice, fine-grained quartz-rich granite (ideally a quartzolite, but they are quite rare) is probably your best granite option. A fine-grained, non-vesicular [] mafic [] igneous rock, such as a basalt [] or diabase/dolerite [], might work well too, although they have higher density and don't have significant quartz (but the very small grainsize partly offsets this).

But why limit yourself to granite or sandstone? You can get all the benefits of a quartz-rich rock by going for a lithology [] that is even more quartz-rich than granites. For example, a metamorphosed [] quartz-rich sandstone will have the sand grains much more tightly cemented together than a typical sedimentary rock. These rocks are known as quartzites [] and are much, much stronger. So strong, in fact, that your biggest challenge will become carving the rock into a wheel shape with the tools you've got (even bronze tools would be a struggle). You might be best off doing it the old fashioned way, and using quartzite tools to carve your quartzite wheel.

To find a nice deposit of quartzite your best bet is probably to find a location where quartz-rich sandstones have been heated up adjacent to a major igneous intrusion, thus producing contact aureole [] in the sandstones where the rocks are metamorphosed into hornfels []. Basically, if you already know what granite looks like and already know what sandstone looks like (and it sounds like you do), then look in the area near the contact between the two. That will probably be the best stuff for the application you have in mind.

You've suggested using rock for your wheel, but in the end, you're almost certainly better off making your wheel out of a modern, light, composite material such as wood. A lighter wheel such as this would also allow you to flee faster should your attempts to throw rocks at the attacking dinosaurs fail.

I hope that helps. Good luck with your project.

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