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I recently saw a video where a glassblower was making something, and one of the first steps was to push the wad of molten glass down into a shaper with a number of vertical spikes, such that the result vaguely resembled a gear.

I am aware that gears can be made of plastics and wood (e.g. https://woodgears.ca/gear_cutting/index.html) as well as metals, but would glass (perhaps a stronger variant such as tempered or borosilicate glass) be suitable to make gears out of? Are there any historical examples of this?

Obviously with modern materials and techniques steel is probably the best choice in an industrial setting, but I am curious as to it's suitability as an intermediate step between cheap but quickly worn down wooden gears for rapid prototyping and metal gears for serious use. I would imagine it could be a lot easier and cheaper to melt glass in a home workshop and pour it into a mold or shape it appropriately as compared to doing the same thing with metals (keeping in mind equipment costs e.g. having to build a foundry capable of melting steel).

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  • $\begingroup$ The trouble with glass is its habit of failing suddenly and completely $\endgroup$ – Jonathan R Swift Aug 10 '20 at 6:44
  • $\begingroup$ A google search should show you if there are any historical examples. $\endgroup$ – Solar Mike Aug 10 '20 at 6:52
  • $\begingroup$ @SolarMike The search results are tainted with other results such as this [1] (which is actually a metal), magnifying glasses, gear train demonstrators for students, etc. - I do not know if I just don't know the right keywords to get useful results or if there is no historical examples to be found. [1] drivesncontrols.com/news/fullstory.php/aid/5302/… $\endgroup$ – flibwib Aug 10 '20 at 7:03
  • $\begingroup$ You need to specify exactly what you mean by "glass". For example polyamides such as nylon are glasses in a technical sense (for example nylon and other common plastic materials such as ABS have glass transition temperatures which are quoted in material specifications) and nylon gears are standard mechanical components. $\endgroup$ – alephzero Aug 10 '20 at 13:43
  • $\begingroup$ @alephzero Just regular glass, as in glass jars or windows. Since the question framed it in the context of home workshopping, let's say soda-lime glass using whatever techniques would be practical for a home workshop e.g. thermal tempering would probably be doable to make tempered glass, but more industrial processes like chemical toughening via molten potassium bath would be excluded. $\endgroup$ – flibwib Aug 11 '20 at 1:48
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the tooth-root stresses in a gear are tensile and the teeth roots have sharp corners. This means if the glass gears were carrying any sort of load, the teeth would shear off right away.

Furthermore, the teeth faces in a meshing gear set are in sliding contact, and if any grit gets into the space between the glass gear teeth, the faces will rapidly get scored and then the teeth will shatter into a million pieces.

Finally, gear teeth have to withstand large shock loads when the gear train is slammed into engagement and starts up and/or reverses during operation. Brittle materials like glass exhibit very low toughness which means they break promptly under a shock load.

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That's only a good idea if the gears will be rotating very slowly with almost no load.

Gear teeth undergo significant tension and compression loading, something glass is not good at handling.

Would you ever make a hammer out of glass?

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    $\begingroup$ It's impact that would damage a hammer, not compression - glass springs exist (e.g. used in very high precision balances since there's less hysteresis compared to a steel spring)... $\endgroup$ – Jonathan R Swift Aug 10 '20 at 14:26
  • $\begingroup$ @JonathanRSwift Glass springs? wut $\endgroup$ – DKNguyen Aug 10 '20 at 20:07
  • $\begingroup$ Per @JonathanRSwift I figured as long as the hardness of the glass of each gear is about the same, it's not likely to chip (unless debris gets into the gears or something like that). I can see stuff about glass-fiber reinforced reinforced plastic springs but nothing about pure glass. $\endgroup$ – flibwib Aug 11 '20 at 1:51
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    $\begingroup$ @DKNguyen - From 'The Laboratory Companion' pg. 128 "Some highly accurate spring balances are typically made out of fused silica (quartz glass) and used in vacuum systems. [This] has two advantages. For one, the material is extremely nonreactive. ... In addition, glass is considered perfectly elastic until the point of failure. Thus, there are no hysteresis problems. These balances simply are a hanging coil of fused silica with a glass basket on the bottom." Link: studfile.net/preview/409197/page:30 $\endgroup$ – Jonathan R Swift Aug 11 '20 at 9:28
  • $\begingroup$ @flibwib the question is, what would you ever use the gears for? Follow up to that would be why not just use a plastic based material instead? It is also very difficult for the home hobbyist to accurately create a mold of a gear profile in their workshop, 3D printing would be much easier with similar results. $\endgroup$ – jko Aug 11 '20 at 12:16
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Another point, that hasn't been mentioned: Over the time you will have some wear, which means scratches on the surface. These scratches can be starting points for cracks. That's why gorilla glass uses pressure stress in the surface.

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