The most important factor for mainspring is, as far as I understand the matter, is their yield point.

In this regard, Kevlar beats steel by at least factor of two. So why aren't we using it instead of steel in watches, well their mainsprings specifically.

I could understand the costs factor, but even premium watches seem to avoid it. As a matter of fact, finding any non-steel mainspring is pretty rare.

Is there any reason for that?

  • $\begingroup$ Cost, size constraints possibly - have you checked? $\endgroup$ – Solar Mike Jul 19 '18 at 8:38
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    $\begingroup$ One reason is that Kevlar is weak in compression and it is a viso-elastic material (did I spell that right ? ), that means it has a nonlinear stress-strain curve. There may be applications where one might need this nonlinear quality but springs for watches methinks is not one of them :-) $\endgroup$ – William Hird Jul 19 '18 at 10:10
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    $\begingroup$ Finding any mainspring is pretty rare. Quartz crystals and much more accurate, and probably cheaper. $\endgroup$ – Olin Lathrop Jul 19 '18 at 11:26
  • $\begingroup$ @OlinLathrop you should check out watches.com . They (and Amazon, for that matter) have zillions of mechanical watches, including the "self-winding" variety. Watches are as much a fashion accessory as a timepiece these days. -- and accuracy better than about 30 seconds/month is worthless since you have to fix daylight savings time anyway. $\endgroup$ – Carl Witthoft Jul 19 '18 at 17:15
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    $\begingroup$ @Carl: You can find vinyl records, film cameras, incandescent bulbs, and floppy disks too. That doesn't mean that spending engineering resources to make any of them better makes much sense. $\endgroup$ – Olin Lathrop Jul 19 '18 at 17:26

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