すごいねじ

このねじは頭の色が赤ならきちんと閉じていない、
黒ならきちんと閉じているとわかる

すごいアイディア
電気も使わずに
実現しているらしい
http://www.smartbolts.com/

Simple
No wires, instruments, complex procedures or electronic black boxes. The only instrument needed is the eye. The indication is based on the most accurate known principle for fastener tension measurement - actual fastener elongation under load.
?????
? Visual Tension Indication
SmartBolts is the only tension indicating fastener system featuring an eye-catching visual indication of a potentially dangerous condition, obvious even from a distance and in moving machinery.

頭は使いようだ