LEONARDO DA VINCI - Worm screw meshing with a toothed wheel (helical gear)

Madrid Ms. I (BNM), fol. 17v

"This lifting device has an endless screw which engages many teeth on the wheel. For this reason the device is very reliable. Endless screws that engage only one of the teeth on the working wheel [lower drawing] could cause great damage and destruction if the tooth breaks. [In such cases] it will be necessary to add a pawl in order to avoid the reversal of the wheel's motion should that tooth break."

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