Study on ZI worm and helical gear drive with large transmission ratio

A ZI worm and helical gear drive is composed of an involute helicoid worm and a helical gear. Its application is limited due to less studies on the transmission performance with a large transmission ratio. In this paper, the design method of the drive is developed as: the parameters of the ZI worm should be the same as those of the helical gear; the helix angle β1 and the reference diameter factor d1∗ of the ZI worm should be greater than the minimum values proposed. The TCA of the drive shows that: the contact areas on the tooth surfaces are narrow; the effective face width of the ZI worm is larger than that of the helical gear; the contact ratio is less than 2; when the number of threads of the ZI worm changes from 1 to 5, the contact ratio decreases slightly, the effective face width of the worm decreases and that of the helical gear increases correspondingly. The instantaneous transmission ratio is proved theoretically to be constant and the assembly errors cause small shifts of the paths of contact.