A duplex or dual lead worm is a worm gear set where the 2 flanks are manufactured with slightly different modules and diameter quotients. As a result, different lead angles on both tooth profiles are obtained so that the tooth thickness continuously increases all over the worm length while the gap between 2 threads decreases. This allows control of backlash.
At the worm wheel, the different modules result in different addendum modification coefficients and rolling circle diameters at both flanks. Because of this, the profiles are other at the front and the rear side. The thickness of each tooth and the tooth gaps remain constant at the wheel’s circumference.
Backlash adjustment is made by shifting the worm axially so that the section of the worm with the needed tooth thickness will be in contact with the wheel, giving the desired backlash (fig. 1).
This way, backlash can be easily adjusted to any desired value when mounting the gear. Even worn gears can be readjusted delicately and continuously without modifying the tooth contact or creating meshing interference.
Besides the above-explained duplex method, there are various possibilities to adjust the backlash of worm gears:
- variation of center distance by rotating an eccentric hub, in which the worm shaft and the worm gear wheel are cradled
- axial shifting of a conical worm (fig. 2a)
- division of the worm in 2 halves (fig. 2b), to be rotated or shifted relative to each other. (system Ott)
- division of the wheel in 2 disks (fig. 2c), to be turned close to each other.
Duplex gearings do not create this kind of problem.
They always permit geometrically accurate teeth contact and, beyond that, very delicate backlash adjustment. The evolved contact area, the load-carrying capacity, and efficiency are affected. In addition, as duplex teeth are executed as the involute gear, they are insensitive in regards to modifications of the center distance, e.g., caused by worm shaft deflections.
Point of caution during assembly
Duplex worm gears diﬀers in the module between the right and left tooth surface; therefore, you must adequately orient the worm and worm wheel. Please carefully verify the following 2 aspects before proceeding with assembly.
1. Verifying the orientation of the assembly
Fig.3 Arrow mark indicates the correct orientation of 2 gears when assembled. As shown, the 2 arrows must point in the same direction.
2. Verifying the reference position
Duplex worms are mainly utilized where any backlash is unwanted or can be harmful, to maintain repeated high precision positioning in both directions, to prevent impulse loaded damage, and when the contact flanks are alternating. Typical applications include rotary and tilting tables, milling machines, and presses.