Duplex worm

A duplex worm or dual lead worm is a worm gear set where the two flanks are manufactured with slightly different modules and/or diameter quotients. As a result of this, different lead angles on both tooth profiles are obtained, so that the tooth thickness is continuously increasing all over the worm length, while the gap between two threads is decreasing. 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 different at the front and at the rear flank. The thickness of each tooth and the tooth gaps remain constant at the circumference of the wheel. 
Backlash adjustment is done 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, and even worn gears can be readjusted at any time delicately and continuously, without modifying the tooth contact or creating meshing interference.
Other possibilities of backlash adjustment.
Besides the above explained duplex method, there are various possibilities to adjust the backlash of worm gears: 
However all these methods demonstrate substantial disadvantages: 
Duplex gearings do not create these kind of problems.
They permit an always geometrically accurate teeth contact and beyond that, very delicate backlash adjustment. Neither the evolved contact area, the load-carrying capacity nor the actual efficiency are affected. In addition as duplex teeth are executed as involute gear they are insensitive in regards to modifications of the center distance, e.g. caused by worm shaft deflections.
Setting of backlash.
Installing and resetting of a duplex worm wheelset is typically done as follows:
Applications.
Duplex gears 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. Common applications include: rotary and tilting tables, milling machines, and presses.