Duplex Worms Gear

A duplex or dual lead worm is a worm gear set where the two 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 two 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 two halves (fig. 2b), to be rotated or shifted relative to each other. (system Ott)
• division of the wheel in two disks (fig. 2c), to be turned close to each other.

Fig.1	Fig.2

 However, all these methods demonstrate substantial disadvantages:

• adjustments and readjustments are interfering with the geometrically accurate meshing.
• They shift the contact profile zone and change its form and size.
• With this, they decrease the load-carrying capacity and deteriorate efficiency.
• Each adjustment causes a tremendous amount of start-up wear.
• The dangers of improper assembly and destruction of the worm gear set are incredible.

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 the actual 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 orient the worm and worm wheel properly. Please carefully verify the following two aspects before proceeding with assembly.1. Verifying the orientation of the assemblyAn arrow indicating the orientation of assembly is stamped on both the duplex worm and worm wheel. When assembling the worm and worm wheel, check the worm wheel of the arrow mark on the front such that the direction of the arrow mark on the worm coincides with that on the worm wheel. Should the assembly be incorrect, the center distance "a" will become more significant than the average distance, resulting in the difficulty of the group and improper gear engagement. (Fig.3)Fig.3 Arrow mark indicates the correct orientation of two gears when assembled. As shown, the two arrows must point in the same direction.

2. Verifying the reference positionA V-groove (60 ゜, 0.3 mm deep line) on the tip peripheral of the duplex worm tooth marks the reference tooth. The gear set is designated to have a backlash of nearly zero ( ± 0.045) when the reference tooth is positioned in alignment with the center of rotation of the worm wheel with the center distance set at the value "a." (Fig.4)

Fig.4

Applications

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.