Worm Gear Load-Carrying Capacity Evaluation According to VDI 2736: Part 3

Standards and Guidelines for Load-Carrying Capacity Evaluation In the past few decades, there has been a gradual evolution in worm and crossed-helical gear design standards. The already noted VDI 2736: Part 3 guideline for plastic non-enveloping worm gears has its foundation in the DIN 3996 standard, which covers the calculation of load-carrying capacity of metal worm gears. The standard covers calculation methods for pitting resistance, wear load capacity, worm shaft deflections, tooth root strength, and thermal stability (Ref. 8). The standard has undergone revisions, with the 2012 and 2019 versions reflecting updates in calculation methods and material properties. Based on the DIN 3996 standard, the ISO/TR 14521:2010 (Ref. 9) standard was formed, which provides calculation methods for assessing wear, pitting, worm deflection, tooth breakage and temperature in metal cylindrical worm gears. The standard was withdrawn and replaced in 2020 by the ISO/TS 14521 (Refs. 10, 11), which covers the same failure modes but omits sections related to worm gear geometry and instead references the standard ISO 10828:2024 (Ref. 12) for geometry specifications. In the field of non-metal gears, the precursor of the VDI 2736: Part 3 guideline is the VDI 2545 (Ref. 13), which was withdrawn in 1996. The 2545 version includes a root stress carrying capacity evaluation method, which was also deemed suitable for use in crossed-helical cylindrical gears and is still proposed in modern gear design software. In the VDI 2736: Part 3 guideline, no comparable root strength rating method is presented, and instead, a fatigue fracture model dependent on the nominal shear stress on the active flank is defined. Since the VDI 2736: Part 3 guideline currently constitutes the state of the art in plastic worm gear rating models, it will be considered in more detail in subsequent sections.

The guideline enables the evaluation of the load-carrying capacity of plastic worm wheels and durability against several key failure mechanisms, typically exhibited by these components during running. A schematic presentation of the complete worm wheel evaluation procedure per noted guideline is shown in Figure 3. As is visible, the complete evaluation procedure is composed of several steps and failure mode criteria, described in more detail in the following pages.

Tooth Root Load Carrying Capacity The guideline presents a model for evaluating worm wheel tooth fracture load-carrying capacity. While the precursor guideline, the VDI 2545, considered bending root stress as the main fatigue failure criterion (leading to root crack induced failure), the updated 2736 guideline assumes that fracture predominantly occurs at the edge of the worm (i.e., at its addendum diameter, see Fig. 4) and considers instead the shear fatigue stress as being the one leading to this type of failure.

Tooth Root Load Carrying Capacity The guideline presents a model for evaluating worm wheel tooth fracture load-carrying capacity. While the precursor guideline, the VDI 2545, considered bending root stress as the main fatigue failure criterion (leading to root crack induced failure), the updated 2736 guideline assumes that fracture predominantly occurs at the edge of the worm (i.e., at its addendum diameter, see Fig. 4) and considers instead the shear fatigue stress as being the one leading to this type of failure.