High-performance plastic gears are increasingly replacing metal gears in several applications due to the many advantages they exhibit. The main ones are lower weight, no need for lubrication, cheaper mass production, significantly better noise, vibration and harshness (NVH) behavior and chemical/corrosion resistance. Most plastic gears are produced by injection molding, which enables great design flexibility, e.g., joining several machine elements into one molded part, while gear geometry modifications like enlarged root rounding or altered profile shapes are also possible (Ref. 1).

Plastic gears have been used since the 1960s, when they were initially used for simple motion transmission applications. Over the years, with the development of new, improved plastic materials, technology started to make its way into power transmission applications. Until recently, plastic gear drives were employed for applications with power up to 1 kW; however, lately, there have been attempts to use high-performance plastics in gear drives exceeding the 10-kW mark.

Along with ever-increasing customer requirements, the NVH behavior of polymer gears is also gaining importance. One of the early studies of the acoustic performance of polymer gears was carried out by Hoskins et al. (Ref. 2), in which the researchers examined the influence of diverse materials used in polymer gears and different operational circumstances on the spectrum of sound frequencies. Parameters such as the texture of the surface, wear, and temperature, stemming from the interaction between tooth surfaces, were recognized as the factors affecting the intensity of sound energy. Trobentar et al. (Ref. 3) compared the acoustic behavior of polymer gears with different tooth profiles, i.e., involute and S-gears. The tooth profile of the S-gears had a convex addendum and concave dedendum, which resulted in a progressively curved (in the shape of the letter S) path of contact. The authors found that S-gears exhibit lower noise than involute gears, which can be attributed to the more favorable contact conditions. Polanec et al. (Ref. 4) studied the noise of coated POM polymer gears. Three physical vapor deposition (PVD) coatings were investigated, i.e., aluminum, chromium, and chromium nitrite. The study revealed that uncoated polymer gears exhibited the lowest sound pressure level, and hence no positive impact of the coating on the reduced noise could be confirmed. Furthermore, the coating started to peel off during operation, causing increased friction and meshing disturbances, which resulted in an increased sound-pressure level.

The broader adoption of polymer gears could be facilitated if standardized design methodologies were established and pertinent material information became accessible. Presently, there is a lack of a global norm that would formalize the calculations, design principles, and recommendations specific to polymer gears. Certain national standards on this topic do exist, for instance, BS 6168:1987 (Ref. 5), as well as the Japanese standard JIS B 1759:2013 (Ref. 6). The latter draws from ISO 6336:2006 (Ref. 7) with some adaptations detailed in Moriwaki et al.’s study (Ref. 8). Additionally, guidelines from diverse engineering associations are at one’s disposal. VDI 2376:2014 (Ref. 9), a successor to VDI 2545 (Ref. 10), was published in 2014, stands as the most comprehensive and commonly employed framework for polymer gear design. It encompasses evaluation techniques for the most recurrent failure modes in polymer gears. Fundamental material data for substances like POM and PA 66 are also encompassed. AGMA (Refs. 11,12) has also issued design guidelines, though these focus solely on potential materials and gear configurations, neglecting design models and essential material data crucial for polymer gear design. Tavčar et al. (Ref. 13) introduced a holistic design optimization for polymer gears that encompasses all plausible failure modes. Efforts have also been made to incorporate machine-learning algorithms into gear design (Refs. 14,15), which have proven beneficial for evaluating non-standard gear designs. Nonetheless, a substantial database of existing instances is requisite to adequately train such models.

When compared to steel gears, polymer ones do also have some disadvantages. The most important ones are a lower load-bearing capacity, poorer thermal conductivity, less temperature stability, and poorer manufacturing precision. While the load-bearing capacity is the most important property, several studies have been conducted that relate to improving it, either with a special gear design (Refs. 1,16,17) or improved materials (Refs. 18,19,20). It is speculated that a significant contribution to the load-bearing capacity can also be applied with sufficient quality of the molded gears. While there are studies available discussing the effects of processing parameters (Ref. 21) and tool design on the geometric quality of injection-molded gears, there is a lack of systematic studies addressing these effects on the mechanical and thermal responses of polymer gears.

An extremely wide selection of different plastic materials is currently available on the market. A major limitation, however, is a huge gap in gear-specific material data on these materials, which is a problem that has been persisting for decades now. Providing a step towards a solution is the German guideline VDI 2736, which proposes design rating methods (Ref. 9) along with testing procedures (Ref. 22) to be followed to generate reliable data required in the gear rating process. This paper delves into the current state of the art in plastic gear testing, providing a comprehensive overview of testing methods and supplemented with case studies.