# Comparison of Selected Parameters of a Planetary
## 1 Introduction
The S-gear tooth flank profile features a concave shape in the lower part of the dedendum such that the mating gears exhibit the convex-concave contact in the vicinity of the contact start and contact end. The shape of the tooth surface of the S-gear also provides higher comparative reduced-curvature radii. Therefore, the contact load is lower [31]. Tests were performed, which led to the confirmation of the assumption that the theoretical tooth flank geometry of S-gears provides more rolling and less sliding between the matching flanks compared to involute gears. The convex-concave contact leads to lower contact stress, which in combination with less sliding, means lower losses due to sliding friction, and consequently, less heat generated [32]. The S-gears were also used by the design of the planocentric gear boxes and devices for large transmission ratios, primarily applied in automation and mechatronic industry, which decrease rotational movement and increase torque to the largest possible extent in the smallest available space [33]. In all applications where the reduction gear must be as small as possible, a planetary gearset is preferred as it has the additional advantage of the input and output shafts being coaxial. The Wolfrom gear is suitable for high speed ratios, and has an efficiency that is not optimal but still acceptable. The version with single-rim satellites has significant design and technological advantages, such as large kinematic capabilities starting with speed ratios of about 50, but displaying a decreasing efficiency with a speed ratio increase. A high grade of compactness is achieved due to internal meshing and planet positioning inside of ring gears. If one-rim planets are used, the manufacturing is greatly simplified, especially by using the carrier-less arrangement [34]. Polymer material are, among others, largely used for the manufacturing of automotive parts, electrical and electronic equipment, and equipment in other industries. The most widely used polymer materials are: polyurethane (PUR), polypropylene (PP), acrylonitrile butadiene styrene (ABS), polyamides (PA), polycarbonate (PC), polyvinyl chloride (PVC), poly(phenylene oxide) (PPO),polyethylene terephthalate (PBT), polyacetal (POM), etc. [35; 36; 37]. In addition, polymer composites can be used for various parts of a vehicle body or, for example, gearbox housings [38; 39; 40; 41]. Injection molding is the most extensively used process for manufacturing plastic gears. It has the capability to produce good-quality, small, nonmetallic gears of various geometries for light-load and noiseless transmissions. Nylon, acetal, polycarbonate, and polyester types of thermoplastic materials are commonly processed using injection molding to make gears. Various additives, such as polytetrafluoroethylene (PTFE), silicon, molybdenum disulfide, and graphite, are also added to improve the strength, inherent lubricity, heat stability, impact and wear resistance, and other properties of the thermoplastic materials [1; 2; 42]. Injection-molded parts, including gears, are manufactured by melting a specific powdered or pelletized polymer, or a combination thereof, before injection into a die containing the desired gear geometry of the gear. The full mechanical properties of the polymer and functionality of the gear is then realized via subsequent cooling, solidification, and ejection from the die and final trimming, if required [43]. The POM-C and PA66 materials are often used in plastic gear manufacturing because of the good dynamic friction coefficient values and the price criterion [31]. As an example of a typical plastic material used for gears, BASF Ultramid A3K (PA66) provides an easy flowing injection molding grade for fast processing and is commonly used in highly stressed technical parts, such as bearings and gear wheels. The convex-concave gearing manufacturing methods and the demands on the gear axes position precision handicap this type of teeth flank relative to the involute ones.