When selecting gears for hard-toothed surface reducers, attention should be paid to their mechanical properties. Gears are mechanical transmission components widely used in reducers. Gear transmission involves the meshing of gear teeth to transmit motion and power between any two shafts in space, enabling changes in both the form and speed of motion. Gear transmissions have a broad range of applications, offer constant transmission ratios, high efficiency, and long service lives. In the design and manufacturing process of mechanical parts, it is not only essential to ensure that the material properties can meet the operating conditions of the part and guarantee its durability, but also to consider better machinability and cost-effectiveness of the materials, thereby improving production efficiency, reducing costs, and minimizing resource consumption. If the gear material is chosen improperly, the part may fail prematurely or even become completely inoperable. Therefore, the rational selection and use of metallic materials is an extremely important task.
 

The mechanical properties of gears—including strength, hardness, plasticity, and toughness—reflect the material characteristics under service conditions. During gear meshing, contact stresses arise at the points of tooth surface contact, while significant bending stresses occur at the tooth roots, potentially leading to failure of either the tooth surface or the tooth body. As each point on the tooth surface slides relative to the adjacent surface, wear is inevitable. The primary failure modes of gears include pitting on the tooth surface, adhesive wear, plastic deformation of the tooth surface, and tooth fracture. Therefore, gear materials must exhibit high bending fatigue strength and contact fatigue strength; the tooth surfaces should possess sufficient hardness and wear resistance, while the gear core must have adequate strength and toughness. When determining the hardness of both small and large gears, it is important to ensure that the tooth surface hardness of the small gear is 30–50 HBS higher than that of the large gear. This is because the small gear bears a load several times greater than the large gear, and its tooth root is thinner, resulting in lower strength compared to the large gear. To ensure that the two gears have approximately equal tooth strength, the tooth surface of the small gear must be harder than that of the large gear.