Gear is one of the most widely used components in mechanical transmission system. Its main function is to transfer power and motion according to the specified speed ratio. The power of motorcycle engine is transmitted to the rear wheel through gears, and then the rear wheel drives the motorcycle to run. The working condition of gear is complex, the tooth root is subject to alternating bending stress, the tooth surface is subject to contact stress and friction, and it will also bear impact load in case of poor engagement [1]. The working environment is poor, and problems such as tooth surface pitting, gluing, wear and tooth fracture often occur during the use process. Therefore, the material type and heat treatment process selected for the gear have a great impact on the internal quality and service performance of the gear.
In order to improve the load-bearing capacity and prolong the service life of gears, the gears generally need to undergo proper heat treatment while the materials are reasonably selected. The gear after heat treatment shall meet the following requirements in performance:
(1) It has higher hardness of tooth surface to improve fatigue resistance and better wear resistance to reduce tooth surface wear.
(2) The gear center has high mechanical properties and enough toughness, so that the gear can withstand large impact load.
(3) The hardness of the matched gear should be poor to reduce the possibility of gluing.
There are many materials suitable for gear manufacturing, such as forged steel, cast steel, cast iron, non-ferrous metal and non-metallic materials. Various heat treatment methods, such as normalizing, tempering, surface quenching, carburizing and nitriding, are widely used in gear manufacturing. Therefore, compared with other mechanical parts, gear material selection and heat treatment methods are more complex [1]. According to the load and rotate speed of motorcycle gear, forged steel is mostly used. The following mainly introduces the selection of motorcycle gear materials and the corresponding heat treatment methods.
Manufacturing materials of motorcycle gears
(1) High quality low carbon steel, such as 20, 08, SPC
(2) High quality medium carbon steel, such as 35, 40, 45
(3) Low alloy steel, such as 15Cr Mo, 20Cr Mo, 20Cr Mn Ti
(4) Medium alloy steel, such as 40Cr, 35Cr Mo
In consideration of the manufacturing cost, when designing and manufacturing the motorcycle gear, the high-quality carbon steel material with lower price should be preferred, and the alloy material with higher price, such as the alloy steel containing chromium, should be used less or not as much as possible. Due to its high strength, toughness and processing performance, medium carbon steel is widely used in the manufacture of motorcycle gears, of which 40 steel and 45 steel are the most widely used. The large chain wheels of QM125 series and QM200GY series motorcycles produced by Ji’nan Qingqi are made of 45 steel.
Selection principle of motorcycle gear material
(1) Generally, 20, 08 and SPC steel are used as engine starting idler gear and starting sector gear
(2) 15CrMo, 20CrMo and 20CrMnTi steel are generally used for engine gearbox gear and main and auxiliary shafts
(3) In addition to the above two items, the driven gear, sprocket, starting motor shaft and tachometer gear with large speed ratio are generally made of 35, 40, 45, 40Cr and 35Cr Mo steel.