What are the types of valve train mechanisms classified by the rotation method of the camshaft?

[Ray]

Valve train mechanisms can be classified into three types based on the rotation method of the crankshaft and camshaft: gear drive, chain drive, and toothed belt drive. Gear drive: In gear drive systems, the transmission between the lower or middle camshaft and the crankshaft mostly uses cylindrical timing gears. Generally, only a pair of gears is needed from the crankshaft to the camshaft. If the diameter of the driving gear is too large, an intermediate idler gear can be added. To ensure smooth meshing and reduce operational noise, timing gears mostly use helical gears. Timing gears have timing marks, which must be aligned during assembly as required. Chain drive: Chain drive is particularly suitable for overhead camshaft valve train mechanisms. To maintain proper tension and prevent chain disengagement, components such as guide plates, upper and lower chain tensioners are installed. For ease of chain adjustment, some engines use a single chain for transmission. The advantage is easy arrangement, and for longer transmission distances, a two-stage chain drive can be used. The disadvantages include higher structural mass and noise, as well as difficulties in ensuring chain reliability and durability. Toothed belt drive: In recent years, toothed belts have been widely used in high-speed engines to replace chain drives. However, some high-power engines still use chain drives. Toothed belts offer advantages such as low operational noise, reliability, and cost-effectiveness. For dual overhead camshafts, the exhaust camshaft is typically driven by the crankshaft via a toothed timing belt or chain, while the intake camshaft is driven by the exhaust camshaft via a metal chain. Alternatively, both intake and exhaust camshafts can be driven directly by the crankshaft through a toothed belt or chain.