1. The main cause of early damage to valve train
1. Poor quality of maintenance. An important problem when carrying out repair work is that quality of processing of valve seat and valve seat cannot meet requirements, which leads to ablation, depression and early damage to seat; match camshaft bearing clearance during scraping, contact area and concentricity of each bearing cannot meet requirements, which will accelerate wear and cause premature damage due to abnormal noise; when valve guide is replaced with a new one, reaming quality cannot meet specified requirements, which directly affects service life of valve and valve seat.
2. Misapplication of maintenance data: Failure to scientifically select maintenance data during maintenance is an important cause of early damage to parts. For example, width of contact surface between valve and valve seat is 1-2.2 mm for an inlet valve and 1.5-2.5 mm for an exhaust valve. But in maintenance, people often think that wider is safer than narrower, and are accustomed to choosing upper limit or close to upper limit, so bevel width of a newly repaired car is close to use limit. Another example is valve stem clearance, which is usually 0.2~0.25mm for automobiles, but it is also mistakenly considered that a larger clearance is better than a smaller clearance in maintenance and adjustment, so it is used beyond upper limit. In fact, if clearance is too large, it will not only reduce engine power, but also cause knocking (valve opening) and early damage.
2. Matters requiring attention during maintenance
Timing mechanism maintenance involves a lot of manual labor. Due to technical differences and cognitive deviations of maintenance personnel, quality of maintenance is difficult to meet specified requirements, so special attention should be paid to quality of maintenance of timing mechanism during maintenance, and take effective measures to improve service quality to prolong its service life.
1. Easy grinding of valve: during maintenance, if valve has ablation, pitting and depression, it should be polished (in serious cases, valve must be replaced). It is usually carried out on a valve grinding machine. When working, four problems should be paid attention to: one is to monitor concentricity of valve head and stem, otherwise it should be straightened first; The better; third is to improve surface finish as far as possible; fourthly, depression at end of valve stem must be smoothed out.
2. Valve seat reaming: Valve seat reaming is usually done by hand and special attention should be paid to three issues: first, based on elimination of depressions and spots and possibility of unfolding entire surface of cone. The smaller sweep, better; secondly, force should be uniform in reaming, knife should be easy to start and close, less reaming and more observation to ensure less reaming and better finishing; Third, try to match with valve to determine work surface position and width. The position must be adjusted to middle and bottom of valve cone surface, and top and bottom reamers can be used to adjust top or bottom position. Width of working surface, inlet valve can be adjusted within 0.9mm (set as 1~2.2mm), and exhaust valve can be adjusted within 1.4mm (set as 1.5~2.5mm). Practice has shown that above width can reach 1mm for intake valve and 1.5mm for exhaust valve after grinding valve and valve seat, both of which are within lower limit of specified width, which can greatly improve their service life.