On June 9, 2009, our company accepted an accident that the rocker arm seat of WD615 engine of Shanghai customer was broken not long after replacing the valve rocker arm seat. Because the customer only brought the broken residual parts, we only analyzed the broken parts:
Because the fracture site is the support shaft root of the inlet valve rocker arm, and the fracture trace belongs to fatigue fracture phenomenon, with obvious crack initiation area and diffusion area. This kind of fracture accident at the root of inlet valve rocker arm support shaft is still relatively rare. We suspect that there are defects in the machining process of the rocker arm seat and the problem of stress concentration in the assembly process.
Therefore, we specially establish a hypothetical model on SolidWorks to analyze the stress of the supporting shaft of the rocker arm seat. (the imaginary rocker arm seat has no transition fillet at the root of the support shaft)
According to the fitting system of WD615 old engine, the rocker arm seat plays a supporting role for the valve rocker arm moving up and down. However, due to the restriction of valve push rod and valve spring, the lower part of valve rocker arm seat is a stress concentration area under the working state.
Because the fit between the rocker arm and the rocker arm seat is clearance fit, when the lubricating oil is fully lubricated, there will be oil film on the lower end face of the supporting shaft of the rocker arm seat with large force, which can play a certain floating effect. However, if the spring force of the valve spring decreases or the valve clearance is not adjusted properly, when the valve rocker arm works, There will be gap impact load at the part of stress concentration.
Through simple finite element analysis, it is found that if there is flutter in the work of the rocker arm, the stress position at the root of the supporting shaft of the rocker arm seat is prone to fatigue. If there is no excessive fillet, this fatigue will lead to local fatigue cracks on the lower circular surface of the root. This is one of the main reasons for the fracture of the valve rocker arm seat.
Why is it controversial? What effect will a simple rocker arm seat fracture have? Hehe, many friends will directly think that at most one cylinder doesn’t work. What else can there be! In fact, when the dealer asked me at that time, he felt that the other party (the user) was blackmailing him; in fact, it was not. Although it was said that the fracture of the rocker arm seat would not have a great impact, if it was not handled immediately, the problem would be great. If it was true, there would be a relatively vicious accident of the crankshaft bushing of the car.
Why do you say that? Because when the inlet valve is closed, although the cylinder is not working normally, the fuel injector continues to inject fuel. For example, if the fuel consumption of the car is 36 liters per hour (constant speed), it means that each fuel injector will inject 6 liters of diesel into the cylinder within one hour.
We all know that the diesel engine has strict requirements for the lubricating oil index of the engine. When the diesel dilutes the engine oil to a certain extent, the lubricating system of the engine will be abnormally worn due to the incorrect viscosity index of the lubricating oil. The crankshaft of WD615 engine is actually a semi floating crankshaft. When the lubricating oil film becomes thinner or even the oil film is damaged during high-speed operation, the heat of local metal friction will increase sharply, resulting in an increase in the probability of engine pad.
Many engine accidents are actually related. If you don’t pay attention to the maintenance process, don’t check carefully in maintenance, and don’t identify when selecting parts, the probability of vehicle accidents will be very high! This accident eventually led to the engine bushing. In fact, we should also see several problems:
1. The phenomenon of mixed eyes in the parts market is very serious.
2. In terms of maintenance, they only know to replace parts without finding the cause of the fault, and their ability to identify parts is very poor.
3. The driver’s car knowledge is very poor. When one cylinder does not work and the engine shakes, he does not find out the cause and continues driving.
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