Maintenance of main parts of valve drive unit
  • Automotive

1. rocker arm and rocker shaft. The function of the rocker is to transfer the movement and force from the push rod and the cam to the valve and turn it on. The rocker arm is mounted on the rocker arm shaft, and the rocker arm shaft is mounted on the rocker arm seat, and the rocker arm seat is fixed on the cylinder head. The rocker arm bears great bending moment during swing, so it should have enough strength and rigidity and smaller quality. The rocker arm is a two arm lever with a rocker shaft as its pivot and two arms unequal. The short arm ends are machined with threaded holes for screwing into the valve clearance adjusting screws. The long arm end is processed into a circular arc surface, which is the working face pushing the valve. 2. tappet. The tappet is a follower of the cam, which causes the cam's turning motion to become a straight-line movement of the tappet and passes the thrust of the cam through the push rod and the rocker arm to the valve. The tappet is simple in structure and light in quality. The radius of the spherical support on the tappet is slightly larger than the radius of the push rod, so that the wedge oil film is formed in the middle of the tappet so as to lubricate the spherical support on the ball head and the tappet. 3. push rod. The push rod is between the tappet and the rocker arm, and its function is to transfer the movement and force from the tappet to the rocker arm. In the lower camshaft air distribution mechanism, the push rod is a slender rod, and the force of transmission is very large, so it is very easy to bend. Therefore, the putter has better longitudinal stability and greater stiffness. 1. common causes of damage are rocker arm and rocker shaft common defects are rocker arm and rocker shaft wear. The reasons for this are: lack of engine oil or lack of maintenance for a long time, uncleanness of oil and low working pressure of oil pumps are important reasons for accelerating the wear of valve rocker. The lubrication of camshaft, rocker arm valve and rocker shaft of engine valve train is lubricated by the oil flowing out of a small hole on the upper corner of cylinder body. In order to ensure the oil pressure of the oil channel and the normal lubrication of other parts, this hole has a specified size, it is an oil hole. If the engine is not maintained for a long time, the engine oil is too dirty and metamorphosed to block the hole, and the light will cause the parts on the cylinder head, such as the rocker arm and so on, speed up wear and tear, and then die the camshaft, the same is the camshaft bitten to death. Causes such as insufficient engine oil or low working pressure of the oil pump will also cause lubrication of the parts such as rocker arm on the cylinder head to be unlubricated and accelerate wear. 2. the fault shows that the wear of the rocker arm and the rocker shaft causes the valve clearance to change. 3. check method (1) use the radius of the arc to measure the arc of the rocker arm, and use the vernier caliper or micrometer to measure the head of the rocker arm, and compare it with the standard value. (2) the wear of the rocker shaft and the rocker arm bushing can be measured by the micrometer and the dial gauge, and the clearance clearance is calculated. The maximum wear clearance between them is 0.20 to 0.25 mm, and the assembly clearance see the instructions of each type. 4. repair the shaft or bushing if the rocker shaft and rocker arm bushing are worn seriously. The working face of the rocker arm should be replaced when worn out. There are many structural forms of tappet, but they can be classified into two categories: flat and stationary. Regular small tappet is often used for small high-speed diesel engines. The common defects of tappet are abrasions, wear and fatigue spalling and so on. Their inspection and repair methods are as follows: 1. common damage causes (1) scratch: the tappet is driven by the cam by the cam and bears the side pressure of the cam at the same time. The bottom will produce greater contact stress. If the lubrication condition between the cam and the tappet is deteriorated, some contact is made. Dry friction will occur in the surface, the local temperature increases dramatically, and even the melting point of the cam and tappet metal can be reached or exceeded, so that the metal melting and tearing of the contact surface of the cam and tappet will cause surface abrasions. (2) wear: the wear of tappet is mainly the wear of bottom and cylindrical surface. The tappet bottom and pits are rubbed and worn with cam and pusher respectively. Poor lubrication and less clean oil will speed up wear. 2. The tappet cylindrical surface is rubbed and worn with tappet installation holes. The phenomenon of tappet wear is affecting the timing of valve timing, reducing valve opening and increasing the working noise of diesel engines. (3) fatigue peeling: at the edge of the tappet, the relative sliding velocity is higher, the contact stress is larger and the contact surface is pressed again and again, so that the fatigue exfoliation is first produced at the defect of the material itself. 2. check method (1) small metal pitting worn out on the bottom of tappet is distributed radially. Under the magnifying microscope, a small piece of small hole that is tearing can be seen clearly. (2) measuring the two upper and lower sections of the cylinder by micrometer, and the diameters of two directions perpendicular to each section. Using micrometer to measure the inner hole of the mating hole on the body, calculate the wear volume, cylindricity and roundness. 3. tappet repair of tappet bottom surface or surface slight abrasions, can be used after grinding stone to continue to use, serious should be replaced. For small diesel engines, when the clearance between the tappet and the mating hole of the body is 0.20 to 0.25 mm, the new tappet should be replaced or repaired with the size method. The assembly gap between tappet and mating hole is shown in the instructions of each type. The phenomenon of metal peeling is found on the bottom of tappet, which is usually not repaired and replaced. After long term use, the rod will bend and end wear. 1. the reasons for the bending and end wear of the main cause of the damage are: (1) the valve clearance is too small, the valve is stuck inside the tube, the valve tube is installed and the cylinder head is exorbitant, which makes the push rod push up the rocker arm to increase, and be bent at the top. (2) there is no pits in the tappet at the time of installation. (3) the timing of the valve timing is incorrect, the valve opening time is too early, the intake valve push rod will be bent, and the valve opening too late will also cause the valve push rod to bend. 2. failure performance, push rod bending and end face wear affect timing of valve timing, and valve clearance changes. 3. the method of inspection is to check the straightness of the push rod and twist the push rod on the platform to judge the degree of its deformation. If the length of the line is more than 0.30 mm/m, it should be straightened. 4. the straightening of the putter should be carried out by cold pressing straightening. The top wear can be repaired by overlaying welding, and the combined push rod can replace worn parts at both ends.

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