Common damage to diesel camshafts and overhaul

    Abstract: This paper introduces the main damage characteristics of diesel engine camshafts, analyzes the causes of the damage and discusses the overhaul methods of camshafts in order to improve the quality of maintenance of injection pump camshafts.
    The camshafts of diesel engines have a distribution mechanism camshaft and an injection pump camshaft. The damage to the injection pump camshaft in use is roughly similar to that of the camshaft in the gas distribution mechanism, mainly in terms of cam surface wear and camshaft bending. These damages will directly affect the timing of the diesel engine's gas distribution and oil supply, affect the normal working quality of the diesel engine, lead to overloading of the other cylinders, and in serious cases damage the engine body and affect driving safety. Therefore, it is especially urgent to overhaul the cam surface damage in time to reduce the failure rate.
    I. Main damage to the camshaft
    When the camshaft is at rated speed, the linear speed is as high as 1.3 m/s. In the fuel supply process of driving the plunger upward, the friction and pressure on the cam surface are considerable due to the fuel compression pressure and spring pressure, and when the dead point is turned, the opposite side of the cam is subjected to the impact of the elastic force, so that the cams are worn to different degrees on both sides, especially on the lift side of the cam, in addition to wear. In particular on the lift side of the cam, in addition to wear, there are also pitting fatigue pits. When the cams are worn to 0.3 mm, this obviously affects the accuracy of the oil supply time and the continuation angle of the oil injection, resulting in poor engine ignition and reduced dynamics and economy. When pits appear on the cam surface and the surface roughness decreases, continued use will accelerate wear.
    2. Cam journal wear: The journal and the inner ring of the ball bearing are a tight fit and generally do not wear out. When the individual shafts are worn, the radial clearance increases and affects the engine's speed control performance.
    3. Cam keyway wear: Generally uncommon, individual cases are due to loose installation of the drive sleeve, resulting in impact during operation and hence wear. After wear, the oil supply time will fall behind.
    4. Broken camshaft: The broken part is mostly on the conical cylinder of the shaft head. This is because a small number of camshafts are inherently defective at the time of manufacture and fatigue wear during work, which eventually leads to fracture or to a significant increase in camshaft transmission torque due to a stuck or broken plunger, exceeding the permissible stress of the camshaft, which is easy to break here due to the small cross-section of the tapered part of the shaft head.
    Defects in the camshaft are common with the first one, and the remaining three are less commonly encountered.
    II. Causes of damage
    1、When the oil pump is assembled, if the axis of the push rod is skewed during the assembly process, it will lead to the roller and camshaft cam not being in line contact, but in point contact, resulting in uneven force, causing the roller to roll poorly and leading to increased wear on the working surface of the roller and cam.
    2. The lower body of the oil injection pump and the cam use pressure lubrication, and oil is supplied to the lower body of the pump through the hollow bolt, the small copper pipe for oil supply and the joint body to lubricate the cam and the roller, and the blockage of the hollow bolt, the small copper pipe for oil supply and the joint body often causes poor oil supply, and the heat generated by the cam and the roller cannot be dissipated in time, which initially leads to the strain on the cam and the roller, and the wear of the copper sleeve of the roller exceeds the limit, and the roller rolling on the cam causes the impact on the roller The impact of the rollers on the cam causes wear on the rollers, increasing the wear on the brass bushings and locating pins, sometimes causing the brass bushings to hold on the pins. Sliding friction occurs between the roller and the cam, resulting in irregular oval formation of the roller and serious damage to the top of the cam.
    3. The clearance between the air distribution roller body and the push rod duct is too large. Due to the improper clearance of the push rod assembly, the clearance between the gas distribution roller body and the push rod conduit is too large, and the clearance between the push rod conduit and the roller pin is too large, which can cause the roller pin to be poorly oriented and the roller axis to deviate from the cam axis, causing the gas distribution roller to turn at a small angle and gnawing the gas distribution cam of the camshaft.
    3. Overhaul
    1. To check the axial clearance of the camshaft, pry the camshaft to one side by holding the end of the camshaft and adjust the gauge needle to "0", then pry the camshaft to the other side and the indicator value of the gauge needle is the axial clearance (0.05-0.15 mm is normal). If the axial clearance exceeds the specified level, it can be adjusted by replacing the shims with different thicknesses. If the axial clearance is too large, a thinner shim needs to be replaced; if the axial clearance is too small, a thicker shim should be replaced. If the camshaft is 0.05 mm to the left, just pull out the shim on the right (also 0.05 mm thick) and put it on the left.
    2. Check the camshaft curvature. If the bend is more than 0.05 mm, it should be straightened repeatedly by cold pressing, and it is not advisable to hit it hard with a hammer (hand hammer). There are only two kinds of camshaft bending: one is the bending of the central part of the camshaft; the other is the bending of the end of the camshaft (the end connected to the oil supply automatic advance), of which the bending of the front end of the camshaft is most common when the oil supply automatic advance is equipped with a gear drive type. This bending causes the tapered roller bearing in the front end of the camshaft to be subjected to the greatest force, resulting in the bearing housing ring "going round", the bearing cover loosening and moving out, the camshaft tampering increasing, the wear of the cam, tappet balls and the timing gears meshing with each other in the gearbox accelerating, and in severe cases, also causing the bearing roller support frame to break and the pump to be damaged. Therefore, the bending of the camshaft must be straightened in time.
    3. Check the journals of the camshaft where the oil seal is mounted. If the depth of the groove is more than 0.10 mm, it should be welded and repaired. Check the fit of the camshaft keyway and the semi-circular key. Check the camshaft taper and threads. If they are rough or damaged, the camshaft should be reground with an oil stone or replaced. Check the cams on the camshaft, if they are damaged, deformed or severely worn, the camshaft should be replaced. Check the radial runout of the camshaft, if it exceeds 0.5 mm, the camshaft should be straightened by cold pressing. Check the axial clearance of the camshaft, if it exceeds 0.15 mm, the camshaft should be adjusted or replaced.
    4. Grind and repair the cam surface and roller, and ensure even contact between the roller and cam surface when assembling; before assembling the push rod body and the lower body of the pump, carefully clean and check the roller surface for pockmarks and abrasions, its rotation and the up and down operation of the roller shaft. Is it flexible and is the clearance too large.

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