10 Precautions for Heavy Truck Maintenance（Part 1）

2. Misuse of butter

(1) Grease the cylinder gasket. Butter is a commonly used grease in the repair of construction machinery, which can play the role of lubrication and sealing. Therefore, when installing the cylinder gasket, some repairmen will apply a layer of butter on the cylinder gasket, believing that this can increase the tightness of the diesel engine. However, this will affect the working performance of the diesel engine. The cylinder gasket is the most important seal between the cylinder block and the cylinder head of the diesel engine. It can not only seal the high-temperature and high-pressure gas generated in the cylinder, but also seal the cooling water and lubricating oil in the cylinder head and cylinder body. Therefore, when disassembling the cylinder gasket, special attention should be paid to its sealing quality. If the cylinder gasket is coated with butter during installation, when the cylinder head bolts are tightened, part of the butter will be squeezed into the cylinder water channel and oil channel. When the cylinder is working, due to the high temperature, part of the butter will flow into the cylinder for combustion, and the other part will remain on the joint surface of the cylinder block and the cylinder head, causing a gap between the cylinder gasket, the cylinder head and the plane of the cylinder body. High temperature and high pressure gas can easily impact the cylinder gasket from here, Destroy the cylinder gasket and cause air leakage. In addition, when the butter is at high temperature for a long time, it will also produce carbon deposits, which will cause premature aging and deterioration of the cylinder gasket. Therefore, do not apply grease when installing the cylinder gasket.

(2) Grease the tire nuts. In order to tighten the nuts easily and prevent corrosion, many repairmen applied oil on the bolts and nuts of the tires. In fact, this is a mistake. Because after the tire nut is tightened, there is a self-locking feature between the threads. This is because the helix angle of thread is less than the equivalent friction angle between threads. In a given bolt connection, the helix angle is a certain value, and the equivalent friction angle changes with the friction state between threads. Obviously, the equivalent friction angle between threads decreases after oil coating, and the self-locking performance of bolt connection becomes worse. Therefore, never apply grease or oil on the bolts and nuts of the tire. Doing so will loosen the nuts and even cause accidents.

3 New cylinder liners and pistons are installed without option

When replacing cylinder liners and pistons, it is considered that the new cylinder liners and pistons are standard parts and interchangeable, and can be used after installation. In fact, there is a certain tolerance range for the dimensions of cylinder liner and piston. If the cylinder sleeve with the largest size is matched with the piston with the smallest size, the matching clearance will be too large, resulting in weak compression and difficult starting. Therefore, the size grouping code of the standard cylinder liner and piston must be checked when replacing. The size grouping code of the standard piston and the size grouping code of the standard cylinder liner must be the same as the size grouping code of the standard cylinder liner. Only in this way can the standard fit clearance between the two be guaranteed. In addition, when cylinder liners and pistons with the same group code are installed in each cylinder, attention should also be paid to the inspection of cylinder plug clearance before installation. In order to ensure the assembly standard, a test should also be carried out before installation to prevent the installation of counterfeit and defective products.

4 Do not check the plunger stroke allowance

During the ironic test of plunger injection pump, many maintenance personnel did not pay attention to checking the stroke allowance of plunger. The so-called stroke allowance of the plunger refers to the amount of movement that the plunger can continue to move upward after being pushed to the top dead center by the cam on the camshaft. After adjusting the start time of oil supply, the reason why we should check the stroke allowance is that the stroke allowance of the plunger is related to the wear of the plunger and sleeve. After the plunger and sleeve are worn, the plunger needs to move upward for a longer period to start oil supply, thus delaying the start time of oil supply. After screwing out the adjusting bolts or replacing with thicker adjusting blocks and gaskets, the lowest position of the plunger will move up to reduce the stroke margin of the plunger. Therefore, when repairing and debugging the fuel injection pump, first check the stroke margin to determine whether the fuel injection pump can be adjusted. During the inspection, the following different methods should be adopted according to the different structures of the injection pump: a) Turn the camshaft, push the plunger to the top dead center, remove the oil outlet valve and valve seat, and measure with a depth vernier. b) After the plunger is pushed to the top dead center, pry up the spring seat of the plunger spring with a screwdriver to raise the plunger to the highest point. Then insert a feeler gauge between the lower plane of the plunger and the adjusting bolt of the tappet. The standard stroke allowance of plunger is about 1.5mm, and the limit stroke allowance after wear should not be less than 0.5mm.

5 Inaccurate measurement of cylinder clearance

When measuring the cylinder clearance, the piston skirt was not measured in the direction perpendicular to the piston pin hole, but in other directions. The structural characteristics of aluminum alloy piston are small at the top and large at the bottom, which is a cone, and the local section is elliptical, so the cylinder clearance along the circumference is not equal. When measuring, it is specified that the clearance in the direction of the ellipse major axis shall prevail, that is, the clearance of the piston skirt perpendicular to the direction of the piston pin hole shall be measured. This measurement is more convenient and accurate. In reciprocating motion, the direction of the piston skirt perpendicular to the piston pin hole is greatly worn due to the effect of lateral pressure. Therefore, when measuring the cylinder clearance, the measurement should be made in the direction of the piston skirt perpendicular to the piston pin hole.