Nine Taboos for Heavy Truck Maintenance and Repair

Avoid adding engine oil without changing it

Engine oil is indispensable in the operation of diesel engines, mainly serving to lubricate, cool and clean. Many drivers pay attention to checking the oil level and adding oil according to the standard, but they neglect the inspection of oil quality and the replacement of deteriorated oil, which leads to the wear and tear of some engine parts. Under normal circumstances, the loss of engine oil is not significant, but it is prone to contamination, thus losing its protective function for diesel engines. During the operation of a diesel engine, many impurities will enter the engine oil. For new or overhauled machinery, there will be more impurities after the trial run. If it is put into use without changing the oil, it is very likely to cause unexpected accidents such as bearing seizure and shaft seizing. Moreover, even when changing the engine oil, some drivers, due to a lack of maintenance experience or for the sake of convenience, do not thoroughly clean the oil passage during the change, leaving mechanical impurities still remaining in the oil pan and oil lines.

2. Do not use or apply grease randomly

Some mechanics apply a layer of grease on the cylinder gasket when installing it, believing that this can enhance the sealing performance of the diesel engine. However, this practice actually affects the working performance of the diesel engine. To make it easier to tighten the nuts and prevent rust, many mechanics apply oil to the bolts and nuts of tires, which is also a wrong practice. Because after the tire nut is tightened, the threads have the feature of self-locking. This is because the helix Angle of the thread is less than the equivalent friction Angle between the threads. In a given bolt connection, the helix Angle is a constant value, while the equivalent friction Angle varies with the friction state between the threads. Obviously, after oiling, the equivalent friction Angle between the threads decreases, and the self-locking performance of the bolt connection deteriorates. Therefore, never apply grease or drip lubricating oil onto the bolts and nuts of the tires. Doing so will instead cause the nut to loosen and even lead to an accident.

3. Do not install new cylinder liners and pistons without proper matching

When replacing the cylinder liner and piston, it is believed that the new cylinder liner and piston are standard parts and interchangeable, and can be used immediately after installation. In fact, both the cylinder liner and the piston have a certain tolerance range for their dimensions. If the largest-sized cylinder liner is matched with the smaller-sized piston, the fit clearance will be too large, resulting in weak compression and difficult starting. Therefore, when changing the cylinder liner and piston, it is necessary to check the size group codes of the standard cylinder liner and piston. The size group codes of the standard piston and the standard cylinder liner used must be the same. Only in this way can the standard fit clearance between the two be guaranteed. In addition, when each cylinder is equipped with cylinder liners and pistons of the same group code, it is also necessary to pay attention to checking the cylinder plug clearance before installation. To ensure the assembly standard, a test should also be conducted before installation.

4. Avoid inaccurate measurement of cylinder clearance

When measuring the cylinder clearance, it was not measured in the direction perpendicular to the piston pin hole at the piston skirt, but in other directions. The structural feature of the aluminum alloy piston is that it is smaller at the top and larger at the bottom, forming a cone, and its local cross-section is elliptical. Therefore, the cylinder clearance along the circumferential direction is not equal. When measuring, it is stipulated that the gap in the direction of the major axis of the ellipse should be taken as the standard, that is, the gap between the piston skirt and the direction perpendicular to the piston pin hole should be measured. This measurement method is more convenient and accurate. Moreover, during reciprocating motion, the direction of the piston skirt perpendicular to the piston pin hole wears out more due to the lateral pressure. Therefore, when measuring the cylinder clearance, it should be done in the direction of the piston skirt perpendicular to the piston pin hole.

5. Do not heat the piston with an open flame

Since the piston and the piston pin are in an interference fit, when installing the piston pin, the piston should be heated and expanded first. At this time, some maintenance personnel will directly heat the piston over an open flame. This practice is very wrong because the thickness of different parts of the piston is uneven, and the degree of thermal expansion and contraction will be different. Open flame heating will cause uneven heating of the piston and easily lead to deformation. Carbon ash will also adhere to the surface of the piston, reducing its service life. If the piston cools down naturally after reaching a certain temperature, its metallographic structure will be damaged and its wear resistance will be greatly reduced, and its service life will also be significantly shortened. When installing the piston pin, place the piston in hot oil and heat it evenly to make it expand slowly. Do not heat it directly with an open flame.

6. Do not use sandpaper to polish the bearing shells

For some inexperienced mechanics, bearing shells are a rather challenging task. Due to the difficulty in mastering the technique of scraping the shells, it is hard for the bearing shells to meet the technical requirements. Therefore, when replacing the bearing shells, some people use sandpaper to grind them instead of scraping them in order to increase the contact area between the bearing shells and the crankshaft. This method is highly unadvisable in actual maintenance because the abrasive grains on the sandpaper are relatively hard, while the alloy of the bearing bush is relatively soft. As a result, during grinding, the abrasive grains can easily embed into the alloy, which will accelerate the wear of the journal when the diesel engine is working and shorten the service life of the crankshaft.

7. Do not tighten the bolts too much

During the disassembly and assembly process, many parts of the bolts have specified torque requirements, such as the transmission box, cylinder head, tires, connecting rods and front axle, etc. The tightening torque is specifically stipulated in the manual and must not be changed at will. However, many users mistakenly believe that tightening it tighter will be safer. But tightening it too much can cause the screws or bolts to break, and it may also lead to malfunctions due to thread slippage.

8. Avoid having the tire pressure too high

The inflation pressure of the tires of wheeled construction machinery is an important factor determining its service life and working efficiency. Both excessively high and low tire pressure can affect their service life and are not conducive to safe driving, especially in the hot summer. The scientific inflation standard should be: based on the standard tire pressure, make slight adjustments to the tire pressure as the temperature changes.

9. Do not add cold water suddenly when the water tank boils

Overloading the engine, poor heat dissipation or lack of water in the radiator can all cause the radiator to boil over. If cold water is added immediately at this time, it will lead to the cracking of the cylinder head and cylinder block. Therefore, once the radiator boils over during use, emergency measures should be taken to stop the operation and allow the diesel engine coolant to cool down by itself.