Ten "minefields" in Excavator Maintenance and Repair! To be honest, how many have you stepped on?

Butter is used and spread randomly

Grease the cylinder gasket. Butter is a commonly used lubricating grease in the repair of construction machinery, which can play a role in lubrication and sealing. Therefore, some mechanics will apply a layer of grease on the cylinder gasket when installing it, believing that this can enhance the sealing performance of the diesel engine. Little do they know that doing so actually affects the working performance of the diesel engine. The cylinder gasket is the most important sealing component between the cylinder block and the cylinder head of a diesel engine. It not only seals the high-temperature and high-pressure gas generated inside the cylinder but also the cooling water and lubricating oil in the cylinder head and cylinder block. Therefore, when disassembling and assembling the cylinder gasket, special attention should be paid to its sealing quality. If grease is applied to the cylinder gasket during installation, when the cylinder head bolts are tightened, some of the grease will be squeezed into the water and oil channels of the cylinder. The grease remaining between the cylinder gaskets, due to the influence of high temperature during cylinder operation, will partly flow into the cylinder and burn, while the other part will remain on the mating surface between the cylinder block and the cylinder head, creating a gap between the cylinder gasket, the cylinder head and the plane of the engine block High-temperature and high-pressure gas can easily impact the cylinder gasket from here, damaging it and causing gas leakage. In addition, if butter is exposed to high temperatures for a long time, it will form carbon deposits, causing the cylinder gasket to age and deteriorate prematurely. Therefore, do not apply grease when installing the cylinder gasket.

The new cylinder liners and pistons can be installed without selection

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 check the cylinder plug clearance before installation. To ensure the assembly standards, a test should be conducted before installation to prevent the installation of counterfeit, substandard and defective products.

Do not check the plunger stroke allowance

In the mock test of plunger-type fuel injection pumps, many maintenance personnel do not pay attention to checking the stroke allowance of the 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 fuel supply start time, the reason why it is necessary to check the stroke allowance is that the stroke allowance of the plunger is related to the wear of the plunger and the sleeve. When the plunger and sleeve wear out, the plunger has to move upwards a bit more before it can start supplying fuel, thus delaying the start time of fuel supply. When the adjusting bolt is unscrewed or a thicker adjusting spacer or gasket is used, the lowest position of the plunger moves upward, reducing the stroke allowance of the plunger. Therefore, when maintaining and debugging the fuel injection pump, the first step should be to check this stroke allowance to determine whether the fuel injection pump still allows adjustment. When inspecting, the following different methods should be adopted according to the different structures of the fuel injection pump: a) Rotate the camshaft, push the plunger to the top dead center, then remove the fuel outlet valve and valve seat, and measure with a depth gauge. b) After the plunger is pushed to the top dead center, use a screwdriver to lift the spring seat of the plunger spring to raise the plunger to the highest point. Then, insert a thickness gauge between the lower plane of the plunger and the tappet adjustment bolt to measure. The standard stroke allowance of the plunger is approximately 1.5mm, and the ultimate stroke allowance after wear should be no less than 0.5mm.

The measurement of cylinder clearance is inaccurate

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. Moreover, the cross-section of the skirt is elliptical, so 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.

The piston is heated by 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.

Polish the bearing bush with sandpaper

For some inexperienced mechanics, scraping the bearing shells is a rather challenging task. Due to the difficulty in mastering the scraping technique, 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 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.

The bolt is tightened too much

During the disassembly and assembly of construction machinery, many parts of the bolts have specified torque requirements, such as the transmission box, cylinder head, wheel hub, connecting rod 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.

The tire pressure is 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. For instance, the pressure in summer should be 5% to 7% lower than that in winter. This is because in summer, the temperature is high and the gas is heated, causing the pressure to increase. Conversely, in winter, it must reach the standard pressure or be slightly lower.

When the water tank boils, add cold water suddenly

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 water tank boils over during use, emergency measures should be taken to stop the operation and allow the diesel engine's cooling water to cool down by itself.

The engine oil is only added and not changed

Engine oil is indispensable in the operation of diesel engines, mainly serving to lubricate, cool and clean.

Therefore, many drivers pay attention to checking the oil level of the lubricating oil and add it according to the standard, but they neglect the inspection of the quality of the lubricating oil and the replacement of deteriorated engine oil. As a result, some moving parts of the engine always operate in a poor lubrication environment, thereby accelerating the wear of each part. 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 contaminants (such as soot, carbon deposits and dirt produced by incomplete fuel combustion) will enter the engine oil. For new or overhauled machinery, there will be more impurities after trial operation. If it is put into use immediately without replacement, 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.