【Repair and Maintenance】Ten taboos in construction machinery repair
1. Adding engine oil but not changing it
Engine oil is essential for diesel engines, providing lubrication, cooling, and cleaning. Therefore, many drivers carefully check the lubricant level and add it according to regulations. However, they often neglect to inspect the lubricant’s quality and replace any deteriorated oil. This causes some engine parts to operate in a poorly lubricated environment, accelerating wear and tear.
Under normal circumstances, engine oil loss is not significant, but it is easily contaminated, thus losing its protective function for the diesel engine. During diesel engine operation, many impurities (such as soot, carbon deposits, and scale generated by incomplete fuel combustion) enter the engine oil. For new or overhauled machinery, impurities will increase after the test run. If the oil is added without changing before being put into use, it can easily cause accidents such as burning bearings and seizing shafts. Furthermore, even when the engine oil is changed, some drivers, due to lack of maintenance experience or to save time, will not thoroughly clean the oil passages during the change, allowing mechanical impurities to remain in the oil pan and oil passages.
2. Misuse of butter
Apply butter to the cylinder gasket. Butter is a commonly used grease in construction machinery repairs, providing both lubrication and sealing. Therefore, some repairmen apply a layer of butter to the cylinder gasket during installation, believing this will improve the diesel engine’s seal. However, this practice actually affects the engine’s performance. The cylinder gasket is the most important seal between the diesel engine’s cylinder block and cylinder head. It not only seals out the high-temperature, high-pressure gases generated within the cylinder, but also keeps the cooling water and lubricating oil inside the cylinder head and cylinder block. Therefore, special attention should be paid to the sealing quality of the cylinder gasket during installation and removal.
If you apply grease to the cylinder gasket during installation, some of the grease will be squeezed into the cylinder water and oil passages when the cylinder head bolts are tightened. The grease remaining between the cylinder gaskets will, during operation, be affected by the high temperatures, causing some of it to flow into the cylinders and burn. Other grease will remain on the interface between the cylinder block and cylinder head, creating a gap between the gasket, cylinder head, and engine block. High-temperature, high-pressure gas can easily impact the gasket through this gap, damaging it and causing leaks. Furthermore, prolonged exposure to high temperatures can cause carbon deposits to form, leading to premature aging and deterioration of the gasket. Therefore, never apply grease to the gasket during installation.
3. New cylinder liners and pistons are installed without selection
When replacing cylinder liners and pistons, some people assume that the new cylinder liners and pistons are standard and interchangeable, allowing them to be installed and used immediately. In reality, the dimensions of cylinder liners and pistons have certain tolerances. For example, pairing the largest cylinder liner with the smallest piston will result in excessive clearance, resulting in weak compression and starting difficulties. Therefore, when replacing cylinder liners and pistons, it is crucial to check the size groupings of the standard cylinder liners and pistons. The cylinder liners and pistons used must match the size groupings of the standard pistons to ensure a standard clearance between the two. Furthermore, when using cylinder liners and pistons with the same groupings for each cylinder, it is important to verify the cylinder-plug clearance before installation. To ensure proper assembly, a pre-installation inspection should be conducted to prevent the installation of counterfeit or substandard products.
4. Inaccurate cylinder clearance measurement
When measuring cylinder clearance, the piston skirt was not measured perpendicular to the piston pin hole, but rather in another direction. Aluminum alloy pistons are characterized by being tapered, smaller at the top and larger at the bottom, and the skirt has an elliptical cross-section. Therefore, cylinder clearances are not uniform along the circumference. When measuring, the clearance along the major axis of the ellipse is used as the reference, meaning the clearance along the piston skirt perpendicular to the piston pin hole. This method is more convenient and accurate. However, during reciprocating motion, the piston skirt wears more perpendicular to the piston pin hole due to lateral pressure. Therefore, when measuring cylinder clearance, the piston skirt should be measured perpendicular to the piston pin hole.
5. Piston open flame heating
Because the piston and piston pin have an interference fit, the piston should be heated and expanded before installing the piston pin. Some repairmen will heat the piston directly over an open flame to allow it to expand. This practice is highly inappropriate because the piston’s thickness varies from part to part, causing different expansion and contraction rates. Heating with an open flame will result in uneven heating of the piston, which can easily cause deformation. It will also cause carbon ash to accumulate on the piston surface, shortening its service life. If the piston cools naturally after reaching a certain temperature, its metallographic structure will be damaged, significantly reducing its wear resistance and significantly shortening its service life. When installing the piston pin, heat the piston evenly in hot oil to allow it to expand slowly. Never heat it directly over an open flame.
6. Polish the bearing with emery cloth
For some inexperienced repairmen, scraping bearings is a difficult task. Because scraping bearings is difficult to master, bearings often fail to meet technical requirements. For this reason, some people use emery cloth to increase the contact area between the bearing and the crankshaft when replacing bearings. This method is highly undesirable in actual repairs because the abrasive grains on the emery cloth are hard, while the bearing alloy is soft. This method can easily embed sand grains into the alloy during grinding, accelerating wear of the journal during operation and shortening the crankshaft’s service life.
7. Bolts are too tight
During assembly and disassembly of construction machinery, bolts in many areas, such as the transmission case, cylinder head, wheel hub, connecting rod, and front axle, have specific torque requirements. These tightening torques are specifically specified in the manual and must not be changed arbitrarily. However, many users mistakenly believe that tightening the screws and bolts is safer. However, overtightening can break screws or bolts and can also cause problems due to thread slippage.
8. Tire pressure is too high
The tire inflation pressure of wheeled construction machinery is an important factor in determining its service life and working efficiency. Tire pressure that is too high or too low will shorten its service life and is also not conducive to safe driving, especially in hot summer.
The scientific inflation standard should be: based on the standard tire pressure, the tire pressure should be slightly adjusted as the temperature changes. For example, in summer, the tire pressure should be 5%-7% lower than in winter. This is because the high temperature in summer causes the air to heat up and the pressure to increase. In contrast, in winter, the tire pressure must be at the standard pressure or slightly lower.
9. Not checking the plunger travel allowance
During the trial of a plunger injection pump, many maintenance personnel do not pay attention to checking the plunger’s travel allowance. The so-called plunger travel allowance refers to the amount of upward movement the plunger can continue after being pushed to the top dead center by the cam on the camshaft.
After adjusting the fuel delivery start time, it’s important to check the travel allowance because the plunger’s travel allowance is related to wear on the plunger and sleeve. Wear on the plunger and sleeve requires the plunger to move upward a certain distance before fuel delivery begins, delaying the start of fuel delivery. Removing the adjusting bolt or replacing a thicker adjusting block or shim shifts the plunger’s lowest position upward, reducing the plunger’s travel allowance. Therefore, when repairing and debugging a fuel injection pump, the travel allowance should be checked first to determine if the pump can still be adjusted.
When checking, the following different methods should be used according to the different structures of the fuel injection pump:
a) Turn the camshaft and push the plunger to the top dead center, remove the oil outlet valve and valve seat, and measure the depth with a vernier gauge.
b) After the plunger has reached its top dead center, use a screwdriver to pry up the plunger spring seat to raise the plunger to its highest point. Then, insert a feeler gauge between the plunger’s lower surface and the tappet adjustment bolt for measurement. The standard travel allowance for the plunger is approximately 1.5 mm, and the maximum travel allowance after wear should be no less than 0.5 mm.
10. Add cold water to the water tank when it boils
Overloading the engine, poor heat dissipation, or lack of water in the water tank can cause the water tank to boil. If cold water is added immediately, the cylinder head and cylinder block will burst. Therefore, if the water tank is found to be boiling during use, emergency measures should be taken to stop operation and allow the diesel engine cooling water to cool down naturally.