Material and manufacturing defects: If the piston rod has internal cracks, uneven hardness or improper surface treatment (such as defects in the chromium plating layer), fatigue cracks are prone to occur under alternating stress. For instance, improper control of heat treatment processes can lead to insufficient toughness of the material, which may cause it to break when it bears the weight of the boom for a long time.
High-frequency stress fatigue: During the excavation and lifting processes, the boom cylinder is subjected to periodic loads. Especially when it operates at full load frequently, the root of the piston rod (at the connection with the cylinder barrel) is prone to fatigue cracks due to stress concentration. If the equipment has been in use for more than five years and has not been inspected regularly, the risk of fatigue fracture increases significantly.
Hydraulic system shock: Sudden changes in the pressure of the hydraulic pump, rapid action of the reversing valve or failure of the safety valve can cause the cylinder to be subjected to overpressure shock instantaneously. For instance, when the operator rapidly switches the boom action, the sudden change in the flow rate of the hydraulic oil may cause a pressure peak that exceeds the yield strength of the piston rod.
Installation and alignment issues: When the installation Angle deviation of the piston rod from the boom and cylinder barrel exceeds 0.5°, additional bending moments will be introduced. Long-term off-center loading operation can lead to excessive local stress and accelerate fracture. In addition, excessive wear of the pin shaft leading to excessive fit clearance can also cause abnormal vibration.
Overload and misoperation: When forcibly excavating on hard ground or lifting overweight materials (such as large rocks), the actual force on the boom cylinder may exceed 120% of the designed load. If there is also a leakage in the hydraulic system causing insufficient pressure, the piston rod will have to bear a greater mechanical load, and the risk of fracture will increase sharply.
Typical case: During the operation of a 36-ton excavator of a certain brand in a mine, due to the operator's long-term loading method of "full shovel + rapid lifting", the piston rod of the boom cylinder broke at the root. Upon inspection, it was found that there were fatigue cracks in this part, and the pressure setting value of the safety valve in the hydraulic system exceeded the standard value by 15%, which intensified the impact load.
Suggested solution：
Regularly conduct non-destructive testing (such as magnetic particle inspection) to check for cracks;
Ensure that the pressure of the hydraulic system is within the rated range (usually fluctuating by ±5%);
Avoid continuous operation under extreme working conditions and check the pin clearance every 200 hours.
Select alloy steel piston rods with better fatigue resistance (such as 42CrMo).
If the fracture occurs in the middle of the piston rod, it is necessary to focus on checking whether there is frequent shock in the hydraulic system. If it breaks at the root, the rationality of the boom structure design and the operating habits need to be evaluated.
