In the overhaul of the unit, the torque wrench is used because the torque is too small and the tightening force of the bolt is insufficient.
After analyzing the historical vibration data of the unit, the adjustments made to the shafting during the overhaul, the repair of several cemented carbide components, and the operating parameters during the start-up of the low-pressure rotor, static and dynamic rubbing between the rotor and stator was still observed. In cases where the balance was not fully achieved, it was suspected that there might be serious faults in the unit's shafting or supporting system. These issues could have reduced the stiffness of the system, brought the resonance speed closer to the operational speed, and increased the vibration response under the excitation force.
Insufficient tightening of the bolts connecting the short shaft to the low-input wheel was identified as a potential main cause of the abnormal vibration. During the shutdown inspection, it was found that 11 of the steam-side connection bolts on the short shaft had insufficient tightening forces, with a torque range of 15-35 degrees. The power generation side connections were within acceptable limits, and the short shaft's swing and end face deviation were also within normal ranges. After re-tightening the bolts to the correct torque, the abnormal vibration was eliminated, and both the bearing bush and shaft vibration levels returned to excellent conditions. This issue has been encountered before in other power plant units during overhauls due to similar problems with bolt tightening.
A significant difference was observed between the fourth wattage vibration and the shaft speed before and after the abnormal vibration event. The primary reason for this change is that the fourth wattage vibration and the shaft vibration increased sharply as the rotational speed exceeded 2700 r/min. In addition to changes in the dynamic stiffness of bearing No. 4, the increase in vibration may also be attributed to a new imbalance in the shaft system, which caused a larger excitation force, or to insufficient or uneven tightening of the bolts connecting the steam-side short shaft.
This case highlights the importance of proper bolt tightening during maintenance, as even small deviations can lead to significant vibration issues. It also emphasizes the need for thorough inspections and precise measurements when diagnosing and resolving mechanical problems in rotating equipment. Ensuring that all components are correctly aligned and tightened can prevent many of these types of failures and improve the overall reliability and performance of the unit.
The hydraulic cylinder is a hydraulic actuator that converts hydraulic energy into mechanical energy and does linear reciprocating motion (or oscillating motion). It has a simple structure and works reliably. When it is used to realize reciprocating motion, the deceleration device can be dispensed with, and there is no transmission clearance, and the movement is stable, so it is widely used in the hydraulic system of various machinery. The output force of the hydraulic cylinder is proportional to the effective area of the piston and the pressure difference between its two sides, and the hydraulic cylinder is basically composed of cylinder barrel and cylinder head, piston and piston rod, sealing device, buffer device and exhaust device.
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