What problems can occur if the pump shaft and motor shaft of a long-shaft pump are not concentric?

A long-shaft pump consists of a motor, coupling, pump shaft, guide bearing, and impeller. The concentricity and alignment accuracy of the shaft system are crucial to the pump's operational stability. If the pump shaft and motor shaft are not concentric (i.e., "misalignment" or "eccentricity"), it will directly affect the mechanical balance and service life of the equipment. This article will analyze the impact of this problem from three aspects: failure mechanism, symptoms, and preventive measures.

 

 

• Main hazards of misalignment between the shaft and the motor shaft:

 

Significantly increased vibration and noise

Misalignment of the shaft system causes periodic eccentric forces to be generated in the coupling during rotation, resulting in unbalanced vibration in the pump system. Slight deviation manifests as high-frequency noise and micro-vibration; larger deviations can cause severe shaking of the entire machine, and even damage to the anchor bolts and foundation.

Typical Phenomena:

* **Vibration exceeding limits during initial operation (>4.5 mm/s RMS);**

* **Periodic metallic friction noise in the pump body or motor;**

* **Unstable outlet water pressure and flow rate.**

 

Premature Bearing Wear and Overheating

Due to shaft misalignment, the guide bearing, upper thrust bearing, and motor bearing all bear additional radial loads, resulting in uneven contact stress. Long-term operation leads to bearing overheating, accelerated wear, and grease deterioration, ultimately causing serious malfunctions such as shaft jamming and seizure.

Consequences:

* **Significantly shortened guide bearing life;**

* **Increased motor temperature and aging of winding insulation;**

* **Increased bearing vibration and oil film damage.**

 

Abnormal Stress on Couplings and Pump Shafts

When the two shafts are not concentric, the elastic element (or rigid disc) of the coupling is subjected to alternating bending moment and torque impact with each rotation. Over time, this can lead to:

* Fatigue rupture of the coupling rubber blocks and diaphragm;

* Fatigue cracks or even breakage of the pump shaft;

* Loosening and shear damage of connecting bolts.

These problems often occur suddenly after prolonged operation and are typical examples of shaft fatigue failure.

 

Increased Energy Consumption and Decreased Efficiency

Misalignment increases the frictional resistance between the rotor and stator, leading to increased mechanical losses and shaft power, resulting in:

* Higher motor current and increased energy consumption;

* A 2-5% decrease in pump efficiency;

* Increased overall system operating costs.

 

• Common causes of shaft misalignment:

1. Uneven installation foundation or uneven anchor bolt preload;

2. Coupling assembly error, not aligned according to standards;

3. Long-term operation leading to bearing wear and axial displacement changes;

4. Thermal expansion and contraction causing deformation of the motor and pump body structure.

 

• Prevention and Adjustment Measures

1. Strict Alignment: Use a dial indicator or laser alignment instrument during installation to ensure radial deviation ≤ 0.05mm and angular deviation ≤ 0.2mm/100mm.

2. Operational Monitoring: Regularly monitor vibration levels, motor current, and bearing temperature changes.

3. Maintenance and Repair: After replacing worn guide bearings, the axis should be readjusted.

4. Foundation Reinforcement: Ensure sufficient rigidity of the pump base to prevent structural displacement after long-term operation.

 

Misalignment between the pump shaft and motor shaft of a long-shaft pump can cause a series of problems, including increased vibration, bearing overheating, coupling damage, and decreased efficiency. Precise installation alignment and regular inspection and maintenance can significantly improve the operational stability and service life of the pump unit.

During the commissioning phase of a long-shaft pump, maintaining shaft concentricity is not only a core indicator of installation quality but also crucial for ensuring the long-term efficient operation of the equipment.