Vacuum pump shaft bending triggers thoughts

A case overview

The vacuum pump equipment in a factory that had been operating for two years recently failed and the drive shaft head was bent and deformed.

2. Causes of shaft bending and gaining experience

After inspection by our maintenance engineer, we found:

1. The vacuum pump has a large load and is overloaded;

2. There are hidden dangers in the starting method of the vacuum pump, and the starting torque is very impactful;

Through this failure analysis, our company summarizes the reasons that may cause the vacuum pump shaft to bend:

1. Problem of centering the connection flange between the motor and the vacuum pump

---1.1 Although centering is guaranteed by flange, there may be initial errors during installation (such as flange stop processing error, coaxial error between the outer circle of the stop and the centerline, and verticality error between the end face of the stop and the centerline). The vibration generated during operation may further aggravate the misalignment, causing the journal to bear periodic bending stress.

---1.2 Temperature changes during operation of vacuum pump may cause the pump body to expand differently from the motor end, destroying the centering accuracy, especially in high temperature or cooling conditions.

Experience gained: When installing the vacuum pump and motor, check for any signs of improper installation or damage that may affect alignment

2. Overload and abnormal load

---2.1 Instant Overload

The vacuum pump may cause the instantaneous torque to exceed the design value due to the system blockage of foreign matter and the pressure of entering the gas suddenly increases or frequent start and stop.

---2.2 Continuous overload

Long-term operation under overload conditions (such as suctioning high-density media or limit vacuum) leads to shaft fatigue deformation

---2.3 Impact load

The coupling buffering capacity is insufficient (such as rigid coupling is selected), the motor start torque or load suddenly changes to the shaft, causing impact deformation

Experience obtained: Pay attention to whether the vacuum pump has load during operation: look at the current curve, look at the reading of the negative pressure gauge, look at the operating parameters of the upstream and downstream machine pumps, and observe whether foreign objects accumulate in the vacuum pump cylinder on the rotor surface

3. Coupling problem

---3.1 Improper selection: The coupling type (such as rigid elastic tooth type) does not match the working condition and cannot compensate for centering deviation or absorbing vibration

---3.2 Damage or wear: Coupling components (such as elastic component toothed parts) age and break or wear, resulting in uneven transmission torque

---3.3 Installation error: The pre-tightening force of the coupling lock bolt is uneven or the bonding is too tight, causing additional stress

---3.4 Large dynamic balance error of the coupling: The drive shaft head generates a large centrifugal force, causing the shaft to bend and deform.

Experience obtained: Review the assembly quality of coupling supporting components

4. Shaft and keyway design/manufacturing defects

---4.1 Material defects: Unqualified shaft material (such as insufficient strength internal cracks) or improper heat treatment (such as residual stress not being eliminated)

---4.2 Structural design problems: The keyway size is unreasonable (such as too shallow or insufficient transitional corners), resulting in local stress concentration

---4.3 Machining error: Inadequate machining accuracy of keyways (such as poor symmetry and roughness), resulting in poor coordination between keyways and keyways and uneven stress.

Experience obtained: Select reasonable materials, optimize keyway design, and improve keyway processing accuracy

5. The rotor dynamic balance is poor:

The imbalance of the rotor is one of the causes of shaft bending. When the rotor rotates at high speed, if its mass distribution is uneven, a large centrifugal force will be generated. If this force exceeds the elastic limit of the shaft, it will cause permanent bending deformation of the shaft.

Experience obtained: Perform dynamic balance correction of the rotor of the vacuum pump before assembly

6. Abnormal vibration of vacuum pump:

The vibration of the main driven rotor is also an important factor in the bending of the shaft. If the rotor vibrates abnormally during operation, the bending of the shaft will be caused by long-term action.

Experience gained: Pay attention to the vibration during operation of vacuum pumps

7. Bearing failure: The bearing of the vacuum pump loses its positioning function, causing the vacuum pump drive bearing to be overloaded, which ultimately leads to the pump shaft bending.

Experience gained: Check and maintain regularly, replace bearings in time to prevent failure

Three solutions to axle bending

1. Cold calibration: By calibrating the shaft under low temperature conditions, the curvature of the material is used to adjust the curved part of the shaft to achieve the purpose of straightening.

2. Thermal calibration: Heat the curved part of the shaft to expand by heating it, and then correct the bending of the shaft by external force or its own cooling and contraction.

3. Mixed calibration: Combining cold calibration and thermal calibration methods, select the appropriate calibration method according to the specific situation to achieve the best calibration effect

4. Replace, process and repair if necessary: ??first consider changing directly. If an emergency occurs, you can correct the coaxiality of the journals at both ends of the rotor, repair the central hole, turn deformation position + laser cladding + restore the journal size, check the coaxiality of the journals at both ends of the rotor, and check the dynamic balance.