After a screw air compressor has been in use for some time, oil may sometimes seep out near the pulley. This usually indicates a shaft seal oil leak. There are many factors that can cause oil leakage in screw air compressor shaft seals, as detailed below:  

1. Dirty Working Environment。Excessive dirt, sludge, or debris coming into contact with the lip of the shaft seal can cause rapid wear.  

2. Poor Oil Return。 Most compressor heads have an oil return passage and a check valve (made of a fluororubber ball) between the shaft seal cavity and the suction chamber. During normal operation, the suction chamber is under negative pressure, drawing excess oil back into the compressor through the oil return passage and check valve. When the suction chamber is under positive pressure, the check valve prevents oil and gas from affecting the seal. If the oil is dirty or clogged with carbon deposits, oil return is hindered, leading to leakage. Additionally, if the rubber ball is lost or forgotten during repairs, oil can press against the seal under internal pressure, causing leakage.  

3. Loose O-Ring Seal。 Some models have an O-ring between the oil seal sleeve and the shaft. If the pulley is not tightened properly during assembly, the sleeve may rotate, wearing out the O-ring and causing leaks.  

4.Paint on the Shaft Sleeve。 During manufacturing or repairs, if paint is sprayed onto the shaft seal sleeve, it can1 interfere with the lip seal, leading to poor sealing and oil leakage.  

5. Excessive Vibration。 Poor dynamic balance in the pulley or coupling at the input shaft end can cause vibration, leading to accelerated wear on the shaft seal and oil leakage.  

6. Axial Force or Misalignment(Common in Gear-Driven Heads)。 Many gear-driven compressor heads use couplings for power input. Improper installation or misalignment can generate axial forces, causing the seal sleeve and lip to experience excessive movement and rapid wear. Additionally, the compressor-side half-coupling must be installed using a thermal fitting method.  

7. Improper Maintenance。If the compressor head is disassembled, care must be taken not to damage the sealing lip. Reinstalling the oil seal requires special tools and proper procedures. If replacing the seal yourself, it is recommended to use original manufacturer parts and replace both the seal and sleeve, as it is difficult to pass pressure tests otherwise. **Bearing Wear** – Poor oil quality or contaminants can cause premature bearing wear, leading to radial displacement of the output shaft and subsequent seal lip wear.  

8.Oil Coking。Since screw compressors operate at high temperatures, using low-quality oil, failing to change the oil on schedule, using the wrong oil (non-specialized compressor oil), or mixing incompatible oils can lead to carbon buildup. This accelerates lip wear, compromising the seal and causing oil leakage.