Identifying Abnormal Noises in Screw Air Compressors

CHANUN Air Compressor

Abnormal noises are the machine’s “distress signals.”
Understanding them early can prevent costly overhauls.

In industrial workshops, any unusual sound coming from an air compressor can immediately raise concern among maintenance staff. These noises are often early warning signs of potential failures. Accurately identifying the source of abnormal noise in a screw air compressor can stop minor issues from developing into major breakdowns.

01

Three Types of Abnormal Noises and What They Mean

Abnormal noises in screw air compressors generally fall into three typical categories: friction squealing, metallic impact noise, and rhythmic knocking. Each sound corresponds to a different type of fault.

• Friction squealing is usually caused by belt slippage or poor lubrication. When the belt is too loose or the belt profile does not match the pulley, abnormal friction occurs between the belt and pulley. This is often accompanied by a burnt smell and belt debris on the floor.
  A sharper, harsher squeal caused by metal-on-metal friction usually indicates insufficient lubrication, leading to direct contact between the male and female rotors.

• Metallic impact noise suggests more serious mechanical problems. Possible causes include foreign objects entering the airend, bearing wear, or excessive gear clearance leading to collision sounds. This type of noise is a strong signal that the machine should be shut down immediately for inspection.

• Rhythmic knocking noise is often related to the inlet valve. If the inlet valve does not close tightly or the check valve vibrates, regular knocking sounds may occur, often accompanied by unstable air pressure.

02

Analysis of Common Noise Sources

The belt drive system is one of the most common sources of abnormal noise. A simple way to check belt tension is to press the middle of the belt with your finger—the deflection should be less than 5 mm. If not, adjustment is required.
When replacing belts, never replace only one belt. Uneven tension between old and new belts will cause rapid wear of the new belt.

Lubrication system issues require special attention. Screw compressors do not use timing gears; instead, they rely on an oil film between the rotors and the housing. If oil starvation occurs, the rotors may contact directly, producing a sharp metal friction sound.
It is important to note that “oil starvation” does not always mean insufficient oil volume. It may also be caused by oil line blockage, oil separator failure, or improper installation of the oil return line, leading to localized lubrication failure.

Internal airend faults are the most complex noise sources. These include excessive clearance between the male and female rotors and bearing end faces, damaged airend bearings, or foreign objects entering the airend.
According to industry data, screw compressor clearance ratios should generally be controlled within 0.008–0.012 mm. During maintenance, adjusting the ratio between the thrust-side clearance and non-thrust-side clearance to 1.0–1.2 is considered optimal.

Inlet valve failures are also common. When excessive air leakage occurs due to poor valve sealing, rhythmic noise and vibration may appear. During inspection, ensure proper contact between valve plate and seat, correct radial clearance, and correct lift. All internal components should be replaced as a complete set.

03

Practical Methods for Diagnosing Abnormal Noises

Listening to locate the source is the most direct method. Use a long-handled screwdriver: place the tip on the suspected component and press your ear against the handle. This helps pinpoint the noise source more accurately.
For example, motor bearing failures often produce dull rolling sounds or noticeable vibration due to contaminated grease or bearing wear.

Observing accompanying symptoms helps confirm the diagnosis. Belt slippage is often accompanied by a burnt smell; oil starvation causes abnormally high operating temperatures; inlet valve faults lead to unstable pressure readings on the gauge. These symptoms provide valuable supporting evidence.

Step-by-step inspection is effective for complex noise issues. Start with simple checks—such as loose bolts or incorrect belt tension—then gradually move to more complex systems like lubrication circuits and internal airend components.

04

Noise Prevention and Daily Maintenance

Regular inspection is the most effective preventive measure. This includes daily checks of oil level and belt condition, monthly tightening of mounting bolts, and quarterly inspections of inlet valves and bearings. Keeping detailed maintenance records helps identify potential issues early.

Proper lubrication prevents most noise-related failures. In addition to monitoring oil level, oil quality must be checked regularly. For compressors that have been shut down for long periods, ensure sufficient oil is added before restarting—at least enough to immerse three-quarters of the rotors—to prevent instant oil starvation.

Professional maintenance should not be overlooked. Complex operations such as internal clearance adjustment and bearing replacement should be handled by qualified technicians. Improper repairs can cause more severe damage.

Diagnosing abnormal noises in screw air compressors requires experience, but once the basic principles are understood, any equipment manager can become skilled at “listening for faults.” Every abnormal sound is a message from the machine. Interpreting these messages correctly and promptly ensures long-term, stable operation and reliable air supply for production.