Symptom, failure mode and root cause are different things
When an airend becomes noisy or hot, three separate questions need answers:
- Symptom – what you observe: noise, vibration, high temperature, oil carry-over, lost capacity, a trip.
- Failure mode – what is physically happening: bearing spalling, rotor contact, seal leakage, clearance growth.
- Root cause – why it happened: contaminated oil, overheating, misalignment, reverse rotation, wrong lubricant, excessive pressure.
Replacing the failed bearing without finding why it failed often produces the same failure again. And many symptoms that look like airend trouble actually come from the package around it: a failing motor bearing, a loose coupling, a sticking inlet valve or a blocked cooler.
How a screw airend works, and where it is vulnerable
An oil-injected screw airend has a male and female rotor turning in a close-fitting housing. Oil is injected to seal clearances, cool the compression and lubricate the bearings. Radial and thrust bearings hold the rotors in position, with very small clearances between rotor lobes and between rotors and housing. A shaft seal keeps oil in at the drive end.
Everything depends on those clearances staying where the designer put them. Bearings that wear let rotors move. Oil that breaks down stops sealing and cooling. Heat expands parts. Once clearance is lost, rotors touch, and contact damage escalates quickly.
The main airend failure modes
| Failure mode | Typical evidence | Common root causes |
|---|---|---|
| Bearing fatigue or spalling | Rising vibration at bearing frequencies, growing noise, wear metals in oil analysis | Contaminated or degraded oil, long running beyond service, overload, normal end of life |
| Thrust bearing failure | Axial movement, discharge-end rotor contact marks, sudden noise | High discharge pressure, frequent start-stop, lubrication loss, reverse rotation |
| Rotor-to-rotor or rotor-to-housing contact | Scoring, smearing, bright marks, metal debris, high current, seizure | Bearing wear, overheating, foreign object ingestion, oil starvation |
| Lubrication breakdown | Dark or varnished oil, sludge, high discharge temperature, sticky valves | Overheating, wrong or mixed oil, extended oil change intervals, moisture |
| Water contamination and corrosion | Milky oil, rust on rotors and bearings, pitting | Running too cool, short running cycles, humid intake, long idle periods |
| Shaft seal leakage | Oil at the drive end or coupling housing | Seal wear, shaft wear, misalignment, high sump pressure |
| Abrasive wear | Gradual loss of capacity, worn rotor tips, fine debris | Damaged intake filter, dusty site, dirt entering during service |
| Foreign-object damage | Sudden noise, dents and gouges on rotor lobes | Broken inlet valve parts, debris left after service or transport |
Reading the symptoms
Each symptom points to a short list of mechanisms, but none is conclusive on its own.
- Noise. A rumble or growl that rises over weeks suggests bearing wear. A sudden knock or metallic scrape suggests contact or a foreign object, and calls for an immediate stop.
- Vibration. Trending vibration is the best early warning of bearing distress. A single reading means little; a rising trend means a lot. Check the motor and coupling too.
- High temperature. Normal discharge on an oil-injected screw is typically about 75–95 °C. Higher readings usually point to cooling, oil level, thermostatic valve or room ventilation, but rising friction from internal damage also adds heat.
- Loss of capacity. Gradual loss with a healthy inlet valve can mean clearance growth from wear. Rule out the inlet valve, intake filter, leaks and control settings first.
- Oil carry-over. More often a separator, scavenge line or minimum-pressure valve problem than an airend fault, though overheated oil foams and carries over more readily.
- High current. Rising motor current at the same pressure can indicate internal friction, but also high pressure, low voltage or a motor problem.
Oil analysis as an early warning
Oil analysis is one of the cheapest ways to see inside an airend without opening it. Trend it rather than reading single samples:
- Wear metals such as iron and bearing-cage materials rising over time point to bearing or rotor wear.
- Silicon suggests dust entering past the intake filter.
- Water shows condensation inside the machine, often from running too cool or short cycles.
- Oxidation and acid number show oil degradation from heat or extended intervals.
- Viscosity change may indicate mixing of incompatible oils or degradation.
Take samples from the same point, at operating temperature, and use the same laboratory so results are comparable.
When to stop the machine immediately
Some conditions turn a repairable airend into a scrap one if running continues. Stop, isolate and investigate if you see any of these:
- A sudden metallic knock, scrape or squeal from the airend.
- A sharp rise in vibration or current with no other explanation.
- Visible metal particles in oil, filters or the drain.
- Repeated high-temperature trips that recur immediately after reset.
- Smoke or a burning smell from the airend or oil system.
Restarting repeatedly to “see if it clears” is the most common way minor bearing damage becomes rotor contact.
Preserving evidence for root-cause analysis
If the airend must be opened, keep what it tells you:
- Photograph and export alarm history, running hours and the last readings before shutdown.
- Take an oil sample before draining, and keep the old oil filter and separator.
- Photograph the airend before and during disassembly, marking orientation.
- Keep failed bearings, seals and debris, bagged and labelled.
- Record rotor and housing marks, clearances and bearing condition.
- Check the surroundings: inlet valve, intake filter, cooler, thermostatic valve, coupling and motor bearings.
This evidence decides whether the fix is a bearing set, a full rebuild or a replacement airend, and what must change in operation or maintenance to stop it happening again. Our airend inspection and measurement page explains the next step.
Problems that look like airend failure but are not
Before condemning an airend, rule out the package around it. These faults are commonly mistaken for internal damage:
| What you see | Often actually caused by |
|---|---|
| Noise from the drive end | Motor bearing wear, worn coupling element, loose belt or pulley |
| Loss of delivered air | Inlet valve not opening fully, choked intake filter, leaks, wrong pressure band |
| High discharge temperature | Fouled oil cooler, failed thermostatic valve, low oil level, hot room or recirculating exhaust |
| Oil in the air line | Separator element, blocked scavenge line, minimum-pressure valve fault |
| High motor current | Low or unbalanced voltage, system pressure above set point |
A careful check of these items takes far less time than opening an airend, and avoids an unnecessary overhaul.
Preventing airend failures
- Change oil, oil filters, separators and intake filters at the maker’s intervals, or sooner on dusty or hot sites. On our hire fleet, machines are serviced every 300 running hours.
- Use the correct lubricant and never mix types without flushing.
- Keep compressor rooms ventilated so discharge temperatures stay in their normal range through the summer.
- Avoid running at pressures above what the plant needs; higher pressure means higher bearing loads and about 7% more power per extra bar.
- Trend vibration and oil analysis so wear is seen before it becomes damage.
- Check rotation after any electrical work.
- Keep the intake clean during service and after transport.
How Reddy Compressors helps
Reddy Compressors, at Tarnaka in Secunderabad and serving industry since 2002, is an independent multi-brand specialist, not an authorised dealer of any brand. We have tie-ups with all major and reputed brands. We assess and support airends on screw compressors from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar, Gardner Denver and many others.
- On-site assessment of noise, vibration, temperature and performance, with checks of the package around the airend.
- Oil sampling coordination and interpretation of trends.
- Airend inspection and overhaul support, with bearings, seals and parts identified from the nameplate and fit confirmed before quoting. Specialist processes such as rotor regrinding or coating are arranged where needed.
- Advice on rebuild, exchange or replacement based on the evidence.
- Rental compressors from our own fleet while the airend is out of service.
- Service and AMC to prevent repeat failures.
Our service teams cover all of Telangana and Andhra Pradesh, and we supply parts and rental machines across India.
What to send us
- Compressor make, model, serial and any airend nameplate, with photos.
- Running hours and the last service date and work done.
- A description or short video of the noise, and the alarm history.
- Recent discharge temperature, pressure and motor current readings.
- Oil analysis reports if you have them.
- Photos of any debris found in oil, filters or drains.
Frequently asked questions
What causes screw compressor airend failure?
The most common causes are lubrication problems, contamination and heat. Degraded, wrong or water-contaminated oil, dust past a damaged intake filter, high discharge temperatures and running above design pressure all shorten bearing life. Worn bearings then let rotors move until they touch the housing or each other, which causes rapid secondary damage.
How do I know if my compressor airend bearings are failing?
Watch for a rising trend in vibration, a growing rumble or growl, increasing wear metals in oil analysis and gradually rising current or temperature at the same pressure. A sudden knock or scrape is more serious. Check the motor bearings and coupling as well, because they produce similar symptoms.
Why is my screw compressor airend making a knocking noise?
A knock can come from severe bearing damage, rotor contact, a foreign object or a loose coupling or mounting. Because rotor contact escalates quickly, stop the machine, isolate it and investigate before running it again. Repeated restarts to see if the noise clears can turn a repair into an airend replacement.
Can high discharge temperature damage the airend?
Yes. Normal oil-injected screw discharge is typically about 75–95 °C. Running hotter breaks down the oil, reducing its ability to lubricate bearings and seal clearances, and causes varnish and sludge. Persistent high temperature shortens bearing life and can lead to rotor contact. Fix the cooling, oil or ventilation cause promptly.
Is oil analysis useful for screw compressors?
Very. Trending wear metals, silicon, water, oxidation and viscosity shows bearing wear, dust ingress, condensation and oil degradation before they cause failure. Samples must come from the same point at operating temperature and go to the same laboratory, so trends are meaningful.
Should a failed airend be rebuilt or replaced?
It depends on the damage found. Bearing wear caught early is usually rebuildable. Heavy rotor or housing damage may need specialist rotor work or a replacement airend. Compare the options against the condition of the whole package, parts availability and remaining life of the machine.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.