What airend bearings and seals actually do
In a rotary screw airend, the male and female rotors run with very small clearances to each other and to the housing. They never touch in normal operation. The bearings are what make this possible: they hold each rotor in its exact radial and axial position while the rotors carry heavy gas forces at full speed.
Two kinds of load act on each rotor. Radial load pushes the rotor sideways, towards the housing bore. Axial (thrust) load pushes the rotor along its axis, towards the discharge end face, because the compressed air at discharge pushes back on the rotor lobes. Most airends therefore use different bearings at each end and, often, more than one bearing at the discharge end.
- Inlet (suction) end: usually cylindrical roller bearings that carry radial load and allow small axial movement as the rotor grows with heat.
- Discharge end: a combination that carries both radial and axial load, commonly cylindrical rollers for radial load plus angular contact ball bearings, four-point contact bearings or tapered roller bearings for thrust.
- Shaft seal: on the drive shaft where it leaves the housing, it keeps the injected oil and pressurised air inside and keeps dust out.
- Static seals and O-rings: at covers, bearing housings and oil connections, they stop leaks between fixed parts.
The discharge-end clearance between the rotor end faces and the housing is set by the thrust bearings and any shims or spacers. It is one of the most important settings in the airend. If it opens up, air leaks back and free air delivery (FAD) falls. If it closes up, the rotors can rub on the end face.
Why a bearing that fits the shaft can still be the wrong bearing
Two bearings can have the same bore, outside diameter and width and still behave very differently in an airend. Size is only the first check. These hidden features decide whether the part is right:
- Internal clearance or preload: bearings are made in different internal clearance groups. Paired angular contact bearings are matched for a set preload or clearance. The wrong group changes rotor float and end clearance.
- Arrangement of paired bearings: back-to-back, face-to-face or tandem mounting changes how thrust is carried. A matched pair must stay a matched pair and be fitted the right way round.
- Precision class: airend bearings are often made to a tighter running accuracy than general-purpose bearings. A standard-grade bearing may fit but run with more runout.
- Contact angle: angular contact bearings come in different contact angles. The angle sets how much thrust the bearing can carry and how stiff it is.
- Cage material and design: brass, steel and polymer cages behave differently with heat, oil and speed.
- Airend-specific references: some airend makers use special bearings with their own part numbers. The catalogue equivalent may differ in clearance or tolerance.
This is why we do not quote bearings from a shaft size or a photo of a worn bearing alone. We work back from the airend identity to the bearing set the airend was built with.
Common airend bearing and seal types compared
| Component | Typical position | Main role | What must match |
|---|---|---|---|
| Cylindrical roller bearing | Inlet end, and radial support at discharge end | Carries radial load, allows thermal growth | Size, clearance group, flange style, cage, precision class |
| Angular contact ball bearing (often paired) | Discharge end | Carries axial thrust and locates the rotor | Contact angle, pairing and preload, mounting arrangement, precision class |
| Four-point contact ball bearing | Discharge end on some designs | Carries thrust in both directions | Split inner ring type, clearance, cage |
| Tapered roller bearing | Discharge end on some designs | Carries combined radial and axial load | Cone and cup set, setting method, shimming |
| Lip-type shaft seal (elastomer or PTFE) | Drive shaft exit | Holds oil and air in, keeps dirt out | Shaft size, lip material, pressure rating, wear sleeve if fitted |
| Mechanical shaft seal | Drive shaft exit on some airends | Seals using lapped faces under spring load | Face materials, secondary seals, fitted length |
| O-rings and gaskets | Covers, bearing housings, oil ports | Static sealing | Size, elastomer compound, hardness |
Seal material matters as much as size. Common elastomers include nitrile (NBR) and fluorocarbon (FKM). PTFE lips are often used where temperature, speed or oil chemistry is demanding. A seal compound that suits one lubricant may swell or harden with another, so we always ask which oil the machine runs on, especially if it has been changed to a synthetic.
Symptoms that point to bearing or seal trouble
Bearings and seals rarely fail without warning. The signs below are worth acting on early, because a bearing that fails in service can let the rotors contact the housing and turn a bearing job into a full airend overhaul.
| Symptom | Possible bearing or seal cause | Other causes to rule out |
|---|---|---|
| Rising noise or a new whine, growl or knock from the airend | Worn or damaged rolling elements or races | Coupling or belt wear, motor bearing, loose guards |
| Rising vibration on trend readings | Bearing wear, loss of preload | Misalignment, soft foot, motor imbalance |
| Oil leaking at the drive shaft | Worn shaft seal lip or grooved shaft | Blocked scavenge line, high oil level, separator issue |
| Falling FAD at the same pressure and speed | Opened end clearance from worn thrust bearings | Inlet valve not opening, leaks, slipping belts |
| High discharge temperature | Rotor rubbing from bearing wear | Cooler fouling, thermostatic valve, low oil level |
| Metal particles in oil or on the oil filter | Bearing distress | Contamination from outside, wear elsewhere |
| Airend hard to turn by hand, or rough when turned | Bearing damage or rotor contact | Seized drive components |
Vibration trending and oil analysis are the most useful early warnings. A single reading tells you little. A steady rise over weeks tells you a lot.
Why airend bearings and seals wear out
Bearings in a well-run airend can last a long time. When they fail early, the cause is usually one of a small number of things, and replacing the bearing without fixing the cause brings the failure back.
- Oil condition: degraded, wrong-grade or contaminated oil, or oil changed too late, thins the lubricating film. Water in oil is particularly harmful.
- Dirty intake air: a damaged or poorly sealed air filter lets fine dust into the oil, and the dust acts as an abrasive inside the bearings.
- High temperature: fouled coolers, high ambient heat in machine rooms, and blocked ventilation raise oil temperature and shorten oil and bearing life. Indian summers make this a regular issue.
- Drive problems: overtight belts load the airend shaft and inlet bearing sideways. A misaligned coupling pushes vibration and load into the bearings.
- Operating pattern: very frequent load and unload cycles, running above rated pressure, and long idle spells that allow condensation all add stress.
- Fitting errors: bearings knocked into place, heated unevenly, fitted the wrong way round, or mixed between pairs.
- Seal wear: a seal lip hardens with heat and age, and the shaft under the lip can wear a groove. A new seal on a grooved shaft may leak again soon after.
How we identify the right bearing and seal set
Our approach is to identify first and quote second. The steps are:
- Identify the compressor: make, model, serial number and nameplate photo. This tells us the package and its build period.
- Identify the airend: the airend itself usually carries its own plate or stamped number. Some compressor models were built with more than one airend type over their life, so the airend identity matters more than the package model.
- Map the bearing and seal positions: inlet and discharge ends, male and female rotors, and the drive shaft seal.
- Collect existing references: markings on the old bearings and seals, parts list or service manual pages if available.
- Check dimensions and features: size, clearance group or suffix codes, pairing marks, seal lip material and design.
- Confirm fit: we confirm the part against the airend specification before quoting. Where the evidence is incomplete, we tell you what is missing rather than guess.
Bearing suffix codes are worth photographing clearly. The letters and numbers after the basic bearing number often show the clearance group, cage, precision and pairing. Losing them by cleaning or grinding the old bearing makes identification harder.
Bearing kit, seal kit or full airend overhaul?
A seal can usually be changed without opening the rotor housing. Bearings are different. Replacing airend bearings means taking the airend apart, measuring, refitting and setting clearances. The decision depends on condition:
| Situation | Usual path | Why |
|---|---|---|
| Oil leak at shaft, bearings quiet, vibration normal | Shaft seal replacement, check shaft surface | Bearings are healthy; seal is the wear item |
| Rising vibration or noise, no rotor contact, FAD near normal | Planned bearing and seal replacement | Catches wear before it damages rotors |
| Falling FAD, high temperature, signs of rotor rubbing | Full airend inspection and overhaul assessment | Rotors and housing may need work beyond bearings |
| Seized airend or heavy rotor damage | Overhaul, exchange or replacement decision | Bearing parts alone will not restore the machine |
Bearing replacement inside an airend needs clean conditions, correct tools, measurement and careful setting of end clearance. We will be clear about what work suits on-site repair, what needs a workshop, and when an airend exchange or specialist rebuild is the sensible route.
Fitting and checks after replacement
Good parts can still fail early if they are fitted badly. The points that matter most are:
- Keep new bearings in their packing until the moment of fitting, and keep the work area clean.
- Use controlled heating for bearings that need it, never an open flame. Press only on the ring that carries the fit.
- Keep matched bearing pairs together and fit them in the stated arrangement.
- Measure and set rotor end clearance as the airend maker specifies.
- Inspect the shaft seal running surface and fit a new seal square, without damaging the lip on keyways or shaft steps.
- Fresh oil and new oil filter, and a check of the separator, air filter and coolers so the new bearings start in good conditions.
- Check coupling alignment or belt tension and pulley alignment before restart.
- After restart, record temperature, pressure, current, noise and a vibration baseline so future readings can be compared.
Where the loads come from inside a screw airend
Knowing where each bearing load comes from explains why airend bearings fail in a particular order, and why a change in pressure or drive shows up first at the discharge end.
- Gas radial force. The compressed air between the lobes pushes the rotors apart. The force rises roughly in proportion to the pressure difference across the airend and is highest at the discharge end, where the pressure is highest.
- Gas axial thrust. Discharge pressure acting on the rotor end faces and helical lobes pushes both rotors towards the inlet end. In most designs the male rotor carries the larger share. Some airends reduce it with a balance piston fed with discharge pressure or oil.
- Drive loads. Belt pull adds a steady radial load on the input shaft. A helical gear drive adds both radial and axial components. A direct coupling adds very little if alignment is correct, and a large cyclic load if it is not.
- Thermal growth. The rotors and casing grow by different amounts as they warm up. The inlet-end bearings are usually allowed to float axially so that this growth does not load the thrust bearings.
| Bearing arrangement | Usual location | Load it is meant to carry | What increases the load |
|---|---|---|---|
| Cylindrical roller bearing | Inlet end and discharge end of both rotors | Radial load only; allows axial float | Higher discharge pressure, belt over-tension, misalignment |
| Angular contact ball bearings in a matched pair or set | Discharge end, mainly male rotor | Axial thrust, with a defined preload or end play | Higher discharge pressure, wrong pairing or preload, low oil viscosity |
| Four-point contact ball bearing | Discharge end, often mounted with radial clearance in the housing | Axial location in both directions | Reverse rotation at shutdown, pressure surges, end play outside limits |
| Tapered roller bearings in pairs | Discharge end in some designs | Combined radial and axial load | Incorrect set-up of end play, heat from excessive preload |
Bearing life in numbers: L10 and the cube rule
Rolling bearing fatigue life is rated as L10: the number of revolutions that 90% of a large group of identical bearings will reach before the first sign of fatigue. The basic relation is L10 = (C ÷ P) raised to the power p, in million revolutions, where C is the bearing’s dynamic load rating, P is the equivalent load, and the exponent p is 3 for ball bearings and 10/3 for roller bearings. Life in hours is L10 × 1,000,000 ÷ (60 × rpm).
Worked example: raising the pressure setting (illustrative). An angular contact thrust bearing on a male rotor running at 2,950 rpm has C = 78 kN and carries an equivalent load P = 4.0 kN at 7.5 bar(g).
- C ÷ P = 78 ÷ 4.0 = 19.5.
- L10 = 19.5³ = 7,415 million revolutions.
- Hours = 7,415 × 1,000,000 ÷ (60 × 2,950) = 7,415,000,000 ÷ 177,000 = about 41,900 h.
- The setting is raised to 10 bar(g). With atmospheric inlet, the thrust rises roughly with gauge discharge pressure, so P = 4.0 × 10 ÷ 7.5 = 5.33 kN.
- New L10 = (78 ÷ 5.33)³ = 14.63³ = 3,128 million revolutions, or about 17,700 h.
A 33% higher load cuts calculated life to (7.5 ÷ 10)³ = 0.42 of the original, a loss of 58%. For a roller bearing the factor is (7.5 ÷ 10) to the power 10/3 = 0.38. A machine built for 7.5 bar and run at 10 bar often loses bearings early while seeming to run normally.
The basic L10 assumes clean oil of adequate viscosity. Modern rating methods (ISO 281) multiply it by a life factor that falls sharply when the oil film is thin or the oil is contaminated. The film depends on viscosity at the bearing’s working temperature. For a typical ISO VG 46 mineral-based compressor oil (46 cSt at 40 °C, 6.8 cSt at 100 °C):
| Oil temperature | Approximate viscosity | Effect on bearing film |
|---|---|---|
| 80 °C | 11.1 cSt | Normal operating range for most airends |
| 90 °C | 8.6 cSt | Film thinner; still acceptable for many designs |
| 100 °C | 6.8 cSt | About 39% below the 80 °C value; life factor reduced |
| 110 °C | 5.5 cSt | Half the 80 °C value; mixed lubrication likely in loaded bearings |
Synthetic oils with a higher viscosity index lose less viscosity with temperature, so read the actual lubricant data sheet. On Indian sites, 45 °C summer ambient with a fouled cooler can push oil temperature from the 80s into the 100s, which costs bearing life as surely as higher pressure. Water in the oil does the same: keep it below the caution limit set by the lubricant supplier, typically a few hundred ppm.
Shaft seal designs and how each one leaks
The input-shaft seal keeps oil in and dirt out where the drive shaft leaves the airend. The design decides how it wears and what a leak looks like.
| Seal design | How it seals | Typical leak pattern | Points to check |
|---|---|---|---|
| Elastomer lip seal (nitrile or fluoroelastomer) | A spring-loaded rubber lip runs on the shaft or a wear sleeve | Slow weep that becomes a steady drip as the lip hardens or the shaft grooves | Groove under the lip, lip hardness after heat, compatibility of the rubber with the oil |
| PTFE lip seal | A PTFE lip, often with a return pattern, runs on a hard, fine-finished surface | Leaks quickly if fitted on a scored shaft, lip folded at fitting, or rotation direction wrong for the return pattern | Shaft hardness and finish, fitting tool, direction arrow |
| Mechanical face seal | A rotating face (often carbon) runs against a stationary hard face (ceramic or silicon carbide), with a secondary O-ring or bellows | Small weep is normal; a sudden leak follows face chipping, dry running or a damaged O-ring | Face condition, spring load, O-ring material, cleanliness at assembly |
| Labyrinth with oil return | Non-contact grooves throw oil back to a drain | Leaks when the drain blocks or the casing behind it is pressurised | Drain path, vent and pressure behind the seal |
Lip seals normally need a plunge-ground shaft or sleeve with no machining lead, a finish of about Ra 0.2–0.8 µm and a hardened surface. A new lip placed in the old groove will leak within days; fit a wear sleeve or a seal that sets the lip at a new position if the design allows. Oil-free airends use separate air seals and oil seals with a vented space between them, so oil at the vent means the oil seal has failed, while air at the vent means the air seal has.
Failure signatures: sound, vibration, oil and temperature
Each bearing fault generates vibration at a predictable frequency that can be calculated from the bearing geometry. For a bearing with N rolling elements, element diameter d, pitch diameter D and contact angle φ, running at shaft frequency fr (rpm ÷ 60, in Hz):
- Outer race defect frequency (BPFO) = (N ÷ 2) × fr × (1 − (d ÷ D) cos φ)
- Inner race defect frequency (BPFI) = (N ÷ 2) × fr × (1 + (d ÷ D) cos φ)
- Cage frequency (FTF) = (fr ÷ 2) × (1 − (d ÷ D) cos φ)
Worked example (illustrative). A cylindrical roller bearing with 12 rollers, d ÷ D = 0.2 and φ = 0° on a shaft at 2,950 rpm. fr = 2,950 ÷ 60 = 49.17 Hz. BPFO = 6 × 49.17 × 0.8 = 236 Hz. BPFI = 6 × 49.17 × 1.2 = 354 Hz. FTF = 24.6 × 0.8 = 19.7 Hz. A four-lobe male rotor at the same speed has a lobe-pass frequency of 4 × 49.17 = 197 Hz, which is normal and must not be mistaken for bearing damage. Use the real bearing geometry from the bearing maker for diagnosis; these values only show the method.
| Stage of damage | Sound | Vibration | Oil analysis | Temperature |
|---|---|---|---|---|
| Early (subsurface fatigue, first spall) | No audible change | Rise in high-frequency enveloped or acceleration readings in the kHz range; defect frequencies appear in the envelope spectrum | Iron trending up slowly; chromium with iron points to bearing steel | No change |
| Developing | Faint rumble or hiss at the discharge end, heard with a stethoscope | Defect frequencies and harmonics visible in the velocity spectrum, with sidebands at shaft speed for an inner race fault | Iron and chromium rising together; ferrous particle count up | Bearing housing a few °C above its usual value |
| Advanced | Clear grinding or knocking; changes with load and unload | Broadband noise floor rises; overall velocity moves into the alarm zone of ISO 20816-type limits | Large wear particles; copper or brass if a cage is breaking up | Housing and discharge temperature rising together |
| Imminent failure | Loud, irregular noise | Defect peaks may fall as damage spreads, while the noise floor climbs | Visible metal in filter or drained oil | Rapid rise; rotor contact risk |
Silicon in the oil points to dust entering through the intake: a cause of bearing wear, not a symptom. Trend the same point, at the same load, with the same instrument. For the wider failure picture see airend failure modes and lubricant and oil analysis.
Bearing handling and measurement checks during a rebuild
Most early bearing failures after a rebuild come from contamination, heat damage or clearances set by guesswork. These measurement checks add detail to the fitting practices above and the full procedure in the airend overhaul and rebuild guide:
- Measure shaft journals and housing bores with an outside micrometer and a bore gauge at two positions 90° apart. Compare the readings with the specified fits. A worn housing bore that lets the outer ring creep will destroy a new bearing.
- Heat bearings with an induction heater or oven to about 110 °C, and never above 120 °C. Never use a flame. Do not press a bearing on through its rolling elements.
- Fit matched angular contact sets in the marked order and orientation (back-to-back, face-to-face or tandem). Never mix rings from different sets.
- Let the assembly cool to room temperature before measuring rotor end play with a dial indicator. Set shims to the maker’s figure, not to a figure copied from a similar airend.
- Fit the shaft seal with a sleeve over keyways and threads. Check that the running surface is undamaged and that a PTFE seal suits the direction of rotation.
- Record the as-measured fits, end play and shim sizes on the job sheet, so that the next overhaul starts from known figures.
How Reddy Compressors helps
Reddy Compressors is an independent multi-brand compressed-air specialist based at Tarnaka, Secunderabad, serving industry since 2002. We support screw compressors from makers such as Atlas Copco, Ingersoll Rand, ELGi, KAESER, Chicago Pneumatic, Gardner Denver, Sullair, Doosan, Kirloskar, Bobcat and many more. We are not an authorised distributor for these brands, and brand names belong to their owners. We have tie-ups with all major and reputed brands.
For airend bearings and seals, we identify the parts from your machine and airend data, confirm fit before we quote, and state on the quote whether each part is genuine or a named aftermarket alternative. Our service teams cover the whole of Telangana and Andhra Pradesh for diagnosis, seal replacement, condition checks and overhaul assessment. Spare parts are supplied across India. If the airend needs more than bearings and seals, we explain the options plainly, including overhaul, exchange or replacement.
What to send us
- Compressor make, model and serial number, with a clear nameplate photo.
- Airend plate or stamped number, with a photo.
- Running hours and the date of the last service or airend work.
- Photos of the old bearings and seals showing all markings, if already removed.
- Symptoms: noise, vibration readings, oil leak location, temperature or FAD changes.
- The oil type currently in use and any recent oil analysis report.
Frequently asked questions
How do I know if my screw compressor airend bearings are failing?
Look for a steady rise in noise or vibration from the airend, metal particles in the oil or on the oil filter, rising discharge temperature, and falling air delivery at the same pressure. Any one of these deserves a check. Vibration trending and oil analysis give the earliest warning. Stopping the machine for inspection early often saves the rotors and housing from damage.
Can I replace airend bearings with standard catalogue bearings of the same size?
Not safely without checking. Airend bearings often have a specific internal clearance, preload, contact angle, cage design and precision class. A catalogue bearing of the same size may differ in these hidden features and change rotor clearances. We identify the bearing set from the airend data and markings, then confirm fit before quoting.
Why is oil leaking from my compressor shaft seal?
The usual causes are a hardened or worn seal lip, a groove worn into the shaft under the seal, or a seal fitted crooked. Other causes include a blocked scavenge line, high oil level or pressure in the bearing area. We check these before replacing the seal, because a new seal on a grooved shaft or with an underlying fault will soon leak again.
Can a shaft seal be changed without overhauling the airend?
On most screw airends, yes. The drive shaft seal can usually be replaced after removing the coupling or pulley and seal cover, without opening the rotor housing. The shaft surface should be inspected at the same time. If the bearings are also showing wear, it is often better to plan both jobs together and avoid a second shutdown.
Should I replace airend bearings or overhaul the whole airend?
If vibration or noise is rising but air delivery and temperature are normal and there is no rotor contact, a planned bearing and seal replacement may be enough. If air delivery has fallen, temperature is high or the rotors show rubbing, a full inspection and overhaul assessment is needed. We explain the options after seeing the condition.
What details do you need to supply airend bearings and seals?
Send the compressor make, model and serial number with a nameplate photo, the airend plate or stamped number, running hours, and photos of any old parts showing markings. Tell us the oil in use and the symptoms. With these, we can identify the correct bearing and seal set and confirm fit before giving a quotation.
How long do screw compressor airend bearings last?
There is no single figure. Airend bearings are designed for a calculated life, and many makers set a bearing overhaul interval in the tens of thousands of running hours. Actual life depends on discharge pressure, oil temperature and viscosity, oil cleanliness, water content, drive alignment and the number of starts. A machine run at a higher pressure than its design, or with oil regularly above 100 °C, can reach the end of bearing life in well under half the expected hours. Trending vibration and oil analysis shows the actual condition better than counting hours.
Does running a compressor at higher pressure shorten airend bearing life?
Yes. Bearing load rises roughly in proportion to discharge pressure, and fatigue life falls with the cube of load for ball bearings and the 10/3 power for roller bearings. Raising a 7.5 bar(g) machine to 10 bar(g) raises the thrust load by about 33% and cuts the calculated life of a ball thrust bearing to about 42% of its original value. If a higher pressure is needed, check the airend’s rated maximum pressure and motor power first. Do not simply change the pressure setting.
Which metals in oil analysis indicate airend bearing wear?
Iron rising together with chromium usually points to rolling bearing steel. Copper, or copper with zinc, can come from brass bearing cages, but it can also come from coolers and fittings, so it needs to be read with the other results. Silicon indicates dust entering through the intake and is a common root cause of bearing wear. Ferrous particle counts help to separate fine normal wear from larger fatigue particles. Look at the trend over several samples taken at the same point and interval, not at one result.
Should the shaft seal be replaced when airend bearings are replaced?
Yes. During a bearing overhaul the shaft seal is disturbed and costs little compared with the labour of opening the airend again. Inspect the shaft or wear sleeve under the old lip. If it is grooved, fit a new wear sleeve or repair the surface, because a new lip on a grooved track will leak. Replace the O-rings and gaskets disturbed during the job as well. A mechanical face seal should be renewed or professionally lapped and checked. It should not be refitted only because it looked clean.
Related pages
Field guide
Airend bearings and seals: Protect the most expensive part
The airend is the heart and the most expensive part of a screw compressor. Its bearings and shaft seal wear with hours, heat and oil condition. Catching wear early can mean a planned overhaul instead of a seized airend.
Warning signs
- New noise from the airend, a whine or rumble that changes with load.
- Rising vibration at the airend.
- Higher motor current or lower delivery at the same pressure.
- Oil leaking at the shaft seal or appearing in the coupling or belt area.
- Metal particles in oil or in the oil filter; wear metals in oil analysis.
What decides bearing life
Clean oil of the right type, correct operating temperature, good alignment and belt tension, avoiding frequent starts and keeping water out of the oil. Neglect in any one of these shortens life considerably.
Repair options
A shaft seal can often be replaced on site. Bearing replacement needs an airend overhaul with correct clearances, usually in a workshop. Depending on condition and age, an exchange airend or a new machine may be the better choice. See rebuild, exchange or replace.
What to send for a quotation
- Compressor make, model and serial number, and airend serial if visible.
- Running hours and service history.
- Symptoms: noise, vibration, temperature, oil leaks.
Send it through find my part or the spare parts enquiry, or call +91 99590 60718 or +91 92468 88812.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.