Why bearings and seals decide airend life
Inside a screw airend, the rotors never touch each other or the housing in normal running. They are held in position by their bearings, separated by thin films of oil and very small clearances. The bearings carry radial load from the compression process and a large axial (thrust) load that pushes the rotors towards the suction end. If a bearing wears, the rotor moves, clearances close up, and the lobes start to rub. A bearing failure that is not caught early often becomes a rotor and housing failure.
The shaft seal has a different job: it stops oil and pressurised air escaping where the drive shaft leaves the airend, and stops dirt getting in. A leaking seal wastes oil, contaminates the drive and coupling, and in dusty Indian site conditions lets abrasive particles reach the bearings.
That is why these parts are replaced as standard in most airend overhauls, and why getting them exactly right matters more than almost any other part of the rebuild.
A bearing that fits is not necessarily the right bearing
Two bearings can share the same bore, outside diameter and width and still behave very differently in an airend. The details that matter include:
- Bearing type and arrangement – cylindrical roller bearings for radial load, angular-contact ball or tapered roller bearings for thrust, often in matched pairs set face-to-face or back-to-back.
- Precision class – airends commonly use bearings made to a tighter running accuracy than general industrial bearings.
- Internal clearance or preload – a different clearance group changes how the rotor runs when the airend reaches operating temperature.
- Cage material and design – brass, steel or polymer cages behave differently at temperature and under oil flow.
- Contact angle on angular-contact bearings, which governs how much thrust the bearing can carry.
- Suffixes and markings – the letters after the basic number often carry this information. Ignoring them is a common cause of repeat failure.
The correct specification comes from the airend manufacturer’s parts data, the markings on the bearing removed and the airend’s identity. Where those disagree, the difference must be resolved before ordering, not after fitting.
Bearing positions and what each one does
| Position | Main load | Typical bearing types | Effect if wrong or worn |
|---|---|---|---|
| Suction end, male and female rotors | Radial | Cylindrical roller bearings | Rotor-to-bore rubs, vibration, noise |
| Discharge end, male and female rotors | Radial and axial (thrust) | Cylindrical roller plus angular-contact ball or tapered roller sets | Loss of discharge end clearance, end-face rubs, overheating |
| Thrust balance arrangement (where fitted) | Reduces net axial load | Balance piston or similar, by design | Higher thrust bearing load and shorter life |
| Drive shaft | Belt or gear load where applicable | Depends on design | Belt-side load damage, seal leakage |
The arrangement varies by manufacturer and model, so the table is a general guide. The actual position and orientation of each bearing must be recorded at disassembly and followed at assembly. A thrust bearing pair fitted the wrong way round can fail very quickly.
Why did the old bearing fail?
A replacement bearing will only last if the cause of the previous failure has been removed. Typical patterns and causes are:
| What the bearing shows | Likely cause | What else to correct |
|---|---|---|
| Fatigue spalling, oil clean | End of life, or load beyond design | Operating pressure, alignment, overhaul interval |
| Small dents across raceways | Hard particle contamination | Intake filter, oil filter, cleanliness at last service |
| Rust or etching | Water in oil | Running pattern, oil temperature, storage |
| Blue or brown discolouration | Overheating, poor lubrication | Oil grade, cooler, thermostatic valve, oil level |
| Fluting (washboard marks) | Electric current through bearing | Earthing, VFD shaft voltage |
| Marks at roller spacing (false brinelling) | Vibration while stationary | Transport, standby machines near running plant |
| Cracked ring or creep marks | Wrong fit, worn seat | Shaft and housing seat dimensions |
The oil itself is often the strongest clue. An oil analysis or simply a careful look at the old oil and filter can show whether dirt, water, wear metal or oxidation is involved.
Shaft seals: types, materials and failure
Airend shaft seals are usually lip seals or mechanical face seals, sometimes in combination with a wear sleeve. Points we check:
- Seal type and material – lip materials such as PTFE-based compounds and elastomers have different temperature and oil-compatibility limits. The replacement must suit the oil in use, including synthetic oils.
- Seal track or sleeve – a grooved shaft surface will leak even with a new seal. The sleeve or track must be replaced or restored.
- Orientation and depth – lip seals are directional and must sit at the correct depth so the lip does not run in an old groove.
- Drain and vent paths – some designs have a drain between seals. If it is blocked, oil is forced past the outer seal.
- Cause of leaks – hardened lips from heat, cuts from fitting over a keyway, or excessive shaft movement from worn bearings.
A seal leak is often a symptom. If the bearings allow the shaft to move, a new seal will not stay dry for long.
How should airend bearings be fitted?
The fitting method affects bearing life as much as the bearing itself. A good procedure looks like this:
- Check the seats – measure shaft and housing seats for size, roundness and fretting before fitting a new bearing.
- Keep parts clean – unwrap bearings only at the point of fitting. Do not wash out the protective oil unless the manufacturer says so.
- Heat evenly where an interference fit is used – an induction heater or controlled oven, within the bearing maker’s temperature limit. Never use a gas flame.
- Press on the correct ring – force through the ring that has the interference fit, never through the rolling elements.
- Set preload or clearance as specified, using the shims, spacers and methods for that design.
- Torque lock nuts and covers to the specified values and secure locking devices.
- Turn by hand and check rotation is smooth before going further.
Seals are fitted with protective sleeves over keyways and threads, lubricated with the correct oil and pressed square to the bore.
After assembly: proving the parts are right
Verification does not end when the covers go on. After the airend is installed, we record a baseline:
- Discharge temperature under load, against the typical 75–95 °C band for an oil-injected screw.
- Vibration on the bearing housings, where measurement is available, as a reference for future checks.
- Noise and smoothness on start, load and unload.
- Shaft seal dryness after the first running hours.
- Oil condition and filter check after a short initial run.
This baseline is what later readings are compared against. It also gives an early warning if a fitting problem has slipped through.
Genuine, equivalent and aftermarket parts
Bearings and seals are available as manufacturer-supplied kits, as the same branded bearings bought through bearing distributors, and as aftermarket kits of varying quality. Our approach is neutral: we identify the exact specification from the airend, compare the options available, and tell you plainly what each one is. A cheaper bearing of the wrong precision class or clearance is not a saving.
How Reddy Compressors helps
Reddy Compressors, at Tarnaka in Secunderabad, has supported industry since 2002 as an independent multi-brand compressor specialist. We are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands. We replace airend bearings and seals on screw compressors from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar, Gardner Denver, Bobcat and other makes.
We identify bearings and seals from the make, model, serial number, nameplate and the parts removed, confirm fit before quoting, and supply spares across India. Our service engineers and technicians cover all of Telangana and Andhra Pradesh, and they look for the cause of failure, not just the worn part. If the machine has to come off line, a rental compressor from our own fleet can keep the plant running.
What to send us
- Compressor make, model and serial number, with a nameplate photograph.
- Photographs of the bearings or seals removed, including all markings.
- Running hours, oil type and the last oil change date.
- The symptom: noise, vibration, oil leak at the shaft, high temperature or a trip.
- Any oil analysis or vibration readings.
Frequently asked questions
How long do screw compressor airend bearings last?
Bearing life depends on the design, operating pressure, temperature, oil condition, contamination and how many hours the machine runs on load. Manufacturers publish overhaul intervals for their airends, and these should be the starting point. Dusty sites, high summer temperatures, poor oil changes or running above design pressure all shorten life, so condition monitoring is a better guide than hours alone.
What are the signs of a failing airend bearing?
Typical signs are a new growl or whine from the airend, rising vibration on the bearing housings, increasing discharge temperature, metal particles in the oil filter and, later, oil leaking at the shaft seal because the shaft has started to move. Catching these early usually means a bearing and seal job rather than a rotor and housing repair.
Can I use any bearing with the same size number?
No. Bearings with the same basic size can differ in precision class, internal clearance, preload, contact angle and cage design. Airends often need a specific combination, shown in the suffix letters and the manufacturer’s parts data. Fitting the wrong variant can cause overheating, rotor rubs or rapid failure even though the bearing physically fits the shaft.
Why is my compressor leaking oil from the shaft seal?
Common causes are a worn or hardened seal lip, a grooved shaft or sleeve, a blocked seal drain, the wrong seal material for the oil, or shaft movement from worn bearings. Replacing the seal alone may not cure the leak. The shaft surface and bearing condition should be checked at the same time to find the real cause.
Should bearings and seals always be changed together?
In most airend overhauls, yes. Once the airend is opened, bearings, shaft seals, O-rings and gaskets are normally replaced as a set, because the labour of opening it again far outweighs the cost of the parts. Reusing a bearing or seal removed from service is rarely justified unless the manufacturer allows it.
Do you supply airend bearing and seal kits for all brands?
We supply bearings, seals and overhaul kits for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar and other airends across India. As an independent specialist, not an authorised dealer, we identify the exact specification from the nameplate, serial number and old parts and confirm fit before quoting.
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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.