How rotor geometry makes compressed air
A screw airend has two helical rotors, a male rotor with convex lobes and a female rotor with concave flutes, turning in a closely machined housing. Air trapped between the lobes and the housing is carried from the suction end towards the discharge end. As the rotors turn, the trapped volume shrinks and the pressure rises until the pocket opens to the discharge port.
Efficiency depends on how little air leaks back from high-pressure pockets to low-pressure ones. The leak paths are the clearances: between the lobe tips and the bore, between the meshing rotor profiles, and between the rotor end faces and the end covers. In an oil-injected airend, the oil film helps seal these gaps. In an oil-free airend, the clearances alone do the sealing, and timing gears keep the rotors from touching.
When clearances grow through wear, internal leakage increases, delivery drops and the airend runs hotter for the same output. When clearances close up through bearing wear or incorrect assembly, the rotors rub and damage follows quickly.
The clearances that matter
| Clearance | Where it is | What controls it | Effect if too large | Effect if too small |
|---|---|---|---|---|
| Discharge end clearance | Rotor end face to discharge end cover | Thrust bearings, shims, spacers | Internal leakage at the highest pressure difference | End-face rubs, heat, seizure risk |
| Suction end clearance | Rotor end face to suction housing | Rotor length, bearing position | Minor leakage | Rubs when rotors expand |
| Tip clearance | Lobe tips to housing bore | Rotor diameter, bore, radial bearings | Leakage, lower delivery | Bore scoring |
| Interlobe clearance | Between male and female profiles | Profile accuracy, centre distance, timing (oil-free) | Leakage along the mesh | Profile contact, noise, wear |
The discharge end clearance is usually the most sensitive, because it sits at full discharge pressure and is set by the thrust bearing arrangement. Small changes here can have a noticeable effect on delivery and temperature.
Why clearances change with temperature and load
Clearances are set cold on the bench, but the airend works hot and under load. Several things happen at once in service:
- The rotors heat up and grow in length and diameter, more towards the discharge end where air and oil are hottest.
- The housing also expands, but not by the same amount or at the same rate as the rotors.
- The axial gas force pushes the rotors towards the suction end, loading the thrust bearings and taking up their internal clearance.
- Radial bearings carry the pressure load, letting the rotors move slightly towards the low-pressure side of the bore.
The manufacturer’s cold settings are chosen so that the running clearances stay open in all these conditions, including start-up, when parts warm at different rates. High ambient temperatures in Indian summers, blocked coolers or a failed thermostatic valve push temperatures beyond normal and use up that margin. That is one reason a typical oil-injected screw is kept at about 75–95 °C discharge temperature.
Never tighten clearances to chase efficiency
It can be tempting, during a rebuild, to set end clearances below specification to recover lost delivery. This is a mistake. A clearance that seems generous on the bench may close completely at full load on a hot afternoon. The result is end-face rubbing, rapid heating and, at worst, a seized airend. If delivery is low after a rebuild, the cause should be found – worn profiles, an inlet valve that does not open fully, a leaking minimum-pressure valve, a slipping belt – rather than compensated by closing gaps.
Reading rotor surface damage
Different marks on rotors mean different things, and the decision depends on what caused them:
| What is seen | Likely cause | Usual decision |
|---|---|---|
| Light polishing on tips | Normal running contact through oil film, minor wear | Usually reusable if measurements are within limits |
| Tip rub concentrated at discharge end | Thrust bearing wear or end clearance lost | Assess depth; correct bearings and shimming |
| Rub along the full length on one side | Radial bearing wear, bore wear, bent shaft | Measure run-out and bore; may need rework or replacement |
| Dents and gouges | Foreign object or bearing debris passing through | Depends on size and location; check source |
| Pitting on profiles | Corrosion from water in oil, or cavitation | Assess extent; correct running pattern |
| Coating loss (where coated) | Rub, debris, chemical attack or age | Specialist recoating may be possible; otherwise replace |
| Heavy smearing or welding | Seizure | Normally reject rotors; check housing |
Many oil-free rotors and some oil-injected rotors carry a coating. Coatings must not be polished, ground or blasted with ordinary workshop methods, and recoating is a specialist process.
How rotor geometry is checked
- Record match marks made at disassembly so rotors return to the same housing and orientation.
- Visual and contact-pattern check – marking compound or careful inspection shows where the rotors have touched each other or the housing.
- Run-out between centres – with the rotor supported on its own centres or bearing seats, a dial indicator shows bend or eccentricity at the lobes and journals.
- Bearing seats and journals – size, roundness and fretting measured with micrometers.
- Housing bores – bearing bores and rotor bores checked for size, ovality and scoring.
- End clearance at assembly – measured with dial indicators or feeler gauges, then adjusted with shims or spacers to the specified value.
- Timing gear backlash on oil-free airends, checked to the manufacturer’s setting.
Measurement datums matter. A reading taken from a worn face or an unsupported shaft can be misleading. Parts should be measured at a stable temperature, and readings should repeat before they are trusted.
Alignment outside the airend
Internal geometry can be spoilt by external alignment. A misaligned coupling or an over-tensioned belt puts side load on the drive shaft, which the bearings must carry and which can push the rotor towards the bore. After an airend rebuild we check:
- Coupling alignment between motor or engine and airend, within the coupling maker’s limits.
- Belt tension and pulley alignment on belt-driven units.
- Soft foot and mounting condition, so the airend is not distorted when bolted down.
- Gearbox condition on gear-driven designs.
Machining, balancing and their limits
Some damage can be corrected – a worn seal track sleeved, a bearing seat restored, a lightly damaged profile dressed. Rotor profile regrinding and recoating are specialist processes that need the correct profile data and equipment. They are only sensible when enough material remains and the geometry can be brought back to a usable standard. Any reworked rotor must be measured again, checked for run-out and, where the process affects it, balanced before reassembly. If the rework cannot be proved, an exchange or new airend is the safer choice.
What worn geometry costs in service
Lost clearance control rarely shows up as one dramatic event. More often a plant notices that the compressor takes longer to reach pressure, runs loaded for more of the shift and sits closer to its temperature trip on summer afternoons. Each of these has a cost. Internal leakage means the airend does more work for each cubic metre of air delivered, so specific power (kW per m³/min) gets worse. If operators respond by raising the pressure setting to keep the far end of the line supplied, power rises again, by about 7% for each extra bar. Tracking delivery, loaded hours and discharge temperature over time gives early warning that internal geometry is changing, well before a rub or seizure forces a stop.
How Reddy Compressors helps
As an independent multi-brand specialist based at Tarnaka, Secunderabad, and serving industry since 2002, we inspect and rebuild screw airends from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar, Gardner Denver and many more. We are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.
We measure rotor condition, run-out, bearing seats, bores and clearances, and set end clearance to the manufacturer’s value with the correct bearings and shims. Where regrinding or coating is needed, we arrange the specialist process and check the parts on return. We also check coupling and belt alignment so the rebuilt airend is not loaded wrongly from outside. Our engineers cover Telangana and Andhra Pradesh, spares are supplied across India, and our rental fleet can provide standby air while the airend is out.
What to send us
- Compressor make, model and serial number, with a nameplate photograph.
- Photographs of the rotors and housing if the airend is already open.
- Discharge temperature and delivery history, including recent changes.
- Whether the machine is oil-injected or oil-free, and its drive type.
- Any previous rebuild records.
Frequently asked questions
What is rotor end clearance in a screw compressor?
Rotor end clearance is the small gap between the rotor end faces and the end covers of the airend, most importantly at the discharge end. It is set by the thrust bearings and shims during assembly. Too large a gap lets compressed air leak back and reduces delivery; too small a gap risks the rotors rubbing on the cover when they expand at operating temperature.
Why do screw compressor rotors rub?
Rotors usually rub because bearings have worn and let them move, because end clearance was set incorrectly during a rebuild, or because the airend ran much hotter than normal. Debris passing through, a bent shaft and external misalignment can also cause contact. The rub pattern shows where the contact happened, which helps identify the cause before a rebuild.
Can damaged screw compressor rotors be repaired?
Light damage can sometimes be dressed, and specialist regrinding or recoating may be possible when enough material remains and the correct profile data is available. Heavy scoring, seizure damage or bent rotors usually mean replacement. Any repaired rotor must be measured again for profile, run-out and clearances before it is accepted for reassembly.
Does reducing clearances improve compressor efficiency?
Not safely. Manufacturers set clearances to stay open at full load and operating temperature. Setting them tighter than specification may raise delivery slightly on a cold test, but it risks rubbing and seizure when the airend heats up, especially in high summer ambient. Low delivery should be traced to its real cause rather than corrected by closing gaps.
How does bearing wear affect rotor clearances?
Bearings locate the rotors both axially and radially. As thrust bearings wear, the rotors move towards one end and the end clearance changes; as radial bearings wear, the rotors move towards the bore. Either can lead to rubbing. That is why bearings are normally replaced during a rebuild and clearances are reset afterwards.
Are oil-free airend clearances different from oil-injected ones?
Yes. In an oil-injected airend, oil helps seal the gaps and cools the compression. An oil-free airend has no oil in the compression chamber, so clearances do all the sealing and timing gears keep the rotors apart. Rotors are often coated, and clearance and timing settings must follow the manufacturer’s data closely.
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.