Airend inspection, disassembly and precision measurement

Why the inspection order matters

Opening an airend is not where diagnosis begins. Once the first bolt comes out, the evidence starts to change: deposits get disturbed, rotor positions are lost, bearing races are separated from the marks that show how they were loaded, and cleaning solvent removes the varnish or debris that tells you why the machine failed. If the external condition is not recorded first, it cannot be reconstructed later.

That is why a sound airend inspection follows a fixed order. It starts on the running (or last-running) machine, moves to controlled disassembly, then to measurement, and ends with a written decision for each part. Skipping a stage usually leads to one of two expensive outcomes: a rebuild that replaces parts that were fine, or a rebuild that reuses a part that caused the failure and fails again.

For a plant engineer the practical question is simple: will this airend give reliable service again, and what will it take? A disciplined inspection is the only honest way to answer it.

What should be checked before the airend is opened?

Most of the useful information about an airend’s condition is available before any spanner touches it. We collect as much of the following as the site can provide:

• Operating history – running hours, load/unload hours, recent service records, oil and filter change history and any fault logs from the controller.
• Discharge temperature trend – a normal oil-injected screw typically runs at about 75–95 °C at the airend outlet. A slow rise points to cooling, oil or internal wear issues; a sudden trip points to something more abrupt.
• Delivery and pressure – whether the machine still reaches rated pressure, how long it takes to load, and whether free air delivery (FAD) appears to have dropped.
• Noise and vibration – changes in tone, a new knock or growl, and vibration readings taken on the bearing housings where available.
• Oil condition – colour, smell, foaming, varnish, metal particles on the filter or magnetic plug, and an oil analysis report if one exists.
• Leaks and seal condition – oil at the shaft seal, weeping joints and oil carry-over downstream.
• Shaft and drive – end float and radial play felt by hand (with the drive isolated), coupling or belt condition and alignment.

Photographs of the airend as received, nameplate details and the serial number are part of this record. They also let us identify parts correctly later.

How is an airend dismantled without losing evidence?

Controlled disassembly means following the manufacturer’s sequence where it is available, or an equivalent documented sequence, and never improvising with force. The main points are:

1. Drain and sample the oil before moving the airend. Keep a sample in a clean, labelled container.
2. Mark orientation – match-mark rotors to each other and to the housing, and mark end covers, shims, bearing positions and spacers. A shim pack placed on the wrong end changes rotor clearance.
3. Remove covers and seals in sequence, noting the condition of each joint and seal lip as it comes off.
4. Pull bearings with correct tooling – pullers and presses that load the correct ring. Hammering or pulling through the rolling elements damages a bearing that may otherwise have shown a clean failure pattern.
5. Withdraw rotors carefully, supported so that lobes do not touch each other or the bore.
6. Photograph before cleaning – deposits, scoring, discolouration, debris and contact marks.
7. Bag and tag every part with its position and the job identity.

Damage caused during disassembly is a real risk. A bent shim, a dented rotor tip or a bearing pulled through its cage can easily be mistaken later for in-service damage, which leads to the wrong root cause.

Cleaning can destroy the failure story

Cleaning is necessary for measurement, but it is the step where evidence is most often lost. Before parts go into a wash, we record and, where useful, collect:

• Varnish or lacquer on rotors and bores (often linked to high temperature or degraded oil).
• Rust or water staining (condensate in the oil, long idle periods, humid storage).
• Foreign particles – sand, metal flakes, gasket material or filter media – which point to intake, upstream or internal sources.
• Rub marks and their location: tip, flank, end face or bore.

The cleaning method itself must suit the material. Aggressive chemicals or abrasive blasting can attack rotor coatings, seals and precision surfaces. Bearings that will be examined for failure analysis should not be spun dry with compressed air, which can damage the rolling surfaces.

What gets measured, and against what?

A measurement only means something when it is compared with an approved limit. Where the manufacturer’s tolerances are available, they govern. Where they are not, the inspection report must say so and state what reference was used, rather than present a guess as a specification.

Measurement conditions matter. Parts should be at a stable temperature, close to that of the instrument, because a precision fit can change noticeably with a few degrees of difference. Instruments should be in calibrated condition and readings should be repeatable; if two readings disagree, measure again rather than average them.

What the evidence usually means

Reading the evidence correctly is what separates an inspection from a parts list. Some common patterns we look for:

• Bearing spalling with clean oil – often end of fatigue life, or load beyond design (for example, sustained high pressure or misalignment).
• Bearing damage with sand or dust dents – intake filtration or oil contamination, very common in dusty Indian site conditions.
• Tip rubs along one end of a rotor – axial bearing wear or incorrect end clearance allowing the rotor to move into the end cover.
• Varnish and dark oil – sustained high discharge temperature, wrong oil, or extended oil change intervals. High summer ambient (40–45 °C, and above 45 °C on open sites) and blocked coolers make this worse.
• Rust and pitting – water in the oil from short running cycles that never warm the oil enough, or from long idle periods.
• Fluting on bearing races – electrical current passing through bearings, worth checking on VFD-driven machines.

If the cause lies outside the airend – intake, cooler, oil, controls or the drive – rebuilding the airend alone will not stop the failure from coming back. The inspection report should say so plainly.

How is each part dispositioned?

Every inspected part should leave the bench with one decision and the reason for it:

Machining, rotor regrinding or recoating are specialist processes. They should be used only where the geometry and material allow it, and every reworked part must be measured again before it goes back into the airend.

What a good inspection report contains

The report is what lets a maintenance manager take a decision on rebuild, exchange or replacement. It should include:

• Airend identity – make, model, serial and the compressor it came from.
• Reported symptoms and operating evidence collected before opening.
• Photographs as received, during disassembly and before cleaning.
• Measurements for each part, with the limit used and its source.
• The decision for each part and the reason.
• The probable cause of failure, with any external causes that must also be corrected.
• A clear scope for the rebuild and the validation that will follow.

Over a fleet, these reports build a useful picture: which machines wear bearings early, which sites have dust or oil problems, and which overhaul intervals actually fit your duty.

How Reddy Compressors helps

Reddy Compressors is an independent multi-brand compressor specialist based at Tarnaka, Secunderabad, serving industry since 2002. We inspect and overhaul screw compressor airends from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar, Gardner Denver and other makes. We are not an authorised dealer of any of these brands. We have tie-ups with all major and reputed brands.

Our team of over 20 service engineers and technicians covers the whole of Telangana and Andhra Pradesh. We collect the operating evidence on site, dismantle under control, record what we find, and give you a written disposition with a clear rebuild, exchange or replacement option. Where a specialist process such as rotor regrinding or coating is needed, we arrange it and reinspect the parts before reassembly.

While your airend is being inspected, we can supply a rental compressor from our own fleet (50–2,000 cfm, 2–30 bar, diesel and electric) so that production continues. Spare parts are identified from the machine and fit is confirmed before we quote, and equipment and spares are supplied across India.

What to send us

• Compressor make, model and serial number, with a nameplate photograph.
• Running hours and the date of the last service or overhaul.
• The symptoms: high temperature, noise, low delivery, oil carry-over or a trip.
• Controller fault history and any oil analysis or vibration reports.
• Photographs of the airend, oil and filters as they are now.
• Site location and whether you need standby air during the work.

Frequently asked questions

How do I know if my screw compressor airend needs an overhaul?

Common signs are a rising discharge temperature that cooling and oil changes do not fix, new noise or vibration from the airend, metal particles in the oil filter, oil leaking at the shaft seal and a drop in delivery or longer loading time. One sign alone may have another cause, so the external checks should be done first before deciding to open the airend.

What is checked during an airend inspection?

The inspection covers the bearings, shaft seals, shafts and bearing seats, housing bores, rotor lobes and end faces, coating condition and the rotor clearances. Each is checked visually and measured where relevant, then compared with the manufacturer’s limits. The oil, filter debris and deposits found before cleaning are also recorded because they often explain why the failure happened.

Can an airend be inspected without dismantling it?

Partly. Discharge temperature trends, vibration readings, oil analysis, noise, shaft play and leak checks reveal a lot about the airend without opening it. They can show that bearings are wearing or that oil is degrading. Measuring rotor surfaces, bearing raceways and clearances directly, however, requires controlled disassembly on the bench.

Why should the airend not be cleaned before inspection?

Deposits, varnish, rust, sand and metal flakes are evidence of the cause of failure. If the airend is washed before they are photographed and recorded, that evidence is lost and the root cause may be missed. The rebuilt airend could then fail again for the same reason, for example an intake filtration or oil problem that was never identified.

Is it better to rebuild an airend or replace it?

It depends on what the inspection finds. If bearings and seals are worn but rotors and housings are within limits, a rebuild is usually sensible. If rotors or housings are badly damaged, an exchange or new airend, or even a new compressor, may be better value. A written inspection report lets you compare these options on facts.

Do you inspect airends of all compressor brands?

Yes. We work on screw compressor airends from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar, Gardner Denver and other makes as an independent multi-brand specialist. We are not an authorised dealer, and we identify the correct parts for each airend from its nameplate, serial number and the parts removed.

Related pages

• Airend overhaul and rebuild guide
• Airend failure modes and their causes
• Airend bearings, seals and wear parts
• Airend rotors, clearances and internal geometry
• Compressor oil analysis and fluid condition
• Rental air during a compressor breakdown