How to tell an airend is wearing and when to overhaul

Why running hours alone are not enough

Every maker publishes a recommended airend or bearing service interval in hours, and it is a sensible planning figure. But two identical compressors can reach that figure in very different condition. One has run cool, on the right oil, with clean intake air and few starts. The other has run in a hot, dusty compressor room, cycled hard, and had a water problem in its oil. The first may have years left; the second may be close to bearing failure well before the interval.

The opposite mistake is just as costly. Airends are often blamed for low pressure, high temperature or high power when the real cause sits in the package or the system. Rebuilding a healthy airend does not fix a blocked cooler, a sticking inlet valve or a leaking pipe network.

A sound overhaul decision therefore rests on three things: the right identity and history of the unit, measurements trended against a baseline, and elimination of the package and system causes first. Once the decision is made, our airend overhaul and rebuild guide explains the workshop process and the rebuild, exchange and replace options.

What are the signs of a worn airend?

Wear rarely announces itself all at once. The useful signs are changes from the machine’s own normal, noticed early.

• Less air for the same power. The plant needs more running time or a second compressor to hold pressure, while the motor current at full load is unchanged or higher.
• Rising discharge temperature. Internal leakage turns work into heat, and worn bearings generate heat themselves. A slow upward trend matters more than a single reading.
• New or changing noise. A rumble, growl or high-pitched whine from the airend, especially a change heard after start-up or under load.
• Rising vibration at the bearing housings. Particularly at bearing defect frequencies or with a growing high-frequency content.
• Wear metals in the oil. Iron, chromium or bearing-cage material rising in oil analysis, or fine metallic glitter in the oil filter.
• Shaft seal leakage. Oil weeping at the drive end of the airend that returns after a seal change can indicate shaft movement from worn bearings.
• Hard turning or roughness. When the machine is isolated, the rotors feel rough, notchy or tight when turned by hand, or end-play is felt at the shaft.

Any one sign is a reason to investigate. Two or more trending in the same direction is a strong reason to plan intervention.

Rule out package and system causes first

Before any symptom is laid at the airend’s door, check the parts around it. Most of these checks are quick and cheap compared with an airend removal.

For discharge temperature and oil carryover in particular, see our pages on cooler service and oil separator performance.

Which measurements show airend condition?

A single snapshot reading is hard to judge. Each measurement below becomes powerful when compared with the same machine’s earlier readings, taken at the same pressure and load.

Free air delivery and specific power

Free air delivery (FAD) is the actual air output referred to inlet conditions, measured by a method based on ISO 1217. Specific power is input power divided by FAD, often in kW per m³/min. A worn airend shows lower FAD and higher specific power at the same pressure. On site, a calibrated flow meter or a receiver pump-up test gives a practical estimate. It is best measured as a trend rather than against the brochure figure, which is taken under test conditions.

Discharge and oil temperature

Log airend discharge temperature at full load with the room temperature alongside it. In oil-injected screw compressors the normal working band is typically about 75–95 °C, with alarm and trip set points higher, depending on the model. A rise of several degrees at the same ambient and load, with the cooling system proven clean, is a meaningful sign.

Vibration

Overall vibration velocity at the bearing housings, read in horizontal, vertical and axial directions, gives a trend. Spectrum analysis and enveloping or high-frequency methods pick up bearing defects earlier. Severity guidance in the ISO 10816 and ISO 20816 series helps classify readings, but the machine’s own baseline matters most. Our vibration monitoring guide explains the methods.

Oil analysis

Regular oil samples show wear metals, water content, viscosity change, oxidation and contamination. A steady rise in iron or bearing-cage elements between samples is often the earliest internal warning. Water and oxidation tell you whether the oil itself is shortening bearing life.

Mechanical checks when stopped

With the machine safely isolated and depressurised, the rotors can be turned by hand for roughness, and shaft end-play and radial play checked with a dial indicator. These checks need care and the maker’s limits, but they confirm what the running data suggest.

How fast is it getting worse?

The rate of change decides urgency. Slow, steady drift in capacity and specific power over many months is typical of gradual clearance wear and can be planned into a shutdown. A fast rise in vibration, a sudden new noise, a jump in wear metals or a run of high-temperature trips suggests a bearing in distress, and the risk of a rotor contact grows quickly.

Plot each measurement over time and mark services, oil changes and repairs on the same chart. A step change straight after an oil change or a separator replacement points to a package cause; a step change with no event behind it deserves attention at the airend. The condition monitoring page shows how to set baselines and escalation points.

Decision table: continue, investigate, plan or act now

Weighing the energy cost of a worn airend

A worn airend that still runs reliably can still be expensive. Over its life, the electricity used by an industrial compressor typically costs many times its purchase price, so a modest rise in specific power adds up. The method is simple: measure today’s specific power, estimate what it would be after overhaul (from the machine’s earlier baseline or the maker’s data), and multiply the difference by annual air produced and your electricity tariff.

If that yearly saving, plus the reduced risk of an unplanned breakdown, is large compared with the overhaul cost, the case for acting is strong. If the saving is small and condition is stable, continued running with monitoring may be the better choice. Our overhaul versus replacement decision tool and our guide to maintenance reliability and condition monitoring help put real numbers on both sides.

Local operating conditions that speed up wear

In Telangana and Andhra Pradesh, summer compressor-room temperatures can be very high, and many plants and sites are dusty. Both shorten airend life. Hot rooms push discharge temperature up and age the oil; dust that passes a tired or poorly sealed air filter acts as an abrasive in the bearings. Coastal sites add humidity and more water in the oil.

Good ventilation, correct air-filter maintenance, the right oil changed on time and checks on the thermostatic valve and cooler do more for airend life than any rebuild. Factor these conditions in when you judge whether a machine is wearing faster than it should.

Typical trigger levels for trending airend condition

The decision table above describes patterns; this table puts typical numbers on them. The levels are common guidance, not fixed rules, expressed as change from the machine’s own baseline at the same pressure, load and room temperature. The maker’s published limits take precedence, and absolute vibration readings should also be checked against the severity zones of ISO 20816 (formerly ISO 10816).

One indicator in the “watch” column rarely justifies removing an airend. Two or more moving together, with package causes already ruled out, is the usual basis for planning.

Measuring FAD loss with a receiver pump-up test

A pump-up test needs no flow meter. It times how long the compressor takes to raise a known volume through a known pressure range. It is not as accurate as an ISO 1217 test, but repeated the same way each time it shows the trend reliably.

1. Calculate the test volume: receiver nameplate volume plus the pipework between the compressor and an isolating valve. For pipe, volume = π × (bore ÷ 2)² × length.
2. Close the isolating valve so no air leaves to the plant. Check that drains are not discharging during the test.
3. Vent the receiver to below the start pressure, start the compressor and let it reach full load. Make sure the unload or VSD setpoint is above the upper test pressure, so the machine stays fully loaded throughout.
4. Time the rise between two fixed pressures, for example 5.0 and 7.0 bar(g), with a calibrated gauge. Note intake and receiver temperatures.
5. Repeat twice and average. Use the same pressures and volume for every future test.

Formula: FAD (m³/min) = V × (p2 − p1) ÷ p0 × (T intake ÷ T receiver) × 60 ÷ t, where V is in m³, pressures in bar, p0 = 1.01325 bar(a), temperatures in kelvin and t in seconds.

Worked example (illustrative): test volume 2.1 m³ (2.0 m³ receiver plus 0.1 m³ of pipework); rise from 5.0 to 7.0 bar(g) in 31 seconds; intake 35 °C (308.15 K); receiver 40 °C (313.15 K).

FAD = 2.1 × 2.0 ÷ 1.01325 × (308.15 ÷ 313.15) × 60 ÷ 31 = 4.145 × 0.984 × 1.935 = 7.89 m³/min (about 279 cfm).

If the same test gave 9.2 m³/min when the airend was known to be healthy, the loss is (9.2 − 7.89) ÷ 9.2 = 14%. That is beyond the “plan intervention” level in the table above, provided the inlet filter, inlet valve and minimum-pressure valve have been checked and the motor speed is correct. A slipping belt or a wrong VSD speed limit reduces FAD in the same way as internal wear.

Overhaul, exchange or replace: worked lifecycle comparison

Once an airend needs major work, there are three usual routes: overhaul your own airend, fit an exchange airend (a ready rebuilt or new unit swapped for yours), or replace the compressor. Repair price alone does not decide it; downtime and energy usually matter more. All rupee figures below are illustrative inputs, not prices.

Common inputs (illustrative): 75 kW class machine running loaded to meet a steady demand of 12 m³/min; 6,000 hours per year; tariff ₹8.50 per kWh; standby rental air ₹9,000 per day while the machine is out; 8-year comparison, undiscounted for simplicity.

The new machine saves 43,200 kWh a year, worth ₹3,67,200. Against the exchange route its extra upfront cost is ₹30,27,000 − ₹8,36,000 = ₹21,91,000, so simple payback is 21,91,000 ÷ 3,67,200 = 6.0 years. Against the overhaul route it is (30,27,000 − 7,85,000) ÷ 3,67,200 = 6.1 years.

What this shows:

• Between overhaul and exchange, the gap here is only ₹51,000. If lost production during downtime costs more than rental air, or no rental is available, the exchange wins quickly.
• Energy is about ₹43 lakh a year, so a 0.6 kW per m³/min difference outweighs the repair price difference within the period. If the machine will run for eight years or more at this duty, replacement is worth costing seriously.
• If demand is falling or the machine runs a single shift, the energy difference shrinks and the overhaul or exchange becomes the better choice.
• If the housing bores are damaged, an overhaul may not be possible at all; the choice is then exchange or replacement.

For a discounted version of this comparison, use the overhaul-versus-replacement calculator further down this page. The route itself, and what each involves, is covered in the rebuild, exchange or replace guide.

If the airend has already failed: the first 24 hours

A seized or noisy airend is often followed by decisions made in a hurry that destroy evidence or add damage. These steps protect the machine and make the repair route clear.

1. Do not restart it to “see what happens”. A locked or rubbing rotor can shear the coupling, damage the motor or crack the housing on a second start.
2. Isolate, lock out and depressurise. Confirm zero pressure in the separator tank before opening anything.
3. Record the controller data before power is removed for long: fault history, last discharge temperature, running and loaded hours, and any trip messages.
4. Take an oil sample from the tank or oil filter housing into a clean bottle, and keep the oil filter when it is removed. Metal type and size point to bearing, rotor or gear failure.
5. Turn the rotors by hand at the coupling or pulley, with the drive isolated. Note whether they are seized, tight in one spot, rough or free with knocking. Check shaft end-play with a dial gauge if one is available.
6. Look upstream for the cause: the inlet filter and inlet valve for debris or a broken part, oil level and colour, cooler blockage, and thermostatic valve condition. A new airend fitted to a package with the original fault will fail again.
7. Arrange temporary air if production depends on it, sized from the failed machine’s actual duty, not its nameplate.

Then send the repairer photos of the nameplate and airend tag plate, the controller fault list, the oil sample result or a photo of the oil and filter, a description of the hand-turn check, and a short video of any noise heard before the failure. With these, a repairer can usually say whether the airend is a candidate for overhaul or whether an exchange or replacement should be costed. The overhaul process itself is described in the airend overhaul and rebuild guide, and common failure mechanisms in airend failure modes.

How Reddy Compressors helps

Reddy Compressors, based in Tarnaka, Secunderabad, and serving industry since 2002, is an independent multi-brand service and spares specialist. We are not an authorised dealer of any brand, so our view on whether an airend needs overhaul is neutral. We have tie-ups with all major and reputed brands. We work on Atlas Copco, Ingersoll Rand, ELGi, KAESER, Chicago Pneumatic, Gardner Denver, Sullair, Doosan, Kirloskar and other screw compressors.

• Site visits to record running condition: pressure, temperature, current, oil condition, noise and vibration.
• Package checks first: filters, inlet valve, separator, cooler, thermostatic and minimum-pressure valves, coupling and settings.
• Air audits to measure demand, leakage and the real performance of your compressors.
• A clear recommendation: continue and monitor, fix the package, plan an overhaul, or consider exchange or replacement.
• Overhaul, spare parts identified from the nameplate, rental compressors to cover downtime, and new or refurbished replacements where needed.

Our service teams cover all of Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India. Call +91 99590 60718 or +91 92468 88812, or email reddycompressors@gmail.com.

What to send us

• Compressor make, model, serial and a nameplate photo; airend plate details if visible.
• Running hours, loaded hours and last airend or bearing work.
• The symptom and when it started: low air, high temperature, noise, vibration, trips or oil carryover.
• Recent controller readings or alarm history, with room temperature.
• Oil type, last change date and any oil analysis reports.
• A short video with sound of the machine running under load, if possible.

Frequently asked questions

How do I know if my screw compressor airend is bad?

Look for a combination of signs: less air delivered for the same power, discharge temperature creeping up, a new rumble or whine from the airend, rising vibration at its bearings, wear metals in oil analysis and metallic particles in the oil filter. First rule out the air filter, inlet valve, cooler, separator and settings, because these cause the same symptoms more often than the airend does.

When should a compressor airend be overhauled?

Overhaul when trended evidence shows bearing or clearance wear, not simply because the hours have been reached. Use the maker’s recommended interval as a planning point, then decide on measured capacity, specific power, temperature, vibration and oil analysis. Act urgently if vibration or wear metals rise quickly, noise changes suddenly or the machine trips repeatedly on high temperature.

Why is my screw compressor running hot?

The most common causes are a fouled oil cooler, a failed or slow fan, a hot or poorly ventilated room, low oil level, a sticking thermostatic valve, a blocked oil filter or degraded oil. A worn airend can also run hotter because internal leakage and bearing friction add heat. Check and correct the cooling and oil system before suspecting the airend.

Why is my compressor not building pressure?

Usually the cause is outside the airend: more air demand or leakage than before, a blocked air filter, an inlet valve not opening fully, a wrong pressure setting, belt slip or a restricted separator. If all of these are proven correct and measured air delivery is still low with normal motor speed, internal clearance wear in the airend becomes a likely cause.

Can oil analysis predict airend bearing failure?

Oil analysis is one of the earliest warnings available. A steady rise in iron and bearing-related metals between samples, along with water or oxidation, suggests wear is under way and the oil may be accelerating it. It works best when samples are taken regularly from the same point and trended, and when combined with vibration and temperature readings.

Is it worth overhauling an old compressor airend?

It depends on the rotor and housing condition, availability of bearings and seals, the energy performance of the machine and how much air you now need. If the compressor is still the right size and parts are available, overhaul is often worthwhile. If it is obsolete, oversized or has poor specific power, compare the overhaul with a replacement compressor.

What is the airend on a rotary screw compressor?

The airend is the compression element of a rotary screw compressor. It contains a pair of meshing helical rotors, male and female, running in a close-clearance housing with bearings at each end and a shaft seal at the drive end. Air trapped between the rotor lobes is carried along and squeezed into a shrinking space until it leaves at the discharge port. In oil-injected machines, oil seals the clearances, cools the air and lubricates the bearings. Its condition sets how much air the compressor delivers for each kW.

The airend on our rotary screw compressor failed. Who can fix it?

An independent multi-brand compressor service specialist or the maker’s service network can usually assess it. Reddy Compressors works on airends of all major makes from Secunderabad. Before calling, isolate and depressurise the machine, do not restart it, keep an oil sample and the oil filter, and note whether the rotors turn by hand. Send nameplate photos, the controller fault list and any recordings of the noise. That evidence decides whether overhaul, an exchange airend or replacement is the sensible route.

Is airend rebuilding cheaper than an exchange airend?

The repair price for rebuilding your own airend is usually lower, but it often takes longer, and the machine is out of service while the rotors and housing are inspected and parts arrive. An exchange airend shortens downtime because the unit is prepared in advance. Compare the total: repair cost plus the cost of rental air or lost production for each day out of service. Where downtime is expensive, the exchange often costs less overall.

How much free air delivery loss justifies an airend overhaul?

There is no single figure, but a common guide is to plan intervention when measured FAD is about 10% below the machine’s own healthy baseline at the same pressure, with package causes already ruled out. The decision should also weigh specific power, vibration and oil analysis trends. Use the same test method each time, such as a receiver pump-up test between fixed pressures, so the trend is reliable even if the absolute figure is approximate.

Related pages

• Airend overhaul and rebuild: process and options
• Airend failure modes and causes
• Compressor vibration monitoring
• Predictive and condition-based maintenance
• Rotary screw compressor service and reliability
• Overhaul versus replacement decision tool