Why acceptance criteria must be set before the job starts
If nobody writes down what “good” looks like before the airend is opened, every result afterwards becomes a matter of opinion. The customer remembers the machine as it was when new; the workshop compares it with the day it came in damaged. Neither comparison is useful.
Agree a short acceptance plan at the quotation stage. It should state which checks will be done, at what pressure and load, with which instruments, and what range counts as acceptable. Where the manufacturer’s data for that package is available, use it. Where it is not, use the best available baseline: commissioning records, previous service readings or readings from a sister machine on the same site.
Checks at each stage of the rebuild
Validation is not a single test at the end. Most problems are cheaper to catch while the element is still on the bench.
| Stage | Check | What it tells you |
|---|---|---|
| After assembly, before fitting | Hand rotation, smoothness and torque feel | Bearings seated, no rotor contact or binding |
| After assembly | End clearance and bearing preload as specified for that element | Rotors positioned correctly in the housing |
| After fitting to package | Alignment of coupling or belt drive, belt tension | No external load forcing the new bearings |
| First start | Direction of rotation, oil pressure build-up, leaks | Basic safe operation |
| Warm-up run | Oil and discharge temperature trend | Lubrication and cooling circuit working |
| Loaded run | Pressure, current, vibration, noise, oil carry-over | Airend and package performing under real duty |
| Follow-up | Repeat readings after early running hours; oil sample where justified | Bedding-in complete, no developing fault |
Temperature: the most useful single reading
On an oil-injected screw compressor, a normal discharge temperature is typically about 75–95 °C once the machine is warm and loaded. A rebuilt element that runs steadily in that band, with the thermostatic valve opening and the cooler doing its job, is a good sign.
Watch for these patterns during the loaded run:
- Temperature climbing steadily without levelling off – suspect the oil cooler, thermostatic valve, oil level, wrong oil or restricted oil filter before blaming the airend.
- Temperature too low on long running – a stuck-open thermostatic valve can let condensate collect in the oil, which harms the new bearings.
- Hot spot at one bearing end – check preload, alignment and lubrication to that bearing.
Record the ambient temperature at the time. A machine tested on a cool morning in Secunderabad will behave differently in a 45 °C afternoon in an enclosed plant room, so readings must be compared at similar conditions or corrected with care.
Vibration and noise
New bearings should make a rebuilt airend quieter and smoother than it was before the failure. Take vibration readings at fixed, marked points on the airend and motor so that later readings can be compared like for like. Note the pressure and load at each reading.
- A steady, low level that does not change with time is normal.
- Rising vibration in the first running hours points to alignment, preload or a soft foot.
- Rough, gritty noise suggests bearing damage, contamination or debris left in the circuit.
- Knocking on load or unload may come from the inlet valve, minimum-pressure valve or coupling rather than the element.
Pressure, flow and power
The real test of a rebuild is whether the package delivers the free air delivery (FAD) it should at the working pressure, without drawing excess power. On site, the practical checks are:
- Confirm the machine reaches its rated working pressure and unloads correctly.
- Time the pressure rise in a receiver of known volume from one pressure to another with all consumers isolated – a simple pump-up test that gives an indication of delivered flow when repeated the same way each time.
- Record motor current or kW at full load and at unload.
- Calculate specific power (kW per m³/min) if you have reliable flow and power readings, and compare with the baseline.
Be careful with comparisons. Pressure has a large effect on power – roughly 7% more for each extra bar – so a test at a different setting does not prove anything. Formal FAD measurement to a standard method needs proper instruments and controlled conditions; where that is required, agree it in the acceptance plan.
When the airend is fine but the package is not
A perfectly rebuilt element can still give poor results if something around it is wrong. Before rejecting a rebuild, rule out these secondary causes:
- Restricted intake filter or inlet valve not opening fully
- Separator element blocked or failed, causing pressure drop or oil carry-over
- Minimum-pressure valve sticking
- Oil cooler or aftercooler fouled with dust and oil
- Belt slip, wrong pulley ratio or motor problems
- Controller set points, pressure transducer drift or unload settings
- Leaks in the plant network that make the compressor appear short of flow
This is why acceptance looks at the whole package. A good report separates what was proved about the airend from what was found elsewhere.
What a good acceptance record contains
- Machine identity: make, model, serial, airend type and serial where visible
- Work done, parts fitted and measurements taken at assembly
- Instruments used and test conditions: pressure, ambient temperature, load
- Readings at each stage, with the agreed acceptance range beside them
- Anything that was not tested, and why
- Package findings and recommended follow-up actions
- Commissioning date, running hours at handover and the first follow-up check date
Keep this with the asset. It becomes the baseline for future service and the evidence if any question arises later.
Exchange and new elements need checks too
It is tempting to skip validation when the element is new or remanufactured, because the work was done elsewhere. That is a mistake. Most early failures of a fresh element come from installation, not manufacture: a coupling left misaligned, the wrong oil, debris left in the oil cooler or pipework from the original failure, or a separator and oil filter that were not changed at the same time.
- Flush or clean the oil circuit and cooler after any failure that produced metal debris.
- Fit a new oil filter, separator element and the correct lubricant before first start.
- Check the element’s rotation direction and drive ratio match the package.
- Run the same warm-up and loaded checks as for a rebuild, and record them.
Common acceptance mistakes
- Testing for a few minutes cold and calling it done.
- Comparing a reading at 7 bar with an old one at 8 bar without allowing for the difference.
- Not recording ambient temperature, which matters greatly in Indian summers.
- Using a gauge of unknown accuracy, then treating its reading as proof.
- Blaming the airend for a network leak or a stuck valve elsewhere.
How Reddy Compressors helps
From our base at Tarnaka, Secunderabad, our team of more than 20 service engineers and technicians works on compressors across Telangana and Andhra Pradesh, with parts and equipment supplied across India. We are an independent multi-brand specialist, not an authorised distributor, and we work on ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar, Gardner Denver and other makes. We have tie-ups with all major and reputed brands.
- We agree the checks with you before the work starts and record them during the job.
- We commission rebuilt and exchange airends on site, including alignment, warm-up and loaded running.
- We check the rest of the package – coolers, valves, separator, controls – so a package fault is not mistaken for a bad rebuild.
- We return for follow-up readings and can include the machine in a service or AMC plan.
- Where formal performance measurement is needed, we say what can and cannot be proved on site.
What to send us
- Make, model, serial and nameplate photo of the compressor
- Details of the rebuild or exchange and who carried it out
- Any previous commissioning or service readings
- Your working pressure and typical air demand
- Current symptoms, if the machine is already running after a rebuild
Frequently asked questions
How do I know if my airend rebuild was done properly?
Look at the record as well as the machine. A sound rebuild comes with measured clearances, a list of parts fitted, and readings from a warm, loaded run: discharge temperature in the normal range, steady vibration, no leaks or oil carry-over, and pressure and power similar to the previous baseline at the same setting. If the only evidence is that it starts and runs, ask for proper checks.
What discharge temperature is normal after an airend overhaul?
For an oil-injected screw compressor, typically about 75–95 °C once warm and loaded, depending on ambient temperature and design. More important than one number is the trend: it should rise during warm-up and then level off. A temperature that keeps climbing usually points to the cooler, thermostatic valve, oil level or oil filter rather than the rebuilt element itself.
Why is my compressor still short of air after an airend rebuild?
Often the cause is not the airend. Check the intake filter and inlet valve opening, the separator element, the minimum-pressure valve, belt slip, controller set points and pressure transducer accuracy. Plant leaks also make a compressor look undersized. Measure at the compressor outlet first so you can tell whether the problem is inside the package or in the network.
Should vibration be lower after a rebuild?
Usually yes, because worn bearings have been replaced. Take readings at marked points on the airend and motor, at a stated pressure and load, and repeat them after early running hours. Steady, low readings are a good sign. Rising vibration suggests alignment, bearing preload or mounting problems, and should be checked before it damages the new bearings.
Can you check a rebuilt airend done by another workshop?
Yes. We can inspect the installation, check alignment and package condition, run the machine warm and loaded, and record temperature, vibration, pressure and current readings. We will tell you what the readings show and what they cannot prove. We cannot vouch for work we did not carry out ourselves, but we can give you clear evidence for discussing it with the workshop concerned.
How long should I monitor a compressor after an airend overhaul?
Plan a closer watch during the early running hours, when bearings bed in and any assembly fault tends to show. Repeat temperature and vibration readings at an agreed point, check for leaks, and take an oil sample where the duty justifies it. After that, the machine can return to its normal service schedule with the acceptance record as its new baseline.
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.