Air Compressor Airend Replacement & Overhaul

Airend is the most critical component of screw compressors, also the most expensive. When airend has problems, you face the decision of repair or replace.

Airend Components

Housing, male and female rotors, bearings, seals, some models also have timing gears. Male rotor 4 lobes driving female rotor 6 flutes is most common configuration. Bearings, imports use SKF, FAG mostly. Domestic use Luoyang LYC or Harbin HRB. Oil-injected machines rely on oil film to transmit power. Oil-free machines must have timing gears.

Core Component

Airend Assembly

Airend Life

Manufacturers' stated design life 40,000 to 60,000 hours. Three shifts, about 6 years to overhaul cycle.

Lubricating oil's impact on life gets underestimated by many. Mineral oil 8,000 hours is the limit. Run longer and oil acidifies. Bearings and seals both get corroded. Full synthetic is expensive but can go 16,000 hours. Overall airend life extends with it.

Intake filtration is another easily overlooked point. Dusty shops with air filter not changed often enough, fine particles get into compression chamber. Rotor coating wears grooves much faster. airends.com technical materials mention some severely dusty installations, air filter blocks in a week, must change.

Machines with discharge temp long-term over 212°F, aging speed is visible. Normal should be controlled between 167 to 203°F.

Analysis

Oil Sample Testing

Monitoring

Discharge Temperature

Fault Symptoms and Diagnostics

Discharge volume drop is easiest to notice. Same pressure setting, loading time gets longer. Internal leakage is definitely increasing.

Noise changes, listen carefully. When bearings start wearing there's continuous humming. Rotor coating peeling is irregular scraping sound. Worst is metal impact sound. That means rotors are already touching. Run more and they'll seize.

Vibration monitoring is more reliable than listening. Can quantify. Bearing housing vibration velocity under 0.18 in/s is normal. Over 0.44 in/s must shut down. A $100-200 handheld vibration meter is enough. Key is measure regularly, keep records, watch trend changes. Last month measured 0.13, this month 0.14, next month 0.16. This steady rising trend is more concerning than one-time reading of 0.2.

Oil analysis also very useful. Oil sample lab test costs about $50. Report has iron content, copper content, particle count. High iron means rotors or housing wearing. High copper points to bearing cage. Kaishan USA technical documents specifically emphasize metal fines appearing in oil sample is signal of impending bearing failure. So regular oil testing is key for judging airend life.

Temperature abnormality is also a signal. Same environment and load, discharge temp 20°F higher than before, internal efficiency is probably dropping.

Oil consumption suddenly goes up, separator differential normal, return line clear, then it's airend internal oil seals leaking.

Repair or Replace

This question gets discussed a lot on foreign tech forums. Practical Machinist forum has a thread, an Atlas Copco machine ran 48,000 hours, airend bearings failed. Called Atlas Copco tech to look. Conclusion was bearing failure, repair probability not good. Tech said bearings can't be bought from Atlas Copco, even if found elsewhere, repair success probability not high. Owner also felt at 48,000 hours, not worth the trouble.

Thread replies, someone says screw machine manufacturers deliberately create barriers on repair and parts support, treating aftermarket as profit center. Want to buy a part and it's difficult. Others say they had similar situation, rented a backup from dealer, fixed their own airend.

airends.com has an Ingersoll Rand SSR 3000 diesel airend gear failure case. Photos show after male rotor discharge end thrust bearing wore, rotor axial float, dragging gear toward discharge end housing moved about 1/8 inch. Gear face and gearbox housing rubbed, leaving obvious burn marks. Roller bearing inner race also discolored. Eventually gear stripped teeth on startup. This situation, thrust bearing is the root cause, but by discovery time whole gearbox needs replacing. Repair cost straight up multiplied several times.

For 50 hp class, Atlas OEM airend is about $6,000, Ingersoll Rand, Sullair also around $4,500, Fusheng, Kaishan around $3,000. European market same power range OEM airend roughly €4,000 to €6,500, North American market $4,500 to $7,000. Overhaul cost depends on damage extent. Just changing bearings and seals is about 30% of new airend price. Rotors need repair, add another 20-30%. Housing boring is another expense.

Fluid Aire Dynamics technical article gave a rough estimate: airend overhaul cost is about 20% to 25% of buying a new complete compressor. Severe damage might approach 50%. Small power machines overhaul cost percentage runs higher. Large power machines overhaul is relatively more economical.

Overhaul Process

Airend removed and shipped to repair shop. Before disassembly, all component positions and orientations marked.

Inspection phase, rotor clearances measured with feeler gauge, surface condition photographed. Bearings checked for play and raceway, problems found, direct replacement. Housing bore checked for scoring.

Inspection

Rotor Condition

Bearings

Replacement Parts

Assembly

Precision Rebuild

Bearings and seals all replaced with new, this is routine. Rotor coating depends on condition. Light cases polished. Heavy cases spray coating repair. More serious sent to specialized factory for grinding. Housing scoring can be bored oversize and fitted with repair rotors.

Atlas Copco website technical note mentions airend repair needs specialized process and tools, only OEM has capability. Rotor coating is core technology. After Teflon coating wears, replacing with new airend is best choice. Disassembling airend without professional knowledge almost certainly leads to increased energy consumption, shortened life, compressed air oil content exceeding spec. This statement has element of advertising for their own airend sales, but also indicates airend overhaul really has technical threshold. Not something random mechanic can do.

Assembly requirements are high. Bearing preload, rotor positioning, all clearances have standards.

Test run first unloaded, listen for sound, measure vibration. No problems then loaded test measuring discharge volume.

Short cycle two weeks. Rotors sent out or waiting for parts can be over a month. Legitimate repair shops give six months to one year warranty, covering bearings, seals, and rotor repair portion. Get this in written contract.

New Airend Replacement

Compatibility needs verification. Interface dimensions, mounting positions, drive method all need to match. OEM parts expensive but hassle-free. Arrives, install directly. Alternative parts cheaper, need to confirm compatibility. Find manufacturer with mature solutions.

Belt drive machines adjust belt tension, press middle section deflection 3/8 to 5/8 inch. Direct drive machines check coupling alignment.

New airend first 500 hours is break-in period. First oil change around 300 hours, carry away metal debris.

Prevention

Record vibration and temperature weekly. Plot trend chart. Catch deterioration signs early. Small fixes solve it. Drag until big problem, downtime loss plus repair cost multiplies several times.

Change oil and filters on time. Air filter clogged, intake restriction increases. Oil filter clogged, both lubrication and cooling affected. Some users run filter until alarm and keep going. Filter money saved isn't enough for a fraction of airend repair.

Sustained over-pressure is chronic damage to airend. Each 15 psi higher, power consumption up about 7%, airend load increases with it. Good enough is good enough. No need to crank it up.

Frequent start-stop is wear on bearings and seals. Can run continuous, try to run continuous. Applications with big load swings, use VFD or increase receiver capacity.