Compressor bearings: arrangement, lubrication and failures
In a screw compressor the gas pressure pushes the rotors hard toward the suction end and sideways. The bearings are loaded all the time, and they run in the same oil that cools and seals the rotors. If the oil is dirty or hot, the bearings pay.
Typical duty
| Speed | Load | Temperature | Typical life |
|---|---|---|---|
| 1,500 – 15,000 rpm (rotor) | Very high axial and radial from gas pressure | Oil 70 – 100 °C | 40,000 – 60,000 h typical bearing life target |
How the bearings are arranged
Oil-flooded screw compressor · male rotor
Used when: Industrial air compressors with oil injection
The male rotor rides on a roller bearing at each end for sideways load. At the discharge end, a pair of angular contact ball bearings takes the heavy axial push from the gas pressure and keeps the rotor positioned within a few hundredths of a millimetre of the casing.
Discharge end · two angular contact balls, face to face
Used when: Moderate load, simple arrangement
Two matched angular contact bearings face each other. The inboard one carries the axial and most of the radial load. The outboard one is a back-up that stops the rotor moving if the thrust reverses.
Discharge end · roller bearing and one angular contact ball
Used when: Compact, popular layout
The roller bearing takes the sideways load and the angular contact ball takes the axial load. A radial gap round the ball bearing outer ring means it cannot take any sideways load, and the roller bearing, which is free to slide, cannot take any axial load. Each does only its own job.
Discharge end · roller bearing and tandem angular contact pair
Used when: High axial load on the male rotor
Two angular contact balls side by side share the axial load, so the axial capacity is higher than with one bearing.
Reverse thrust options for ball bearing arrangements
Used when: Thrust can reverse at start-up or high speed
At start-up the rotor can be pushed towards the discharge end. At very high speed the balls push the bearing rings apart. A third bearing prevents the rotor from rubbing on the housing.
Tapered roller pair · male and female rotor
Used when: High axial and radial load, male rotor
Two tapered roller bearings face to face. The inboard bearing takes the radial and axial load. The outboard one sets the end clearance and acts as the reverse thrust bearing.
Tapered roller · single bearing, back-up and ball options
Used when: No reverse load, or lighter duty
A single tapered roller bearing works only if the gas thrust is much larger than the reverse thrust that the radial load itself induces. Otherwise add a back-up bearing: same size, or a spring-loaded deep groove ball.
Suction end · roller, NJ, ball or spring loaded angular contact
Used when: The lightly loaded end of the rotor
The suction end sees less load. The most common choice is a single cylindrical roller bearing. Other choices suit light loads or add reverse thrust capability.
Gear shaft of a geared compressor
Used when: Speed increasing gear between motor and rotors
A gear shaft driven by the motor turns a pinion on the male rotor. Two layouts are common.
Common failures and how to prevent them
| Failure | What you see | Why it happens | Prevention |
|---|---|---|---|
| Oil contamination Abrasive wear | Scored raceways, premature wear on rollers, dark oil. | Clogged or bypassed oil filter, dirty intake air, wear debris. | Change the oil filter and separator on schedule and never run with a bypassed filter. |
| Hot oil Overheating & lubricant breakdown | Blue tint on rings, hard varnish deposits. | Cooler fouled, wrong oil, or running at high ambient temperature. | Keep the cooler clean, use the correct compressor oil, and watch the discharge temperature. |
| Smearing of lightly loaded rollers Adhesive wear (smearing & skidding) | Smeared bands on rollers and raceway. | Low radial load at part load or when unloaded, with a thin oil film at start-up. | Keep oil warm before loading, and follow the maker’s start and unload settings. |
| Cage damage in the axial pair Cage damage | Broken cage, ball smearing. | Liquid carry-over, sudden pressure changes, or the pair mounted the wrong way. | Avoid liquid slugging, drain condensate, and mount the pair per the marking. |
| Fatigue at high load Subsurface-initiated fatigue | Spalling on the thrust balls. | Running at higher pressure than the compressor was designed for, or loss of preload. | Do not exceed rated pressure and replace the pair as a set. |
| Fluting on drive-fed compressors Current leakage erosion (fluting) | Washboard marks on raceways. | Shaft currents from a variable-speed drive. | Insulated bearing at one end or a grounding ring. |
| Rotor end clearance drift and rub at start-up Excessive endplay and loose setting (scalloping) | Bright rub marks on rotor ends, rising discharge temperature, noise at start-up. | Thrust reverses at start-up or at high speed, or axial bearing clearance is larger than the rotor end clearance. | Use a reverse thrust or back-up bearing, keep axial bearing clearance smaller than the rotor end clearance, and set end clearance with matched spacers. |
Lubrication
Compressor oil
Mineral or synthetic compressor oil with the right viscosity grade for the machine. The same oil lubricates the bearings and cools the gas.
Filtration
Oil filters protect the bearings from particles. Follow the maker’s change interval.
Temperature
Hold the oil temperature in the maker’s range. Too cold lets water condense in the oil; too hot oxidises it.
Refrigerant compressors
Oil and refrigerant mix. Polyolester (POE) oils go with HFC-134a. The oil thins when diluted, so use the mixture viscosity in the life calculation.
Synthetic oils
PAG, PAO and POE give better heat stability. Their lower pressure-viscosity coefficient is balanced by higher viscosity at temperature, so film thickness is about the same.
Sealing
- Shaft seal: Lip seal or carbon-ring seal at the drive end. A worn seal leaks oil outside, or in the other direction draws air in.
- Internal sealing: Rotor tip clearance and oil injection seal the compression chamber. Bearing wear opens the clearance and drops efficiency.
- PTFE seals: Placed between the screws and the bearing housing to stop oil and gas moving across. Dry running and water injected compressors rely on them.
What to watch
- Discharge temperature and oil temperature.
- Differential pressure across the oil filter.
- Vibration spectrum, especially at the screw’s lobe-pass frequency and bearing defect frequencies.
- Oil analysis for wear metals and water.
- Watch oil temperature and pressure: a compressor bearing sees the same oil as the rotors.
- After any rebuild, verify rotor end clearance and look for contact marks on the rotor ends.
Questions
Which bearings are used in screw compressors?
Oil-flooded screw compressor · male rotor: Industrial air compressors with oil injection. Discharge end · two angular contact balls, face to face: Moderate load, simple arrangement. Discharge end · roller bearing and one angular contact ball: Compact, popular layout. Discharge end · roller bearing and tandem angular contact pair: High axial load on the male rotor.
What are the common bearing failures in screw compressors?
Oil contamination: Clogged or bypassed oil filter, dirty intake air, wear debris. Hot oil: Cooler fouled, wrong oil, or running at high ambient temperature. Smearing of lightly loaded rollers: Low radial load at part load or when unloaded, with a thin oil film at start-up.
How should screw compressors bearings be lubricated?
Compressor oil: Mineral or synthetic compressor oil with the right viscosity grade for the machine. The same oil lubricates the bearings and cools the gas. Filtration: Oil filters protect the bearings from particles. Follow the maker’s change interval.