Pump bearings: arrangement, lubrication and failures
A pump bearing sees two loads at once: a sideways force from the impeller that depends on where on the pump curve it runs, and an axial push from the pressure difference across the impeller. Running a pump far from its best point is the quickest way to shorten bearing life.
Typical duty
| Speed | Load | Temperature | Typical life |
|---|---|---|---|
| 1,450 – 3,600 rpm | Radial thrust that changes with flow, plus hydraulic axial thrust | Oil sump 50 – 80 °C | 17,500 h (ASME B73.1) · 25,000 h (API 610) |
How the bearings are arranged
End-suction process pump (ASME B73.1)
Used when: Chemical and general process duty, light to medium service
The pump shaft rides on two bearings in an oil-filled frame. The one nearest the coupling takes the axial push and holds the shaft in place. The one nearest the impeller only carries sideways load and is free to slide.
Heavy duty refinery pump (API 610)
Used when: Refinery and high-temperature or high-pressure duty
The standard asks for a more robust version: a matched pair of 40° angular contact bearings takes the thrust, and a roller bearing carries the sideways load. The roller bearing lets the shaft grow lengthwise as the pump heats to process temperature.
Vertical deep-well pump · thrust head
Used when: Vertical turbine and deep-well pumps driven from the top
The whole weight of the pump shaft plus the hydraulic thrust hangs from one big thrust bearing at the top of the motor. A tall column of shaft below is guided by plain bushings.
Vertical inline pump · sealed for life, spring preloaded
Used when: Light duty chemical and paper mill service
The impeller is open or semi-open and the bearings are small, greased for life and sealed. A spring pushes on one of them so the shaft end play stays small and the balls do not skid.
Medium duty process pump · paired angular contact thrust bearings
Used when: Refinery service where reliability matters, closed impeller
The impeller is closed and has wear rings, so the axial thrust is small and controlled. A matched pair of angular contact bearings carries it, and a ball or roller bearing carries the radial load. The oil bath or oil ring cools the bearings.
Slurry pump · matched tapered roller bearings
Used when: Heavy slurry service with big combined radial and axial loads
Slurry pumps push very hard on the bearings. A pair of steep-angle tapered roller bearings, face to face or back to back, carries both the sideways and the axial load in one compact set.
Slurry pump · spherical roller and spring preloaded thrust bearing
Used when: Very heavy loads, thrust can reverse at start or stop
Spherical roller bearings carry the radial load. A spherical roller thrust bearing carries the axial load, and a spring keeps it under load even when the thrust reverses at start-up or stop.
Magnetic drive pump · ball bearings on the drive shaft
Used when: Where shaft sealing has been a problem
There is no shaft seal, so no leak. Inside the pump the impeller shaft runs in plain bearings lubricated by the pumped liquid. Outside, the drive shaft sits in ordinary ball bearings which can be spring preloaded.
Common failures and how to prevent them
| Failure | What you see | Why it happens | Prevention |
|---|---|---|---|
| Water in the bearing oil Moisture corrosion | Milky oil, rust on the raceways, rust-coloured pitting. | Leaking mechanical seal, wet plant washdown, or condensation in a breather. | Fix seal leaks promptly, use bearing isolators and a sealed breather or expansion chamber, and check oil for water regularly. |
| Oil too hot Overheating & lubricant breakdown | Dark, burnt-smelling oil, blue tint on the rings. | Oil level too high, oil ring not turning, or process heat conducted down the shaft. | Set the oil level correctly, check that the oil ring runs, and add cooling or a heat deflector for hot service. |
| Dents on standby pumps False brinelling | Evenly spaced dents on a pump that mostly sits idle. | Vibration from neighbouring running machines while the standby pump is stopped. | Rotate standby pumps regularly or start them on a schedule, and isolate them from vibration. |
| Pipe strain and misalignment Misalignment & distorted seats (edge loading) | Off-centre wear track, hot bearing at one end. | Pipe forces on the nozzles, soft foot, or poor coupling alignment. | Check pipe strain at the nozzle, soft foot and alignment, and remeasure when hot. |
| Fatigue from running off the best point Subsurface-initiated fatigue | Spalling at the thrust or radial bearing well before the rated life. | Operating far from the pump’s best efficiency point raises radial thrust and recirculation loads. | Operate within the recommended flow range and avoid prolonged operation at minimum flow. |
| Cage damage in the thrust pair Cage damage | Broken or worn cage pockets, smearing of the balls. | Pair mounted the wrong way round, insufficient minimum axial load, or sudden thrust reversal. | Mount the pair as marked on the bearing outer rings and keep to the minimum load. |
| Fluting on inverter-driven pumps Current leakage erosion (fluting) | Washboard marks on the raceways. | Shaft currents from the drive. | Same measures as for the motor: insulated bearing or grounding ring. |
Lubrication
Oil bath
Most process pumps use mineral or synthetic oil in a sump. The oil level is set at the centre of the lowest rolling element. A constant-level oiler keeps it there.
Viscosity
ISO VG 46 or 68 is typical. Hot service and high speed change the answer, so follow the pump maker.
Oil change
Often every 3 to 6 months or a few thousand hours on mineral oil. Synthetic oil can run longer. Look at colour and water content between changes.
Oil mist and ring oilers
API pumps may be ring-oiled or oil-mist lubricated. A stopped oil ring is a sign of trouble: check the ring, shaft level and speed.
Grease
Used on small pumps and on vertical thrust heads in some designs. Quantity and interval follow the bearing and speed.
Oil mist
Directed mist when ndm is above 300,000. The housing needs a 3 mm vent opposite the entry. Purge mist keeps an oil bath; pure mist has no sump.
Air-oil
Metered oil carried by air to each bearing for low temperature and high speed.
Grease
Lithium soap grease, NLGI 2 for most, 3 for vertical shafts or heavy vibration, 1 for cold or large slow bearings. Do not mix thickeners.
Sealing
- Bearing isolators: Two-piece labyrinth with a rotor and stator that never touch. They keep spray out and oil in and do not wear the shaft. The standard choice for process pumps.
- Lip seals: Low cost and tight at low speed, but they wear a groove in the shaft and add friction. Replace at every overhaul.
- Mechanical seal: Seals the pumped fluid at the stuffing box. When it leaks it is the usual source of water and chemicals in the oil.
- Breather: Use a desiccant or expansion-chamber breather so that the sump can “breathe” without drawing in humid air.
- Radial lip seals: Spring-loaded rubber lip. Needs oil at the lip and a smooth shaft. Life is usually short and friction is high.
- Labyrinth seals: No contact, little friction, long life. Some designs vent the housing naturally. Fit them where oil mist or dust is present.
What to watch
- Vibration at the bearing frame in horizontal, vertical and axial directions.
- Oil temperature and oil condition (colour, water, particles).
- Where the pump runs on its curve. Low-flow operation raises load on the bearings.
- Sight glass and oiler level on a daily round.
- Evaluate bearing loads at the best efficiency point, and at the minimum and maximum rated conditions, not just at BEP.
- Watch the oil level at rest and the water content (below 200 ppm).
Questions
Which bearings are used in centrifugal pumps?
End-suction process pump (ASME B73.1): Chemical and general process duty, light to medium service. Heavy duty refinery pump (API 610): Refinery and high-temperature or high-pressure duty. Vertical deep-well pump · thrust head: Vertical turbine and deep-well pumps driven from the top. Vertical inline pump · sealed for life, spring preloaded: Light duty chemical and paper mill service.
What are the common bearing failures in centrifugal pumps?
Water in the bearing oil: Leaking mechanical seal, wet plant washdown, or condensation in a breather. Oil too hot: Oil level too high, oil ring not turning, or process heat conducted down the shaft. Dents on standby pumps: Vibration from neighbouring running machines while the standby pump is stopped.
How should centrifugal pumps bearings be lubricated?
Oil bath: Most process pumps use mineral or synthetic oil in a sump. The oil level is set at the centre of the lowest rolling element. A constant-level oiler keeps it there. Viscosity: ISO VG 46 or 68 is typical. Hot service and high speed change the answer, so follow the pump maker.