Electric motor bearings: arrangement, lubrication and failures
An electric motor is the simplest bearing arrangement in industry. That is exactly why it fails from simple causes: the wrong grease, too much grease, currents through the shaft, and a bearing that cannot slide when the rotor warms up.
Typical duty
| Speed | Load | Temperature | Typical life |
|---|---|---|---|
| 750 – 3,600 rpm | Light to moderate radial, small axial | 60 – 90 °C at the bearing | 40,000 h and up (sealed-for-life small frames) |
How the bearings are arranged
Standard motor · ball bearings both ends
Used when: Direct-coupled or lightly belt-driven motors, frames 56 to about 225
The rotor shaft needs to be held in one place along its length, but it also grows longer as it warms. So one bearing is clamped (the locating bearing) and the other is allowed to slide a little, with a spring pushing it gently so the balls always roll rather than skid.
Large motor · roller bearing at the drive end
Used when: Belt-driven or heavily loaded motors, frames 160 to 355 and up
A belt pulls the shaft sideways with a lot of force. Ball bearings carry that poorly, so the drive end gets a roller bearing, which has a much larger contact area. Rollers cannot take axial load, so the other end keeps a ball bearing that holds the shaft in position.
Inverter-fed motor · protected from shaft currents
Used when: Any motor driven by a variable frequency drive, especially above about 30 kW
A drive switches voltage very fast, which puts a small voltage on the rotor shaft. When that voltage is high enough it jumps through the bearing’s oil film as a tiny spark. Thousands of sparks a second slowly pit the raceway into a washboard pattern. The cure is to give the current somewhere harmless to go.
Small motor · cross-locating pair
Used when: Short shafts, frames up to about 132
Both bearings are the same simple type. Each one stops the shaft moving in one direction, so together they hold it in place both ways. The tiny axial clearance inside the bearings absorbs the shaft growing as it warms.
Medium and large motor · relubricated, with grease escape
Used when: Frames from about 160 up, bearings that need regreasing
Open bearings can be topped up with fresh grease while the motor runs. The old grease must have somewhere to go, so a rotating disc throws it into a cavity in the end cover and out of a hole underneath.
Aluminium end shield · O-ring against ring creep
Used when: Motors with aluminium housings and heavy loads or high temperatures
Aluminium grows with heat more than twice as much as steel or cast iron. The bearing seat gets looser as the motor warms and the outer ring can slowly turn in it, wearing the seat away. A rubber ring in a groove presses on the outer ring and holds it still.
Motor with axial load both ways · angular contact pair
Used when: Gear or pump driven motors that push the shaft both directions
Two angular contact bearings face each other and clamp the shaft in both directions. The other end floats. If the radial load is big, a roller bearing takes it. If not, a ball bearing with a spring does.
Large motor · two roller bearings and a free ball bearing
Used when: Large machines with heavy radial load and some axial load
Roller bearings carry the heavy sideways load but cannot hold the shaft lengthways. A deep groove ball bearing is added only to locate the shaft. It sits loose in the housing so it never shares the radial load.
Very large machine · spherical roller and CARB
Used when: Very large motors and generators, shaft deflection or misalignment
A spherical roller bearing locates the shaft and tolerates tilting. At the other end a CARB toroidal roller bearing lets the shaft slide inside the bearing itself, so it does not need a sliding fit in the housing.
Small vertical motor · sealed ball pair
Used when: Small vertical motors, light axial load both ways
Two sealed deep groove balls. The lower one locates the shaft, the upper one is spring loaded and floats.
Larger vertical motor · angular contact locating bearing
Used when: Vertical motors, pump drives with downward thrust
Downward thrust from the rotor or pump has to be taken by a bearing that handles axial load well. An angular contact ball bearing does that. Three versions are common, depending on how heavy the thrust is and whether it can reverse.
Large vertical machine · spherical roller thrust bearing
Used when: Large vertical motors with heavy downward thrust
A spherical roller thrust bearing takes a very heavy downward force and tolerates some tilt. A cylindrical roller bearing carries the sideways load.
Common failures and how to prevent them
| Failure | What you see | Why it happens | Prevention |
|---|---|---|---|
| Fluting from shaft currents Current leakage erosion (fluting) | Evenly spaced stripes across the raceway, black or grey grease, a whining or humming noise. | Voltage on the rotor shaft discharges through the bearing, usually on inverter-fed motors. | Insulated bearing at the non-drive end, shaft grounding ring, drive filters and proper cable grounding, or hybrid bearings. |
| Grease breakdown from heat Overheating & lubricant breakdown | Hard, dark or dry grease. Blue or brown colour on the rings. Bearing hotter than the rest of the motor. | Wrong grease for the temperature, or too much grease churning, or a blocked fan cover. | Use the grease the motor maker specifies, keep to the relubrication quantity, and keep cooling air paths clear. |
| Dents from vibration while stopped False brinelling | Shiny, evenly spaced dents in the raceway at the spacing of the balls, on a motor that sat idle. | Vibration from nearby machines or from transport while the rotor was not turning. | Lock the rotor for transport, turn spare motors by hand regularly, and isolate standby machines from vibration. |
| Misalignment and soft foot Misalignment & distorted seats (edge loading) | Wear track that is off-centre or heavier on one side of the raceway. | Uneven feet, misaligned coupling, bent shaft or a skewed end shield. | Check soft foot, align with a laser or dial gauge, and check shaft runout. |
| Early fatigue from belt overtension Subsurface-initiated fatigue | Spalling at the drive-end bearing on belt-driven motors. | Belt tension too high or sheave too small, which multiplies the radial load. | Check belt pull against the motor’s permitted radial load and move to a roller bearing at the drive end if needed. |
| Skidding of lightly loaded rollers Adhesive wear (smearing & skidding) | Smeared, dull bands on the roller bearing raceway. | Cylindrical roller bearing carrying less than its minimum radial load. | Use the roller bearing only where a belt or coupling provides enough radial load, or fit a ball bearing instead. |
| Contamination through the seal Abrasive wear | Dull, scratched raceways and gritty grease. | Damaged or worn seals, missing drain plugs, dusty environment. | Replace seals at every overhaul, fit the correct IP-rated seal, and open drain plugs only as the motor maker specifies. |
| Outer ring creep in aluminium end shields Ring creep and loose fits | Polished or fretted outside surface of the outer ring, a worn and enlarged seat, rising vibration. | Aluminium expands more than twice as much as steel, so a seat that is tight cold is loose hot and the ring turns slowly in it. | Cut an O-ring groove off-centre in the seat and fit a rubber O-ring of about 70 Shore A, or use a steel insert. |
Lubrication
Grease type
Most motors use a mineral-oil grease with a lithium complex or polyurea thickener. Do not mix thickeners unless the maker confirms compatibility.
Sealed-for-life
Small frames are packed at the factory and not meant to be relubricated. The catalogue table below gives the grease life by frame.
Relubricable motors
Grease nipple at each end and a relief plug. Regrease while the motor is running with the plug open, then let it run an hour or two before closing the plug.
How much and how often
A common rule of thumb for the amount is G (grams) = 0.005 × D × B, where D is the bearing outside diameter and B its width, both in mm. The interval roughly halves for every 15 °C the bearing runs above 70 °C.
Vertical shafts
Use a stable grease and half the relubrication interval of a horizontal shaft. Contact seals are the right choice.
Motor greases
Greases made for electric motors (suffix GJN or WT on the bearing) give much longer life than standard grease at motor temperatures.
Sealing
- Shielded bearings (2Z): Pressed steel shields on both sides. Low friction and cool running, they keep out coarse dirt and hold grease in. Standard on small and medium motors.
- Contact-sealed bearings (2RS): Rubber seals on the bearing itself. Better against fine dust and splash but they add friction and heat, so check the speed limit.
- End-shield seals: Lip seal or V-ring at the drive end for the IP rating. A flinger or labyrinth ring is used when the seal must not touch the shaft. Dusty plants may use a labyrinth with a grease-filled taconite-style cover.
- Typical large motor: Labyrinth seal at the drive end, a V-ring at the non-drive end, and felt seals in the inner covers to keep grease out of the rotor area.
- Grease escape valve: A disc rotating with the shaft flings old grease into a cavity in the end cover and out through a hole at the bottom.
What to watch
- Vibration at the DE and NDE bearings, with envelope or acceleration measurements for early bearing signals.
- Surface temperature of each end shield. A difference of more than about 10 °C between ends is worth investigating.
- Ultrasonic listening while greasing, to stop when the noise level stops rising.
- Shaft voltage on inverter-fed motors if bearing damage has already shown up.
- Check the grease escape hole on relubricated motors: a blocked hole is a common cause of hot bearings after greasing.
- Listen for a change of noise after long standstill. Spring preload reduces false brinelling in transit.
Questions
Which bearings are used in electric motors?
Standard motor · ball bearings both ends: Direct-coupled or lightly belt-driven motors, frames 56 to about 225. Large motor · roller bearing at the drive end: Belt-driven or heavily loaded motors, frames 160 to 355 and up. Inverter-fed motor · protected from shaft currents: Any motor driven by a variable frequency drive, especially above about 30 kW. Small motor · cross-locating pair: Short shafts, frames up to about 132.
What are the common bearing failures in electric motors?
Fluting from shaft currents: Voltage on the rotor shaft discharges through the bearing, usually on inverter-fed motors. Grease breakdown from heat: Wrong grease for the temperature, or too much grease churning, or a blocked fan cover. Dents from vibration while stopped: Vibration from nearby machines or from transport while the rotor was not turning.
How should electric motors bearings be lubricated?
Grease type: Most motors use a mineral-oil grease with a lithium complex or polyurea thickener. Do not mix thickeners unless the maker confirms compatibility. Sealed-for-life: Small frames are packed at the factory and not meant to be relubricated. The catalogue table below gives the grease life by frame.