The rotor of an impact crusher is mounted on a main shaft that turns in two large bearing housings. The main shaft and rotor bearings carry the rotor’s weight, crushing impacts, belt pull and any imbalance forces. Running at high speed under shock loads, these bearings can fail quickly without the right grease, clean seals and good alignment. A bearing failure often also means damage to the shaft and bearing seats, and therefore a long shutdown.
In this guide we cover in detail the construction of the main shaft and rotor bearings, lubrication principles, monitoring methods, failure types, replacement and mounting steps, common mistakes and spare part selection. For the rotor, see our rotor guide, and for general impact crusher operation, our impact crusher guide.
Construction and Components
- Main shaft: Made from high-strength forged steel; the rotor, pulley and bearings are mounted on it.
- Bearings: Impact crushers usually use heavy-duty spherical roller bearings; bearings with a tapered bore are fixed to the shaft with adapter or withdrawal sleeves.
- Bearing housings: Carry the bearings; one side is usually designed as fixed (taking the axial load) and the other as floating (allowing thermal expansion).
- Sealing: Labyrinth seals and dust covers prevent dust and moisture ingress.
- Pulley and belts: Transmit motor power to the shaft.
Lubrication Principles
- The right grease: Use the extreme pressure (EP) grease of suitable consistency recommended by the manufacturer.
- The right quantity: Too little grease causes friction and heating; too much causes churning and temperature rise.
- The right interval: The greasing interval is set by the manufacturer according to speed, temperature and environmental conditions.
- Cleanliness: The grease nipple and gun must be clean; dust must not get into the grease container.
- Labyrinth grease cushion: A slight overflow of grease at the seals gives extra protection against dust and moisture.
Monitoring: Seeing a Failure Coming
| Parameter | Interval | What it tells you |
|---|---|---|
| Bearing temperature | Continuous / every shift | Lubrication problem, overload, misalignment |
| Vibration | Continuous or weekly route | Bearing damage, imbalance, looseness |
| Sound (ultrasonic) | Monthly | Need for lubrication, early bearing damage |
| Belt tension and alignment | Weekly | Radial load on the bearings |
| Seal condition | Weekly | Risk of dust/moisture ingress |
For more on vibration and temperature monitoring, see our predictive maintenance guide.
Bearing Failure Types and Causes
| Failure | Appearance | Possible cause |
|---|---|---|
| Fatigue spalling | Flaking on the rolling surface | Normal end of life or overload |
| Abrasive wear | Dull, worn surfaces | Dust ingress, dirty grease |
| Lubrication failure / overheating | Discolouration, smearing | Too little grease, too much grease, high speed |
| Brinelling | Regularly spaced indentations | Incorrect mounting (impact), vibration at standstill |
| Corrosion | Rust spots | Moisture ingress |
| Misalignment marks | Skewed load path | Misaligned bearing housings |
Bearing Replacement Step by Step
- Preparation: New bearings, sleeves, seals, grease, an induction heater and hydraulic nut/puller equipment are prepared.
- Safe shutdown: LOTO; confirm the rotor has stopped completely.
- Removing the belts and pulley.
- Supporting the rotor: Before the bearing housings are removed, the rotor and shaft are safely supported or lifted.
- Removing the bearings: The sleeves are released and the bearings are removed with a puller; no impact is applied to the shaft.
- Shaft and housing inspection: The shaft seats and bearing housing bores are measured.
- Mounting: The bearing is heated by induction and fitted; with tapered bore bearings, the sleeve is tightened while measuring the reduction in radial clearance against the manufacturer’s value.
- Greasing and seals: The bearing and housing are filled with the correct amount of grease and the seals fitted.
- Alignment: The bearing housings are aligned; the fixed/floating arrangement is checked.
- Commissioning: Idle run with temperature and vibration monitoring; a brief temperature rise that then stabilises in the first hours is normal.
Common Mistakes
- Hammering the bearing on: Brinelling and early failure.
- Tightening the sleeve without measuring radial clearance: Too tight causes heating; too loose causes slipping.
- Locking the floating side: Thermal expansion strains the bearings.
- Over-greasing.
- Over-tensioning the belts.
- Running with an unbalanced bar set.
Troubleshooting Table
| Symptom | Possible cause | Solution |
|---|---|---|
| High bearing temperature | Too little/too much grease, sleeve too tight, belt tension | Correct the grease quantity, check the mounting and belts |
| Increased vibration | Imbalance, bearing damage, loose bolts | Check the bar set, bearings and bolts |
| Noisy running | Rolling surface damage, lack of lubrication | Ultrasonic check, greasing, replacement if necessary |
| Dust/grease leaking from the seal | Seal damage | Seal replacement |
| Slip marks on the shaft surface | Sleeve not tightened enough | Shaft repair/replacement, correct mounting |
An Example from the Field
On one impact crusher, the rotor bearings were being replaced every few months. The removed bearings showed abrasive wear and signs of overheating. It turned out that the grease gun was kept in an open bucket, there was fine rock dust in the grease, and the operators were pumping in large amounts of grease every shift on the basis that “too much grease does no harm”. The plant switched to sealed cartridge grease guns, set the grease quantity according to the manufacturer’s table and renewed the labyrinth seals. Bearing life increased considerably.
OEM or Aftermarket?
| Part | Check |
|---|---|
| Bearing | Type, dimensions, internal clearance class, supply from an authorised source (risk of counterfeit bearings) |
| Main shaft | Material certificate, dimensional report, ultrasonic testing |
| Bearing housing | Bore tolerance, seal compatibility |
| Sleeves and nuts | Dimensional and taper compatibility |
Stock Planning
- Keep a set of rotor bearings, sleeves and seal sets as critical spares.
- At large plants, a spare bearing housing (with the bearing fitted) shortens replacement time.
- Suitable grease and greasing equipment should always be ready.
Pre-Purchase Checklist
- Have the crusher model and bearing types/numbers been identified?
- Are the sleeves, nuts and seals being ordered as a set?
- Are the bearings being sourced from an authorised/reliable supplier?
- Are a heater and measuring equipment ready for mounting?
Safety
- The rotor and shaft are very heavy; they must only be moved with suitable lifting equipment.
- Wear gloves against burns when using an induction heater.
- Do not work in the bearing area until the rotor has stopped completely and been locked.
Frequently Asked Questions
How long does a rotor bearing last?
With correct lubrication, clean sealing and good balance it lasts a long time; the most common causes of shortened life are contamination, incorrect greasing and imbalance.
Grease or oil?
Grease is mostly used for impact crusher rotor bearings; some large designs have oil systems. Follow the manufacturer’s recommendation.
When does temperature count as an alarm?
The manufacturer’s limit and your own reference values should be the basis; sudden increases should always be investigated.
Can a damaged shaft be repaired?
Slight damage on the bearing seats can be repaired by suitable methods; serious damage requires shaft replacement.
How Are Grease Quantity and Interval Determined?
Bearing manufacturers provide tables that usually set the grease quantity according to the bearing’s outside diameter and width, and the interval according to speed, bearing type and operating temperature. Practical principles:
- High speed and high temperature shorten the greasing interval.
- Dusty and humid environments may require more frequent greasing in small quantities to maintain the seal.
- Greasing should, where possible, be done while the bearing is turning (safely, via an extended grease line) so the grease is distributed evenly in the bearing.
- There should be an outlet for old grease to escape; otherwise excess grease creates pressure and heating in the bearing.
Fixed and Floating Bearing Arrangement
The rotor shaft lengthens as it heats up. So that this expansion does not damage the bearings, the bearing on one side is designed to be axially fixed and the other floating. If the floating side is accidentally fixed during assembly (for example by fitting the locating rings incorrectly), the shaft’s thermal expansion is prevented and excessive axial load is created on both bearings. This soon results in heating and bearing damage.
Effect of Operating Conditions on Bearing Life
| Condition | Effect | Recommendation |
|---|---|---|
| Rotor imbalance | Increased dynamic load, fatigue | Matching the bar set, balancing |
| Excessive belt tension | Increased radial load | Correct tension |
| Dust and water ingress | Abrasive wear, corrosion | Seal maintenance |
| Coarse feed and impact | Shock loads | Feed size control |
| Long shutdowns | Static marks, corrosion | Periodic turning and protection during shutdowns |
Maintenance Record Form
| Field | Record |
|---|---|
| Bearing temperatures | Fixed and floating side |
| Vibration | Radial and axial values |
| Greasing | Date, quantity, grease type |
| Belt tension | Measured value |
| Bearing replacement | Date, bearing number, findings on removal |
Learning from Removal Findings
Every replaced bearing is evidence of the cause of failure. Before a removed bearing is thrown away, it should be cleaned and examined, and the marks on the rolling surfaces, the condition of the cage and the colour of the grease recorded. The same type of failure recurring (e.g. continual abrasive wear) shows that the root cause (sealing, grease cleanliness) has not been solved.
More Questions
Should the sleeve also be replaced when a bearing is changed?
If the threads of the sleeve and nut are damaged or there are slip marks on the sleeve surface, they should be replaced together; if they are in good condition, they can be reused according to the manufacturer’s recommendation.
Is bearing heating normal in the first hours?
With a new bearing and fresh grease, a brief temperature rise may be seen in the first hours; the temperature should then stabilise. A continuous rise is not normal.
Can vibration analysis show bearing failure in advance?
Yes. Bearing damage produces characteristic frequencies in the vibration spectrum and can often be detected weeks in advance.
Spare Parts and Service for the Main Shaft and Rotor Bearings
CSP Mühendislik supplies main shafts, rotor bearings, bearing housings and seal sets for impact crushers as part of its impact crusher spare parts range, and provides service support for bearing replacement, alignment and vibration analysis. Send your crusher model on WhatsApp and we will prepare a quote quickly.
