1. Scope & Purpose
This guide describes the standardized procedure for changing rolling bearings in electric motors. It can be used for all common designs of three-phase asynchronous motors, direct current motors and permanent magnet synchronous motors in industrial use. The aim is to maximize the service life of the motors, prevent unexpected failures and ensure the operational safety of the systems. A bearing change must be carried out during planned preventative maintenance, after the expected bearing service life has been reached or if abnormalities such as increased noise, vibrations or operating temperatures are detected in accordance with DIN EN 60034-1.
2. Safety instructions
DANGER! Before starting any work on the electric motor, the system must be switched off in accordance with the VDE 0105-100 regulations. The “Five Safety Rules” (disconnecting, securing against being switched on again, determining the absence of voltage, grounding and short-circuiting, covering or barriering off adjacent live parts) must be strictly adhered to. A lockout/tagout (LOTO) procedure is mandatory to prevent accidental power-on. Residual energy in capacitors must be discharged and mechanical tensions secured (e.g. by springs).
WARNING! Personal Protective Equipment (PPE) is mandatory throughout the maintenance process. This includes at least: Cut-resistant protective gloves (according to EN 388), safety glasses with side protection (according to EN 166), safety shoes with toe caps and puncture protection (according to EN ISO 20345) and, if necessary, hearing protection (according to EN 352-1). When handling cleaning agents or lubricants, the relevant safety data sheets must be observed and suitable gloves must be worn.
WARNING! There is a risk of burns when heating bearings. Always use heat-resistant protective gloves (according to EN 407) and ensure that there are no flammable materials nearby.
3. Required Tools & Materials
| Tool / Material | Specification | quantity |
|---|---|---|
| Torque wrench | Range: 10-200 Nm, accuracy: ±4% (according to DIN EN ISO 6789-1) | 1 |
| Socket wrench set / combination wrench set | Metric, SW 8 - SW 32, chrome vanadium steel | 1 sentence |
| Puller (2-arm / 3-arm) | Size: 50-200 mm clamping depth, internal/external extraction, hydraulic for large bearings | 1-2 (as needed) |
| Induction heater / hot air cabinet | For storage up to 15 kg, temperature control up to 200°C | 1 |
| Mounting sleeve set / impact tool set | For precise bearing assembly without damage | 1 sentence |
| Feeling theory | Range: 0.02 - 1.00 mm, Gradation: 0.01 mm (according to DIN 2275) | 1 |
| Dial gauge with magnetic stand | Measuring range: 0-10 mm, resolution: 0.01 mm | 1 |
| Micrometer / outside micrometer | Measuring range: 0-25 mm, resolution: 0.001 mm (according to DIN 863) | 1 |
| IR thermometer / thermal imaging camera | Measuring range: -30 to 600°C, accuracy: ±2°C | 1 |
| Cleaning agent (grease-dissolving) | Industrial cleaner, evaporates without residue, non-corrosive | 1-2 liters |
| Industrial paper / lint-free cloths | Absorbent, tear-resistant | 1 roll |
| Lubricant (bearing grease) | Follow the manufacturer's recommendations, e.g. lithium complex soap grease, NLGI 2, ISO-L-XBCDB2 | 1 cartridge / can |
| New rolling bearings | Type, size, tolerance, bearing clearance (e.g. 6309 C3), manufacturer quality | 2 (drive side DE, non-drive side NDE) |
| New sealing rings/shaft seals | Dimensions, material (NBR, FKM), type (e.g. radial shaft seal DIN 3760) | 2 |
| Screw locking (optional) | Medium strength, e.g. Loctite 243 | 1 bottle |
| marker pen | Permanent marker | 1 |
4. Pre-inspection checklist
This inspection serves to assess the condition before replacing the bearing and helps to identify possible causes for the bearing damage.
| Item | Check | Acceptance/rejection criteria | Notes |
|---|---|---|---|
| Engine environment | Cleanliness, humidity, temperature, ventilation | Dry, low-dust, ambient temperature within manufacturer specifications | ___________________ |
| Motor mounting | Location of fastening screws, vibration | Safe, no loose screws, no unusual vibrations (hand test) | ___________________ |
| Cables & connectors | Insulation integrity, tightness of terminals, oxidation | No visible damage, tight fit, clean | ___________________ |
| Fan & fan cover | Cleanliness, integrity, free rotation of the fan | Free from dirt and damage, fan rotates smoothly | ___________________ |
| Acoustic test (during operation) | Unusual noises (grinding, whining, knocking) | Just normal operating noise. Deviations >3 dB(A) are critical. | ___________________ |
| Thermal test (during operation) | Motor surface temperature (housing, end shields) with IR thermometer | Within manufacturer specifications (typical max. 80°C housing), no hotspots. Delta-T between end shields < 5°C. | ___________________ |
| Vibration test (during operation) | Measurement on bearing seats (optional, vibration measuring device if available) | Total vibration speed RMS < 2.8 mm/s for class N (according to DIN ISO 10816-3) | ___________________ |
| Shaft feedthrough | Visual inspection for leaks (lubricant), damage to the seal | No leaks, seal intact | ___________________ |
5. Step-by-step instructions
5.1. Preparation and safety
Secure the work area: Mark the work area with barrier tape and warning signs. Make sure there is enough space for disassembly and handling of the engine.
Disconnect motor from power supply (LOTO):
- Disconnect the motor from the power supply at the main switch.
- Secure the switch against being switched on again (e.g. with a padlock and a warning sign with the name and date).
- Use a suitable multimeter (tested according to VDE 0413) to check that there is no voltage at the motor connection terminals. Measure between all phases and ground.
- Common error: Assuming no voltage without verification. This is a safety hazard and could result in serious injury.
Ensuring engine cooling: Allow the engine to cool to ambient temperature if it was previously in operation. This makes handling easier and reduces the risk of burns.
Documentation: Take photos of the actual condition of the engine, especially the arrangement of the attachments and the wiring. This is helpful for correct reassembly.
Mark positions: Mark the position of the end shields to the stator housing with a center punch or waterproof pen. This ensures correct alignment during assembly.
5.2. Dismantling
Dismantle attachments:
- Loosen and remove all protective devices, e.g. fan cover and terminal box cover.
- Remove couplings, pulleys or fan blades from the motor shaft. Use a suitable puller for this. If parts are stuck, careful heating can help, but avoid direct flames on the shaft. Improper use of tools can lead to shaft damage.
- Remove any keys and store them safely.
Remove bearing shields:
- Loosen the mounting screws of the end shields (drive side DE and non-drive side NDE). Pay attention to any fitting screws that may be present.
- Carefully remove the bearing shields. If they are stuck, use rubber hammers or plastic wedges to loosen them without damaging the mating surfaces.
- Torque value Example: For M8 screws on end shields the tightening torque is typically 25-30 Nm, for M10 screws 45-55 Nm. The exact values can be found in the engine documentation.
Remove rotor:
- Position the motor so that the rotor can be safely removed. For larger engines, suitable lifting equipment (crane, hoist) must be used.
- Pull the rotor carefully and evenly out of the stator. Be careful not to damage the stator windings. Protect the rotor shaft to avoid damage to the bearing fit.
- Place the rotor on a clean, soft surface.
Dismantle old bearings:
- Use a suitable puller to remove the old bearings from the motor shaft. For bearings with a tight fit on the shaft, a heating ring or an induction heater can be helpful to heat the inner ring to around 80-100°C, which makes disassembly easier.
- Remove the bearings from the bearing shields.
- Important note: Never use brute force or a hammer and chisel as this can cause irreparable damage to the shaft or bearing seats.
5.3. Inspection and analysis
Examine old warehouses (damage analysis):
- Inspect old bearings for discoloration (overheating), tracking (wear, misalignment), galling, cracks, or rolling element marks. Identify possible damage patterns according to DIN ISO 15243.
- Signs such as corrosion indicate the ingress of moisture, while deep running marks indicate a lack of lubrication or overloading.
- The analysis of the damage pattern is crucial for finding the cause and preventing future failures.
Check shafts and bearing seats:
- Thoroughly clean the bearing mating surfaces on the shaft. Measure the shaft diameter at the bearing seats with a micrometer (accuracy 0.001 mm). Compare the values to the motor manufacturer's tolerances (typically h6 or j6 for bearing fits).
- Check the shaft for runout and runout using a dial indicator and magnetic stand. The concentricity should be a maximum of 0.02 mm (according to VDI 2060).
- Check the surface quality for damage, scoring or fretting corrosion. A surface roughness of Ra < 1.6 µm should be aimed for.
- Deviations from the shaft diameter tolerance or excessive concentricity can lead to bearing damage and vibrations.
Check bearing housings and bearing shields:
- Thoroughly clean the bearing holes in the bearing shields. Measure the hole diameters with an inside micrometer or a calibrated caliper. Compare with the manufacturer's tolerances (usually H7 or G7).
- Check the end shields for cracks, deformation or damage.
- Make sure the lubrication holes (if any) are clean and unobstructed.
Check stator and windings:
- Remove loose particles and debris from the stator pack and windings.
- Check the windings for damage to the insulation, signs of overheating or foreign objects.
- Carry out an insulation resistance measurement according to VDE 0701-0702 if an appropriate test device is available.
5.4. Assembly of the new bearings
Preparation of the new warehouses:
- Only remove the new bearings from the original packaging immediately before assembly to avoid contamination and corrosion.
- If the bearings are not already greased, carefully clean them with a quickly evaporating, residue-free cleaning agent.
- Heat the bearings for assembly onto the shaft (if press fit). A temperature of 80-100°C (maximum 120°C) is ideal. Use an induction heater or hot air oven. Never use open flames or heat the bearing above 120°C, as this can negatively affect the bearing structure and lubricant quality.
- Visual inspection: After heating, the bearings should be evenly expanded and slide easily onto the shaft.
Mount bearings onto the shaft:
- Quickly push the heated bearing onto the cleaned shaft seat until it stops. Make sure you sit straight.
- For smaller bearings that are cold mounted (seat F6, G6), use a mounting kit with matching sleeves to apply force exclusively to the inner ring. Never hit or press on the outer ring as this will damage the bearing.
- Visual control: The bearing must lie flat and free of play in its seat.
Installation of the bearings in the bearing shields:
- Insert new sealing rings and shaft seals into the bearing shields. Make sure the installation direction is correct (lips to the lubricant side).
- Lightly grease the sealing lips with the recommended bearing grease.
- Fill the bearing shields with the manufacturer's recommended amount of bearing grease. Be careful not to overfill the lubrication chamber (typically 30-50% of free volume) to avoid overheating.
5.5. Reassembly of the engine
Insert rotor:
- Carefully insert the rotor into the stator without touching or damaging the windings.
- Make sure the shaft is centered to avoid damaging the bearings when placing the signs.
Install bearing shields:
- Carefully place the bearing shields on the shaft and stator housing. Note the markings made previously for correct alignment.
- Insert the fastening screws and first tighten them hand-tight.
- Tighten the screws crosswise and gradually using the torque wrench to the values specified by the manufacturer. Over-tightening or uneven tightening can cause housing distortion and bearing damage.
- Torque value example: M8 = 25-30 Nm, M10 = 45-55 Nm, M12 = 80-95 Nm.
- Visual inspection: The end shields must lie flat on the stator housing, without a gap.
Check radial and axial play:
- After assembling, rotate the motor shaft by hand. It must rotate smoothly and without grinding noises or unusual resistance.
- Measure the axial play of the shaft (if permitted, e.g. for cylindrical roller bearings or compensating bearings) with a dial indicator. The value must be within the manufacturer's specifications (typically 0.1 - 0.3 mm for free-floating bearings). For deep groove ball bearings, this is primarily determined by the bearing clearance (e.g. C3 bearing clearance approx. 0.011-0.025 mm radial for 6309).
- Low or missing clearance indicates improper seating or assembly errors, leading to overheating and rapid bearing failure.
Reinstall add-on parts:
- Insert keys and mount couplings or pulleys.
- Pay attention to the precise alignment of couplings (e.g. with a laser alignment device according to VDI 2060). A misalignment of 0.05mm radial or 0.02mm angular can cause significant vibration and bearing damage.
- Assemble the fan blades and fan shroud. Make sure the fan rotates freely and does not drag.
- Connect the electrical connections in the terminal box according to the circuit diagram and tighten the terminals with the specified torque.
6. Post-maintenance inspection checklist
This test serves to verify the correct assembly and function of the motor after the bearing change.
| Test | Expected result | Current value | Passed / Failed |
|---|---|---|---|
| Manual shaft rotation | Smooth running, noiseless, without resistance | ||
| Electrical testing | Insulation resistance > 1 MΩ (according to VDE 0701-0702), winding resistances in tolerance (multimeter) | ||
| Test run (without load) | Engine starts and runs smoothly, no unusual noises or vibrations | ||
| Temperature measurement (after 30 minutes test run) | Bearing housing temperature stable, max. 70°C (IR thermometer), no significant difference between DE and NDE | ||
| Vibration measurement (after 30 minutes test run) | Total vibration speed RMS < 2.8 mm/s for class N (DIN ISO 10816-3) | ||
| Noise level test | Normal noise level, no grinding, knocking or whining noises | ||
| tightness | No visible lubricant leaks on shaft bushings | ||
| Direction of rotation | Correct direction of rotation of the motor |
7. Troubleshooting
Common problems after a bearing change and their possible solutions.
| Symptom | Probable cause(s) | corrective action(s) |
|---|---|---|
| Motor runs hot (bearing) | overfilling with lubricant; Insufficient bearing clearance (wrong bearing or assembly error); Assembly errors (tension in the bearing); Fake fat | Check/reduce the amount of lubricant; Check bearing clearance, replace bearing if necessary; Check assembly, reassemble if necessary; Check manufacturer's instructions for grease |
| Increased vibrations | imbalance (rotor, clutch, pulley); Misalignment (clutch, pulley); Loose component; Bearing damage (assembly error) | Balancing the rotor/clutch/pulley; Check and correct alignment (laser); Check all fasteners; Check storage status, replace if necessary |
| Unusual noises (howling, grinding) | lack of inventory; bearing ran dry; foreign bodies in the bearing; Wrong camp; Damaged bearing (assembly) | Check and supplement lubrication; Remove, clean or replace bearings; Check bearings for correct type and condition |
| Shaft is difficult to turn | Press fit too high; Bearings mounted tight; waves; Too much lubricant | Check fit, edit shaft if necessary or select bearing with a different fit; Check assembly, reassemble if necessary; check wave runout; Check the amount of lubricant |
| Lubricant leaks | Damaged shaft seal; Incorrect shaft seal; Too much lubricant; Overpressure in the storage room | replace shaft seal; Select correct seal type; Reduce the amount of lubricant; Check whether a relief hole is blocked |
8. Recommended maintenance schedule
A proactive maintenance plan extends the life of your electric motors and avoids unplanned downtime.
| Task | Frequency | Estimated duration | Skill level |
|---|---|---|---|
| Visual inspection (external) | Monthly / every 750 operating hours | 15 minutes | Maintenance technician |
| Acoustic/thermal testing | Quarterly / every 2,000 operating hours | 30 minutes | Maintenance technician |
| Vibration measurement (according to DIN ISO 10816) | Every six months / every 4,000 operating hours | 60 minutes | Vibration diagnosis expert |
| Relubrication of the bearings | According to manufacturer information (typically 1,000 - 5,000 operating hours) | 15-30 minutes | Maintenance technician |
| Bearing change | Preventive: every 20,000 - 40,000 operating hours (3-5 years); Depending on requirements if bearing damage is detected | 4-8 hours (depending on engine size) | Experienced maintenance technician |
| Electrical testing (insulation, winding) | Annually / every 8,000 operating hours | 60 minutes | Electrician |
9. Spare parts reference
High-quality spare parts are crucial for the reliability and service life of your electric motors. UNITEC-D offers a wide range of industrial bearings and seals.
| Part Description | Typical specification | UNITEC category |
|---|---|---|
| Deep groove ball bearing (DE side) | 62xx, 63xx, 2RS/C3 | Rolling bearings |
| Deep groove ball bearing (NDE side) | 62xx, 63xx, 2RS/C3 | Rolling bearings |
| Cylindrical roller bearings (for larger radial loads) | NUxx, NUPxx, C3 | Rolling bearings |
| Insulated bearings (for shaft currents) | 63xx-J20AA, INSOCOAT | Special warehouse |
| High temperature bearing | S2, S3 (up to 350°C) | Special warehouse |
| Radial shaft seal | NBR, FKM (Vitron®), according to DIN 3760 | Sealing technology |
| Bearing grease | Lithium complex soap grease, NLGI 2, -30 to +140°C | Lubricants |
| Lock washers/circlips | DIN 471 (shaft), DIN 472 (bore) | Standard parts |
Visit our e-catalog for detailed specifications and ordering: www.unitecd.com/e-catalog/
10. References
- DIN EN 60034-1: Rotating electrical machines – dimensioning and operating behavior
- VDE 0105-100: Operation of electrical systems
- DIN ISO 10816-3: Measurement and assessment of machine vibrations - industrial machines
- DIN ISO 15243: Rolling bearings - damage and causes - terminology, classification and examples
- VDI 2060: Criteria for assessing the balance of rotating rigid bodies
- VDE 0701-0702: Inspection after repairs, changes to electrical devices
- DIN EN ISO 4287: Geometric product specification (GPS) – surface quality: stylus method
- SKF, FAG (Schaeffler), NSK: Manufacturer documentation and assembly instructions for rolling bearings
