Bearings Don’t Fail — We Fail Them
Rolling element bearings have a calculated design life — the L10 life — that typically exceeds 100,000 hours for properly selected and applied bearings. That’s over 11 years of continuous operation. Yet the average bearing in industrial service lasts a fraction of that. SKF’s own studies suggest that only about 10% of bearings reach their calculated L10 life. The rest fail prematurely from contamination, improper lubrication, mounting errors, and overloading.
This means 90% of bearing failures are preventable through better practices. No other single improvement opportunity in maintenance offers a comparable return on effort.
Bearing Selection: Matching the Bearing to the Application
When replacing bearings, use the OEM-specified bearing number unless engineering analysis justifies a change. Bearing selection involves balancing load capacity, speed capability, lubrication method, operating temperature, and available space. Changing one parameter affects the others.
Load Ratings
Every bearing has a dynamic load rating (C) and a static load rating (C0). The dynamic rating determines fatigue life under rotating load per ISO 281. The static rating indicates the maximum load before permanent deformation of the raceway — relevant during shock loads, starting, and very slow rotation.
For general industrial applications, the equivalent dynamic load (P) should not exceed 50% of the dynamic load rating (C). This provides a calculated L10 life of at least 1 million revolutions — typically translating to several years of continuous operation depending on speed.
Speed Limits
Bearing catalogs list two speed values: the kinematic limiting speed and the thermal reference speed. The limiting speed is the absolute maximum for the bearing geometry. The thermal reference speed is the speed at which heat generation equals the bearing’s ability to dissipate heat under reference conditions. Don’t operate above the thermal reference speed without enhanced cooling or a modified lubrication system.
Bearing Types for Common Applications
- Deep groove ball bearings — The default for moderate loads, high speeds, and simple mounting. Used in electric motors, pumps, fans, and general machinery. Handle both radial and moderate axial loads.
- Spherical roller bearings — Heavy radial and axial loads with tolerance for misalignment (up to 1-2 degrees). Common in conveyor pulleys, vibrating screens, gearbox shafts, and equipment with alignment challenges.
- Tapered roller bearings — Combined radial and heavy thrust loads. Require careful setting of internal clearance (preload or endplay) during installation. Used in large gearboxes, heavy-duty conveyors, and equipment with significant axial thrust.
- Cylindrical roller bearings — High radial load capacity, limited axial capability. Used where space is limited and loads are purely radial. Common in gearbox intermediate shafts and high-load applications.
Handling and Storage: Before Installation Even Begins
Bearings are precision components manufactured to tolerances measured in microns. Treat them that way.
- Keep bearings in original packaging until ready to install. The manufacturer’s packaging includes rust preventive coatings and moisture barriers. Opening a bearing and leaving it on a workbench overnight exposes it to contamination and moisture that reduce its life before it’s even installed.
- Store bearings flat in a clean, dry, temperature-stable environment. Vibration from nearby equipment can cause false brinelling in stored bearings. If the storage area is near heavy machinery, use vibration-isolating shelving.
- Rotate stock using first-in, first-out. Bearing preservatives have a shelf life — typically 2-3 years for standard packaging. Bearings stored beyond this need inspection and possible represervation before use.
- Never wash bearings with solvent before installation unless specifically required by the OEM. The factory-applied preservative is compatible with most greases and oils. Solvent washing introduces contamination risk from residual solvent and airborne particles during the drying and handling process.
Installation: Where Most Failures Are Born
Mounting Methods
The cardinal rule: apply mounting force only to the ring being pressed onto its seat. Never drive a bearing onto a shaft by pressing on the outer ring — the force transmits through the rolling elements and damages the raceways before the bearing turns a single revolution.
- Cold press fitting — Use a bearing fitting tool kit (hollow tubes and impact rings) that applies force evenly around the ring circumference. Never use a drift punch or hammer directly on the bearing ring — uneven force causes ring distortion and raceway damage.
- Heat mounting — The preferred method for interference fits. Heat the bearing to 80-110°C (175-230°F) using an induction heater designed for bearings. Never use an open flame — it creates localized hot spots that can alter the bearing steel metallurgy. The heated bearing slides onto the shaft easily and locks in place as it cools and contracts.
- Hydraulic mounting — For large bearings on tapered seats, hydraulic oil injection expands the inner ring for mounting while a hydraulic nut drives it to the correct position. This method requires specific equipment and training but produces the most controlled and repeatable mounting results.
Fit Tolerances
The fit between the bearing bore and the shaft — and between the outer ring and the housing — determines bearing performance and life. Too loose, and the ring creeps on the seat, generating heat and fretting. Too tight, and internal clearance is consumed, preloading the rolling elements and reducing life.
Standard fits for rotating inner ring applications (the most common configuration):
- Shaft: j5 to m5 for ball bearings, m5 to n6 for roller bearings (interference fit)
- Housing: H7 for stationary outer ring (clearance fit)
Measure the shaft diameter at the bearing seat before every installation. Use a calibrated outside micrometer — calipers lack the accuracy needed for bearing fit assessment. Record the measurement. If the shaft is undersize from wear, repair or replace the shaft. Running a bearing on a worn shaft guarantees premature failure from creep.
Internal Clearance Verification
After mounting, verify that adequate internal clearance remains. For ball bearings, this is difficult to measure directly on the installed bearing. For spherical and cylindrical roller bearings, use feeler gauges to measure radial clearance at the top of the bearing (the unloaded zone). Compare measured clearance to the bearing manufacturer’s recommendations for the application. Insufficient clearance after mounting means the fit is too tight or the bearing was heated insufficiently during installation.
Lubrication: The Ongoing Commitment
Grease Selection
Use the grease specified by the equipment manufacturer. If no specification exists, select based on:
- Base oil viscosity — Must be adequate for the bearing speed and operating temperature. Higher speed = lower base oil viscosity needed. Higher temperature = higher base oil viscosity needed (to compensate for thinning).
- Thickener type — Lithium complex is the default for most industrial applications. Polyurea for electric motor bearings (better high-temperature performance and longer life). Never mix incompatible thickener types — the result can be softening, hardening, or separation that destroys lubrication effectiveness.
- NLGI grade — Grade 2 for most applications. Grade 1 for centralized grease systems. Grade 3 for vertical shafts where grease migration is a concern.
Relubrication Intervals and Quantities
Bearing manufacturers publish relubrication guidelines based on bearing type, size, speed, and temperature. SKF’s relubrication interval calculator and the Timken lubrication guide are reliable references.
Apply the correct amount. The formula from earlier bears repeating: volume in ounces = 0.114 x bore (inches) x width (inches). Applying more does not provide better lubrication — it causes churning, heat generation, and seal pressure that can push grease past the seals and into the process.
Use a calibrated grease gun. Know how many grams or cubic centimeters your gun delivers per stroke and calculate the number of strokes needed. A “couple of shots” is not a lubrication specification.
Contamination Exclusion
Contamination is the number one killer of bearings in most industrial environments. Seal the housing. Upgrade from lip seals to labyrinth seals or bearing isolators on critical applications. Keep the area around the fill fitting clean before and during greasing. Cap grease fittings with dust covers.
For oil-lubricated bearings, install desiccant breathers, use high-quality shaft seals, and maintain oil cleanliness through proper filtration. The bearing’s operating environment is under your control — dirty environments produce short bearing lives unless you take active steps to keep contamination out.
Every hour invested in proper bearing selection, clean installation, and disciplined lubrication returns hundreds of hours of reliable operation. The practices described here are not complicated or expensive. They’re just easy to shortcut — and those shortcuts are why 90% of bearings never reach their design life.