Introduction
Bearings are one of the most common mechanical elements, yet selecting the wrong type or size is a frequent cause of premature failure. This guide covers the core concepts a mechanical designer needs to select bearings correctly.
Bearing Type Overview
| Type | Load Direction | Typical Use |
|---|---|---|
| Deep groove ball bearing | Radial + moderate axial | Motors, general machinery |
| Angular contact ball bearing | Combined radial + axial | Ball screws, spindles |
| Cylindrical roller bearing | Radial (high) | Gearboxes, heavy shafts |
| Tapered roller bearing | Radial + axial (heavy) | Wheel hubs, bevel gear shafts |
| Needle roller bearing | Radial (space-constrained) | Thin-section applications |
| Thrust bearing (ball/roller) | Axial only | Vertical shafts, screw jacks |
Load Direction: The First Selection Criterion
Before selecting a bearing, determine the load direction:
- Radial load: perpendicular to the shaft axis (most common)
- Axial (thrust) load: parallel to the shaft axis
- Combined load: both radial and axial simultaneously
Deep groove ball bearings handle moderate combined loads. For heavy axial loads, use angular contact or tapered roller bearings. For very heavy radial loads with minimal axial, use cylindrical roller bearings.
Basic Rating Life (L10)
Bearing life is calculated using the L10 life formula — the number of revolutions (or hours) at which 90% of bearings will still be running:
L10 = (C/P)^n × 10^6 revolutions
Where:
- C = dynamic load rating (from bearing catalog, in kN)
- P = equivalent dynamic bearing load (calculated from actual radial and axial loads)
- n = 3 for ball bearings, 10/3 for roller bearings
Target L10 life depends on the application. Industrial machinery typically targets 20,000–50,000 hours; high-speed spindles may target 10,000 hours.
Speed Limits
Bearing catalogs list a reference speed — the maximum speed for standard lubrication conditions. Exceeding this speed requires:
- Upgraded lubricant (lower viscosity grease or oil mist lubrication)
- Reduced preload
- Cooling provisions
Mounting and Fits
Bearing fit to shaft and housing is critical:
- Rotating inner ring (most common): shaft fit = press fit (k5, m5, or similar); housing = sliding fit (H7)
- Stationary inner ring: shaft fit = sliding fit (h6); housing = press fit (M7, N7)
Incorrect fits cause either fretting (too loose) or excessive preload leading to early failure (too tight).
Lubrication
| Type | Speed Range | Notes |
|---|---|---|
| Grease | Low to medium | Simple, sealed bearings, infrequent relubrication |
| Oil bath | Medium to high | Continuous lubrication; housing must be sealed |
| Oil circulation | High | Cooling as well as lubrication; complex system |
FAQ
Q. How do I choose between a ball bearing and a roller bearing?
A. Ball bearings are lighter, handle moderate combined loads, and run at higher speeds. Roller bearings carry heavier radial loads and are more tolerant of shaft misalignment (spherical rollers). If load is the dominant factor, use rollers; if speed or compactness is priority, use balls.
Q. What causes premature bearing failure?
A. The most common causes are: incorrect fit (too tight or too loose), contamination (dirt or water in the bearing), insufficient or incorrect lubrication, overload beyond rated capacity, and misalignment.
Q. Where can I find bearing load ratings?
A. Major bearing manufacturers (SKF, NSK, NTN, FAG, Timken) publish comprehensive engineering catalogs — available free online — with detailed load ratings, life calculations, and mounting recommendations for every catalog bearing.
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