IDB-BRG-028
Bearings · L10 life · load rating · fits · lubrication
Bearing selection guide
Choosing and sizing a bearing — rolling vs plain, the L10 life calculation, mounting fits and the locating/floating arrangement, lubrication, and the failure modes to design against.
Abstract
A bearing carries load between moving parts with low friction. Getting it right is a sequence: the load direction and speed choose the bearing type, the dynamic load rating and the L10 life equation set the size, and the mounting fits, arrangement and lubrication determine whether it reaches that life.
Section 1 covers bearing types and when to use each. Section 2 is load, rating and the L10 life calculation. Section 3 is speed and lubrication. Section 4 is mounting fits and the locating/floating arrangement. Section 5 covers plain bearings and the PV limit. Section 6 is failure modes and a selection checklist.
1.Bearing types and when to use each
The first split is rolling-element (low friction, defined life, needs space and cleanliness) vs plain/bushing (cheap, quiet, shock-tolerant, friction- and PV-limited). Within rolling bearings, the load direction picks the type.
| Type | Radial | Axial | Speed | Self-align | Use when |
|---|---|---|---|---|---|
| Deep-groove ball | high | moderate | high | no | Versatile default, combined loads |
| Angular-contact ball | high | high (1 dir) | high | no | Spindles; mount in preloaded pairs |
| Cylindrical roller | very high | ~none | high | no | Heavy radial, axial float |
| Tapered roller | high | high | moderate | no | Wheels, gearboxes; pairs |
| Needle roller | high | none | moderate | no | Tight radial envelope |
| Spherical roller | very high | moderate | low–mod | yes | Heavy load + misalignment |
| Thrust ball | none | high | low | no | Pure axial load |
| Plain bushing | PV-limited | flanged | low–mod | slight | Low cost, quiet, shock, dirty |
1.1Terms
2.Load, rating and L10 life
Rolling bearings fail by subsurface fatigue — so life is statistical, quoted as L10 (the life 90% survive):
L10 = (C / P)ᵖ million revolutions, with p = 3 for ball, p = 10/3 for roller bearings.
In hours: L10h = L10 × 10⁶ / (60 · n) where n is rpm.
Example: ball bearing, C = 30 kN, equivalent load P = 3 kN, at 1500 rpm → L10 = (30/3)³ = 1000 million rev → L10h = 1000×10⁶ / (60·1500) ≈ 11,000 h.
- Equivalent load P combines radial and axial via catalogue factors X, Y (which depend on the axial/radial ratio and the bearing's contact angle).
- Reliability: L10 is 90% survival. For higher reliability multiply by a₁ (≈0.62 for 95%, ≈0.21 for 99%); modern ratings add an aISO factor for lubrication and contamination.
- Static check: also verify C₀/P₀ ≥ ~1–2 so shock or standstill loads don't brinell the raceways.
3.Speed and lubrication
- Limiting speed is set by heat and lubricant; compare ndm (speed × mean diameter, mm·rpm) to the catalogue limit. Grease is simpler and sealed-for-life up to moderate ndm; oil (bath, mist, jet) for high speed or heat.
- Grease for most applicationschoose base-oil viscosity for the speed/temperature and an NLGI grade (typically 2). Re-greasing interval falls with speed and temperature.
- Plain bearings run in a lubrication regime: boundary (start/stop, metal-to-metal, needs additives or self-lube liner), mixed, then hydrodynamic (a full oil film at speednear-zero wear). Design journal bearings to reach hydrodynamic at running speed.
4.Mounting fits and arrangement
The ring that sees a rotating load must be an interference fit or it creeps and wears its seat; the ring with a stationary load can be looser.
- Rotating inner ring (most common): interference on the shaft (k5 / k6 / m6 by size and load), looser in the housing (H7 / J7).
- Rotating outer ring: interference in the housing, looser on the shaft.
- Locating / floating: fix one bearing axially (both rings clamped) to locate the shaft; let the other float (one ring free to slide) so thermal expansion doesn't preload the pair. Cylindrical-roller and deep-groove bearings make good floating bearings.
- Preload angular-contact and tapered pairs for stiffness and running accuracybut over-preload kills life and overheats.
- Provide square, supported shoulders and the catalogue fillet/chamfer clearance.
5.Plain bearings and the PV limit
Plain bushings are limited by frictional heat, captured by the PV value (contact pressure × sliding velocity). Stay under the material's PV limit and check P and V individually:
| Material | Max P (MPa) | Max V (m/s) | Notes |
|---|---|---|---|
| Oil-impregnated sintered bronze | ~14 | ~6 | Self-lubricating, low cost |
| PTFE-lined metal (e.g. DU) | ~250 (static) | ~2 | Dry/marginal lube, thin |
| Filled PTFE / composite | ~100 | ~2 | Chemical, dry running |
| Acetal / nylon | ~10 | ~3 | Cheap, quiet, moulded |
| Carbon-graphite | ~4 | ~5 | High temperature |
Prefer a plain bearing for low speed, oscillating or shock loads, quiet running, dirty environments, or cost — and a rolling bearing for defined life, high speed, low starting friction, or precise location.
6.Failure modes and checklist
| Mode | Cause | Fix |
|---|---|---|
| Fatigue spalling | normal end of L10 life | size for required L10; reduce load/speed |
| Contamination dents / wear | dirt, debris (the #1 premature cause) | better sealing, clean assembly, filtration |
| Brinelling | static overload / impact at standstill | check C₀; cushion shock; handle carefully |
| False brinelling / fretting | vibration while not rotating | preload, secure in transit, lubricate |
| Smearing / skidding | too light load at high speed | minimum load, lighter preload |
| Lubrication failure | wrong/insufficient lube, overheat | correct grease/oil, re-lube interval, cooling |
| Electrical fluting | shaft currents (VFD drives) | insulated bearing or shaft grounding ring |
| Misalignment wear | shaft/housing not coaxial | self-aligning type, tighter alignment |
6.1Selection checklist
- Loaddirection (radial / axial / combined / moment) and magnitude, including shock.
- Speedrpm and ndm vs the limiting speed.
- Life & reliabilityrequired L10h and survival % (apply a₁).
- Environmenttemperature, contamination, moisture, electrical.
- Constraintsenvelope, precision/runout, stiffness/preload, noise.
- Then: pick the type (table), size to L10, choose fits and the locating/floating arrangement, select lubrication and sealing, and specify mounting (shoulders, clamping, fits) on the drawing.