Bearings

Bearings support rotating shafts with low friction. Learn rolling-element (ball, roller) and plain (journal) bearings, load ratings, L10 life, equivalent dynamic load, selection from catalogues, lubrication, mounting and failure modes.

Every rotating machine rides on bearings. Rolling-element bearings (balls or rollers between rings) are compact and nearly frictionless; plain bearings (a shaft on an oil film) carry huge loads smoothly in engines and turbines.

Bearing selection means estimating the loads, choosing a type, then picking a size from a catalogue so its fatigue life meets the design requirement.

An analogy

Moving a heavy box on round logs instead of dragging it: rolling replaces sliding friction with far smaller rolling resistance.

Types

Deep-groove ball (general purpose, radial + some axial), angular-contact ball (combined loads, precision spindles, in pairs), cylindrical roller (heavy radial), tapered roller (combined radial + axial — wheel hubs), spherical roller (heavy load, misalignment), needle (compact), thrust bearings (axial only). Plain bearings: bushings and hydrodynamic journal bearings.

Life and load rating

Rolling bearings eventually fail by subsurface fatigue (spalling). C is the load giving 1 million revolutions at 90 % survival. Life falls with the cube of load for ball bearings: doubling load cuts life to one-eighth.

Arrangements and mounting

Typically one 'locating' bearing takes axial load and one 'floating' bearing allows thermal expansion. Rotating rings get interference fits; stationary rings sliding fits. Seals and shields keep lubricant in and dirt out.

Modified life and lubrication

ISO 281 modified life L_nm = a₁ a_ISO L₁₀ accounts for reliability, lubricant film ratio κ (viscosity vs required), contamination and fatigue load limit — clean, well-lubricated bearings can last far beyond L₁₀. Hydrodynamic bearings are designed with the Sommerfeld number and Raimondi–Boyd charts.

Formulas

• Rating life: L₁₀ = (C/P)^p [million revolutions]
• Life in hours: L₁₀h = 10⁶ L₁₀ / (60 n)
• Equivalent load: P = X F_r + Y F_a
• Required rating: C_req = P · (60 n L_h / 10⁶)^(1/p)

Worked example: Ball bearing life

Given: C = 30 kN, P = 3 kN; n = 1440 rpm

Find: L₁₀ in revolutions and hours.

• L₁₀ = (30/3)³ = 1000 million revs
• L₁₀h = 10⁹/(60 × 1440) = 11 574 h

Answer: ≈ 11 600 h

Worked example: Roller vs ball

Given: Same C/P = 10

Find: Roller bearing life.

• L₁₀ = 10^(10/3) = 2154 million revs

Answer: ≈ 2.15× the ball bearing life for the same C/P

Common mistakes

• Wrong: Using radial load only when axial load exists. — Right: Compute P = XF_r + YF_a from catalogue factors.
• Wrong: Fixing both bearings axially on a long hot shaft. — Right: Use one locating and one floating bearing for thermal expansion.

Applications

• Electric motors: Deep-groove ball bearings, often sealed-for-life.
• Vehicle wheels: Tapered roller or double-row angular-contact hub units.
• Turbines & engines: Hydrodynamic journal and tilting-pad bearings.

Key takeaways

• L₁₀ = (C/P)^p million revs; p = 3 ball, 10/3 roller.
• P = XF_r + YF_a for combined loads.
• Locate one bearing, float the other.
• Lubrication and cleanliness extend life dramatically.

Going further

• Preload: Angular-contact pairs are preloaded to remove clearance and raise stiffness in machine-tool spindles.
• Condition monitoring: Vibration analysis detects bearing defect frequencies (BPFO, BPFI) long before failure.
• Electrical erosion: Inverter-driven motors can pass currents through bearings; insulated or hybrid ceramic bearings prevent fluting.