Bearings, Lubrication, Seals and Tribology
What this domain tests
Bearing type selection and identification, fits, lubrication method and grade, seal selection, and failure diagnosis.
Core rules
- Plain bearings rely on a hydrodynamic oil film; they need a minimum speed and correct clearance. Rolling bearings give low starting friction but are sensitive to misalignment and shock.
- Bearing number 6205: 6 = deep-groove ball, 2 = light series, 05 = bore code; bore = 05 × 5 = 25 mm. For codes 04 and above, bore = code × 5 mm; 00, 01, 02, 03 mean 10, 12, 15, 17 mm.
- Fit rule: the ring that rotates relative to the load gets an interference fit; the stationary ring gets a light push fit. A rotating inner ring therefore needs a k5/m5 shaft, not a loose fit.
- Lubrication: oil for high speed and heat removal, grease for sealed long-life applications. ISO VG grade is set by viscosity at 40 °C; NLGI number sets grease consistency. Over-greasing a high-speed bearing causes churning and overheating.
- Seals: lip seals for shafts (lip facing the medium), O-rings for static joints, labyrinth seals for high speed, and felt or brush seals for dust.
- Failure signatures: fatigue spalling on the load zone, brinelling from shock, false brinelling from vibration while stationary, and blue discolouration from overheating.
Worked example
A 6205 bearing (C = 14 kN) carries P = 2 kN at 1000 rpm. L10 = (C/P)³ × 10⁶ rev = 7³ × 10⁶ = 343 × 10⁶ rev. Life in hours = 343 × 10⁶ ÷ (60 × 1000) ≈ 5717 h.
Traps that cost marks
- Reading bore code 05 as 5 mm instead of 25 mm.
- Giving the rotating ring a loose fit, which causes fretting and creep.
- Choosing grease for a hot, high-speed application that needs oil circulation.