Bearings, seals, lubrication and maintenance
Bearings, seals, lubrication and maintenance questions ask you to identify a bearing from its designation, choose the correct fits, mount it without damage, select a seal and lubricant, and diagnose a failure from its appearance.
Bearing designations
In 6205: 6 = deep-groove ball bearing, 2 = light series, 05 = 5 × 5 = 25 mm bore (from 04 upwards multiply by 5; 00, 01, 02, 03 mean 10, 12, 15, 17 mm). Suffixes: 2RS rubber sealed both sides, ZZ metal shielded, C3 increased internal clearance, N/NU cylindrical roller, 30206 tapered roller. The rotating ring takes the interference fit - shaft k5/m5 for the inner ring, housing H7 for a stationary outer ring; reverse this if the outer ring rotates.
Mounting
Press only on the ring being fitted, on the inner ring when mounting on a shaft, and never transmit force through the balls. Heat by induction or in an oil bath to 80–110 °C; never exceed 120 °C for sealed bearings. Check clearance or end float after mounting.
Seals
A radial lip seal faces the medium it retains, with the lip pre-lubricated, the shaft hardened to about 55 HRC and ground to Ra 0.2–0.8 µm, and a 15–30° lead-in chamfer. O-rings sit in a groove with 20–25 % squeeze: too little leaks, too much shears the ring. Labyrinth seals suit high speed and dirty environments; gaskets and sealant close static joints.
Lubrication
Grease fills 30–50 % of the free space in the bearing and housing - over-greasing churns and overheats. In a splash-lubricated housing the oil level reaches the centre of the lowest rolling element; ISO VG 32–68 is typical. Keep to change intervals, filter the oil, and monitor vibration (mm/s RMS, ISO 10816), temperature and oil condition.
Worked example
A 6308-2RS C3 on a rotating shaft: bore 08 × 5 = 40 mm, so the shaft is machined k5/m5 and the housing H7, the bearing is heated to 90 °C, and no extra grease is added beyond the factory fill. The seals retain the fill, C3 accommodates thermal expansion in a warm housing, and heating avoids pressing through the balls.
Traps
Pressing on the outer ring while mounting on a shaft, heating a sealed bearing, over-greasing, fitting a lip seal backwards, and using normal clearance in a hot application.