Rolling bearings
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Frequently asked questions
How do I read a bearing code such as 6205-2RS C3?
The 6 means a deep groove ball bearing, the 2 is the dimension series, and 05 times 5 gives a 25 mm bore. The suffix 2RS means two rubber seals, 2Z or ZZ means two steel shields, and C3 means radial clearance larger than normal. Boundary dimensions follow ISO 15, so bearings with the same basic code from different makers are dimensionally interchangeable.
When do I need C3 clearance instead of normal clearance?
C3 is used when the inner ring has a tight fit on the shaft, at high speed, or when the shaft runs much hotter than the housing, because thermal expansion reduces clearance in operation. Electric motors commonly use C3 for this reason. Replacing a C3 bearing with normal clearance in such applications can make the bearing run hot and fail early.
The bearing code is worn off. What do I measure to find a replacement?
Measure the bore, outside diameter and width with a caliper, then look up the series in the ISO 15 dimension tables. Also note the sealing type, the rolling element type (ball, cylindrical roller, tapered, spherical) and whether there is a snap ring groove. Several bearing types share the same three dimensions, so a photo of the cross section helps.
Which installation mistakes cause early bearing failure?
Hammering on the outer ring while pressing the inner ring onto the shaft dents the raceways, and is a very common cause of damage. Mounting force must act on the ring with the tight fit, or the bearing can be heated with an induction heater to around 80 to 110 degrees C. Overgreasing also makes the bearing run hot and pushes the seals out.
How do rubber seals and steel shields compare when choosing a bearing?
Rubber seals (2RS) keep out dust and water better but have contact friction, so the speed limit is lower. Steel shields (2Z) are non-contact and allow higher speed but only stop coarse dirt. Damp areas with fine dust favour 2RS, while high-speed motors in clean surroundings suit 2Z.
How often should a bearing be regreased?
The regreasing interval depends on speed, bearing size, temperature and contamination, and is normally calculated from the bearing maker's chart or formula. Faster and hotter bearings need shorter intervals, and running above about 70 degrees C typically halves grease life. Bearings sealed or shielded on both sides (2RS, ZZ) are greased for life and are not relubricated.
How much grease is enough, and is more better?
No, overfilling causes churning friction, which heats the bearing and breaks the grease down faster. At first fill, grease usually occupies about 30 to 50 percent of the free space inside the bearing. When relubricating, pump slowly in small amounts and let the old grease escape through the purge path.
Can the damage pattern on a bearing reveal the cause?
Yes, the failure pattern usually points to the root cause. Flaking of the raceway after long service is normal fatigue, dents spaced at ball pitch come from impact during fitting, and fluting with parallel grooves comes from electric current passing through the bearing. Blue or brown discoloration usually means lubricant starvation or overheating.
