How to Choose a Bearing
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Most bearing trouble traces back to a handful of choices: the type, the size for the load and the life you need, the speed, the seals and lubrication, and the fits. Work through them in that order.
Match the type to the load
- Deep groove ball bearings: radial loads plus moderate axial load in both directions. The default for motors, pumps and general machinery.
- Angular contact ball bearings: combined radial and axial loads at high speed, usually mounted in pairs. Common in machine tool spindles.
- Cylindrical roller bearings: heavy radial loads with little or no axial load.
- Tapered roller bearings: heavy combined loads, as in wheel hubs and gearbox shafts. Mounted in pairs and adjusted for preload or end play.
- Spherical roller bearings: heavy loads where the shaft deflects or the housings don't line up exactly. Self-aligning ball bearings do the same job at lighter loads.
- Thrust ball bearings: axial loads only.
- Needle roller bearings: radial loads where there's very little room.
Size it for life, not just load
Every catalog bearing has a basic dynamic load rating, C. The standard life formula gives L10, the life that 90 percent of a group of identical bearings will reach or exceed:
L10 = (C/P)p million revolutions, where P is the equivalent load and p is 3 for ball bearings and 10/3 for roller bearings.
A worked example: a 6205 deep groove ball bearing (25 mm bore) is rated at C = 14.8 kN in SKF's catalog. With a steady 2 kN radial load at 1,500 rpm, L10 = (14.8/2)³, about 405 million revolutions, or about 4,500 hours. Double the load to 4 kN and the life drops to an eighth, about 560 hours.
The cube works in your favor too. A bearing with a 26 percent higher rating doubles the life, which is why moving up one size often fixes a recurring failure. If there's an axial load as well, the catalog gives X and Y factors to combine it with the radial load into P. Catalogs also adjust life for lubrication and cleanliness, and those factors can move the answer a lot in either direction.
Check the speed
Catalogs list a reference speed and a limiting speed. SKF lists the open 6205 at 18,000 rpm, but seals, cage, lubrication and load all change what's practical. Contact seals (2RS) keep dirt out best but add friction and lower the speed limit. Shields (2Z or ZZ) add almost no drag but only keep out larger particles. Past the limits of grease, look at oil lubrication and at precision or hybrid ceramic bearings.
Seals, lubrication and contamination
Many bearings fail early from dirt, water or the wrong lubricant rather than from overload. Sealed, greased-for-life bearings suit most small machines. Washdown, dusty and wet areas need better seals or a sealed housing. Stainless bearings resist corrosion but carry lower load ratings than standard bearing steel. On motors run from variable frequency drives, current passing through the bearing can pit the races, and insulated or hybrid ceramic bearings stop it.
Get the fits and clearance right
The ring that rotates relative to the load needs an interference fit, or it will creep and wear its seat. On a typical motor shaft that's the inner ring, often on a k5 or m5 shaft, with the outer ring in an H7 housing. An interference fit takes up some of the bearing's internal clearance, and so does a shaft running hotter than its housing, which is why electric motors often use C3 bearings, with more clearance than normal.
Allow for misalignment
Deep groove ball bearings tolerate only a few minutes of arc of misalignment before their life drops. Spherical roller and self-aligning ball bearings tolerate around a degree or more. If the housings bolt to a frame that flexes, pick a bearing that can follow it.
When the bearing isn't the problem
If the same position keeps failing, look at the shaft, the housing and the load before buying another bearing. And if the part around it, like a ball screw or a housing, is worn or obsolete, it can often be measured and remade. See our ball screw case study, or the bearings, ball screws and linear rails on our bearings page.