Rolling Bearing Selection Basics: Types, Numbering and L10 Life

Every rotating shaft needs bearings, and a wrongly chosen bearing is one of the most common reasons machines come back for repair. Selecting a rolling bearing involves three decisions: which type suits the load direction and speed, what size fits the shaft and housing, and whether its rated life is long enough for the duty. This article explains the main bearing types, how to read a standard bearing number, and how to calculate bearing life using the basic dynamic load rating from a catalogue.

Main Types of Rolling Bearings

Type	Radial load	Axial load	Speed	Typical use
Deep groove ball	Good	Moderate, both directions	Very high	Electric motors, fans, gearboxes
Angular contact ball	Good	High, one direction (used in pairs)	High	Machine tool spindles, pumps
Cylindrical roller	Very high	Little or none (most designs)	High	Gearboxes, large motors, rolling mills
Tapered roller	High	High, one direction (used in pairs)	Moderate	Wheel hubs, differentials
Spherical roller	Very high	Moderate	Moderate	Heavy machinery, tolerates misalignment
Thrust ball	None	Pure axial	Low to moderate	Vertical shafts, turntables
Needle roller	High for its size	None	Moderate	Compact spaces, rocker arms

Ball bearings make point contact and run with low friction at high speed. Roller bearings make line contact and carry heavier loads in the same space but generally at lower speeds. If the shaft may bend or the housings may not be perfectly aligned, choose a self-aligning or spherical roller bearing.

Reading a Bearing Number

Most metric bearings follow the ISO basic designation system. For a number such as 6205, the first digit gives the type, the second the dimension series, and the last two the bore code.

Bore code	Bore diameter
00	10 mm
01	12 mm
02	15 mm
03	17 mm
04 and above	Code x 5 mm (04 = 20 mm, 05 = 25 mm, 10 = 50 mm)

So 6205 is a deep groove ball bearing (6) from the light series (2) with a 25 mm bore. A 6305 has the same bore but belongs to the medium series (3), so it is larger in outside diameter and width and carries more load. Suffixes such as 2RS (rubber seals both sides) or ZZ (metal shields both sides) describe sealing; their exact letters vary slightly between manufacturers.

Bearing Life and Load Rating

Even a perfectly installed bearing eventually fails by fatigue of the rolling surfaces. Because fatigue is statistical, life is defined as L10: the number of revolutions that 90 percent of a large group of identical bearings will complete or exceed before the first sign of fatigue.

Basic rating life    L10 = (C / P)^p        (million revolutions)
                     p = 3      for ball bearings
                     p = 10/3   for roller bearings
Life in hours        L10h = L10 * 10^6 / (60 * N)
Required rating      C = P * (L10)^(1/p)
Equivalent load      P = X * Fr + Y * Fa

C  = basic dynamic load rating (from catalogue)
Fr = radial load, Fa = axial load, N = speed in rpm

For a pure radial load P is simply Fr. When there is also an axial load, X and Y factors from the manufacturer's table are used; for deep groove ball bearings X is 0.56 when the axial load is significant, and Y depends on the ratio of axial load to the static rating C0.

Worked Example: Checking a Motor Bearing

A bearing on a motor shaft carries a radial load of 2 kN at 1440 rpm. The selected deep groove ball bearing has a catalogue dynamic load rating C = 14.8 kN. Check its life, then find the rating needed for 20,000 hours.

Life in revolutions:
  L10 = (14.8 / 2)^3 = 7.4^3 = 405 million rev
Life in hours:
  L10h = 405e6 / (60 * 1440) = 4690 h

Required for 20,000 h:
  L10 = 20,000 * 60 * 1440 / 1e6 = 1728 million rev
  C   = 2 * 1728^(1/3) = 2 * 12.0 = 24.0 kN

About 4700 hours is under two years of single-shift use, which is too short for an industrial motor. You would pick a bearing from the catalogue with C of at least 24 kN in a bore that fits the shaft, usually by moving to a heavier series or a larger bore.

The cube law makes life very sensitive to load. Halving the load on a ball bearing increases its life eight times; a 25 percent overload cuts it roughly in half.

Machine duty (typical guideline)	Suggested L10h (hours)
Household appliances, occasional use	300 to 3000
Machines used 8 h a day, not fully loaded	10,000 to 25,000
Machines used 8 h a day, fully loaded	20,000 to 30,000
Continuous 24 h operation	40,000 to 50,000

Practical Selection Points

Locate the shaft axially with one bearing and let the other float to allow thermal expansion.
Check the limiting speed in the catalogue, especially for sealed bearings and grease lubrication.
Use the correct fits: usually an interference fit on the rotating ring and a transition or clearance fit on the stationary ring.
Contamination and poor lubrication shorten life far more than the L10 formula predicts; modern catalogues add life modification factors for this.
Never hammer a bearing through the rolling elements during fitting; press on the ring that is being fitted.

Summary

Choose the bearing type from load direction, speed and alignment; decode the bore from the last two digits of the number; and confirm life with L10 = (C/P)^p. Because life varies with the cube of load ratio for ball bearings, a modest increase in rating gives a large gain in service life. Always finish by checking speed limits, fits and lubrication.