KEVOS® MAKEOS · Power Transmission
Ball Bearing Load Ratings from Geometry (Basic Dynamic and Static Ratings, L10 Life, Static Safety)
From a ball bearing’s geometry — rows, contact angle, number and diameter of balls — what are its basic dynamic and static load ratings, what L10 life does it give at my equivalent load and speed, and is the static load safe?
Basic load ratings of radial, angular contact and thrust ball bearings by the AFBMA formulas the handbook gives: C = fc (i cos α)^0.7 Z^(2/3) D^1.8 for balls up to 25.4 mm, or 3.647 fc (i cos α)^0.7 Z^(2/3) D^1.4 for larger balls; thrust bearings Ca = fc Z^(2/3) D^1.8 at 90° or fc (cos α)^0.7 Z^(2/3) D^1.8 tan α otherwise; rows or bearings in tandem by the 0.7 power; the static ratings Co = fo i Z D² cos α and Coa = fo Z D² sin α; then the rating life L10 = (C ÷ P)³ in millions of revolutions and hours, and the static safety Co ÷ Po. The factors fc and fo are read from the handbook tables for D cos α ÷ dm, which the tool shows.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Bearing type | choice | Radial and angular contact bearings, or thrust bearings. |
| Rows of balls i | rows | Number of rows of balls (for a thrust bearing, rows carrying load in the same direction). |
| Nominal contact angle α | ° | Nominal contact angle: 0° for a deep groove bearing, 90° for a pure thrust bearing. |
| Balls per row Z | balls | Number of balls in each row. |
| Ball diameter D | mm | Diameter of the balls. |
| Pitch diameter of the ball set dm | mm | Pitch diameter of the ball set, used only to read fc and fo from the tables. |
| Factors from the handbook tables | ||
| Dynamic factor fc (metric, for D cos α ÷ dm) | Dynamic rating factor read from the handbook table for the bearing type and D cos α ÷ dm (metric column). | |
| Static factor fo (metric, for D cos α ÷ dm) | Static rating factor read from the handbook table for the bearing type and D cos α ÷ dm (metric column). | |
| Similar bearings in tandem | bearings | Number of similar single-row radial or angular contact bearings mounted in tandem and sharing the load. |
| Loads and requirements | ||
| Equivalent dynamic load P | N | Equivalent dynamic load, X Fr + Y Fa, from the bearing life tools. |
| Speed | rpm | Rotational speed of the bearing. |
| Life you require | h | Rating life the application needs. |
| Static equivalent load Po | N | Static equivalent load, the greater of Xo Fr + Yo Fa and Fr. |
| Least static safety Co ÷ Po you accept | Least static safety factor you accept. | |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Bearing type
- What it is
- Whether the ball bearing carries mainly radial load or mainly thrust.
- Units and format
- Choose one of 2 options from the list: Radial or angular contact; Thrust.
- Example
- The form opens with “Radial or angular contact” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Changes the rating formulas and the static rating.
- Get it right
- Angular contact bearings count as radial when the contact angle is below 90°.
What this means: Rows of balls i
- What it is
- Number of rows of balls in the bearing.
- Units and format
- Enter a number in rows: rows. The form accepts 1 rows to 8 rows. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 rows as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More rows raise the rating less than in proportion.
- Get it right
- For a thrust bearing, count only rows carrying load in the same direction.
What this means: Nominal contact angle α
- What it is
- Nominal contact angle between the load line and the plane square to the axis.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 90°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger angle lowers the radial rating and raises the thrust rating.
- Get it right
- 0° for deep groove and cylindrical bearings, 90° for pure thrust bearings.
What this means: Balls per row Z
- What it is
- Number of balls carried in each row of the bearing.
- Units and format
- Enter a number in balls: balls. The form accepts 3 balls to 500 balls. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 9 balls as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The rating rises with a power of the number below one.
- Get it right
- Count them, or take the number from the bearing drawing.
What this means: Ball diameter D
- What it is
- Diameter of the balls, all the same size in a bearing.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12.7 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The dynamic rating rises with D^1.8 and the static rating with D².
- Get it right
- Measure a ball or take it from the bearing drawing; above 25.4 mm a different formula applies.
What this means: Pitch diameter of the ball set dm
- What it is
- Pitch diameter of the ball set, from bearing centre to ball centre and back.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 63.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets D cos α ÷ dm, the value for reading the factors.
- Get it right
- About the mean of the bore and the outside diameter of the bearing.
Factors from the handbook tables
What this means: Dynamic factor fc (metric, for D cos α ÷ dm)
- What it is
- Dynamic rating factor from the handbook table for the bearing type.
- Units and format
- Enter a plain number with no unit. The form accepts 0.000001 to 1000. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 59.9 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The dynamic rating rises in proportion.
- Get it right
- Read the metric column at the D cos α ÷ dm the tool shows; interpolate between rows.
What this means: Static factor fo (metric, for D cos α ÷ dm)
- What it is
- Static rating factor from the handbook table for the bearing type.
- Units and format
- Enter a plain number with no unit. The form accepts 0.000001 to 1000. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12.1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The static rating rises in proportion.
- Get it right
- Read the metric column at the D cos α ÷ dm the tool shows; interpolate between rows.
What this means: Similar bearings in tandem
- What it is
- Number of similar bearings mounted in tandem and sharing the load.
- Units and format
- Enter a number in bearings: bearings. The form accepts 1 bearings to 10 bearings. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 bearings as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The dynamic rating rises by a power below one; the static rating in proportion.
- Get it right
- Only when they are made and mounted to share the load equally.
Loads and requirements
What this means: Equivalent dynamic load P
- What it is
- Equivalent dynamic load on the bearing.
- Units and format
- Enter a number in N: newtons. The form accepts 0.000001 N or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2000 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Life falls with the cube (ball) or the 10/3 power (roller) of the load.
- Get it right
- X Fr + Y Fa with the factors from the handbook tables, or from the bearing life tools.
What this means: Speed
- What it is
- Rotational speed of the bearing.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 1000000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1500 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Converts the life in revolutions to hours.
- Get it right
- Use the mean speed over the duty cycle.
What this means: Life you require
- What it is
- Rating life the application needs, in hours.
- Units and format
- Enter a number in h: hours. The form accepts 0.000001 h to 10000000 h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20000 h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Fails the bearing when its life is shorter.
- Get it right
- The handbook lists typical design lives for many machines.
What this means: Static equivalent load Po
- What it is
- Static equivalent load on the bearing at rest or turning slowly.
- Units and format
- Enter a number in N: newtons. The form accepts 0 N or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2000 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the static safety.
- Get it right
- The greater of Xo Fr + Yo Fa and Fr; 0 if there is no static load.
What this means: Least static safety Co ÷ Po you accept
- What it is
- Least static safety factor Co ÷ Po you accept.
- Units and format
- Enter a plain number with no unit. The form accepts 0.000001 to 100. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Fails the bearing when the static safety is lower.
- Get it right
- Take it from the bearing maker for the duty and the quietness you need.
Method and formulas
- Radial and angular contact, D ≤ 25.4 mm: C = fc (i cos α)^0.7 Z^(2/3) D^1.8; D > 25.4 mm: C = 3.647 fc (i cos α)^0.7 Z^(2/3) D^1.4; tandem bearings × n^0.7.
- Thrust, D ≤ 25.4 mm: Ca = fc Z^(2/3) D^1.8 (α = 90°) or fc (cos α)^0.7 Z^(2/3) D^1.8 tan α; D^1.4 with 3.647 for larger balls; i similar rows × i^0.7 (formula 10 with equal rows).
- Static: Co = fo i Z D² cos α (× number in tandem); thrust Coa = fo i Z D² sin α.
- L10 = (C ÷ P)³ million revolutions; hours = L10 × 10⁶ ÷ (60 n); static safety = Co ÷ Po.
- Example: the bearing’s dynamic rating is 25,144 N, giving 22,079 h at 2000 N and 1500 rpm.
Limitations
- The formulas apply to bearings of good quality, hardened steel and the groove radii the standard assumes; fc and fo come from the handbook tables.
- The rating life is for 90 % reliability; adjust it for reliability, material and lubrication with the bearing life tools.
- Catalogue ratings from the bearing maker take precedence over these estimates.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Estimate the load rating of a special or non-catalogue ball bearing from its geometry
- Check how ball size, number and contact angle change the rating
- Find the L10 life and static safety of a bearing at the design loads
Who it is for
- Machine designers
- Maintenance and reliability engineers
- Bearing application engineers
What you get
- D cos α ÷ dm for reading fc and fo
- Basic dynamic load rating C or Ca
- Basic static load rating Co or Coa
- Rating life L10 in millions of revolutions and hours
- Static safety Co ÷ Po
Interpreting the result
- Ball size has the strongest effect: the rating rises with D^1.8 and the static rating with D².
- A long life at the equivalent load does not prove a bearing is suitable; check speed, lubrication and fits as well.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2405–2409, 2415–2418. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Useful alongside · Library collection
Selecting bearings and lubrication for reliable machines: types, life, fits, sealing and failure causes
In Machine Elements and Mechanical Design
What it covers: Most bearings fail from contamination, poor lubrication or bad fitting, not fatigue. How to choose bearing types, size them for life, fit, seal and lubricate them, and diagnose failures.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.