BIZOMAT MAKEOS · Engineering
Rolling Bearing Adjusted Rating Life
How many hours will this ball or roller bearing last at the required reliability?
Equivalent dynamic load from radial and axial loads, basic L10 life for ball or roller bearings, and life adjusted for reliability and a life modification factor, against a required life.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Bearing type | choice | Ball or roller bearing. |
| Basic dynamic load rating C | kN | Basic dynamic load rating from the bearing catalogue. |
| Radial load | kN | Radial load on the bearing. |
| Axial load | kN | Axial load on the bearing. |
| Radial factor X | Radial load factor from the catalogue for the Fa/Fr ratio (1 for pure radial load). | |
| Axial factor Y | Axial load factor from the catalogue (0 for pure radial load). | |
| Speed | rpm | Shaft speed (use a weighted mean for variable duty). |
| Reliability | choice | Required reliability; a₁ values follow ISO 281. |
| Life modification factor a_ISO | Life modification factor for lubrication and contamination from the bearing maker’s method (1 if not assessed). | |
| Required life | h | Required service life from the machine specification. |
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 bearing has balls or rollers, which sets the life exponent in the rating life equation.
- Units and format
- Choose one of 2 options from the list: Ball bearing (exponent 3); Roller bearing (exponent 10/3).
- Example
- The form opens with “Ball bearing (exponent 3)” selected as a worked example. Choose the option that matches your case.
- Get it right
- Needle and tapered roller bearings count as roller bearings.
What this means: Basic dynamic load rating C
- What it is
- The basic dynamic load rating of the bearing from the maker’s catalogue: the load that gives a basic rating life of one million revolutions.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0.000001 kN to 100000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 52 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Life rises with the cube of C for ball bearings and the 10/3 power for roller bearings.
- Get it right
- Use the dynamic rating C, not the static rating C₀.
What this means: Radial load
- What it is
- The load acting on the bearing at right angles to the shaft, from a shaft reaction calculation.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0 kN to 100000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter the load on this bearing, not the total load on the shaft.
What this means: Axial load
- What it is
- The load acting on the bearing along the shaft axis, such as gear thrust or preload.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0 kN to 100000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the thrust on this bearing only; enter 0 for a purely radial load.
What this means: Radial factor X
- What it is
- The radial load factor used to combine radial and axial loads into an equivalent load, from the catalogue for this bearing and its Fa/Fr ratio.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.56 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the catalogue value for the actual Fa/Fr ratio; for pure radial load it is 1.
What this means: Axial factor Y
- What it is
- The axial load factor used to combine radial and axial loads into an equivalent load, from the catalogue for this bearing.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.6 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the catalogue value for the actual Fa/Fr ratio; for pure radial load it is 0.
What this means: Speed
- What it is
- The rotational speed of the bearing, which converts life in revolutions into hours.
- 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
- Life in hours falls in proportion as speed rises.
- Get it right
- For variable duty use a time-weighted mean speed.
What this means: Reliability
- What it is
- The reliability the life must achieve: the share of identical bearings expected to reach the calculated life. It selects the reliability factor a₁.
- Units and format
- Choose one of 6 options from the list: 90% (a₁ = 1); 95% (a₁ = 0.64); 96% (a₁ = 0.55); 97% (a₁ = 0.47); 98% (a₁ = 0.37); 99% (a₁ = 0.25).
- Example
- The form opens with “90% (a₁ = 1)” selected as a worked example. Choose the option that matches your case.
- Get it right
- Higher reliability gives a shorter life for the same bearing.
What this means: Life modification factor a_ISO
- What it is
- The life modification factor for lubrication, contamination and fatigue load limit, from the bearing maker’s method.
- Units and format
- Enter a plain number with no unit. The form accepts 0.01 to 50. 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
- Adjusted life changes in direct proportion.
- Get it right
- Use 1 only if it has not been assessed; work it out with the maker’s method for the lubricant and cleanliness.
What this means: Required life
- What it is
- The service life the bearing must reach in operating hours, from the machine specification or customer requirement.
- Units and format
- Enter a number in h: hours. The form accepts 0 h to 10000000 h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15000 h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Moves the pass or fail line only; the calculated life does not change.
- Get it right
- Count running hours, not calendar time.
Method and formulas
- P = X Fr + Y Fa (P = Fr when X = 1 and Y = 0; P is never taken below Fr).
- L10 = (C ÷ P)^p million revolutions, p = 3 (ball) or 10/3 (roller); hours = L10 × 10⁶ ÷ (60 n).
- Lnm = a₁ × a_ISO × L10.
- Example: C = 52 kN with 4 kN radial and 1.5 kN axial gives P = 4.64 kN and L10h = 15,639 h.
Limitations
- X and Y depend on the bearing type and Fa/Fr ratio; take them from the catalogue for the actual bearing.
- Static capacity, speed limit, fit and lubrication must be checked separately.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Select a bearing for a required life
- Check bearing life after a load or speed change
- Compare ball and roller bearings
Who it is for
- Design engineers
- Maintenance engineers
- Reliability engineers
What you get
- Equivalent dynamic load
- Basic L10 life (millions of revolutions and hours)
- Adjusted life Lnm in hours
- Comparison with the required life
Interpreting the result
- If life is short, check the a_ISO factor (lubrication and cleanliness) before choosing a larger bearing.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2400–2420. Topic reference; no source tables or recommended settings are reproduced.
- ISO 281:2007 Rolling bearings — Dynamic load ratings and rating life — https://www.iso.org/standard/38102.html
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