BIZOMAT MAKEOS · Power Transmission
Rolling Bearing Selection Check
Does this bearing give the life I need at the reliability I want, and is it safe statically and at speed?
Selection check of a ball or roller bearing: dynamic equivalent load from radial and axial loads, required dynamic rating for the life at the chosen reliability (Weibull reliability factor) and life modification factor, the life of the chosen bearing, static equivalent load and safety factor, and speed against the limiting speed.
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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 (life exponent 3 or 10/3). |
| Radial load | kN | Radial load. |
| Axial load | kN | Axial load. |
| Radial factor X | Radial load factor for the bearing type and Fa/Fr ratio (from the bearing catalogue). | |
| Axial factor Y | Axial load factor (from the catalogue; use X = 1, Y = 0 when Fa/Fr is below e). | |
| Speed | rpm | Rotational speed. |
| Required life | h | Life required in hours. |
| Reliability | % | Required reliability (90% is the basic rating). |
| Reliability factor basis | choice | ISO 281:2007 gives slightly higher factors than the older ANSI/ABMA values printed in the handbook; bearing makers now use ISO 281. |
| Life modification factor | Life modification factor for lubrication and contamination (from the bearing maker’s method; 1 for basic life). | |
| Chosen bearing | ||
| Dynamic load rating | kN | Dynamic load rating of the chosen bearing. |
| Static load rating | kN | Static load rating. |
| Static radial factor | Static radial factor. | |
| Static axial factor | Static axial factor. | |
| Required static safety factor | Static safety factor required (higher for shock and quiet running). | |
| Limiting speed | rpm | Limiting speed of the bearing with its lubrication. |
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 as rolling elements. This sets the life exponent: rollers have line contact and a flatter life curve.
- Units and format
- Choose one of 2 options from the list: Ball; Roller.
- Example
- The form opens with “Ball” selected as a worked example. Choose the option that matches your case.
- Get it right
- Tapered, cylindrical, spherical and needle bearings are all roller types.
What this means: Radial load
- What it is
- Load acting at right angles to the shaft on this one bearing, from the shaft’s reaction forces at the bearing position.
- 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.
- Effect on the result
- Both the dynamic and static equivalent loads rise with it.
- Get it right
- Work out the reaction at each bearing; do not enter the total load on the shaft.
What this means: Axial load
- What it is
- Load acting along the shaft axis on this bearing, such as thrust from helical gears, pumps 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
- Assign the thrust to the bearing that actually locates the shaft in that direction.
What this means: Radial factor X
- What it is
- Catalogue factor that converts the radial load into its share of the dynamic equivalent load, for this bearing type and its axial-to-radial load ratio.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 2. 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
- Read X and Y together from the catalogue for your Fa/Fr ratio; the tool never lets the equivalent load fall below Fr.
What this means: Axial factor Y
- What it is
- Catalogue factor that converts the axial load into its share of the dynamic equivalent load, for this bearing and its axial-to-radial load 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 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 pair of X and Y that belongs to your Fa/Fr ratio; the catalogue lists different values either side of its limit e.
What this means: Speed
- What it is
- Rotational speed of the bearing’s rotating ring. It converts life in revolutions into hours and is checked against the limiting speed.
- 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 varying speed use a time-weighted mean speed; for oscillating motion this tool does not apply directly.
What this means: Required life
- What it is
- Fatigue life you need from the bearing in operating hours, used to work out the dynamic rating required.
- 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.
- Get it right
- Base it on real running hours per year times the service years expected for the machine.
What this means: Reliability
- What it is
- Percentage of a large group of identical bearings that must survive the required life. Higher reliability reduces the life factor a₁.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 90% to 99.95%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 95% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher reliability lowers the life of the chosen bearing and raises the rating required.
- Get it right
- Type the percentage itself (for example 99), not a fraction.
What this means: Reliability factor basis
- What it is
- Which formula the tool uses for the reliability factor a₁: the current ISO 281:2007 one, or the older ANSI/ABMA 1990 values printed in the handbook.
- Units and format
- Choose one of 2 options from the list: ISO 281:2007; ANSI/ABMA 1990 (as in the handbook).
- Example
- The form opens with “ISO 281:2007” selected as a worked example. Choose the option that matches your case.
- Get it right
- Match the basis your bearing maker uses for its catalogue life calculations.
What this means: Life modification factor
- What it is
- Life modification factor for lubrication, contamination and fatigue limit, from the bearing maker’s method. It multiplies the basic rating life.
- Units and format
- Enter a plain number with no unit. The form accepts 0.05 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
- Life rises in proportion, and the rating required falls.
- Get it right
- Use the value from the bearing maker’s calculation; it can be well below or above 1 depending on lubrication and cleanliness.
Chosen bearing
What this means: Dynamic load rating
- What it is
- Basic dynamic load rating of the bearing you have chosen, from its catalogue entry. Life is calculated from the ratio of this rating to the equivalent load.
- 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 70 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 C to the power 3 for balls or 10/3 for rollers.
- Get it right
- Use the dynamic rating C, not the static rating C0, for the exact bearing designation.
What this means: Static load rating
- What it is
- Basic static load rating of the chosen bearing: the load that causes a set amount of permanent deformation at the contacts. It is used for the static safety check.
- 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 45 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The static safety factor rises in proportion.
- Get it right
- Take C0 from the catalogue for the same bearing designation as C.
What this means: Static radial factor
- What it is
- Catalogue factor converting the radial load into its share of the static equivalent load for the chosen bearing.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 2. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.6 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Static factors X0 and Y0 differ from the dynamic X and Y; read both pairs from the catalogue.
What this means: Static axial factor
- What it is
- Catalogue factor converting the axial load into its share of the static equivalent load for the chosen bearing.
- 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 0.5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Read it from the catalogue for this bearing type and contact angle.
What this means: Required static safety factor
- What it is
- The lowest acceptable ratio of static rating to static equivalent load. It protects against permanent dents from peak or shock loads.
- Units and format
- Enter a plain number with no unit. The form accepts 0.5 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Take it from the bearing maker’s guidance for your duty; shock loads and quiet running call for higher values.
What this means: Limiting speed
- What it is
- Highest speed the chosen bearing may run at with its lubrication, from the catalogue. The operating speed is checked against it.
- 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 8000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Catalogues may list separate grease and oil speeds; use the one for your lubrication.
Method and formulas
- P = max(F_r, X F_r + Y F_a); life exponent p = 3 (ball) or 10/3 (roller).
- Reliability factor: ANSI/ABMA 1990 a₁ = [ln(1 ÷ R) ÷ ln(1 ÷ 0.9)]^(2/3) (handbook values 0.62 at 95%, 0.21 at 99%); ISO 281:2007 a₁ = 0.95 × that + 0.05 (0.64 at 95%, 0.25 at 99%).
- Life L_h = a₁ a_ISO (C ÷ P)^p × 10⁶ ÷ 60n; required C = P [L_h,req 60n ÷ (10⁶ a₁ a_ISO)]^(1/p).
- Static: P₀ = max(F_r, X₀ F_r + Y₀ F_a); s₀ = C₀ ÷ P₀.
- Example: 4 kN radial and 1.5 kN axial at 1500 rpm need C ≥ 65.568 kN for 20000 h at 95% reliability.
Limitations
- X, Y and the life modification factor come from the bearing maker for the exact bearing.
- Fatigue life only; wear, lubrication failure and contamination often end bearings sooner.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Pick a bearing size for a shaft load and life
- Check a bearing against a higher reliability requirement
- Confirm static safety for shock loads
Who it is for
- Design engineers
- Maintenance engineers
- Power transmission engineers
What you get
- Equivalent load and required rating
- Life of the chosen bearing
- Static safety and speed checks
Interpreting the result
- Raising reliability from 90% to 99% cuts the expected life to about a fifth.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2365–2422. Topic reference; no source tables or recommended settings are reproduced.
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