KEVOS® MAKEOS · Maintenance
Bearing Fleet Spares Plan (Rating Life, Weibull Failures, Spares for a Service Level)
For a group of identical bearings in service, how many are expected to fail over the planning period, and how many spares must I hold to be this sure of not running out?
Spares plan for identical rolling bearings: the basic rating life L₁₀ from the dynamic rating, equivalent load and speed (as the bearing life tool), the Weibull characteristic life that puts 10 % of failures at L₁₀ for the slope you enter, the probability that one bearing fails within the planning period (as the Weibull reliability tool), the expected number of failures in the group, and the smallest number of spares whose binomial probability of covering all failures reaches your service level, against the spares held.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Bearing type | choice | Ball or roller bearing (sets the life exponent). |
| Basic dynamic load rating | kN | Basic dynamic load rating from the bearing maker’s catalogue. |
| Equivalent dynamic load | kN | Equivalent dynamic load on each bearing in service. |
| Speed | rpm | Running speed of each bearing. |
| Weibull slope of bearing life | Weibull slope of bearing life scatter; the reliability factor a₁ in the bearing selection check corresponds to a slope of 1.5. Use your own failure records where you have them. | |
| Fleet and plan | ||
| Identical bearings in service | bearings | Number of identical bearings running under the same load and speed. |
| Running hours per year | h/year | Running hours of each bearing per year. |
| Planning period | years | Period the spares must cover (often the resupply time or a stock-review period). |
| Chance of not running out | % | Probability that the spares cover every failure in the period. |
| Spares held | bearings | Spares in stock now. |
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 bearings are ball or roller bearings, which sets the life exponent in the rating life formula: 3 for ball and ten-thirds for roller bearings.
- 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.
- Effect on the result
- The roller exponent makes life more sensitive to the load ratio.
- Get it right
- Tapered, cylindrical and spherical roller bearings all use the roller exponent.
What this means: Basic dynamic load rating
- What it is
- The basic dynamic load rating of the bearing from the maker’s catalogue: the load at which 90 % of a group of bearings reach 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 30 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 (ball) of the rating, so fewer bearings fail and fewer spares are needed.
- Get it right
- Use the rating for the exact bearing designation and maker; ratings differ between makers.
What this means: Equivalent dynamic load
- What it is
- The constant radial load that would give the same life as the actual combination of radial and axial loads on each bearing in service.
- 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 3 kN 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) of the load, so failures and spares rise sharply.
- Get it right
- Work it out from the loads with the maker’s X and Y factors (the bearing selection check does this).
What this means: Speed
- What it is
- The rotational speed of each bearing in service, which turns the rating life in revolutions into running 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 1450 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher speed shortens the life in hours, so more bearings fail in the period.
- Get it right
- Use the shaft speed of the bearing itself, after any belt or gear ratio.
What this means: Weibull slope of bearing life
- What it is
- The Weibull shape parameter describing how widely the lives of identical bearings scatter; it fixes how the 10 % failure point at the rating life spreads into failures over time.
- Units and format
- Enter a plain number with no unit. The form accepts 0.3 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.
- Effect on the result
- With running time well below the rating life, a steeper slope predicts fewer early failures.
- Get it right
- Use your own failure records where you have them; the reliability factor in the bearing selection check corresponds to 1.5.
Fleet and plan
What this means: Identical bearings in service
- What it is
- The number of identical bearings running under the same load and speed that the spares must cover, such as the same bearing on a row of pumps or conveyor idlers.
- Units and format
- Enter a number in bearings: bearings. The form accepts 1 bearings to 1000 bearings. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 bearings as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Expected failures and the spares needed rise with the number in service.
- Get it right
- Group only bearings with similar duty; bearings on different loads need separate plans.
What this means: Running hours per year
- What it is
- The running hours of each bearing per year, which with the planning period gives the running time over which failures are counted.
- Units and format
- Enter a number in h/year: h/year. The form accepts 0.000001 h/year to 8784 h/year. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4000 h/year as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More hours raise the chance of failure per bearing and the spares needed.
- Get it right
- Count only hours turning; a machine on one shift runs about 2000 hours a year.
What this means: Planning period
- What it is
- The length of time the spares must cover, such as the resupply time for an imported bearing or the period between stock reviews.
- Units and format
- Enter a number in years: years. The form accepts 0.01 years to 50 years. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 years as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer period raises the chance of failure per bearing and the spares needed.
- Get it right
- For a long-lead bearing use at least the resupply time; fractions of a year are accepted.
What this means: Chance of not running out
- What it is
- The probability that the spares held will cover every bearing failure in the period, the service level the stock is sized to.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 50% to 99.99%. 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 service level raises the spares needed.
- Get it right
- Critical bearings whose failure stops production justify a high level; the spares needed grow quickly near 100 %.
What this means: Spares held
- What it is
- The number of these bearings in the store now, compared with the number needed and used to find the chance that they cover all failures.
- Units and format
- Enter a number in bearings: bearings. The form accepts 0 bearings to 1000 bearings. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 bearings as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line and the cover reported; the failures expected do not change.
- Get it right
- Count only bearings in good condition, correctly stored and in date for their grease.
Method and formulas
- L₁₀ = (C ÷ P)^p × 10⁶ ÷ (60 n) hours, p = 3 for ball and 10/3 for roller bearings (as the bearing life tool).
- Characteristic life η = L₁₀ ÷ [ln (1 ÷ 0.9)]^(1/β); chance one bearing fails in t hours F = 1 − exp[−(t ÷ η)^β] (as the Weibull reliability tool).
- Failures in the group follow a binomial distribution with N bearings and probability F; spares needed = smallest s with P(X ≤ s) ≥ the service level.
- Example: L₁₀ is 11,494 h; each bearing has a 2.1397 % chance of failing, 0.85589 failures are expected and 3 spares give 95 % cover.
Limitations
- All bearings start the period as new and a replaced bearing is not counted failing again; with a high chance of failure per bearing, renewals add more demand.
- Rating-life model only: lubrication, contamination, misalignment and installation damage often decide real bearing life.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set stock levels for critical bearings
- Check whether a store holds enough spares for a fleet of pumps or conveyors
- Show how load and speed drive spares demand
Who it is for
- Maintenance planners
- Reliability engineers
- Stores and purchasing staff
What you get
- L₁₀ life and Weibull characteristic life
- Chance of failure per bearing and expected failures
- Spares needed for the service level against spares held
Interpreting the result
- Life varies with the cube of the load ratio for ball bearings, so a small overload multiplies the spares a fleet needs.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2403–2415, 128–133. Topic reference; no source tables or recommended settings are reproduced.
- NIST/SEMATECH e-Handbook of Statistical Methods, 1.3.6.6.18 Binomial Distribution — https://www.itl.nist.gov/div898/handbook/eda/section3/eda366i.htm
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Explains the concept · 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.
Useful alongside · Library collection
Maintenance that prevents breakdowns: choosing a strategy for each machine in a small factory
In Lean, Maintenance and Continuous Improvement
What it covers: Running equipment until it breaks feels cheap until the breakdown comes at the worst time. How to rank machines, match maintenance to each, involve operators and learn from repeat 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.