BIZOMAT MAKEOS · Engineering
Bearing Life for a Load and Speed Duty Cycle
What is the equivalent load and rating life of this bearing over a varying duty cycle?
Mean equivalent load and speed over up to four duty-cycle steps (load, speed, share of time), and the resulting L10 life in hours for ball or roller bearings.
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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. |
| Dynamic load rating C | kN | Basic dynamic load rating from the catalogue. |
| Step 1 | ||
| Equivalent load | kN | Equivalent dynamic load in step 1 (see the adjusted life tool for X and Y factors). |
| Speed | rpm | Speed in step 1. |
| Share of time | % | Share of operating time in step 1. |
| Step 2 | ||
| Equivalent load | kN | Load in step 2. |
| Speed | rpm | Speed in step 2. |
| Share of time | % | Share of time in step 2. |
| Step 3 | ||
| Equivalent load | kN | Load in step 3. |
| Speed | rpm | Speed in step 3. |
| Share of time | % | Share of time in step 3. |
| Step 4 | ||
| Equivalent load | kN | Load in step 4 (0 if not used). |
| Speed | rpm | Speed in step 4. |
| Share of time | % | Share of time in step 4. |
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. It sets the life exponent used both to average the loads and to calculate life.
- Units and format
- Choose one of 2 options from the list: Ball (exponent 3); Roller (exponent 10/3).
- Example
- The form opens with “Ball (exponent 3)” selected as a worked example. Choose the option that matches your case.
What this means: Dynamic load rating C
- What it is
- The basic dynamic load rating of the bearing from the manufacturer’s catalogue: the load at which it reaches one million revolutions of rating life.
- 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 of C for ball bearings, so a slightly larger bearing lasts much longer.
- Get it right
- Use the dynamic rating C, not the static rating C₀, and enter kilonewtons.
Step 1
What this means: Equivalent load
- What it is
- The equivalent dynamic bearing load during the first step of the duty cycle, combining radial and axial loads with the catalogue X and Y factors.
- 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
- Combine radial and axial loads first; entering only the radial load understates it.
What this means: Speed
- What it is
- The shaft speed during the first step. A step with zero speed or zero time share is left out of the averages.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0 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.
What this means: Share of time
- What it is
- The share of total operating time spent in the first step of the cycle.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- The shares are normalised, so they do not have to add up to 100 %, but keeping them to 100 % avoids confusion.
Step 2
What this means: Equivalent load
- What it is
- The equivalent dynamic bearing load during the second step of the duty cycle, combining radial and axial loads with the catalogue X and Y factors.
- 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 8 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Speed
- What it is
- The shaft speed during the second step. Steps with zero speed are ignored.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0 rpm to 1000000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 750 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Share of time
- What it is
- The share of total operating time spent in the second step of the cycle.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Step 3
What this means: Equivalent load
- What it is
- The equivalent dynamic bearing load during the third step of the duty cycle, combining radial and axial loads with the catalogue X and Y factors.
- 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 2 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Speed
- What it is
- The shaft speed during the third step. Steps with zero speed are ignored.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0 rpm to 1000000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Share of time
- What it is
- The share of total operating time spent in the third step of the cycle.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Step 4
What this means: Equivalent load
- What it is
- The equivalent dynamic bearing load during the fourth step of the duty cycle. Leave the step’s speed or time at zero if it is not used.
- 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 0 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Speed
- What it is
- The shaft speed during the fourth step. Zero leaves the step out.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0 rpm to 1000000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Share of time
- What it is
- The share of total operating time spent in the fourth step. Zero leaves the step out.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 100%. 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.
Method and formulas
- Mean speed nm = Σ nᵢ tᵢ ÷ Σ tᵢ.
- Mean load Pm = (Σ Pᵢᵖ nᵢ tᵢ ÷ Σ nᵢ tᵢ)^(1/p), p = 3 (ball) or 10/3 (roller).
- L10 = (C ÷ Pm)ᵖ million revolutions; hours = L10 × 10⁶ ÷ (60 nm).
- Example: the default duty cycle gives Pm = 4.1857 kN and L10h = 3,409.1 h.
Limitations
- Basic L10 life; apply reliability and lubrication factors with the adjusted life tool.
- Steps are treated as long compared with a revolution.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Find bearing life for machines with changing loads and speeds
- Compare duty cycles
- Check a bearing selection against a mission profile
Who it is for
- Design engineers
- Reliability engineers
What you get
- Mean speed
- Mean equivalent load
- L10 life in revolutions and hours
- Share of damage from each step
Interpreting the result
- The heaviest step usually dominates damage because life falls with the cube of load.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2400–2420. Topic reference; no source tables or recommended settings are reproduced.
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Open the Bearing Life for a Load and Speed Duty Cycle calculator