KEVOS® MAKEOS · Power Transmission
Ball Screw Life over a Duty Cycle (Mean Load, Mean Speed, Life)
Over my machine’s duty cycle of different loads and speeds, what mean load and speed does the ball screw see, and how long will it last?
Life of a ball screw working through a duty cycle of up to four load–speed steps: the time-weighted mean speed and the cubic mean load (as the bearing duty cycle tool, ball exponent), the rated life in revolutions and hours at that mean load with the load factor, and the travel in kilometres (as the ball screw life tool), against the life you need.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Dynamic load rating Ca | kN | Dynamic load rating of the ball screw nut from the maker. |
| Load factor fw | Load factor for shock and vibration (1.0–1.2 smooth, up to 2 or more with shock). | |
| Lead | mm | Lead of the screw. |
| Required life | h | Life the screw must reach. |
| Step 1 | ||
| Axial load | kN | Axial load in step 1 (including preload effect). |
| Speed | rpm | Screw speed in step 1. |
| Share of time | % | Share of operating time in step 1. |
| Step 2 | ||
| Axial load | kN | Axial load in step 2. |
| Speed | rpm | Speed in step 2. |
| Share of time | % | Share of time in step 2. |
| Step 3 | ||
| Axial load | kN | Axial load in step 3. |
| Speed | rpm | Speed in step 3. |
| Share of time | % | Share of time in step 3. |
| Step 4 | ||
| Axial load | kN | Axial load in step 4 (0 if unused). |
| Speed | rpm | Speed in step 4. |
| Share of time | % | Share of time in step 4. |
Method and formulas
- Mean speed n_m = Σ nᵢ tᵢ ÷ Σ tᵢ; mean load F_m = [Σ Fᵢ³ nᵢ tᵢ ÷ Σ nᵢ tᵢ]^(1/3) (as the bearing duty cycle tool with exponent 3).
- Life L = (C_a ÷ f_w F_m)³ × 10⁶ revolutions; hours = L ÷ (60 n_m); travel = L × lead (as the ball screw life tool).
- Example: the screw runs at a mean 1,800 rpm and 4.1857 kN, giving 9,133.6 h (9,864.3 km of travel).
Limitations
- Basic rating life at 90% reliability; poor lubrication or contamination shortens it.
- Preload must be included in the step loads.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a ball screw for a machine’s real duty cycle
- Compare screws for a new axis
- Estimate replacement intervals
Who it is for
- Design engineers
- Machine builders
- Maintenance engineers
What you get
- Mean speed and cubic mean load
- Life in revolutions, hours and travel
- Check against the required life
Interpreting the result
- Life varies with the cube of load: a step with double the load uses up life eight times as fast.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2400–2420, 2561–2568. Topic reference; no source tables or recommended settings are reproduced.
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Open the Ball Screw Life over a Duty Cycle (Mean Load, Mean Speed, Life) calculator
