BIZOMAT MAKEOS · Power Transmission
Ball Screw Rating Life
How long will this ball screw last at its mean load and speed?
Rating life of a ball screw in revolutions, travel distance and hours from its dynamic load rating, mean axial load, lead and speed, against a required life.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Dynamic load rating Ca | kN | Basic dynamic load rating from the ball screw catalogue. |
| Mean axial load (including preload effect) | kN | Mean axial load over the duty cycle (cubic mean of loads, with preload included per the maker). |
| Load factor fw | Load (operating condition) factor for shock and vibration (about 1.0–1.2 smooth, 1.2–1.5 normal, up to 2–3 with impact). | |
| Lead | mm | Screw lead. |
| Mean speed | rpm | Mean screw speed. |
| Required life | h | Required service life. |
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: Dynamic load rating Ca
- What it is
- The basic dynamic load rating of the ball screw nut from the manufacturer’s catalogue: the axial 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 15 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 the rating.
- Get it right
- Use the dynamic rating Ca, not the static rating C₀a.
What this means: Mean axial load (including preload effect)
- What it is
- The cubic mean axial load on the nut over the whole duty cycle, including the effect of preload as the screw maker describes.
- 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 2 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 of the mean load.
- Get it right
- Average the loads by the cube rule, weighted by revolutions; a simple average understates the damage from high loads.
What this means: Load factor fw
- What it is
- The load (operating condition) factor that increases the mean load to allow for shock and vibration, from the ball screw maker’s catalogue.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 3. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.2 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 of this factor.
- Get it right
- Take it from the maker’s table for your operating conditions.
What this means: Lead
- What it is
- The distance the nut travels along the screw for one full revolution of the screw.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Travel life rises in proportion to lead; life in hours is unaffected.
- Get it right
- Lead equals pitch only for a single-start screw; a multi-start screw has a lead of pitch times starts.
What this means: Mean speed
- What it is
- The average screw speed over the duty cycle, including slow moves, weighted by time.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 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 is inversely proportional to the mean speed.
What this means: Required life
- What it is
- The number of running hours the ball screw must last in service, from your design specification. Used only for the pass or fail check.
- 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 4000 h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- L = (Ca ÷ (fw Fm))³ × 10⁶ revolutions.
- Travel life = L × lead.
- Hours = L ÷ (60 × mean speed).
- Example: Ca 15 kN at 2 kN mean load lasts 4,069 h, about 2,441.4 km of travel.
Limitations
- Basic rating life (90% reliability); the maker’s reliability and hardness factors may apply.
- Critical speed and buckling are separate checks (ball screw limits tool).
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a ball screw for a machine’s service life
- Compare screw sizes and preloads
- Estimate travel life in kilometres
Who it is for
- Automation engineers
- Machine designers
What you get
- Life in revolutions
- Travel life
- Life in hours
Interpreting the result
- Life falls with the cube of load: reducing the mean load by 20% roughly doubles life.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2561–2568. Topic reference; no source tables or recommended settings are reproduced.
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