Overview
ISO 3408-5:2006 - "Ball screws - Part 5: Static and dynamic axial load ratings and operational life" specifies the calculation schemes used to determine basic and modified static and dynamic axial load ratings, and to predict operational life for ball screws. The standard establishes uniform methods for calculating comparable values for design, procurement and lifetime assessment of ball screw assemblies. It is based on established rolling-contact life theory (Lundberg & Palmgren) and applies under defined preconditions (elastic deformation, minimum track hardness HRC 58, conformity factors > 0.5, bearing‑grade steel and optimum lubrication).
Key Topics
- Scope and preconditions: Applicability limits such as material quality, surface hardness and lubrication.
- Symbols, terms and notation: Standardized symbols and subscripts for consistent calculations.
- Basic axial load ratings: Methods to compute the basic static axial load rating (C0) and basic dynamic axial load rating (Ca) per loaded turn.
- Modified load ratings: Correction factors for real-world influences:
- Surface hardness corrections (hardness factor).
- Accuracy factor (f_ac) - tabled values: classes 0,1,3,5 → f_ac = 1; class 7 → 0.9; class 10 → 0.7.
- Material/melting process factor (f_m) - e.g., air‑melted = 1.0; vacuum‑degassed = 1.25; electro‑slag remelted = 1.44; vacuum remelted = 1.71.
- Operational preload considerations (average operational preload taken as 60% of original preload).
- Life calculation procedures:
- Equivalent rotational speed and equivalent axial load definitions for variable speed/load duty cycles.
- Treatment of different assembly conditions: backlash between nut and shaft, and preloaded ball nuts.
- Inclusion of reliability factors to obtain life at specified reliability levels.
- Tables and reference data: Accuracy classes, material factors and other reference parameters required for calculations.
Applications
- Component selection and sizing: Use ISO 3408-5:2006 to size ball screws for target life and load capacity.
- Life prediction and reliability analysis: Estimate nominal and modified life under variable loads and speeds.
- Supplier data validation and comparison: Compare manufacturers’ rated C0/Ca and life predictions using a common calculation basis.
- Design optimization: Account for surface hardness, accuracy class, preload and melting process when optimizing performance and cost.
- Maintenance planning and failure investigation: Interpret load ratings and predicted life to plan replacements and root-cause analysis.
Who should use it
- Mechanical design engineers and machine-tool designers
- Ball screw and actuator manufacturers
- Procurement/specification engineers
- Reliability and maintenance engineers
- Test laboratories and standards compliance teams
Related standards
- ISO 3408-1: Vocabulary and designation (see also ISO 3408 Parts 2–4 for diameters, acceptance tests and static rigidity).
Keywords: ISO 3408-5:2006, ball screws, static axial load rating, dynamic axial load rating, operational life, preload, equivalent axial load, hardness factor, accuracy class.
