Two load ratings, two questions
Manufacturer catalogs list two load ratings for each linear guide, which answer different questions:
- Dynamic load rating C – the load at which a guide series achieves its nominal L10 service life (10⁵ m of travel) during operation. Basis for service life calculation (next section). Note: Some manufacturers base C on 50 km; such values are approximately 1.26times larger (for ball bearings) and must be converted before comparison.
- Static load rating C₀ – the load at which a permanent total deformation of the rolling element and raceway at the most heavily loaded contact point occurs, equal to approximately 0.0001times the rolling element diameter (ISO 14728-2) – this is considered the absolute limit of tolerability. It is particularly relevant during standstill, slow movement, and shock loads.
The static safety factor S₀
The safety factor indicates whether a guide can safely support a given load under static conditions:
S₀ = C₀ / P₀
S₀ = static safety factor · C₀ = static load rating (from the catalog) · P₀ = maximum load on the most heavily loaded carriage, including brief peak loads
If additional torques act on the system, the torque component (as an equivalent force according to the manufacturer’s catalog) is included in P₀—more on this in Unit 5.
Guideline values for S₀
| Operating Conditions | S₀ |
|---|---|
| All loads known; smooth operation without shocks | 3–5 |
| Typical design: Loads partially estimated, moderate shocks/vibrations | 5–8 |
| High dynamic loads, severe shocks, contamination | 8–12 |
| Suspended (overhead) installation or high risk potential | ab 12 |
Suspended installation: A high S₀ value protects against permanent deformation, but not against falling. For overhead installations, provide an additional fall protection device and verify the fastening screws for tensile strength (according to the manufacturer’s specifications).
Safety Notice
These guidelines are conservative and follow European manufacturers. Other manufacturers (e.g., from Japan and Taiwan) specify lower minimum values using their own calculation methods (correction factors, separate torque verification). The calculation method and guideline values specified by the manufacturer in question are binding.
Mnemonic
Worked example
Given
According to the catalog, a carriage has a static load rating of C₀ = 28.6 kN. The maximum load acting on it (labeled F in the sketch) is P₀ = 8 kN.
Calculation
- S₀ = C₀ / P₀ = 28.6 / 8 ≈ 3.58
This is sufficient only if all loads are known and the guide runs smoothly without shocks (guideline value 3–5). For typical designs with partially estimated loads or moderate shocks (guideline value 5–8), a larger size or a second carriage would be necessary.
Knowledge check
Answer all three questions, then click "Check". From 2 of 3 correct answers, the unit counts as completed. You can retry at any time.
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