Glossary: What is a safety factor in product design?

1. The Basic Concept of the Safety Factor

In product design, the **safety factor (SF)** is a ‘margin of safety’ established to prepare for unforeseen loads and uncertainties when a structure or component is actually in use. Put simply, it acts as ‘insurance against failure’ and reduces the risk of accidents or breakdowns by ensuring a margin between the design strength and the actual operating conditions.

For example, if a force of 100 N is expected to be applied to a component, and the material’s breaking strength is 200 N, the safety factor is calculated as follows.

In this case, it means ‘allowing for a margin of safety of two times’.

2. The purpose of setting a safety factor

There are three main reasons for establishing a safety factor.

Accounting for variations in materials and manufacturing

Even with the same material, strength can vary from batch to batch, and performance can also change depending on manufacturing precision and surface finish.

Uncertainties in the operating environment

It is necessary to prepare for factors that cannot be fully anticipated at the design stage, such as temperature fluctuations, impact, wear and corrosion.

Human error and unforeseen loads

This accounts for the possibility of loads not anticipated at the design stage, as well as user error.

3. Guidelines for safety factors

Safety factors vary significantly depending on the product’s application and the required level of reliability.

Machinery components (general industrial machinery): approximately 1.5–3

Building structures (bridges, buildings, etc.): approximately 2–4

Aerospace sector: approximately 1.2–1.5 (due to strict weight restrictions, the margin is reduced through precise analysis and testing)

Life-critical medical devices and lifts: may be set at 5 or higher

4. Specific Examples

Example 1: Automotive suspension arms

Design load: 5 kN (force expected during normal use)

Permissible load calculated from the material’s yield strength: 15 kN → Safety factor = 15 ÷ 5 = 3

In other words, the design ensures that the product will not break even if subjected to three times the force expected during normal use.

Example 2: Mountaineering carabiner

Assuming the combined weight of the user and the load is approximately 1 kN (≈ 100 kg)

Carabiner breaking load: 25 kN → Safety factor = 25 ÷ 1 = 25

As mountaineering equipment is a matter of life and death, extremely high safety factors are applied.

5. The relationship between safety factor and cost

The higher the safety factor, the safer the product becomes; however, this simultaneously gives rise to issues such as ‘increased weight’, ‘increased cost’ and ‘reduced efficiency’. Consequently, designers must strike the following balance:

Ensuring safety

Balancing safety with cost, weight and efficiency

Selecting the optimal value according to the product’s intended use and operating environment

Summary

In product design, the safety factor is a numerical indicator that quantifies ‘how much margin is built in’ to account for uncertainties. There are general guidelines such as 2–3 for industrial machinery, 2–4 for buildings, 1.2–1.5 for the aerospace sector, and 5 or higher for life-critical applications; designers are required to strike the optimal balance between safety and cost.