Reliability /Safety Index Page

Safety Factors..

Basic Notes on Factor of Safety

The factor of safety also known as Safety Factor, is used to provide a design margin over the theoretical design capacity to allow for uncertainty in the design process.   The uncertainty could be any one of a number of the components of the design process including calculations, material strengths, duty, manufacture quality.  The value of the safety factor is related to the lack of confidence in the design process.   The simplest interpretation of the Factor of Safety is

FoS = Strength of Component / Load on component

If a component needs to withstand a load of 100 Newtons and a FoS of 4 is selected then it is designed with strength to support 400 Newtons...

The selection of the appropriate factor of safety to be used in design of components is essentially a compromise between the associated additional cost and weight and the benefit of increased safety and/or reliability.   Generally an increased factor of safety results from a heavier component or a component made from a more exotic material or / and improved component design

The factors of safety listed below are based on the yield strength..

Factor of SafetyApplication
1.25 - 1.5 Material properties known in detail.  Operating conditions known in detail   Loads and resultant stresses and strains known with with high degree of certainty.   Material test certificates, proof loading, regular inspection and maintenance.    Low weight is important to design.
1.5 - 2Known materials with certification under reasonably constant environmental conditions, subjected to loads and stresses that can be determined using qualified design procedures. Proof tests, regular inspection and maintenance required
2 - 2.5 Materials obtained for reputable suppliers to relevant standards operated in normal environments and subjected to loads and stresses that can be determined using checked calculations.
2.5 - 3 For less tried materials or for brittle materials under average conditions of environment, load and stress.
3 - 4 For untried materials used under average conditions of environment, load and stress.
3 - 4 Should also be used with better-known materials that are to be used in uncertain environments or subject to uncertain stresses.

Repeated Cyclic loads :
The factors established above must be based on the endurance limit ( fatigue strength ) rather than to the yield strength of the material.  The strength calculations should also include for stress concentration factors.

Impact Shock forces :
The factors given in items 3 to 6 are acceptable, but an impact factor (the above dynamic magnification factor) should be included.

Brittle materials :
The ultimate strength is used as the theoretical maximum, the factors presented in items 1 to 6 should be approximately doubled.

Impact Shock forces :
The higher factors of safety given above (2.5 to 4) may be used but based on stress levels calculated based on the resulting dissipated energy at impact.

Where higher factors might appear desirable, a more thorough analysis of the problem should be undertaken before deciding on their use.

Extreme care must be used in dealing with vibration loads, more so if the vibrations approach resonant frequencies.   The vibrations resulting from seismic disturbances are often important and need to be considered in detail.

Use of Standards and Codes

A convenient method of ensuring safe confident design is to use design codes; A good standard used by mechanical engineer is

BS 2573-Pt 1:1983 Rules For Design of Cranes.   Specification for Classification, stress, Calculations and design criteria for structures.

This standard (together with BS 2573 part 2) includes rules for completing calculations and applying factors and the relevant allowable stresses to be used for the different grades of materials.   This standard is primarily used for design of cranes and associated equipment but it is used widely for design of similar mechanical systems.   When designing systems based using the rules from this standard it is not generally necessary to include additional margins of safety.
When design engineering structures using structural steel section a useful standard is..

BS 5950-1:2000-Structural use of steelwork in building. Code of practice for design. Rolled and welded sections.

This standard together with BS 5950-Part 2,3-1,4,5,6,7,8 & 9 provide service factors and design stresses relevant to structural design.

In designing many equipment items including vessels, pumps, valves, piping systems there are equivalent standards and codes which should be followed.  These documents generally identify the necessary design procedures and the safety margins to be included.

Use of Proprietary Items
A mechanical design often includes rolling element bearings, gearbox units, shaft couplings, belt /chain drives etc.  When using these items it is necessary to strictly follow the design rules provided in the suppliers technical documents.  The operating duties and service factors to be used are generally clearly specified.   It not correct to simply use oversized equipment for convenience.  It is also recommended that the supplier is consulted on the duty.  

Links on Safety Factors
  1. STRESS, STRENGTH AND SAFETY ... DANotes - Very useful notes of safety ...

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