Thursday, June 22, 2006

Factor of Safety

It is common practice to size the machine elements, so that the maximum design stress is below the UTS (Ultimate Tensile Stress) or yield stress by an appropriate factor - the Factor of Safety, based on UTS(Ultimate Tensile Stress) or Yield Strength. 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. Factor of safety is reccomended by the conditions over which the designer has no control, that is to account for the uncertatinities involved in the design process.

The uncertainities include (but not limited to),

  1. Uncertainity regarding exact properties of material. For example, the yield strength can only be specified in between a range.

  2. Uncertainity regarding the size. The designer has to use the test data to design parts which are much smaller or larger. It is well known that a small part has more strength than a large one of same material.

  3. Unceratinity due to machining processes.

  4. Uncertainity due to the effect of assembly operations like riveting, welding etc.
  5. Uncertainity due to effect of time on strength. Operating environments may cause a gradual deterioration of strength, leading to premature and unpredictable failure of the part.

  6. Uncertainity in the nature and type of load applied.

  7. Assumptions and appoximations made in the nature of surface conditions of the machine element.

Selection of factor of safety

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 or/and 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. An appropriate factor of safety is chosen based on several considerations. Prime considerations are the accuracy of load and wear estimates, the consequences of failure, and the cost of over engineering the component to achieve that factor of safety. For example, components whose failure could result in substantial financial loss, serious injury or death usually use a safety factor of four or higher (often ten). Non-critical components generally have a safety factor of two. 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. Where higher factors might appear desirable, a more thorough analysis of the problem should be undertaken before deciding on their use.

  • 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 - 2 - Known 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. Should also be used with better-known materials that are to be used in uncertain environments or subject to uncertain stresses.

Usually the factor of safety is kept larger, except in aerospace and automobile industries. Here safety factors are kept low (about 1.15 - 1.25) because the costs associated with structural weight are so high. This low safety factor is why aerospace parts and materials are subject to more stringent testing and quality control. Now computers are being used to provide more accurate simulation of stresses that occur in components, particularly in the case of high value products where safety and saving weight is essential.


At Friday, June 23, 2006 1:48:00 AM, Anonymous Gopu said...

hey, pratz,, this is really a good and informative blog for we mechs!!,,then, why did u choose the name as Basic mechanical engg.??,,

At Friday, June 23, 2006 12:30:00 PM, Blogger Abhi said...

nice post buddy, but then begin frm the basics. Not Desgin 1st.

At Tuesday, May 22, 2007 12:21:00 AM, Blogger Spice Boy said...

Hey... can you write an article about stirling engines. I feel its the technology that will drive us into clean pollution free world.

At Wednesday, August 08, 2007 1:26:00 AM, Blogger sweet appy said...

hi da, nice information. Put some basic mechanical things. Need basic mechanical things

At Thursday, November 22, 2007 10:32:00 PM, Anonymous Anonymous said...

hi, it is a good try. but u try to explain from basics like what is inertia, moment of inertia and principal moment of inertia and the relationship between this. I expect more from u.

At Sunday, January 13, 2008 2:53:00 AM, Anonymous Anonymous said...

Would anyone please tell me. Why is the factor of safety is kept larger for brittle materials than that of ductile Materials?

Response with reference is recommended.

At Tuesday, May 06, 2008 6:23:00 PM, Anonymous Anonymous said...

Sweet info. This info is pretty basic already.

At Thursday, May 15, 2008 11:49:00 PM, Anonymous Digicad said...

Hi Good Work... I appreciate people to come forward bring more awareness what Mechi's can do...

I welcome you to visit my blog

Which is all About Mechanical Engg Design.. Sure you will like it..

If possible my link to your blog..

Thanks a lot..


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At Monday, March 02, 2009 8:24:00 PM, Blogger vinod said...

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At Friday, September 10, 2010 12:58:00 AM, Anonymous Anonymous said...

thats a good start. lets build a database of basic terms, alphabetically for quick ref. Thanx

At Monday, October 11, 2010 9:38:00 AM, Anonymous Anonymous said...

could you advise a standard or other document on the dynamic safety factor

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My name is Pratik; I'm a Web Associate for I came across your site and I notice you make mention of Mechanical Engineering Related articles.

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If you have a moment, could you please review the article and see if it's worthy of a mention on your site as an additional resource for your users?

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