Factor of Safety/ Margin of Safety 1

[Lee.Conti]

Hi All,

I am new to automotive and would like to know what standard is usually used when checking strength? Is it using factor of safety or margin of safety? Sometimes people confuse with safety factor with factor of safety...

If it is not a structural part and it is more concerned of quality check, do we still use factory of safety/ margin of safety when checking whether max stress exceeds allowable strength? Thank you!

 

[MintJulep]

How well do you know your loads?

How well do you know your population of parts?

 

[Lee.Conti]

Hi MintJulep, the loading is pretty clear. Can you explain further?

In other experience, we used MoS/ FoS but those parts are structural parts which safety concern. But now the parts I am dealing with are all non-structural parts. However, we still check if the mounting fail or any failure happens under different load cases for quality purpose... so, wondering if using FoS is appropriate or it is subject to company policy?

 

[MintJulep]

If you know the loads with 100% certainty and you know the weakest possible way that any part in the population will be manufactured then you can design to that without any factor of safety.

It's pretty impossible to know either of those two things.

Factory of safety is proportional to your uncertainty.

 

[Lee.Conti]

Hi MintJulep,

I think that is not the factor I am mentioned but it does confuse sometime. I was checking the ratio of allowable to max stress. Some people use Margin of Safety and some people use Factor of Safety...

 

[MintJulep]

MoS = FoS - 1

Is that your question?

 

[Lee.Conti]

Hi MintJulep, yeah... But I am new to Automotive and wonder if automotive also using this for strength analysis?

 

[CapriRacer]

I think it varies with component. For example, tires have a bursting strength 4 to 5 times the max pressure, mainly to account for fatigue. I'll bet that - say - suspension components, use FOS/MOS as a way to deal with shock loads.

But there are other reasons. For example, by increasing the panel thickness of a body part, you reduce the vibrational characteristics making the panel less likely to vibrate in sympathy to road input. That FOS/MOS is probably off the charts.

 

[GregLocock]

There is always one very simple answer. Ask your customer.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[michaelwoodcoc]

I read a race chassis design book one time, the advice was something like any suspension corner should be able to take 10g's of the car's weight. Car weights 1 lb? any wheel should be able to take 10 lb, can't remember all the specifics.
this is all IIRC

Also, in racing it's possible to go airborne and have one wheel land first, unlikely but possible in mall crawlers.

Also, in racing, it is not unheard of for a car to have half it's weight in downforce, Although I don't know I suspect some cars have more than that.
I think this book was race car handling & chassis design. Worth a read I suppose.

I think another aspect to consider is, and perhaps even the most important, how will a component fail?

Ball joints should have the ball pressing into the socket, not the other way around. Wan't to know why?

https://www.youtube.com/watch?v=OVzIiczOJ04

Also please tell me your definition of structural vs. non structural. Almost everything in cars can be considered structural in the event of a crash, for example, or a roof with an aftermarket rack installed. Even a hood is part of the crumple zone.

Also in the event of a crash some components are designed to fail a certain way. That is to say that you need to know how much force a component can take under all operating conditions and design for that, and know how much it will take in a serious crash, and design it to fail at that point.

I think I remember engine mounts being designed in such a way as to drop the block.

IIRC The dodge challenger (is this based on the charger?) was poorly designed so that when the suspension failed, the wheels swung into the passenger compartment, breaking their legs.

https://www.youtube.com/watch?v=LHXoxZRSv-I

Link

If you design stuff I want to see your front wheel pop off like the Camero in the above video.

Engineering student. Electrical or mechanical, I can't decide!
Minoring in psychology

 

[BrianPetersen]

The Chrysler LX/LD vehicles (Charger, Challenger, 300, Magnum) were designed years before the IIHS thought about the small-overlap test (which started IIRC 2012-ish). Most vehicles that were designed prior to that test coming into existence, don't do well at it. Newer vehicles are getting enormously strengthened door-ring structures, often using hot-stamped steel in the structural layer under the skin, to stop the A-pillar from collapsing the way the Challenger did.

The Camaro is a much newer design.

 

[CWB1]

Like anything else, it depends on the part and the company. Some parts are designed based on G-loading, others to a specific known limit.

 

[RodRico]

Yours is a very general question, so I'll point you to very general guidance. Note that safety factors are related to risk and consequence. Risks include uncertainty in peak load, material defects, etc. and risks encompass customer annoyance to financial loss to human fatality. It's a really hard question, and you'll probably get a different answer from every designer. That's why many companies, industries, and codes specify safety factors.

 

[BUGGAR]

However you design it, document your calculations with ALL of your design assumptions and get these cleared with client and legal. (CYA!)