JB
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| posted on 1/10/04 at 05:19 PM |
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Avoiding Stress Raisers-----Advice.
In response to my post about lightening holes being bad here is a short piece on the subject of general design and stress raisers.
If you want to know more about this subject I recommend you read Engineer To Win by Carrroll Smith. In fact I recommend you read all his books.
When we build our own cars we want them to be safe and reliable (in fact reliability is the main criteria for my cars). If we can make them light then
we will go faster. If we make them too light then they will break. Things break at points of highest stress, so we should try to minimise these areas
of high stress by making sure the stress is as evenly distributed as possible. We can do this by removing material from places of low stress and / or
add material to areas of high stress.
FEA and computers can do this or we can learn to identify stress raisers. Stress raisers basically occur anywhere there is a change in section that
interrupts the free flow of stress along the component. Common raisers are holes, scratches (scribe lines), notches, abrupt changes in section
(square edges or corners, internal and external), machining marks, threads, splines, joints (including welds which is why top bicyles use butted
tubes, ones that are thicker wall near the joints), corrosion etc.
So wherever we have a stress raiser we should take steps to reduce the stress in that area. Sometimes this means adding material and other times
removing material. For example if we have splines on the end of a half shaft or drive shaft then these splines should be on a larger radius than the
rest of the shaft, or we can reduce the rest of the shaft diameter, taking care to have a nice blend between the section changes. The same applies to
bolts. You can generally identify a highly optimised bolt because the shank will be a smaller diameter than the threads.
If we are putting a hole to bolt a part in a shaft or piece of plate the area around the bolt is thicker than the rest of the area.
However sometimes we can remove material to reduce stress. If we are cutting sheet material into a corner you should cut to a drilled hole, the bigger
the better. The end of the cut leaves a sharp notch, a stress concentration. The hole distributes the stress over a larger area.
Components generally fail due to an overload, how long this takes to happen depends on the size of the component. If you have made the part too small
then it will fail first time the load is put onto it. If you make it stronger we run into fatigue. This is basically a reversal of load and will
eventually cause a small crack to appear, this crack will propagate until the part is weakened and it fails. If you inspect the part often enough you
will spot the crack and avoid failure. This is why components are lifed.
I try to design and make my parts to avoid failure by fatigue because I have better things to do than crack test my critical components every week /
month. Consequently my parts are heavier than they could be but they meet my criteria of reliability.
The following drawings have come from Carroll Smiths Engineer To Win. The subject is covered in greater detail. It is a good book as are all his
books.
Finally a couple of drilled brake pedals I have seen in magazines.
JB
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phelpsa
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| posted on 1/10/04 at 05:22 PM |
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Wow, longest single post ever?
Adam
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Spyderman
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| posted on 1/10/04 at 06:01 PM |
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I admire your enthusiasm.
However I question your point in making these statements without explaining the technicalities.
Surely it would be more beneficial if you explained one area in greater detail and then expanded on it to include other relevent areas.
A good example to start with would be the spline on a shaft as it is symetrical and easily grasped. The other examples then become more obvious.
Not meant as critisizm! 
Terry
Spyderman
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silex
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| posted on 2/10/04 at 01:33 PM |
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JB,
Again you are being way too general in your statements, also the brake pedals are a bad example as there are really under a bending load not direct
tension as some of your other pics. As I am sure you are aware the moment of area will be pretty decent for those pedal arms. As the stress is zero in
the neutral axis and maximum at the outer edge of the plate, taking holes out of the centre can be easily achieved whilst maintaining more than
sufficient strength. After all, have you never seen " I " gurders in buildings with parts of the centres taken out. Take a look at a brake
pedal on a lotus elise.
Yes people need to be sensible in their use of holes to lighten structures. But it is another thing saying that they are all bad.
If you want to enlighten people, do it in a sensible manner explaining fully the pro's and the cons of the design.
I know that your heart is in the right place and I understand where you are comming from but for as many examples you give for holes as stress
raisers, I could probably match with holes giving stress relief, improved fatigue, etc.
Silex.
[Edited on 13/10/04 by silex]
Murphy's 2 laws
1. If it can go wrong it will
2. In case of emergency - refer to rule 1.
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JB
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| posted on 13/10/04 at 11:57 AM |
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My Best Effort
I have tried to give my best explanation on this subject where there seems to be a lot of discussion and ideas.
Perhaps other people could contribute their own explanations in a manner that people will find useful?
I do not confess to be an expert in this field and would be willing to learn as much as possible from others.
JB
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