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Old 01-29-2005, 12:19 PM's Avatar Moderator
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Mechanical design note

This subject raises its ugly head in a lot of seemingly unrelated topics but is very important for DIY rodders to understand. In some cases it means the difference between life and death.

In designing ANY mechanical element that will see even moderate stress and especially vibration of any magnitude, be sure there are no sharp INNER corners in the design. The sketch below show a couple cartoon examples of what I am talking about. Note the shaft and weld joint at the left with the sharp inside corners and the cracks drawn in. These are very bad designs since the sharp inner corner is in essence a crack already initiated. It takes little added stress and/or fatigue from vibration to extend the crack into the body of the part, leading to total failure.

To understand why, just think of what stress is; it is a force borne by an area thus the term 'pounds per square inch' or 'psi'. The equation is simple,

force, #/area, sqin

Using this equation to analyze a sharp V-intersection that by definition has zero area, the equation is

X#/0.0sqin = infinite stress!

Thus it take very little force to propagate a crack. However when you design in a fillet or radius in a machined element or perfect your welding technique or at least grind proud weld beads to a fillet, you spread that force over a much greater area and eliminate the stress riser and inevitable crack.

The failure of the Heidt's MII X-members using strut rod eliminators that has been discussed many times on this board is due to a poor design that incorporates the 90deg welded intersection of two steel plates at a highly stressed point of the design. See the picture below. By simply continuing the bottom plate of the X-member up the side of the riser, those sharp stress-riser intersections are eliminated and the danger of cracks and catastrophic failure goes away.


Last edited by; 08-08-2005 at 09:15 PM.
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Old 02-10-2005, 02:46 PM's Avatar Moderator
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Here is an example of a shaft that engineers designed with sharp internal corners. It is an automatic transmission input shaft and broke due to a crack forming at the corner of an O-ring groove. Thanx for posting this picture Crosley. In applications like this the need for the function provided by the notch in the shaft over-rides the extra stress that the notch introduces. Engineers pretty much overcome this design 'flaw' by over designing material strength for the part. Unfortunately, there is no way to totally eliminate this type of failure in a part like this 'cause as the equation shows, there is approaching infinite stress there!
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Old 03-07-2005, 06:01 PM
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(Psst... by the way, engineers call them fractures, failure is a bad word.)

In a production environment, manufactured parts usually don't have sharp corners often anyway.. Usually a cutter will have a radius on its corner to improve tool life, or else it might tend to develop one just due to wear anyway.

If I get a chance I might see if I can send ya some FEA pics. On a convertible top, not too long ago we had a ball stud for a hydraulic cylinder that fractured before test completion. We had ordered the parts for an evaluation from a machine shop that didn't have experience with ball studs, they machined the ball studs without a radiused transition from the ball to the shaft or from the shaft to the base. Analysis of the design intent vs. the parts as received showed about a 10X increase in stress levels.
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Old 07-07-2005, 08:32 AM
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To anyone doing any fabricating or modification of structural parts, drivelines, etc., I highly recommend Carroll Smith's book Engineer to Win. Carroll goes into great detail about the whys and wherefores of metallurgy, part design, and generally how to make sure your stuff holds together. Although he writes from a racing focus, the same principals hold true for any vehicle.

His website closed down after his death, I think, but the book is available from a lot of the mail order houses. I think I got my copy from Summit.
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