Machinery and Localized Stresses

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MolaKule

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A basic rod and disk are forged and machined to make a wheel axle, with the rod at a right angle to the disk. The other end of the rod is splined inside the differential housing. The disk is further machined and tapped to make a wheel hub. It was later found that cracks were occurring where the rod meets the disk. Supply the missing word: A ___________ geometry at the rod-disk interface would have prevented cracks from forming. Open to all.
 
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Not sure of the exact nomenclature you're looking for but you need a radiused transition at that location in order to prevent a localized stress riser. Also, depending on the raw material being forged and the post forge quenching process, a tempering or solution annealing heat treatment will also aid in preventing cracking of the finished product.
 
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I would like to amend my answer to concave radius. Reading the question again, I think that fits the answer for geometry better than the term fillet.
 

MolaKule

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Lots of good responses and thanks for contributing your answers. See the PDF file below, Page 8, bottom illustration. Stress Concentrations See Page 8 Stress concentration is a problem of load-bearing mechanical parts, which is reduced by employing fillets in areas of expected high stress. The fillets distribute the stress over a broader area and effectively make the parts more durable and capable of bearing larger loads. So any answer that gave a "radius" or a "fillet" gets a definite thumbs up because those types of geometries reduces stress concentrations! thumbsup In the casting of the axle, it would have been better to allow a greater thickness of metal at the rod-hub interface and then use a rounded-tip end mill to machine a fillet. After that, proper heat treating could increase the overall strength of the axle.
 
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