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Re: Motor shaft steels
Re: Motor shaft steels -- roadking525	Post Reply	Top of thread	Forum

Posted by: randykimball ® 06/13/2006, 21:27:44 Author Profile eMail author Edit

yes, Your shaft broke, most likely from cyclic bending. Every shaft bends some, perhaps hardly measurable but metal flexes to absorb loads, vibrations, and many other forces,(if there is much induced vibration it too could be the cause) This flex must be absorbed in the material. At 58 RC you left little room for the material to flex. When metal flexes is starts to work harden. Think of a strip of plywood being bowed over and over, eventually the plywood gives up. Steel is somewhat like plywood, it has a crystalline structure. Now add to that the fact that the flexes cause work hardening. The harder the flexing area gets the less tollerant it becomes to flexing. Then small cracks form and begin to grow. At some point the shaft breaks. This is exactly what happens when you break a spoon shaft of a paper clip. Perhaps torque flex was also a big factor. If you look closely at the broken shafts you should see signs that there were stages of breakdown. You will most likely find evidence that what I have explained has happeded. You will most likely get better results with a less hard material or one that is only case hardened. 4140 HT is a very popular shaft material, because of its combination of strength and ability to take flex. 4140 HT comes "off the shelf" already heatreated (thus the HT) just right for shafting. Another good choice for small shafts, where you want stiffness, is Thompson Shafting. Thompson is a stress proof material with a case hard outer layer. It is hard to machine a keyway into, but it can be done. I believe it is available in 5/8" diameter. Thompson shafting is "good stuff". The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. Modified by randykimball at Tue, Jun 13, 2006, 21:49:31

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