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#16
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I’ve gotten a few private notes from other FRC teams that have experienced fatigue failures originating from the snap ring groove on longer length 2024 aluminum axle shafts. Received a message from AndyMark stating they are transitioning from 2024 to 7075. “Our fabricator had a hard time in the past finding 7075 shafts, so we settled on 2024. From now on, we are fabricating these shafts out of 7075.”
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#17
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Re: pic: Snap Rings Or Shaft Collars?
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Andy |
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#18
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Re: pic: Snap Rings Or Shaft Collars?
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If anyone finds it, please do share! |
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#19
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Re: pic: Snap Rings Or Shaft Collars?
Try Yarde Metals - they have 7075-T6 in 1/4", 5/16", 3/8", and 1/2" and 7075-T651 in larger sizes.
Never bought from them, but I've seen a lot of people online buying from the "drop zone" (surplus/remnant) page. https://www.yarde.com/catalog/cat296.html |
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#20
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Re: pic: Snap Rings Or Shaft Collars?
We purchased quite a bit of 0.375 and 0.25 7075 hex from onlinemetals.com in 2011 and we still have at least another seasons worth left. I confirmed with a call to them that they discontinued it last year for reasons unknown to their rep.
My place of employment, Oshkosh Corp, has a supplier that will supply this flavor of shaft, but will only supply to goverment contractors. Weird. I'll make it a personal quest to find a supplier... |
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#21
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Re: pic: Snap Rings Or Shaft Collars?
Strictly speaking, there is a good reason not to use aluminum in shafts. As a material, aluminum doesn’t handle fatigue well.
From wikipedia: Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the surface. Eventually a crack will reach a critical size, and the structure will suddenly fracture. Since shafts are rotating, they are almost always subject to fatigue. Actually, the lab test for fatigue involves a bending load applied to the end of a rotating shaft. As one would expect, the lower the stress on the material, the longer it lasts. And here is where materials differ: With steel, there is what is referred to as the “Endurance Limit” at which point the degradation of the material is so minimal, it will last indefinitely. With aluminum, there isn’t an endurance limit. Here’s a graph showing a typical fatigue response of steel and aluminum: http://en.wikipedia.org/wiki/File:S-N_curves.PNG Notice that at a stress of 30 ksi, the curve for steel levels out, while aluminum continues down. This being said, fatigue is not an exact science, but is based on lab tests and observation. The results offer guidelines for good design. Here is a great primer on fatigue: http://www.epi-eng.com/mechanical_en..._in_metals.htm Another important point about fatigue is that it begins at the surface. So grooves for snap rings act as stress risers, and are often the point of failure if they are in an area of high stress. I suspect that the original shaft in question failed due to bending. A cantilevered load on the end of the shaft would cause bending stress between the bearing supports, which is right where the snap ring groove is. Switching to a higher strength aluminum would make it last longer, but getting rid of the snap ring groove altogether would be an even better solution. Obviously a lot of teams use aluminum shafts in FRC with great success, and our team has as well. If the stress is low, they will last long enough. Keep in mind that the average lifetime of an FRC robot is measured in hours. It’s a calculated risk. But in the real world, you would almost never see an aluminum shaft because of fatigue. Last edited by Rob Stehlik : 26-07-2012 at 13:47. |
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#22
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Re: pic: Snap Rings Or Shaft Collars?
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http://www.knovel.com/web/portal/bas...LAY_bookid=754 Pages 3-382, 3-383, and 3-384 have fatigue data for notched and un-notched 7075 in several different types of fatigue loading (fully reversed, partly reversed, etc). It is most illuminating. |
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#23
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Re: pic: Snap Rings Or Shaft Collars?
We have a respected Metallurgist in my group at work. I sat down with him the other day and we ran the numbers comparing strength to load. Study showed our design was sound and should have survived the entire 2012 competition season. He said calculating fatigue through a snap ring groove isn’t always an exact science and using 2024 in this application was a calculated risk. His question to me, what is my plan for the three that haven’t broke. A steel shaft would be his choice for the application. High strength aluminum without a groove would be better, not ideal. He noted how the drive hub had imprinted on the failed shaft from all the torque reversals in our sport. Looks like we have some steel axle shafts to make before our next event at Kettering.
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#24
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Re: pic: Snap Rings Or Shaft Collars?
I'd suggest steel too, your transmission shaft is a mission critical part that can't be easily changed between matches so I vote for overkill. Back in Breakaway we had a similar failure though in that case the snap ring groove wasn't even needed for our end application. It happened to us the night before ship date and twisting at other components meant I had to get the sawzall out to fully disassemble and we were blessed by the fact that due to the heavy snows on the east coast that season the withholding allowance was increased. We shipped our robot with a lot of things missing and had a lot of fun at the event putting it together with the new steel shafts that did not have snap ring grooves.
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#25
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Re: pic: Snap Rings Or Shaft Collars?
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Thanks! -RC |
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#26
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Re: pic: Snap Rings Or Shaft Collars?
We had this identical failure in one of our SuperShifters during our first unbagging window this season. Discovering the issue at that time literally saved our season (otherwise, we likely would have failed the shaft during our first event).
We were cantilevering the load on the shaft (chain run in either direction, but not exactly horizontal) and were surprised when the aluminum shaft failed. I know that the snap ring groove is a stress concentrator, but was still surprised by the failure. We changed to steel and have not had problems since (this is not an indictment of aluminum shafts, but we wanted to go super-conservative after one failure). Here is a close-up of our drive train geometry for reference: http://www.flickr.com/photos/team341...57628843002609 |
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#27
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Re: pic: Snap Rings Or Shaft Collars?
if i were you i would drill hole then thread it and put a bolt on with spacers to keep the wheel in place
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