23-07-2012 20:59

Cory

Re: pic: Snap Rings Or Shaft Collars?

More info is needed on how the shaft was loaded and what caused the failure.

23-07-2012 22:58

FrankJ

Re: pic: Snap Rings Or Shaft Collars?

It does not look like fatigue to my admittedly under-trained eye. What are the details around the failure?

23-07-2012 23:37

sanddrag

Re: pic: Snap Rings Or Shaft Collars?

2024 or 7075?

24-07-2012 00:17

ratdude747

Re: pic: Snap Rings Or Shaft Collars?

Quote:

Originally Posted by FrankJ It does not look like fatigue to my admittedly under-trained eye.

24-07-2012 07:18

GDG 2337

Re: pic: Snap Rings Or Shaft Collars?

Took the broken axle shaft to a Metallurgist this morning, he described a normal cycles to failure type of break. Two cracks formed 150 degrees apart from irregularities in the snap ring groove and spread toward the center. He suggested changing alloy from 2024 to 7075 for added strength. We’ve played a lot matches in the Michigan District system, Championships and two off season events. Bot got a new set of pneumatic tires for IRI which increased load on an already weakened axle shaft.

24-07-2012 08:54

Brandon Holley

Re: pic: Snap Rings Or Shaft Collars?

One understated feature of IRI is robot lifetime. By the point the competitors get to IRI, some robots have seen 100-120+ matches and are now playing some of the hardest matches they've ever seen. Things start to fail that you don't necessarily expect and team's aren't necessarily ready for.

Thanks for sharing. Hopefully your team got something out of a rough situation.

-Brando

24-07-2012 11:04

JamesCH95

Re: pic: Snap Rings Or Shaft Collars?

If the snap ring groove was cut in a torqued or stressed part of the shaft it was bound to fail and the design should be reconsidered.

As some have said, more context is needed, and a clear picture would be nice (macro mode FTW).

24-07-2012 15:04

Rob Stehlik

Re: pic: Snap Rings Or Shaft Collars?

I would just switch to a steel shaft and call it done. I don't think the minimal weight savings you get from aluminum shafts is worth the risk. We have used aluminum shafts in the past, and while we didn't see any catastrophic failures like you show here, we bent a few of them.
In response to your original question, usually snap rings are more reliable than shaft collars, but they do weaken the shaft. You want to avoid transmitting torque past a snap ring groove if possible. Generally I like to use snap rings on the ends of the shaft to keep things on, and spacers in the middle of the shaft where it will see torque loading.

24-07-2012 15:09

Madison

Re: pic: Snap Rings Or Shaft Collars?

I am presuming, based on the description, that this is:
http://www.andymark.com/product-p/am-0396.htm

I don't recall how the Toughbox gears are arranged, but I don't believe that they transmit torque across the snap ring groove -- the short end is there for a bearing and for the encoder. It's probably difficult to find a different application for this shaft that puts torque on both sides of the groove.

24-07-2012 18:23

Joe Finkel

Re: pic: Snap Rings Or Shaft Collars?

Totally agree with Rob:steel shaft way to go. Also, when grooving for snap rings
one should refer to machinist handbook.

24-07-2012 20:55

trilogy2826

Re: pic: Snap Rings Or Shaft Collars?

This general topic has been discussed several times before. General theme: Use the right tool for the job. If you understand what material types can be used where, you can get away with a robot that has no business weighing less than 120 lbs. Not saying we have it all figured out. We still have a lot to learn, but here is what we have learned during our short time in FRC.

We just started using the FEA tools embedded in Inventor this last year and this helped shave our sheet metal thicknesses by at least 20 %. We also found a couple problems with fatigue and were able to fix it before we even manufactured a single part.

2828 converted over to 7075 AL hex shaft in 2010. We have used .375, .5, .625 and .75 for anything from gearbox shafts and cantilevered axles to arm joints and shooter axles. All of these had e-clip style snap rings as the main retaining fastener. We had one issue this year that was similar to the OPs shear at the groove, but it turned out to be that the material used was accidentally 2024 instead of 7075. After the switch, no problems.

I highly suggest purchasing from onlinemetals.com. They have a great selection, fast shipping and prices that are hard to beat.

If anything, please don't "just go to steel". In most cases it isn't necessary.

24-07-2012 21:37

R.C.

Re: pic: Snap Rings Or Shaft Collars?

Quote:

Originally Posted by trilogy2826 If anything, please don't "just go to steel". In most cases it isn't necessary.

24-07-2012 22:11

IanW

Re: pic: Snap Rings Or Shaft Collars?

Quote:

Originally Posted by trilogy2826 2828 converted over to 7075 AL hex shaft in 2010. We have used .375, .5, .625 and .75 for anything from gearbox shafts and cantilevered axles to arm joints and shooter axles.

25-07-2012 01:38

DampRobot

Re: pic: Snap Rings Or Shaft Collars?

One of my favorite ways to retain elements on shafts is to just use spacers stacked up between them. Yes, you usually have to manufacture them yourself, but they can make assembly and disassembly much easier. They maintain components the correct distance apart without a long alignment process (shaft collars) or weakening the shaft (retaining clips). In my designs, there is usually just a stack up of spacers and components captured between two bearings, or there is a shaft collar that you just press up against one end of the stack up.

On a side note, if you do decide to use shaft collars, use two piece ones. It can be a life saver to be able to drop a shaft collar off a shaft to get at something rather than disassemble a whole mechanism to get it off.

25-07-2012 10:20

GDG 2337

Re: pic: Snap Rings Or Shaft Collars?

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.”

25-07-2012 10:31

Andy Baker

Re: pic: Snap Rings Or Shaft Collars?

Quote:

Originally Posted by GDG 2337 “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.”

25-07-2012 20:01

AdamHeard

Re: pic: Snap Rings Or Shaft Collars?

Quote:

Originally Posted by IanW Where do you get your .375" 7075 hex bar? Onlinemetals does not supply any in this size. Fry Steel's website says that they have it, but when I called them about a month ago, they said that they didn't have any in stock and didn't have any plans to stock it. Thanks for your help!

25-07-2012 23:12

scottandme

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

26-07-2012 05:42

trilogy2826

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...

26-07-2012 11:05

Rob Stehlik

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.

26-07-2012 13:57

JamesCH95

Re: pic: Snap Rings Or Shaft Collars?

Quote:

Originally Posted by Rob Stehlik 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.

26-07-2012 15:59

GDG 2337

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.

26-07-2012 16:07

Mr V

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.

26-07-2012 16:22

R.C.

Re: pic: Snap Rings Or Shaft Collars?

Quote:

Originally Posted by trilogy2826 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...

27-07-2012 11:34

Jared Russell

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

11-09-2012 08:21

greasemonkey

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