OCCRA
Go to Post In 2012, we built our best machine ever. Some visitors to our pit were visibly disappointed to find that our machine was a pile of extruded aluminum and urethane belting, just cleverly arranged and programmed. - OZ_341 [more]
Home
Go Back   Chief Delphi > ChiefDelphi.com Website > Extra Discussion
CD-Events   CD-Media   CD-Spy   FRC-Spy  
portal register members calendar search Today's Posts Mark Forums Read FAQ rules

 
Reply
Thread Tools Rate Thread Display Modes
  #16   Spotlight this post!  
Unread 07-25-2012, 09:20 AM
GDG 2337 GDG 2337 is offline
Registered User
FRC #2337 (ENGINerds)
Team Role: Mentor
 
Join Date: Feb 2011
Rookie Year: 2003
Location: us
Posts: 30
GDG 2337 has a spectacular aura aboutGDG 2337 has a spectacular aura about
Smile 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.”
Reply With Quote
  #17   Spotlight this post!  
Unread 07-25-2012, 09:31 AM
Andy Baker's Avatar Woodie Flowers Award
Andy Baker Andy Baker is offline
President, AndyMark, Inc.
FRC #3940 (CyberTooth)
Team Role: Engineer
 
Join Date: May 2001
Rookie Year: 1998
Location: Kokomo, Indiana
Posts: 3,364
Andy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond repute
Send a message via AIM to Andy Baker
Re: pic: Snap Rings Or Shaft Collars?

Quote:
Originally Posted by GDG 2337 View Post
“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.”
... and we are using the same supplier that Jake pointed out above for this 7075 aluminum, onlinemetals.

Andy
Reply With Quote
  #18   Spotlight this post!  
Unread 07-25-2012, 07:01 PM
AdamHeard's Avatar
AdamHeard AdamHeard is offline
Lead Mentor
FRC #0973 (Greybots)
Team Role: Mentor
 
Join Date: Oct 2004
Rookie Year: 2004
Location: Cal Poly San Luis Obispo
Posts: 4,487
AdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond repute
Send a message via AIM to AdamHeard
Re: pic: Snap Rings Or Shaft Collars?

Quote:
Originally Posted by IanW View Post
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!
We're in the same boat, we used to get it from Fry but they no longer stock it. I've hunted it at other suppliers with no success so far.

If anyone finds it, please do share!
__________________
Need help? Check out 973 Remote Assistance and Mentorship Program.

Like us on Facebook!

2014 Galileo Champions
2011 World Champions

Like our Robots? Check out the CAD files of MOST of our robots!
Reply With Quote
  #19   Spotlight this post!  
Unread 07-25-2012, 10:12 PM
scottandme's Avatar
scottandme scottandme is offline
Registered User
AKA: Scott Meredith
no team
Team Role: Teacher
 
Join Date: Jan 2012
Rookie Year: 2009
Location: Hightstown, NJ
Posts: 166
scottandme has a reputation beyond reputescottandme has a reputation beyond reputescottandme has a reputation beyond reputescottandme has a reputation beyond reputescottandme has a reputation beyond reputescottandme has a reputation beyond reputescottandme has a reputation beyond reputescottandme has a reputation beyond reputescottandme has a reputation beyond reputescottandme has a reputation beyond reputescottandme has a reputation beyond repute
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
Reply With Quote
  #20   Spotlight this post!  
Unread 07-26-2012, 04:42 AM
trilogy2826's Avatar
trilogy2826 trilogy2826 is offline
Mentor - 2826
AKA: Jake Fischer
FRC #2826 (Wave Robotics)
Team Role: Mentor
 
Join Date: Nov 2008
Rookie Year: 2006
Location: Oshkosh, WI
Posts: 104
trilogy2826 has a reputation beyond reputetrilogy2826 has a reputation beyond reputetrilogy2826 has a reputation beyond reputetrilogy2826 has a reputation beyond reputetrilogy2826 has a reputation beyond reputetrilogy2826 has a reputation beyond reputetrilogy2826 has a reputation beyond reputetrilogy2826 has a reputation beyond reputetrilogy2826 has a reputation beyond reputetrilogy2826 has a reputation beyond reputetrilogy2826 has a reputation beyond repute
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...
Reply With Quote
  #21   Spotlight this post!  
Unread 07-26-2012, 10:05 AM
Rob Stehlik's Avatar
Rob Stehlik Rob Stehlik is offline
Registered User
FRC #0610 (Coyotes)
Team Role: Engineer
 
Join Date: Sep 2009
Rookie Year: 2009
Location: Toronto
Posts: 73
Rob Stehlik is a glorious beacon of lightRob Stehlik is a glorious beacon of lightRob Stehlik is a glorious beacon of lightRob Stehlik is a glorious beacon of lightRob Stehlik is a glorious beacon of light
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 : 07-26-2012 at 12:47 PM.
Reply With Quote
  #22   Spotlight this post!  
Unread 07-26-2012, 12:57 PM
JamesCH95's Avatar
JamesCH95 JamesCH95 is offline
Hardcore Dork
AKA: JCH
FRC #0095 (LRT-UVR Grasshoppers #95)
Team Role: Engineer
 
Join Date: Dec 2004
Rookie Year: 2001
Location: Enfield, NH
Posts: 1,458
JamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond repute
Re: pic: Snap Rings Or Shaft Collars?

Quote:
Originally Posted by Rob Stehlik View Post
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.
To expand on this, here is a reference that I use at my job occasionally, it is a good reference for material properties and fatigue data on nearly every metal one could ever want.

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.
__________________
Theory is a nice place, I'd like to go there one day, I hear everything works there.

Maturity is knowing you were an idiot, common sense is trying to not be an idiot, wisdom is knowing that you will still be an idiot.
Reply With Quote
  #23   Spotlight this post!  
Unread 07-26-2012, 02:59 PM
GDG 2337 GDG 2337 is offline
Registered User
FRC #2337 (ENGINerds)
Team Role: Mentor
 
Join Date: Feb 2011
Rookie Year: 2003
Location: us
Posts: 30
GDG 2337 has a spectacular aura aboutGDG 2337 has a spectacular aura about
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.
Reply With Quote
  #24   Spotlight this post!  
Unread 07-26-2012, 03:07 PM
Mr V's Avatar
Mr V Mr V is offline
FIRST Senior Mentor Washington
FRC #2046 (Bear Metal)
Team Role: Mentor
 
Join Date: Feb 2011
Rookie Year: 2009
Location: Maple Valley Wa
Posts: 533
Mr V has a reputation beyond reputeMr V has a reputation beyond reputeMr V has a reputation beyond reputeMr V has a reputation beyond reputeMr V has a reputation beyond reputeMr V has a reputation beyond reputeMr V has a reputation beyond reputeMr V has a reputation beyond reputeMr V has a reputation beyond reputeMr V has a reputation beyond reputeMr V has a reputation beyond repute
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.
Reply With Quote
  #25   Spotlight this post!  
Unread 07-26-2012, 03:22 PM
R.C.'s Avatar
R.C. R.C. is offline
2014... Hella Parts
AKA: Owner, WestCoast Products
FRC #1323 (MadTown Robotics)
Team Role: Mentor
 
Join Date: Feb 2008
Rookie Year: 2006
Location: Madera, CA
Posts: 1,927
R.C. has a reputation beyond reputeR.C. has a reputation beyond reputeR.C. has a reputation beyond reputeR.C. has a reputation beyond reputeR.C. has a reputation beyond reputeR.C. has a reputation beyond reputeR.C. has a reputation beyond reputeR.C. has a reputation beyond reputeR.C. has a reputation beyond reputeR.C. has a reputation beyond reputeR.C. has a reputation beyond repute
Re: pic: Snap Rings Or Shaft Collars?

Quote:
Originally Posted by trilogy2826 View Post
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...
If you can find a supplier, a ton of teams will be buying. Please do post up if you find someone willing to sell some. We use quite a bit of 3/8 Hex, but 2024 hasn't cut it in some applications. So we've been using 12L14 Steel.

Thanks!

-RC
__________________
R.C.
Owner, WestCoast Products || Twitter
MadTown Robotics Team 1323
Reply With Quote
  #26   Spotlight this post!  
Unread 07-27-2012, 10:34 AM
Jared Russell's Avatar
Jared Russell Jared Russell is offline
I feel better now.
AKA: Jared341
FRC #0254 (The Cheesy Poofs)
Team Role: Engineer
 
Join Date: Nov 2002
Rookie Year: 2001
Location: San Francisco, CA
Posts: 2,639
Jared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond repute
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
Reply With Quote
  #27   Spotlight this post!  
Unread 09-11-2012, 07:21 AM
greasemonkey's Avatar
greasemonkey greasemonkey is offline
Truss me I'm a Doctor
AKA: Fred Parker, fredword, fredwardo, phred, frederick
FRC #3467 (windham windups)
Team Role: Mechanical
 
Join Date: Oct 2011
Rookie Year: 2011
Location: windham, NH
Posts: 26
greasemonkey is an unknown quantity at this point
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
Reply With Quote
Reply


Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -5. The time now is 02:00 AM.

The Chief Delphi Forums are sponsored by Innovation First International, Inc.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Copyright © Chief Delphi