|
|
Thread Tools | Display Modes |
#51
|
|||
|
|||
Residual stress, fatigue and stress relief
Peter Cole wrote:
If there are residual stresses, there must be both tension and compression residuals. Since the working load is tension, tension residuals are bad for fatigue, compression residuals are harmless or beneficial. By overloading in the working direction, tension residuals -- wherever they occur -- are reduced. There still seems to be some confusion about stress relief. Where beneficial residual stresses exist (e.g. rolled threads), stress relief will not reduce them, if anything, it will increase them. Since residual compression stress is created by yielding other parts of the section from tension, a uniform tension overload will reach yield in the areas with residual tension before those with residual compression. overload tension + load tension + residual tension yield tension, overload tension + load tension - residual compression yield tension This is the basis of overload stress relief. If you don't grasp that then everything else remains obscure. |
Ads |
#52
|
|||
|
|||
Residual stress, fatigue and stress relief
Peter Cole wrote:
Peter Cole wrote: If there are residual stresses, there must be both tension and compression residuals. Since the working load is tension, tension residuals are bad for fatigue, compression residuals are harmless or beneficial. By overloading in the working direction, tension residuals -- wherever they occur -- are reduced. There still seems to be some confusion about stress relief. Where beneficial residual stresses exist (e.g. rolled threads), stress relief will not reduce them, if anything, it will increase them. Since residual compression stress is created by yielding other parts of the section from tension, a uniform tension overload will reach yield in the areas with residual tension before those with residual compression. overload tension + load tension + residual tension yield tension, overload tension + load tension - residual compression yield tension This is the basis of overload stress relief. If you don't grasp that then everything else remains obscure. the only obscurity here is why some people are still confused about relevance of "stress relief" where it's not observed to be the cause of fatigue initiation!!!! buy a magnifier and examine a broken spoke. then correlate the observed fatigue initiation point to the residual stress profile of a spoke bend. |
#53
|
|||
|
|||
Residual stress, fatigue and stress relief
On Apr 24, 9:59*pm, jim beam wrote:
manufacturers actually state that spokes /shouldn't/ be bent. Everybody I know who uses Sapim spokes bends them. Dave Ornee, what do you do? |
#54
|
|||
|
|||
Residual stress, fatigue and stress relief
On Apr 24, 11:58 pm, jim beam wrote:
Peter Cole wrote: If there are residual stresses, there must be both tension and compression residuals. Since the working load is tension, tension residuals are bad for fatigue, compression residuals are harmless or beneficial. By overloading in the working direction, tension residuals -- wherever they occur -- are reduced. actually, 1. overload can introduce more residual stress than it relieves. 2. miner's cumulative damage rule says you're simply reducing fatigue life. jim beam is mis-applying Miner's rule. Miner's rule is intended to give approximate estimations of fatigue life for repeated patterns of cyclical loads. AFAIK, it's never been intended for evaluating the effect of a single application of a load causing yielding. From http://en.wikipedia.org/wiki/Metal_fatigue: "Though Miner's rule is a useful approximation in many circumstances, it has two major limitations: 1. It fails to recognise the probabilistic nature of fatigue and there is no simple way to relate life predicted by the rule with the characteristics of a probability distribution. 2. There is sometimes an effect in the order in which the reversals occur. In some circumstances, cycles of low stress followed by high stress cause more damage than would be predicted by the rule. It does not consider the effect of overload or high stress which may result in a compressive residual stress." Note that last sentence. - Frank Krygowski |
#55
|
|||
|
|||
Residual stress, fatigue and stress relief
Ron Ruff wrote:
On Apr 24, 9:59�pm, jim beam wrote: manufacturers actually state that spokes /shouldn't/ be bent. Everybody I know who uses Sapim spokes bends them. that's probably because of jobst's bad advice about "correcting the spoke line". there's no technical reason to do it though because in reality, once the spoke becomes tensioned, and the hub holes are deformed, it becomes unnecessary. especially on modern hubs with canted flanges. http://www.flickr.com/photos/38636024@N00/331112190/ that's why spokes are formed with 95 degree bends, not 90. it's not like manufacturers are so dumb they couldn't form it right first time. Dave Ornee, what do you do? |
#57
|
|||
|
|||
Residual stress, fatigue and stress relief
jim beam wrote:
Ron Ruff wrote: On Apr 24, 9:59�pm, jim beam wrote: manufacturers actually state that spokes /shouldn't/ be bent. Everybody I know who uses Sapim spokes bends them. that's probably because of jobst's bad advice about "correcting the spoke line". there's no technical reason to do it though because in reality, once the spoke becomes tensioned, and the hub holes are deformed, it becomes unnecessary. especially on modern hubs with canted flanges. http://www.flickr.com/photos/38636024@N00/331112190/ that's why spokes are formed with 95 degree bends, not 90. it's not like manufacturers are so dumb they couldn't form it right first time. http://www.flickr.com/photos/38636024@N00/327722444/ Dave Ornee, what do you do? |
#58
|
|||
|
|||
Residual stress, fatigue and stress relief
Ron Ruff Wrote: On Apr 24, 9:59*pm, jim beam wrote: manufacturers actually state that spokes /shouldn't/ be bent. Everybody I know who uses Sapim spokes bends them. Dave Ornee, what do you do? I bend the heads in ones to a slightly smaller angle (depending on the hub, crossing pattern and rim ERD) with spokes laced and nipples turned 5 full turns. Some hubs like Phil Wood have canted flanges so I don't bend spokes on them unless it looks like it will improve the spoke line. Yes, I saw what Sapim says on their web site about not bending, but I find that the improved spoke line is benenficial in the long run. I have had one returned wheel from over 1,000 built with a broken spoke... and it broke at the end of the spoke shaft just at the elbow. I tossed the spoke and all 31 of the others in the wheel (no pictures as I forgot to take the time and wanted to get the wheel back to the rider) and rebuilt it with all new spokes and new nipples. Hub is Dura Ace 7700 rear (25th Annivesary) rim is Velocity Fusion. I saw the spokes in Jim Beams pictures. That is interesting. Some day I will photograph spokes before & after... inbound and outbound + spoke holes prebuild and post build, including post stabilizing. If any of you have such pictures already I would like to see them and get some suggestions how to get the details photographed such that you can really discern what is going on. -- daveornee |
#59
|
|||
|
|||
Residual stress, fatigue and stress relief
daveornee wrote:
Ron Ruff Wrote: On Apr 24, 9:59*pm, jim beam wrote: manufacturers actually state that spokes /shouldn't/ be bent. Everybody I know who uses Sapim spokes bends them. Dave Ornee, what do you do? I bend the heads in ones to a slightly smaller angle (depending on the hub, crossing pattern and rim ERD) with spokes laced and nipples turned 5 full turns. Some hubs like Phil Wood have canted flanges so I don't bend spokes on them unless it looks like it will improve the spoke line. Yes, I saw what Sapim says on their web site about not bending, but I find that the improved spoke line is benenficial in the long run. I have had one returned wheel from over 1,000 built with a broken spoke... and it broke at the end of the spoke shaft just at the elbow. I tossed the spoke and all 31 of the others in the wheel (no pictures as I forgot to take the time and wanted to get the wheel back to the rider) and rebuilt it with all new spokes and new nipples. Hub is Dura Ace 7700 rear (25th Annivesary) rim is Velocity Fusion. I saw the spokes in Jim Beams pictures. That is interesting. Some day I will photograph spokes before & after... inbound and outbound + spoke holes prebuild and post build, including post stabilizing. this is the hub those spokes came from: http://www.flickr.com/photos/38636024@N00/104463818/ that hub hole deformation is *definitely* going to affect exit angle. and thus "correcting" the spoke line /before/ tensioning [and stabilizing], as most people do, is going to be premature. If any of you have such pictures already I would like to see them and get some suggestions how to get the details photographed such that you can really discern what is going on. |
#60
|
|||
|
|||
Residual stress, fatigue and stress relief
jim beam Wrote: daveornee wrote: Ron Ruff Wrote: On Apr 24, 9:59*pm, jim beam wrote: manufacturers actually state that spokes /shouldn't/ be bent. Everybody I know who uses Sapim spokes bends them. Dave Ornee, what do you do? I bend the heads in ones to a slightly smaller angle (depending on the hub, crossing pattern and rim ERD) with spokes laced and nipples turned 5 full turns. Some hubs like Phil Wood have canted flanges so I don't bend spokes on them unless it looks like it will improve the spoke line. Yes, I saw what Sapim says on their web site about not bending, but I find that the improved spoke line is benenficial in the long run. I have had one returned wheel from over 1,000 built with a broken spoke... and it broke at the end of the spoke shaft just at the elbow. I tossed the spoke and all 31 of the others in the wheel (no pictures as I forgot to take the time and wanted to get the wheel back to the rider) and rebuilt it with all new spokes and new nipples. Hub is Dura Ace 7700 rear (25th Annivesary) rim is Velocity Fusion. I saw the spokes in Jim Beams pictures. That is interesting. Some day I will photograph spokes before & after... inbound and outbound + spoke holes prebuild and post build, including post stabilizing. this is the hub those spokes came from: http://www.flickr.com/photos/38636024@N00/104463818/ that hub hole deformation is *definitely* going to affect exit angle. and thus "correcting" the spoke line /before/ tensioning [and stabilizing], as most people do, is going to be premature. If any of you have such pictures already I would like to see them and get some suggestions how to get the details photographed such that you can really discern what is going on. Here are links to 6 pictures I took today of a Dura Ace HB-7700 32H wheel I built today with Sapim Race 14/15 DB spokes. One side I used my previous pattern of "correcting the spoke line" and the other side I made no such correction. Judge for yourself. http://www.flickr.com/photos/17085834@N08/2443679334/ http://www.flickr.com/photos/17085834@N08/2443679236/ http://www.flickr.com/photos/17085834@N08/2442851273/ http://www.flickr.com/photos/17085834@N08/2443679416/ http://www.flickr.com/photos/17085834@N08/2442851411/ http://www.flickr.com/photos/17085834@N08/2442851349/ After this experiment I think the next build will be done without correcting the spoke line. -- daveornee |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Your best solution to relieving residual stress! | liverliver | Techniques | 2 | December 28th 07 05:48 PM |
yet another residual stress redux | jim beam | Techniques | 0 | March 17th 07 09:41 PM |
Spoke stress relief test | [email protected] | Techniques | 32 | December 27th 06 06:21 PM |
Stress-relief demonstration suggestions? | [email protected] | Techniques | 73 | January 8th 05 02:09 AM |
RR: Stress relief | Mike Kennedy | Mountain Biking | 3 | October 5th 04 06:59 PM |