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markhr 07-13-08 04:35 AM

Random question #274
 
Given over head power cables for trains, trams and trolley busses are under constant tension and must experience a fair amount of vibration when in use:
  1. do they fatigue,
  2. if so, how fast,
  3. has anyone ever had to re-hang/-string them (why haven't I/we seen it happen)
  4. and are there any reports of fatigue breakage before the maintenance schedule catches the problem?
...and, yes, I should be out riding my bicycle.

supton 07-13-08 06:01 AM

I found this:
http://en.wikipedia.org/wiki/Overhead_lines

It sounds like they wear down, but I'm going to guess that if wear is anticipated, then they are replaced prior to wearing out. Wearing out probably takes decades, given the hardship of shutting down a line to replace 1000's of feet of cable. Although, they might be able to do sections at night time, so as to avoid the downtime.

Also, it sounds like they rarely break, given the second wire (messenger wire, etc) supporting it (which is much more mechanically robust). But they probably do break: it's probably just not exactly leading news on the telly when it does occur.

markhr 07-13-08 06:49 AM

Quote:

Originally Posted by supton (Post 7049743)
I found this:
http://en.wikipedia.org/wiki/Overhead_lines

It sounds like they wear down, but I'm going to guess that if wear is anticipated, then they are replaced prior to wearing out. Wearing out probably takes decades, given the hardship of shutting down a line to replace 1000's of feet of cable. Although, they might be able to do sections at night time, so as to avoid the downtime.

Also, it sounds like they rarely break, given the second wire (messenger wire, etc) supporting it (which is much more mechanically robust). But they probably do break: it's probably just not exactly leading news on the telly when it does occur.

Cool, thanks. I'll check that out. The night time replacement sounds most likely, especially in dense urban areas, but you're right unless it shuts down multiple blocks we may never hear about it.

As for fatigue, I had a materials prof. who showed us a fairly simple application of a fatigue theorem that could be used to predict fatigue limits, e.g., in bolts. That is, when to schedule maintenance to replace them before they broke as well as how many turns to tighten them to minimise fatigue. It was suprising how precise it could be. Now if only I could remember any of it.


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