Rail buckling

[nw-mailing-list at nwhs.org]

On the other hand Under mention the rail is actually more stable, 
However broken rails are common in the Spring an fall where you have hot 
day and cold nights. the rail will expand in the day and the the 
shrinking at night literally pulls it apart.

Nathan Simmons
trainman51 at gmail.com



nw-mailing-list at nwhs.org wrote:

>

> Rail buckling is caused by the compressive forces generated by the 

> heat expanded rails in one section of rail.

>

>  

>

> The buckling is a stability issue in engineering, not one of strength 

> or internal temperature.  Simplified, as the rails heat up on a hot 

> day, they increase in length and push against their neighbors causing 

> a compressive load in the rail (two neighbors push inward on one chunk 

> of rail causing an internal compressive load).  Taking the 'chunk' as 

> dead straight, the compressive loads can be increased up to a critical 

> load where the rail will immediately bow sideways.  This critical load 

> value changes as the two compressive loads are applied through the 

> centerline of the rail or just slightly off centerline, eccentricity, 

> in the rail.  More eccentricity gives lower critical loads.  Also the 

> slenderness ratio, very crudely, length over width, matters, too.

>

>  

>

> As the problem is, more-or-less, load and geometry of the system, no 

> buckling will occur until the critical load is reached, then lots of 

> deflect can occur for a slight increase in load. 

>

>  

>

> Since rail and track is rarely straight and the rail will grow at 

> different rates depending on local conditions, sun angle and all sorts 

> of other stuff, the critical load will vary bunches over a long length 

> of track.  In other words, unpredictably.  Hence the track inspection 

> trucks. 

>

>  

>

> Ribbon rail and jointed rail have mostly similar geometries over a 

> long length, so the critical buckling loads are not much different.  

> Never the less, there are a large number of variables to consider in 

> this problem.

>

>  

>

> The simple model is to stand a soda straw on end and carefully press 

> downward on the end with your hand.   Slowly increasing the force will 

> cause nothing much to happen until the cross section of the straw 

> folds and the straw will easily bend sideways with the slightest 

> increase in load.

>

>  

>

> Gary Rolih

>

>  

>

> ------------------------------------------------------------------------

>

> *From:* nw-mailing-list-bounces at nwhs.org 

> [mailto:nw-mailing-list-bounces at nwhs.org] *On Behalf Of 

> *nw-mailing-list at nwhs.org

> *Sent:* Wednesday, October 04, 2006 9:51 PM

> *To:* NW Mailing List

> *Subject:* Re: Trackwork

>

>  

>

> Asketh Ron:

>

>  

>

> >> 

>

> Thanks Abe, but wouldn't that have happened with jointed rail also?

>

> << 

>

>  

>

> I'm an operating guy, Ron, and not a track expert.  And I'm a 

> philologist, not an engineer. So I speak from very limited knowledge.

>

>  

>

> Did you ever walk along "jointed track" in the Summertime, and notice 

> how much the rail had "run" longitudinally in the tie plates, as 

> indicated by the shiney abrasions of the spike heads on the base of 

> the rails?  Each rail joint is in effect an "expansion joint" for 

> taking up and relieving longitudinal stresses. 

>

>  

>

> That doesn't happen on track with welded rail.  The rail is anchored 

> tightly to the ties with Woodring anchors or Pandrol clips, so that it 

> will not "run" longitudinally.  When the expansive forces and 

> pressures build up under these conditions, the rails move laterally 

> and move the ties with them, which amounts to a "kink" in the track.

>

>  

>

> I remember some years ago, not long after ribbon rail became popular, 

> that railroads were having numerous derailments on "buckled track," 

> but most of them were happening about 15 or 20 car lengths behind the 

> engine.

>

>  

>

> The DOT Pueblo facility fitted some welded rail track with all kinds 

> of instrumentation and even took movies of track buckling in the hot 

> sun.  What they discovered is that the sun heated the rail to the 

> high-130's, and then the friction caused by a train moving over the 

> track added a couple more degrees to the rail temperature and pushed 

> the track over the critical temperature threshold, and the 

> track buckled horizontally right under the train.  I recall seeing an 

> AAR movie (viedeo ???) of these experiments in the late 1970s.

>

>  

>

> This showed the railroads why they were having "track buckled 

> derailments," and the response was to "regulate" gobs of ballast 

> against the ends of the ties on continuously welded track, to keep the 

> ties from moving laterally.

>

>  

>

> That's a brakeman's explanation for you.  I'm sure a civil or 

> mechanical engineer could put perfume and a ribbon my my pig of an 

> explanation.

>

>  

>

> -- abram burnett

>

> ------------------------------------------------------------------------

>

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