Friday, October 24, 2008

It's deja vu all over again

As Yogi Berra is reputed to have said (I'm sure he can't be the only one).

This caught my attention in the news yesterday:
Passengers at risk from track design flaw



A design flaw found in thousands of places on the country’s rail network is putting passengers at risk of a catastrophic derailment similar to the Potters Bar and Grayrigg disasters, according to a secret analysis by rail safety inspectors.
[...]
It says that the joint “may not withstand normally encountered forces imposed by the operating railway, resulting in the potential for loose fasteners and consequent changes to the configuration of the points leading to further points degradation and subsequent train derailment”.
[...]
A source close to the investigation said: “We should not just be relying on track workers to spot loose bolts in time. We need to stop the bolts from coming loose. This is about making sure that the points are fail-safe.”
Sound familiar? Of course, it's exactly the same process that I uncovered with quick-release failures on disk brake-equipped bicycles - an underspecified bolted fastener working loose due to an applied load that exceeds the capacity of the design.

Here is the full report on the Grayrigg crash. A few excerpts:

The fasteners in the third permanent way stretcher bar joint failed by unwinding. This occurs when the applied load exceeds the clamping force on the joint. The RAIB has concluded that, in this case, such loadings were a result of the passage of trains.
[...]
The RAIB has concluded that the design of the joint between the permanent way stretcher bar bracket and the switch rail at 2B points, and of other similarly configured points, was such that it could have been subjected to forces beyond its design capability and therefore the points system had significant reliance on regular inspection and maintenance to maintain safe operations.
[...]
The RAIB concludes that this incomplete understanding of the performance of S&C with non-adjustable stretcher bars, and the relationships between its design, usage, loadings, inspection and maintenance, led Network Rail to consider that the risk associated with the design was low and was being adequately controlled. This also resulted in an absence of clear and properly briefed standards for the setting up and adjustment of S&C.

[S&C - switches and crossings, a generic term for various railway track junctions]

There is lots more to wade though for anyone who is interested. But there is hardly any need. It is hardly rocket science - it took me literally seconds to find out what I needed to know via Google and this web site, once I guessed at what was going on back in 2003 (in relation to disk brakes). It is worrying to think that there are supposedly qualified engineers out there who don't know this stuff like the back of their hands...

5 comments:

Chuck said...

Bolts?

Jesus fucking Christ, what century are they in? Back in the twentieth, railroads learned about this wonderful new invention called welding. Send the lot of them to Japan- or France, and force feed then wasabi-flavored snails until they learn how to run a modern rail system.

James Annan said...

The main rails are continuously welded AIUI, but this problem seems to be specifically at points. I'm not enough of a trainspotter to know if bolts are completely unavoidable even here.

But anyway there is no problem with a properly designed and installed bolted joint. There is, however, a problem with a safety-critical bolted joint that relies on regular retightening to stay together!

Chuck said...

Use a rivet, then. Or weld the nut on (basically a weak rivet).

James Annan said...

Maybe they need to take them apart? But I'm just guessing. It is notable that Network Rail's main defence appears to be "they hadn't killed anyone before"...which is certainly a valid point in terms of the overall risk (taking a train is much less dangerous than driving a car, for example) but hardly the final word on the matter.

David B. Benson said...

Petroski Henry:
To Engineer is Human: The Role of Failure in Successful Design

Vintage Books Usa , 1992
Quality paperback, 272 pages

strongly recommended