Half of them must be in tension because they're pulling the wire to one side. The other half must be in compression.
They are *lightly* in tension - usually significantly less than the 2.5kN load limit for the Series 1 registration arm, which is a tiny load compared to the 16.5kN tension in the contact wire. There is a special arm capable of taking a small compression load but thats rarely used because it may perform less well.
My point was that they are not called tension arms, and thats because their purpose is not to provide tension. They are there to register the contact wire, i.e. provide horizontal restraint. Hence "registration".
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oh god, the Alan Baxter report is in the public domain. *shudders*
Health warning: that document was not written by an OLE engineer, and although it has some good stuff it also contains a number of factual errors. For instance it talks about 2.75m public area clearance when the UK standard (GL/RT1210, available on RSSB website) now require 3.5m.
If the forum can hold on about a month I will be publishing a free ebook on the subject which is more comprehensive, factually correct and importantly has been written and peer-reviewed by electrification engineers. Give me a prod mid November if a link hasnt appeared here.
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I'm fairly sure that the stagger is achieved by rotating the arm around the wire, rather than having arms alternately in tension and compression.
Eg at Didcot.
yes you're correct. We don't put reg arms in compression if we can possibly avoid it because it can result in an area of increased stiffness which can affect dynamic performance.
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