Re: Pantographs (was Re: MBTA BREDA Light Rail Cars)

[matthew@mail.usyd.edu.au (Matthew Geier)]

In article <368833E3.148B@REMOVEiprolink.co.nz>,
David McLoughlin  <davemcl@REMOVEiprolink.co.nz> wrote:
>Silas Warner wrote:
>
>> There are usually two reasons why a locomotive might have two pantos.
>> One is to bridge "dead spots" in an overhead wire, but this is not the
>> usual reason.  The usual reason is that the pantos are designed for
>> one-way operation, and cannot be asily reversed.  This was true of
>> early Faiveley (one-arm) pantographs. which could only be operated with
>> the "elbow" facing backward: a forward-facing wlbow would be forced up
>> by the air rush of the panto's passage and might snag the wire.
>
>Melbourne in Australia is in the final stages of converting its large
>tramway (streetcar) system from trolleypoles to pans. They are
>single-arm pans, one to a tram. It seems there is a resulting problem
>with pans breaking or bending over, causing major delays in services. 

 I dont think the single arm pans are generally uni-directional - our
'Tangara' suburban trains have single arm pans, 2 per set - with the
elbow facing inwards. The train requires both raised to operate.

 A tram, metro or Suburban train wouldnt get up enough speed for the
aerodynamics to be much of a problem.

 Our electric locomotives had 2 pans (mostly diamond types) due to current
demands - if they were pulling a large load up a hill, both pans would be
raised to allow more current ibn with out risking overheating the carbon
contact strip. Most of the time they ran with only the rear pan raised -
I belive on the basis that if the rear pan failed, the debris wouldnt 
foul the front and you stood at least a chance of continuing. Running
front raised risked damage to the rear due to debris fly across the roof.
Some of these electric locos were over time fitted with single arm pans,
big heavy monsters, not light the light stuff on a tram or suburban EMU.

 When the Tangara's introduced a new lighter single arm pan to Sydney,
there was a spate of overhead entanglements the next summer. I assume
the light single arm pans tended to twist on marginal overhead and then
rip something else down. In the summer the situation got worse due to
expansion of the copper wires. 10 years later this doesnt seem to happen
so much any more.

 Melbourne's main problem is probably marginal overhead - the system has been
for years trolley pole operated - and the trolley poles can follow some
pretty badly strung wire. With a 'quick' conversion to pan operation on
some routes, they probably didnt re-string the wire tight enough - which
would also tend to get worse in summer as the wire expands and droops more.
All you need is a loose wire to twist the pan head a little and the head
catches the next cross stringer.