Re: [MELB] Trams with Trolley--Poles

[Jim Holland <pghpcc@pacbell.net>]

> "McAndy B(tm) with cheese" <laurelb@dingoblue.net.au> wrote in message

> Why have 2 poles on a tram? :)  Obviously there is some reason, otherwise
> they'd just use the same one, and turn as necessary :)

	Quite a few reasons why 2-trolley-poles are on a trolleycar.

	This is most common on double-ended equipment and it allows the car to
change directions by a simple X-over on double track or just change ends
on single track.

	But that also begs the question implied in McAndy's question - why not
still use one pole and just swing it around.  As mentioned in other
posts, this is what is done with shorter equipment, but when the
trolleycar is 40-60-feet long, it is very difficult to get the trolley
pole back on the wire when it is mounted in the center of the car and
the trolley pole is only 14-feet long!
	The obvious answer to that  *might*  be to make the trolley pole
longer.  In this case, the trolley pole would have to be 23-30-feet long
to reach the end of the car.  To prevent shimmy and vibration of the
trolley pole under tension, the pole would have to be much stouter which
means much heavier and to keep sufficient pressure against the overhead
to maintain reliable contact, the pressure would have to be much
greater!  As it stands, trolley poles equipped with shoes have a
recommended pressure against the overhead of 25-30-pounds at a nominal
height of 18-feet.  It would probably have to be at least twice that
with an heavier and longer pole.

	Additionally, the geometry and layout of the overhead wires would be
considerably different.  On the American PCC, the trolley pole is
mounted over the forward axle of the rear truck so that the trolley pole
base is slightly to the inside of the center of a turn.  And on single
truck cars the pole is mounted in the center of the car which would make
it ever-so-slightly toward the inside of center on a turn.  Thus the
trolley wire is placed in a similar position - slghtly toward the inside
of center on a turn.
	But for practical purposes, let's say that the trolley pole base on a
single-end car is immediately above the king pin making it essentially
right on top of the centerline of the track, tangent and curved.

	On a 45-60-foot car with a trolley-pole mounted in the center of the
car, the trolley pole base could possibly be over the inside rail on a
turn, and even inside the inside rail on a turn.  (Remember that in city
streets, a radius of 35-50-feet is quite common!) And the wire would
have to be in a similar position to keep the pole on the overhead.
	The latter is fine if  ALL  equipment is similar, but as most
trolleycar systems started with single truck bobbers, this dictated
where the trolley wire was placed.  And since the trolley wire was
placed near the center line of the track on a turn to accommodate single
truck bobbers, all future equipment needed to comply.  Thus the longer
double-ended cars use 2-trolley poles so they can operate effectively on
overhead placed near the center line of a turn.

-- 
James B. Holland

        Pittsburgh  Railways  Company  (PRCo),   1930  --  1950
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