Speaking of Air Brakes on 1600 B-2s and B-3s

[Fred Schneider]

The PCC specification (a performance spec) called for three different
braking systems that needed to achieve a specific retardation rate ... seems
to me it was about 4.5 miles per hour per second in service and around 9.0
mphps in emergency.  

Service braking consisted on all PCCs of a dynamic brake (i.e. the car's
momentum was dissipated into heat energy in the resistors by using the
motors to generate against each other in a closed wiring loop).  Immediately
when an operator lifts his foot off the accelerator pedal, the motoring
contactors drop out and the braking contactors are energized setting up this
wiring loop.  The rate of braking is controlled by the same limit relay that
controls rate of acceleration.  If one motor lead should become disconnected
(as happened in the last tunnel runaway), the loop is broken and dynamic
braking is zilch.

As the car decerates, the current produced is reduced, and resistance needed
to be cut out to maintain an even braking rate.  Eventually the car is going
to slow that dynamic braking is negligible.  On air cars, this happens in
the 5 to 9 miles per hour range.  On cars with extended dynamic braking (the
all-electrics), dynamic braking was extended down to a very slow walk.

When dynamic braking fails, a lockout relay drops out allowing a friction
brake to be applied.  Friction brakes were only used on the PCC to bring the
car to the final stop and to hold the car once stopped.  There was
considerable variety here.  All of the earliest air cars used truck mounted
air cylinders (like those on a highway vehicle) to apply air brake shoes
against the wheels.  With all-electrics, brake drums were mounted on the
motors ... drums similar to those on an automobile.  They could either be
internal expanding (like the auto) or external contracting.  In general,
springs were used to apply them and electric solenoids were used to release
the drums.  Some cars built with air after 1940 used air drums (the
Pittsburgh 1200s) and some cars were rebuilt (most of the Washington DC
fleet for example).  

So, if all goes as planned, the operator uses only dynamic brakes to slow
the car and some form of friction brake to finish the stop.  

Four track brake shoes, two per truck, were installed for emergency use when
energized from the car's 32 volt battery.  The motorman's brake pedal
progressively controlled the brake systems ... you got dynamic and track
brakes if the brake pedal was pushed more than half-way down.  Using both,
you should be able to stop a car doing 30 miles per hour on dry rail in
about 3.5 seconds or about 1.5 times its own length.  

If a motor lead tore ... you were down to air and track brakes only ... and
the air (or spring applied brakes) were never intended to stop the car.  If
the air failed, the car would slow to about 7 mph and then just roll onward
at that speed.  If the batteries were aged and would not hold a charge, I
suspect the motorman also perspired a tad.  I don't know if the MG set alone
can handle the entire track brake load if the batteries are shot ...
Holland, help me out on that.  You ran the critters for many a moon.  

There was a 1940-ish study by Transit Research Corp. comparing air brakes on
a Pittsburgh 1200 with spring drums on a St. Louis 1500.  A conclusion was
made that the springs could never possibly hold a car on Pittsburgh's hills.
I can personally guarantee that the spring drums would not hold a Pittsburgh
1700 on Henderson Street (21 FINEVIEW) because I staged such a fantrip in
1958.  Track brakes were ineffective too.  The car coasted backward until
the track shoes were canted upward at a 30 degree angle.  

Jim:  How could you hold a Muni 1016 class in Mission Park on the grade (not
in the level car stop)?   And what is the grade there?  Are there any
steeper ones in SF?      


 

		-----Original Message-----
		From:	mrb190 [mailto:mrb190 at pop.pitt.edu]
		Sent:	Monday, February 28, 2000 11:46 AM
		To:	pittsburgh-railways at dementia.org
		Subject:	Speaking of Air  Brakes  on  1600  B-2s  and
B-3s

		I need to take a crash course in PCC Braking.  The story I'm
about to
		relate would be a bit more interesting if I knew the
mechanics better.

		And I wish I could be more specific about the car series
number, but I
		think it was a 1600...anyway, while staying at my aunt's
house in the
		summer of 1966, there was the occasion where a car on the 39
Brookline
		route lost one of its braking capabilities.   We came down
Brookine
		Blvd. slowly onto West Liberty, and continued coasting down
West
		Liberty, not very fast, but we all knew something was up.   
		First we went through a red light.  Then, we passed some
very befuddled
		folks standing on the safety islands on West Liberty.  We
just kept
		gliding without too much incident down West Liberty and up
to South
		Hills junction.   I don't remember what track we turned off
on up there
		- OR, even if we did (I was 8 or 9 years old at the time).
I remember
		hearing what I thought was a lot of crossing over or onto
special work,
		and getting a kick out of the fact that we were all getting
off the car
		near all these parked PCCs.
		Apparently, the brakes that slowed the car worked well
enough to stop
		the car up in the yards, but the brake that actually grips
the track to
		a complete stop, normally on the downgrade of West Liberty,
did zilch. 
		What I recall of out conductor was his soaked blue-gray
shirt.  He was
		perspiring, to say the least!   My aunt was laughing as we
passed all
		those people waiting on the West Liberty islands, (this was
a.m. rush
		hour!), and so I guess I wasn't too worried.  

		Anybody else remember anything about this incident?  Since
it went off
		without much trouble, i.e., nobody hurt, no derailment,
perhaps it was
		never reported in the press.  In fact, I think we watched
the news that
		night to see if it was mentioned to no avail.

		Matt

		Fred Schneider wrote:
		 
		> I have a tabulation of brake variants done about 1951 by
Homewood
		> engineering staff ... I think the 1200s were the only air
cars with spring
		> applied drums and they came from the factory that way.
		> 
		>                 -----Original Message-----
		>                 From:   HRBran99 at AOL.COM
[mailto:HRBran99 at AOL.COM]
		>                 Sent:   Monday, February 28, 2000 10:25 AM
		>                 To:     pittsburgh-railways at dementia.org
		>                 Subject:        Re: Air  Brakes  on  1600
B-2s  and  B-3s
		> 
		>                 In a message dated 02/28/2000 5:25:43 AM
Eastern Standard
		> Time,
		>                 pghpcc at pacbell.net writes:
		> 
		>                 << Which way did the brakes work on the
1600s? >>
		> 
		>                 The brakes were air-applied and
spring-released. Hence, the
		> hand brake to the
		>                 right of the operators seat in case air
pressure was lost
		> and the car had to
		>                 be stopped with that device. This did
happen to me one time
		> coming down New
		>                 Arlington Avenue on the 49 line.
		> 
		>                 HrB