[PRCo] Braking systems

[Fred Schneider]

Those not interested in technology, please delete immediately.

(I should make this a word document and just send it out every time  
someone new asks but I guess I'm too dumb to do that....)

1.   Earliest braking systems were usually hand brakes.  By using a  
lever or a crank, leverage is applied to brake shoes pressed against  
the wheels of the car to retard its motion.   Hand brake systems  
usually disappeared from streetcars in the early years of the 20th  
century.   However, most cars still retained hand brakes as parking  
brakes right up through the air-brake equipped PCC cars except for  
certain air-PCCs that had spring applied air released shoe brakes.   
One of the more usual hand brake schemes that I've seen used a  
leather strap around a brake drum pulled tight by a lever; this  
arrangement appeared on a former Market Street Railway work car  
inherited from an underlier.   The car is now at Western Railway  
Museum in Rio Vista, CA.    The museum with the largest collection of  
hand brake cars in day-to-day operation is probably the Baltimore  
Streetcar Museum, which has six cars, four of which are normally  
used.   Car 554, a open car by Brownell dates to 1896; another single  
truck closed car number 1050 dates to 1898.  A double truck  
Narragansett was built in 1902.   Those three run almost any summer day.

2.   Air brake systems came into being in the early part of the 20th  
century.   There were cars with storage air systems that could be  
charged at termini.  There were also cars with compressors that could  
be operated from the motion of the car itself.   Most of the PCC cars  
with air systems used compressors that ran continuously off the motor- 
generator set and and were loaded or unloaded by engaging the valves  
on demand.   And the most common system, that which won out in the  
end, were compressors driven by attached motors that were turned on  
and off by pressure sensitive governors.   The most common compressor  
manufacturers were Westinghouse Air Brake (those with DH series part  
numbers, like DH-16) or General Electric (CP series, such as  
CP-27).   One minority compressor manufacturer that comes to my mind  
was Allis Chalmers.

Air Brake Systems came in essentially two varieties, straight air and  
automatic air:

2a.   Straight air systems were those designed initially by George  
Westinghouse.   You compress air (in his case a steam air pump on a  
steam locomotive was used), store it in a reservoir (a cylinder or  
tank), feed it through a control valve into a brake cylinder  
containing a piston, which forces the piston out, moving linkage  
under the car to press shoes against the revolving wheels to slow and  
stop the train.    A simple straight air valve has three positions:   
release, lap, apply.   In he apply position, air is fed into the  
brake cylinder.   In the lap position, the air is held in the  
cylinder.   In the release position, springs on the other side of the  
piston, force the brakes to release and push the air out into the  
atmosphere.

2a1.  The flaw with straight air systems comes when running trains.    
If the train brakes in two, you cannot stop it because when you apply  
the air, it just blows out the end of the pipe.

2b.  The second system is automatic air ... Each car has a triple  
valve and its own reservoir.   You charge the reservoir by pumping up  
a train line.   When you apply brakes, you take air out of the train  
line and the triple valve then feeds air from the car reservoir into  
the brake cylinder on each car.   If the train breaks in two, the  
brake pipe also splits, dumps all the air out and the train goes into  
an emergency brake mode.

2c.   You can have self-lapping brake valves with straight air  
systems.   They simply put the handle in a different position to  
graduate brake pressure.   Self lapping automatic brakes were a new  
development near the end of the 20th century.

3.  Electro-pneumatic Brakes.   You can also control the brakes on  
each car electrically.  The advantage is electricity moves with the  
speed of light, i.e. 186,000 miles per second.   Air can take forever  
to evacuate a pipe.  Therefore with a EP brake system, when a subway  
or MU car motorman applies brakes, all cars go on simultaneously.    
Much higher braking rates are permissible without discomfort to  
passengers.   On older equipment such as the Pennsylvania Railroad  
multiple unit electric car fleet, the electric features were  
superimposed on top of a conventional automatic air brake system so  
that if a fuse blew, the train would still stop using the  
conventional system but it might over shoot the platform before the  
brakes on the last cars came on.   The Delaware River Port  
Authority's Lindenwold High Speed Line use a very low current  
( miliamperage ) loop through the couplers of the entire train for  
both acceleration and braking control as part of the ATO system that  
is fail safe because no current flow is emergency braking and maximum  
current is maximum speed.

4.  Dynamic braking, Bob, was used in two ways.  It was the common  
way to stop a car with direct current, series wound motors in an  
emergency such as finding out you had neglected to read your air  
gauge and suddenly, at the least opportune moment, found yourself  
without air.   On a drum control car, you can manually set up dynamic  
braking by, turning off the canopy switch to create a brake loop,  
pulling the reverse key to the opposite direction, and then notching  
up the controller.  It should begin to brake immediately on a four  
motor car or in the parallel points on a two-motor car.  Did any cars  
actually do that as a matter of actual operating practice?   Yes.    
The European systems did it almost universally.   Almost every car in  
the National Tramway Museum in Crich, England is designed for dyanmic  
braking.   Most of the older cars in museums in other European  
countries are the same.  However, the practice was uncommon in the  
United States.  Prior to 1936, one of the few companies that used  
cars with dynamic braking was West Penn Railways.   If you see a B  
controller, it is simply a drum controller set up for both motoring a  
braking.  The B-50 was a K-35 (designed for 4 35 hp motors) but for  
dynamic braking too.    The PCC cars also used dynamic braking.   The  
current generated by the motors in dynamic braking was dissipated in  
the car's on resistors.

5.  Regenerative braking, in which the energy generated by motors in  
a braking mode is fed back into the line is turning up more often  
recently in rapid transit schemes.   With rotary converter equipment  
such as the Milwaukee railroad used, it was touted as a way of  
holding back trains in the mountains of Idaho and Montana and  
Washington while helping other trains up the mountains or feeding  
energy back into the electric grid.  With today's solid state  
systems, as I understand it (and I could be wrong), a lot of systems  
are not reversible and regenerative braking only works if there is  
another train accelerating at the same time.  It may be useful in  
Washington or Chicago or New York  but probably not practical in  
Minneapolis or Pittsburgh.

6.  The shoes you saw hanging between the wheels on PCC cars were  
magnetic track brakes.   They were energized from the 32 volt  
batteries on the car.   On a PCC they were used only in emergencies,  
i.e. only when the brake pedal was more than half way to the floor or  
when the deadman pedal was released.  West Penn Railways also used  
track brakes but I think they may have used regenerative current from  
dynamic braking.   Some other applications such as the track brakes  
on the Buffalo and Lake Erie's Cincinnati lightweight cars may have  
used 600 volts, or perhaps they had four shoes in series across 600  
volts (I've never seen the wiring diagram).

7.  There have also been examples of hydraulic brakes.

8.  If you really want the references, back about 1949-1950 there  
were several issues of CERA Trolley Sparks written by Dave Blain on  
the subject of air brake systems.  I can probably move a mountain in  
this den and find them and give you a reference to the actual issues  
if you want to know what to look for in used book sales.



On Sep 1, 2007, at 10:54 AM, robert simpson wrote:

>
>   Did the Direct Current cars use resistive braking on long  
> downhill grades?
>
>   I have recollections of a rather large, flat metallic object  
> hanging between the wheels suspended only about an inch above the  
> rails.  Was this part of the braking system?
>
>   Bob Simpson
>   from Krazy California
>