[PRCo] Re: Braking systems

[robert simpson]

Thanks, Fred!
   
  I am saving this document as a Word file for future reference.  I am still "digesting" all the information but it appears you have explained a lot of the questions I asked.
   
  Robert Simpson
  from Krazy California
  
Fred Schneider <fwschneider at comcast.net> wrote:
  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
>