From: BRANDT_JOBST/HP1900_01@hplomg.hpl.hp.com
Newsgroups: rec.railroad
Subject: Re: Brakes on SF cable cars
Date: 17 Aug 92 18:58:51 GMT

Robert Coe writes:

> About 30 years ago "Road & Track" magazine did a somewhat tongue-in-cheek
> "Road Research Report" on the SF cable car.  (Excellent acceleration up to
> its top speed of 9 mph; etc.; etc.)  :^)  Analyzing its braking capability,
> the article noted that a cable car has three separate braking systems, the
> most drastic of which is designed to drive a steel plug into the slot with
> such force that it has to be cut out with a torch.  Is this the one that
> failed?  What was the nature of the failure?

I recall the article that was part of a series in which non sports cars
were tested and reported in the standard R&T format.  Amusing but for the
cable car not exactly accurate with respect to the emergency brake.
All tests of the cable car emergency brake ever made were essentially static
tests that ignored dynamic heating. For most who have seen steam locomotives
start the effect of sliding drivers is probably the most dramatic display
of lubrication by molten steel.  The drivers continue to coast after power
has been shut off.  The steel wedge in the cable slot is similarly
lubricated.

A brake is an energy dissipating device and the heat of a glowing brake shoe
on a train or a cherry red disk brake is not questioned yet the cable car
emergency brake has about 1/2 square inch of contact, steel on steel, with
no heat dissipating surface and this is expected to work on a runaway cable
car without question.

Another writer mentions:

> The cars have two other more frequently used brakes.  Pine
> blocks slide along the rails and iron shoes press against the
> wheels.  The cable itself is also a brake of sorts.

None of the brakes has any substantial effect on the speed of a car that
lost contact with the cable on the Hyde Street hill.  It is this hill on
which most of the runaways occurred, and I use the past tense because they
were caused by a errors of operation that I believe has been corrected.
The belief that the slot wedge worked is a case of mistaken cause and
effect.  THe molten wedge slid down the hill until striking a vehicle on
Bay Street or below.  At his time the steel cooled and welded solid.  The
removal by cutting torch was always shown to prove its great stopping power
when in fact it proved the opposite to the critical observer.

Several years ago, after the last Hyde Street runaway, I made a great effort
with fresh data to once more attack this problem.  At that time it occurred
to me that the labor union was one avenue that I had not tried.  All other
people from the State PUC, SF PUC, Municipal RR management, and Harre
DeMorro didn't want to talk about it.  Harre DeMorro is the rail
reporter for the SF Chronicle and noted rail author.  From his refusal
to respond to written communication and saying that he was out when he
was in on the phone, greatly disappointed me about his objectivity.
The gripman's union turned out to be a viable path.

The reason for most runaways was that gripmen eased the car over the
sharp crest of hills to spare the passengers the queasy feeling as
the car went "over the cliff" so to speak.  To do this the cable must
be allowed to slip through the grip.  At the crest of the hill the
cable has an enormous downward pull that is normally supported by a
"crown wheel".  When the car crests the hill the grip must lift the
cable off the crown wheel.  The partial grip engagement that allows
the car to travel at a speed that is less than synchronous with the
cable also allows the cable to be pulled from the grip.  For safety,
cars are also required to stop at the top of the hill and wait for a
signal that indicated when traffic on Bay Street had been stopped
so that an eventual runaway would not plow into heavy traffic.  The
stop line was also too close to the crest of the hill for a full car
readily to be brought to full speed before cresting the hill.

Through my efforts I was able to convince the union that, contrary to
the official findings, their gripman was no at fault and that the
operating rules needed to be changed to require full speed (synchronous)
grip engagement when cresting any hill.  I also got the opportunity
after these meetings to talk to the chief engineer about the emergency
brake, something that could not be discussed previously.  The conversation
was guarded to put it mildly.  There is an extreme fear of litigation
that makes all discussion of this matter highly sensitive.  I proposed that
the wedge mechanism, that has substantial force, be used to press a rubber
tread similar to a pair of dual tires on a bus, against the pavement on
either side of the slot.   Although this is not a good energy sink, its
ablative nature makes a skidding rubber tire reasonably effective and
certainly better than a steel knife edge against steel.

As I said, I have hope that something is being done about the brake
after seeing the imaginative solution to the depression pulleys.  The main
thing is that the cars should not run away now that the operating rules
recognize the hazard.  I have not pursued this mater in a few years so
any new information would interest me.  From experience, I am sure that
the Muni isn't going to say a word one way or the other.

jobst_brandt@hplabs.hp.com