I don't understand the comments about air pressure having to build up to
provide braking. Unless things have changed since I did a lot of
driving with air brakes, the braking pressure comes from springs, and
the air pressure is required to release the brakes. Air-pressure
failure results in the brakes begin applied rather than resulting in no
brakes. I believe this is the way it works on buses, trucks, as well as
trains.
I never drove trucks or trains, but in a former life (read this as
"summer job for four summers during college") I was a school bus
inspector at a large school bus manufacturer. I inspected about 25
buses a day for about 12 weeks times four summers. That means I drove a
whole bunch of school buses (like about 6000 different buses, can anyone
top that?) of all chassis makes (Chevy, Gimmy, Ford, International,
Dodge, Mercedes) and all sizes. Perhaps half of them had air brakes.
I spent a fair amount of time sitting in the bus with the engine running
before I could move it. After rolling off the assembly line the buses
were parked in a large field and didn't get inspected until several days
later. I had to drive each bus to an inspection hut and perform the
inspection. By the time I got to each bus in the field, there was no
air pressure in the tank. Once I got the engine started there was no
way that bus was going to move until the air pressure got high enough to
release the brakes. Building up a couple-cubic-foot tank from zero psi
to 100 psi takes a while. Some of the compressors were fairly slow and
took 100 to 150 seconds to build up enough pressure in the air tank to
get the brakes released. I know 2 minutes is not really a long time;
however, when you start your engine but can't move for two minutes, it
seems like a really long time.
I see that Brian W. said something about this a few minutes ago,
including that the pressure can be dumped in about half a second. My
memory is it was at least that fast if not faster. If my mind was
wandering and I forgot I was driving a bus with air brakes I would press
the brake pedal too fast and too hard. An empty school bus with air
brakes can stop pretty fast. I heard the hiss, muttered "crap", and hit
my chest or gut on the steering wheel all about the same time. Most
buses at that time did not have seat belts and definitely did not have
shoulder straps. The steering wheels were big, especially when there
wasn't power steering, and they were oriented more horizontally than on
a car. That meant the "bottom rim" could be pretty close to your
chest/gut. I got in and out of so many buses that it could be a blur as
to what I was currently driving, but I tried real hard to remember
whether it had air brakes or not.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu