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An
Introduction to Train Brakes
By
John Bentley
Introduction
Imagine a
vehicle that is a mile in length. It is so long that the front
of the vehicle might be climbing a grade while the back is descending,
or perhaps the front and back are turning left while the middle
is turning right. This same vehicle is more than 500 times as
long as it is wide. Next, imagine that it weighs more than 8 million
pounds or 4000 tons. Onboard the vehicle are televisions, foodstuffs
and hazardous material. Now, visualize the vehicle is traveling
at 60 MPH and the operator wants to stop.
This is a
complex and challenging problem, but a situation that occurs thousands
of times every day. The vehicle of course is a typical freight
train. This short paper will introduce the reader to the principles
of how train brakes accomplish this remarkable task.
Background
Freight train
brake systems have not changed in basic operation since the 1930's.
They are controlled and actuated by compressed air. For those
tempted to think that train brakes operate the same way as large
truck brakes read on. You might be surprised.
Each power
unit (locomotive) has an air compressor that supplies air for
the entire train's braking system. A feed valve in the locomotive
regulates the desired pressure that is supplied to the train.
This pressure must be at least 70 psi (although most modern systems
use 90 psi). A "brakepipe" runs the full length of the
train. The brakepipe carries the compressed air from the control
unit to the rest of the train. Unlike truck brakes (and passenger
train brakes for that matter) this single source of air carries
both the air that powers the brakes as well as the signal to control
them.
Details
of Brake Operation
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Each
rail car has its own brake system. The brake components
include a brake cylinder, brake shoes, a dual air reservoir,
and a control or AB valve. The AB valve is used to route
air from the reservoirs (auxiliary and emergency) to the
brake cylinder. The brake cylinders are connected through
rods, levers and slack adjusters to the brake shoes. While
these components are similar to truck brakes their operation
is very different. Unlike truck brakes, train brakes are
normally off, or unapplied. The return spring in the brake
cylinder is used to return the piston and pull the brake
shoe away from the wheel and allow the wheel to roll freely.
So, in order to apply the brakes, air must be ported from
the reservoir to the brake cylinder.
There
are several ways the engineer can apply braking to the train.
He selects the type of braking depending on the nature of
the stop desired.
SERVICE
BRAKES: This is the type of brake application normally
used for braking. This level of braking is achieved with
a 6psi to a 26psi reduction in the brake pipe pressure.
When the AB valve senses the difference in pressure air
is ported from the reservoir to the brake chamber. Air pressure
acts against the piston and brakes are applied. Braking
with the Service Brakes offers up to 75% of a train's Emergency
Brake capability.
INDEPENDENT
BRAKES: These are the brakes on the locomotive units
only and do not apply brakes on any of the cars. While this
brake method would effectively slow the locomotives if operated
alone, this type of braking has only a minimal effect on
a fully loaded train. These brakes are used in train handling,
standing or any time a small brake level is needed on a
train. They can give a braking level from none up to full
independent, which is 75% of the locomotive's Emergency
Brake capability.
POWER
BRAKING: This means just what it says. When an engineer
anticipates a problem may develop or desires to control
the speed of the train, an application of the service brakes
is made without reducing the throttle. When the train has
slowed or the problem does not arise, then the train brakes
are released and the train continues on, with the throttle
still set. This type of braking has the advantage of reducing
the time necessary to achieve Emergency Braking. This results
in a quicker stop than an Emergency stop that was not preceded
with Power Braking.
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Flexible
Hose Carries the
Brakepipe between Cars
A
Brake Cylinder and associated Hardware
A
Brake Shoe Removed from its Retaining Hardware and Held
Against the Braking Surface of a Wheel
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DYNAMIC
BRAKES: Dynamic braking is using the traction motors of the
units in a reverse flow so that they act to stop or slow the train.
This type of braking is used primarily for train handling as it
only slows the train via the locomotives. This type of braking
cannot compare to train brakes.
EMERGENCY
BRAKES: This is all the brake capability that a train has.
It is utilized, as implied, when there is an emergency. Application
of Emergency opens the brake pipe to atmosphere on all cars and
units sequentially from front to rear. As a result, the AB Valves
ports pressure from the Auxiliary and Emergency reservoirs to
the brake chamber and all brakes slow the train. This type of
brake use applies the brakes as fast as possible. An emergency
application will cancel throttle to idle (see Power Braking above).
Special
Considerations
Utilizing
air brakes on a vehicle that is more than 1 mile in length poses
some interesting problems. Under ideal circumstances the air signal
travels at about 921 fps. This occurs in Emergency, when the brakepipe
is vented to the atmosphere. Thus, if a train was 5,526' long,
it would take 6 seconds for the last car to sense the pressure
drop and begin to start applying the brakes. The Federal Railroad
Administration, under CFR Part 49, specifies the maximum time
each car can take to achieve maximum braking. On trains operating
at 70psi brakepipe pressure this maximum application time is 10
seconds. So, our hypothetical 5,526' freight train would take
16 seconds to attain full emergency braking. During this 16 seconds
the train will have steadily increasing brake application taking
effect between 1.5 (the time for braking in the first car to begin)
to 16 seconds (the time the last car achieve full braking).
A second counter-intuitive
situation exists with train brakes. Reconstructionists are accustomed
to ignoring vehicle weight when calculating stopping distances.
This assumption has validity for vehicles that skid to a stop.
Trains on the other hand are designed with a maximum brake force
that is below the force necessary to lock the wheels of an unloaded
train. The effect of this is that maximum braking force is the
same for loaded and unloaded trains and stopping distance is roughly
proportional to weight. Stated another way, a train weighing twice
as much will take about twice as far to stop. While this idea
may be counter intuitive, it of course makes perfect sense for
a vehicle with fixed maximum braking force.
Finally, train
acceleration rates are severely restricted when viewed from a
road vehicle's perspective. Cars and large trucks are capable
of stopping at nominal rates of .75g. This stopping force originates
at the tire-road contact area. It is limited by the friction coefficient
between these two surfaces, rubber and asphalt, rubber and concrete,
etc. All of these surfaces have dry surface friction coefficients
near .75. Just like road vehicles, trains gather their slowing
force from the wheel-track contact area. A typical friction coefficient
for steel on steel is .25. This value is near the value of rubber
on ice. So, it is not improper to view trains as perpetually driving
on a surface that is equivalent to ice. The actual slowing or
stopping is controlled by the wheel/shoe adhesion. A train can
stop up to 40% shorter in distance using wheel/shoe adhesion rather
than steel wheels on steel rails. The wheel/shoe adhesion drops
off rapidly as speed increases. The result is, when compared to
road vehicles, trains change their speed very slowly. Despite
the fact that a train reacts slowly, an engineer does have the
ability to make meaningful changes in speed that could result
in avoiding a collision. The following example will detail such
an analysis.
A Typical
Grade Crossing Analysis
An example
of a time distance analysis follows. In this case a train was
approaching a road crossing at 29 M/H. The sight distance available
to the engineer was 484'. The question posed is how much could
the engineer have delayed the train's arrival at the crossing
by placing the train in emergency? That analysis follows.
Total Available
Distance = 485'
Initial speed
= 29 M/H (42.5 ft/sec)
Estimated
reaction time = 1.5 Seconds
TRAIN
DATA
Number of
units (Locomotives)
= 2
Length of units (Locomotives)
= 136.66 feet
Weight of units (Locomotives)
= 350.5 tons
Number of cars
= 18
Length of cars
= 902 feet
Gross weight of cars
= 980 tons
Empty weight of cars
= 540 tons
Train length
= 1038 feet
Gross weight of train
= 1330 tons
Empty weight of train
= 890 tons
Brakepipe pressure
= 90 psi
Emergency propagation
= 1.128 sec.
Emergency braking efficiency
= 0.736
Grade
= -.00173
First lets
determine the Engineer's reaction distance using an average reaction
time of 1.5 seconds.
Reaction distance
= (Reaction Time) x (Velocity)
Reaction distance
= (1.5 seconds) x (42.5 ft/second) = 64 feet.
Subtracting
this reaction distance from the total distance of 485' leaves
421' feet for the train to slow.
Next let's
determine how long it would take the train to arrive at the crossing
if the engineer did not act.
Distance/Rate
= Time
(421 feet)
/ (42.5 ft/second) = 9.89 seconds
Next, we must
calculate the actual slowing for the train. This is not a simple
calculation. Recall we must account for the time for the air to
propagate the length of the train. Next, the actuation time of
the brakes for each car must be considered. We must also determine
the weight of the train then compare it to brake force. The results
of these calculations will be presented in the table below without
support.
Initial Speed
(mph): 29
Stopping Distance (feet): 712.1
Time to Stop (sec): 25.89
The first
thing that is apparent is that the available stopping distance
of 421' is well less than the 712.1' feet required for the train
to stop. The conclusion: The train can't stop before it gets to
the crossing. But perhaps more interesting is the comparison of
the time it takes the train to reach the crossing with and without
braking. Again, this involves detailed calculations beyond the
scope of this treatment. The result will be presented for purposes
of comparison. Given the 421' brake distance the train arrives
at the crossing at 24.14 M/H. The time it arrives is 10.46 seconds
after the point the brakes were first applied. This time should
be compared to the time required to reach the crossing if no action
was taken. That time calculated above was 9.89 seconds. The difference
is .57 seconds. This difference in time is not much, but perhaps
sufficient for a car to clear the crossing.
Conclusion
When compared
to other modes of ground transportation trains have some unique
characteristics that require special analytical consideration.
The length of a train and its associated pneumatic brake systems,
determining the train weight and calculating brake force are all
variables that appear in stopping distance calculations. While
running steel wheels on steel tracks greatly increases a train's
load-carrying capability, these materials limit the ground forces
available so that speed changes in trains occur relatively slowly.
These problems
notwithstanding, this truly massive vehicle travels thousands
of miles daily with infrequent incident.
John
Bentley is a former Texas Highway Patrol expert in accident reconstruction.
In 1964, he entered into a private consulting practice. This practice
steadily evolved toward train specific accidents and is now limited
to train performance. He has participated in train testing, developing
parameters, training and is a frequent lecturer on the topic.
Mr. Bentley
can be contacted at jb@train-dynamics.com
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