Here's a long explanation of how air brakes work.
I did not write it but I like it.
I do a lot of research (Messing around on line?) and I save and archive the interesting
stuff I find.
In The ShopThe Antique Truck Club of America, Inc.
In The Shop
Sometimes I think Iím my own worst enemy. In fact I know I am! After
seeing my last column in print I realized that I set myself up again. Air
brakes Ö what was I thinking of; Noooo.
First, air brakes are a tough subject. Either people are totally mystified
by them or the knowledge they have is somehow flawed. Second, do to
federal laws; we now have two types of basic air systems being used; pre
and post 121. Third, with all of the "voodoo changes" and "modifications"
done by the various owners of our iron; how does one try to get this all
sorted out? Fourth, my stack of brake information fills 14 binders and a
stack of paper 4 foot high. What do I use and what do I ignore??? This is
the hole I dug for myself. Smart! Real SmartÖ Quit laughing GregÖ
George Westinghouse invented air-operated brakes for railroad use in 1869.
The first air brake system for trucks came in 1919. George Lane developed
the Lane Air Brake System for logging trucks in the Northwest. It was a
major improvement over currently available systems. It used an accumulator
to store combustion gasses instead of a compressor. Throughout the 1920ís
Westinghouse Automotive Air Brake Co. developed a reciprocating compressor
and other air system control and safety components. Sold to Vincent Bendix
in 1930 the "BW" brand continued development of relay, foot, emergency,
relay governors, trailer air system and controls. Two more major events
happened in the 1930s to aid the development of air brakes. Timken-Detroit
Axle introduced the first tapered lining for heavy-duty brakes in 1935 and
in 1938 they introduced the "P" series or ĎS" cam type of brake actuation.
By the 1940ís, the basics of the modern air brake system were in place.
Many refinements and improvements have taken place over the years. Mainly
driven by state or federal laws, or in some instances by serious
accidents, such as one in 1955 that caused the ICC (Interstate Commerce
Commission) to mandate tractor protection valves on all combination
vehicles in interstate service. The first state or federal law came in
1933. The State of Rhode Island required air-braked vehicles to stop in 50
feet from a speed of 20 MPH. This law also required all vehicle have an
independent hand-operated emergency brake. This law required major
improvements in braking at the time. Throughout the 50s, 60s and early 70s
improvements, refinements and new components were introduced and
integrated into the brake system. In 1970 Ford motor Co. introduced the
first split braking system for air brakes with the introduction of the new
L Series truck line. This major innovation would be mandated in 1976 by
Federal Motor Vehicle Safety Standard 121 (FMVSS 121). Our iron now is
legal with early antilock and split brake systems. We have come a long
To truly understand air brakes you need to fully understand the term
"Naugahyde Factor". To quote from a C W McCall song " itís sorta like
steppin on a plum Earl". By now I am sure some veterans of the windshield
university school of driving know exactly where Iím going. The official
definition of the "Naugahyde Factor" is as follows. The amount of grip
generated on the seat upholstery by the drivers posterior in direct
proportion to the amount of loss of air brake effectiveness. To fully
understand the Naugahyde Factor you need to have three more miles of hill,
smoking brakes and the low air buzzer going off. There have been some of
us who have gotten out of the cab dragging the seat and floor mat with us.
A true Naugahyde rating of 1000. After that, nothing will faze youÖ
Iím going to use my usual format of fundamentals first, and then weíll
look at the troubleshooting as we go along. Iím going to try something
different for illustrations though. One of the pages in this column will
be a blank diagram you can copy and fill in as we go. Donít mess up your
book; copy it (the drawing) and make a few extra copies just in case. Hit
the kids (or grandkids) crayon box for Blue, Red, Green, Orange, and
Black. Personally I like to use Crayola colored pencils. In case the
crying gets too loud or you donít have leg-climbers around, hit the "big
box stores" (Wal-Mart) etc. I know this will help you understand how the
subsystems are put together, and figure out where the trouble is when it
ainít right. Yea I just said subsystems. Donít you just hate when I say
things like that? Coffee ready? ChCh chuuu.
Air Brakes Part 1
You just know I have some fundamentals that we have to deal with first.
Without these basic principles, the nightmare would be complete. All air
brake configurations, even the most complex start with the following.
The energy used in an air brake system is a volume of air under pressure.
The fact that it is under pressure is more important than volume. The
system operates by metering amounts of pressure from a reservoir to where
it is required. A dash gauge wonít tell you how much air is in the
reservoir or the weight, only how tightly it is packed in. A reservoir
weighing 30 lbs. empty will weigh just about the same with 125 lbs. of air
pressure inside. It will make no difference if different size reservoirs
are plumbed together at the same pressure. Volume of air only relates to
the amount of air supply you have in reserve.
A quick primer on reservoir and pressure will help. One hundred (100)
pounds of pressure simply means that one hundred (100) pounds of force is
exerted on every square inch of the confining surface. This force will
only change if the volume (size) of the reservoir or pressure changes. If
we have a constant pressure and volume and we compared a steel reservoir
to a balloon, we would find the pressure in the balloon would be lower.
Because it expands, the increase in surface area will reduce the force per
If I were to connect a reservoir to a brake chamber and install an on-off
inlet valve and an on-off exhaust valve (your foot valve), I would have
just created a simple air brake system. I could write 17 paragraphs to
describe the following action. Hereís the short version. When you step on
the foot valve, the exhaust valve is closed then the inlet valve is
opened. A volume of air now flows through the lines to a brake chamber and
begins to push on a diaphragm that rests on a plate connected to a push
rod. The air pressure now has more space to occupy thus reducing the
pressure, although it will be equalized through out this basic system. How
much strictly depends on reservoir size and the amount of room added by
the interior space of the lines, valves and chamber. More added space
equals a bigger pressure drop. By design truck reservoirs are considerably
larger than the total volume of all the lines, valves and chamber interior
areas. With this fact in mind I hope you can see how when the foot valve
is opened the actual change in system volume is very small so the
resulting pressure drop is negligible. If you hold the foot valve down
long enough the pressure will equalize all over the system.
At this point I better talk about the brake chamber and the pounds per
square inch of force generated here. If I take the total effective area of
a piston or diaphragm and multiply it by the applied air pressure, I will
get the output force generated. Truck diaphragms, are sized by a type
number. (i.e. 24, 15, 30) Doing the math gets these results. An
application pressure of 100 PSI applied to a diaphragm of 30 sq, in.
generates a force of 3000 pounds of force on chamber push rod. To get the
chamber to release, we must first close the inlet valve to prevent dumping
the reservoirs then opening the exhaust valve to release the air trapped
in the lines and chamber. As you will learn in future columns, the foot
application valve sequences the action of the inlet and exhaust valves for
us when the brake treadle is pushed. OkayÖ at least it is supposed to.
Another requirement of an air brake system is Pneumatic Balance. Unlike
hydraulic brakes, when the pedal is pushed things happen almost
instantaneously. Air brake seem to work that way but in fact they work
slower, tenths of a second slower. This happens because it takes time for
air to move and equalize. The chamber nearest the brake valve will
activate the quickest and the others will follow in relationship to the
distance from the application valve. In time all chambers will reach equal
pressure. The same thing happens when you release the brakes. The nearest
chamber releases first then the rest in order of distance. As you may well
imagine, this can and will cause a host of serious braking concerns such
as skidding, jack-knifing or bogie-hop.
Torque Balance is the system balance that the correct axle is applied
first, and the braking efforts are distributed equally side to side.
Normally the pressure is delivered to the front brake chambers directly
from the brake valve only. In order to have the rear brakes or trailer
brakes apply slightly ahead of the front axle. This is done in hundredths
of a second by relay valves, which are installed into the system. By tying
the air reservoirs directly to the relay valve and then operating it with
the foot valve the balance is achieved. I like thinking of relay valves as
a "drivers remote foot", or an air operated foot valve. This enables the
driver to stop the truck or tractor-trailer smoothly and safely.
These basic fundamentals are incorporated in every air brake system. The
engineers have to know the operational speeds, port sizes, tubing run
lengths, diaphragm sizes, and capacities of every component in the trucks
air brake system to achieve pneumatic and torque balance. To sum it up, we
An Air source
Correct Size and Number of Reservoirs
In the real world our iron gets "modified" during its working life and
then by us in our restoration efforts. Sometimes we have to modify air
brake systems because of obsolescence or weíre changing the basic function
of the chassis from as built. There is usually enough remaining of the
original system to keep us out of trouble. Surrrre DougÖ all the time.
Have you ever seen diagrams like these on a shop wall or in a service
manual? One is for Pre 121 systems; the other is for 121 systems. The
first things techs and most people say is that it doesnít look anything
like my irons air system. In fact it wonít match exactly, each
manufacturer has their own routing and components, but the basic layout is
as shown on these charts.
We will break these complex diagrams into the subsystems that really make
up the air brake system. One note of caution though, there is differences
between a straight truck and a tractor or truck tractor. They are minor
but important differences. Iíll get you to the point of understanding
these schematics or the ones in your service manuals. Scouts honorÖ
Pre 121 Tractor
In the upcoming columns Iíll get into the four subsystems. They are as
Air supply system
Air delivery system
Parking and emergency system
The first will require a whole column, as it causes the most confusion.
(that is where most money is spent ) and the last will cover whatís
different about a tractor or if you want to add a tow package to your
chassis. We will look at components, how the basic ones work, how they are
plumbed, and what happens when they oops. Youíll follow along on your own
Time to hit the trail. I have a ton of marker light signals to give out
for this column and the ones coming. Thanks to Knorr-Bremse (Bendix),
Arvin Meritor (Rockwell), Haldex North America (Haldex Midland Grey-Rock),
Ford Motor Co., General Motors Corp., Freightliner LLC, Volvo North
America (Volvo, Mack), International Trucks. Blink...Blink Blink.
Getting a question or a suggestion for an article to me is as easy as
Snail-mail Doug Rodgers
P.O. Box 306
405 Cedar Ave.
Richland, NJ 08350-0306
Keep Your Taillight LitÖDoug