An Air Brake Primer
An Air Brake Primer
- by Pete Snidal (C)2001 - revised Nov 2004
This little text file is an excerpt from an email I wrote outlining
the most basic "need-to-knows" about air brakes. it is not
intended to be a substitute for proper air brake training and
certification, but it may be better than nothing.
First, let me say that in all of Canada, and I'm sure in most if
not all of the States, you must have an air endorsement on your
driver's licence to be permitted to drive an AB equipped vehicle.
Although I'm hardly a fan of regulations, I almost have to agree
with this one. It'd be a very irresponsible thing to do to drive
one without having taken the time to master the details of Air
Brakes. Following is a brief discussion of the differences between the
two types of systems. This is not intended to make you an expert, but
rather to motivate the reader to seek the education - and certification
- to run an airbrake-equipped vehicle intelligently - and most important
- safely.
1. The Differences
The first and most major difference is the complete lack
of "feel" in the brake pedal, a feature we all become accustomed
to with the otherwise much inferior hydraulic variety.
Air - A Valve, Not A Pump
 |
The main and majorly important difference between air and the "normal"
hydraulic brakes with which all are familiar, is in the fact that the
air brake pedal is not a Pump, as with hydraulic brakes, but rather a
Valve, which is entirely different. Operation of the brake pedal -
correctly called the Treadle Valve - admits air to the brake chambers,
which applies the brakes. Ther linkage between the air chamber and the
brake shoes is external, to permit regular and frequent checking and
adjustment. |
With hydraulic brakes, if your linings are worn, and therefore getting too
far away from the drums (need adjustment), you get a "low" pedal, which
can be "pumped up" - you can actually correct the bad adjustment by pumping
more fluid into the system, which will move the brake shoes closer to the
drums. Not so with Air Brakes! - the pedal is simply a valve which allows
air flow to the brake chambers. Once the chambers are full, they've
travelled their full extent, and if the shoes are still too far from the
drums, there's NOTHNG that can be done from the cab! And there's no
difference in "feel," - whether the brake chambers need only a little bit
of air - tight brakes, or almost all they can stand - loose brakes.
One At A Time
As loose brakes wear just a little bit more, you get a situation
in which the chamber moves its maximum, but the brake on that
wheel still isn't engaged! This "feels" just the same to the driver -
depressing the pedal allows airflow to the brake chambers, but when the
chamber is full and stops moving, the brake for that wheel is not
applied. So now, you're stopping with one less brake than you realize -
in the light applications you use while driving normally, you might not
even notice the difference.
A Wear Spiral
But of course, the remaining brakes which are still working are taking
more load, and wearing faster than previously, thus running out of _their_
adjustment!
And of course, your stopping distance in a panic situation
has dropped by the power of one brake. Worse yet this situation will
continue for the next brake to run out of adjustment as well, and now
you're _two_ brakes short!
The Air Supply
Another problem which crops up as the brakes wear is that as more
movement is required on the part of the brake pots due to wear, more
actual air is needed for each application, meaning your compressor is
called upon to work harder. So we can agree, I'm sure, that keeping your
brakes properly adjusted is very necessary! Uncorrected, the wear
spiral will go on until your last brake's adjustment goes to nothing, and
then you have, not Hydraulic Brakes, not Air Brakes, but NO Brakes! - And
this entire scenario can easily play out on one good-sized hill!!
And remember - when this happens with ABs, all the "pumping" in the world
will do you no good; the chambers are at their full extension, and the
brakes are too loose to care. This is why you see those "Trucks Stop -
Check Brakes" signs at the tops of the big hills on the highways. This
is also the reason for the laws, which don't seem to have stopped the
articles in the news we still see regularly about trucks "losing their
brakes" on hills once in a while and wiping out carloads of civilians.
Happens here in BC a few times a year, even though the drivers have passed
the air brake test, which has come to be another government formality
which needs to be gotten around. (You can lead a horse to water, but
you can't make him think!)
What To Do?
Avoiding this ugly scenario is clearly the responsibility of everyone
who drives any airbrake vehicle - even across town or around the block.
You must at all times know for certain what the adjustment
status of your brakes is - as well as the state of your air supply.
Fortunately, the latter is simple enough to check - you will have a
reservoir gauge on your instrument panel. This gauge tells you
the pressure of your reservoir - it will fall just a bit each time you
apply the brakes, and rise as the compressor "kicks in" and
repressurizes to its pre-set limit. More on this later.
The adjustment of the brakes themselves can be checked in only one way -
getting out and getting under, and physically checking the slack in the
linkage between air chamber and brake linkage.
Danger!
In order to check the brakes, they must be off - ie, no pressure
to the brake chambers, the diaphragms being fully back. This means the
vehicle can move, possibly rolling over the brake checker, unless some
other means of limiting movement is used. Thus, even on level ground,
only the very foolish will fail to carry a pair of wheel chocks, which
will be in place before any rolling under the wheels is done. If you
have a reliable mechanical driveshaft brake, or an automatic with
Park position, or standard gearbox in gear with engine off, these will
also suffice. But you can't leave an air maxi-brake (spring brake) on
and adjust the rears - more on this later.
The Single-Pot Brake In The Off
Position.
Note that both the shaft clevis on
the shaft and the pushrod length
are adjustable. |
Tightening the worm adjusting nut rotates the clevis about the
cam shaft, taking out the slack. Note the two adjustments. |
The On Position. Note that the
brake clevis is
perpendicular to the push rod when the slack
is taken up. Note also that the pot is at the limit of its travel.
- Past Adjustment Time! |
As shown by the drawings above, (cadged from the BC Air Brake Manual -
with value added)
the diaphragm in the brake chamber has only a certain amount of
movement. There must be enough slack in the linkage to allow the brake
shoes not to contact the drums when the pressure is off, but there must
never be too much slack, which will allow the diaphragm to reach
the limit of its travel before contact is made. Note also that the
ideal angle between the pushrod and the brake shaft clevis is 90 degrees
at the point of shoe-drum contact. This gives the greatest mechanical
advantage.
Diaphragm distension is not limited by applied pressure - the movement
limit is reached with only a few psi; after that, increasing pressure
(by pedal movement) increases brake action.
Checking Adjustment
Once out and under, checking the actual adjustment of each brake pot -
there is one behind each wheel - is done by physically manipulating the
brake clevis from the no-pressure position to the limit of full
movement. This is most easily accomplished with a lever-type tool -
also an adjustment wrench, called a "Brake Buddy" (TM). But you can do
it in a number of other ways - the strong will do it with their bare
hands. Important thing is to ensure that you check the full
movement! If the pushrod travel exceeds 1/4-3/8", it's time to tighten
the worm nut and re-check, repeating the process until the
adjustment is correct.
How Often?
Legally, alll trucks must stop at every trucks-stop-check brakes area
and check slack. While walking around, you should also listen for any
hissing air - sign of a leak developing or developed. It may be argued
that Motor Homes and Converted Busses, being lighter, stress their
brakes less and can handle more extended periods between checks. What
is important and indisputable, however, is that you must check
often enough to be sure that none of your brakes is
ever too slack! Check them often, until you get a "feel" for how
much wear you're doing on the road. Obviously, cruising the prairie,
you'll likely find you can go for days without finding much wear/need
for adjustment, whereas a few hours in the mountains can wear them down
enough to need it. Use your head - and when in doubt, check!
"Two-Man" Adjustment Check
There is an easier way to check your adjustment - if you have a
co-driver, that is. That way is just to stop, chock the wheels or
otherwise prevent movement, and have the co-driver(ette) give a light
application of the brakes each time you position yourself to observe the
movement of the brake mechanism for each wheel. If the movement exceeds
the limit, then you have to get under there and tighten/re-check, but if
you're within limits, you can just carry on until the next check. There
are two advantages to this method: the first is, it's easier, but don't
miss the second: this ease should motivate you to stop and check them
more often - each time you stop for fuel or a snack, for example. It
takes little time and gives you a whole lot of peace of mind!
Pushrod Adjustment
Pushrod adjustment is required only when the final angle between clevis
and pushrod at the point of brake contact gets too acute. Fortunately,
this doesn't happen nearly as often as excess slack in the mechanism.
Adjustment of this parameter is usually taken care of in the shop. But
you'll want always to make observation of the angle at brake contact a
part of your checks.
Spring Brakes - Parking Brakes for Air-Equipped Vehicles
Since it would be ridiculous to expect an air
chamber to maintain pressure over long periods of parking, the
spring brake has just that - a spring applies the rear brakes,
which are therefore always on, unless the spring is "caged" by a
second additional air chamber, "piggy-backed" onto the one already
there for the regular service brakes. To de-apply the parking brakes,
the system must first be brought up to pressure (the compressor) and
then the spring chambers are charged with air by means of the
dash-mounted parking brake valve. This offsets the springs and
lets the vehicle move. (In most cases, there is also a mechanical
means to cage the springs in case of emergencies - you get out and
under, remove the dust cap, and screw the cage bolts up by hand.)
No-Pressure (Park) position -
spring is "uncaged," over-riding regular brake chamber and applying
full brake |
Normal Driving Position -
spring is caged by pressure to spring chamber, service brake chamber is
unpressurized - no braking action |
 |
Service Brake on during regular
driving - Spring brake is caged, but application pressure to service
chamber applies brake
(Danger Note! -
Spring brake chambers, obviously, have big, heavy springs in them.
Attempting to dismantle them without proper information and precautions
will earn you a Darwin Award!
|
This kind of parking brake has a major difference over the brakes we'
re all used to - if the system loses its pressure, the de-application
pressure is also lost, and the parking brakes will come on as the
pressure decreases - providing an "automatic" setting of the parking
brakes if system pressure is lost on the road. This is another reason
we must pay attention to system pressure - if it drops to the danger
level, you need to pull over in the first safe place, while you still
have enough air to maintain control over your brakes, or you just may
find yourself stopped in the middle of the freeway!
But Not Always...
That is, provided the deapplication pressure is provided from the
reservoir of the regular system. In some cases, the de-application
pressure is provided from a second special reservoir, supplied from the
first one through a one-way check valve, so that pressure loss in the
main system will not affect the pressure in the spring brake reservoir.
In this case, the spring brakes will only be applied when the dash-
mounted valve is operated to exhaust the air from the spring brake
chambers. This eliminates the "automatic" aspect of spring brake
application in the case of loss of pressure in the service brake
system, but allows greater operator control in that the second
reservoir can allow the operator to de-apply the brakes for such
things as small movements once the spring brakes
have brought the vehicle to a stop.
What About The Flx Clippers?
By the late '50's the last years of the Flx Clippers/Visicoaches, spring
brakes still weren't standard equipment - a driveshaft drum brake was the
parking brake. Many owners have upgraded their rear brake chambers
to spring brakes, installing the necessary lines and dashboard valve, thus
providing an extra margin of safety in the form of an excellent extra
brake for emergency situations involving loss of air, as well as a better
and easier-to-apply parking brake.
Your Flx may already have been converted. When you check your
slack on the rear brakes, have a look for double air chambers.
If they're there, then you'll have to find the application valve
on your dash somewhere to release the brakes.
More On The Air Supply
Obviously, you need at all times to have enough air on hand to make your
brakes work. This is stored in the brake reservoirs (two tanks,
and is "topped up" as required by the compressor, which cycles on and
off between two pressure limits. The compressor "kicks out" at the high
limit, and "kicks in" at the lower one.
Compressor volume is important. As a compressor gets old, its
ability to make air fast enough deteriorates. It HAS to make
air faster than you can be using it, in every situation, long
hills or whatever. The way you get a handle on this one is by
its build-up time when you first start the engine with empty
air tanks. The compressor needs to get your air pressure into
the green (over 80 psi, in the case of the Clippers) within 3
minutes, by law, (at least in BC). Mine does it in 90 seconds at
fast idle.
The air compressor must disconnect (it has a governor arrangement)
at the top pressure - 120 psi in my case. You can hear it with
a gas engine, with a Diesel you probably won't be able to, but
the gauge should stop climbing close to the 120 mark. It also needs to
start climbing again (the compressor governor cutting back in) when it
drops down to the low limit - around 80 psi in the case of the Flx.
The BC air brake manual calls for "fanning" the brakes to release
enough air to get the pressure down, and then watching for the governor
to kick back in (motor running, of course) when you do your "Pre-Trip
Inspection" every morning before starting out. I haven't met too
many truckers who do this every morning, but it's not a bad idea,
in the case of a rig you don't use every day, such as an RV. And
of course, on the road, after you've used your brakes enough to get the
pressure down to the 80 psi range, you certainly want to see that needle
start to climb again as the governor cuts back in. If it doesn't, you'll
need to use what little air you have left by the time you notice to
STOP IMMEDIATELY and find out why the air isn't building back up. Your
perception will be aided by:
Spare Belt!
Needless to say, if your compressor drive belt (if fitted - some
diesel compressors are gear-driven) goes south, your safe driving time
is measurable in minutes. You will notice, if you're observant, that
the gauge goes down past the "kick-on" pressure and keeps dropping. If
you've failed to notice that, there will be other indications (see
below) which hopefully will give you enough time to find a safe place to
pull over and investigate. And if it's a blown compressor belt, will
you have one on board? This is obviously as important as a spare water
pump/fan belt, and even moreso than an alternator belt. So you'll want
to be sure to have one of each of these with you when travelling.
