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Can
Truck Brakes lead to Accidents?
Most automobiles utilize hydraulic brakes. When you step on
the brake pedal, hydraulic fluid pressure is generated in a
master cylinder that is transmitted to the actuators, i.e. the
wheel cylinders and caliper pistons throught the brake lines
thus applying the brakes. Big rigs do not use hydraulic brakes,
they utilize air brakes.
Air brakes use high pressure air which is supplied by an
engine driven air compressor. This air is stored in tanks
on the tractor. When the brakes are applied, the air comes
from the tanks at about 100 psi and is forced into an air
chamber in the brake drum. A push rod moves out turning a
slack adjuster which in turn rotates an "S" cam
which forces the brake shoes into the drum.
Most people assume that because trucks are big, they must
have the best brakes. What they fail to realize is that large
trucks cannot stop nearly as quickly as a passenger vehicle.
A car driving at 55 miles per hour can stop in about 225 feet
with its hydraulic brakes. However, a truck traveling at the
same 55 mile per hour speed will take more than 400 feet to
stop.
This is because the pneumatic and mechanical lag between
the time when a driver applies his brakes until the time the
air travels through a 40 foot hose to reach a valve can be
more than two seconds. The stopping time of a truck may be
further delayed if it is fully loaded.
Because most air braking systems do not automatically adjust
for wear, they must be well maintained and inspected by the
operator. If not, the performance of the air braking system
can rapidly deterioriate. In fact, truck inspections demonstrate
that a large percentage of trucks have some form of defect
in their braking systems. These defects are usually the result
of air leaks or poor adjustments.
Air brakes should be adjusted in a garage with the wheels
off the ground. However, drivers are trained in making adjustments
on the road by observing the adjusting arm with air brake
pressure.
When air brakes are applied it generates heat. A full stop
from 60 miles per hour might raise the drum temperatures to
600 degrees farenheit. If the brakes are not properly adjusted
these temperatures could exceed 1000 degrees farenheit thereby
causing a potentially dangerous situation.
When brake lininings get hot the friction provided by the
linings decreases. Since the lininings do not offer the same
resistance to the rotating drums they become slick. Moreover,
as the drum heats up it expands and moves away from the brake
shoes. In air brakes, the distance that the shoes can move
is limited. Thus, if the brakes are improperly adjusted, when
they get hot it is possible that the shoes will not make good
contact with the drums. This can be disasterous.
Another potential problem with big rigs is downhill braking.
Some drivers fail to appreciate the severe demands put on
the brakes by long downhill runs. Negotiating a 6% downgrade
in elevation for 6 miles is the equivalent of trying to stop
the truck from a speed of 238 miles per hour, or 16 stops
from 60 miles per hour.
Today there is almost universal agreement that the appropriate
braking technique to be utilized by a driver is to apply the
brakes intermittently or "snub" them as opposed
to making a continuous application to the brakes while proceeding
downhill. |
