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SECONDARY BRAKING SYSTEMS

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SECONDARY BRAKING SYSTEMS Powered By Docstoc
					SECONDARY BRAKING SYSTEMS: A
SECOND CHANCE
SECONDARY BRAKING SYSTEMS: A SECOND CHANCE

BY WILLIAM C. PETERS


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The members of Squad 4 were having a particularly busy
day, responding to alarms from one end of the city to the
other. With the unrelenting summer heat, drivers were
cruising around with windows closed and air conditioners on
high, seeking relief. The apparatus operator remarked to the
captain that it seemed as if no one was paying any attention
to their warning lights and siren. He had to continuously
accelerate and brake to maneuver around the complacent
drivers.

Since the squad`s response to a minor fire was canceled,
the alarm center dispatched it to a confirmed "working fire"
in another part of the city. The plume of smoke off in the
distance indicated squad members would be going to work!
As the operator threaded the heavy rig through traffic, he
began to notice that the brake`s reaction to his foot
pressure seemed different; he had to press harder to slow
the apparatus down. A quick check of the gauges showed a
steady 120 psi of air pressure, and he began to think that
perhaps it was fatigue he was experiencing.

The final approach to the fire scene involved descending a
long steep grade. About halfway down the hill, the operator
was "standing" on the brakes, and the apparatus still was
not stopping. He looked at the captain and said, "Hang on!"

***

SECONDARY BRAKING DEVICES

The upcoming (1996) edition of NFPA 1901, Pumper Fire
Apparatus, if approved by the organization, will recommend
a secondary braking device on apparatus weighing more
than 31,000 pounds and require it on units exceeding
36,000 pounds.

Secondary braking devices in the forms of transmission
retarders, driveline retarders, and engine brakes, when
activated by the driver, apply a retarding force to the
vehicle`s drive wheels without the use of friction. These
auxiliary braking devices increase safety as well as economy
when used properly.

The foundation or friction brakes on an apparatus were
designed to fulfill two main requirements--to maintain the
vehicle in a stationary position for indefinite periods of time
when the parking brake is applied and to bring the
apparatus to a standstill in the shortest possible distance
without loss of control such as is experienced in a panic stop
condition.

The braking duty cycle of fire apparatus on a response is
such that the vehicle is accelerated and braked in rapid
succession. The friction involved in braking causes heat to
rapidly build up in the brake block and drums. This excessive
heat can cause a major loss of efficiency in the braking
system by affecting the friction surface of the brakes and
expanding the drums. Some estimate that brakes that heat
to over 520 degrees lose 60 percent of their efficiency. This,
of course, increases stopping distance dramatically. On a
long downgrade, this results in brake fade. Disc brakes are
somewhat less prone to fading, as they dissipate heat better
than drum brakes.

Some secondary braking devices provide up to 85 percent of
the vehicle`s deceleration prior to the service brake
engagement. By keeping the friction brakes cool, they are
ready to respond to panic stop conditions, and stopping
distances are greatly reduced.

An additional benefit is the reduction of downtime for brake
maintenance. Many high-use emergency vehicles require
frequent brake lining changes due to premature wear. Using
auxiliary braking devices increases lining life by decreasing
brake temperatures. The savings on brake maintenance can
provide a substantial return on an investment in an engine
brake or retarder.

TYPES OF SECONDARY BRAKING SYSTEMS

Secondary braking devices basically fall into three
categories: the engine brake, electronic driveline retarder,
and automatic transmission retarder. Although each
operates differently, the result is the same--a reduction in
stopping distance.

All of the auxiliary braking devices interface and are
compatible with the antilock braking system (ABS). When
wheel lockup is detected, the ABS automatically disconnects
the auxiliary brake device and restores it when the situation
is under control. A word of caution: Each system has specific
operating instructions that apparatus operators should read
and understand. Some of these instructions are particularly
important on slippery or wet road surfaces.

Engine brake. The Jacobs engine brake, or "jake brake" as it
is commonly called, has been in service on most models of
commercial trucks and fire apparatus for many years. The
jake brake is a hydraulic engine attachment that converts
the diesel engine into a power-absorbing air compressor,
which in turn provides a retarding effect to the drive wheels.
The jake brake can be installed on two- and four-cycle diesel
engines.

To understand how the engine brake provides retarding
power, let`s compare each cycle of a four-cycle engine, with
and without an engine brake.

Stroke 1, intake. Both with and without the engine brake,
the intake valve opens and air is pulled into the cylinder.

Stroke 2, compression. Without the engine brake, air is
compressed, heat rises, fuel is injected, and combustion
occurs, resulting in a rise in pressure. With the engine
brake, as the air is compressed and the piston reaches near
the top of its travel, the engine brake opens the exhaust
valve and the compressed air is released through the
exhaust system. No combustion occurs, since the engine
brake operates only during a "no-fuel" situation.

Stroke 3, power. Without the engine brake, the high
pressure resulting from the firing of the air-fuel mixture
drives the piston downward, imparting power to the drive-
train. With the engine brake, no power is produced. The
energy required to return the piston to the bottom of the
cylinder is now derived from the momentum of the vehicle.
This two-step process--the elimination of the compressed air
in stroke 2 and the use of the vehicle momentum in stroke
3--develops the engine brake`s retarding capabilities.
Stroke 4, exhaust. With and without the engine brake, the
piston is in an upward motion pushing out air or exhaust.

The Jacobs engine brake is activated by an electrical dash-
mounted switch. On some applications, a multiposition
switch is used to provide variable retarding capabilities. In
most automatic transmission applications, when the
accelerator is released and a no-fuel condition is present,
the engine brake engages.

Electrical driveline retarders. The Telma electrical driveline
retarder operates on a completely different principle but
obtains the same results. Rather than using engine power, it
uses a powerful electromagnetic field to slow down and
retard the turning of the driveline. The unit is made up of a
circular stator that surrounds two rotors and, depending on
the application, is securely mounted to the vehicle chassis or
rear axle housing. Several electrical coils of alternating
polarity are located in the stator.

The vehicle driveshaft is cut, balanced, and attached to the
input side of the two rotors. When the retarder is activated,
a current flows through the stator coils and a magnetic field
is created. This field passes through the rotors, which are
spinning inside the stator. The field produces eddy currents
in the rotors, which oppose rotor rotation and slow the
driveline. The rotors have specially engineered vanes to
dissipate heat generated during braking; however,
understand that no friction surfaces are involved.

In most applications, the brake pedal controls the retarder.
The driver automatically uses the retarder whenever the
brake pedal is depressed. Retardation is applied in four
progressive levels, and all four are automatically activated in
sequence before the vehicle brakes make contact with the
drum. An automatic cutoff switch deactivates the retarder as
soon as the stop is completed.
The electrical retarder can use up to 200 amps of current in
the stopping process; however, this discharge is for a short
duration only. Another benefit of the electrical driveline
retarder is that it is basically maintenance-free.

Automatic transmission retarders. Integrated into the
vehicle`s automatic transmission, transmission retarders
have been shown to increase brake life by as much as three
times the normal service period, depending on the vehicle
and application.

Two types of transmission retarders are the input and output
(the latter is more prevalent). The input version operates at
the input section of the transmission, between the torque
converter housing and the main housing. It is particularly
effective in work cycles where downhill speed control is
required. An output retarder is mounted at the tail-shaft
(output shaft) of the transmission and may be modulated by
the vehicle operator.

Typical automatic transmission retarders are made up of a
combination of an oil-filled rotor/stator chamber and a clutch
pack. The rotor blades are attached to, and turned by, the
transmission input/output shaft. When the retarder is
activated, fluid enters the cavity and provides resistance to
the turning of the rotor blades. This effectively slows the
vehicle to a point where the service brakes are needed only
for final stopping.

Transmission retarders are operated by air pressure or
electrical control activated by the application of the brake
pedal, the release of the accelerator, or a separate control
lever, depending on the preferred installation. Some
purchasers choose to have a percentage of the retarder
activate when the accelerator is released and the remainder
when the brake pedal is depressed.
Additional transmission fluid-cooling equipment might be
necessary when an automatic transmission retarder is
specified.

Safely bringing heavy apparatus to a stop places a greater
burden on the brakes. A secondary brake system not only
will reduce costly brake maintenance but also can give you a
second chance when confronted with a panic stop situation.

				
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