Most Army wheeled
vehicles are adapted from commercial models.
However, the Army is not keeping up with the best commercial
automotive
technology in one important area: the brakes.
The Army has a fleet of vehicles that numbers approximately
500,000. Combat vehicles operate on both unpaved roads and
paved highways. Normally, light trucks, heavy-haul trucks,
and high-mobility,
multipurpose wheeled vehicles use paved concrete or asphalt
roads for everyday duties. Nearly all of the
engines, transmissions, differentials, and brake systems used
in Army vehicles are commecially produced.
Why are so many commercial systems integrated into Army vehicles?
The reason is that Army vehicles are adaptations of commercially
marketed vehicles. Army light trucks are modified civilian
vehicles, heavy-haul dump trucks are modified commercial dump
trucks, and semi-tractors and -trailers are modified over-the-road
rigs—the same type we see moving freight down our Nation’s
highways. They have the same frame designs, the same driveline
designs, and the same brake systems. In fact, the number of
vehicle manufacturers is limited, and many use all of the same
components.
It would be neither efficient nor effective for the Army to
design, create, and test new engines or transmissions from
scratch, and the costs would be prohibitive. Minimizing risks
to personnel is a priority, and tests on vehicle equipment
previously conducted by industry provide data and engineering
information that can be used to reduce the danger of injuries
or death. System upgrades contribute to risk reduction, but
occasionally state-of-the-art advances in the private sector
are overlooked. The best example is the use of technologically
superior disc brakes over traditional drum brakes.
European
and Domestic Use
Most European heavy over-the-road trucks use disc brakes
on all axles, with over 1.5 million disc brakes produced
in Europe. U.S. commercial truck manufacturers entered the
disc brake market with front-axle discs, but they were not
promoted in the industry. (I worked on Mack Truck disc brakes
in the 1970s.) Four-wheel disc brakes have been available
for American-made light trucks for several years, and they
have demonstrated superior stopping efficiency. FWD/Seagrave,
Pierce Manufacturing, Spartan Motors, and Sutphen use air
discs on fire, emergency, and rescue vehicles. Motor Coach
Industries and Prevost Car order them installed on intercity
coaches.
Currently, the Army uses disc brakes only on M998-series
high-mobility, multipurpose wheeled vehicles, with four-wheel
hydraulic discs, and a limited number of commercial light
trucks adapted for military use, with front disc brakes.
Drum Brakes
Drum brakes operate using two steel shoes with a friction material
bonded or riveted onto the faces of the shoes. The two shoes
are mounted at the outer end of an axle on a backing plate.
When hydraulic or air pressure is applied to the shoes, they
expand inside the brake drum, which is attached to the wheel
and tire. The force of the shoes against the inside of the
drum produces friction and stops the rotation of the drum-wheel-tire
unit, thus stopping the vehicle.
This is a satisfactory system for some applications, such as
light-duty vehicles. However, the larger and heavier the vehicle,
the larger and heavier the drum brake unit must be to dissipate
the enormous heat generated in stopping a large vehicle. This
means a large vehicle needs large shoes, a
massive (150 pounds or more) cast-iron drum, heavier gauge
steel backing plates, and large supporting operation activators.
The shoes also must be enclosed in a drum, which greatly reduces
or eliminates the air circulation needed to remove heat buildup.
The drum sheds heat only from the surface area on its outer
circumference. When the shoes get wet, their ability to brake
weakens (a phenomenon known as “brake fade”) because
the drum cannot sling water out of the unit; the water is trapped
and held inside the drum by centrifugal force as the drum rotates
until it is evaporated by heat. Air drum brakes have approximately
20 internal moveable parts and another 20 external parts for
operation.
To maintain air drum brakes on a heavy rig, time must be allotted
to inspect material wear of the shoes caused by friction. The
maintainer must crawl under the vehicle to see the wear. If
all 40 parts are mechanically acceptable, the brake is adjusted
using one wrench while the wheel is raised off the ground.
If the problem diagnosed requires that the brake be torn down,
the wheel must be raised; if it is a dual wheel, a wheel jack
must be used to remove the duals and drum—a 600-pound
package—before the tear-down can be started.
|
|
| The M998-series
high-mobility, multipurpose wheeled vehicle is one
of the few Army vehicles equipped with disc brakes. |
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Disc Brakes
Disc brakes were developed in England in the 1890s and patented in 1902. They
were extensively used on fighter planes during World War II and were adapted
to automobiles in the 1940s and 1950s. By 1970, front disc brakes were installed
on most American cars. In the late 1970s, light American trucks and a limited
number of heavy trucks used front axle discs. Today, use of disc brakes has become
common in the U.S.
automotive industry.
Heavy truck air disc brakes have many advantages over drum brakes. Disc brakes
operate with a large cast-iron rotor attached to the vehicle spindle or axle
that rotates. A steel wheel and tire are bolted to the rotor. A stationary caliper
is mounted to the axle housing, with two steel pads made of friction material
bonded or riveted to one side of the pads. The pads, operated by an internal
cylinder, float outward when air or hydraulic pressure is applied. This forces
the pads against the rotor, stopping the vehicle. Disc rotors are not enclosed,
so they sling water when wet and thus dry quickly with minimum brake fade. They
do not pull the vehicle to one side, as drums can do; all stopping is in a straight
line.
Disc brakes dissipate heat quickly because the rotors and pads are open and the
rotors have large internal ventilation air passages. Because discs run cooler
than drums, less heat is transferred to tires, which increases tire longevity.
Disc brakes are 30 to 40 percent lighter in weight than drum brakes, which also
reduces tire wear and, with less rebound over potholes and rough roads, keeps
tires in contact with the road more effectively. Consistent tire-to-road contact
produces consistent steering efficiency.
Disc brakes have approximately one-fourth the number of parts of drum brakes.
They require inspection, but not adjusting, and that inspection can be performed
without lifting the axle. Discs last four times longer than drums, making them
the more cost-effective alternative. The most important vehicle functions are
braking and steering. All-wheel disc brakes immediately improve both. They decrease
stopping distances by up to 50 percent and, with less brake weight, reduce the
unsprung weight of axles and wheel bounce, thereby maximizing tire-to-road contact
and increasing steering control. [“Unsprung weight” is the weight
of all components of a vehicle that are not supported by the vehicle’s
springs. These components include wheels and tires. The lower the weight of these
components, the better they can handle bumps and potholes in the road.]
I believe that future tactical military vehicles should be ordered with all-axle
disc brakes. Whether current brake systems are hydraulic- or air-operated, a
retrofit developmental program could be initiated to fit tactical vehicles and
trailers with all-axle disc brakes. Several major brake manufacturing corporations
are able to provide the necessary expertise and components to reach this objective.
Requiring disc brakes on Army vehicles would reduce the long-term logistics footprint
by helping the Army to reduce brake maintenance time, extend brake replacement
intervals, increase tire life, and consume fewer brake replacement parts. If
a single vehicle component can provide all these benefits, I believe that serious
consideration must be given to including that system on all Army vehicles.
ALOG
Louis J. Gorenc is an equipment specialist with the Combat Vehicle Evaluation
Team at the Integrated Logistics Support Center, Tank-automotive and Armaments
Life Cycle Management Command, at Warren, Michigan. He has a bachelor’s
degree in criminal justice administration from Concordia College in Michigan
and worked as a heavy equipment mechanic for 25 years.
