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Vehicle duty cycles,
type of operation,
drivers, inspection
practices and weather
all play a part in
establishing good brake
maintenance procedures.
Brake maintenance,
particularly maintenance
intervals for heavy duty
trucks, depends more
than anything else on
the vehicle duty cycle.
There is a world of
difference between a
refuse truck that
experiences 200-300
heavy brake applications
in a shift and an
over-the-road truck that
may have two
applications a day - one
for lunch and the other
at the end of the day.
Oddly enough, the
easy-duty application
may dish up more
problems than severe
service, because very
little heat is generated
in the brakes. OTR
trucks might see 125-150
degrees in the brakes on
average, where a refuse
truck may well have
brakes running 450-500
degrees. These severe
applications result in
shoe lining wearing out
in as little as two
months, with drum
replacements maybe every
three times the shoes
are exchanged. But at
least it's predictable
and service can be
scheduled. (Much better
is a hydraulic hybrid
system that captures the
energy when stopping,
instead of throwing it
away as heat. But that's
another story.) Related:
Are Your Brakes Up to
Snuff? As far as
over-the-road trucks are
concerned, brake service
is usually done when
mechanics routinely
lubricating the chassis
peer through the access
holes in the dust
shields - when they are
fitted - to discover the
linings are worn thin.
For a conventional S-cam
brake this might be
around the
250,000-300,000 mile
mark - although good
drivers can make a set
of linings go out to
double this mileage. But
this is a rather
hit-and-miss inspection
process, says John
Hawker, the primary
field engineer for
Bendix Spicer Foundation
Brake. Far better, he
says, is to use a wear
gauge provided by the
brake manufacturer. The
correct procedure is to
look at the date the
truck is placed in
service and the mileage
at which 50 percent of
the brake block has been
worn away as indicated
by the gauge. So when a
7/8-inch block show
7/16-inch remaining with
the gauge, the mileage
to full lining wearing
out can be estimated
fairly accurately.
Hawker uses the example
of a 50 percent
indication at 300,000
miles, at which point he
strongly suggests
pulling drums at 500,000
to 550,000 miles to see
what is going on inside
the drum and how much
lining is left on the
shoes. Depending on the
truck's suspension,
peering or access
through the inspection
holes may not be
possible. Such
circumstances argue for
spec'ing brakes without
dust shields, and
according to
ArvinMeritor's Joe Kay,
engineering manager for
Brake Systems, something
like half the fleets opt
to delete the shields.
He says Canadian fleets
like them because more
sand is used up there
during the winters, and
dust shields keep some
of the abrasives out of
the drums. Bendix's
Hawker thinks dust
shields are a good idea,
because they can cut
down on other issues.
But, he says, mechanics
must be dissuaded from
levering them aside to
get in to the brakes to
assess wear. Corrosion
Issues For U.S.-based
fleets, a different
winter issue can spell
big problems for brakes:
winter chemicals used to
de-ice the highways.
Rust jacking of linings
surfaced as a problem
around five years ago
after some states
adopted calcium and
magnesium chloride as a
de-icer in preference to
the old road salt
(sodium chloride). Years
of experience and steps
to minimize salt spray
corrosion have been
negated by the adoption
of magnesium chloride.
The new chemical is
virtually impossible to
wash away and is
hygroscopic - that is,
attracts water from
vapor in the air. It
also shows a remarkable
ability in solution to
wick its way into wiring
and the tight space
between the brake shoe
table and the lining.
Once in there, the
chemical pulls in water
and sits as a corrosive
chemical, attacking the
shoe table if it is not
fully protected. As rust
flakes form, they jack
the lining off the
table, leading to a host
of problems. Regularly
hosing off the underside
of the truck during and
after winter is a good
preventive maintenance
practice, helping to
prevent some of the
issues with wiring, but
cleaning out the brakes
is especially difficult,
if not impossible. Some
suggestions include
pulling drums and using
a brush with a soap
solution, but most
fleets wouldn't welcome
the downtime. And
pulling drums can
exacerbate wheel seal
problems if technicians
are not careful.
According to Hawker,
original equipment shoes
shipped with the tractor
tend to be pretty good
and go to the warranty
point without problems.
The issue is more with
trailer brakes and with
relined shoes after
brake service. The major
line of defense to avoid
the rust jacking problem
is to buy relined shoes
from quality sources
that not only supply the
correct lining material
to OE spec - important -
but also clean and then
apply an anti-rust
barrier to the shoe
table before linings are
attached. Trailer brakes
are a different problem,
with most being purely a
low-cost commodity.
There, voids between the
shoe table and the
lining allow for capture
of the corrosion. The
age of the linings also
works against the
longevity of the shoes
in a corrosive
environment. Curiously,
another problem is
surfacing, especially on
trailers that are parked
for a long time after
being exposed to winter
chemicals. Corrosion of
the drum's friction
surface extends into the
lining to freeze the
brakes. According to
Kay, if the problem is
severe enough, the
trailer can drag the
tires, especially if the
trailer is unloaded,
which is likely when
it's sitting for a long
time. Corrosion can also
affect other brake
parts, resulting in
failures. Spring brake
chambers are a case in
point. Considerable care
must be exercised when
checking out and
servicing these
components. The tightly
compressed spring within
can do considerable harm
if it is accidentally
released. Unfortunately,
brake jobs are usually
performed by technicians
new to the job, leaving
experienced mechanics to
do the higher-level
troubleshooting and
repair of other truck
systems. Supervision is
definitely needed
whenever any spring
brake repair is called
for. Other components at
the wheel end that can
cause problems, often
through the effects of
corrosion, are the slack
adjusters, auto slack
adjusters and S-cam
bushings. Automatic
Slack Adjusters The most
immediate indicator of a
brake problem is a brake
stroke out of range.
Automatic slack
adjusters have been
required for trucks and
tractors since 1994 and
trailers since 1995. For
tractors, it is likely
that between 93 percent
and 98 percent will have
auto slacks. For
trailers, this
percentage could be
considerably less,
because the regulation
went into effect later
and trailers stay in
fleets a lot longer.
However, trailers that
have seen brake work may
have been retrofitted
with ASAs, since using
them results in far
fewer DOT out-of-service
conditions. A Canadian
study showed vehicles
without ASAs are twice
as likely to be put out
of service for brake
stroke outside of
mandated limits. There
are two things really to
be understood about
ASAs. They work, and
work well - until they
stop working. And when
they stop working, they
must never be manually
adjusted, because the
ASA is telling you
there's likely something
else wrong with the
brake. Several
fatalities have occurred
because of ASA manual
adjustment, and the
National Transportation
Safety Board has come
out with a very strongly
worded prohibition
against tampering with
the auto slacks. Drivers
have a duty to inspect
brakes for proper
adjustment during a
pre-trip inspection.
Drivers are qualified to
inspect brakes - Section
5 of the CDL Study
Guide, accepted by all
50 states, recommends
the procedure. However,
a driver is actually
forbidden from working
on truck brakes unless
he or she has the
training or experience
required under FMCSR
Part 396.25. So if a
driver finds a brake out
of adjustment, he has a
duty to report it and
get it fixed by a
qualified mechanic. (See
driver pre-trip check on
page 87). ArvinMeritor's
Kay and Bendix's Hawker
say that brake chamber
overstroke is the first
and most obvious sign
that something is wrong
at the wheel end. ASAs
out of their stroke
range are most often a
symptom, not the cause,
of a problem, says Kay.
"The only time an ASA should need adjustment is if some kind of brake work has been performed and the adjuster has been backed off manually. " Then the
correct procedure must
be followed. Hawker says
that an adjuster with a
clutch mechanism must be
backed off only as far
as it is necessary to
remove the drum. In
fact, he says, if the
brake drum is being
removed, the ASA should
also be removed,
cleaned, checked for
operation and lubricated
before reassembly on to
the brake assembly. In
terms of general
maintenance, during a
brake inspection, the
mechanic should place
the appropriate wrench
on the adjuster nut
while a buddy applies
the brakes. The wrench
should index as the
brake applies. If it
fails to move, there is
something wrong with the
adjuster and it must be
removed for service. If
the wrench swings, the
adjuster is working and
should be left alone.
Some fleets, he says,
put a wrench on the
adjuster and actually
manually adjust them up
whenever a truck comes
in for service. This is
absolutely wrong; the
technician is
overloading and slipping
the clutch, which will
ultimately damage the
ASA. Lubrication
Problems with S-cam
rollers or S-cam
bushings might cause
issues with ASAs
overstroking. S-cam
bushings are susceptible
to corrosion if seals
are defective or if an
inferior grease is used
in maintenance. Kay
points out that in the
ArvinMeritor long-life,
low-maintenance wheel
ends, high quality
synthetic grease is
used. He recommends
these type of lubricants
be used on camshafts.
Hawker says to use the
best lubricants
available on ASAs and to
keep pumping grease
until all old grease is
purged from the
component. The cheap
dollar-per-pound grease
will wash out quickly,
where a $6 per pound
synthetic will stay in
place and do its job.
This is not a place to
skimp on quality, they
say. Since the clevis
points are critical in
the process of detecting
stroke or clearance, the
clevis pins must always
be free in any ASA. For
that reason, they should
be treated to an
anti-binding lubricant
whenever the truck is in
for a chassis lube
service. Servicing the
Brake There can be no
better place to learn
all there is to reading
brake problems and the
correct repair
procedures than the
Technology and
Maintenance Council's
Recommended Practices on
braking. The S.6 Chassis
Study Group, which
includes both Kay and
Hawker among its
contributors, has
produced RP-604A through
RP-609A, RP-612B,
RP-617, RP-619A, RP-625,
RP-627 through RP-629
and RP-635. They cover
everything from system
description, through
brake shoe
reconditioning and
relining, to the
inspection and repair of
S-cam brakes. A
particularly useful RP
concerns the condition
of the drum and, where
appropriate, the rotor
of an air disc brake.
Detailed analysis with
pictures of various drum
and rotor conditions
help any brake
technician read the
problems and likely find
the causes for failures
of these parts. Other
topics cover brake
balance, brake and
lining compatibility,
failure analysis of
linings and things like
lining classifications
and even long-stroke
brake chambers. Discs
Coming? Changes to
braking regulation that
will require as much as
a 30 percent shorter
stopping distance for
truck tractors will see
new braking equipment on
the front axle of heavy
trucks. There has been
some discussion that the
additional braking load
that will be carried by
the front axle to
achieve the shorter
stopping distances will
finally see the
introduction of the long
awaited air disc brake,
with both ArvinMeritor
and Bendix poised with
proven disc-brake
products to capitalize
on demand. But both
companies also have
bigger S-cam brakes that
have the capacity to do
the job, so neither is
betting on a wholesale
shift to different types
of brakes because of the
regulation change.
ArvinMeritor's Joe Kay
says fleets could well
go from today's almost
standard 15 x 4 steer
axle brake to a 16-1/2 x
5 to gain the additional
brake torque to meet the
regulation. As such,
little retraining of
technicians would be
required to service the
new components. However,
fleets that have
experienced the air disc
brake have reported
significant benefits in
reduced maintenance from
the very long brake life
of the latest air disc
models. Also, when it
comes time to change out
pads, the process is
quick with no need to
disturb wheel bearings.
The latest generation
brakes also present few
problems with rotors or
interference with
existing wheels. Disc
steer-axle brakes are
now optional from many
truck OEMs. SOURCE:
https://www.truckinginfo.com/153150/brake-maintenance-1
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