BENDIX BW7532 User Manual

THE CONSEQUENCES OF USING LOW-QUALITY, LOW-COST

SPRING BRAKES: A LOOK AT THE CRITICAL ISSUES.

OEM Quality Spring Brakes Maintain OEM Parking Capability, Deliver Longer Service,

Reduced Downtime, and Reduced Operating Costs Compared to Low-Quality/Low-Cost Alternatives

In the last several years, the commercial vehicle marketplace has been flooded with low-quality, low-cost spring
brakes. With today’s emphasis on protecting the bottom line, cost conscious consumers may purchase these lesser
quality products without full awareness of their potential pitfalls in the areas of safety and performance. When
choosing replacement spring brakes, it’s important for fleet operators to know what to look for in terms of brake
design and components.

Responsible for both service and parking brake applications, spring brakes are a critical

Designing the spring
for durability is critical,
as a broken power
spring remains the
leading cause of spring
brake malfunction.
Any spring brake issue
means certain vehicle
downtime and
increased maintenance
costs.

Safety Standard (FMVSS) 121 parking requirements. However, no such standards exist for aftermarket products,
leaving the parking ability of low-cost aftermarket replacement spring brakes to chance.

To ensure that our spring brakes meet safety standards and deliver durability, reliability and safe operation, for
years Bendix has subjected these products to extensive ongoing research and benchmark testing in order to
compare elements related to such items as functional performance, corrosion resistance, and structural durability.

Among the battery of tests regularly undertaken, Bendix also conducts real world performance comparisons, of our
spring brakes versus low-quality spring brakes in a “hill hold” situation. While vehicle manufacturers traditionally
perform the FMVSS 121 park test using a drawbar, an alternative test for parking performance can also be
measured on a hill with a 20 percent grade. Using a fully-loaded single axle park tractor, a driver moves the vehicle
to the top of the grade and applies the parking brake to test how it holds on the hill. Bendix performed this test
using its OEM-specified Bendix® EverSure® Spring Brake with No Touch Technology. The brakes held fast, and the
truck did not move. Bendix then performed the test using a popular low-cost replacement spring brake. The results
were concerning, as the truck almost immediately lost its hold and started to roll back down the hill. Video of this
test can be found in the brakes and wheel-end section of the Bendix video channel on YouTube.com. To view the
footage, visit www.youtube.com/user/bendixvideos.

This test demonstrates how low-cost chambers can fail to maintain sufficient force output to hold a vehicle in place.
To avoid the uncertainty and safety risks that may be associated with low-cost, low-quality alternatives, fleets
should look to durable, long-lasting OEM products that will help keep their vehicles on the road.

component in a commercial vehicle’s air brake system. The service side is used to slow
or stop a vehicle, while the parking side holds a vehicle in place when parked.

The longevity and operability of the parking side of a spring brake rests on its ability to
maintain sufficient force, over time, to hold a vehicle in place. The most critical
component for providing this force is a large, powerful spring that, in normal operating
conditions, stays compressed while the vehicle is in motion, and is released once the
vehicle is parked. Designing this spring for durability is critical, as a broken power
spring remains the leading cause of spring brake malfunction. Any failure means
certain vehicle downtime and increased maintenance costs.

Fortunately, owners of new vehicles can rest assured that their vehicles will stay in
place, as OEMs must certify that their braking systems meet Federal Motor Vehicle

2

For new vehicles, OEMs must

The increased

Protecting the Spring

With broken power springs representing the leading cause of spring brake
malfunction, it only makes sense to protect the spring at all costs. This may be
easier said than done.

A typical T30/30 spring brake is open to the environment via holes found on the
park-side housing. These openings serve as entry points for contamination such as
dirt, water, and de-icing chemicals. These corrosive contaminants can threaten the
power spring. A damaged spring may cause a leak if the diaphragm gets
punctured, or it may affect the stroke of the pushrod. Regardless of the damage,
the failed unit will need to be replaced immediately.

Because of corrosion concerns, most spring brake suppliers coat the springs to
provide some level of protection. However, the durability of such coatings is
compromised in lower-cost designs, which tend to compress the spring such that
the coils come in contact with one another. This type of design results in “coil clash,” which can wear away the
protective coating. Once the coating is worn, the bare spring is susceptible to corrosion, which can lead to
breakage.

The best protection against premature spring coating wear is to avoid coil clash. If the coils don’t rub together, the
coating will last longer. That’s the solution Bendix arrived at for its Bendix® EverSure® Spring Brake with No Touch
Technology. The springs are designed to virtually eliminate coil clash, protecting the premium epoxy coating used
on all Bendix power springs. This results in a longer lasting product and translates into fewer repairs, less
downtime, and lower fleet operating costs.

Ensuring Parking Force Output

A secondary issue caused by over-compressing the spring is a rapid decrease in force output. This reduces any
safety margin when parking a vehicle.

When evaluating spring brakes, a force output test can be used to measure the amount of

height of the
compressed
spring in the
Bendix® EverSure®
Spring Brake
design reduces
stresses on the
spring, allowing it
to better maintain
force output over
time. This means
drivers can park
their vehicles with
the same level of
confidence for
every stop.

Protecting the Center Seal

In addition to the power spring, another important feature on a spring brake is the center seal. This seal separates
the service and parking chambers of the unit and serves two important roles:

1. Preventing leakage from one side of the unit to the other

2. Acting as a bearing that helps to guide the spring brake’s pushrod

A robust and smart design of the center seal components is therefore necessary to reduce any issues resulting
from damage or premature wear. Operating a spring brake places a large amount of force on the pushrod, some of
which may be lost due to friction. An improperly guided pushrod could significantly diminish the force output
delivered by the spring brake (due to high friction losses) and could also wear the seal or diaphragm prematurely,
typically resulting in a leak.

force the spring brake can produce throughout its stroke. This force output is measured
against SAE International’s recommendations. Having a high force output right out of the
box is important, but equally important is a spring’s ability to maintain high force output over
time, as all springs will degrade due to repeated compressions. This loss of force over time
is called sag loss.

Testing the force output of a spring brake out of the box and then after holding the spring in
the compressed position for specific periods of time will provide an indicator of how much
force output may be lost. Low-cost/low-quality spring brakes typically show low levels of
force out of the box, sometimes just meeting or even falling below SAE’s recommended
minimums. Even worse, these units show greatly diminished force output capability after
being compressed.

The increased height of the compressed spring in the Bendix® EverSure® Spring Brake
design reduces stresses on the spring, allowing it to better maintain force output over time.
This means drivers can park their vehicles with the same level of confidence for every stop.

certify that their braking
systems meet Federal Motor
Vehicle Safety Standard
(FMVSS) 121 parking
requirements.

No such standards exist for
aftermarket products, leaving
the parking force of low-quality
spring brakes to chance.