The average residential refuse collection truck may make up to 5,000 stops in a typical week of service. Picking the right type of braking and retarder systems for the vehicle could mean the difference between stopping on a dime and costing you millions in a lawsuit.

By Lynn Merrill

A brake system on a collection truck gets an incredible workout each week under some of the most severe environments and circumstances. The truck has to maintain normal braking functions in traffic conditions ranging from the freeway to heavy, urban traffic to the repetitive stop-and-go of the collection route. With each stop the weight of the truck increases until it reaches its payload—with the expectation that the braking characteristics of the truck will remain fairly consistent from empty to full. Add the need to make those unexpected panic stops for the motorist who suddenly cuts in front of the truck or the child on a tricycle who darts unseen from between two parked cars to see the "big truck," and you've got to have a system that is fail-safe.

In a nutshell, braking and retarder systems work on the concept of offering selective resistance to a moving component, either on a turning wheel or rotating shaft in a frictional force equal and opposite to the movement until the object stops. As a result of the braking function, the energy contained in the moving object is turned into heat that subsequently dissipates into the atmosphere. Braking systems are primarily mechanical in nature, while retarder systems operate either through resistance of fluid or electromagnetic forces. Retarders are used to supplement the main braking system by reducing the amount of effort that the primary braking system must assert to bring a vehicle to a stop.

A Primer on Braking Systems

On a refuse-collection truck, the braking system consists of two sub-systems: the service brake, which is designed to operate through the foot pedal and be applied during the normal operation of the vehicle; and the parking brake, which is a mechanical system designed to hold the brakes whenever the vehicle is parked and the motor is turned off. In situations where the driver must repeatedly exit the vehicle while placing the vehicle in a parked situation, a work-brake system, which combines the features of both the service brake and the parking brake, may be used without generating significant wear on the spring-applied parking brake.

Braking systems use either hydraulic fluid or air to operate. While both systems have appropriate applications on various vehicles, the primary braking systems on refuse-collection vehicles are air-operated. This requires a compressor for generating air at the proper pressure and a storage tank to hold the compressed air. The air is then treated to remove moisture, which can reduce the effectiveness of the braking systems. The braking system is often divided into circuits that are designed to provide a redundancy to the system. This redundancy is meant to allow the vehicle to retain greater than 50% of its braking function in the event of a circuit failure. Each circuit has an appropriate valve system that distributes air to the foundation brakes on each wheel assembly.

Air-brake systems are the dominant platform because of their reliability and lower cost of maintenance. "An air-brake system is a forgiving system," states Ron Bailey, technical sales manager for Bendix Spicer Foundation Brake LLC of Elyria, OH. "You can operate an air-brake system with some leakage in the system and you do not lose the brake system. I mean, both air and hydraulic systems have redundancy, in that if you lose part of the system, the other half of the system works. However, an air system can work with minor leakage in the system and still operate, and you can't really do that with a hydraulic. You get a leak in a hydraulic system and you've lost part of the system."

Brake components for collection trucks have to be rugged to stop the heavy vehicles on their daily rounds. Clockwise from upper left are an air disc brake; a valve actuator, which uses air from the parking brakes to keep the foot pedal applied during inspections; and a compressor, which is used as a source of energy to charge the air for the air-brake system. At right, a technician works on an air disc brake.

Foundation brakes can consist of either disc or drum brakes. The majority of braking systems on refuse-collection vehicles are S-cam drum brakes, which consist of a brake chamber and slack adjuster turning an S-shaped cam that applies the shoes against the drum to create the braking force. Disc brakes use a caliper to apply pressure through the pads to a rotor. Drum brakes push outward against the drum surface that's moving parallel with the road surface, while disc brakes apply pressure to a disc rotating perpendicular to the road surface.

According to Bailey, there are some distinctive differences between the two systems. "Less than 2% of the heavy vehicles in the States are air disc brakes," he says. "I think the prime reason behind that is the economics. Disc brakes are generally more expensive up-front, but we believe there are life-cycle payments that will reduce that, and make it a more reasonable opportunity for air disc brakes through longer lining life and easier maintenance. In comparison, drum brakes have a couple of features that disc brakes overcome. Number one is stability. There can be as much as 30% variation from right to left [in drum brakes]. Disc brakes inherently have much more stability—probably less than 10% variation from brake to brake. The other thing with disc brakes over drum brakes is what we call brake fade, which would be the loss in brake effectiveness. With increased temperature, drum brakes will certainly have efficiency losses as the brakes get hot. You can have as much as 30% to 40% fade in a drum brake from a cold situation to a hot situation. Disc brakes probably get less than 10% variation from cold to hot. Refuse is considered a heavy-duty hotbrake operation. We've been seeing up to 60 stops per mile on refuse. The drum brakes get hot, they fade, and you start losing break effectiveness and get increased lining wear with increased temperatures. Disc brakes are much less sensitive to those conditions and can offer improvement in lining wear and a much quicker service time when you do need to replace the pads."

Determining which type of braking system (disc or drum) is appropriate for a specific application depends on a variety of factors, says John Hall, vice president of product engineering at Webb Wheel Products in Cullman, AL. Some of these factors are duty cycle, driver habits, type of cargo, weight, and cost.

Today, the cost of an air disc brake is substantially higher than that of a drum brake. The major reason for this is that the components of a drum brake are manufactured in much greater volume than the components of an air disc brake. The cost of components is extremely sensitive to volume. As the volume of components for the air disc brake increases, the difference between the cost of an air disc brake and that of a drum brake will decrease.

Improving Brake Drum Life

The life of replacement brake drums may be increased by not following the common practice of "trueing up" a new brake drum before putting it into service. Today most drum manufacturers machine the drum in one chucking. This allows the runout between the braking-surface diameter and the pilot diameter to be minimized. The practice of further machining a brake drum reduces the wear life of a new drum by increasing its braking-surface diameter. Also, this practice usually increases the runout of the braking-surface diameter in relation to the pilot diameter, which can contribute to the formation of heat checks on the braking surface. This can ultimately reduce drum life.

The use of vented drums along with brake turbines may reduce the operating temperature of brake drums in severe service. Lower operating temperatures correlate to increased drum life.

Adding a work brake to the system allows the braking system to be used more efficiently, says Chuck Eberling, senior staff engineer with Bendix Commercial Vehicle Systems. "One of the biggest enemies of a braking system on a refuse vehicle is the application of the park brake on house-to-house routine stops," he observes. "Through studies with the industry, we have learned that it is literally impossible for the charging systems that are typically used on these vehicles to keep up with an 800- to 1,200-stop route where the driver actually applies the park brakes at every stop. It's very important that he have what is known as the work brake that will actually employ the service brake. In doing so, this will reduce the air consumption by three times and therefore afford the opportunity for the charging system to keep up with the demand. If the charging system can't keep up with demand, it's going to saturate the whole system and have problems down the road with internal corrosion and freeze-ups and that sort of thing."

Retarders Aid Braking

Anyone who has stood along a downhill freeway off-ramp as a heavy truck slows has heard the familiar grumbling whine of an engine-retarder system. Coupled with driveline and transmission retarders, they are designed to assist the existing braking system by converting the energy generated by the vehicle's drive train into a braking system.

When a collection truck's throttle is depressed the engine accelerates as fuel is pushed through the cylinders and ignited, creating work. When the throttle is released, the engine coasts. An engine retarder captures the exhaust from the engine and compresses it so that the engine must work harder to push the gas out of the cylinders, and it therefore retards and slows the engine through resistance. This causes the vehicle to slow, requiring less braking force on the service brakes to bring the vehicle to a halt.

Transmission retarders perform a similar function in that they slow the rotational forces of the driveline by creating resistance within the transmission. Steve Spurlin, chief engineer for Allison Transmissions in Indianapolis, IN, describes how the system operates: "The retarder consists of a vaned rotating element called a rotor. On either side of the rotor, there are vaned elements that do not rotate called stators. The cavity between the rotor and stators is pressurized with oil. The rotor is splined to the output shaft of the transmission. The pressurized oil working between the rotor and the stator slows down the vehicle when the operator asks for retardation. The retarder pumps oil up to about 60 gallons a minute and it draws oil from the normal transmission oil supply."

Driveline retarders are installed between the transmission and the rear end of the truck, and use electromagnetic energy to counter the energy in the rotating driveline, similar to an electric motor. "In the refuse market, it is primarily purchased for brake-life extension," says Joe Gawlik, regional sales manager for Telma Inc. of Elk Grove Village, IL, a subsidiary of the Valeo Group. "Given the amount of brake jobs that are performed in the refuse market, it'll save on brake life and the associated brake expenses as well as the tire life. [The system consists of] two rotors that are attached to the driveline that rotate at the same speed as the driveline. The stator is bolted to the frame rail and that's stationary. As the retarder is activated, there's an electromagnetic field created using eddy currents that slow down the rotation of those rotors, which means you're slowing down the truck without using your brakes." According to John Gillespie, general manager for Telma, "No other auxiliary braking technology is as well-suited to the refuse industry as electromagnetic. We've seen as much as a five-fold increase in brake life and a reduction in heat-related tire failures using this technology."

According to Spurlin, there are two reasons to have any type of retarder system on a collection vehicle. "One of those is speed control on grades, which isn't really what a refuse-collection vehicle would want it for," he says. "The second purpose is to provide additional braking to the vehicle that could, in fact, help save the normal service brakes so to extend brake life. The most prevalent benefit is in a high stop/start vocation like a refuse truck. In normal residential refuse collection, whether it's a sideloader, rearloader, or even a frontloader, when they're doing the low-speed work you're not going to get much benefit from any type of a retarder because you're running at low speeds. The braking energy that you're putting into the service brakes is very small because the speed is low. But every one of those trucks goes out of the neighborhood. They go in normal traffic and, in most cases, they go out on a highway several times a day at high speeds to landfills or transfer stations. They would get the benefit of the retarder in terms of brake savings because they are at higher speeds now and there is more braking energy going into service brakes. If the retarder can absorb that energy versus the service brakes, then that's how you're going to extend brake life."

Activation of retarder systems can be set up based on the needs of the operating system. "The drivers and the companies have a choice with how they want the system set up," says Gawlik. "The majority of the units that go out in the refuse market are with foot control, which is as soon as the driver hits the brake, the retarder is activated in four different stages with 25% for each stage. It is activated off of an air-pressure switch and it's activated at 3, 5, 7, and 10 pounds of air in the braking lines. That's the most popular option in the refuse market. However, there are other options such as off-throttle control. A few refuse companies choose that option where approximately 25% or 50% of the power is activated as soon as the accelerator is released, with the additional stages being activated off the brake pedal. As soon as you let off the accelerator, boom, you've got 50% of the retarder working immediately."

Future Systems

Currently, all braking systems must meet or exceed the National Highway Traffic Safety Administration's (NHTSA) requirements to be able to bring a Class 8 vehicle to a stop in 355 feet from 60 miles per hour. The NHTSA currently has a nine-step program to reduce the fatalities involved with heavy-duty vehicles by 50% by the year 2010. "In one of the major efforts, they are planning a reduction in that stopping-distance requirement from that current 355 down to 248 feet," Bailey says. "Drum brakes may be able to get to that level, but that's where this effectiveness factor really comes into play when you start putting much larger drum brakes on front axles. Disc brakes can handle that. We've got information that shows the stopping distance on a typical tractor with disc brakes all the way around down to 185- to 215-foot stopping distances."

Since activation of braking systems may cause stability and handling issues, especially in severe weather or panic applications, future systems will incorporate such features as stability controls and more advanced automated braking systems. "The advanced systems that we're talking about are actually those which you would find more prevalent in a full electronic braking system, which is fairly commonplace in Europe," says Eberling. "However, due to the absence of shorter stopping distance [requirements] and the cost involved in having a fully electronic brake system with the pneumatic redundancy required by law, what we've actually done is incorporate the advanced features of an all-electronic brake system into the current anti-lock brake system platform. We think our system is to the point, with these advanced concepts of roll stability and enhanced electronic stability, that there is no real value in a full-electronic brake system."

Lynn Merrill is director of public services for the City of San Bernardino, CA.

MSW - November/December 2004