Air vs. Hydraulic Brakes: A Large Truck Comparison

Fri May 29, 2009 - National Edition
David Ewel




Air vs. Hydraulic Brakes is a popular topic when looking to figure out the best brake system, whether an air brake system or a hydraulic break system. All heavy truck drivers need to choose. Below you will find out what can happen and how they can affect stopping distances if not properly serviced.

The Mining Safety and Health Administration (MSHA) offers some pretty sobering statistics and investigations. Take one recent incident where a 50-year-old haul truck operator was fatally injured while dumping coal waste material at a spoil dump. “Condensation in the air lines froze and the brakes would not release,” the MSHA reported. “The victim exited the cab of the haul truck. The brakes finally released. The victim sustained fatal crushing injuries …”

Examples like this push engineers to build better brakes that will stop a vehicle faster and operate more reliably. Fortunately, things have come a long way from the days when wooden blocks, cotton and even camel hair were incorporated into brake systems. Hydraulics, anti-lock brake systems (ABS) and traction control have continued to advance automotive technology, and can now be incorporated on the largest of trucks for the first time ever.

Predominance of Air Brakes

For quite some time the large-truck standard has been an air brake system. Initially developed for railcars and patented for road vehicle use in 1872, this system made coupling a tractor and trailer easy since the lines could be disconnected and reconnected while losing nothing but air. Multiple units could be controlled from the cab with a single brake pedal. Even when multiple units weren’t necessary, as with large haul trucks and other off-highway vehicles, air brakes remained the standard.

Air brakes have had their share of problems, though. Any system that incorporates a light, alarm or “wig wag” to indicate that brake pressure is too low leaves many wanting a better system that doesn’t warrant such devices. To find a better solution, one must understand two simple laws of nature that hinder air brakes.

The first is that air is compressible. A relatively large amount of air must flow from the reservoir to the brake chamber before the air compresses enough to build pressure to decelerate the vehicle.

The second immutable law is that air contains moisture, and when air expands — as it does when it crosses the pressure-regulating valve in a brake system — it cools. And when air cools enough, it condenses. In cold environments, this condensed water can freeze and, in turn, cause brakes to fail, as the MSHA accident report detailed. Drain valves, air dryers and alcohol evaporators must work to alleviate the problem.

Clearing the Air

While air brakes’ 135-year history proves the system’s overall success, transportation has changed significantly during that time. Speeds are faster. Loads are heavier. Drivers’ initial experiences behind the wheel are vastly different. New braking systems have begun proactively addressing these changes.

The safety and convenience of hydraulic brakes have been available in automobiles since 1922. Anti-lock brake systems improved that safety in the 1980s. While the two systems have performed well in improving automobile safety, they have not previously co-existed in large trucks. Although the technical know-how has been there, it’s taken time for the demand to build.

As with many things, consumers drive demand. After experiencing the safety and convenience of hydraulic-powered anti-lock brake systems with traction control in automobiles, many air brake users desire the same convenience in their larger trucks. The next generation of truck drivers has learned — and will continue to learn — how to drive in vehicles with ABS, traction control and electronic stability control. Allowing time for air pressure to build or pumping hydraulic brakes on ice is an antiquated idea for many of today’s young drivers. Familiarity with these newer systems breeds habit, and habits can be hard to break, especially in an emergency-driving situation.

Full-power hydraulic brake systems with ABS result in shorter stopping distances, which also contributes to safer truck operation. Based upon physical design alone, hydraulic systems with ABS will stop in shorter distances than comparable air brake systems. The compressibility of air contributes to delays in braking performance. Hydraulics, however, are virtually incompressible, resulting in reduced delays in brake application.

What does this mean to a driver? Picture a million-dollar haul truck heading down a steep grade in an open pit mine. At just 30 mph (48 kmh), the truck is covering 44 ft. (13 m) in a second. If a hydraulic system causes the truck to come to a stop just one quarter of a second sooner than a comparable air brake system, that’s a difference of 11 ft. (3.3 m). That quarter of a second and corresponding 11 ft. can make a major difference — the difference in work going on or an investigation beginning, the difference in equipment repair costs or business as usual, the difference in life or death.

Hydraulic Advantages

While safety poses the greatest reason for choosing a hydraulic system over an air brake system, a number of other factors also should be considered, such as money, time, ease of use and maintenance.

Hydraulic systems typically cost less to purchase and install than comparable air brake systems. Additionally, hydraulic systems are smaller and simpler than air brakes. The fewer components a system has, the more dependable it will be since reliability is a compounding factor. For example, if one component in a series is 99 percent reliable, and a second component also is 99 percent reliable, the overall reliability becomes 98 percent, not 99 percent. Better reliability means fewer maintenance costs and less downtime over the life of the system. There also are fewer preventative maintenance steps involved in the upkeep of hydraulic systems. Air brakes may have manual drains to open and alcohol evaporators to refill. As with the brakes on the family sedan, hydraulic brakes require comparatively little preventative maintenance.

When it comes to haul trucks relying on hydraulics for dumping loads, a hydraulic brake system is even more logical and cost saving. Any vehicle that uses hydraulics for an application besides braking will have a hydraulic pump already installed, making hydraulic brake installation simple and relatively inexpensive.

With the greater power density of hydraulic brake systems, vehicle designers find it easier to integrate the hydraulic components into their design. By using hydraulic brake systems, engineers have the ability to control a large amount of power while using only a small amount of space, another benefit over air brake systems.

The development of leak-free couplers now enables hydraulic systems to overcome the main roadblock from the past — air brakes could be uncoupled and only air would leak, whereas hydraulic line uncoupling risked fluid leakage. Air brakes also can now be coupled with hydraulic brakes, allowing tractors the benefits of hydraulics while maintaining a trailer’s air brake system.

After more than a century of air-brake predominance in large vehicles, hydraulic ABS systems offer a way to catch up with the changing world of driving. Hydraulics react faster to an operator’s needs, leading to a safer environment for not only the driver, but for others onsite, as well.

It’s rare that a system that works better will cost less, but hydraulics offer that advantage. Additionally, hydraulic systems offer greater power density in the same or less space and have a track record of decreased maintenance. But perhaps the greatest benefit of hydraulic brake systems is simply safe operation. With ABS capabilities, hydraulic systems can reduce accident-related repairs and investigations. But more importantly, the reduced delay and shorter stopping distances provided by hydraulic brake systems with ABS will result in safer vehicle operation.

The history of brake development and technology spans more than a century. What matters most, however, is not the decades of development but rather the milliseconds of the application. That split second that took years to develop may be the difference between a serious collision and a safe stop.

David Ewel is director of engineering of MICO Inc.