Air vs. Hydraulic Brakes: A Comparison for Large Trucks

The Mining Safety and Health Administration offers some pretty sobering statistics and investigations. Take one recent incident, for example. 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 such as 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.

Air Predominance

For quite some time the large-truck standard has been an air brake system. Initially developed in 1872 for railcars, 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:

Air is compressible. Therefore, 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.
Air contains moisture. 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, and new braking systems have begun proactively addressing these changes.

While hydraulic brakes with ABS have been improving the safety of automobiles since the 1980s, the demand for their use in large trucks has not yet developed. Unfortunately, in the time that demand has been building, accidents have been caused by the long stopping distances of air brakes, and lives have been lost by some drivers’ inexperience with the old system.

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. Furthermore, young drivers who only have experience with hydraulic systems may not know how to react properly with air brakes, resulting in increased stopping time.

What does this mean to a driver? Picture a million-dollar haul truck travelling at 30 mph, or 44 feet per 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 feet. That quarter of a second and corresponding 11 feet 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; or possibly the difference in life or death.

Safety Aside

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:

Cost: Hydraulic systems are smaller, simpler and typically cost less than comparable air brake systems.
Integration: Any vehicle that uses hydraulics for an application besides braking will have a hydraulic pump already installed, helping to keep the design simple and costs at a minimum.
Reliability: The fewer components a system has, the more dependable it will be. A simple hydraulic brake system consists of a hydraulic reservoir, master cylinder, brake lines and caliper assemblies, whereas air brakes incorporate a compressor, reservoir, air dryer, brake lines, brake assemblies and multiple valves.
Maintenance: While air brakes may have manual drains to open and alcohol evaporators to refill, hydraulic brakes require comparatively little preventative maintenance.
Couplers: Leak-free couplers reduce the risk of oil loss. Also, air brakes can now be coupled with hydraulic brakes, allowing tractors the benefits of hydraulics while maintaining a trailer’s air brake system.

It’s rare that a better working system 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 is the reduced delay and shorter stopping distances provided by hydraulic brake systems with ABS, resulting in safer vehicle operation, fewer accidents and fewer injuries.

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, MICO, Incorporated.)