Tought Leader
Understanding the Differences Between Air Brakes & Hydraulic Brakes
WRITTEN BY ROBERT T. PUDLEWSKI
A
ir versus hydraulic brakes in the Type C school bus is a popular topic that is often debated. When looking to figure out the best brake system for your fleet of school buses, there is no shortage of testimonials or
opinions editorialized from many forums and conference presenta- tions as to why school bus operators choose one over the other. While cost and personal preference sometimes prevail in making choices, certain states prevent choice by having long-standing school bus construction regulations that dictate what brake foundation system is required. A few states use a standard that dictates the brake system by the combined gross vehicle weight (GVWR) rating of the completed bus, which includes loaded capacity (average student weight multiplied by capacity, including the driver) and body and chassis weight. Other states may just use the student capacity of a school bus, which then mandates the use of air brakes. Te science behind the logic was that only air brake systems
offered bigger braking surfaces. Tis, of course, has changed with the introduction of four-wheel disc brakes in air and hydraulic disc systems that are now available. It is an appropriate time to challenge those states with school bus construction standards that have man- dates for brake systems based on capacity alone. Te OEMs will follow stringent engineering design and component manufacturer capability when developing brake systems for Type C buses in the 19,501- to 33,000-pound GVWR range, or truck class 6 and 7.
AIR BRAKES While the 135-year history of air brakes proves the system’s
overall success, transportation has changed significantly during that time. Traffic and congestion are different, and driver experiences and expectations behind the wheel of today’s school buses are vastly different. New braking systems have begun proactively addressing these changes by introducing air dryers, automatic brake slack adjusters, ABS traction control, and electronic stability control (ESC) that are now available on Type C hydraulic disc and air brake systems.
38 School Transportation News • AUGUST 2018
ADVANTAGES OF AIR BRAKES Te supply of air is unlimited, so an air brake system can never
run out of its operating fluid, as hydraulic brakes can. Minor leaks do not result in brake failures. Air line couplings are easier to at- tach and detach than hydraulic lines, thereby eliminating the need for hydraulic fluid and related service. Air not only serves as a fluid for transmission of force, but it also
stores potential energy. So, it can serve to control the force that is applied, such as in compressing the spring in a chamber to control the emergency and parking brake.
Air brake systems include an air tank that stores sufficient energy to stop the vehicle if the compressor fails. Te systems are effective even with considerable leakage—the air brake system is designed with sufficient “fail-safe” capacity to stop the vehicle safely even when leaking. Tere is also the wig wag, or the warning device for low air pressure.
ADVANTAGES OF HYDRAULIC BRAKES Hydraulic disc brake systems typically cost less to purchase and install than comparable air brake systems. Additionally, hydraulic disc systems are smaller and simpler to use and maintain than air brakes. Te fewer components a system has, the more dependable it will be, since reliability is a compounding factor. Tere are also fewer preventive maintenance steps that are involved in the upkeep of hydraulic disc systems. Air brakes may have manual drains to open, alcohol evaporators to refill, or air dryer maintenance. With the greater power density of hydraulic disc 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, which is a benefit over air brake systems. It’s rare that a system that works as well as another will cost less, but
hydraulic discs offer that advantage. Hydraulic disc brake systems also offer greater power density in the same or less space and demonstrate decreased maintenance. More importantly, the reduced
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