Air Brake Basics Part 7
Other Braking Designs
Although the S-cam brakes described above are the current standard of the industry for heavy trucks, disc brakes like those used on automobiles are also an option for a heavy truck braking system . Disc brakes offer several advantages:
1. Disc brake maintenance is easier than drum brake maintenance.
2. Disc brakes produce more stopping power than drum brakes when they are hot.
3. Disc brakes are lighter in weight than drum brakes.
Wedge brakes are another type of heavy truck braking system. They usually do not require frequent manual adjustment. The primary difference between wedge and cam brakes is that instead of having an S-cam rotate, wedge brakes have a plunger which pushes a wedge between the brake rollers, and thus forces the linings in contact with the drum. This system is becoming obsolete.
Braking Enhancements
ABS Brakes
The purpose of an anti-lock brake system (ABS) is to keep the brakes on a vehicle from locking during a hard braking situation. A locked wheel can cause the vehicle to lose directional control and extend braking distances. To prevent a brake from locking, ABS is designed to control brake application so that the maximum amount of retarding force is achieved. This point occurs just before the brake locks. Since the brake does not lock, the driver maintains complete directional control.
ABS functions similarly to a drivers foot during normal brake application. Due to the nature of air brakes, a driver cannot feel when the tire is about to stop rotating and therefore cannot adjust the pressure to the wheels fast enough to attain optimal braking. In an ABS system, brakes are activated through electrical impulses, and ABS pumps the brakes faster than any human can.
The system works by having three basic parts:
1. a wheel speed sensor that detects the rotational speed of the individual tire or wheel assembly
2. an electronic control module (ECM) that analyzes information and
3. the modulator valve assembly that adjusts the air pressure to the wheel
As the wheel turns, the sensor mounted next to it generates voltage. This voltage is transmitted to the electronic control unit (or module) which monitors the pulse rates. During normal brake application, ABS is inactive. The ABS system activates only when the system detects that the wheels are approaching lock-up.
During a brake application, if a wheel is approaching lock-up or stops rotating, the ECM sends a signal to the modulator valve. The modulator valve releases air from the brake chamber on that particular wheel until the wheel rotates to the proper rpm and then the modulator valve reapplies the brakes. This again increases braking resistance to the point of almost locking up. The system does this repeatedly up to five times per second.
There are five different kinds of anti-lock braking systems currently in use on most over-the-road trucks. The most common are manufactured by Rockwell Wabco and Bendix. Both use an indicator light or an ABS warning lamp mounted next to the gauges on the cab’s instrument panel. The warning lamp lights when the key is first turned on and the truck is started. The light remains lit until the vehicle reaches a speed of approximately four miles per hour. It then turns off, indicating normal operation. If the light remains lit at a speed above four miles per hour, the system has a problem or malfunction and needs to be checked by a service technician.
ABS brakes are also available on trailers and dollies. Typically, if the trailer has ABS, a warning lamp is mounted on the trailer’s left front comer, or an extra ABS warning lamp is mounted on the instrument panel in the cab.
Manufacturers offer different models of the same basic ABS system. For example, the number of wheel sensors and relay valves can be specified to coincide with particular applications. The use of sensors on all brakes assures that the vehicle can come to a safer and straighter stop.
ABS was required on new tractors built after March 1, 1997 and on new trailers built after March 1, 1998. New straight trucks and buses (hydraulic brakes) built after March 1, 1999 are required to have ABS.
Supplemental Braking Systems
Spring Brakes
In 1960, spring brakes were first available. Trucks and tractors equipped with air brakes must have a parking brake that is independent of the air pressure to hold the vehicle in place during loading. Trucks and tractors with air brakes have at least one axle equipped with spring brake chambers. The spring brake’s design and function make it suitable as a parking brake as well as an emergency brake.
Spring brakes have various names: dual chamber, piggybacks, MGM’s, or Maxi’s. They all refer to a service air brake chamber with an attached spring brake chamber, both of which can apply the brakes. Figure 2-11 reveals the inside of both chambers and Figure 2-15 shows where they are located on a truck.
The spring brake is aptly named as it is a spring that applies the brakes. The supply line air is plumbed to the spring brake air chamber. When there is a safe supply of air pressure available, the air in the chamber will compress the large spring so that it does not apply the brakes.
When the parking control valves in the cab are pulled, which empty the air lines, or in an emergency when the air line pressure drops below about 35 psi, the springs are no longer held compressed. They extend and force the pushrod to also extend, thereby engaging the brakes.
Fully extended spring brakes will apply approximately the same braking force as 60 psi of air pressure in the air brake chamber.
In order to disengage a spring brake, the air in the spring brake air chamber needs to be increased to a pressure that will once again compress the spring. Under normal conditions this is accomplished by starting the engine, waiting for the air compressor to build air pressure, then pushing in the parking control valves on the dash. Since this will not work when an air system failure exists, spring brakes can be disengaged by a method called caging.