OOSC, Part II, Item 1. Brake Systems - Caging Port Dust Covers

As a result of an investigation of a vehicle that was unable to stop at the stop sign at the bottom of the ramp. The 79,040-pound combination unit was traveling approximately 48 mph when it entered the intersection and collided with the right side of a westbound, 65-passenger, 1990 Blue Bird Corporation school bus. The Safety Board determined that the probable cause of the accident was the truckdriver’s inability to stop the tractor semitrailer at the stop sign at the bottom of the ramp due to the reduced braking efficiency of the truck’s brakes, which had been poorly maintained and inadequately inspected. Of six brakes on the tractor semitrailer equipped with spring brakes for emergency/parking brake application (axles 3, 4, and 5), three had broken springs. The 3L brake spring was broken in three pieces, restricting total stroke by 3/8 inch. Thus, even though the 3L brake appeared to be within adjustment at 1 7/8 inches, it was not. The 4L brake spring was broken in two pieces, and dynamic testing of the vehicle showed that the broken spring did not prevent service brake application; the service brakes even locked during one test. When the emergency-parking brake was applied during another test, the 4L brake provided some braking force (the service brake locked on gravel but not on concrete), indicating the emergency-parking brake force was reduced due to the broken spring, but was not completely eliminated. The 5L brake spring was also broken and blocking the pushrod, thereby limiting pushrod stroke and preventing it from reaching the minimum 1 1/2 inches necessary for the automatic adjuster to begin readjustment. During testing, 5L did not provide any braking force when either the emergency-parking brake or the service brake was applied. In other words, both the emergency-parking brake and the service brake were nonfunctional. Thus, a broken spring, in addition to reducing the braking ability of the emergency/parking brake or rendering it inoperable, can have a detrimental effect on the service brakes, as was the case in this accident. Broken springs on two of the vehicle’s three brakes prevented proper brake adjustment, thereby contributing to a reduction of the tractor semitrailer’s braking efficiency. The caging that houses brake springs makes detection of broken springs difficult because access to the closed chamber is restricted. If the spring brake is equipped with an integrated caging bolt, then it has no port for accessing the spring. Brake springs are neither a CVSA out-of-service item nor an inspection item. However, as this accident demonstrates, broken springs can have safety consequences when they prevent proper adjustment of the service brake or decrease the braking capability of the emergency-parking brake. The Safety Board concludes that because of the spring brake design, examining the springs to determine whether they were broken was difficult on three of the truck’s brakes.

Springs break for various reasons, including exposure to the elements, number of brake applications, age, or material properties. Contaminants can enter spring brake chambers through the caging ports unless dust covers are in place to prevent contaminants from entering. These contaminants, such as salt and water, can weaken the material properties of the spring, making it more susceptible to breakage. On the accident vehicle, dust covers were missing on all four spring brakes that had caging ports. Evidence of a white substance, probably salt deposits, was present inside the 4L spring chamber, and salt can corrode a spring, leading to its failure. The Safety Board concludes that dust covers on the caging ports of the accident vehicle’s spring brakes would have reduced the chance of corrosion to the spring, possibly prolonging the life of the spring and, in turn, the life of the emergency-service brakes.

The National Transportation Safety Board recommends that the Commercial Vehicle Safety Alliance include spring brake caging port dust covers as an inspection item during Motor Carrier Safety roadside inspections.