1 Air Brake failure - weak return springs or fractured parking Brake power springs. Automatic Slack Adjuster (ASA) - Air This paper discusses several defects and methodologies for failure analysis Brake chamber failure important to maintenance professionals, analysis CMV inspectors and collision reconstruction professionals. David A. Stopper, Director Stopper & Associates All of the examples discussed have been Commercial Vehicle Safety found on vehicles involved in serious Highway Collision Research & injury and fatal traffic collisions Reconstruction involving CMVs. June, 2005. 1994 FMVSS Requirements Abstract Air braked vehicles subject to Federal Motor Vehicle Safety Standard Air Brake equipped heavy trucks, trailers (FMVSS) 49 CFR and and buses manufactured since late 1994 Commercial Motor Vehicles (CMV). were originally equipped with self subject to Federal Motor Carrier Safety adjusting brakes. Many commercial Regulation (FMCSR) 49 CFR.
2 Motor vehicle (CMV) drivers believe (b),(c), manufactured after there is little need to thoroughly inspect October 20, 1994 are required to be self adjusting air brakes while still others equipped Automatic Brake adjusters . In are in the habit of manually adjusting addition, if the foundation brakes are these brakes, which in some cases can equipped with air Brake chambers that actually damage the mechanisms. have exposed air chamber push rods, a visual cue is required to indicate an A disturbing number of CMVs involved under adjustment condition. in injury and fatal traffic collisions equipped with self adjusting air brakes The FMVSS regulation uses the term are still found with brakes out of Automatic Brake adjusters or ABAs. adjustment. This term is inclusive of multiple current designs and allows for alternate new Many air braked vehicles equipped with designs. The most common version of Automatic Slack adjusters (ASA's) are the Automatic Brake Adjuster is the still found in an under adjustment Automatic Slack Adjuster (ASA).
3 These condition with inoperative or are applicable both as original malfunctioning self adjusting equipment manufacture (OEM) and after mechanisms. market retrofit on pre 1994 vehicles. Manual Slack adjusters and ASAs are an Adjustment failures can be as obvious as essential component to S cam, T . a disconnected adjusting mechanism cam and some air disc Brake (a/k/a: adjusting link) or as subtle and applications. concealed as air Brake chambers with Code of Federal Regulations: (c) Brake adjustment indicator (air Brake systems). On each commercial motor vehicle manufactured on or after October 20, 1994, and 49 CFR applies to equipped with an air Brake system which trucks, buses and truck tractors. contains an external Automatic adjustment 49 CFR while identical, mechanism and an exposed pushrod, the applies to trailers. condition of service Brake under adjustment shall be displayed by a Brake adjustment indicator conforming to the requirements of 49 CFR (FMVSS 121) is Federal Motor Vehicle Safety Standard No.
4 121. applicable to manufacturers. (49 CFR , ) applicable to the vehicle at the time it was manufactured. 2. 49 CFR Brake distribution and Automatic adjustment. Each vehicle shall be Discussion equipped with a service Brake system acting on all wheels. First and foremost, a properly installed, (a) Brake Adjuster . Wear of the service brakes lubricated and maintained ASA. shall be compensated for by means of a system connected to properly functioning air of Automatic adjustment. When inspected chambers and foundation Brake pursuant to , the adjustment of the service components will not need manual brakes shall be within the limits recommended by the vehicle manufacturer. adjustment. If manual adjustment is required to maintain proper adjustment (b) Brake indicator. For each Brake equipped levels the ASA function should be with an external Automatic adjustment diagnosed, repaired and/or replaced. mechanism and having an exposed pushrod, the condition of service Brake under-adjustment shall It is important to understand that a be displayed by a Brake adjustment indicator that is discernible when viewed with 20/40 vision malfunctioning ASA does not always from a location adjacent to or underneath the result in a complete Brake failure , but vehicle, when inspected pursuant to can contribute to Brake imbalance, uneven Brake wear and ultimately failure 49 CFR 393 - PARTS AND ACCESSORIES or reduced efficiency of the subject NECESSARY FOR SAFE OPERATION is Brake .
5 As ABAs are required equipment applicable to motor carriers. on vehicles subject to the FMCSR. manufactured after October 1994 it is a 49 CFR Automatic Brake adjusters and Brake adjustment indicators. violation of those regulations to operate a CMV with defective ABAs even if (b) Automatic Brake adjusters (air Brake they can be manually adjusted. systems). Each commercial motor vehicle manufactured on or after October 20, 1994, and No employer shall operate a equipped with an air Brake system, shall meet the commercial motor vehicle, or cause or permit it Automatic Brake adjustment system requirements to be operated, unless it is equipped in of Federal Motor Vehicle Safety Standard No. accordance with the requirements and 121 (49 CFR , ) applicable to the specifications of this part. Ibid2. vehicle at the time it was manufactured. 2. Transportation; 49 CFR Parts 200 399 ;. 1. Transportation; 49 CFR Parts 400 999 ; FMCSR 393 - Parts and Accessories Necessary FMVSS for Safe Operation; Government Printing Office, Government Printing Office, Washington, DC Washington, DC.
6 ASA Air Brake failure analysis / Stopper & Associates Page 2 of 12. Many of the problems to be discussed procedures in service and replacement of were published in the NTSB Highway ASAs. Accident Report, Mountainburg, AR3. involving multiple Brake failures which In some cases, motor carriers and resulted in a fatal truck tractor, semi owners operators simply remove ASAs, trailer crash with a school bus resulting replacing them with non OEM manual in 3 fatalities and 8 injuries. Slack adjusters . This is a violation of 49. CFR & (a)(1) if the Personal involvement in this Brake vehicle was manufactured on or after failure analysis and accident October 20, 1994. reconstruction as well as the NTSB. investigation revealed a combination of While many failures are found on older poor maintenance as well as component units, do not assume ASAs are failures. These allowed multiple brakes functioning properly on new vehicles. to operate with reduced efficiency and According to the Commercial Carrier progressively fail contributing to this Journal Incorrect factory installation of fatal collision.
7 ABAs is a common problem on new vehicles. Add this item to your pilot and Reconstruction analysis in the pre delivery inspections .4. Mountainburg, AR case revealed that had any one of the many Brake defects While properly maintained ASAs can not been present, the driver could have have an operating life of million miles developed sufficient Brake force, or more, it is not unusual to find over the although severely compromised, to have road CMVs accumulating 600,000 miles avoided the collision with the school in less than three years. Trucks and bus. buses used in heavy duty and urban service will often require ASA service or Over ten years after the ABA replacement in much shorter intervals. requirement, most air Brake vehicles are now equipped with ABAs, the vast CVSA Brake Inspections majority being ASAs. Still a disturbing number of air Brake vehicles are found Each year, the Commercial Vehicle with brakes beyond allowable Safety Alliance (CVSA) coordinates an adjustment limits.
8 Announced nationwide roadside CMV. Brake inspection over a three day period Many failures are the result of ASA combining enforcement and educational equipped units now showing significant efforts. This annual inspection is widely normal wear and requiring preventative published throughout the trucking maintenance. Many are being replaced industry, months in advance, specifying by mechanics that do not have specific the dates and in some cases, the training or fail to follow critical locations of the inspection sites. 3. National Transportation Safety Board, Collision Between Truck-Tractor Semitrailer and 4. School Bus Near Mountainburg, AR on May 31, Taking Out The Slack ; Gerald Standley;. 2001, NTSB/HAR-02/03; PB2002-916203; Commercial Carrier Journal; Air Brake Book 6 th Adopted September 4, 2002. Revised Edition; 1998. ASA Air Brake failure analysis / Stopper & Associates Page 3 of 12. In spite of this well known annual dependent on the amount of rotation of enforcement effort, the September 2004 the cam shaft or power shaft relative to announced Operation Air Brake the air chamber push rod stroke.
9 Enforcement drive found nearly one out of five ( ) of CMVs inspected to be The most popular design is similar to the operating with defective brakes to the manual Slack Adjuster with an adjusting level of being declared Out Of service . link attached between an extended push rod clevis and the adjusting mechanism The CVSA, whose committee members connected to internal adjusting linkage. include law enforcement and industry representatives, publish the standards for the OOS (Out Of Service) criteria typically endorsed by the FMCSA. (Federal Motor Carrier Safety Administration). The OOS criteria allow a CMV to continue in operation with up to 20% of the brakes with identified defects. The OOS will not count a defective ASA so long as the push rod stroke is within Type 30/30 Air chamber & Meritor ASA. allowable adjustment limits at the time Note: Central Wheels-On Drum Gauge in background of inspection. Brake application results in the geometry Defective ASAs are often not noted or between the cam shaft, Slack Adjuster simply overlooked in typical CVSA and adjusting link to change, causing the Level I inspections.
10 As a result, air adjusting link to rise and fall in relation braked vehicles with defective and to the Slack Adjuster housing. inoperative ASAs can continue to be operated indefinitely provided the As push rod stroke increases by operator of the vehicle continues to over foundation Brake component wear, the ride the Automatic Brake adjustment adjusting link movement becomes function and manually adjust the Brake . greater. At a predetermined point, the adjusting link rotates the adjusting nut to Operation of Automatic Slack the next adjustment level where the adjusters internal mechanism locks the adjusting nut in place. Each adjustment is ASAs have been available for over 20 typically turn of the adjustment nut. years as optional equipment before becoming mandatory for air Brake Due to manufacturer design vehicles in October 1994. considerations, some models of ASAs perform the adjustment on the While some improvements and application stroke (Gunite , Allied additional competitive products have Signal/Bendix ) while others adjust on been developed, ASAs all operate by two basic hardware styles, both ASA Air Brake failure analysis / Stopper & Associates Page 4 of 12.