Automatic Slack Adjuster (ASA) and Air Brake …

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.

2 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. 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 .

3 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.

4 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). 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.

5 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.

6 121. 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.

7 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.

8 Uneven Brake wear and ultimately failure 49 CFR 393 - PARTS AND ACCESSORIES or reduced efficiency of the subject NECESSARY FOR SAFE OPERATION is Brake . 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.

9 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. 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.

10 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.