S-cam air brake system is provided in almost all commercial vehicles having tonnage above 7.5-ton. In S-Cam brake system, drum to brake lining gap (henceforth referred to as ‘brake lining gap’ or simply ‘gap’ for convenience) range is an important factor which can impact braking behavior during brake application. Different OEMs (Original Equipment Manufacturers) define different brake lining gap ranges between S-cam brake lining and drum. This range depends majorly on the internal mechanism deployed in ASA (Auto Slack Adjuster). When these brake lining gaps start lowering i.e. when they fall in the range of 0 to 0.4 mm, or they become unstable (checked by feeler gauge at inspection window provided on dust cover of S-cam) then it starts impacting brake behavior in the subject vehicles. Unstable gaps can be defined as the phenomenon where gaps vary by a good margin of approx. 0.4 mm or more (as per subject vehicles study), between top and bottom brake linings or measured at different points (S-cam end, center & abutment end) of same brake lining.
This paper defines a systematic methodology for root-cause analysis of zero/low/unstable brake lining gap concern between S-cam brake lining and brake drum in air brake system, by investigating a field concern. It gives a first-hand idea as to how manufacturer can ensure the appropriate brake lining gap range during concept stage of design, so that field issues related to low gaps like brake grabbing, brake drag, premature brake lining wear etc. are mitigated significantly. The study also gives an insight in identifying the design factors that must be considered during selection of auto slack adjuster as well as help in sorting out the low gap concern which may occur due to design error or assembly failure.