Automatic Slack Adjuster (ASA) And Air Brake Chamber Failure

the release stroke (Arvin/Meritor ,Rockwell , Crewson-Brunner ).5Haldex S-ABA Creson-Brunner “Auto-Check” ASAThe Crewson-Brunner ASA hasincorporated a unique permanentlymounted template which easily allows adriver to evaluate brake adjustment.The second ASA design unique to theHaldex S-ABA (Self SettingAutomatic Brake Adjuster) utilizes ananchor bracket. This alternate designattaches the internal adjustmentmechanism via an anchor arm thatencircles the cam shaft. The anchor armis then attached to a fixed point by abracket typically attached to the axle,brake spider or to a mounting bolt of theair brake chamber.65The Air Brake Book; 6th Revised Edition,Commercial Carrier Journal, 1998; CahnersBusiness Information6TMC-Technology and Maintenance Council,Recommended Practice – RP 609; Manual andAutomatic Slack Adjuster Removal, Installationand Maintenance; 2002 Edition.ASA Air Brake Failure Analysis / Stopper & AssociatesPage 5 of 12Haldex ASA – Control arm worn through anchor bracket& excessive wear at clevis pin bushingAdjusting Link GeometryThe geometry, more commonly referredto as installation angle, is critical toproper function of ASAs equipped witha pushrod clevis actuated adjusting link.This includes all current designs with theexception of the Haldex S-ABA .The angle created by the three centerpoints between the cam shaft, clevis pinand adjusting link clevis pin are critical

to insure proper adjustment as well as toprevent over adjustment.The ASA must be checked with atemplate supplied by the manufacturer.various air chamber “Type” sizes. Inrecent years the color coded nylon straphas been eliminated however the “color”is still identified on a label on the ASA.Meritor ASA identification labelBrake maintenance mechanics andprofessionals making a forensic analysiscan not assume that successfulconnection of the linkage has resulted inproper ASA adjustment. Confirmationof installation angle must be confirmedwith the appropriate template.Push Rod LengthInstallation inspection with Meritor templateThere are subtle differences in manydesigns between ASAs for trucks, trucktractors, trailers and air disc designs. Itis also important to insure the ASA is theproper model for the air chamber towhich it will be connected.7Meritor (formerly Rockwell) uses acolor coding system. The nylon strap tiethat secures the rubber boot on at theadjusting link shows a different color for7Meritor Automotive Systems, FieldMaintenance Manual 4-B,ASA Air Brake Failure Analysis / Stopper & AssociatesPage 6 of 12New and replacement air brakechambers generally are provided with a12 inch push rod which must be cut toproper length depending on the specificinstallation requirements.If cut too short, when the clevis isconnected, the slack adjuster it maycontact axle components, restricting fullretraction. This prevents the ASA fromcompleting a full apply to release cycleand will not allow proper automaticadjustment.Even without contact with axlecomponents a push rod cut too short willgenerally require the installer to pull thepush rod out to allow connection of theadjusting link to the clevis. When thisoccurs, the adjusting link in the released

position, will reach its mechanical limitscausing the pushrod to remain in apartially applied position.In the Mountainburg, AR fatality caseNTSB wrote:“During the post accident inspection,investigators discovered that the anglebetween the pushrod and the slack armon brake 4R (semitrailer left leadingaxle) was greater than the 90 degreesthat specifications allowed. Whenmeasured against a slack adjustertemplate, the pushrod was about 1 inchshorter than the pushrod on the left sideof the trailer, causing the greater angle.According to the manufacturer, theautomatic slack adjuster was bottomingout, thus preventing full release of thepushrod and preventing the brakes fromautomatically adjusting. The 4R brakedrum was rusty and the brake did notappear to be functioning.”In this circumstance, when the push rodstroke is measured in accordance withCVSA recommended practice, the pushrod stroke may measure withinadjustment limits when in fact it isbeyond the limits. The effective pushrod stroke will be the measured push rodstroke plus the distance the push rod isbeing suspended in the partially releasedposition.Due to the shortened push rod stroke, theASA will not sense the need foradjustment and will allow an out ofadjustment condition to develop.At the time of this improper installationand periodic maintenance, if themechanic notices excessive brake blockto drum clearance and manually adjuststhe automatic slack adjuster, thisASA Air Brake Failure Analysis / Stopper & AssociatesPage 7 of 12condition could continue indefinitelyeven though the ASA is not functioning.If cut too long, the adjusting link willeither be restricted in cycle movement soas not to adjust or only adjust well afterthe push rod stroke is beyond theadjustment limit.Defective Air Brake ChambersOften overlooked and difficult todiagnose are worn out air brakechambers. Most inspections of air brakechambers only involve a check forleaking air diaphragms and connectingair hoses. Few drivers, mechanics andCMV inspectors are sensitive to or evenaware of the brake adjustment indicatorsrequired since 1994 and used by most airchamber manufacturers for several yearsbefore the FMVSS and FMCSRrequirement for an external automaticadjustment identifier. Ibid1 2Air brake chambers manufactured forthe last ten years are typically marked intwo ways. First with a “stroke alertindicator”, marking which visuallyidentifies when a push rod has reachedthe adjustment limit. This is typically ared or orange ring marking the last ½inch of the push rod which is exposedwhenever it is at or beyond theadjustment limit.Stroke Alert IndicatorMGM uses a green (within adjustmentlimit), yellow (nearing the adjustmentlimit) and red (at or over adjustment

limit) color code. This style is pasted onthe push rod and is subject todeterioration as a result of extendedoperation.poorly publicized and unknown to mostdrivers of air brake vehicles. It is notcurrently discussed in CommercialDrivers License Manuals.The second common marking is referredto as a “visual identifier groove”. This isan identification point on the push rodwhich should be flush with the face ofthe air chamber when the brakes arereleased.Inspection of combination “Service /Spring brake” air chambers haverevealed fractured power springs in theparking brake segment of the dualsection chambers.Typical Type 30/30 Spring-Service Brake & ASA9An investigation by the author and theNTSB of the collision between a loadedtruck tractor, semi trailer and school busnear Mountainburg, Arkansas in 2001revealed numerous brake deficiencies.Exposed “Stroke Alert Indicator”Brake at maximum strokeThe adjustment indicator was discussedfor many years and was finally made arequirement after the NationalTransportation Safety Board (NTSB)recommendation in the 1992 HeavyTruck Airbrake Performance SafetyStudy.8This long overdue safety enhancementwas welcome but unfortunately has beenThe brake deficiencies leading tomultiple fatalities in the school busstruck included this summary from theNTSB “Factual Information” report:“The parking/emergency brake springwas found to be in three pieces whenbrake disassembled. After a manualcaging bolt was installed, the chamberretracted an additional 5/8 inch,indicating that the broken spring waspreventing full pushrod release. Thespring was fractured in such a way thatit prevented full return of the pushrod;thus, the automatic slack adjuster didnot have the minimum 1½ inches of8“Heavy Vehicle Airbrake Performance,” Safety Study,National Transportation Safety Board, P892-917003,Washington, DC, Adopted April 29, 1992.ASA Air Brake Failure Analysis / Stopper & AssociatesPage 8 of 129United States Patent #5002164; US Patent Office,Arlington, VA

stroke necessary to activate theadjusting mechanism.” Ibid 3In this case it became apparent threebrake chambers inspected were at ornear maximum stroke (bottomed out) butstill measured to be at or near allowablepush rod stroke limits. Furtherexamination through the “Caging bolt”hole at the end of the brake chamberrevealed a tilted pressure plate. NTSBinvestigators removed the suspect airbrake chambers and had amanufacturer’s laboratory disassemblethem exposing the fractured powersprings.First, with the wheels of the vehiclechocked, release the parking brakes withnormal air pressure (100 – 125 PSI).Insert a narrow ruler along the push rodto the base of the push plate. Measurethe distance from the retracted push plateto the face of the chamber.The thickness of the internal mountingbolts and the chamber body is normally½”.If the chamber push rod is retractingproperly, the measurement should beequal to the maximum push rod strokeplus ½”. A standard Type 30/30chamber push rod stroke adjustmentlimit is 2” with a maximum stroke of2 ½”. If fully retracted, the distancefrom the face of the chamber to theinternal push plate should measure 3”.If the measured distance is less, theeffective push rod stroke will be themeasured stroke at an 85 – 90 PSI brakeapplication, plus the difference betweenthe proper fully retracted measure andthe actual measure.Disassembled Type 30/30 brake chamberwith fractured power springInspection of a service-spring brakechamber suspected of a fractured powerspring requires great care. Disassemblyshould NEVER be attempted except by aprofessional equipped with a propercompression press and safety cage.Failure to do so could result in seriousinjury or death. Visual inspection,without dismantling can be conductedwith a bore scope.Measurement of the push rod stroke onan air brake chamber with a suspectedrestriction of movement can accuratelybe attained by inserting a narrow ruleralong the push rod.ASA Air Brake Failure Analysis / Stopper & AssociatesPage 9 of 12Current maintenance and inspectionprocedures do not directly addressinspection for fractured power springs.NTSB reported:“A broken spring, which is difficult todetect, can reduce emergency-parkingbrake forces or render the emergencyparking brake inoperable, and thebroken spring pieces can be displaced,thus shortening the pushrod stroke orpreventing the automatic slackadjuster from functioning. The extent ofthe broken spring problem isundetermined.Radlinski and Associates, a brakeconsulting firm, reported to investigators

that in an inspection of 11 five-axlecombination trucks at a large firmwith an excellent maintenanceprogram, an inspector found an averageof two broken spring brakes on eachtractor semitrailer.”push rod excessive movement withoutthe ASA sensing the need foradjustment.Broken spring brakes are not an out-ofservice item if detected during CVSAinspections, nor is visual examination ofspring brakes an inspection item.Regulations do not require use of dustcovers over the caging port to preventcontaminants from getting into thespring brake assembly.“Clearance between the slack adjusterclevis pin and its bushing should notexceed 0.020”.Weak Return SpringsBoth service and service/spring air brakechambers are equipped with returnsprings to retract the push rod and slackadjuster to its fully released position.ASAs must return to the fully retractedposition to accurately sense the push rodstroke and determine when automaticadjustment is required. The mostpopular ASAs adjust on the returnstroke. To achieve full retraction, thecam shafts and slack adjusters must beproperly lubricated and the air chamberreturn spring needs a minimum of 32pounds of spring force to effect fullretraction and adjustment.Paul Richards, Executive Editor forCommercial Carrier Journal wrote:“Check cam shaft splines for free playbetween the slack adjuster and thecamshaft. The cam shaft and/or slackadjuster should be replaced if there ismore than 0.020” free movementbetween slack and camshaft.”10Cam bushing play must be checked withthe brakes released. With brakesapplied, the pressure and torque of thebrake application will take the play outof the system masking excessive play.“S” cam shaft bushing play should notexceed 0.040”. Ibid 7Cam Bushings & Clevis PinsAnother area often overlooked is thecondition of the cam bushings and clevispins. ASAs require proper alignmentand lubrication to maintain brakeadjustment levels. They have to besensitive to brake wear while not overadjusting.As a result, excessive play in theattaching components will permit theASA Air Brake Failure Analysis / Stopper & AssociatesPage 10 of 12Disassembled Rockwell ASA10Restoring S-Cam performance; Richards, Paul;CCJ/The Air Brake Book, 6th Edition, 1998

Internal adjusting mechanisms aresensitive to the introduction of excessivedirt and corrosion from road dirt.Meritor has reduced the size of theadjusting nut in an attempt to prevent thedrivers from using a large enoughwrench to back off the brakes withoutreleasing the pawl. They also added a“Pull-pawl”, a spring loaded cap on thepawl nut that can be lifted withoutremoving the pawl allowingdisengagement of the adjusting linkduring manual adjustment.Worn adjusting link rubber bootWorn rubber boots, protecting internalparts must be inspected. If worn or torn,the ASA must be disassembled, cleanedand the boot replaced. Often labor costexceeds the value of simply replacing aworn ASA.Manual Adjustment an ASADrivers and mechanics often insist onmanually adjusting ASAs. In the case ofMeritor ASAs, the adjusting pawl, whichlocks the adjusting sleeve preventing theworm gear from backing off, like aratchet wrench, can be damaged bymanual adjustment.It is common practice in manualadjustment for the brake shoes to bebrought to firm contact with the brakedrum then back off the adjustment nut ¼to ½ turn. This allows for minimum shoeto brake drum clearance.The manual adjustment nut on the end ofthe Rockwell / Meritor ASA will allowthe brakes to be tightened but the pawlmust be released or removed beforebacking the brakes off.ASA Air Brake Failure Analysis / Stopper & AssociatesPage 11 of 12Worn/damaged Rockwell / Meritor ASA adjusting pawlOther manufacturers of ASAs use avariety of clutch spring assemblies tohold the automatic adjustment while stillallowing manual adjustment.Excessive manual adjustment can causethese clutch springs to weaken and failto maintain adjustment. To test theclutch spring, a torque wrench must beused to test the holding strength of themanual adjustment nut. These varybetween manufacturers and servicemanuals need to be consulted for thesetorque values.

Push Rod Strokes for ASAsConclusionThe general thought since the onset ofmanual slack adjusters was to adjust thebrakes so that the push rod stoke is asshort as possible without the brakesdragging the brake drums, in some casesas little as 1/2” push rod stroke. This isnot necessary the case with ASAs.CVSA Operation Air Brake dataindicates CMVs equipped with ASAsstill have a substantially lower numberof brake violations than those equippedwith manual slack adjusters. Thebenefits are self evident as thepercentage of all CMV brake violationshave substantially dropped since the1992 NTSB Heavy Vehicle AirbrakePerformance Safety Study.The ASA needs enough push rod stroketo allow full application but still enoughbrake shoe to drum clearance so thebrakes don’t tighten during brake drumheat expansion and then drag when thedrums cool. As a result, properlyinstalled and functioning ASAs willtypically operate with push rod strokesmuch closer to the adjustment limit. Inthe case of a Type 30 brake chamber, 11/2” to 1 7/8” are typical with 2” as theadjustment limit and 2 1/2” at maximumstroke.The importance of this is when an ASAmechanism does fail to adjustautomatically, it will run beyond theadjustment limit sooner than a manualadjustment that is left unattended.One safety solution to provide anadditional measure of safety is to equipair braked vehicles with “Long Stroke”air chambers. These chambers typicallyhave ½” of additional push rod strokeallowing an additional margin of safety.“Long Stroke” chambers are generallymarked with a plastic tab identifyingthem. In the absence of the plastic tab,they often have “LS” or “Long Stroke”embossed in the air chamber body.Square air hose inlet ports provide anadditional visual identification of “LongStoke” air chambers.ASA Air Brake Failure Analysis / Stopper & AssociatesPage 12 of 12It is still unknown what percentage ofthe relative decrease in violations is dueto properly functioning ASAs / ABAsand how many are maintainingadjustment by manually adjusting andover riding the self adjustingmechanism.It has become apparent there is a generallack of knowledge as to the criticalnature of proper installation, lubricationand periodic inspection of these criticalsafety devices.It is very important that collisioninvestigators and CMV maintenancepersonnel be trained and becomefamiliar with the operation and criticalfactors that can render ASA / ABAsinoperative leading to diminished brakeforce and/or brake failure.Presentation toARC Network – Collision Science Institute ConferenceLas Vegas, NVJune, 2005ByDavid A. StopperStopper & Associates, LLC

Code of Federal Regulations: 49 CFR §571.121 S5.1.8 applies to trucks, buses and truck tractors. 49 CFR §571.121 S5.2.2 while identical, applies to trailers. 49 CFR §571.121 (FMVSS 121) is applicable to manufacturers. 49 CFR 571.121 S5.1.8 Brake distribution and automatic adjustment. Each vehicle shall beFile Size: 541KB