These do need to be checked periodically for shoe drag due to other failures.Ģ. Some S-CAM brakes have automatic self adjusters. It is important that students realize that even though S-CAM brakes are adjusted properly, broken or worn return springs and/or a worn or non-lubricated backing plate can also cause S-CAM shoes to drag.ĪNSWERS TO INFORMATION CHECK 1. A slow releasing or non-releasing shoe will not only wear out faster but will reduce the vehicle's mpg. As a result, the shoes drag, especially when the drums are expanded from braking induced heat gain. Students, in their zealousness to make sure the brakes respond rapidly, have a tendency to reduce travel to less than the minimum. TEACHER'S NOTES Excessively loose slack adjusters are dangerous. Worn or improperly lubricated backing plate D). If false, please explain why it is false.ġ._ All S-CAM brakes must be periodically adjusted.Ģ._ One important S-CAM brake adjustment check is to check to see that the pushrod and slack adjuster are at 90° to each other with maximum brake application.ģ._ As long as the wheel turns freely, no adjustment is needed on an S-CAM brake.Ĥ._ A properly adjusted S-CAM brake will typically drop the system air pressure 18 to 22 psi.ĥ._ Which of these S-CAM brake problems will decrease mpg? A). INFORMATION CHECK Indicate whether the statements below are true or false. If the wheel still does not turn freely, remove the brake drum for an internal inspection.Ĭonsult the general S-CAM brake diagnostic chart for assistance. If the wheel and tire assembly does not turn freely, recheck the adjustments. If any of these adjustments are out of specifications, readjust according to manufacturer's specification. The slack adjuster set at 90° to the brake chamber with full brake application. Correct decrease of air tank pressure for a full brake application - usually 8 to 12 psi. Proper stroke or travel of the slack adjuster, usually 1 - 1 1/2 inches. By hand, try to rotate the wheel and tire assembly. Raise the axle and place a safety stand at an appropriate place to support the weight of the vehicle.ĥ. Release the parking brake and place transmission in neutral.Ĥ. Block the wheels so the vehicle will not roll. Locate an appropriate manual to determine correct adjustment procedure.ģ. USE THE CORRECT EQUIPMENT AND PROCEDURES.Ģ. ALWAYS CONSULT THE MANUFACTURER'S SHOP MANUAL FOR EXACT BRAKE ADJUSTMENT PROCEDURES.ĬAUTION: ALL BRAKE WORK HAS THE POTENTIAL FOR ASBESTOS HAZARDS. DO NOT ATTEMPT TO OPEN OR DISASSEMBLE AIR BRAKE CHAMBERS SERIOUS INJURY MAY RESULT.Ģ. Remember, a dragging shoe causes reduced mpg and premature shoe wear.ĪCTIVITY: WARNINGS: 1. Also, excessive corrosion or lack of proper lubrication of the shoe mount and shoe to backing plate may also cause an S-CAM shoe to drag. A broken return spring or a return spring that has lost its tension will cause a brake shoe to drag. Figure 2 Illustration courtesy of the Government of the Province of British Columbia.īesides an improperly set slack adjuster, two other abnormalities may cause an S-CAM brake to drag. WARNING: ALWAYS CONSULT THE MANUFACTURER'S SHOP MANUAL FOR EXACT S-CAM BRAKE ADJUSTMENT PROCEDURES. The adjusting nut allows the shoe to be positioned closer or farther from the brake drum. Figure 2 illustrates a typical S-CAM brake. Trailer brake adjustment on fifth wheel units is also important to synchronization and lateral force equalization. It is important to note that all S-CAM brakes on a given vehicle be adjusted properly to avoid undue lateral forces or improper rear to front synchronization. It is critical for both safety and maximum fuel economy that S-CAM brakes be properly adjusted. This drag will reduce mpg and result in premature brake shoe and/or drum wear. The Figures below shows the comparison of non-self adjusting and self adjusting S-CAM brakes.Ībove Illustrations courtesy of Mack Trucks, Inc.Īn S-CAM brake with excessive shoe to drum clearance will engage too late while one which is too tight will engage too soon and may drag. Many S-CAM brakes are non-self adjusting and therefore require periodic readjusting to compensate for brake shoe wear. LESSON / INFORMATION: S-CAM brake systems have been used in medium and heavy trucks for decades. Demonstrate the correct procedures for adjusting an S-CAM brake. Understand how an excessively tight S-CAM brake can reduce fuel economy. Learn to properly identify an improperly adjusted S-CAM brake. GOAL: To understand the relationship between excessively tight S-CAM brakes and reduced fuel economy.ġ. S-CAM Brake Adjustment and Fuel Economy S-CAM BRAKE
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