After a Brake Job: Adjustment and Balance on a Tandem Axle
A shop has just replaced the rear brake shoes and drums on a 2012 tandem-axle dump truck. The truck has manual slack adjusters and Type 20 clamp-type chambers on both rear axles. The apprentice sets the right rear pushrods to 2 1/4 inches (57 mm) and the left rear to 1 3/4 inches (44 mm) with the brakes applied at 100 psi. The parking brake is released, the wheels are chocked, and the system is at 120 psi. The apprentice then turns each adjusting nut a few clicks until the lining just touches the drum, but does not recheck the applied stroke. The truck has a load of gravel and will leave for a 40 km haul. The shop's inspector hears a light rubbing noise from the right rear and sees the right drum at 90°C after a 5 km test drive, while the left is at 55°C. The manual specifies a Type 20 standard stroke limit of 2 inches (51 mm) and a free stroke of about 3/8 to 1/2 inch (10-13 mm).
1. Are the apprentice's pushrod strokes acceptable? Explain the Type 20 limit and why the left and right settings are not equivalent.
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For a standard clamp-type Type 20 chamber, the maximum pushrod stroke is 2 inches (51 mm). A long-stroke Type 20 chamber allows 2 1/2 inches (64 mm), so the chamber type must be confirmed. If these are standard chambers, the right rear at 2 1/4 inches is over the limit and is not acceptable. The left rear at 1 3/4 inches is within the limit, but that does not make the pair correct. A shorter stroke can mean the brake is over-adjusted, with too little running clearance, and it may drag and overheat. The left and right settings are not equivalent because the brakes must be balanced; one side long and one side short can cause a pull, uneven wear, and unequal braking. The correct approach is to set both sides to the same specification and verify with an applied stroke measurement.
2. Describe the correct manual slack adjuster adjustment procedure and how to check free stroke and applied stroke.
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Chock the wheels, release the parking brake, build the system to 100-120 psi, and shut the engine off. Turn the adjusting nut in the correct direction until the lining contacts the drum, so the wheel cannot be turned by hand. Then back the adjuster off about 1/2 turn until the drum turns with a slight drag. Check free stroke at the slack adjuster; it should be about 3/8 to 1/2 inch (10-13 mm). Next, apply the brakes fully at 100 psi and measure the pushrod stroke on each chamber. Compare the measurement with the Type 20 limit, and repeat the procedure on both sides. After adjustment, road test at low speed and check drum temperatures; the brakes should be balanced and should not drag.
3. What do the temperature difference and rubbing noise indicate, and what should be done before the truck leaves?
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The right drum at 90°C versus the left at 55°C, plus the rubbing noise, indicates the right rear brake is dragging. That is consistent with over-adjustment, insufficient running clearance, a slack adjuster that is not returning, or a restricted return spring. A dragging brake can overheat, fade, crack the drum, damage the lining, and even start a fire. The truck should not leave with a load of gravel until the right rear brake is readjusted to the correct clearance, the pushrod stroke is rechecked, and the slack adjuster and return springs are inspected. After the repair, the truck should be road tested again and both drum temperatures compared; they should be close, with no rubbing noise and no excessive heat.