How To Use a Slack Adjuster

2016 Red Semi Truck With Automatic Slack Adjuster

If you drive a commercial vehicle or are planning to pursue a commercial driver’s license, you have no doubt heard about a slack adjuster. These common mechanisms are necessary for commercial vehicles that have air brakes with an S cam.

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Properly using a slack adjuster is important for both driver safety and vehicle condition. Demonstrating how to use a slack adjuster is an essential component of most CDL examinations. In this article, we tell you how slack adjusters work. We also discuss the difference between automatic and manual adjustments. Finally, we tell you how to properly use a slack adjuster.

What Is a Slack Adjuster?

​A slack adjuster is a clearance-sensing brake mechanism that maintains optimal clearance between a brake’s lining and its drum. It also functions as a lever to move the brake shoe to the drum through the S cam. Commercial vehicle drivers rely on the slack adjuster to get full use of brakes while components wear and age. When there is excessive distance between a brake’s drum and its lining due to wear, the adjuster eliminates the slack.

How Does a Slack Adjuster Work?

The slack adjuster on commercial vehicles with S cam air brakes mounts directly on the S cam shaft. One end of the adjuster connects to the brake chamber’s push rod. It is the push rod that moves the slack adjuster.

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​When a driver applies pressure to the brake pedal inside the truck, a control arm attached to an anchor bracket allows the mechanism to rotate through the clearance angle. At the same time, the brake shoe makes contact with the drum. The harder the driver presses on the brake pedal, the more torque the system produces. The increased torque causes the shaft to move. This makes the heavy coil spring compress, disengaging the clutch gear. When the driver lets up on the brake pedal, torque decreases. This causes the clutch mechanism to advance, rotating the shaft and reducing clearance. Adjustment of the system occurs after the driver depresses and releases the brake pedal a few times.

​As you probably know, depressing the brake pedal on a commercial vehicle creates linear motion. The slack adjust converts linear motion to rotational motion. The designers of the slack adjuster use this process to discourage over-engagement of the braking system.

What Is the Difference Between a Manual Slack Adjuster and an Automatic Slack Adjuster?

If you need a CDL, you should know about both manual slack adjusters and automatic slack adjusters. Chances are good, though, as a commercial vehicle driver, you will probably not have to deal with manual slack adjusters. Because all vehicles made after 1994 have automatic slack adjusters, manual adjusters are becoming less common every day.

Commercial vehicle mechanics encounter manual slack adjusters frequently. While late-model commercial vehicles have automatic slack adjusters, a manual adjustment is necessary during installation.

Know your local CDL rules

​Depending on where you drive, your trip inspector may ask you to demonstrate competency with using a manual slack adjuster. This is possible even if your vehicle is equipped with an automatic slack adjuster. In some Canadian provinces, this test is common. As such, you must either show the inspector how to use a manual adjuster or describe the process. If you can’t, you may be prohibited from leaving the station or truck yard.

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A manual slack adjuster requires a nine-sixteenth-inch box wrench, a hammer, a prybar and some know-how. If you want to pass your safety inspection, you should also wear gloves and safety glasses. Moreover, you want to chock the wheels of your truck before you begin manual slack adjustment.

​An automatic slack adjuster does the work for you. You should know how a properly aligned slack adjustment feels and looks. You should also know how to spot the signs of an unadjusted or misadjusted braking system.

How Do You Use an Automatic Slack Adjuster?

Before heading out on a haul, commercial truck drivers sign a trip inspection form. Part of this form asks drivers to certify that they have checked brakes for proper adjustment. Again, adjustment includes ensuring that there is not too much distance between the brake line and brake drum on vehicles with S cam air brakes. If you want to pass inspection, you simply can’t shortcut the slack adjustment process. Doing so could result in a costly fine or suspension of your CDL.

​To properly complete the trip inspection form, you must test your truck’s brakes. To do so, you can release the spring brakes and build up pressure inside the braking system to 100 psi. Then, fully apply the brakes and pay attention to the drop in pressure. This helps you measure slack. A 10 psi drop for a five-axle truck is acceptable. Alternatively, you can look at the slack adjuster. Generally, if the adjuster is at no greater than a 90-degree angle to the push rod when the brake pedal is fully depressed, brake adjustment is acceptable. Both of these tests are subjective, though. Remember, your CDL is on the line. If an inspector sees something you don’t, your truck may be decommissioned.

Since your truck likely has an automatic slack adjuster, you must know how to properly use it. Automatic slack adjusters do a fantastic job of keeping brakes sufficiently adjusted. That is, they keep stroke within normal operating parameters. Still, every year, an alarming number of automatic slack adjusters are found to be out of adjustment.

​To keep your truck’s slack adjuster in adjustment, you must stay on top of it. Every week, you must make six full-pressure brake applications. Simply depress brakes completely and release. Do this six times in succession. You may also want to encourage the adjuster to turn over and visually inspect it. This will tell you immediately if the slack adjuster is correctly adjusted.

​Another way to keep your trucks slack adjuster working properly is to drive cautiously. By managing speed and staying aware of traffic and road conditions, you avoid aggressive braking. If you never depress your truck’s brakes more than 15 or 20 psi, your commercial vehicle’s slack adjuster won’t have to engage. As such, the brakes on your vehicle wear naturally without needing assistance from the slack adjuster.

You may want to use a ramp or a floor jack to raise your car more easily and in a stable, safe position.

How Do You Use a Manual Slack Adjuster?

To use a manual slack adjuster, you must first think about your safety. Put on a pair of safety glasses and wear gloves to protect your hands. Also, chock the wheels of your rig to keep it from rolling over you during the process.

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​First, release your truck’s brakes and verify that system pressure is above 100 psi. Then, inspect the braking system to determine if it is out of adjustment. Next, disengage the locking sleeve and establish adjusting nut direction. Remember, the air brake S cam must rotate in the same direction as brake applications. If you aren’t unsure which direction to turn the nut, you can check yourself.

In most cases, turning the nut in the wrong direction more than a couple full rotations means either you are turning it the incorrect direction or your brakes are extremely out of adjustment. If the latter were true, you probably would have noticed braking problems before you started the slack adjustment.

Turn the adjustment nut until it is snug. Be careful not to over-torque the nut, as this could cause stripping. You don’t want that. When the nut is snug, the brakes should be fully applied. Verify this with a visual inspection. Finally, unwind the nut about a third or a half of a turn. This should be the sweet spot where your truck’s brakes are properly adjusted. Finally, reengage and check the locking sleeve.

​If you have never used a manual adjuster before, you probably don’t want to do your first adjustment alone. Instead, ask an experienced driver or mechanic to supervise.

Final Thoughts on Slack Adjusters

Whether you want to pass inspection, successfully complete a CDL examination or simply drive responsibly, knowing about both manual and automatic slack adjusters is important.

If your commercial vehicle was made after 1994, it probably has an automatic slack adjuster. These vehicles usually don’t require manual slack adjustment. Nonetheless, to pass a CDL exam or successfully complete a trip inspection in some places, you must know how to use a manual slack adjuster. At a minimum, you should be able to describe the process of using a manual slack adjuster to adjust the brakes on a commercial vehicle with S cam air brakes.

Drive Responsibly

The best way to avoid out-of-adjustment brakes is to drive responsibly. Coasting to traffic lights and avoiding hard brakes encourages your truck’s brakes to wear naturally. Natural-worn brakes don’t require engagement of an automatic slack adjuster.

Still, if your truck’s tires brakes are over-stroking, you probably have a problem with the automatic slack adjuster on your rig. This could be due to improper installation or damage. Automatic adjusting likely won’t fix the problem. Manual adjustment, meanwhile, may provide only a temporary solution. While many drivers attempt to use slack adjustment to as a workaround for over-stroking, a mechanic’s intervention is likely the only responsible repair.

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