Heavy truck brakes are a crucial component of keeping the truck driver and the motoring public safe on the road. Brakes are built to stop trucks in all kinds of traffic, weather and terrain without creating crash conditions. But in order to do so, they must be properly maintained.
What are Slack Adjusters?
Automatic Slack Adjusters (ASA) are one key part of your braking system that is often overlooked or ignored. They get their name from taking up slack in the pushrod stroke of air brake applicators and keeping the brake linings within optimum clearance between linings and brake drums to ensure safe stopping distance of large trucks. An 80,000 pound loaded tractor trailer requires tremendous energy to be applied by the air brake system to avoid dangerous incidents. Slack adjusters handle the job of applying the air brakes by keeping brakes adjusted.
A Luxury for Heavy Trucks
ASAs first appeared in 1980, although they were not mandated for heavy trucks until 1994. ASAs were pioneered and produced to automatically adjust the stroke of the pushrod travel for achieving the best possible clearance between the brake lining and drum and to keep brakes in proper adjustment during normal driving conditions.
ASAs are a luxury on trucks these days, though few drivers will recall what it was like before they came into existence. Before, with manual slack adjusters, over-the-road drivers were required to crawl under the tractor and trailer axles to adjust the brakes by taking a wrench and adjusting the travel of each pushrod on every slack adjuster. And they had to check and adjust every day to prevent accidents, especially in high traffic conditions and difficult terrain such as mountains. Now, ASAs are on every axle with drum brakes, and the slack adjusters automatically adjust while driving!
Today, ASAs lessen maintenance requirements by doing away with manual adjustments. ASAs also reduce brake application time, thereby shortening stopping distance for greater safety.
However, they still need daily care and maintenance in order to operate properly. Let's first look at the dangers of manual adjustment, and then some maintenance and care tips.
The Dangers of Manual Adjustment
The important thing to note is that it is not recommended to manually adjust ASAs, despite some drivers and technicians ignoring the industry experts and choosing to do so. Just like their name, they are, after all, automatic.
Some drivers see excess pushrod travel as needing manual adjustment and just go ahead and do it. But excess travel is most likely due to wear and tear between drum and brake linings or wear on S-cam splines and bushings. In this case, manual adjustment is likely to cause premature wear between brake drums and linings and can create brake drag which generates heat and quickly leads to brake fade, a condition where brakes become incapable of stopping the truck and will undoubtedly lead to an accident.
Nowadays experts agree that an ASA with excess pushrod stroke is due to an underlying brake problem or wheel issue, or even failure of the ASA itself. They also agree that regular manual adjustment is the likely cause of ASA failure as it shortens the life of the unit.
The only times an ASA needs to be manually adjusted is during initial installation when it is new, or when brake linings and/or drums are replaced. Manual adjustment may also be required in an emergency if air pressure is unable to release the brakes. However, it should be performed only by a certified technician.
What to Do if Adjustment is Needed
If you are experiencing excess pushrod travel, you most likely have an underlying brake wear condition or alignment problem. When this happens, have your brakes inspected for the source of the problem, rather than adjusting pushrod travel yourself. It could be incredibly dangerous both for you and others.
Dragging brakes caused by manual adjustment also increases fuel consumption which adds to the operating cost of your vehicle, especially for Owner-Operators and fleets when it results in expensive brake repairs or worse.
If you discover excess pushrod travel on your truck, try to correct it by applying a few service brake applications to automatically adjust the brakes. If that doesn’t work, have your brakes inspected immediately. Never try to repair an ASA. If the automatic slack adjuster doesn't bring pushrod travel into normal limits and you know there is no underlying brake issue causing it, always opt for replacement of the ASA. And always have it installed by a certified and knowledgeable technician.
Keeping a Close Eye on your ASAs
The National Transportation Safety Board (NTSB) states that improperly adjusted ASAs have been found to cause heavy truck accidents. It's important to keep a close eye on ASAs. Check your push rod travel daily. Push rod travel should be less than one inch.
Additionally, the Commercial Vehicle Safety Alliance (CVSA) catches a lot of brake issues during its annual spring inspection blitz. We've got you covered with more detail in our post, CVSA Inspections: Understanding and Preparing for Them. The information below also details the role of inspectors when performing brake system checks so you can be prepared:
- Check for missing, non-functioning, loose, contaminated or cracked parts on the brake system.
- Check for S-cam flip-over.
- Listen for audible air leaks around brake components and lines.
- Check for improper connections and chafing of air hoses and tubing.
- Ensure slack adjusters are the same length (from center of S-cam to center of clevis pin) and the air chambers on each axle are the same size.
- Ensure the air system maintains proper air pressure.
- Look for non-manufactured holes (e.g., rust holes, holes created by rubbing or friction, etc.) and broken springs in the spring brake housing.
- Mark and measure pushrod travel.
- Inspect required brake system warning devices, such as anti-lock braking system malfunction lamp(s) and low air-pressure warning devices.
- Inspect the tractor protection system, including the bleed-back system on the trailer.
- Ensure the breakaway system is operable on the trailer.
Maintenance Tips for Automatic Slack Adjusters
Just because ASAs are automatic, does not mean that maintenance and care can be ignored. There are several things you can do to ensure they can do their job properly and keep you and others safe. Here are a few pointers:
Ensure regular lubrication
Lubrication of ASAs is one key to a long life for adjusters. If not greased regularly, contaminants from the roads and moisture can enter the units and cause deterioration and failure of internal components. During maintenance you must clean the purge slot in the boot and pump in new grease until the old grease is purged, leaving clean, uncontaminated lubricant in its place.
Be careful not to over grease as doing so could harm seals and allow contaminants to enter and cause unnecessary wear. Always heed manufacturer's specifications for mileage intervals and grade of lubricant to supply for your ASAs. Grease the S-cam spline and clevis pin to complete regular maintenance.
Invest in high quality units
When you must replace ASAs, always go for high quality units. Don't waste money on bargain priced parts as you get what you pay for. High quality ASAs will serve you better, be your safest option, and cost less in the long run if regularly maintained and not manually adjusted.
Brush up on the regulations
This GovRegs.com resource will provide you with Federal Regulations and guidelines regarding the safe and legal operation of a CMV.
Although it is an important part of your daily pre-trip inspection to check your braking system for audible leaks, broken, cracked or missing components as well as other items listed above under "Keeping a Close Eye on your ASAs" it is important to understand that you are not permitted to make repairs or completed an official inspection unless you meet the FMCSA guidelines for Inspector/repair listed in the two following resources:
Keep up with daily pre and post-trip inspections
The absolute best policy toward maintaining not only your brakes, but your entire unit is to complete thorough daily pre and post-trip inspections and have any and all defects corrected immediately. In addition, both the tractor and trailer should be inspected by a certified inspector with any noted defects completed by a certified technician immediately. Failure to do so will cost more in the long run and add unnecessary risk to you, to the company and to the motoring public.
Be smart, be safe, be courteous, and be successful.
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