Properly adjusted air brakes on semi-trailers are critical for safety, preventing accidents, and ensuring compliance with federal regulations. Effectively adjusting trailer air brakes involves understanding the brake system components, applying the correct adjustment procedures, and verifying the adjustment using the pushrod stroke method.
Understanding the Importance of Properly Adjusted Air Brakes
The braking system on a semi-trailer is a complex interplay of pneumatic and mechanical components. When everything is functioning correctly, it allows for safe and controlled stopping, even under heavy loads. However, when the air brakes are out of adjustment, several dangerous situations can arise:
- Reduced Braking Power: Out-of-adjustment brakes require longer stopping distances, increasing the risk of collisions, especially in emergency situations.
- Uneven Braking: Uneven brake application can cause the trailer to pull to one side during braking, leading to loss of control.
- Premature Brake Wear: Over-adjusted brakes can cause excessive heat buildup, leading to premature wear of brake linings, drums, and other components. Under-adjusted brakes require more force to achieve the same stopping power, also accelerating wear.
- Out-of-Service Violations: Federal Motor Carrier Safety Administration (FMCSA) regulations mandate properly adjusted air brakes. Violations can result in costly fines and negatively impact a carrier’s safety rating.
- Increased Operating Costs: Inefficient braking leads to higher fuel consumption and increased maintenance expenses.
Step-by-Step Guide to Adjusting Semi-Trailer Air Brakes
This section outlines the standard procedure for adjusting S-cam air brakes, the most common type found on semi-trailers. Always refer to the specific manufacturer’s recommendations for your trailer’s braking system.
Required Tools and Materials:
- Brake Adjustment Tool (Slack Adjuster Wrench): Designed to fit the specific slack adjuster.
- Wheel Chocks: For safety while working on the brakes.
- Safety Glasses: To protect your eyes from debris.
- Gloves: To protect your hands.
- Measuring Tape or Stroke Indicator: To measure pushrod stroke.
- Pen and Paper: To record adjustments.
- Service Manual (Optional): For specific brake system details.
Procedure:
- Safety First: Chock the wheels on both sides of the trailer to prevent movement. Release the parking brakes.
- Inspect the Brake Components: Before adjusting, visually inspect all brake components, including the brake linings, drums, air chambers, slack adjusters, and pushrods. Replace any worn or damaged parts.
- Locate the Slack Adjuster: The slack adjuster is located on the brake chamber and connected to the S-cam.
- Identify the Adjustment Mechanism: Most slack adjusters have an external adjustment nut or bolt.
- Adjust the Brakes:
- Using the brake adjustment tool, rotate the adjustment mechanism to move the pushrod.
- Tighten the adjustment until the brake shoes contact the brake drum. You should feel resistance when turning the wheel by hand.
- Loosen the adjustment until the wheel turns freely, but with minimal drag.
- Measure the Pushrod Stroke: With the air system charged to its normal operating pressure (around 100-120 PSI), fully apply the brakes.
- Measure the distance the pushrod travels from its released position to its fully applied position. This is the pushrod stroke.
- Verify the Adjustment: Compare the measured pushrod stroke to the maximum allowable stroke specified by the brake manufacturer or FMCSA regulations (this information is often found on a sticker near the air chamber). If the stroke exceeds the maximum, the brakes are out of adjustment.
- Repeat on All Brakes: Repeat the adjustment procedure on all brakes on the trailer.
- Final Inspection: After adjusting all brakes, re-inspect all components to ensure proper function and tightness.
- Road Test: Perform a road test to verify that the brakes are functioning properly and evenly.
Understanding Automatic Slack Adjusters (ASAs)
Many modern semi-trailers are equipped with Automatic Slack Adjusters (ASAs) designed to maintain proper brake adjustment automatically. While ASAs reduce the need for manual adjustments, they still require regular inspection.
ASA Inspection:
- Visual Inspection: Check for leaks, damage, or excessive wear.
- Stroke Test: Perform the pushrod stroke test as described above. If the stroke exceeds the maximum allowable, the ASA may be malfunctioning and require repair or replacement.
- Functionality Test: After a full brake application, the ASA should automatically adjust the brake shoes to maintain the correct clearance.
ASA Troubleshooting:
- Excessive Stroke: Indicates a potential malfunction of the ASA or excessive wear of brake linings.
- Failure to Adjust: Could be due to a clogged air line, a faulty ASA mechanism, or a damaged brake component.
Frequently Asked Questions (FAQs)
Q1: What is the maximum allowable pushrod stroke for a Type 30 air chamber?
The maximum allowable pushrod stroke for a Type 30 air chamber typically ranges from 2 inches to 2.5 inches, depending on the manufacturer and specific application. Always consult the brake manufacturer’s specifications or the FMCSA regulations for the exact value.
Q2: How often should I adjust my semi-trailer air brakes?
Air brakes should be inspected and adjusted (if necessary) during every pre-trip inspection, as well as during routine maintenance intervals. Regularly checking and maintaining your brakes is crucial for safety.
Q3: What causes air brakes to go out of adjustment?
Several factors can cause air brakes to go out of adjustment, including brake lining wear, drum wear, worn S-cam bushings, and faulty slack adjusters.
Q4: Can I adjust my air brakes myself, or do I need a professional?
While some drivers may have the skills and knowledge to adjust air brakes properly, it is generally recommended to have a qualified mechanic perform this task. Improper adjustment can lead to serious safety consequences.
Q5: What is the difference between a manual and an automatic slack adjuster?
Manual slack adjusters require periodic manual adjustment to compensate for brake lining wear. Automatic slack adjusters are designed to automatically maintain proper brake adjustment, reducing the need for manual intervention.
Q6: What should I do if I suspect my automatic slack adjusters are not working correctly?
If you suspect your automatic slack adjusters are not working correctly, take your trailer to a qualified mechanic for inspection and repair. Do not attempt to repair ASAs yourself unless you are properly trained and equipped.
Q7: What are the penalties for having out-of-adjustment air brakes?
Having out-of-adjustment air brakes can result in fines, out-of-service orders, and negatively impact a carrier’s safety rating.
Q8: Where can I find the pushrod stroke measurement guide for my trailer?
The pushrod stroke measurement guide is often located on a sticker near the air chamber, in the brake manufacturer’s specifications, or in the trailer’s service manual.
Q9: What are the signs that my brake linings need to be replaced?
Signs that your brake linings need to be replaced include excessive pushrod stroke, audible squealing or grinding noises during braking, and visible cracking or crumbling of the brake linings.
Q10: What is the role of the S-cam in the air brake system?
The S-cam is a rotating cam that converts the force from the air chamber into mechanical force to push the brake shoes against the brake drum.
Q11: Can I use any type of grease on the slack adjuster?
No, you should only use high-temperature brake grease specifically designed for slack adjusters. Using the wrong type of grease can damage the adjuster.
Q12: How do I bleed the air from the air brake system after making adjustments?
Bleeding the air from the system after making adjustments is usually not necessary for simple slack adjuster adjustments. However, if other repairs have been made that introduced air into the system, refer to the trailer’s service manual for the proper bleeding procedure. Improper bleeding can negatively impact braking performance.