Manual Slack Adjusters
How it works - general principles:
In a S-cam type foundation brake, the final link between the pneumatic system and the foundation brake is the brake adjuster. The arm of the brake adjuster is fastened to the push rod of the chamber with a clevis and the spline end is installed on the brake camshaft. Primarily, the brake adjuster is a lever that converts the linear force of the air chamber push rod into a torque which turns the brake camshaft and applies the brake.
Two types of brake adjusters are in use; manual type brake adjusters, which periodically require a manual adjustment; and self-adjusting brake adjusters, which automatically adjust during normal service braking applications. All brake adjusters use the worm and gear principle and fundamentally differ only in their torque limit specification.
INSTALLATION AND REMOVAL
To avoid possible injury, proper precautions must be taken to prevent automatic actuation of the brake chambers while removing or installing slack adjusters. Always block the wheels or mechanically secure the vehicle. Spring brakes must be mechanically caged or released with air. All the brakes should be released.
A. Manual Brake Adjuster Removal
1. Remove the brake chamber push rod clevis pin.
2. Remove the retaining mechanism from the end of the brake camshaft.
3. Rotate the adjusting hex to back the brake adjuster out of the clevis.
4. Remove the brake adjuster from the spline end of the brake cam shaft.
B. Manual Brake Adjuster Installation
Install the brake adjuster on the cam shaft so that the adjustment hex and grease fitting (if so equipped) are accessible for servicing.
Align the brake adjuster arm with centre of the push rod clevis. Install the clevis pin and secure it with a new cotter pin.
Check and make sure the angle formed by the brake adjuster arm and the brake chamber push rod is greater than 90° when the brake adjuster is in the released position.
Install the brake adjuster retaining mechanism at the end of the brake camshaft and make sure to shim it to less Tan 0.060 inch of end play.
Tighten the jam nut on the push-rod-to-clevis attachment (1/2-20 300-400 in.lbs.5/8-18-400 in.lbs)
After installation, make sure that there is adequate clearance in both the fully applied and fully released positions. Check to ensure that all the brake adjusters rotate freely and without binding. Adjust the brakes by following the procedure in the next section entitled Brake Adjustment Procedure.
NOTE: All adjustments should be made with cold brake drums and brakes fully released.
BRAKE ADJUSTMENT PROCEDURE
A. Manual Slack Adjuster
Brake adjusters with locking collar (positive lock type) - Jack up the vehicle. Thoroughly clean the adjusting hex and locking sleeve area. Position a wrench or socket over the adjusting hex and disengage the locking sleeve by depressing it. With the locking sleeve fully depressed, adjust the brakes while rotating the tire and wheel. Use the wrench or society to turn the adjusting hex until the shoes contact the drum. Then back off the adjusting hex until the tyre and wheel turn freely. The actuator should be as short as possible without the brakes dragging.
If the vehicle cannot be jacked up, thoroughly clean up the adjusting hex and locking sleeve area. Position a wrench or socket over the adjusting hex and disengage the locking sleeve by depressing it. With the locking sleeve fully depressed, use the wrench or socket to turn the adjusting hex until it will not go further indicating that either the shoes have contacted the drum or the adjusting hex has been turned in the wrong direction. Pull on the brake adjuster to make sure it will not move. If there is movement, adjustment was made in the wrong direction and the adjusting hex must be turned in the opposite direction until it will not go no further. After establishing solid shoe-to-drum contact, back off the adjusting hex 1/4 turn for worn linings and 1/2 turn when relining brakes.
The actuator stroke should be as short as possible without brakes dragging. Measure the chamber power stroke at 80-90 psi as described in subsection “B”, “Self-Adjusting Brake Adjuster Brake Adjustment Procedure”, below. Take a free stoke measurement as outlined in the section entitled Failure Analysis. Ensure there is at least 3/8 ” of free stroke. Free stroke less than 3/8” can cause brake drag. If you can’t maintain maximum legal stroke and the free stroke is less than 3/8”, contact the brake manufacturer for foundation or brake geometry problems.
When the manual brake adjuster brake adjustment is completed, the adjusting hex should be positioned so that the locking sleeve engages it, thus locking it in place. If the locking sleeve does not engage the adjusting hex, the brake adjuster can back itself off.
Brake adjuster with lock screw ball Indent type lock mechanism-Back off (turn counter clockwise) the worm shaft lock screw (if applicable). Make the necessary adjustment by turning the adjusting hex as described in item number 1 of this section. Following brake adjustment, make sure that the lock ball or plunger engages the worm shaft indent. Without such engagement, the slack adjuster can back itself off.
Roadside Brake Adjustment
If the driver has to adjust brakes on the road, the following procedure is recommended: If the vehicle is equipped with a self-adjusting brake adjuster, use pry bar to put on the brake adjuster. If the movement is more than 5/8”, a manual adjustment should be made following the same procedure described below for a manual brake adjuster. If the self-adjusting brake adjuster is equipped with a pawl, remove the pawl for the brake adjustment and then properly reinstall the pawl. If the self-adjusting brake adjuster needs adjustment, inform maintenance personnel.
1. Block the wheels or mechanically secure the vehicle. On the brakes to be adjusted, spring brakes must be mechanically caged or released with air.
2. Rotate the adjusting mechanism until the brake shoes contact the drums. Using a pry bar, pull on the brake adjuster by hand to make sure it will not move. If there is a movement, adjustment was made in the wrong direction and the adjusting mechanism must be turned in the opposite direction. Tap the brake with a wrench; you should hear a dull clunk indicating the brake linings are tight against the drum.
3. Back of the brake adjuster a small amount at a time, while tapping on the brake drum with a wrench in between adjustments. Stop backing of the adjuster when you hear a clear ringing sound from the brake drum when tapped with a wrench.
4. Using a pry bar, pull on the slack adjuster by hand. If movement is more than 5/8", adjustment was not done properly or here is a problem with foundation brake.
Brake Adjuster Replacement (Slack Adjuster)
When replacing a brake adjuster, it is recommended that the replacement be of the same size as the original equipment.
Arm length (centre of spline to centre of arm hole to be used)Type, width, number, and diameter of splines. Clevis pin diameter (do not drive out bushing to accommodate a larger clevis pin. Brake chamber push rod size (5/8” or 1/2”)
If offset configuration, determine the offset dimension (right or left side)
Manual Brake Adjuster Failure Analysis
Manual slack adjuster should be inspected for gear set wear. To do this, back off the adjusting hex until all spring pressure is relieved from the clevis. Work the adjusting nut 1/4 turn back and forth while watching for cam rotating. If you have 1/8 to 1/4 turn of play without the cam rotating, the manual brake should be replaced. Repeat this procedure every 1/4 turn of the adjusting nut to check the whole gear set.
Self-Setting Brake Adjuster Failure Analysis
If the power stroke is at or more than the maximum stroke, measure free stroke to determine if the slack adjuster is operational.
Every month 8,000 kms, or 300 operating hours, check brake chamber push rod travel; stroke should be as short as possible without the brakes dragging or the push rod binding. Adjust manual slacks if necessary. Due to the different operating conditions, adjustments may be necessary at earlier intervals.
Every 6 months, 50,000 kms, or 1,800 operating hours, lubricate all slack adjusters and clevis pins with manufacturer's recommended lubricant. Check for worn clevises, clevis pins, clevis pin bushings and control arm/bracket wear. Failure to replace worn components will increase chamber stroke. Lubrication and inspection may be necessary at earlier intervals due to different operating conditions.