Slack Adjusters
Slack Adjusters, also known as Automatic Brake Adjusters or Automatic Slack Adjusters, are ingenious devices that compensate for the wear of the friction material on the brake shoes. In the beginning were manual slack adjusters but they have evolved into reliable self adjusting adjusters which are mandatory on new vehicles. It has been said that because of improved automatic adjusters, the out-of-adjustment brakes found during roadside inspections have decreased by more than 4% per year for the last ten years. See this October 2008 article by Truck Parts & Service magazine which makes the point that the devices still must be periodically given a fresh supply of grease and inspected, and second, if you do find that the automatic adjuster is out of adjustment, it should NOT be adjusted but rather all other related foundaton brake components should be closely checked.
Two different approaches have been used for designing the automatic adjustments: stroke sensing and clearance sensing (now probably preferred due to its ability to more accurately adjust for lining wear). Due to space constraints, the shape of the slack adjustor must also sometimes be specialized for left or right handed application; otherwise a slack adjuster is "unhanded". Finally, different installation brackets are often required depending on the use of the device at a drive, steer, or trailer axle. For these reasons many part numbers are required. Plaza carries slack adjusters of all types as well as the camshafts that connect directly to the "slacks". Meritor, Haldex, and Gunite contribute to offer a comprehensive inventory.
Two different approaches have been used for designing the automatic adjustments: stroke sensing and clearance sensing (now probably preferred due to its ability to more accurately adjust for lining wear). Due to space constraints, the shape of the slack adjustor must also sometimes be specialized for left or right handed application; otherwise a slack adjuster is "unhanded". Finally, different installation brackets are often required depending on the use of the device at a drive, steer, or trailer axle. For these reasons many part numbers are required. Plaza carries slack adjusters of all types as well as the camshafts that connect directly to the "slacks". Meritor, Haldex, and Gunite contribute to offer a comprehensive inventory.
Meritor R806016 clearance sensing slack adjuster for Mack drive axle
Brake Adjustments: Common in Roadside Inspections
One of the CVSA criteria for roadside inspections is the proper adjustment of brakes. The current inspection guideline is to determine that the “applied stroke” (also referred to as “power stroke”) of a brake (at 90-100psi reservoir pressure) is within specified limits. For example, the limit for a standard type 30 brake chamber is 2”. The following is the procedure recommended by the Technology & Maintenance Council (TMC) for measuring the applied stroke.
1. With the brakes released measure from the brake chamber face to the center of the clevis pin. The push rod is fully retracted into the chamber.
2. Now make a series of brake applications to reduce pressure in the reservoir to 90-100 psi. (The assumption here is that a pressure higher than 100 psi leads to deformation and does not measure true push rod stroke.)
3. At this point make and hold a full brake application (with a board or a second set of hands). Now measure from the brake chamber face to the center of the clevis pin. The push rod is fully extended out of the chamber.
The difference between the brakes released (push rod fully retracted) and applied measurements (push rod fully extended) is the applied stroke or power stroke. If this stroke is less than the maximum specified for the chamber size, the brake adjustment is proper.
1. With the brakes released measure from the brake chamber face to the center of the clevis pin. The push rod is fully retracted into the chamber.
2. Now make a series of brake applications to reduce pressure in the reservoir to 90-100 psi. (The assumption here is that a pressure higher than 100 psi leads to deformation and does not measure true push rod stroke.)
3. At this point make and hold a full brake application (with a board or a second set of hands). Now measure from the brake chamber face to the center of the clevis pin. The push rod is fully extended out of the chamber.
The difference between the brakes released (push rod fully retracted) and applied measurements (push rod fully extended) is the applied stroke or power stroke. If this stroke is less than the maximum specified for the chamber size, the brake adjustment is proper.
Preventive Maintenance for Automatic Brake Adjustors
TMC’s recommended maintenance procedure states that “ a self-adjusting brake adjuster should not have to be manually adjusted except for initial installation and at brake reline”. They recommend instead that the above procedure for measuring the power stroke be performed routinely.. They have concluded that the failure of the test at the left may be the best sign of a significant problem with the foundation brakes. When the power stroke is found to exceed the limit, a second measurement of the “free stroke” should be performed as follows.
1. (With the brakes released) using a lever or “pry bar” extract the push rod and move the brake adjuster arm until the brake shoes just contact the drum. Then measure again the distance from the brake chamber face to the center of the clevis pin.
2. The difference between this new measurement (using only a lever) and the fully retracted measurement (brakes released) is the “free stroke” which should normally be in the range 3/8” – 5/8”.
If your measurement of the free stroke falls within the normal range, then the slack adjuster is probably operating as it was designed and there is another problem with the foundation brakes. See the attached Haldex document that shows the CVSA limits for the "applied stroke" and suggests several causes for the free stroke to fall either within or outside of its normal range.
1. (With the brakes released) using a lever or “pry bar” extract the push rod and move the brake adjuster arm until the brake shoes just contact the drum. Then measure again the distance from the brake chamber face to the center of the clevis pin.
2. The difference between this new measurement (using only a lever) and the fully retracted measurement (brakes released) is the “free stroke” which should normally be in the range 3/8” – 5/8”.
If your measurement of the free stroke falls within the normal range, then the slack adjuster is probably operating as it was designed and there is another problem with the foundation brakes. See the attached Haldex document that shows the CVSA limits for the "applied stroke" and suggests several causes for the free stroke to fall either within or outside of its normal range.
Haldex 40010144 clearance sensing slack adjuster for Mack drive axle
Haldex 40010212 slack adjuster for trailer axle
Meritor R810019 Clevis Kit
Meritor R806013 slack adjuster for trailer axle
Slack adjusters currently in stock
| PN | Spline | Lever | Features |
| MERITOR clearance sensing | |||
| R806016 | 1 1/2" -28 | 5 1/2" | For Mack drive axle |
| R806019 | 1 1/2" -28 | 5 1/2" | For Meritor, Eaton, Spicer trailer axles |
| R806013 | 1 1/2" -28 | 6" | For Meritor, Eaton, Spicer trailer axles |
| MERITOR stroke sensing | |||
| R801073 | 1 1/2" -28 | 5 1/2" | unhanded w. clevis fits chamber sizes 24,30 |
| R801079 | 1 1/2" -28 | 5 1/2" | unhanded w. clevis fits chamber size 20 |
| R801080 | 1 1/2" -28 | 5 1/2" | unhanded w. clevis, air disc brake, chamber 30 |
| R801074 | 1 1/2" -28 | 6" | unhanded w. clevis fits chamber size 30 |
| R801102 | 1 5/8" -37 | 6" | unhanded w. clevis for Fruehauf, chamber size 30 |
| MERITOR manual | |||
| R824003B10 | 1 1/2" -28 | 5,6,7" | straight clevis, economy |
| R824000B10 | 1 1/2" -10 | 5,6,7" | straight clevis, economy |
| R824004B10 | 1 1/2" -28 | 5.5, 6.5" | straight clevis, economy |
| R824007 | 1 5/8" -37 | 6" | straight clevis, economy |
| HALDEX clearance sensing | |||
| 40010212 | 1 1/2" -28 | 6" | for trailer axle |
| 40010144 | 1 1/2" -28 | 6" | for Mack drive axle |
| 40010141 | 1 1/2" -28 | 5.5, 6.5" | for drive axle |
| 40010156 | 1 1/2" -28 | 5 1/2" | For Kenworth with 8 Bag Air Ride, Drive Axle |
| 40020243 | 1 1/2" -28 | 6" | self adjusting S-ABA for Mack drive axle |
| GUNITE clearance sensing | |||
| AS1140 | 1 1/2" -28 | 5 1/2" | with clevis for steer and drive axles |
Example of Left and Right Handed Slacks
Example of Manual Slack Adjuster
