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Slack adjusters are mechanical links between the brake-chamber push rod and the camshaft on cam type brakes. Slack adjusters are not used with wedge-type brakes.
Slack adjusters are used to manually (Fig. 51) or automatically (Fig. 53) maintain proper brake chamber stroke and lining-to-drum clearance during normal operation.
Slack adjusters are available in a variety of arm configurations, lengths, torque ratings and spline types.
The entire slack adjuster operates as a unit, rotating with the brake camshaft as brakes are applied or released. The most efficient braking occurs when push rod travel is held to a minimum, therefore it is important that brake adjustments are made often.
When you sign the trip inspection form, you need to check the brakes to make sure they are in adjustment. This means marking and measuring the brake stroke which is part of the trip inspection. While there are some short-cut methods to determine brake adjustment but they will not be accurate enough to pass a DOT inspection.
Some say if the slack adjuster is at no more than a 90-degree angle to the push rod when the spring brakes are applied, the adjustment is good. At the same time, many believe that if, when using brake stroke indicators, the pointer is inside the markers when the spring brakes are applied than the adjustment is good. Unfortunately, neither is true, and neither way will measure the brake stroke accurately.
Vehicles made after 1994 have automatic slack adjusters that adjust themselves during full brake applications and should only require manual adjustments during installation. However, all brake systems, whether manual or automatic slack adjusters, should receive regular maintenance checkups.
It adjusts and takes up the slack in the stroke as the friction surfaces wear away. In a “normal” U.S. highway environment a manual slack adjuster would probably need adjustment approximately every 10,000 miles. But since there are no such things as normal truck brakes, slack adjusters are designed to be adjusted. You cannot go by the mileage interval, like an oil change, you have to make the adjustment when it’s required.
You can release the spring brakes and build the system pressure to 100 psi, make a full pressure application and note the pressure drop. According to some a 2-psi per axle drop in system pressure, or 10-psi on a 5-axle unit, is acceptable. The problem is, how do you know that two brakes are adjusted well below the limit and three are over-stroking by an eighth of an inch? You cannot tell, but an inspector can.
Slack Adjuster Preventative Maintenance and Inspection
When the brake lining or brake pad wears, the gap between it and the brake drum increases. Automatic slack adjusters have an adjusting mechanism that adjusts for brake lining wear. However, manual slack adjuster periodically needs to be adjusted to properly function and operate at peak performance and compensate for lining wear. At Wheelco, we suggest adjusting your manual slack adjuster approximately every 10,000 miles. However, this is just a general guideline since truck braking systems vary. When in doubt, call our service technicians for further assistance.
After 1994, all vehicles made have automatic slack adjusters installed. However, if you are driving a vehicle that predates this regulation, manual adjusters require fine-tuning. Read more about Brake FMSCA Regulations.
Be safe and check the slack adjuster movement. Often, improper adjustment is due to operator negligence. Should you require any additional information, contact Wheelco customer service to assist you.
Automatic Slack Adjusters operate in the same manner that manual slack adjuster does but with one key difference. They automatically adjust for the gradual brake lining wear. Therefore, it does not require periodic manual adjustment and is highly recommended not to adjust unless absolutely necessary or in an emergency situation.
Automatic slack adjusters are fast becoming a more reliable and consistent operational part, which is why Wheelco recommends using this type of slack adjusters. There are two types of slack adjusters: the stroke and clearance adjuster.
The stroke adjuster measures the stroke of the pushrod. It keeps the stroke constant, no matter if you brake hard or soft.
The clearance adjuster measures the clearance between the drum and brake lining and keeps the wear consistent (insufficient or excessive clearance between the brake lining and the drum). Once the lining meets the drum the adjusting mechanism disengages. This means that the brake adjusters’ automatic adjustment is not affected by the driver’s braking method (hard or soft).
Depending on your specific needs we offer several braking solutions.
FAQ
What are slack adjusters CDL quizlet?
What are slack adjusters on a vehicle?
What are 2 Functions of slack adjusters?
- To convert the forward linear motion of the push rod into rotary motion of the camshaft. …
- To act as a lever, thus generating greater force;
- To provide a means of reducing the clearance between the brake shoes and the brake drum.
