Getting to Know Your Brakes

getting to know your brakes

Trucks, trailers and buses demand a lot from their brake components as they have to perform in the toughest of applications and it is important for a truck workshop to work hard on getting to know your brakes. 

Meritor is one of the few foundation brake manufacturers that can offer a complete portfolio of air disc, hydraulic disc and air drum brakes. One of the most popular brake set-ups in Australia is the Meritor Q-Plus brake. Here are some tips on how to get the best out of this braking system.

The Q-Plus shoe is designed to contact the centre of the shoe at the new lining condition. This designed “crown” is necessary to control brake torque. Due to tolerance stack ups, the crown is designed to contact the drum first. 

The crown is necessary to control the brake torque caused by the moment between anchor pin, point of lining contact and the cam roller. This condition will exist until the linings wear to the drum’s inside diameter over several brake applications. 

This is often called “bedding”, “burnishing” or “green lining condition”. Once the bedding is complete, a full contact will exist between the lining and the drum.

Without the crown, the brake torque output will vary depending on the point of contact. Varying initial contact may cause vehicle instability, which is highly undesirable. 

There are various versions of foundation brakes and shoe assemblies available on the market today.  Some assemblies will grind the profile to tighter tolerances, thus reducing the amount of cam and anchor end clearance compared to the centre. Grinding, of course, comes at a cost to the user. Even the high-tolerance grinds will have a crown. The bedding essentially takes less time. 

getting to know your brakes

Brake Maintenance and Diagnostics

The point at which the brakes need to be relined is when the lining thickness reaches 0.25 inch (6.3mm) at the thinnest point. To avoid damage to the components, rivets or bolts must not touch the drum. 

Meritor recommends that you replace the springs, rollers, camshaft bushings and anchor pins at each reline. Check the drum and perform major inspection when you reline the brakes.

Check the up-and-down and side-to-side end play of the camshaft to determine if you must replace the camshaft bushings. If the total movement is more than 0.03 inch (0.76 mm), replace the bushings.

If axial end play exceeds 0.06 inch (1.52mm), remove the snap ring and add an appropriate number of spacing washers between the slack adjuster and the snap ring. Correct specification is 0.005 – 0.06 inch (0.127 – 1.52mm). 

Brake force imbalance will result in premature lining and drum wear from the brakes being over-worked. Brake imbalance can be defined as each brake in a vehicle or a combination vehicle not doing its equal share of the braking.

Brake imbalance can be caused by pneumatic imbalance, well-adjusted and poorly-adjusted brakes, incorrect lining mix for the application, using the trailer brakes to stop the combination vehicle, worn foundation components, etc.

A defective brake, (out-of-service) is defined as brake with one or more of the following conditions:

  • Lining cracks or voids over 1/16 inch (2mm) in width observable at the lining edge.
  • Cracks that exceed 1.5 inch (38mm) in length.
  • Missing portions of a lining segment such that a fastener is exposed when viewed from the lining edge.
  • Cracks extending across the lining face through the lining edges.
  • Loose lining segments.
  • Complete lining segment missing.getting to know your brakes