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Resource Information for Auto Repair Projects:

Maintaining the Integrity of the Brake Lining Assembly

Brake safety is an issue that everyone is concerned about. Motorists want the assurance that the brakes on their vehicles are safe and dependable. Installers want to be sure the brake parts they install on their customers’ vehicles are well-built, reliable and trouble-free. And most brake suppliers want to offer products that are engineered to deliver the best possible value, performance and safety.

Most brake products today are safe, and many have been rigorously tested in the laboratory and in the field to make sure there are no surprises down the road. But normal wear and tear and time all take a toll on brake system components, and eventually some of these components have to be replaced.

Brake pads and shoes are both wear items that can only last so long. One concern with these type of products is how to maintain the integrity of the lining assembly for the service life of the linings. The friction lining on a disc brake pad is mounted on a steel backing plate. The plate provides stiffness and rigidity, and allows the caliper to squeeze the pads against the rotor.

One way manufacturers attach the pad to the steel backing plate is with rivets. For many years, this allowed re manufacturers to rebuild disc brake pads. But today, remain disc brake pads are not as common.

Though rivets have long been used to attach the pads, they do have some drawbacks. One is that using rivets requires punching holes in both the pads and backing plate. Over time, these holes can become a focus point for stress cracks that may cause the pads to flake apart or even separate from the backing plate.

Another limitation with using rivets to hold the pads in place is that over time rivets can loosen up. This can cause unwanted vibrations and noise.

The rivet heads are recessed in the pads, but their installed height still limits the amount of overall wear the pads can tolerate before replacement becomes necessary. Once the pads are worn down to the rivet heads, the rivets can score the rotors. And if the rivet heads are worn completely off, there’s not much left to hold the pads in place.

Another common method of attaching pads to their backing plates is using a high temperature adhesive. Bonding the pads to the backing plates eliminates the need to punch rivet holes, so theoretically the pads can tolerate more wear before replacement becomes necessary. Bonding also, in theory, reduces the risk of the pad coming loose because of rivets stretching or wearing off. But over time, the combination of corrosion and high temperatures may cause the adhesive to lose strength. If the adhesive fails, the pad will delaminate and separate from its backing plate. Fortunately, this doesn’t happen very often.

Molding pads is another method of mounting the friction material to the backing plate. Integrally molded pads are made by pressing, heating and curing the friction material right on the backing plate. To get the friction material to stick to the plate, a combination of adhesives and some type of mechanical lock is often used. One type of mechanical lock is to have holes in the backing plate so the friction material fills the holes when the pad is molded. This creates a mechanical link to the plate as well as a chemical bond.

The latest approach, though, is to eliminate the adhesive altogether and go with a purely mechanical locking system.

One such mechanical locking system for molded pads is the NRS system. It relies on dozens of tiny curved hooks that project from the surface of the backing plate. Some of the hooks face one way, and some the opposite way. The shape, distribution and angle of the hooks act almost like Velcro to provide maximum adhesion between the plate and friction material. Consequently, the strength of the bond becomes a function of the strength of the friction material itself.

The hooks only project about 1/16th of an inch above the surface of the plate, so the pads can wear quite a bit before the tops of the hooks are exposed. Even then, tests have shown that the hooks cause no damage to the rotor. Because of their small size and distribution, they act as if they are part of the friction material and actually help increase braking performance.

Many brake systems today are having to handle heavier loads and higher operating temperatures. Understanding how each brake pad is constructed allows technicians to make educated purchase decisions for their customers.

 

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