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As you may remember, drum brakes are a type of brake in which friction is generated by brake shoes rubbing against the inside of a drum. There are advantages and disadvantages to using drum brakes in a vehicle. Drum brakes can easily incorporate a parking brake mechanism into the drum brake assembly to hold a vehicle when it is parked. Drum brakes also have lower noise levels and are self-energizing. Self-energizing means that when applying the drum brakes, one brake shoe helps to apply the other brake shoe. Drum brakes, however, have less resistance to brake fade, which is the loss of friction between the brake shoes and the drum. Drum brakes do have the tendency to pull and grab. They dissipate heat poorly and require a special linkage for self-adjustment.


Return Springs
The return springs hold the brake shoes against the anchor pin and away from the drum until the brakes are applied. There are two return springs, one for each brake shoe. The springs have ends in the shape of a hook. One end of the spring is placed into a hole drilled into the web of the shoe, while the other end is placed on the anchor pin.

Wheel Cylinder
The wheel cylinder applies the brake shoes by converting hydraulic pressure from the master cylinder to mechanical force. Inside the wheel cylinder, hydraulic fluid pushes on the cup seals. The cup seals push against the pistons to apply the brake shoes. A spring is located between both cup seals. The spring maintains tension between the cup seals, the pistons, and the brake shoes when the brakes are not applied. The mechanical force from the pistons pushes on the upper portion of the brake shoes to push the brake linings against the drum surface.

Primary Shoe
The primary shoe is the first shoe to come in contact with the drum during forward braking. The lining of the primary shoe is typically shorter in length than the secondary shoe, because the secondary shoe provides a greater percentage of the total braking power in a duo-servo brake. The brake shoes have three parts: web, table, and lining. The web has the anchor and adjusting ends and supports the table. The table and web are welded together. Once the table and web are welded together, it is common to refer to them as the frame. The brake lining is attached to the frame by bonding or riveting.

Connecting Spring
The connecting spring holds the primary and secondary brake shoes against the adjusting screw. The connecting spring has hooks at both ends, which are placed into a slot at the bottom of each brake shoe. The connecting spring is typically small in diameter compared to other springs in the drum brake.

Brake Adjuster Assembly
The task of the brake adjuster assembly is to ensure correct clearance between the brake shoe linings and the drum. The major components of a typical brake adjuster assembly are a star-wheel adjuster screw, adjusting cable or rod, adjusting lever, and a lever return spring. Because the star-wheel adjuster screw is not attached to the backing plate, it transfers the servo action from one shoe to the other during braking. Typically, the adjusting lever turns the star-wheel adjuster screw when you apply the brakes with the vehicle moving in reverse. The turning action of the star-wheel adjuster screw causes the brake shoes to move closer to the drum, achieving the correct clearance while holding the bottom of the brake shoes apart.

Retaining Springs
The retaining springs hold the shoes to the backing plate, while allowing for some flexibility when the brake shoes move during braking. They also reduce noise and vibration in the drum brake.

Secondary Shoe
The secondary shoe’s construction is the same as the primary shoe, with the exception of the brake lining. The lining of the secondary shoe is longer than the primary shoe. The bottom of the secondary shoe is forced against the drum brake surface during braking.

Parking Brake
You apply the parking brake to hold the vehicle in a stopped position. You engage the parking brake by using a hand lever or a foot pedal. Pulling on the hand lever or depressing the foot pedal moves the cable, which moves the parking brake lever. The parking brake lever moves away from the shoe it is attached to, causing the strut to push against the forward brake shoe. Once the brake shoe makes contact with the drum, the strut can’t move forward any further and then acts as a pivot to move the rear brake shoe toward the brake drum.

Anchor Pin
The anchor pin is attached to the backing plate and prevents the shoes from rotating with the drum. The shoes rest on the anchor pin while the brakes are not in use. The secondary brake shoe pushes on the anchor pin when the brakes are applied.

Return Springs
The return springs hold the brake shoes against the anchor pin and away from the drum until the brakes are applied. There are two return springs, one for each brake shoe. The springs have ends in the shape of a hook. One end of the spring is placed into a hole drilled into the web of the shoe, while the other end is placed on the anchor pin.

Wheel Cylinder
The wheel cylinder applies the brake shoes by converting hydraulic pressure from the master cylinder to mechanical force. Inside the wheel cylinder, hydraulic fluid pushes on the cup seals. The cup seals push against the pistons to apply the brake shoes. A spring is located between both cup seals. The spring maintains tension between the cup seals, the pistons, and the brake shoes when the brakes are not applied. The mechanical force from the pistons pushes on the upper portion of the brake shoes to push the brake linings against the drum surface.

Primary Shoe
The primary shoe is the first shoe to come in contact with the drum during forward braking. The lining of the primary shoe is typically shorter in length than the secondary shoe, because the secondary shoe provides a greater percentage of the total braking power in a duo-servo brake. The brake shoes have three parts: web, table, and lining. The web has the anchor and adjusting ends and supports the table. The table and web are welded together. Once the table and web are welded together, it is common to refer to them as the frame. The brake lining is attached to the frame by bonding or riveting.

Connecting Spring
The connecting spring holds the primary and secondary brake shoes against the adjusting screw. The connecting spring has hooks at both ends, which are placed into a slot at the bottom of each brake shoe. The connecting spring is typically small in diameter compared to other springs in the drum brake.

Brake Adjuster Assembly
The task of the brake adjuster assembly is to ensure correct clearance between the brake shoe linings and the drum. The major components of a typical brake adjuster assembly are a star-wheel adjuster screw, adjusting cable or rod, adjusting lever, and a lever return spring. Because the star-wheel adjuster screw is not attached to the backing plate, it transfers the servo action from one shoe to the other during braking. Typically, the adjusting lever turns the star-wheel adjuster screw when you apply the brakes with the vehicle moving in reverse. The turning action of the star-wheel adjuster screw causes the brake shoes to move closer to the drum, achieving the correct clearance while holding the bottom of the brake shoes apart.

Retaining Springs
The retaining springs hold the shoes to the backing plate, while allowing for some flexibility when the brake shoes move during braking. They also reduce noise and vibration in the drum brake.

Secondary Shoe
The secondary shoe’s construction is the same as the primary shoe, with the exception of the brake lining. The lining of the secondary shoe is longer than the primary shoe. The bottom of the secondary shoe is forced against the drum brake surface during braking.

Parking Brake
You apply the parking brake to hold the vehicle in a stopped position. You engage the parking brake by using a hand lever or a foot pedal. Pulling on the hand lever or depressing the foot pedal moves the cable, which moves the parking brake lever. The parking brake lever moves away from the shoe it is attached to, causing the strut to push against the forward brake shoe. Once the brake shoe makes contact with the drum, the strut can’t move forward any further and then acts as a pivot to move the rear brake shoe toward the brake drum.

Anchor Pin
The anchor pin is attached to the backing plate and prevents the shoes from rotating with the drum. The shoes rest on the anchor pin while the brakes are not in use. The secondary brake shoe pushes on the anchor pin when the brakes are applied.

     
 
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