Drum Brake Systems Brake Shoe Energization When the
Drum Brake Systems
Brake Shoe Energization • When the brake shoes are forced against the rotating drum, they are pulled away from their pivot point by friction • This self-energizing action draws the shoes tighter against the drum • Servo action results when the primary shoe helps apply the secondary shoe
Servo • Anchor at top acts as a pivot • Shoes linked by adjuster • Primary shoe forced against secondary shoe
Non-Servo • Anchor prevents energization • Most of wear is in top half of shoes • Typically smaller vehicles
Which style increases braking force? 1. Servo 2. Non-servo
Type of brake? 1. Servo 2. Non-Servo
Backing Plate • Holds the springs, wheel cylinder, and other parts inside the drum • Helps keep road dirt and water off the brakes • Bolts to the axle housing or the spindle support
Drum Brake Components • Backing plate – Holds the drum components
Backing Plate
Wheel Cylinder Assembly Uses master cylinder pressure to force the brake shoes out against the drum
Wheel Cylinder Components • Boots – keep road dirt and water out of the cylinder • Pistons – metal or plastic plungers that transfer force out of the cylinder to the brake shoes • Cups – rubber seals that keep fluid from leaking past the pistons
Wheel Cylinder Components • Springs – hold the rubber cups against the pistons when the wheel cylinder assembly is not pressurized – metal expanders may be used on the ends • Bleeder screw – provides a means of removing air from the brake system
Purpose of the spring? 1. 2. 3. 4. 5. 6. 7. 8. Retract pistons Retract shoes Retract cups Keep cups out against pistons Keep pistons from retracting too far Maintain correct pedal travel 1– 3 4– 6
Wheel Cylinder Inspection
Brake Shoes • Rub against the revolving brake drum to produce braking action • Made by fastening organic friction material onto a metal shoe – rivets or bonding agents may be used • The front shoe is the primary shoe – uses the shortest lining • The rear shoe is the secondary shoe – uses the largest lining surface area
All brake shoes are glued on? 1. Yes 2. No
Brake Springs • Retracting springs – pull the brake shoes away from the brake drums when the brake pedal is released • Hold-down springs – hold the brake shoes against the backing plate when the brakes are in the released position
Different size brakes will have different size springs? 1. True 2. False
Brake Shoe Adjusters • Maintain the correct drum-to-lining clearance as the brake linings wear • Automatic adjusters normally function when the brakes are applied with the vehicle moving in reverse • If there is too much lining clearance, the brake shoes move outward and rotate with the drum enough to operate the adjuster lever
Adjuster Screw Parts
Brake Shoe Adjusters Cable-type star wheel adjuster
Brake Shoe Adjusters Lever-type star wheel adjuster
Service Precautions • Allow the brakes to cool before servicing them • Familiarize yourself with the brake system before attempting service • Follow the manufacturer’s service procedures
Service Precautions • Do not reuse damaged parts • Do not reuse drums that are damaged or beyond service limits
Drum Brake Service Tools • Vacuum enclosure or wet sink – Used to clean brake dust from brake components
Clean Dust – DO NOT BLOW IT OFF!!!!!!
Approved Method
Remove As Much As Possible
Vacuum Enclosure
Tools • • Spoon – adjusting Clamp – holds pistons Hold down tool Return spring tools
Lining Minimum Thickness
Lining Thickness Measurement
Lining Thickness Gauges
Adjustment
Drum Brake Service Tools • Drum brake micrometer – Used to measure drum diameter and check for out of round
Drum Measurement 1. Determine the maximum drum diameter and the machine-to limit
Drum Measurement 2. Set the bar of the micrometer to the nominal diameter 3. Place the micrometer into the drum and move until the highest reading on the dial is obtained
Drum Measurement 4. Add the gauge reading to the reading on the bar 5. Repeat the procedure at ninety degree points around the drum
Brake Drum Measurement Click on image above to view animation
Drum Defects
Drum Wear Patterns
Drum Brake Concerns • Noise – Can be caused by: • Brake dust accumulation • Worn linings • Dry backing plate studs • Broken parts
Drum Brake Concerns • Grabbing – Can be caused by: • Overly tight shoes • Fluid contamination on the linings
Drum Brake Concerns • Parking brake does not hold – Can be caused by: • Improperly adjusted shoes • Stuck or binding lever or cables
Drum Brake Concerns • Pulsation – Commonly caused by an out of round brake drum
Drum Brake Concerns • Low, soft, or spongy brake pedal – Usually caused by a leak in the hydraulic system
Wheel Cylinder Inspection
Leaking Wheel Cylinder or Axle Seal?
Drum in Hat
Drum in Hat
- Slides: 60