Drum Brakes

Drum Brakes Points : What is Drum Brakes, Backing Plate, Wheel Cylinder Assembly, Drum Brake Shoes, Retracting Spring, Hold Down Spring, Brake Shoe Adjusters, Brake Drums, Brake Shoe Energization Drum brakes use many of same principles already covered for disc brakes. However drum brakes have a large drum that surrounds the brake shoes and hydraulic wheel cylinder. A drum brake assembly, brake shoes and lining, retracting springs, hold down springs brake drum and automatic adjusting mechanism. Backing Plate The brake backing plate holds the shoes springs, wheel cylinder and other parts in side the brake drum. It also helps to keep away road dirt and water of the brakes. The backing plate holds to the axle housing or spindle support. Wheel Cylinder Assembly The wheel cylinder assemblies use master cylinder pressure to force the brake shoes out against the brake drums. They bolt to the top of the backing plates. A wheel cylinder consists of a cylinder or housing, expander spring, rubber cups, pistons, dust boots and a bleeder screw. The wheel cylinder housing from the enclosure for the other parts of the assembly. It has a precision hole or cylinder in it for the pistons, cups, and spring. The wheel cylinder boots keep road dirt and water out of the cylinder. They snap in to grooves on the outside of he housing. Seals that keep fluid from baking past the pistons. They fit in the cylinder and against the piston. The wheel cylinder pistons are metal or plastic plungers that transfer force out of the wheel cylinder assembly. They action push rods connected to the brake shoes or directly on the shoes.

Wheel cylinder spring helps hold the rubber cups against the when not pressurized. Sometimes, the end of this spring has metal expanders called cups expanders, they help press the outer edges of he cups against the wall of the wheel cylinder.

The bleed screw provides a means of removing air from the brake system it threads in to a hole in the back of wheel cylinder. When the screw is loosened, hydraulic pressure can be used to force the air and fluid out of the system. Drum Brake Shoes The brake shoes rub against the revolving brake drum to produce breaking action. Drum brake shoe assemblies are made by fastening asbestos lining (friction) material on to the metal shoes. The living may be held on the shoes by either rivets or a bonding agent (glue). Like disc brake pads the as besots lining serves as a head resistant surface that contacts the brake drum. The metal shoe supports and hold the soft being material. The primary brake shoe is the front shoe. It normally has a slightly shorter lining than the secondary shoe. The secondary shoe is the rear shoe. It has the largest lining surface area. Retracting Spring Retracting springs pull the brake shoe away from the brake drum when the brake pedal is released. They push the wheel cylinder. Pistons inward usually the retracting springs fit in holes in the shoes and around an anchor pin at the top of the backing plate. Hold Down Spring The springs hold the shoes against backing plate when the brakes are in the back of backing plate. A metal cup locks on to these pins to sense the hold down springs to the shoes. Other springs are used on the automatic adjust mechanism. Brake springs are high quality capable of with standing the high temperature encountered in side the brake drum. Brake Shoe Adjusters Brake shoe adjusters maintain correct drum to being clearance as the brake being wear. Many cars use a star wheel (screw type) type brake shoe adjusting mechanism. This type includes a star wheel (adjusting screw assembly) adjuster lever, adjuster spring and either an adjuster cable, lever arm, or link (rods) Automatic brake shoes normally function when the brakes are applied when the car moving in reverse.

If there is too much lining clearance, the brake shoes move out ward and rotate with the drum enough to operate the adjusting lever. This lengthens the star wheel assembly. The linings or moved closer to the brake drum maintaining the correct lining to drum clearance. Brake Drums Brake drums provide a rubbing surface for the brake shoe lining. The drum usually fits over the wheel by studs. A large hold in the middle of the drum canters the drum on the front hub or rear a x b flange the wheel and drum turn together as a unit. Brake Shoe Energization When the brake shoes are forced against the rotating drum, they are pulled away front then pivot point by friction. This movement called self energization action, draws the shoes tighter against eh drum. With most drum brake designs shoe energization is supplemented by servo action, and servo action results when the primary (front) shoe helps apply the secondary (rear) shoe.

The hacking plate anchor pin holds the secondary show during brake application. However the primary shoe is free to float out and push against the end of the secondary shoe through the star wheel assembly. This action presses the secondary shoe in to the drum with extra force.