Drum Brakes

                                     DRUM BRAKE

                       

PRINCIPLE  OF OPERATION-

principle of operation of drum brake part-1
principle of operation of drum brake part-2

DRUM BRAKE COMPONENTS-

Modern drum brakes are sometimes called internal expanding brakes, since the brake components are internal (inside a drum), and the shoes must expand, or move out­ward, to contact the drum. Drum brake components vary only slightly between manufacturers. Most design differ­ences are in the methods of brake shoe anchoring and spring placement.

Backing Plate -

To provide a foundation for the drum brake COMPONENTS and to act as a splash shield against water and road debris, a backing plate (sometimes call ed a support plate) is used. The wheel cylinder,brake shoes, and springs are attached to the backing plate.The flat spots on the backing plate are cal led support pads. The wheel cylinder is usually solidly attached to the top of the backing plate. Most wheel cylinders are bolted to the backing plate.

Most backing plates are discs of stamped steel, bolted or riveted to the spindle assembly on front brakes and to the axle flange on rear brakes.A few backing plates are made of aluminum or plastic. These plates are used primarily as splash shields with the wheel cylinder and anchors mounted on a steel support plate.

ANCHORS-

Anchors, which are solid attaching points for the brake shoes and springs, are also installed on the backing 

plate. Anchors are often called anchor pins or anchor plates, depending on their construction. Some anchors support only the shoes, whiIe others are connecting points for the shoes and return springs.

Brake Shoe Assembly-

brake shoe assemblies consist of two major parts, the steel shoe, and the friction material lining. The steel brake shoe holds and supports the brake lining. Wheel cylinder movement is transmitted to the lining by the shoe. The two parts of a brake shoe are the table and web. They may be welded together or stamped from a single piece of steel. The table and web vary in thickness and the web mayor may not be reinforced.

Nibs and Support Pads-

Some shoes have raised spots on the table which con­tact raised spots on the backing plate when the shoe is 

installed. These are called nibs. The nibs and support pads provide a bearing surface for the shoe to slide on. Some shoes do not have nibs and the edge of the shoe table slides on the backing plate support pads.

Primary and Secondary Shoes- 

There are two types of brake shoes in every drum brake assembly. The leading or primary shoe faces the 

front of the vehicle, and the trailing or secondary shoe faces the rear of the vehicle. Since the secondary shoe per­forms more of the braking effort, the primary shoe is often smaller.

Linings -

The brake lining is the friction material which con­tacts the drum. Brake linings must be able to produce     fric­tion in order to properly stop the vehicle. Some of these factors are the ability to stop well in cold and wet weather, and resistance to compression and swelling when hot. The lining should not fade under normal braking conditions. 

Lining Size and Attachment -

Brake lining thickness and surface area varies with vehicle size and anticipated demands. Older vehicles with 

front drum brakes have linings with a large surface area for more braking power. These linings are relatively thin to reduce heat buildup. Smaller brake shoes are used on the rear. On modern front- and rear-wheel drive vehicles, the rear brake linings have a smaller surface area, since they carry a smaller portion of the braking load. These linings are relatively thick for longer wear. To perform their job, the brake linings must be securely attached to the shoe table. They must also be attached tightly enough to allow heat transfer between the lining and the shoe. The brake lining is attached to the shoe table in one of two ways.

1. Some linings are attached with rivets. The rivets are usually made of brass or copper to reduce drum damage .Some rivets are tubular, with a hole in the center. This allows brake dust to exit the pad and drum surface.

2.Other linings are attached by bonding. Bonding is a method of gluing the lining to the shoe table by pressure and heat. A high temperature cement is placed on the back of the lining. Then the lining is tightly held against the shoe table as heat is applied. This creates a tight bond, securely holding the two pieces together.

lining contact method-

Lining-to-drum contact is very important for proper brake function in any given design.The contact can be
designed by arc grinding the shoes. Arc grinding is actually a sanding process done at the factory which removes small amounts of the lining's top layer until it has the proper curve. so that at the time of braking the line is in full contact with the drum and The contour lining is thicker at the center. When the brakes are first applied and the lining moves into contact with the drum, the thicker center section contacts the drum first.
and there is a clearance of about 0.25 mm between lining and drum.