AUTOMOTIVE BRAKES
Braking system is the most important thing in the vehicle system. It is operate as a safety for vehicle. The braking system must to function properly to make sure that the vehicle is safe to drive. There were two type of braking system in the vehicle, disc brake and drum brake. Normally, the
service brakes, operated by a foot pedal, which slow or stop the vehicle while
parking brakes, operated by a foot pedal or hand lever, which hold the vehicle stationary when applied. Most automotive services brakes are hydraulic brakes. They operate hydraulically by pressure applied through a liquid. The service or foundation brakes on many medium and heavy duty trucks and buses are operated by air pressure (pneumatic). These are air brakes. Many boat and camping trailers have electric brakes. All these braking systems depend on friction between moving parts and stationary parts for their stopping force.
Friction In Vehicle Brakes
When the brakes are applied by pushing down on the brake pedal, a fluid flows through tubes or brake lines to the brake mechanisms at the wheel. The mechanisms apply force on rotating parts so the wheels are slowed and stopped. There are two types of wheel-brake mechanisms, drum and disc. In the drum brake, fluid pressure pushes lined brakes shoes against a rotating drum. In the disc brake, the fluid pushes lined brake pads against a rotating disc.
Friction between the stationary shoes or pads and the rotating drum or disc produces the braking action that slows or stops the wheels. Then friction between the tires and road slows and stops the vehicle. If the brakes are applied so hard that the wheels lock, the friction between the tires and road is kinetic friction. If the brakes are applied less hard, the wheels continue to rotate. The resulting friction is static friction. since static friction is greater than kinetic friction, the vehicle stops in a shorter distance if the wheels do not lock. However, the brakes must be applied to the point at which the wheels are almost ready to lock.
HYDRAULICS
Hydraulics is the use of a liquid under pressure to transfer force or motion, or to increase an applied force. Pressure on a liquid is called hydraulic pressure. It is hydraulic pressure that force the brake shoes or pads into contact with the rotating drum or disc to produce braking.
Incompressibility of Liquid
Increasing the pressure on a gas will compress it into a smaller volume. However, increasing the pressure on a liquid will not compress it. The liquid is incompressible. This makes it possible to use the pressure on liquids in hydraulic system to transmit force or motion.
Transmission of Motion by a Liquid
When the input or apply piston is pushed is 8 inches (203 mm), the output piston will be pushed the same distance. Motion can be sent from one cylinder to another by a tube. As the apply piston moves, it forces pressurized fluid through the tube and into the other cylinder. This forces the output piston to move.
Transferring Force by a Liquid
The force that is applied to a liquid is transmitted by the liquid in all directions to every part of the liquid. The piston has an area of 1 square inch [6.45 cubic centimeters (cc)]. It is applying a force of 100 pounds [445 N]. This is 100 pounds per square inch (psi) [690 kPa]. Note that regardless of position, all gauge show a pressure of 100 psi. the size of the piston also determine pressure, the piston area is increased to 2 square inches. When the same force of 100 pound is applied over double the area, the resulting pressure is only 50 psi [345 kPa]. The bigger the output piston, the greater output force. For example, in a disc-brake system an apply force of 100 pound causes a smaller-diameter piston in the master cylinder to travel a relatively long distance. The resulting hydraulic pressure the moves a much larger piston in the disc brake a relatively short distance. However, the larger piston has a greatly increased output force.
BRAKES AND BRAKING
The service braking system includes two basic part. These are the master cylinder and the drum and the disc wheel brake mechanism. The master cylinder is a reciprocating-piston pump. It pressurize the hydraulic system when the driver depress the brake pedal.. This convert the mechanical force from the brake pedal into hydraulic force that applies the brakes at the wheels.
Braking begins at the brake pedal. When the brake pedal is push down, brake fluid is forced from the master cylinder into the lines to the wheel brakes. As hydraulic pressure increase, brake shoe or pads are force against the rotating drums or disc. the resulting friction slows or stop the wheels and the vehicle.
There are two pistons in the master cylinder. The spaces ahead of the piston form two pressure chamber. When the pedal is push down, the piston is pushed toward the closed end of master cylinder. this send fluid from the front pressure chamber to the front wheel brakes. Fluid from the rear chamber is send to the rear wheel brakes. All four then operate to slow or stop the vehicle. This arrangement using a two piston tandem or dual master cylinder is a dual braking system.
Dual Braking System
In a dual braking system, the hydraulic system has primary section and secondary section. the primary section is always closed to the firewall. however the dual braking system is hydraulically separate or split in different ways. Most vehicle with rear wheel drive use the front wheel split. Many front wheel drive vehicle use the diagonal split.
splitting the hydraulic system into two sections improves vehicle safety. One section will continue to work and stop the vehicle if the other section leak and fail. Both section seldom fail at the same time. in early braking systems, there was only one piston in the master cylinder and one hydraulic system. A leak or failure anywhere in the hydraulic system usually mean there were no brakes.
DRUM BRAKES
Drum-Brake Construction
The drum brake has a metal brakes drum that encloses the brakes assembly at each wheel. Two curved brake shoe expand outward to slow or stop the drum which rotates with the wheel. The brakes assembly attaches to a steering knuckle, axle housing , or strut-spindle assembly. older car and truck has drum brakes at all four. Newer vehicles using drum brakes have them only in the rear.
Brake shoe are made of metal. A facing of friction material called brake lining is riveted or commented (bonded) to the shoes. The lining are usually made of non asbestos material such as fiberglass or a semimetallic material that can withstand the heat-producing braking action. Asbestos lining has been used, but is beeing phased out because it danger to human health.
Wheel Cylinder
when the driver depress the brake pedal, brake fluid flows from a pressure chamber in the master cylinder thorough brake line to the wheel cylinder. It convert the hydraulic pressure from the master cylinder into the mechanical movement. The wheel cylinder has two pistons, with seals or cups, and a spring in between. As the pressure increase, the piston overcome the brake shoe return spring and push the shoe outward into contact with the drum.
in vehicle with four wheel drum brakes, the front wheels cylinder piston are usually larger than the pistons in the rear wheel cylinder. This produce the greater braking force required on the front wheel. braking transfer more of the vehicle weight to the front wheels.
Drum Brakes Operation
The leading-trailing or non-servo brakes is used on the rear wheels of many front-drive vehicle. Rear-drive vehicles usually have duo-servo rear brakes. In a duo-servo brakes, the action of one shoe reinforce the action of the other shoe. In a leading-trailing brakes, the action of one shoe does not effect the other.
1. Leading-trailing Drum Brake
The return retracting springs hold both shoes against the wheel cylinder at the top, and against fixed anchor pins at the bottoms. depressing the brakes the brakes pedal the wheel-cylinder pistons to move the tops of the shoes outward against the drum. friction between the forward or leading shoe and the drum causes the leading shoe try to rotates with the drum. This self energize action of the leading shoe force the bottoms of the shoe against the anchor pin. As a result, the leading shoe does most of the braking.
When the rear or trailing shoe contact the drum, drum rotation tries to force the shoe away from the drum.there is no self energize action. therefore, the trailing shoe usually wear less than the leading shoe. The leading and trailing shoes swap jobs when the vehicle is braked while moving in reverse. This brake is less self energizing and more dependent on the force supplied by the wheel cylinder than the duo-servo described below.
2. Duo-Servo Drum Brake
The tops of the shoe rest against a single anchor pin. the bottoms of the shoe are link together by a floating adjusting screw. the show toward the front of the vehicle the primary shoe. The show toward the rear is the secondary shoe. The primary shoe normally has shorter lining than the secondary shoe.
when the shoe contact the rotating drum, the friction cause both shoe try to rotate with the drum. The top of the primary shoe tends to pull into the drum and move downward. The bottoms of the shoe then pushes the adjusting screw rearward. This forces the bottom of the secondary shoe against the drum with moves the secondary shoe upward against the anchor pin. further drum rotation tend to pull both shoe more tightly into the drum. This further increase the self energizing action of both shoe make total braking forces greater than the amount supply by the wheel cylinder. The secondary shoe has longer lining because it provides about twist as much braking force as the primary shoe. the function of each shoe change when braking with the vehicle moving in reverse.
Drum-Brake Self-Adjusters
Most drum brake self adjust to compensate for lining wear. This prevent the brake pedal from getting lower and lower during normal use. Two types of self-adjuster used on leading-trailing brakes are the one shot and the incremental. the one shot makes a single adjustment when a clearance between the lining and drum reaches the preset gap.Then no additional adjustment can be made. The shoe must be replace and the self adjustment reset.
It moves the shoes outward whenever the gap is large enough to turn the adjusting screw. Adjustment occur when the vehicle is brake while moving either forward or rearward. As the brake shoe moved outward, the adjusting screw retracting spring cause the adjusting lever to pivot outward. If the lining is worn enough , the lever movers above the end of the next tooth on the adjusting wheel. when the brakes are release, the adjuster lever pivots downward. this turn the tooth. the adjusting screw then lengthen slightly to moves the shoes closer to the drum.
An adjusting lever attaches to a self adjuster cable that pass around a cable guide and fastens to the anchor pin. The adjustment is made when the vehicle is moving-backward and the brakes are applied. then friction forces the upper end of the primary shoe against the anchor pin. The wheels cylinder forces the upper end of the secondary shoe away from the anchor pin and downward. This causes the capable to pull the adjusting lever upward by pivoting in a hole in the secondary shoe. If the brakes lining have warm enough, the lever passes over and engages the end of a new tooth on the adjusting wheel.
when the brakes are release, the adjuster spring pulls the adjusting lever downward . this turn the tooth and slightly lengthens the adjusting screw. the brakes show moves closer to the drum.
