Disk Brakes

The disk brake is the best brake we have found so far. Disk brakes are used to stop everything from cars to locomotives and jumbo jets. 

Disk brakes wear longer, are less affected by water, are self adjusting, self cleaning, less prone to grabbing or pulling and stop better than any other system around.

The main components of a disk brake are the Brake Pads, Rotor, Caliper and Caliper Support. 

·         Brake Pads
There are two brake pads on each caliper. They are constructed of a metal "shoe" with the lining riveted or bonded to it.   The pads are mounted in the caliper, one on each side of the rotor.  Brake linings used to be made primarily of asbestos because of its heat absorbing properties and quiet operation; however, due to health risks, asbestos has been outlawed, so new materials are now being used.  Brake pads wear out with use and must be replaced periodically. There are many types and qualities of pads available. The differences have to do with brake life (how long the new pads will last) and noise (how quiet they are when you step on the brake). Harder linings tend to last longer and stop better under heavy use but they may produce an irritating squeal when they are applied. Technicians that work on brakes usually have a favorite pad that gives a good compromise that their customers can live with.

Brake pads should be checked for wear periodically. If the lining wears down to the metal brake shoe, then you will have a "Metal-to-Metal" condition where the shoe rubs directly against the rotor causing severe damage and loss of braking efficiency. Some brake pads come with a "brake warning sensor" that will emit a squealing noise when the pads are worn to a point where they should be changed.  This noise will usually be heard when your foot is off the brake and disappear when you step on the brake.  If you hear this noise, have your brakes checked as soon as possible.

·         Rotor
The disk rotor is made of iron with highly machined surfaces where the brake pads contact it.  Just as the brake pads wear out over time, the rotor also undergoes some wear, usually in the form of ridges and groves where the brake pad rubs against it.  This wear pattern exactly matches the wear pattern of the pads as they seat themselves to the rotor.  When the pads are replaced, the rotor must be machined smooth to allow the new pads to have an even contact surface to work with.  Only a small amount of material can be machined off of a rotor before it becomes unusable and must be replaced.  A minimum thickness measurement is stamped on every rotor and the technician doing the brake job will measure the rotor before and after machining it to make sure it doesn't go below the legal minimum.  If a rotor is cut below the minimum, it will not be able  to handle the high heat that brakes normally generate.  This will cause the brakes to "fade," greatly reducing their effectiveness to a point where you may not be able to stop!  

Caliper & Support
There are two main types of calipers: Floating calipers and fixed calipers. There are other configurations but these are the most popular.  Calipers must be rebuilt or replaced if they show signs of leaking brake fluid.

• Single Piston Floating Calipers are the most popular and also least costly to manufacture and service. A floating caliper "floats" or moves in a track in its support so that it can center itself over the rotor. As you apply brake pressure, the hydraulic fluid pushes in two directions. It forces the piston against the inner pad, which in turn pushes against the rotor. It also pushes the caliper in the opposite direction against the outer pad, pressing it against the other side of the rotor. Floating calipers are also available on some vehicles with two pistons mounted on the same side. Two piston floating calipers are found on more expensive cars and can provide an improved braking "feel". 

• Four Piston Fixed Calipers are mounted rigidly to the support and are not allowed to move. Instead, there are two pistons on each side that press the pads against the rotor.  Four piston calipers have a better feel and are more efficient, but are more expensive to produce and cost more to service.  This type of caliper is usually found on more expensive luxury and high performance cars.

Parking Brakes

·         The parking brake (a.k.a. emergency brake)  system controls the rear brakes through a series of steel cables that are connected to either a hand lever or a foot pedal.  The idea is that the system is fully mechanical and completely bypasses the hydraulic system so that the vehicle can be brought to a stop even if there is a total brake failure.
On drum brakes, the cable pulls on a lever mounted in the rear brake and is directly connected to the brake shoes. this has the effect of bypassing the wheel cylinder and controlling the brakes directly.
Disk brakes on the rear wheels add additional complication for parking brake systems.  There are two main designs for adding a mechanical parking brake to rear disk brakes. The first type uses the existing rear wheel caliper and adds a lever attached to a mechanical corkscrew device inside the caliper piston.  When the parking brake cable pulls on the lever, this corkscrew device pushes the piston against the pads, thereby bypassing the hydraulic system, to stop the vehicle.  This type of system is primarily used with single piston floating calipers, if the caliper is of the four piston fixed type, then that type of system can't be used.  The other system uses a complete mechanical drum brake unit mounted inside the rear rotor. The brake shoes on this system are connected to a lever that is pulled by the parking brake cable to activate the brakes. The brake "drum" is actually the inside part of the rear brake rotor.

·         On cars with automatic transmissions, the parking brake is rarely used.   This can cause a couple of problems.  The biggest problem is that the brake cables tend to get corroded and eventually seize up causing the parking brake to become inoperative.  By using the parking brake from time to time, the cables stay clean and functional. Another problem comes from the fact that the self adjusting mechanism on certain brake systems uses the parking brake actuation to adjust the brakes.  If the parking brake is never used, then the brakes never get adjusted.

 

Power Brake Booster

·         The power brake booster is mounted on the firewall directly behind the master cylinder and, along with the master cylinder, is directly connected with the brake pedal.  Its purpose is to amplify the available foot pressure applied to the brake pedal so that the amount of foot pressure required to stop even the largest vehicle is minimal.  Power for the booster comes from engine vacuum. The automobile engine produces vacuum as a by-product of normal operation and is freely available for use in powering accessories such as the power brake booster.  Vacuum enters the booster through a check valve on the booster. The check valve is connected to the engine with a rubber hose and acts as a one-way valve that allows vacuum to enter the booster but does not let it escape. The booster is an empty shell that is divided into two chambers by a rubber diaphragm. There is a valve in the diaphragm that remains open while your foot is off the brake pedal so that vacuum is allowed to fill both chambers.  When you step on the brake pedal, the valve in the diaphragm closes, separating the two chambers and another valve opens to allow air in the chamber on the brake pedal side.  This is what provides the power assist.   Power boosters are very reliable and cause few problems of their own, however, other things can contribute to a loss of power assist. In order to have power assist, the engine must be running. If the engine stalls or shuts off while you are driving, you will have a small reserve of power assist for two or three pedal applications but, after that, the brakes will be extremely hard to apply and you must put as much pressure as you can to bring the vehicle to a stop.

 

Master Cylinder

·         The master cylinder is located in the engine compartment on the firewall, directly in front of the driver's seat. 

·         A typical master cylinder is actually two completely separate master cylinders in one housing, each handling two wheels.

·         This way if one side fails, you will still be able to stop the car.

·         Master cylinders have become very reliable and rarely malfunction; however, the most common problem that they experience is an internal leak.

·         This will cause the brake pedal to slowly sink to the floor when your foot applies steady pressure. Letting go of the pedal and immediately stepping on it again brings the pedal back to normal height.

How Brakes Work

We all know that pushing down on the brake pedal slows a car to a stop. But how..... 

1. How does this happen?
2. How does your car transmit the force from your leg to its wheel?
3. How does it multiply the force so that it is enough to stop something as big as car?

 

Of all the systems that make up your car, the brake system might just be the most important. In the olden days it was also one of the simplest. Over the years as improvements have been made, the system that has evolved isn't so simple anymore. (It's also about a zillion times more reliable and safer.)

Ø Brakes convert the kinetic energy to heat energy. All of this heat is generated between the friction surfaces of your brake pads and your rotors.

Ø Brakes operate on a simple hydraulic principle. If a force is exerted on the piston putting pressure on the fluid confined in the left hand container, the fluid is forced out through the narrow tube at the bottom and into the right hand container, exerting a force on the second piston, forcing it to move upward.

Ø This is how the force from your foot gets to the four corners of the car. If we add a lever to magnify the force applied to the first (master) cylinder, and maybe even a power booster unit to increase that force even more.

• When you depress your brake pedal, your car transmits the force from your foot to its brakes through a fluid. Since the actual brakes require a much greater force than you could apply with your leg, your car must also multiply the force of your foot.
  

FUNDAMENTAL OF BRAKING SYSTEM

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.