1、Lesson 13 Braking System The typical brake system consists of disk brakes in front and either disk or drum brakes in the rear connected by a system of tubes and hoses that link the brake at each wheel to the master cylinder.Other systems that are connected with the brake system include the parking b
2、rakes,power brake booster and the anti-lock system.Sketch map of the braking system When you step on the brake pedal,you are actually pushing against a plunger in the master cylinder,which forces hydraulic oil(brake fluid)through a series of tubes and hoses to the braking unit at each wheel.Since hy
3、draulic fluid(or any fluid for that matter)cannot be compressed,pushing fluid through a pipe is just like pushing a steel bar through a pipe.1 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
4、 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.1.1 Brake Pads There are two brake pads on each caliper.The
5、 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.Brake pads wear out with use and must be replaced periodically.1.2 Rotor The disk rotor is made of iron with highly machined
6、 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.Disc brake 1.3 Caliper&Support The main type of calipers is floating calipers.Calipers must be rebuilt o
7、r 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 hydrauli
8、c 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.Caliper On a disk brake,the fluid from the master cylinder is
9、forced into a caliper where it presses against a piston.The piston,in-turn,squeezes two brake pads against the disk(rotor),which is attached to the wheel,forcing it to slow down or stop.2 Drum Brakes Drum brakes consist of a backing plate,brake shoes,brake drum,wheel cylinder,return springs and an a
10、utomatic or self-adjusting system.When you apply the brakes,brake fluid is forced under pressure into the wheel cylinder,which in turn pushes the brake shoes into contact with the machined surface on the inside of the drum.When the pressure is released,return springs pull the shoes back to their res
11、t position.Drum brake Typical drum brake parts 2.1 Brake Shoes Like the disk pads,brake shoes consist of a steel shoe with the friction material or lining riveted or bonded to it.2.2 Backing Plate The backing plate is what holds everything together.It attaches to the axle and forms a solid surface f
12、or the wheel cylinder,brake shoes and assorted hardware.2.3 Brake Drum Brake drums are made of iron and have a machined surface on the inside where the shoes make contact.2.4 Wheel Cylinder The wheel cylinder consists of a cylinder that has two pistons,one on each side.Each piston has a rubber seal
13、and a shaft that connects the piston with a brake shoe.When brake pressure is applied,the pistons are forced out pushing the shoes into contact with the drum.2.5 Return Springs Return springs pull the brake shoes back to their rest position after the pressure is released from the wheel cylinder.A go
14、od technician will examine the springs during a brake job and recommend their replacement if they show signs of fatigue.2.6 Self Adjusting System The parts of a self adjusting system should be clean and move freely to insure that the brakes maintain their adjustment over the life of the linings.With
15、 drum brakes,fluid is forced into the wheel cylinder,which pushes the brake shoes out so that the friction linings are pressed against the drum,which is attached to the wheel,causing the wheel to stop.3 Parking Brakes The parking brake(emergency brake)system controls the rear brakes through a series
16、 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.Disk brakes on the rear wheels add additional co
17、mplication 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.The other system uses a complete mechanic
18、al drum brake unit mounted inside the rear rotor.4 Master Cylinder The master cylinder is located in the engine compartment on the firewall,directly in front of the drivers seat.A typical master cylinder is actually two completely separate master cylinders in one housing,each handling two wheels.Thi
19、s way if one side fails,you will still be able to stop the car.The brake warning light on the dash will light if either side fails,alerting you to the problem.Master cylinders have become very reliable and rarely malfunction;however,the most common problem that they experience is an internal leak.Th
20、is 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.Typical master cylinder 5 Brake Fluid Brake fluid is a special oil that has specific properties.It is d
21、esigned to withstand cold temperatures without thickening as well as very high temperatures without boiling.Brake fluid must meet standards that are set by the Department of Transportation(DOT).The brake fluid reservoir is on top of the master cylinder.Most cars today have a transparent reservoir so
22、 that you can see the level without opening the cover.Exposure to air will cause the fluid to absorb moisture,which will lower that boiling point.Never put anything but approved brake fluid in your brakes.Anything else can cause sudden brake failure!Any other type of oil or other fluid will react wi
23、th the brake fluid and very quickly destroy the rubber seals in the brake system causing brake failure.6 Brake Lines The brake fluid travels from the master cylinder to the wheels through a series of steel tubes and reinforced rubber hoses.The rest of the system uses non-corrosive seamless steel tub
24、ing with special fittings at all attachment points.7 Anti-Lock Brakes(ABS)The most efficient braking pressure takes place just before each wheel locks up.While the wheels are locked up,you loose all steering control so that,if you have an opportunity to steer around the obstacle,you will not be able
25、 to do so.Another problem occurs during an extended skid is that you will burn a patch of rubber off the tire.Anti-lock brake systems solve this lockup problem by rapidly pumping the brakes whenever the system detects a wheel that is locked up.In most cases,only the wheel that is locked will be pump
26、ed,while full braking pressure stays available to the other wheels.This effect allows you to stop in the shortest amount of time while maintaining full steering control even if one or more wheels are on ice.The system uses a computer to monitor the speed of each wheel.When it detects that one or mor
27、e wheels have stopped or are turning much slower than the remaining wheels,the computer sends a signal to momentarily remove and reapply or pulse the pressure to the affected wheels to allow them to continue turning.This pumping of the brakes occurs at ten or more times a second,far faster then a hu
28、man can pump the brakes manually.If you step on the brakes hard enough to engage the anti-lock system,you may feel a strong vibration in the brake pedal.The system consists of an electronic control unit,a hydraulic actuator,and wheel speed sensors at each wheel.If the control unit detects a malfunction in the system,it will illuminate an ABS warning light on the dash to let you know that there is a problem.If there is a problem,the anti-lock system will not function but the brakes will otherwise function normally.Thanks for your presence!
