018 AUTOMOTIVE Brake Systems and Components – ABS

018 AUTOMOTIVE Brake Systems and Components – ABS


bleeding means removing air from a hydraulic system when pressure is applied to liquid in a hydraulic system the liquid does not compress into a smaller volume pressure is transmitted without loss gases however are compressible pressure applied to air changes its volume and some pressure is lost that is why if air enters a hydraulic braking system it can be dangerous pressure on the brake pedal will not be transmitted in full through the system to apply the brakes the brakes will be spongy bleeding the brakes means removing this air so that only liquid is left in the system friction is a force that resists the movement of one surface over another it can be desirable that often is not it is caused by surface rough spots that lock together these spots can be microscopically small which is why even surfaces that seem to be smooth can experience friction friction can be reduced but never eliminated friction is always measured four pairs of surfaces using what is called a coefficient of friction a low coefficient of friction for a pair of surfaces means they can move easily over each other a high coefficient of friction for a pair of surfaces means they cannot move easily over each other [Music] hydraulic pressure is transmitted through liquid since liquid is effectively incompressible pressure applied to a liquid is transmitted without loss throughout the liquid in a braking system this allows a force applied to the brake pedal to act upon the brakes at the wheels hydraulic pressure can transmit increased force since pressure is force per unit area the same pressure applied over different areas can produce different forces larger and smaller this system has cylinders of different sizes when the brake pedal is pressed the force against the piston in the master cylinder applies pressure to the fluid this same pressure is transmitted throughout the fluid but it has a different effect on each piston in the other cylinders the top cylinder is smaller than the master cylinder so the force it exerts will be less than the force applied to the master cylinder the middle cylinder is the same size as the master cylinder so the force from it will be the same to the bottom cylinder is larger than the master cylinder and so is its force most of our vehicles weight is supported by its suspension system it suspends the body and associated parts so that they are insulated from road shocks and vibrations that would otherwise be transmitted to the passengers and the vehicle itself however other parts of a vehicle are not supported by the suspension system such as the wheels tires brakes and steering and suspension parts not supported by Springs these parts are all called unsprung weight generally unsprung weight should be kept as low as possible [Music] this section examines basic principles of the hydraulic braking system several factors can influence vehicle braking road surface road conditions the weight of the vehicle the load on the wheel during stopping different maneuvers and of course the tires on the vehicle an effective braking system takes all these factors into account a basic hydraulic braking system has two main sections the brake assemblies at the wheels and the hydraulic system that applies them there is a brake for when the vehicle is in motion usually a foot brake and a part brake for when it’s stationary usually operated by hand some systems have all drum brakes some have disc brakes from the front wheels and drum brakes on the rear others have all disc brakes a basic braking system has a brake pedal a master cylinder to provide hydraulic pressure brake lines and hoses to connect the master cylinder to the brake assemblies fluid to transmit force from the master cylinder to the wheel cylinders of the brake assemblies and the brake assemblies drum or disc that stop the wheels the driver pushes the brake pedal it applies force to the piston in the master cylinder the piston applies pressure to the fluid in the cylinder the Lyons transfer the pressure to the wheel cylinders and the wheel cylinders at the wheel assembly’s apply the brakes force is transmitted through the fluid four cylinders the same size the force transmitted from one is the same value as the force applied to the other by using cylinders of different sizes for bosses can be increased or reduced in an actual braking system the master cylinder is smaller than the wheel cylinders so the force at all of the wheel cylinders is increased when brakes are applied to a moving vehicle they absorb the vehicle’s kinetic energy friction between the braking surfaces converts this energy into heat drum brakes the wheel cylinders force brake linings against the inside of the brake drum in disc brakes pads are forced against a brake disc in both systems heat spreads into other parts and the atmosphere so brake linings and drums pads and discs must withstand high temperatures and high pressures on modern vehicles this basic system has some refinements such as a power booster this helps the driver apply the brakes this section examines divided systems for ABS modern cars use tandem master cylinders to suit divided or Joule line braking systems a divided system is safer in the event of partial failure fluid loss in one half of the system still leaves the other half able to halt the vehicle although with an increase in stopping distance a wheels braking ability depends on the load its carrying during braking so the type of vehicle is a major factor in how its system should be divided a front-engined rear-wheel drive car has around 40% of its load on its rear wheels so it’s braking system can be divided in a vertical or front rear split this puts the front wheels in a different system from the rear wheels if one half of the system fails the front or the back there’s still enough separate braking capability left in the other half to stop the vehicle this doesn’t work well for a front-wheel drive vehicle a load of about 20% on the rear wheels can’t provide enough braking force to stop the vehicle front-engined front-wheel drive vehicles user braking systems split in a diagonal or X the left-hand front brake unit is connected to the right-hand rear unit and the left-hand rear to the right-hand front if one system fails a 50% braking capability is left in the other system dual proportioning valves maintain optimum braking in each system a system that partially failed would cause severe braking pool on a vehicle suspension so suspension geometry is usually revised to counter this an alternative arrangement for front-engine rear-wheel drive vehicles is an L split the front disc brake units have four piston two of the Pistons on each front unit connect to the right-hand rear and the other two Pistons of each unit connect to the left-hand rear as with the xsplit if there is failure of either half of the system it still leaves 50% braking capability this section examines the anti-lock braking system applying brakes too hard or on a slippery surface can cause the wheels to lock when wheels lock steering control is lost and in most cases it produces longer stopping distances the anti-lock braking system prevents wheels locking or skidding no matter how hard brakes are applied or how slippery the road surface steering stays under control and stopping distances are generally reduced it consists of a brake pedal a master cylinder wheel speed sensors the electronic control unit or ECU and the hydraulic control unit also called a hydraulic modulator the wheel speed sensor consists of a notched or toothed rotor that rotates with each wheel and a pickup as the wheel turns a small voltage pulse is induced into the pickup and sent to the electronic control unit when the brakes are applied the wheel speed of rotation changes this sends a new signal to the ECU if the control unit detects that a wheel might lock it sends a signal to the hydraulic control unit in a three-channel system the hydraulic control unit uses three solenoid valves to control brake pressure and prevent them locking the valves are in series with the brake master cylinder and the brake circuits one operates for each of the front wheels and one controls both rear wheels at the start of a journey the ABS automatically checks itself any failure in the system lights up a warning lights in the dash panel [Music] during normal braking as the rotational speed of the wheel Falls no electric current flows from the ECU to the hydraulic unit the solenoid valve is not energized at the brake master cylinder hydraulic pressure is applied to the brake unit and the ABS is not involved however even though the ABS is passive during normal braking its control module is constantly monitoring for rapid deceleration of any of the wheels if a wheel speed sensor signals severe wheel deceleration which means the wheel is likely to lock up the ECU sends a current to the hydraulic unit this energizes the solenoid valve the action of the valve isolates the brake circuit from the master cylinder this stops the braking pressure at that wheel from rising and keeps it constant if the sensors signal the wheel is still decelerating too rapidly the ECU sends a larger current to the hydraulic unit the armature moves even further and opens the valve it opens a passage from the brake circuit brake fluid is sent from the brake circuit back to the master cylinder pressure in the brake caliper circuit is reduced so that the wheel is brakes less heavily if the wheel sensors indicate that lowering the brake pressure is letting the wheel accelerate again the ECU stops sending current to the hydraulic unit and de energizes the solenoid valve this lets the pressure increase so that the wheel is again decelerated this cycle repeats itself about four to six times per second it is normal in an ABS for the valves and the hydraulic control unit to keep changing position as they change the brake pressure that’s being applied these changes in position may cause rapid pulsations to be felt through the brake pedal this section looks at the brake pedal the brake pedal uses leverage to transfer the effort from the driver’s foot to the master cylinder different lever designs can alter the effort the driver needs to make it is usually suspended from a bracket between the dash panel and the firewall in many vehicles the brake pedal is either connected to a switch or in contact with one it operates the stop lights when the brake pedal is depressed this section looks at the master cylinder in ABS the master cylinder is connected to the brake pedal via a pushrod the ABS master cylinder is similar to the tandem master cylinder used in divided systems it has a primary piston and a secondary piston the secondary piston incorporates a center valve this controls the opening and closing of a supply port drilling in the piston at rest the supply port is open and connects the reservoir with the front brake circuits the primary piston still has an inlet port and a compensating port when the brake is applied the primary piston moves which closes its compensating port fluid pressure in the primary circuit Rises it acts with the primary piston spring to move the secondary piston forward closing the center valve pressure builds in the secondary circuit pressure keeps building in both circuits and applies the brakes in both circuits if the secondary circuit fails the secondary piston is forced to the end of the cylinder when it reaches the end pressure builds in the primary circuit if the primary circuit fails the primary piston contacts the secondary piston and pushes it to operate the secondary circuit in normal operation when the pedal is released the springs in the master cylinder pushed the Pistons back more quickly then the fluid can flow back from the wheel brake units this creates a low-pressure area in front of each piston such low pressures can cause air to be drawn into the system to prevent this there are recuperating grooves in the primary piston and the seal fluid at atmospheric pressure flows through the inlet port and past these grooves when the primary piston is returned fully any extra fluid coming back from the brake units displaces fluid into the reservoir through the compensating port in the secondary circuit fluid also at atmospheric pressure is forced back into the inlet port the inlet port connects with the supply port drilling in the piston any difference in pressure lifts the center valve from its seat and lets fluid enter the chamber ahead of the secondary seal and prevents low pressures developing when the piston has returned to the rest position the seal is pulled off its seat by the action of the link and spring this lets fluid still returning from the wheel units displace fluid back to the reservoir if breaking conditions are such that the hydraulic modulator must return brake fluid to the master cylinder then for the front brake circuits fluid is returned to the front section this forces the secondary piston back against the force of the primary piston spring and the rear brake pressure if enough fluid returns the center valve opens and allows fluid to return to the reservoir if fluid is returned from the rear brake circuit the secondary and primary Pistons tend to be forced apart the amount of fluid that returns to the master cylinder is determined by the degree of anti-lock braking control with approximately four to six ABS control cycles per second the rapid changes in pressure caused pulsations that can be felt by the driver at the brake pedal this section examines break lines break lines carry break fluid from the master cylinder to the brakes they are basically the same on all brake systems for most of their length they are steel and attached to the body with clips or brackets to prevent damage from vibration a flexible section must be included between the body and suspension to allow for steering and suspension movement these flexible lines are made of reinforced tubing to protect them from objects that could be thrown by the tires in some vehicles the brake lines are inside the vehicle to protect them from corrosion this section examines the proportioning valve it divides up the braking effort applied to front and rear wheels under heavy braking according to how load is distributed across a vehicle the effectiveness of braking force is determined by tire to road friction and this increases as load increases applying the brakes causes the front of this vehicle to dip this causes greater tyre to road friction on the front tires and less on the rear this kind of change of load is called load transfer so if equal braking force is applied to the front and rear wheels the smaller rear load can make the rear wheels lock and perhaps skid the braking force applied to the wheels needs to be adjusted to allow for changes in load the proportioning valve adjusts braking force to allow for load transfer it can be pressure sensitive or load sensitive the pressure sensitive valve can be in the master cylinder or in a separate unit in the rear brake circuit the load sensitive type can be in the body or the axle where it can respond to load changes and change the braking effort as needed master cylinder applications usually combine the proportioning valve with a pressure differential switch in normal braking the poppet piston is held in a relaxed position by a large pressure spring the poppet valve is held against its retainer by a light return spring and fluid passes freely through the valve to the rear brakes in heavy braking master cylinder pressure can reach a valves crack point the pressure applied to the two different areas of the poppet piston creates unequal forces that moves the poppet piston against the large pressure spring this action holds the conical section of the valve against the seat which limits the pressure increase to the rear brakes as greater pedal force increases pressure in the master cylinder fluid pressure rises on the smaller end of the piston this combines with the force of the pressure spring to overcome the lower pressure now on the larger end this forces the piston back clear of the poppet valve the increased pressure now acts on the larger end of the poppet piston and again forces the piston forward to contact the valve when the pedal is released the pressure of the rear brake fluid unseats the poppet valve letting fluid return to the master cylinder the pressure spring now returns the poppet piston to its relaxed position should the front brake system fail the warning lamp spool moves forward taking the poppet valve with it pressure in the rear brakes rises and the piston moves forward but it can’t seal on the vow should the rear brake system fail the warning lamps pool will move backwards to activate the warning light the proportioning valve doesn’t operate in this situation on a diagonally divided system the pressure sensitive proportioning valve is usually located away from the master cylinder there is one for each circuit they each operate in a similar way to the pressure sensitive proportioning valve located in the master cylinder but without the pressure differential warning light circuit this section examines the power booster a power booster or power brake unit uses a vacuum to multiply the drivers pedal effort and apply that to the master cylinder this increases the pressures available from the master cylinder units on petrol engines use the vacuum produced in the intake manifold vehicles with diesel engines cannot use manifold vacuum so they are fitted with an engine-driven vacuum pump the most common booster now operates between the brake and master cylinder it increases the force that acts on the master cylinder whenever the pedal is depressed the power brake unit assists the driver the level of assistance depends on the pressure applied when the driver moves the brake pedal pushrod it transmits movement through the power unit to the master cylinder piston to apply the brakes it also operates a control valve that admits air at atmospheric pressure to the rear of the unit how it works depends on the position of the push rod her hose connects the intake manifold to a vacuum check valve on the power unit with the engine running the vacuum in the intake manifold is used to evacuate the power unit this valve is held off at seat and a vacuum is produced in both chambers of the unit the chambers are separated by a flexible rubber diaphragm attached to the diaphragm plate it is held in the off position by a diaphragm return spring the master cylinder push rod and the control valve assembly are centrally located on each side of the plate as the brakes are applied the pedal push rod and plunger move forward in the diaphragm plate this brings the control valve into contact with the vacuum port seat it closes the vacuum port sealing off the passage connecting the chambers further movement of the push rod and plunger moves the air valve away from the control valve to open the atmospheric port air at atmospheric pressure comes into the air filter and passages and enters the chamber at the rear of the diaphragm the difference in pressure now on both sides of the diaphragm moves the diaphragm plate forward and it takes the master cylinder push rod with it [Music] hydraulic pressure builds up in the brake system to operate the brakes as pressure rises a counter force acts through the master cylinder push rod and the reaction disc this counter force acts against the plunger and pedal push rod it tends to move the plunger slightly to the rear and it closes off the atmospheric port if the vacuum source is interrupted then as the pedal is pushed down the pedal push rod and plunger assembly come in contact with the reaction disc this forces the master cylinder push rod forward to operate the brakes the pedal force needed then is much greater than with vacuum assistance during application the reaction force against the valve plunger works against the driver to close the atmospheric port with both the atmospheric and the vacuum ports closed the power unit is in a holding position it stays this way until increased pedal force reopens the atmospheric port or a drop in pedal force reopens the vacuum port with the force on the pedal held constant the valve returns to the holding position but if the pedal is fully applied the plunger moves away from the control valve to open the atmospheric port and give full power application when the brakes are released vacuum returns to both sides of the diaphragm so the spring releases the brakes when the engine has switched off or stops for any reason no vacuum is available the vacuum remaining in the booster held by the non-return valve will provide for at least one power boosted application after this the brakes will still operate but without power assistance they require more effort from the driver this section examines brake fluid the brake fluid transmits hydraulic pressure from the master cylinder to the wheels it is a special fluid with special properties most are a mixture of glycerin and alcohol called glycol with additives to give it the characteristics that are needed it must have the correct viscosity for hot and cold conditions its boiling point must be higher than the temperature reached by the system it must not damage seals gaskets or hoses all cause corrosion glycol-based fluids meet most requirements although they do damage paint and they absorb moisture hence the warnings this is important because as moisture is absorbed it lowers the boiling point of the fluid brake fluid should not be mixed with mineral based oils or solvents if contamination is suspected the braking system must be drained and flushed with a suitable solvent and rubber components replaced this section examines the hydraulic control unit or modulator of the anti-lock braking system the ABS control module or ECU sends commands in the form of electrical signals to the hydraulic control unit this unit executes them using three solenoid valves connected in series with the master cylinder and the brake circuits one valve for each front wheel hydraulic circuit and one for both of the rear wheels hydraulic circuit this is a simplified diagram of one solenoid valve operating on just one wheel in normal non ABS braking brake pedal force is transmitted to the master cylinder then through the solenoid valve to the brake unit at the wheel the signals from the wheel speed sensor show no tendency for the wheel to lock up so the ECU is not sending any control current to the solenoid coil the solenoid valve is not energized and the hydraulic pressure from the master cylinder is supplied to the brake unit at the wheel when the control unit detects any lockup tendency perhaps from two rapid wheel deceleration it sends a command current to the solenoid coil this causes the armature and valve to move upward and isolate the brake circuit from the master cylinder that keeps the pressure between the solenoid and the brake circuit constant whether or not the master cylinder hydraulic pressure rises if the sensor signal continuing excessive wheel deceleration the control module sends a larger current to the solenoid valve this lowers the braking pressure by moving the armature up further opening a passage from the brake circuit to an accumulator a temporary reservoir for any brake fluid that flows out of the wheel brake cylinders because of the fall in pressure a return pump sends this brake fluid back to the master cylinder if the sensors then signal the lower pressure has left the wheel speed up the ECU stops all command current which de energizes the solenoid valve the pressure rises and the wheel is again slowed down no matter the phase of operation pressure in the circuit can never rise above master cylinder pressure this section examines wheel speed sensors a wheel sensor consists of a toothed rotor that rotates with the wheels and a pickup as each tooth of the rotor passes the pickup a small voltage is induced in the pickup these pulses are sent as input signals to the electronic control unit which processes them to operate the hydraulic control unit this section examines the electronic control unit the electronic control unit receives signals from different sources a switch at the brake pedal provides a break operating signal another in the ignition system signals the engine is operating this sets off the automatic check the ABS conducts every time the engine starts another input is from the wheel speed sensors these signals are used to control the hydraulic control unit and anticipate wheel lock if a wheel starts to lock the electronic control unit operates the solenoid valves to reduce hydraulic pressure appropriately


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