The Brake System:
Brake Types: An Overview
Disc Brake
Disc brakes are found on most recently manufactured passenger automobiles.
Disc brakes are comprised of a brake rotor (disc) and caliper assembly. The rotor is a flat circular piece of metal, which is firmly attached to the wheel hub and provides a braking surface for the brake pad. The rotor turns with the wheel of the vehicle and is straddled by the caliper assembly.
Braking is accomplished by forcing friction pads against both sides of a rotating metal, or rotor.
When the brake pedal is depressed, hydraulic fluid forces the pistons and friction linings (pads) against the machined surfaces of the rotor. The pinching action of the pads quickly creates friction to slow down or stop the vehicle.
Drum Brake
Drum brakes have a drum attached to the wheel hub, with disc brakes a disc attached to the wheel hub maybe clamped between two brake pads, and ABS prevents the wheels from locking while braking.
Drum brakes have a drum attached to the wheel hub, and braking occurs by means of brake shoes, expanding against the inside of the drum.
A drum brake is a brake in which the friction is caused by a set of shoes or pads that press against the inner surface of a rotating drum. The drum is connected to a rotating wheel.
Air Brakes
Air-operated braking systems are used on heavy vehicles where large braking forces are required. Compressed air, operating on large-diameter diaphragms, provides the large forces at the brake assembly that are needed.
An air compressor pumps air to storage tanks. Driver-controlled valves then direct the compressed air to different wheel units, to operate the friction brakes.
On articulated vehicles, any delays in applying the trailer brakes should be minimized. This is achieved using a relay valve, and a separate reservoir on the trailer.
This arrangement also applies the brakes if the trailer becomes disconnected from the prime-mover.
Exhaust Brakes
An exhaust brake works by restricting the flow of exhaust gases through the engine, which slows engine rotation.
Heavy goods vehicles can often require increased braking, in situations where friction brakes could overheat and fail. This is achieved by using an exhaust brake.
An exhaust brake works by restricting the flow of exhaust gases through the engine. It achieves this by closing a butterfly valve located in the exhaust manifold. This maintains high pressure in the exhaust manifold and the engine cylinders, which in turn acts as a brake against the engine rotating. This then slows the road wheels through the transmission, or power train.
Other heavy goods vehicles use an engine brake that operates by altering valve timing, and stopping fuel being injected into the engine.
Electric Brakes
Electric brakes are commonly used with trailers and are activated when the brakes in the towing vehicle are applied.
Trailers and caravans towed by light vehicles must have a braking system if the trailer gross mass exceeds a certain value. An electric braking system is commonly used to activate the drum-type friction brakes on the trailer.
Braking effect can be increased or reduced by the driver, adjusting a control unit to suit the load on the trailer. This manual override can be used in certain driving conditions to dampen a trailer's tendency to sway.
When the brakes in the towing vehicle are applied, the brake-light circuit sends the signal to the control unit. The control unit then sends an appropriate current to the trailer brake actuators, to operate the trailer brakes, at the level selected.
In some places, trailers above a specified weight must also have an auxiliary battery-powered braking system that automatically applies the brakes and keeps them applied if the trailer ever breaks away from its towing vehicle.
Parking Brakes
Most light vehicles use a foot brake that operates through a hydraulic system on all wheels, and a hand-operated park brake that acts mechanically on the rear wheels only.
All vehicles must be fitted with a foot brake and a park brake.
Most light vehicles use a foot brake that operates through a hydraulic system on all wheels, and a hand-operated brake that acts mechanically on the rear wheels only.
The hand brake system holds the vehicle when it is parked. Some vehicles incorporate a drum brake for the hand brake, in the center of the rear disc brake. Others use a mechanical linkage to operate the disc brake from the hand brake system, or separate hand brake calipers with their own pads.
Some vehicles have the hand brake operating on the front wheels. Some vehicles use a single drum brake on the rear of the gearbox as a hand brake. That's sometimes called a transmission brake.
On a duo-servo drum brake, the cable for the hand brake lever pulls on an actuating lever inside the brake drum assembly. The actuating lever is connected to the secondary brake shoe by a pin, and to the primary shoe by a strut. Movement of the lever forces both shoes against the drum.
Engine Brakes
Engine braking, or compression braking, uses the resistance of the engine during the compression stroke to slow the momentum of a vehicle. A 'jake' brake on larger vehicles controls a second exhaust valve on the engine to maximize the braking effect of the engine.
Principles of engine braking
When a vehicle is in gear, the crankshaft of the engine and the wheels are mechanically connected. When turning force is applied to the crankshaft by the engine, the transmission applies that force to the wheels. Conversely, if force is applied to the wheels, the transmission applies that force to the crankshaft and through that to the engine. This is the principle behind 'push starting' a vehicle. Force is applied to the wheels by pushing the vehicle; when the vehicle has some momentum, engaging a gear turns over the engine, enabling it to fire without the assistance of a battery or starter motor.
Engine braking uses this same principle. If you take your foot off the accelerator, so that the engine ceases to apply force to the wheels, the engine begins to act as a brake on the vehicle. As the engine slows down, the momentum of the wheels will keep it turning over. The compression stroke of the engine will soak up some of this driving energy, slowing the vehicle down. Engaging a lower gear will slow the vehicle down more quickly, as the engine will be turned over more rapidly by the movement of the wheels.
Uses of engine braking
Slowing a vehicle down a steep or very long decline just using normal wheel brakes alone can lead to heat build up and potentially dangerous brake fade. It is good practice to slow the vehicle initially with the brakes, then to shift the engine into lower gears and let the inertia of the engine assist in slowing the vehicle, and prevent it from accelerating again. This is safer, because engine braking can be sustained for longer without reducing its effectiveness, and it reduces the wear and tear on the wheel brakes.
"Jake" Brakes
In heavy diesel vehicles, engine braking is less effective because more of the compression energy in the cylinders is returned to the crankshaft after the piston reaches top dead center. A 'jake' brake uses an extra lobe on the camshaft to control an auxiliary exhaust valve at the top of each cylinder. When engaged, the jake brake releases the compression stroke pressure before it can be transmitted back to the wheels, thus increasing the braking effectiveness of the engine. The disadvantage with this form of braking is that it is very noisy, producing a chattering, machine-gun sound, usually requiring additional muffling. |