The Brake System:
Operation Of Straight Air Brake System
So far, we have discussed the components contained in air-brake systems and what they do. Knowledge of how the components function is important so that a mechanic can diagnose troubles without making errors.
As mentioned before, the components of the compressed air system are about the same as those used in 2 1/2- and 5-ton trucks. The operation of air compressors, governors, reservoirs, high-pressure relief valves, and drain cocks of air-brake systems is the same as those discussed in earlier lessons for air-hydraulic systems. Again, in this system, the maximum pressure is about 105 PSI.
The brake valve (or brake application valve) is the device that an operator uses to control pressure to the brakes. It is mounted under the floor of the cab and is controlled by brake pedal movement. It is made so that the driver can vary the air pressure admitted to the brake chambers. As we will see later, the more air pressure there is in the brake chamber, the more the brake shoes will be forced against the brake drum. There are several types of brake valves, but they all do about the same job. The main difference between the types is that some are operated by the foot pedal only, while others are operated by a foot pedal but have a hand-operated limit control. Standard brake valves are fitted with a lever that is connected and operated by a foot pedal.
As the lever is moved toward its fully applied position, mechanical force is applied to the top of the diaphragm in the brake valve. This is done by the action of the plunger and pressure regulating spring assembly. As the diaphragm moves downward, a force is applied to the middle of the rocker arm and onto the inlet and exhaust valve. Because the exhaust valve spring is weaker than the inlet valve spring, the exhaust valve is forced down onto its seat before the inlet valve is forced down to open.
When the inlet valve opens, air pressure flows from the reservoir through the valve to the brake chambers. This applies the brakes. When the air pressure being delivered to the brake chambers, from the opening below the diaphragm, overcomes the mechanical force being applied to the top of the diaphragm, the diaphragm lifts. This permits the inlet valve to close preventing any further rise of air pressure in the brake chambers, while the exhaust valve remains closed and prevents any escape of air pressure.
If the driver pushes the foot pedal farther down, more mechanical force is applied to the top of the diaphragm. When this happens, more pressure is delivered to the brake chamber and is applied to the brakes.
If the driver lets the pedal move toward its released position, the force on top of the diaphragm is reduced. Air pressure below the diaphragm overcomes the mechanical force on top of it, and the diaphragm lifts slightly. When this happens, the inlet valve remains closed and the exhaust valve opens. This exhausts air pressure from the brake chambers until the air pressure below the diaphragm again balances the mechanical force on top of it.
If the driver lets the foot pedal return to the fully released position, the exhaust valve remains open. Thus, all the pressure from the brake chamber is exhausted, and the brakes are fully released.
If the driver pushes the pedal down to the fully applied position, the pressure regulating spring is compressed until the spring guide strikes the spring slat.
This holds the rocker arm down, the inlet valve is held open, and full reservoir pressure is allowed to pass through the brake valve to the brake chambers.
The trailer brake control valve is used to apply and release the trailer brakes without applying the brakes of the towing vehicle. This type valve is usually mounted on the steering column or on the instrument panel. The driver may put the handle in any one of several positions between the released and fully applied position.
As the handle of the brake valve is moved toward the applied position, force is applied to the top of the pressure regulating spring. When this happens, the spring and piston assembly move downward and the exhaust valve seat engages the exhaust valve. This closes the passage to the exhaust port. The exhaust and inlet valves, being part of the same assembly, cause both valves to move together.
After the exhaust valve is closed and the piston assembly continues to move downward, the inlet valve is forced off its seat. This lets air pressure from the reservoir pass through the inlet valve and out the brake chamber port to the connection leading to the service line and the brakes on the trailer.
As soon as the air pressure below the piston assembly overcomes the mechanical force on top of it, the piston assembly lifts and the intake valve closes. The closing of the inlet valve stops the airflow from the reservoir. The exhaust valve remains closed preventing any loss of air through the exhaust port.
If the brake valve handle is moved farther toward the fully applied position, it adds more mechanical force on top of the piston assembly and increases the delivered air pressure.
If the brake valve handle is moved toward the released position, the mechanical force on top of the piston assembly is decreased. The air pressure below the piston assembly lifts it slightly, opening the exhaust valve and permitting air pressure to exhaust from the service line. When the air pressure under the piston and the mechanical force on top of the piston is again equal, the exhaust valve will close. When the brake valve handle is moved to the released position, the intake valve is closed, the exhaust valve is opened, all air pressure is exhausted from the brake cylinders, and the brakes are released.
Another device in air-brake systems is the quick-release valve. The purpose of this valve, as the name suggests, is to speed up the release of the applied brakes.
When air pressure from the brake valve enters the brake valve port of the valve, the diaphragm moves down and closes the exhaust port. Air pressure then goes around the outer edges of the diaphragm and flows out the side connections (brake chamber ports) to the brake chambers. This applies the brakes.
As soon as the pressure in the brake chamber and connecting lines equals the brake valve pressure above the diaphragm, the force of the spring below the diaphragm forces the outer edge of the diaphragm back up against the body. At the same time, the center of the diaphragm keeps the exhaust port closed. This is the holding position.
If the brake valve pressure on top of the diaphragm is reduced, the brake chamber pressure raises the diaphragm. This opens the exhaust port and lets the brake chamber pressure escape through the exhaust port. If the brake valve pressure on top of the diaphragm is only partially released, the diaphragm goes to the holding position as soon as the pressure above and below it is equal.
As stated earlier, a relay valve is used on the rear wheels of trucks to speed up the application and release of the rear brakes.
When the valve is in the released position, the reservoir pressure is in the cavity below the supply valve which is closed. The diaphragm is in its normal position resting on the diaphragm guide.
When the brake valve is applied, it sends air through the brake valve port into the cavity above the diaphragm. The pressure pushes the diaphragm down. When the diaphragm moves down, its outer edge seals the exhaust port and the center of the diaphragm forces the diaphragm guide and the supply valve down. This opens the supply valve and allows air pressure from the reservoir to flow through the supply valve and into the cavity below the diaphragm. This cavity is connected through the brake chamber port to the brake chambers. With the mechanism in this position, air pressure is flowing directly from the reservoir through the relay valve into the brake chambers applying the brakes.
As soon as the air pressure below the diaphragm equals the air pressure above the diaphragm, the force of the supply valve spring lifts the center of the diaphragm and closes the supply valve. This limits the air pressure being delivered to the brake chambers by the relay valve to the same pressure as that being delivered by the brake valve to the relay valve. In this position, the supply valve is closed and the force of air pressure on top of the diaphragm keeps the outer edge of the diaphragm down sealing the exhaust port. An increase in brake valve pressure repeats the action (as in the applying position) until the two pressures are again equal.
If the brake valve pressure above the diaphragm is reduced, the brake chamber pressure below the diaphragm lifts the diaphragm. This opens the exhaust port under the outer edge of the diaphragm, and the pressure in the brake chamber is exhausted until a lower balanced pressure is reached. If the air pressure delivered by the brake valve drops to zero, the relay valve releases all pressure from its brake chamber. This releases the brakes and the valve returns to its released position.
A relay emergency valve is used in the air-brake system of trailers. This valve acts as a relay during operation of the brakes. It also automatically applies the trailer brakes when the air lines to the towing vehicle are disconnected or broken. The operation of the relay portion of the valve is much the same as the truck relay valve during normal operation. However, what happens if the air line is disconnected or if the trailer breaks away from the towing vehicle is quite different.
The relay emergency valve is made to go into its emergency position and apply the trailer brakes when there is a quick drop in pressure in the cavity below the emergency diaphragm. For example, if the emergency line is broken, air pressure in the cavity below the emergency diaphragm would flow out of the broken emergency line.
Air pressure above the emergency diaphragm will push the diaphragm down and pull the upper emergency valve down, closing it. Air pressure above the check valve will hold it down and closed.
Air pressure from the trailer reservoir can now flow across the top of the depressed emergency diaphragm into the cavity leading to the brake chambers applying the brakes.As air pressure escapes from below the emergency diaphragm, pressure above the pressure regulating diaphragm drops instantly. The spring below the pressure regulating diaphragm pushes it up, thus, closing the pressure regulating valve. Air pressure in the trailer reservoir is prevented from escaping through the broken line by the closed upper emergency valve.
To release the trailer brakes, the air reservoir will have to be drained of air or the emergency line reconnected to the towing vehicle.
When the trailer emergency line is connected to the emergency line of the towing vehicle, air flows from the tractor reservoir, through the emergency line, and into the relay emergency valve. This airflow lifts the check valve and flows over the top of the emergency diaphragm into the supply line to charge the trailer reservoir. At the same time, air has also been flowing below the emergency diaphragm and into the cavity above the pressure regulating diaphragm. Air continues to flow in this manner until the air pressure in the cavity above the pressure regulating diaphragm reaches about 70 PSI.
Brake chambers are mounted at each wheel. Their purpose is to convert the energy of compressed air into the mechanical force and motion necessary to operate the brakes. One type of brake chamber consists mainly of a housing, diaphragm and pushrod, and a spring. As compressed air enters the brake chamber behind the diaphragm, the diaphragm forces the adjuster lever which rotates a shaft and cam to apply the brakes. The higher the air pressure admitted to the brake chamber, the greater the force on the pushrod. When the air pressure is released from the brake chamber, the spring returns the pushrod and diaphragm to the released position.
Instead of the brake chamber discussed above, some vehicles use a rotochamber. As air pressure enters the rotochamber behind the diaphragm, it moves the diaphragm forward. The diaphragm moves along the inside wall of the cylinder body with a rolling motion. This motion of the diaphragm forces the pushrod forward. The higher the air pressure admitted to the rotochamber, the greater the force on the pushrod. If all the air pressure is released from the rotochamber, the spring returns the diaphragm and push-rod to the released position.
The air cylinder is another version of a brake chamber. When air enters the compressed air opening and goes behind the piston, the piston will move, driving the pushrod and compressing the piston. When air pressure in back of the piston is released, the piston spring returns the piston and pushrod to the released position.
A slack adjuster is used in air-brake systems to convert back and forth motion to rotary motion and to provide a quick and easy way to adjust the brakes to compensate for brake lining wear. All slack adjusters consist mainly of a worm and gear contained in a body having a lever arm. The gear is splined to fit the brake camshaft. In normal braking, the entire slack adjuster moves as one solid unit. It acts as a simple lever to transmit brake chamber forces to the brake camshaft as the brakes are applied. All adjustments are made by turning the worm shaft. This turns the worm gear and brake camshaft and moves the brake shoes either closer to or farther from the brake drum. |