The Brake System:
Operation Of The Air-Hydraulic Brake System
5 Ton and 6x6 Trucks
Now that you are familiar with the components that make up air-hydraulic brake systems, let's see how they work together to stop a vehicle. First, let's consider a truck that is not connected to a trailer. We will use the brake system of the 5-ton, 6x6 truck for our discussion.
When the brake pedal is pushed down, brake fluid is pushed through the hydraulic system. The fluid pressure is transmitted from the slave cylinder to the relay piston in the control unit. The relay piston changes the hydraulic pressure to mechanical motion, which pushes the diaphragm assembly against the atmospheric poppet to close the air exhaust passage and to open the air pressure poppet.
Compressed air can now flow from the reservoir, through the compressed air line, and past the open air pressure poppet in the control unit. The air-flow is directed into two control lines leaving the control unit. One line is to the service outlet to control the trailer brakes. The other control line carries the air pressure to the power cylinder of the air-hydraulic cylinder. The air pressure forces the power piston and the attached pushrod toward the slave cylinder.
The extending pushrod contacts the slave cylinder hydraulic piston and pushes it forward. This closes the check valve in the hydraulic piston and brake fluid under pressure is forced through the outlet to the wheel cylinder to apply the brakes. The fluid pressure to the wheel brakes is now being produced from two sources. The air pressure that is pushing the power piston ahead is one source. The second source is from the pressure applied on the brake pedal. Hydraulic pressure from the master cylinder pushes on the slave cylinder piston, along with the pushrod, so the two forces add together.
The amount of air pressure permitted to enter the power cylinder is regulated by the pressure applied to the brake pedal. To see how this is possible, assume that the driver applies enough pressure on the brake pedal to slow the truck down but not enough to lock the wheels.
The air pressure that is permitted to enter the control unit pushes on the diaphragm assembly in an attempt to close the air inlet poppet valve. Recall that hydraulic pressure from the master cylinder on the relay piston opened the air inlet poppet. Therefore, the air pressure against the diaphragm assembly is opposing the hydraulic pressure on the relay piston. When the air pressure reaches a point where it overcomes the hydraulic pressure, the diaphragm assembly and relay valve move slightly allowing the air inlet poppet to close shutting off incoming air. But the atmospheric poppet remains closed so the controlled air pressure is trapped in the power cylinder. This is known as the "holding" or "lap" position.
The air-hydraulic cylinder will remain in the holding position maintaining an unchanging amount of controlled air pressure as long as the same amount of foot pressure is applied to the brake pedal. The amount of brake application is determined by the amount of controlled air pressure trapped in the power cylinder. If more foot pressure is applied on the brake pedal, more hydraulic pressure is applied on the relay piston. This opens the air inlet poppet and the controlled air pressure increases until it is great enough to overcome the increased hydraulic pressure and move the relay piston back allowing the air inlet poppet to close.
When the brake pedal is released, hydraulic pressure on the relay piston is removed. This allows the diaphragm return spring to return the diaphragm assembly to the released position opening the atmospheric poppet. The control pressure is released to the outside air by passing through the drilled center of the diaphragm assembly and the exhaust port. The piston return spring returns the power piston, pushrod, and hydraulic piston to the released position. As the hydraulic piston nears the released position, the check valve in the center of the piston opens.
The residual check valve assembly in the outlet of the slave cylinder maintains a slight pressure in the lines and wheel cylinders, just as the master cylinder check valve does in straight hydraulic brakes.
To operate properly, the air-hydraulic cylinder must have a supply of compressed air. But if the air supply should fail, the vehicle brakes will still be applied when the brake pedal is pressed. Brake fluid from the master cylinder will flow through the check valve in the center of the slave cylinder hydraulic piston to the wheel brakes. There will be no boost from the air-power cylinder, but the vehicle could be operated under emergency conditions.
Now let's discuss the complete compressed air system of the 5-ton, 6x6 truck. The truck has service and emergency trailer couplings at the front as well as at the rear. If a truck must be towed, the trailer couplings at the front can be connected to the rear trailer couplings of the towing truck. With the two brake systems connected in this manner, the brakes of both trucks can be controlled from the towing truck. Three double-check valves direct the flow of controlled air pressure.
One double-check valve is located at the control line connection of the air-hydraulic cylinder. The center connection of the valve is connected to the power cylinder. The control line from the air-hydraulic cylinder control valve is connected to an end connection of the double-check valve. The control or service line from the front of the truck is attached to the remaining end connection of the double-check valve.
When the control unit of the air-hydraulic cylinder sends air pressure to the double-check valve, the air pressure moves a sliding piston in the valve closing the service line passage to the front of the truck. The air can move freely from the control unit, through the double-check valve, into the power cylinder to apply the brakes.
When the truck is being towed and the brakes are applied, the double-check valve prevents the escape of controlled air from the towing truck. Brake air controlled pressure from the tow truck flows through the service line to the double-check valve. The controlled air moves the sliding piston closing the passage to the air-hydraulic control unit. Controlled air pressure now flows freely from the tow truck, through the double-check valve, and into the power cylinder.
The two remaining double-check valves direct the controlled air flowing to the trailer service coupling at the rear of the truck. They are connected so that controlled pressure furnished from one control unit cannot flow to another control unit. This must be done to prevent the escape of controlled pressure. For instance, if controlled pressure from the air-hydraulic cylinder is allowed to flow through the control lines to the hand control valve, the pressure will be released through the open-hand control exhaust valve.
The double-check valves used on the 5-ton truck make it possible for the brakes of a towed vehicle to be applied by using either the brake pedal or the hand control of the towing truck. When the brake pedal is pushed down, the brakes of both the towing and the towed vehicle are applied. When the hand control valve is applied, the double-check valves direct the controlled air so only the brakes of the towed vehicle are applied.
2 1/2 Ton Trucks
The principles of operation of the air-hydraulic brakes of the 5-ton, 6x6 trucks can be applied to the air-hydraulic brakes of the 2 1/2-ton-series trucks. However, there is enough difference in the design of the two brake systems so that a short discussion on the 2 1/2-ton truck brakes is needed.
The air-hydraulic cylinder of a 2 1/2-ton truck looks different from that of a 5-ton truck, but the operation is about the same. Brake fluid applied pressure from the master cylinder is transmitted to the slave cylinder and the fluid passage at one end of the hydraulic control valve piston. The control valve piston then pushes on the slave cylinder compensator piston moving it to close the air exhaust valve and open the air inlet valve. Air pressure from the reservoir flows through the air inlet opening and out one control line to the power cylinder and a second control line to the trailer service coupling.
Air pressure on the compensator piston and hydraulic pressure on the control valve piston oppose each other to regulate the controlled air pressure. When the brake pedal is released, hydraulic pressure is removed from the control valve piston and the compensator piston spring returns the compensator piston and control valve to the released position. This allows the air inlet valve to close and opens the air exhaust valve. Controlled pressure is exhausted through the hollow compensator piston and out the air breather port.
The power cylinder and hydraulic slave cylinder work just like the power and slave cylinders on the 5-ton truck with two exceptions. The hydraulic piston is returned by inner and outer slave cylinder piston springs. There is no residual line pressure check valve in the outlet of the 2 1/2-ton truck's slave cylinder like on the 5-ton truck. The 2 1/2-ton M35-series truck has the residual line pressure check valve in the outlet of the master cylinder.
The M35-series cargo trucks do not have a hand control valve or trailer couplings at the front of the truck as the 5-ton truck does. However, they do have service and emergency trailer couplings at the rear of the truck. The 2 1/2-ton tractor trucks have a hand control valve so the trailer brakes can be operated separately. |