BENDIX M-21-M-22 ANTILOCK MOD ASSY Manual de servicio descargar pdf (Pagina 18)

Antilock Operation—General

If a service brake application is made and the antilock system detects an impending

wheel lockup, the antilock controller will immediately begin modification of the brake

application using the modulator valve.

In order to modify the brake application, the coils of the two solenoid valves contained

in the modulator valve are energized or de-energized in a pre-programmed sequence

by the antilock controller. When a solenoid coil is energized, a shuttle within the sole-

noid moves and, depending upon the function of the specific solenoid, it either opens

or closes, thereby causing the exhaust or re-application of air pressure to the brake

actuator. The solenoids in the modulator are controlled independently by the antilock

controller. By opening and closing the solenoid valves in the modulator, the antilock

controller is able to simulate what the driver does when he “pumps the brakes.” It

must be remembered, however, that unlike the driver, the antilock controller is able to

“pump” each modulator along with the brakes connected to it, independently and with

far greater speed and accuracy.

Antilock Exhaust

When wheel lock is detected, the antilock controller simultaneously energizes both the

supply and exhaust solenoids in the modulator valve. Energizing the supply solenoid

causes its exhaust to close and inlet to open. With the inlet of the supply solenoid

open, application air is permitted to flow to the control side of the supply diaphragm.

Air pressure acting on the supply diaphragm causes it to close the supply and prevent

further delivery of air to the brake chamber.

Energizing the exhaust solenoid closes its inlet and opens its exhaust. By closing the

exhaust solenoid inlet, application air is prevented from flowing to the control side of

the exhaust diaphragm. Air pressure which was present on the control side of the

exhaust diaphragm flows out the exhaust port of the modulator valve. With control

air pressure removed from the exhaust diaphragm, brake application air forces the

exhaust diaphragm to unseat, which allows it to flow out the modulator valve exhaust

port. The modulator valve will remain in the Antilock Exhaust mode until the antilock

controller senses that wheel speed has increased. The modulator valve can enter the

Antilock Hold or Re-Apply mode from the Antilock Exhaust mode.

Antilock Hold

The antilock controller will place the modulator valve in the Hold position when it sens-

es that the correct wheel speed (braking force) has been attained. The antilock con-

troller will also place the valve in the Hold position prior to entering the Re-Apply,

when it detects recovery from a locked wheel condition. In this mode of operation, the

modulator valve supply solenoid remains oversized while the exhaust solenoid is de-

energized. De-energizing the exhaust solenoid opens its inlet and closes its exhaust.

Opening the exhaust solenoid inlet allows application air to flow to the control side of

the exhaust diaphragm, which then seals the exhaust passage. With the exhaust

diaphragm seated, further exhaust of the brake chamber air pressure is prevented.

Because the supply solenoid remains energized the supply diaphragm remains seated,

thus preventing application air from flowing to the delivery port and out to the brake

chamber. The modulator valve can enter the Antilock Exhaust or Re-apply mode from

the Antilock Hold mode.

Air Brake System

Revised 11/15/99

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BENDIX M-21-M-22 ANTILOCK MOD ASSY Manual de servicio Pagina 18

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